Manual for Theory

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Training package "SENSORIC" Manual for theory

1

Introduction

2

Inductive Sensors

2.1

Fundamental Principles

2.1.1 2.1.2 2.1.3 2.1.4 2.1.4.1 2.1.4.2 2.1.5

Basic Construction Reduction Factor Coil Size and Sensing Range Installation Problems Housing Flush Mounting Electronic Circuit

2.2

Types

2.21 2.2.1.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7

Cylindrical and Rectangular proximity switches Definitions Slotted Types Ring Types Bistable Switches Sensors for use in Welding Magnetic Fields Sensors to distinguish between different materials Inductive Analogue Sensors

2.3

Interfaces for Inductive Proximity Switches

2.3.1 2.3.1.1 2.3.1.2 2.3.1.3 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.3 2.3.4

Electrical Types and their Positive Effects Direct Current Switches Alternating and All voltage Switches Sensors to DIN 19234 (NAMUR) Protected and Safety Switches Reverse Polarity and Over Voltage Protection Overload Protection Safety Circuits Loads and their Characteristics Bus Connection

2.4

Manufacturing Technology

2.5

Applications

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Capacitive Sensors

3.1 3.1.1 3.1.2 3.1.3

Fundamental Principles Sensor Construction Sensitivity Reduction Factor

3.2 3.2.1 3.2.2 3.2.2.1 3.2.2.2 3.2.2.3 3.3.3

Practical Model RC Oscillator Interference Suppression Interference Effects Contamination Compensation Cutting out Interference Pulses Models

3.3

Applications

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Ultrasonic Sensors

4.1 4.1.1 4.1.2 4.1.2.1 4.1.2.2 4.1.2.3 4.1.2.4

Fundamental Principles Propagation of Sound Waves in Air Generation of Ultrasonic Waves Electrostatic Converter Bending Oscillator Membrane Oscillator L/4- Oscillator

4.2

P&F- Oscillator

4.3

Methods of Operation

4.4

Distance Measuring Ultrasonic Sensors

4.5

Ultrasonic Sensors in Through-Beam Mode

4.6

Possible Errors in distance measurements with Ultrasonic Sensors

4.7

Operating Conditions

4.8

Sensor Types

4.9

Applications

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Photoelectric Sensors

5.1 5.1.1 5.1.1.1 5.1.1.2 5.1.2 5.1.2.1 5.1.2.2 5.1.2.3

Fundamental Principles Emitter Element Light Emitting Diodes Solid State Laser Diodes Receiver Element Photodiodes Phototransistors Position Sensitive Diode

5.2 5.2.1 5.2.2 5.2.3

Methods of Operation of Photoelectric Sensors Direct Detection Photoelectric Sensor Reflex Photoelectric Sensor Through-Beam Photoelectric Sensor

5.3 5.3.1 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.3.3 5.3.3.1 5.3.3.2 5.3.4

Signal Processing in Photoelectric Sensors Interference with Photoelectric Sensors Stages in the Interference Suppression Interference Suppression using Optical Modulation Interference Suppression with Band Pass Interference Suppression using Blanking Interference Suppression using Digital Filtering Function Reserve Static Function Reserve Dynamic Function Reserve Protection against Mutual Interaction

5.4 5.4.1 5.4.1.1 5.4.1.2 5.4.1.3 5.4.2 5.4.3 5.4.3.1 5.4.3.1.1 5.4.3.1.2 5.4.3.1.3 5.4.3.2 5.4.4

Types Reflex Photoelectric Sensor with Polarising Filter Polarising Filter Retro-Reflector Through-Beam Detection Direct Detection Photoelectric Sensor with Background Screening Direct Detection Photoelectric Sensor with Light Guides Light Guides Principle of Operation Glass Fibre Light Guides Plastic Light Guides Sensors with Light Guides Output Stage of Photoelectric Sensors

5.5

Triangulation Sensors

5.6

Phase Correlation Sensors

5.7

Applications

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Magnetic Sensors

6.1

Fundamental Principles

6.2 6.2.1 6.2.2

Principal of Operation Hall Effect Sensors Magnetic Resistive Sensors

6.3 6.3.1 6.3.2 6.3.2.1 6.3.2.2 6.3.2.3

Saturated Core Probes Construction and Mode of Operation Function and Measurement Circuit Evaluation using an Oscillator Evaluation using Impulse Current Evaluation using Impedance Measurements

6.4

Applications

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This handbook is a part of the training pack „SENSORIK“ (SP1). This training pack part of the complex Automation Technology includes: - Training case - Handbook of Theoretical Explanations - Collection of Experiments - Solutions and Evaluation of Results - Data Sheets - Folio Set SENSORIK - Video „New Photoelectric Sensors“ - PLC programs - CBT "Industrial Sensors 1.0" The training case is the central part of the training pack; with this set of demonstrations and exercises, experiments with different levels of difficulties, which demonstrate the function, specific characteristics, parameters and typical application for each sensor type can be performed. The theory required for the training pack is contained in the handbook covering: inductive, capacitive, photoelectric, ultrasonic and magnetic sensors. The documentation is not only to aid the further education programme, but is also suitable for self study. The theory presented covering the fundamental physical principles, method of operation, type and possible uses of the sensors has been designed for use with the training case but could be used independently to study the application of sensors in automatic control.

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INTRODUCTION

1.1

Technical - economic importance of sensors

Automatic control has been introduced in production, process engineering, warehousing, materials handling or administration. The following are the main aims in doing this: * * * * *

Improvement in product quality Savings in energy and raw materials Increase in productivity Reduction in damage to the environment Humanization of the work place

Usually the required control engineering is achieved using a computer or an PLC as the central element. In the end the system can only fulfill the required tasks if it is supplied with reliable process information. This is achieved with the use of sensors, which operate according to the most widely different physical principles. These sensors convert non-electrical process measurements such as distance , angle, position, level, temperature or pressure into electrical signals in order that the controller or regulator can operate. At the present time over 100 physical, chemical and biological effects are known for which „technical feelers“ are on the market or under development. The sensors, because of their different operating principles, are only suitable for specific range of applications. This must be taken into account during the planning stage of an installation.

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1.2

Definitions

As stated above, sensors are signal converters, which change a non-electrical magnitude into an electrical magnitude ( only in a few applications will pneumatic output signals be produced). In automatic control sensors replace the function of the human sensory organs.

non-electrical signals mechanical

p

l

2

1

E

R

Q

chemical

ω

v

pH

thermal

%

magnetic

γ

B,H

T

3

∆t C

optical

4

E

∆t

U

E

U

R

U

R

U

5

W

electrical signal p l v ω

= = = =

pH %

= =

T B H γ

= = = =

Pressure Distance, Gap Speed Angular velocity, Speed of rotation Ion concentration Volume % Gas concentration Temperature Flux density Field strength photon

U R Q ∆t C E W 1 2 3 4 5

= = = = = = =

Voltage Resistance Quality factor of a resonant circuit Time interval Capacitance Electric field Electrical energy Ultrasonic sensor Inductive sensor Capacitive sensor Magnetic sensor Photoelectric sensor

Diagram 1.1: Survey of signal conversion with sensors

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Often in automatic control a binary signal is required to signal that an object is in a particular position or not in that position. Proximity switches, which are a special category of sensors, are used for such tasks. In these sensors the output is obtained as follows; the signal converter output is connected to a threshold circuit (e.g. Schmitt trigger), which operates when the converter signal is greater than or less than a preset value, or an adjustable value, allowing the output circuit to operate. Sensors, which operate without physical contact, have a number of advantages over mechanical contacts: no power required, no feedback and no contact bounce Greater number of switching operations and high switching frequency No contact wear Maintenance free Resistant to harsh environments

Subsequently explanation of some terms: Sensor:

other names are primary element, detector, measuring transformer, measuring transducer, pick-up Initiator: Referred to as proximity switches Sensor element: the part of the sensor, which detects the quantity to be measured, but cannot operate alone as the signal processing element and the connectors are also required. Example: Coil of the saturated core of a magnetic sensor,or the transducer of an ultrasonic sensor. Multi-Sensor System:A sensor system in which a number of the same type of sensors or a number of different types of sensors are used together to complete the required task. Due to the concentration the analysis of individual elements is achieved electronically, by the use of logic or mathematics. Example: The combination of a number of initiators to distinguish between production parts of different shapes and materials or a combination of gas analyses sensors; where the operating ranges of the sensors overlap and the total of their measurements by intelligent analysis gives more information than that obtained from individual sensors.

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1.3

Typical criteria for application

Inductive Sensor:

Magnetfeld-Sensor:

-

-

metal objects sensing range up to 50 mm switching frequency up to 5 kHz up to 250 °C up to IP 68 high noise immunity DIN 19234 (NAMUR)

-

magnetic objects sensing range up to 60 mm switching frequency up to 1 kHz up to 70 °C up to IP 67 high noise immunity DIN 19234 (NAMUR)

Capacitive Sensor:

-

metallic, non metallic objects, solids and fluids sensing range up to 50 mm switching frequency up to 100 Hz up to 70 °C up to IP 68 DIN 19234 (NAMUR)

Ultrasonic Sensor:

Optical Sensor:

-

-

-

objects which reflect or absorb sound sensing range up to 15 m reaction time > 50 ms up to 70 °C up to IP 67 lower noise immunity color independent not sensitive to dirt

-

objects which are lightreflecting or non-transparent sensing range up to 100 m switching frequency up to 1,5 kHz up to 300 °C (fibre optic) up to IP 67 detect smallest objects (fibre optic) DIN 19234 (NAMUR) fibre optic, adaptierbar

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1.3

Forecast for the Future

The Sensor element (that is the measurement detector) and the signal processing are already very closely connected in sensor systems today. In this sector integration is also time and money. The technology and experience in the miniaturisation of electronic circuits can be incorporated in and are ideal for the sensor development. It is only a matter of time before an integrated circuit and miniature sensor element can be produced on a piece of silicon, gallium arsenide or another semiconductor material. An interesting development is the use of enzymes, microbes or whole cells as sensor elements; known as „Bio-Sensors“. By the end of this decade contact element, that is control elements, will also be come part of this development. Finally, mechanical parts, such as pressure jets, switches or even motors with gears are already available based on the micro-electronic technology in mini-format.

Sensor materials • ceramic • amorphous metal • Fibre optic • Bio-components

Technology • Surface mount, hybrid • IC design technology • Laser alignment • micro-machining

NEW SENSORS

Sensor Idea • Micro-structure • Smart transmitter • Intelligent sensors • Multi- sensor systems

Communication • 2 conductor technology • Programmed wiring • Interfaces • Bus connections

Figure 1.2: Forecast

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Inductive Sensors

2.1

Fundamental Principles

2.1.1 2.1.2 2.1.3 2.1.4 2.1.4.1 2.1.4.2 2.1.5

Basic Construction Reduction Factor Coil Size and Sensing Range Installation Problems Housing Flush Mounting Electronic Circuit

2.2

Types

2.21 2.2.1.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7

Cylindrical and Rectangular proximity switches Definitions Slotted Types Ring Types Bistable Switches Sensors for use in Welding Magnetic Fields Sensors to distinguish between different materials Inductive Analogue Sensors

2.3

Interfaces for Inductive Proximity Switches

2.3.1 2.3.1.1 2.3.1.2 2.3.1.3 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.3 2.3.4

Electrical Types and their Positive Effects Direct Current Switches Alternating and All voltage Switches Sensors to DIN 19234 (NAMUR) Protected and Safety Switches Reverse Polarity and Over Voltage Protection Overload Protection Safety Circuits Loads and their Characteristics Bus Connection

2.4

Manufacturing Technology

2.5

Applications

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11 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given.

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Fundamental Principles

Inductive sensors, in particular in the form of inductive proximity switches, also known as initiators, are widely used in automation and the process industry. 2.1.1

Basic Construction

The active elements of an inductive sensor are the coil and ferrite core (see diagram 2.1). an alternating current is passed through the coil producing a magnetic field, which passes through the core in such away that the field only leaves the core on one side; this the active face of the proximity switch. When an metallic or magnetic object is near to the active face the magnetic field is deformed. An exact picture of the magnetic field can be obtained from computer simulation (see diagram 2.2). The effect on the magnetic field of a conducting material can be seen, in this case a steel plate. The change in the magnetic field due to the steel plate, also produces a change in the coil so that it’s impedance changes. This change in impedance is evaluated by the integrated sensor electronic and converted to a switch signal. Eddy currents are induced in electrically conducting materials present in the alternating magnetic field. The damping plate may be considered as short circuited winding, and the arrangement of damping material and sensor coil can be considered as a transformer.

Ferrite core damping plate

coin

Diagram 2.1: Principle of an inductive sensor

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damping plate

Diagram 2.2: Diagram showing the lines of force of the magnetic field of an inductive sensor with and without damping plate made from ST37

13 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given.

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The sensor coil forms the primary winding and the metal plate the short circuited secondary winding, see diagram 2.3. Because of the inductive coupling, represented by the mutual inductance M12 the current flowing in the secondary circuit i2 is reflected in the primary circuit. This manifests itself in the change of the coil impedance Z. This can easily derived from a comparison with the ideal transformer. Primary side:

u1 = (R1+j·w·L1)·i1 + j·w·M12·i2

Secondary side:

0 = u2 = (R2+j·w·L2)·i2 + j·w·M12·i1.

From the above we have: u1 ω2·M212 Z = — = R1 + j×ω×L1 + (R2 - j×ω×L2)· __________ i1 R22 + (ω×L2)2

Re (Z) = R1 + R2·

ω2·M212 _________ R22 + (ω⋅L2)2

ω2·M212 Im (Z) = ω⋅L1 - ω⋅L2· _________ R22 + (ω×L2) 2 It can be seen that in the presence of a conducting material the real part of Z is increased above the resistance of the coil R1 the increase is dependent on R 2, L2, M12 and w. Experience shows that the imaginary part of Z only shows a measurable change with very small separation between the coil and the metal plate; it is only necessary to draw on the change in the real part of Z to detect an object made of conducting material. i1

Z =

U1

R1

R2

M 12

L1

i2

L2

Diagram 2.3: Equivalent circuit of the transformer

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Reduction Factor

The increase in the real part of Z by the damping piece is largely dependent on the distance between the plate and the coil assembly and the material from which the plate is made, in particular from the material conductivity and permeability u. The largest change is obtained with damping pieces manufactured from mild steel (St37). The sensing range s of various materials is standardised against sn, which is the sensing range obtained with St37 and define a reduction factor, also known as correction factor: reduction factor = s/sn. Diagram 2.4 shows the dependence of reduction factor on the quotient of electrical conductivity divided by the relative permeability of the test piece; the example is for a proximity switch with 5mm sensing range ( no account is taken of the hysteresis loss of the test piece). The curve varies for each type of proximity switch, however it always has the same tendency. Diagram 2.4: Reduction factor of a proximity switch as a function of the quotient electrical conductivity / permeability of damping piece.

2.1.3

Coil size and Sensing range

Diagram 2.2 shows that the magnetic field only extends over a limited distance, which in the end determines the maximum possible sensing range of an inductive proximity switch. It is evident that the extension of the field and the sensing range sn increase with increasing coil diameter. There is a moderate increase in the sensing range with increase in core diameter for proximity switches with standard sensing range (diagram 2.5).

Diagram 2.5: Nominal sensing range sn for an inductive proximity switch, with standard sensing range, as a function of core diameter d.

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Installation Problems

The surroundings of the coil system, of inductive proximity switches, which includes conducting material outside of the active area creates a problem, in that this also has an effect on the shape of the magnetic field and therefore the impedance of the coil. 2.1.4.1

Housing

Where a stainless steel housing is used for a proximity switch, the induced eddy currents. in the housing, causes an initial damping in the coil system and the oscillator, which in turn reduces the maximum sensing range. The effect can be reduced by mounting a copper ring shell core in the steel housing; the magnetic field in the housing is reduced in this way (see diagram 2.6). The eddy currents which now flow in the copper ring instead of the housing produce a lower loss, because the electrical conductivity of copper is approximately 40 times that of usual housing material V2A (see also diagram 2.4). The pre-damping is reduced to such a degree that it is possible that the sensing range is increased.

coil shell core copper ring V2A-Housing

Diagram 2.6: Lines of force of the magnetic field of an inductive sensor with integrated copper screening

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Flush mounting

Further undesirable losses are produced when a sensor is flush mounted in a conducting material, e.g. machine parts made from steel. The sensing range is reduced due to the additional pre-damping of the sensor magnetic field. In unfavourable cases the initiator may switch by the installation. In this situation the screening produced by the copper ring has a positive effect in that the eddy currents produced in the installation material are reduced. Sensors with increased sensing range, for flush mounting are normally provided with copper ring screening. The effect of the screening however is reduced with sensors with larger diameters, so that the flush mounting of larger sensors remains a problem. A possible solution for the future could be that the proximity switch senses it’s surroundings, this will require an increased technical effort in both the construction and the control circuit of the sensor.

2.1.5

Electronic circuit The coil system of the proximity switch together with a capacitor forms a parallel resonant circuit. A simplified equivalent circuit is shown in diagram 2.7, L represents coil inductance and Rv = Re (Z) the coil resistance, which is dependent on the damping piece (object sensed). C is the parallel capacitor considered as an ideal capacitor. The resistance R v determines the Quality Factor Q of the resonant circuit. Diagram 2.7: Simplified equivalent circuit for the resonant circuit of an inductive sensor.

A block diagram of an inductive proximity switch is shown in diagram 2.8. The resonant circuit is part of an oscillator and the quality factor of the resonant circuit Q = wL/Rv determines the amplitude of the resulting HF oscillations. With the approach of the damping piece the quality factor of the coil is reduced due to the increase in the loss resistance Rv and therefore a reduction in the amplitude of oscillation. When the amplitude falls below a preset value a comparator operates, which in turn operates the output circuit and the sensor switches.

comparator

output stage

Diagram 2.8: Block diagram of an inductive proximity switch

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In diagram 2.9 the change in quality factor Q, as a function of distance s from damping piece, for a coil system of a flush mounting proximity switch with a nominal sensing range of 10 mm is presented. Diagram 2.9: The change in quality factor Q, as a function of distance s from damping piece, for a coil system of a flush mounting proximity switch with a nominal sensing range of 10 mm.

Diagram 2.10 Shows the relative change in quality factor ∆Q/Q for the same case, with reference to the undamped coil. The change in quality factor, which is taken as the switch point, is in the region of 10% to 50% (in this example 10%) for sensors with standard sensing range. In the case of initiators with double the sensing range only a change in quality factor of 1% to 6% is available, which demands a higher specification of the sensing electronic particularly with regard to temperature sensitivity. Diagram 2.10: Relative change in quality factor ∆Q/Q, of the coil system of an inductive sensor with 10mm sensing range, as a function of the sensing distance s of the damping piece, with respect to the undamped system.

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A basic oscillator circuit is shown in diagram 2.11. The resonant circuit comprises L and C. Transistor T is connected in the common collector configuration and operates as an noninverting amplifier with a voltage amplification less than 1; because of this the transformer feedback is necessary to produce the required voltage boost. The transformer is formed by tapping on to the coil. Rb and diode D determine the DC operating point of the transistor. Continuous oscillation of the oscillator is ensured by RE, which is also used to adjust the switching point. In practice this circuit exhibits a number of disadvantages, in particular with reference to temperature stability; because of this a slightly modified version is used as shown in diagram 2.12.

Diagram 2.11: Principle of the oscillator circuit

Diagram 2.12: Oscillator circuit

Here the diode is replaced by the base emitter of a second transistor. When both transistors are at the same temperature, which can be best obtained with a dual transistor, the temperature drift of one is compensated for by that of the other. The capacitor C in the resonant circuit is connected so that the inductance of both coil windings is used. In this way the required capacitance is reduced for a given frequency of oscillation f. This is given by : 1 f = ————. 2·p·(LC)½ Depending on the switch type this ranges from a few kHz to a few MHz and is to a large extent dependent on the size of the coil core and therefore the sensing distance sn (diagram 2.13). The current taken by the output of the oscillator is high in the undamped condition and low in the damped condition. Diagram 2.14 shows the current taken by the oscillator, for an initiator using this circuit, with 10 mm switching distance, as a function of the distance of the damping object.

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The switch point lies in the area of the rapid change in current, which offers the greatest sensor sensitivity. Various temperature effects cause unwanted drift in the coil quality factor. The ohmic resistance of the coil, which is wound with copper wire, increases with temperature. The hysteresis loss in the core, which increases with frequency, is also effected by temperature, this can be positive or negative depending on the ferrite material. These effects determine, together with other effects e.g. skin effect in the coil, frequency and temperature behaviour in the coil system. An attempt is made, by experiment, to determine a frequency at which the counter acting temperature effects cancel one another so that a constant quality factor of the coil is obtained. Diagram 2.15 shows the change in Q, of a coil system for an initiator with 10 mm switching distance, as a function of frequency for different temperatures. The curves are closest together at a frequency of approximately 550 kHz, therefore this the operating point at which the least temperature drift occurs.

Diagram 2.13: Oscillator frequency f, of inductive sensors, as a function of the rated sensing range s n

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Diagram 2.14: Current taken I of an inductive proximity switch, based on the circuit of diagram 2.12, with 10 mm switching distance as a function of the distance from the damping piece

Diagram 2.15: Quality factor Q of the undamped coil system, of an inductive proximity switch with 10 mm sensing range, as a function of frequency f, for different temperatures.

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Types of Proximity Switches

2.2.1

Cylindrical and Rectangular Proximity Switches

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The basic form of an initiator is the cylindrical form. One of the end faces of the cylinder being the active face. The same circuit is also supplied in a rectangular housing (Diagram 2.16).

Diagram 2.16: Examples of Cylindrical and Rectangular Sensors

Cylindrical proximity switches have a steel or plastic housing. The coil system with the ferrite core is mounted at the front active face and is protected by a plastic cap. Behind this is the electronic circuit mounted on a printed circuit board or as a thick film circuit assembly. An LED serves to indicate the sensor switched condition. The housing is sealed with an end cap, which holds the connecting cables. The whole of the inner space is filled with plastic encapsulating material (Diagram 2.17). Encapsulating material

Coil Plastic cap

Covering paste

Ferrite core Support

IC

LED

Component carrier

O- Ring-Seal

Housing

Holding Ring

Diagram 2.17: Assembly principles of a cylindrical inductive sensor

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Definitions

The terms used in the specification and classification of inductive proximity switches, as well as the methods used to measure the most important parameters are defined in DIN EN50010 and 50032. A standard test plate is required made from a square piece of 1mm thick mild steel grade 37; the side length is dependent on the nominal sensing range sn of the initiator. The nominal sensing range is a theoretical characteristic, which is used to classify proximity switches, without taking account of tolerances in the devices (diagram 2.18). The actual sensing ranges s r is determined with rated voltage and at an ambient temperature of 20 °C. A deviation of + 10% from the nominal sensing range sn is permitted. 0,9·sn < sr < 1,1·sn. The effective sensing range s u is the useful sensing range, which can be set, within the specified temperature and voltage range. It must not deviate more than + 10% from the actual sensing range: 0,9·sr < su < 1,1·sr. The operational sensing range s a is the sensing range within which the sensor operates under the permissible operating conditions. The value lies between 0 and the smallest value of the effective sensing range. 0 < s a < 0,81·s n. Test Plate 1,21 sn 1,1 sn 0,9 sn 0,81 sn

sn

+10 % +10 % -10 % -10 %

Sensing range sa sn sr su sa

= = = =

su max sr max sn sr min su min = sa

Nominal sensing range Actual sensing range Effective sensing range Operational sensing range

Diagram 2.18: Definitions of Sensing Range

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Slotted Initiators

Coil 1

The slotted initiator consists of two coil systems facing each other, this forms a transformer with a large air gap and and poor flux linkage (diagram 2.19). The two coils each represent a winding of the transformer in the oscillator circuit diagram shown in diagram 2.12. In the undamped condition the coupling between the two coils is sufficient to allow the oscillator to oscillate. The inductive coupling is reduced when a metal object is placed in the slot between the two coils. At a particular depth of the object in the slot the feedback in the oscillator reaches a critical value and the oscillations cease, the initiator then switches. Due to it’s construction the slot initiator is insensitive to changes in position of the metal object in the direction of the core axis, so that in this direction the system is inexact. The sensitive direction is perpendicular to the core axis.

Coil 2

In this type of sensor it is mainly the change in coupling between the two coils which is evaluated; the increase in resistive loss is of little importance. For this reason the material parameter of the damping object has much less effect on the switching point, as compared to the case of cylindrical initiators. Housing

2.2.3

Diagram 2.19: Construction of a Slotted initiator (principle)

Ring Initiators

Ring initiators have a toroidal core instead of a pot core, which is mounted cylindrically around the coil (diagram 2.20). It shields the magnetic field from the surroundings, so that the active area lies within the coil. Here again the oscillator circuit of diagram 2.12 is used. The oscillator circuit is damped as soon as a metallic object enters the space inside the ring. One application is the recognition and counting of small metallic objects, which pass through the initiator. Ferrous and non-ferrous metals can be detected, as with the reduction factor for cylindrical proximity switches, the smallest non-ferrous object must be larger than the smallest ferrous object in order to initiate switching. Ferrite Ring Metal Object

Diagram 2.20: Coil system and section through a Ring Initiator

Coil

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Bistable Switch

The bistable switch has two stable switched conditions, in which it can remain unchanged, even when the initiating object is removed. In principle this achieved with a bistable ring initiator; the coil system and schematic block diagram is shown in diagram 2.21. Two separate coils are mounted within a ferrite ring, each is connected to a separate oscillator. The two oscillators are linked to oppose each other so that only one can oscillate at a time. The circuit design ensures that on switching the supply on oscillator 1 operates. When a metal object approaches the initiator, from the left, coil 1 is damped and oscillator 1 stops oscillating and oscillator 2 commences oscillation; if the object enters coil 2 this will also be damped and oscillations will cease. As soon as the damping is removed from coil 1, by further movement of the object, the oscillations in oscillator 1 will recommence. When a conducting material passes from left to right through the bistable initiator, oscillator 1 will oscillate according to the initial stable condition; when the object passes from right to left through the initiator oscillator 2 oscillates, the second stable condition. It is therefore possible to use bistable initiators for direction detection. The oscillators are so designed that they require different operating currents, therefore the switched condition of the initiator can be detected from it’s operating current. Ferrite Ring Metal Object

Coil 1

Coil 2

Coil 1

Coil 2

Diagram 2.21: Coil system and Block Diagram of a bistable Ring Initiator

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Proximity Switches for use in A.C. and D.C. Welding Fields

When inductive proximity switches are used near to electric arc welding equipment, two detrimental effects occur. The strong alternating magnetic fields produced by the welding currents influence the magnetic core of the proximity switches, effecting the core up to the point of saturation or at least shifting the operating point, since the reversible permeability is noticeably reduced, thereby reducing the Q factor. In other words the coil system is damped, which may cause the sensor to switch. This can be remedied by using special cores made from sintered iron granules, which saturate at a flux density 2 or 3 times the saturating flux density of the conventional ferrite cores. However the cores have a lower permeability, so that the coil Q Factor is reduced. The second detrimental effect is that the alternating magnetic field produced by the welding equipment induces voltages in the sensor coil. These voltages effect the oscillator and may lead to an uncontrolled switching characteristic, which must be prevented by effective circuit design. Proximity switches for use in welding applications indicate the rough working conditions encountered particularly by their robust mechanical construction.

2.2.6

Sensors for distinguishing between different materials

Diagram 2.2 illustrates the principle of an inductive sensor, which is capable of distinguishing between ferrous and nonferrous metals. In addition to that shown in diagram 2.2 the sensor in this case has a closed metal ring is fitted around the core to ensure pre-damping.

Object

Pre-damping

Coil

Core

Diagram 2.2: Principle of the Material distinguishing Sensor

The operating current of these sensors in the undamped condition can be reduced to approximately half of that of a standard sensor, by the choice of the ring material and it’s dimensions. If the sensor is damped by a ferrous material, e.g. mild steel St37, then the operating current falls to the minimum value.

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Coil Core

Object

Pre-damping

Type 2

Object: Aluminium Object: Iron

Diagram 2.23: Current draw with and without pre-damping

When a nonferrous material enters the sensor field the current draw increases, with decreasing distance from the sensor, up to the maximum value. Technically this behaviour can be used in various applications. This switch is suitable for safety circuits, where the opposite switching characteristic is required to that of a standard sensor. In this case the damping material must be a nonferrous material e.g. aluminium. Another application is as a selective inductive sensor. Here the evaluation unit two switching thresholds are defined, one lies above and one below the current draw in the undamped condition. By including two independent outputs, which are controlled from the different evaluation units, the assigned output will operate depending on the damping material.

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Inductive Analogue Sensors

The inductive analogue sensor occupies a special position among inductive sensors, because instead of a switch signal at a particular distance of the damping piece from the sensor, an output signal is produced, which is proportional to the distance from the sensor. The output current, of the sensor, is proportional to the distance s of the object from the sensor over a definite operating range (see diagram 2.24). The mechanical construction and the coil system takes the form of a cylindrical proximity switch. The principle of operation is shown in diagram 2.25. The oscillator circuit supplies the resonant circuit with an alternating current of constant amplitude i. The following is true for the voltage u of the resonant circuit: u ~ (1 + Q2)½ . For Q factor greater than 10 u is almost proportional to Q and within certain limits to the distance of the damping piece from the sensor. In some types of analogue sensors an additional linearisation circuit is included, which increases the useful upper end of the operating range; this is not necessary in other types. In the output circuit the sensing signal is converted to a current which is proportional to the distance s. As in the case of standard proximity switches the data for analogue sensors is with reference to a standard mild steel test plate. Where nonferrous materials are used the operating range shifts and is reduced correspondingly. The inductive analogue sensors, as well as being suitable for contactless distance measurement, are also suitable for the identification of different materials. with linearisation Ia/mA 20 without linearisation 10

s/mm

0 0

3

5,5

Diagram 2.24: Output current IA of an inductive analogue sensor as a function of the distance s of the damping piece.

8

i = constant Linearisation

Output Signal

Output circuit IA

proportional to distance

Diagram 2.25: Block circuit diagram of an Analogue Sensor

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Inductive Proximity Switch Interfaces

Inductive proximity switches are divided into two basic groups, AC sensors and DC sensors. Two, three and four wire sensors are available. They may have normally open, normally closed or changeover functions. On the sensor side the interface is provided by the output stage of the sensor, which provides the link between the sensor and the customer interface (diagram 2.26) and fulfills numerous tasks: - Energy supply of the sensor - Interpret the sensor signal - Changing voltage level and amplification - Interference suppression (filter) - Optical Indicator (LED) - Protection against incorrect connection - Suppression of erroneous signals (e.g. due to switch on impulse) - Drive different loads on different circuits.

PLC

Sensor

2.3.1

Output Stage

Diagram 2.26: Function of an Inductive Sensor Output Stage as the link between the Sensor and the Customer Interface.

Customer Circuit

Electrical Types and Effective area of operation

The standard DC switches are available for the operating voltage ranges 10-30 V and 10-60 V. AC Voltage switches work over the range 20 V to 250 V. All current sensors (AC/DC sensors) operate over the range 20-300 V with DC voltage and 20-250 V with AC supplies. Initiators with an interface to DIN 19234 (NAMUR) are a special case.

2.3.1.1

Direct Current Switches

DC switches are available in two, three-or four-wire versions (Diagram 2.27). Two wire switches are operated in series with the load and require only two connecting cables for this. They can be connected with reverse polarity and are therefore similar to a mechanical switch.

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In order to supply the sensor itself with electrical energy a small residual current flows in the OFF condition through the load and in the ON condition a voltage drop is present. This must be taken into account in the selection of suitable loads. Three and four wire switches have separate supply connections and one or two outputs for the load; so the limits referred to above are removed. The decision to use positive or negative switching versions depends on whether the switch output connects the load to the positive or negative of the supply (diagram 2.28). Many two or three wire switches are available as normally closed or normally open switches. In the case of normally closed the load is switched off when the oscillator is in the damped condition, in the case of normally open the load is switched on. Four wire sensors have both functions, that is each has a normally closed and a normally open output.

Load Output

Diagram 2.27: Principle of the output stage of a three wire DC Switch, positive and negative switching versions.

Output Load

Diagram 2.28: Various Connections of Inductive DC Voltage Switches 2 - Wire - Technology

3 - Wire - Technology

Load p-switching

4 - Wire - Technology

Load Load p-switching

Load Load

n-switching

Load Load

n-switching

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AC and AC/DC Voltage Switches

AC and AC/DC voltage switches are available in two wire and three wire versions. What has been said above for the DC sensor types is also applicable for these sensor types.

2.3.1.3

Sensors to DIN 19234 (NAMUR)

These sensors are simple 2-wire DC sensors without the output stage. They contain only the oscillator as shown in diagram 2.12. DIN 19234 describes how the 2-wire sensor works together with a switching amplifier; it specifies the characteristics of the amplifier, the sensor and the switch point. The amplifier supplies the sensor with power, which in turn controls the amplifier due to a variable internal resistance which results in a variable current consumption. The operating values are kept so small that it is possible to install these proximity switches in potentially explosive atmospheres, taking account of the pertinent regulations and guiding principles for intrinsically safe apparatus in ignition protection zones. The sensor produces a output signal which is proportional to the distance of the influencing object to the sensor. An example of a stable output characteristic is described in diagram 2.29.

I/mA 3 Switch points

Difference in distance of switch points

Difference in current at the switch points 2,1

1

1,2

∆I

2

∆S

0 s/mm

Diagram 2.29: Relationship Distance/current according to DIN 19234

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Here is defined that the characteristic curve must be identical regardless of the direction of movement. The switching point, which is determined by the switching amplifier, must lie in the range 1.2 mA to 2.1 mA and must exhibit a difference in switching current (hysteresis) of 0.2 mA (typical 1.6 mA and 1.8 mA). The required switching distances are dependent on the damping material and the nominal sensing range of the sensor. The two wire connecting cable represents a resistance between the sensor and the switching amplifier; this resistance should not exceed 50 ohms. When the sensor operates in potentially explosive areas the maximal cable length is limited by it’s inductance and capacitance. The power supply of the amplifier, which supplies the sensor usually has a linear output characteristic with an open circuit voltage of approximately 8.2 volts and a short circuit current of approximately 8.2 mA. The sensor design is such that the internal resistance of the sensor is approximately 10kohms in the damped condition and approximately 1kohm in the undamped condition, which results in a maximal current in this circuit of approximately 4.1 mA. If the sensor current falls below the above value by 0.15 mA, this will be taken as an indication that there is an open circuit in the wiring or that a fault has developed in the sensor. If the current demand of the sensor rises above 6.0 mA a short circuit will be diagnosed. Both faults can be recognised by the monitoring circuits built into the amplifier, indicated and further processing of the information prevented.

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Protection and Safety Circuits

Various protection circuits are used to protect inductive proximity switches from damage from external sources by overloads or incorrect handling. The safety circuits guarantee that no incorrect signal appears at the sensor output, which could cause incorrect operation of the next stage.

2.3.2.1

Reverse polarity and Over voltage Protection

In the case of sensors with reverse polarity protection swapping the connecting cables at will does not lead to damaging the sensor. This is achieved by wiring protection diodes, or diode bridges, to the connections. An over voltage impulse on the supply voltage due to poor regulation of the power supply on switch on, or random disturbances, do not damage or cause incorrect operation of a switch with over voltage protection. Over voltage protection is achieved with a resistor and zener diode or by means of a varistor. From time to time in the electrical wiring of motor vehicles with generators (alternator) high voltages are produced, particularly where mechanical regulators are used. For example, if at maximum charging current the battery connection is loose causing an intermittent connection, which could result in a voltage transient on the supply of 100 to 200 volts approximately, due to the inertia of the regulator. Even in normal operating conditions over voltage transients can occur due to on and off switching of system components. Special circuit techniques such as a larger series resistor in the over voltage protection, higher voltage rating of the semiconductors and a higher rated over voltage protection element prevent damage to the switches intended for use in motor vehicles.

2.3.2.2

Overload Protection

Sensor with overload protection are not damaged when the load resistance reduces even to a short circuit, this is true for the complete specified voltage and temperature range of the device. The danger of overloading the output stage is the increased power loss in the output semiconductor and the increase in the device temperature above the maximum allowable temperature, which could result in damage to this component. The cheapest overload protection is the use of a thermistor with a positive temperature coefficient in series with the load; this had however certain disadvantages, a very high peak current flows in the case of a short circuit, the switch off current is very much dependent on the ambient temperature and this results in a high thermal load on the switch. This type of protection therefore can only be usefully applied with small load currents (Il < 100 mA) and low supply voltages (Us < 30 V). The recovery time after an operation is very long (approx. 1 minute).

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The principle is robust and free from interference, because of it’s sluggish operation. Because of it’s inertia it can be used to switch large capacitive loads. Another method of overload protection is to limit the output current to a constant value. This is the cheapest electronic solution; this leads however to a large power dissipation, particularly in the case of a short circuit. For this reason it is only used for small load currents (Il < 10 mA) and low supply voltages (Us < 30 V). It’s advantage is that it is immediately ready to operate once the overload is removed. In applications where large loads must be switched it is of particular importance that overloads are immediately detected and switched off. The switch off can be self-locking, that is after the removal of the fault the sensor does not automatically start to operate but requires a reset signal. This allows a simple localisation of the fault and in addition is desirable in relevant safety applications. In this case there is no thermal load on the sensor. The most flexible, also the most costly, solution is the pulsed overload protection. When an overload occurs the output switches off and after a short period (tp) switches on again, if the overload is still present the the current is limited to a value Ik and switch off again after a short time (9tk milliseconds), see diagram 2.30. The cycle is repeated as long as the fault is present. This produces an autonomous switch on after the fault is removed. The time required for the sensor to be again operational is the off period tp. There is only a small thermal load on the switch as the ratio of impulse (I = Ik) to off period (I = 0) can be small (tk /tp » 1/100).

Diagram 2.30: Principle of the pulsed overload and short circuit protection of a direct current switch.

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Safety Circuits

In the case of high resistance loads, that is without a definite On/Off signal level, e.g. measurements with a digital voltmeter, the reverse current of the semiconductor switching component in the output, which is approximately 10 µA, must be safely by-passed. In addition three and four wire switches for DC voltages have a base-load, which without an external load, cause a current of approximately 1 mA to flow through the conducting output stage; so that a break in various cables does not produce incorrect switching, undefined switching impulses are suppressed internally. During the starting period of the sensor circuit, after the supply voltage is switched on all outputs are suppressed in order to prevent any undefined output pulses. After the so called initialisation period, approximately 10 ms, the sensor is ready to operate.

2.3.3

Loads

Pure resistive loads add no special demands on the output stage of an inductive proximity switch. Neither over-currents or over-voltages occur during switch on or switch off. On the other hand inductive loads produce problems due to induced voltages. During switch off the load current IL continues to flow, due to the inductance L, through the overvoltage protection components (e.g. zener diode, varistor), the current decreases exponentially. The energy transferred during this time is proportional to L and IL2 so that the maximum allowable inductance must be specified. If this is exceeded the overvoltage protection components will be damaged and therefore the switch will be damaged, independent of whether the output stage is protected against overload or not. For this reason to switch high inductance loads a free-wheel diode should be mounted in parallel with the load, however this increases the dropout time of the relay, or contactor, because the stored energy, W = 0,5 × L × IL2 is slowly dissipated in heat. The inductance can then be as large as required. The requirement for reverse polarity protection prevents the diode being mounted in the sensor. Relays behave as inductive loads however it should be taken into consideration that the inductance in the pull-in condition is different to that in the drop-out condition. Since switch off occurs in the pull-in condition this is the determining inductance. It should be noted where contactors are used as AC loads that the impedance in the Off condition is very much smaller than that in the On condition, since the inductive portion of the impedance is very much larger than the resistive part; this results in a pull-in current 5 to 8 times the rated current.

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The pull-in time is in the region of 10 ms. If the power switch is a thyristor or a triac it is only possible to break the circuit in the region of zero current (IL < 20 mA), because of the device holding current; therefore at switch off only a small amount of energy must be dissipated in the overvoltage protection components. In practice the load inductance does not have to be taken into consideration. Special consideration must be given to capacitive loads. At switch on the capacitor load appears as a short circuit, the load current is only limited by the output stage design. Often in the case of switches without short circuit protection the short circuit current is not defined and in these cases only small capacitances in the region of 100 nF can be switched. By exceeding the maximum allowed capacitance causes the overload protected switch to revert to pulse operation and leads to damage in the case of switches without overload protection. Incandescent lamps also require special consideration. The data provided by the lamp manufacturer, rated current and wattage, refers to lamp in the illuminating condition. On switch on the tungsten filament is cold and the lamp draws 8 to 12 times the rated current in the case of vacuum lamps or gas filled lamps and in the case of Halogen lamps 10 to 15 times the rated current. The cold start current falls to about twice the rated current after 10 ms. Example: From this:

Lamp: Rated voltage Rated current Cold starting current

Un = 24 V; Pn = 2 W. In = 83 mA, P n/U n = 83 mA I k = 12 83mA » 1A

This means that the output stage in the switch must carry 1A for short period, without damage or change over into pulse mode operation. Output stages with overload protection,which are not designed especially for incandescent lights warm the lamp filament with a number of over current pulses, however analysing units such as relays, SPS or counters record these pulses. The effect is that relays oscillate, incorrect counter pulses generated etc..

2.3.4

Bus Connection

As automatic production systems are become increasingly complicated, the trend is more and more to decentralised systems. Thereby the communications requirements, at all levels, increases, down to the level where the sensors are established. On the other side components found at this level sensors, multiplexers etc. are increasingly supplied with digital electronics it would be advantageous to provide these with a serial Bus interface. This results in a number of advantages; one the system is clearer as with a star shaped individual wiring of all components; the system remains flexible, since modifications and extensions are possible without great expense.

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In addition the bidirectional Bus system makes it possible to transmit additional information, such as configuration, initialisation and parameter data, status and fault messages. It is increasingly possible to perform functions, which are at present handled by the central control, at the sensor, examples are signal pre-processing, linearisation, temperature compensation, calculating the mean value and analogue to digital conversion. Lastly not to ignore the saving in cable. In the future these advantages, and the introduction of standardised Bus-systems for this level, will lead to basic sensors, such as inductive proximity switches or distance indicators having a bus interface available.

2.4

Manufacturing technology

The various rising electronic manufacturing technologies are to be found in proximity switches. Therefore parallel to standard printed circuit board technology are found surface mount technology, hybrid and integrated circuit design technology. The complexity and reliability of the circuits increases in the order of the technologies given above. In the past ,because of the limited space available in a proximity switch, only relatively simple circuits with few components could be produced. Today with Standard- or Custom ICs technology it is possible to have hundreds or thousands of transistors on one chip with a few millimeters edge length. To a large scale SMT- and hybrid technologies are used in the manufacture of inductive sensors today. For many years standard integrated circuits have also been available, which contain in addition to the oscillator the signal evaluation and conversion to a switching signal functions. in addition these ICs offer auxiliary functions such as voltage regulation, suppression of switch on transients, short circuit and overvoltage detection and processing, which enables a simple design of a high quality sensor. On the other hand the use of customer specific ICs for inductive proximity switches is in it’s infancy.

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Applications

Application 1: Determination of position with the use of two inductive sensors.

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Application 2: Interrogation of a camshaft gear with inductive sensors.

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Application 3: Determinationof the speed of rotation with a slotted inductive sensor.

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Capacitive Sensors

3.1

Fundamental Principles

3.1.1

Sensor Construction

3.1.2

Sensitivity

3.1.3

Reduction Factor

3.2

Practical Model

3.2.1

RC Oscillator

3.2.2 3.2.2.1 3.2.2.2 3.2.2.3

Interference Suppression Interference Effects Contamination Compensation Cutting out Interference Pulses

3.2.3

Models

3.3

Applications

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Basic principles

Capacitive sensors, as do inductive sensors, without touching, non-interacting and contactless. They add to the range of sensor applications, where the inductive operating principle is unsuitable. Capacitive sensors can also detect nonconducting materials. Capacitive sensors are mainly available as proximity switches, recently however analogue sensors have also become available, which give an output signal proportional to the separation. 3.1.1

Sensor Construction

The active component of a capacitive sensor is the arrangement of a disc shaped electrode inside a cup-shaped screen (Diagram 3.1). These two electrodes form a capacitor with a basic capacitance Cg. When a target approaches the sensor (distance s) the capacitance changes by an amount ∆C. The capacitor is part of a RC oscillator, the output voltage of which is dependent on the effective capacitance Ca= C g + ∆C between the sensor electrode and the screen potential. A block diagram of a capacitive proximity switch is given in diagram 3.2. The oscillator output voltage is rectified, filtered and interference pulses suppressed. This forms a switch signal which is converted to an output signal in the output stage.

Screening Sensor Electrode

Target

Diagram 3.1: Principle of a capacitive sensor

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In principle there are three different possible methods of operating a capacitive sensor: with a nonconducting target, an isolated conducting target or an earthed conducting target. A nonconducting target (e.g. glass or plastic) can only increase the capacitance Ca by changing the dielectric in the field area of the capacitor. This increase in capacitance is very small and depends on the size and permittivity εr of the target. This enables only small switching distances. If an insulated conducting target (metal) approaches the sensor in addition to the basic capacitance C g two series connected capacitors are formed, that is between the target and sensor electrode and between the target and the screening. The increase in capacitance dC is greater than with the nonconducting target; this produces an average response sensitivity. The largest increase in capacitance, therefore the greatest sensing range, is obtained with an earthed metal target. The additional capacitance between the sensor electrode and the target is in parallel with the capacitance C g. Sensor electrode

Target

s

Screening

Diagram 3.2: Block diagram of a capacitive sensor. Target

Target

Screening

Sensor electrode

Sensor electrode

Sensor electrode

a)

Screen

Target

Screening

b)

Sensor electrode Screen

Screening Sensor electrode

c)

Sensor electrode Screen

Diagram 3.3: Methods of activating capacitive sensors a) non conducting target b) isolated conducting target c) earthet conducting target

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Sensitivity

The sensitivity is found by determination of the change in capacitance ∆Cs, at which the switch signal at the output of the sensor occurs. In order to have an impression of the order of magnitude of the change, we consider the case of an earthed conducting target. The problem is reduced to a plate capacitor with round plates of d= 30 mm diameter. The switch point for axial approaching targets should be s1 = 15 mm and the switch hysteresis h = 1. The switch point for targets moving away is then s2 = s1 + h = 16 mm. The capacitance of a plate capacitor is calculated from: ε0× A C = _____ ; s A = Area of plate, s = distance apart of the plates, εr = 1. From this the capacitance at the switch point s1 is

ε0⋅ π × d2 C1 = ________ = 0,4 pF ; 4×s At switch point s2 the capacitance has a value of

ε0⋅ π ⋅ d2 C2 = ________ = 0,39 pF. 4 × s2 The change in capacitance which produces a signal change at the output is therefore ∆Cs = C1 - C2 = 0,03 pF. Due to parasitic elements the basic capacitance Cg = 5pF approximately, this gives a relative change in capacitance of :

∆Cs ∆Cs 0,03 pF ____ × 100% = ________ × 100 % = _______ × 100 % = 0,5 %. Ca Cs + C1 5,42 pF

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Reduction Factor

Depending on the material of the non-conducting target, as shown in chapter 3.1, different changes in capacitance ∆C are produced. This effect can be observed at the capacitive sensors output as a change in the switch point. A material dependent reduction factor is defined analogues to that of the inductive proximity switch. The factor indicates by how much the nominal switching distance sn, which is obtained using an earthed metal target, must be reduced for a given material. In diagram 3.4 is shown this reduction factor = s/sn, as a function of the permittivity r, for various materials. Where the permittivity is temperature dependent a drift in the switching distance must be taken into consideration. Some sensors have the facility of adjusting the sensor range in order to compensate for the different sensing ranges, resulting from the reduction factors of various materials. For reliable operation of the sensor care should be taken not to set the sensing range to too high a value, as in this condition the RC oscillator can become unstable. This condition would become noticeable through an increase in the hysteresis (h > 0,1·s).

Water

Alcohol

Ice PVC Ceramic Glass

Oil

Reduction factor

Diagram 3.4: Reduction factor of a capacitive sensor as a function of the permittivity εr of the target.

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3.2

Practical Example

3.2.1

RC Oscillator

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The circuit used is a two stage RC oscillator (Diagram 3.5). The amplification of the first stage is: U1 Z1 + Z2 V 1 = ____ = ______ ; (U3 » U2). Z2 U2 The second stage, a common collector circuit, has an amplification of : Ua V 2 = _____ = 1. U1 Feedback of the output voltage is achieved via P and C k; with P is the ratio: U2 A = —— adjusted. Ua Setting the switch point with the potentiometer P, in the absence of a target, the following condition is produced: Z1 + Z2 V 1·V 2·A = ———— · A < 1 Z2 This means the oscillator cannot oscillate. The approach of a target leads to a reduction of Z2; with this V1 increases and the circuit amplification becomes V1 . V2 . A > 1. The oscillator starts to oscillate. The relationships are opposite to those of the inductive proximity switch, in which the oscillator without target oscillates and is damped by the approaching target. In the case of the capacitive sensor there is no oscillation without the target, in the presence of a target the system oscillates.

first stage

second stage

Diagram 3.5: Principle of the RC oscillator of a capacitive sensor.

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Interference suppression

3.2.2.1

Interference Effects

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Important interference factors are alternating electrical fields. These are coupled in the high resistance input circuit of the oscillator through the sensor electrode and can cause oscillation. The source of these interference fields are, for example, fluorescent lamps, solenoid valves, thyristor drives and radio transmitters. Continuous interference can only be eliminated by changing the oscillator frequency, providing the field is not too strong. Transient interference can be eliminated by the interference filter, which is described in 3.2.2.3 below, providing the pulse length lies within an adjustable time window. Another source of interference is the temperature effect. Changes in temperature effect the RC oscillator particularly. This effect can be minimised by setting a suitable operating point. Humidity, dust and other forms of contamination effect the sensor by changing the permittivity in the area of the active surface. Compensating for the contamination, described in 3.2.2.2 below, leads to a satisfactory improvement in many applications.

3.2.2.2

Contamination Compensation

The aim of the contamination compensation is to maintain a constant sensing range s, when the surface of the sensor is contaminated (e.g. by drops of water or a film of water). This is achieved through an additional cup shaped compensation electrode between the sensor electrode and the screen, which is connected to the oscillator output (diagram 3.6). The contamination increases the capacitance between the sensor electrode and the screen; this leads to an increase in the amplification V1. At the same time the capacitance between the sensor electrode and the compensation electrode increases. This effect reduces the circuit amplification V = V1. V2 . A. The amplification V remains constant by suitable geometric design of the sensor, compensation and screen electrodes, providing the contamination is homogeneous. Housing Contamination

probe

Sensor electrode Compensation electrode

screen

Diagram 3.6: Principle of contamination compensation

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Interference Filter

As described in section 3.2.2 electric fields can lead to malfunction of the oscillator. Following the rectification and the low-pass filtering of the oscillator output signal it passes through an interference filter (see diagram 3.2); which suppresses interference pulses, by the use of nonlinear filter components providing these do not exceed a maximum, selectable, period of time. This has however has the disadvantage that required switching signals, which have a longer pulse width, can not be detected; this means that the maximum possible switching frequency of the capacitive sensor is reduced. Normally the frequency is in the range 1Hz to 10 Hz. 3.2.3

Models

Capacitive sensors are mainly available as cylindrical or rectangular proximity switches, with an active face at the front end (diagram 3.7). The construction principle of a cylindrical sensor is shown in diagram 3.8. There are however special forms , for example, flexible sensors, which can be glued to level or curved surfaces. The manufacture of sensor electrodes on sheets or flexible copper laminated foil offers a large choice in the sensor construction. All the familiar electrical interfaces of the inductive proximity switches can be used. Available are two, three and four wire models for DC and AC voltages with normally open , normally closed and changeover functions. Also models to DIN 19234 (NAMUR) are available. Detailed information about the different interfaces can be found in the chapter „ Inductive Sensors“.

Diagram 3.7: Examples of a cylindrical and a rectangular capacitive sensor.

Sensor electrode

head housing

Housing Case

Screen cup (brass)

Diagram 3.8: Principle of the capacitive sensor construction.

component carrier

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Applications

Application 1: Position recognition with a capacitive sensor

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Application 2: Determination of the full limit for plastic components.

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4

ULTRASONIC SENSORS

4.1 4.1.1 4.1.2 4.1.2.1 4.1.2.2 4.1.2.3 4.1.2.4

Fundamental Principles Propagation of Sound Waves in Air Generation of Ultrasonic Waves Electrostatic Transducer Bending Oscillator Membrane Oscillator λ/4- Oscillator

4.2

P+F- Oscillator

4.3

Methods of Operation

4.4

Distance Measuring Ultrasonic Sensors

4.5

Ultrasonic Sensors in Through-Beam Mode

4.6

Possible Errors in distance measurements with Ultrasonic Sensors

4.7

Operating Conditions

4.8

Sensor Types

4.9

Applications

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Fundamental Principles

4.1.1

Propagation of a sound wave in air

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Ultrasonic waves denotes sound waves in the range above 20 kHz; the outside of the human hearing limit. As opposed to electro-magnetic waves sound waves can only be propagated through matter. The sound wave is dependent on changes of density ρ in time and space, the pressure P and the temperature T of the medium and with local changes and changes of speed of the medium particles. All the above values vary around a fixed average value. A prerequisite for sound waves in a medium is it’s elastic properties. The propagation velocity for an ultrasonic wave in gas is given by: c = (k · P/ρ) ½ = λ · f, P denotes the gas pressure and k is the adiabatic coefficient of the gas. For air the adiabatic coefficient k = 1.4 and the density r has the value of 1.29 Kg/m3 at an air pressure of 1013Pa. Since the density of a gas decreases with increasing temperature the velocity of sound is also temperature dependent. For air the relationship is given by: c = c0·(1+T/273)½, where c0 = 331.6 m/s (velocity of sound at T = 0°C) and T is the temperature in degrees centigrade. The change in the velocity of sound per K at room temperature, from this formula, is approx. 0.17%/K. The following table summarises the values of the velocity of sound against temperature.

T [°C]

-20

0

20

40

60

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331,6

343,8

355,3

366,5

377,5

In addition to being temperature dependent the velocity of sound is also heavily dependent on the air pressure, such that the speed increases with increasing pressure. The relative change in the velocity of sound with the normal changes in the atmosphere is approximately 5%. The following diagram, diagram 4.1, shows clearly the relationship between temperature, air pressure and the velocity of sound. 52

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velocity of sound Diagram 4.1 Effect of temperature and air pressure on the velocity of sound

In addition to these dependencies, the velocity of sound is also dependent on the air mixture, for example, the percentage of CO2in the air and on the relative humidity. The effect of relative humidity is less than that of temperature and pressure and produces an additional change in the sound velocity of about 2% between dry and moisture saturated air.

4.1.2

Production of Ultrasonic sound in air

In ultrasonic sensor technology the majority of transducers use a piezoelectric ceramic transducer. Ultrasonic transducers which use the magnetostriction effect are only used in ultrasonic welding technologyand therefore will not be further discussed here. Apart from the piezo electric transducer the electrostatic transducer is widely used; because of this it will be briefly covered here. Piezo-electric crystals have the property of changing there dimensions when a voltage is applied to the surface, also electrical energy can be converted to mechanical energy. Conversely when pressure is applied to the outer surface a charge is produced on the upper surface which can be measured as a voltage, which is typically in the order of 100V-. The materials used for these piezoelectric crystals are lead titanium oxide (PbTiO3) and lead zirconium trioxide. Because the production technology to grow macro crystals is difficult piezo ceramics have found many applications.

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Piezo ceramics are obtained from the sintering of piezoelectric crystals with additives (binding agent). The ceramic produced by the sinter process must be polarised by applying a high polarisation voltage at high temperature, since at first the dipole of the micro crystals are arranged in a random manner. The elongation in the polarisation axis is a maximum due to the polarisation. Typical elongation in such ceramics, by the application of a few hundred volts, is dl/l = 10-4 , during this the forces produced are in the region of 106 Pa.

The transition between the ultrasonic generator and the surrounding air is very important during the production of ultrasonic waves in air. To obtain efficient radiation of ultrasonic waves in air the ultrasonic generator must produce a large surface amplitude. An adaption mechanism is necessary, which transforms the high energy, but small amplitude, of the piezoelectric ceramic into a low energy, large amplitude movement. In the following different adaption methods are compared:

4.1.2.1

Electrostatic Ultrasonic Transducer

The transducer (diagram 4.2) in principle consists of a thin metallized plastic foil and a grooved metal plate, which together form a capacitor. When voltage is applied an electrostatic force acts on the foil. Metal support Metallized Plastic Foil Perforated Metal Plate

Grooved Metal Plate Flat Spring

Diagram 4.2 Schematic representation of a electrostatic ultrasonic transducer.

Foil and plate attract each other. An alternating voltage, which is superimposed on a DC voltage, causes the foil to oscillate at the same frequency. The DC voltage is necessary because the force on the foil is proportional to the square of the applied voltage, and with a pure AC voltage the frequency of oscillation would be twice the frequency of the applied voltage. The foil is held under a constant pressure by means of a flat spring. A frequency tuning up to approx. 500 kHz is possible through the air cushion ,which is trapped between the foil and grooves in the metal plate.Characteristics: Wideband, very short decay time and build-up period, relatively low acoustic pressure, open construction ( disadvantage in that a high voltage is applied to the outside). 54

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Bending Oscillator

A piezo ceramic disc is glued to a metal disc. When voltage is applied the diameter of the piezoelectric disc changes, creating shearing forces and bending of the whole system occurs, with a large amplitude. Metal (Alu) Diagram 4.3: Schematic representation of a bending oscillator

Ceramic

Characteristics: Wide radiation characteristic, relatively low frequency, low sound level, narrow-band, because it is a resonant system, very long decay time, encapsulated construction is possible. 4.1.2.3

Membrane Oscillator

An elastic membrane, for example made from metal, is activated into it’s natural period of oscillation by a piezoelectric ceramic.

Metall membrane Diagram 4.4: Schematic representation of a membrane oscillator

Ceramic Characteristics: Wide radiation characteristic, relatively low frequency, low sound level, narrow-band since it is a resonant system, very long decay time, open construction (high voltage). 4.1.2.4

λ/4 - Oscillator

During the transition of an acoustic wave from a piezo ceramic to air the acoustic wave passes through materials of different acoustic impedances. The transmission coefficient is the decisive factor for the efficiency which can be obtained. The transmission factor between ceramic and air however is in the region of 10-5 to 10-4, that is very small, so that no radiation of any consequence occurs. The transmission coefficient is considerably increased by means of an intermediate layer between the piezo ceramic and air.

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A material which comes close to meeting these requirements is a mixture of hollow glass beads and epoxy resin. This material is a compromise with regard to impedance matching, but it is suitable because of it’s resistance to environmental effects, it’s small internal damping and other mechanical properties. In addition to the measures taken for the impedance matching the decoupling layer is designed to have a thickness of exactly λ/4. The λ/4 layer causes amplitude gain, at the surface, by resonance rise so that large surface amplitudes are achieved. λ/4-decoupling layer

Diagram: 4.5 Schematic representation of a λ/4 oscillator

Ceramic Characteristics: High acoustic pressure, narrow radiation characteristic, average decay time, small waveband, high frequencies achievable, no conducting parts on the surface.

4.2

P&F-Oscillator (according to Becker)

In most cases of distance detection with ultrasonic sensors a narrow radiation characteristic is required. The direction characteristic of an ultrasonic sensor depends on the dimensions of the radiating surface, particularly the size, the transmitted frequency and the phase relationship of the oscillating surface. If a good directional beam is to be achieved with a fixed wave length, then the diameter of the emitting surface must be chosen, which is large compared to the wave length in air. In practice the problem exists that with increasing ceramic diameter the natural frequency decreases. In order to maintain the condition L 0,15 > 0,6 > 2,5

300 bis 3000 500 bis 4000 4000 bis 6000

Seperation "X" must be experimentelly determined

X m > 1,2 > 2,0 > 2,5

Detection range mm

XX m

Detection range mm

XX m

60 bis 300 200 bis 1000 800 bis 6000

> 1,2 > 4,0 >25,0

300 bis 3000 500 bis 4000

> 8,0 >16,0

Diagram 4.15: influence between simular sensors

Diagram 4.16: influence of sensors opposite each other

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Surface inclination: the surface of the object should be large, if possible , flat and not tilted more than 3° to the sensor axis, so that the sound wave is not reflected in an unwanted direction (diagram 4.18). This can also cause problems with round objects or wavy surfaces of liquids (agitating).

Diagram 4.18: Surface tilting

Angle of slope: Granulated material and heaped material can also be detected with ultrasonics. The surface of the heaped material should not have a slope of more than 45° to the sound cone axis. The grain size or the rough surface is responsible for the diffused reflection producing an echo which can be evaluated. However at too great a distance the echo is so weak that the object can no longer be reliably detected. Sonic beam deflection: a sonic beam can be reflected easily by simple reflectors made from almost any material (diagram 4.19). The detection range remains the same, if the reflectors are large enough and the sonic beam is not be deflected more than twice. The reflectors must be accurately aligned. In this way the sensor, for example, be kept away from corrosive media or the close range suppressed.

123 123 123 123 123 123 123 123 123

Diagram 4.19: Sonic beam deflection

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Sensor Types

Analogue output: Since the direct detection ultrasonic sensors for distance measurement determine the propagation time of the sound wave, from which the object distance can be immediately calculated; then the sensors are also suitable as analogue sensors. Combinations are offered which have an analogue output as well as a switched output, with an adjustable window range. The evaluation window/ the measuring range is adjusted differently in different sensors: - with two potentiometers - via a coding switch - parameters entered via the interface The analogue output can be supplied with a current output (4-20 mA) or with a voltage output (2-10 V); also it is possible to have automatic switch over from current to voltage output depending on the load. Digital interface: in ultrasonic sensors the evaluation of the signal is almost always a digital calculation. For this reason particularly with this type of sensor a digital interface is provided; usually the serial interface RS232 is used. By using the RS232 interface a wide range of possibilities are available, since this enables a dialogue between sensor and controller. In this way the parameters required for the evaluation of the signal can be entered. For example the range limits for the evaluation (switching range), the switching behaviour of the output (normally closed/normally open), continuous or single interrogation or the parameters for the sound velocity, e.g. temperature, can be changed externally. It is possible with only one sensor available to check if an object is present in the detection zone and when the object is present to determine the distance of the object. Here two additional switched outputs are also planned. 8-bit-output: The basic digital interface has 8 bit parallel logic output, the object distance is represented by two 8 bit words. The individual solutions correspond to 1/256 of the adjustable evaluation range. Intelligent Sensors:As opposed to sensors with adjustable parameters which have to be adjusted to the surroundings by the controller, there are sensors which have their own „intelligence“. These self teaching sensors are able to store the echo pattern at switch on or by activating the learn input. After the learning process is complete the echoes received are compared with those which have been stored. In this way interference objects in the detection region can be cut out. The sensor responds only to echoes which are different to those stored. Example: Ultrasonic sensors which have been specially designed for level detection store the echo of the empty tank, this profile detects all the interference echoes from objects built into the tank such as mixer, heating coils or safety ladders. When the level is measured the stored pattern is compared to the measured echo; only changing signals are evaluated. Sporadic interference signals are cut out with „plausibility control“

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Applications

Application 4.1: Monitoring the filled height in a vibration transport container.

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Application 4.2: Blockage control on a conveyer belt for the transportation of loose material

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5

PHOTOELECTRONIC SENSORS

5.1 5.1.1 5.1.1.1 5.1.1.2 5.1.2 5.1.2.1 5.1.2.2 5.1.2.3

Fundamental Principles Emitter Element Light Emitting Diodes Solid State Laser Diodes Receiver Element Photodiodes Phototransistors Position Sensitive Detector

5.2 5.2.1 5.2.2 5.2.3

Methods of Operation of Photoelectric Sensors Direct Detection Photoelectric Sensor Reflex Photoelectric Sensor Through-Beam Photoelectric Sensor

5.3 5.3.1 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.3.3 5.3.3.1 5.3.3.2 5.3.4

Signal Processing in Photoelectric Sensors Interference with Photoelectric Sensors Stages in the Interference Suppression Interference Suppression using Optical Modulation Interference Suppression with Band Pass Interference Suppression using Blanking Interference Suppression using Digital Filtering Function Reserve Static Function Reserve Dynamic Function Reserve Protection against Mutual Interaction

5.4 5.4.1 5.4.1.1 5.4.1.2 5.4.1.3 5.4.2 5.4.3 5.4.3.1 5.4.3.1.1 5.4.3.1.2 5.4.3.1.3 5.4.3.2 5.4.4

Types Reflex Photoelectric Sensor with Polarising Filter Polarising Filter Retro-Reflector Through Beam Detection Direct Detection Photoelectric Sensor with Background Screening Direct Detection Photoelectric Sensor with Light Guides Light Guides Principle of Operation Glass Fibre Light Guides Plastic Light Guides Sensors with Light Guides Output Stage of Photoelectric Sensors

5.5

Triangulation Sensors

5.6

Phase Correlation Sensors

5.7

Applications

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Fundamental Principles

5.1.1

Emitter Element

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The fundamental characteristics of the components described here is the conversion of an electric current into an electromagnetic wave (light), or the reverse. Under the heading light is understood the electromagnetic spectrum from near to the ultra-violet range ( λ = 0.3µm) through the visible range (0.38µm < λ < 0.78 µm) up to the infra-red region ( λ = 1.2 µm). Important modern emitter components are light emitting diodes (LED,IRED) and solid state laser diodes as emitter components; receiver components are photodiodes (P-N diodes, PIN-diodes), photo-transistors and lateral effect diodes (PSD). 5.1.1.1

Light emitting diodes (LED, IRED)

Light emitting diodes are basically semiconductors components, which consist of a PN junction. When a voltage is applied in the forward direction of a PN junction the electrons are excited and move easily into the p-side. Gallium Arsenide is the semiconductor material mainly used to produce light emitting diodes, which have a high efficiency. Gallium Arsenide has a wavelength of λ = 0.9µm. This wavelength lies near the infra-red region, for that reason GaAs is suitable for infrared diodes (IRED) with a high quantum efficiency (efficiency of a luminous source). An important representative of indirect semiconductor is GaP;by doping with Nitrogen (N) or Zinc Oxide (ZnO) recombination of electron-hole pairs is achieved with emission of light ,which is related to these impurities. The rest of the energy is lost as heat. It can be seen from this that the efficiency as a luminous source( quantum efficiency) of an impure or extrinsic semiconductor is less than for an intrinsic semiconductor. Through the choice of semiconductor and by doping with equipotential recombination centres it is possible to adjust the wavelength. The quantum efficiency of light emitting diodes in the visible spectrum is very much less compared to that of an IR-Diode. Providing the power dissipation limit and the maximum junction temperature of the semiconductor crystal are taken into consideration the light emitting diode can be modulated with a high impulse current Id; the associated momentary radiated power is many times that produced in continuous operation. Diagram 5.1 shows a typical maximum permissible pulse current for given duty cycles and known pulse widths ti.

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Infrared light emitting diodes have typical rise and fall time in the region of 400ns to 1µs and are therefore suitable for optical modulation.

Diagram 5.1 : Pulse Loading

One distinguishes between component types with an optical glass window and those with an optical system. The first have a very large aperture angle (diagram 5.2).

window

window

crys window

crystal

crystaltal

Diagram 5.2: LED with optical glass window; left: sketch of package, right: Intensity Distribution

These components indicate a relatively small radiated intensity, but with the addition of an optical system they demonstrate a well defined radiated intensity distribution (directivity characteristic). For reflex photoelectric sensors, whichrequire as far as possible a parallel radiation distribution, light emitting diodes with an optical glass window are particularlysuitable. In the case of components fitted with a lens the radiated intensity is relatively high and the aperture angle is small (Diagram5.3). Another area of application for these components can be found in the direct detection sensors for middle and smalldetection ranges, equally they can be adapted for use with light guides.

Lens

Crystal

Diagram 5.3: LED with lens; left: sketch of package, right: radiated intensity distribution.

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Semiconductor Laser diode

In the simplest example a semiconductor laser consists of highly doped p-n junction made from Gallium Arsenide (intrinsic semiconductor). Two important effects give the semiconductor laser it's typical property of emitting coherent light, they are the so called induced emission and the optical resonator in the semiconductor crystal. Coherence means that the wave trains of light have the same frequency and have a rigid phase relationship to each other. As opposed to the spontaneous emission of light emitting diodes in the case of induced emission the recombination process is started by the external influence of light with the correct frequency. For example an electron can start to emit at the moment when the influencing light wave oscillation rises, in this way all the emission processes are automatically coherent. Amplification also occurs, in that a weak primary radiation induces a strong secondary radiation. An optical feedback must be provided to maintain this process. An optical resonator, which is tuned exactly to the transition frequency fulfills this requirement, since a standing fundamental frequency wave is produced, which is again a fundamental condition for induced emission. In semiconductor lasers the optical resonator is produced by the parallel planes of the end surfaces of the Gallium Arsenide crystal in which the p-n junction is formed. The reflection at these cleavage planes is about 30% and therefore large enough to achieve the required feedback effect. The remainder of the light passes out of the crystal at both ends (Diagram 5.4). Metal Contact Mirror 2

Mirror 1 Active Zone p-n junction

Heat Sink

Diagram 5.4: GaAs Semiconductor Laser

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In contrast to light emitting diodes the emission spectrum of semiconductor lasers is very much smaller, as a result of the induced emission and the amplification in the resonator. The spectrum of the laser is different to that of light emitting diode, which has a continuous spectrum, in that the spectrum in most cases consists of discrete spectrum lines, produced by a large number of natural oscillations of the fundamental frequency together. By the use of special light guide techniques the spectrum can be compressed to prac-tically a single line. (Diagram 5.5) Semiconductor lasers are very sensitive to changes in temperature. The threshold current has a temperature coefficient of typically 1.5%/°C. Especially by falling Diagram 5.5: Spectrum of an :LED and a Laser temperatures is this effect critical, here the laser characteristic curve is very steep, because of this the diode reaches a region of high power, which results in damage. For this reason it is necessary to provide sufficient temperature stabilisation for the crystal. A further possibility is to control the output power and keep it constant. Many semiconductor lasers have an integrated monitor diode to enable the output power to be regulated. Semiconductor laser diodes have typical rise times and fall times in the region of 1ns to 5ns, which makes them especially suitable for high frequency optical modulation. In the case of laser diodes the beam forming exit slit is very small compared to that of the usual light emitting diode, so that by the inclusion of suitable optics almost parallel radiation can be produced. As well as laser diodes with integrated monitor diode, there are com-plete laser units with diode and optics available (Diagram 5.6).

Optics

Laser

Diagram 5.6: Laser diode with Optics

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Receiver Elements

5.1.2.1

Photodiodes (p-n and PIN diodes)

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The task of the photodiode is the conversion of a received optical signal into an electric current. In light emitting diodes a radiating recombinations process is brought about by the injection of charge carriers in the p-n junction, in the photodiode the opposite process occurs (Diagram 5.7). Photons of different Wavelengths Optical Compensation

blue

red infra-red

Contact p + - region Oxide

Space Charge Region RLZ N-Region N+ -Region Metal contact

Diagram 5.7: Method of operation of a photodiode

Due to the different carrier concentrations in p- and n-regions a, so called, space charge region is produced, without any external influence, which is free from moving charge carriers. Penetrating photons lead to the production of electron hole pairs close to the p-n junction. Carrier pairs, which are produced in the space charge region, are separated by the electric field present and at the same time transported to the other side. Holes are attracted to the p-region and electrons to the n-region. In this way a photocurrent (drift current) flows in the reverse direction, without an external voltage being applied. Holepairs, which are created outside of the space charge region must first diffuse into the space charge region, there separated and contribute eventually to the photocurrent (diffusion current). While in the case of the drift current the separation and transport of the carrier pairs takes place quickly in the case of the diffusion current the carrier pairs must first reach the space charge zone by the comparatively slow diffusion process. By means of a suitable internal construction of the photodiode the type of photocurrent and therefore the dynamic behaviour of the device can be controlled.

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In the case of the, so called, PN-diode the space charge zone is very small. Charge carrier pairs are formed outside of the space charge zone, mainly in the border region. For this reason the diffusion current is predominant, so that PN diodes are distinguished by a relatively low frequency limit and a large rise time. On the other hand the so called Dark Current is relatively small. Hence the PN-diodes are particularly suitable for measuring very low levels of illumination. PN-diodes have rise times and fall times in the range of 1µs to 3µs and junction capacities from 100pF to 1nF. In the case of PN-diodes with wide space charge regions the resulting small junction capacitance Cj together with a selected load resistance produces a low-pass characteristic, which influences significantly the frequency behaviour of the system. These PIN-diodes have a higher frequency limit and a small rise time. As in the case of light emitting diodes the photodiodes are separated into two important types: Photodiodes with plane window have a very wide directional characteristic (Diagram 5.8); therefore they are suitable for measuring intensity of illumination. A narrower and better defined directional characteristic can be obtained by the introduction of an optical system, so that these elements can be used in reflex photoelectric sensors, where this directional characteristic is required.

Radiation Sensitive Surface Cristsal

Diagram 5.8: Photodiode with plane window, left: sketch of package, right: Intensity distribution

Photodiodes with integrated lens have a relatively narrow directional characteristic (Diagram 5.9). These elements are preferred in direct detection optical sensors with small and medium detection ranges; especially when a possible adaptation for use with light guides is required.

Lens

Cristsal

Diagram 5.9: Photodiode with lens; left: sketch of the package right: Intensity distribution.

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Phototransistors

Phototransistors are basically photodiodes connected to a transistor which amplifies the photo current. The dynamic behaviour compared to the photodiode is comparatively poor. A phototransistor has a rise and fall time typically of 20us. The reason for this is to be found in the amplification mechanism, here the junction capacitance, due to the Miller effect, is increased by a factor B, because of this the maximum frequency, which can be achieved, is greatly reduced. As opposed to the photodiode the relationship between the incident radiated power and the resulting photocurrent is not strictly linear in the case of the phototransistor and can vary between 4 % to 20% from the ideal characteristic. Similarly detrimental is the temperature dependency. This large temperature dependency can also be an advantage. Namely, if a optical sensor is made with an IR-light emitting diode and a phototransistor the two temperature relationships almost cancel each other out. Phototransistors are available of similar construction and with similar optical properties to photodiodes. Simple and especially small phototransistors have only collector and emitter connections. In addition phototransistors are available with an additional base connection, which enables the operating point to be adjusted.

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Position Sensitive Detector (PSD)

An interesting variation of a photodiode is the so called position sensitive detector (PSD). In principle the PSD is a photodiode with a strip shaped light sensitive surface. Contacts K1, and K2 are mounted at each end of the device; the common substrate contact K0 is connected on the bottom face of the device (Diagram 5.10). The PSD has, in addition to the blocking layer resistance, a so called cross section resistance R q , in the longitudinal direction parallel to the light sensitive surface, therefore this resistance lies between K 1 and K2 If the PSD is radiated with a spot beam of light from a light source a current Iges is produced. The cross section resistance Rq is divided at this point into two section resistances Rq1 and Rq2. Similarly the current Iges is divided into two current components I 1 and I2, which can be measured at the terminals K1 and K2. Of interest is the relationship between the point p1, radiated by the spot beam, on the light sensitive surface and the components of current I1 and I2.

Light sinsitive surface

Diagram 5.10: Position sensitive detector

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Light sensitive surface

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In diagram 5.10 a normalised abscissa p is inserted of which the points 0 and 1 correspond to the ends of the light sensitive surface. The position of the spot of light p1 can be defined using this abscissa. The following is true for the component resistances Rq1 and Rq2: R q2 = p1·R q und Rq1 = (1-p1)·Rq

From the relationship

which finally gives

R q2 — R q1 p1

=

I1 — I2

=

I1 —— . I 1+I2

From this it can be clearly seen that by measuring the two components of current I1 and I2 the position p1 of the spot of light, on the PSD, can be calculated. The relationship between the penetrating radiated power Fe and the photocurrent is almost linear, as in the case of a conventional photodiode. Here it is interesting to note, that changes or variation in the incident radiated power have theoretically no effect on the position calculation from the above relationship, since these variations effect both components of current I1 and I2 in the same proportion and are therefore eliminated from the quotient. In most cases PIN diodes are preferred, for large surface area position sensitive detectors, in order to keep the rise and fall times small. Depending on the size of the optical surface switching times from 500ns to 50µs are to be found. In addition to the one dimensional position sensitive diodes described here there are also two dimensional devices. It is possible with these components to set up a two dimensional coordinate system, which can be used to determine the position on a surface.

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5.2

Methods of Operation of Photoelectric sensors

5.2.1

The Direct Detection Photoelectric sensor

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In the case of the direct detection photoelectric sensor (Diagram 5.11)light is radiated from the emitter E, diffused light is reflected back to the receiver R from the optically rough object O. When the received amplitude exceeds a fixed value the switch output Q is activated. Direct detection sensors have a typical range from 0mm to 500mm. Special models are available, which have a sensing range up to 10 metres. Direct detection sensors can detect all optically rough objects. Since a simple alignment of the sensor with the object is sufficient the installation and adjustment required is minimal. When combined with light guides the detection of the smallest objects is possible. Since here an evaluation of the received signal amplitude takes place dirty optics or changes in the objects reflection characteristics can have a detrimental effect on the stability of the sensing range. The received light intensity after a diffused reflection is very small; for that reason the individual sensing distances are relatively small. Due to the operating principle of the direct detection sensor, namely the evaluation of the reflected light from the object, transparent or reflecting objects cannot, or can only partially, be detected.

Direct detection E Sensor R

Diagram 5.11: Direct detection Photoelectric Sensor

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The Reflex Photoelectric Sensor

In the case of the reflex photoelectric sensor (Diagram 5.12) light is radiated from the emitter E and reflected from the retro-reflector back to the receiver R. When the optical path is broken by an object O the sensor switched output Q is activated. Ranges of from 0.1m to 20 m, or more, can be achieved with reflex photoelectric sensors. Reflex photoelectric sensors can detect all non-transparent objects. As opposed to the diffused reflection obtained with direct detection sensors here a much greater radiated power is returned to the receiver because of the retro-reflector,so that the detection range is relatively large. Dirty optics and changes in the optical properties of the object have much less effect than in the case of the direct detection sensor. However the adjustment and installation costs are higher, especially by greater distances between sensor and retro-reflector, because accurate alignment is required. Transparent objects can only be partially detected, the eventual reduction in reflected light, when an object enters the optical path, may be insufficient to allow detection. Reflecting objects can produce an inadmissible condition in the optical path; this occurs when the emitted light is reflected back to the receiver by the reflecting object. In this case there is no difference between the object and the retro-reflector.

Reflex photoelectric Sensor

E R

Diagram 5.12 Reflex Photoelectric Sensor

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Through Beam Photoelectric Sensor

The operating principle is shown in diagram 5.13. Light is emitted from the emitter E and reaches the receiver via the optical path. When the optical path is broken by an object O the switch output is activated. With through beam sensors distances up to 100 metres can be bridged. As with reflex photoelectric sensors all non-transparent objects can be detected; in addition with the through beam principle reflecting objects can be detected without difficulty. Dirty optics and changes in the properties of the object have the least effect using this operating principle. In most cases an electrical connection is required between the emitter unit and the receiver unit. In general the installation costs are highest with through beam photoelectric sensors. As with reflex sensors the alignment cost is also high. Here again transparent objects cannot, or can only partially, be detected.

E

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Diagram 5.13: Through beam Photoelectric Sensor

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5.3

Signal processing in Photoelectric Sensors

5.3.1

Interference sources with optical sensors

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Diagram 5.14 shows clearly that an opto-electronic system can be placed in a very hostile environment. Good signal processing should be able to suppress or eliminate the interference effectively. For a closer examination it is an advantage to separate optical from other interference mechanisms.

Interference from constant light source Object

optical switch

Interference from variable light-intensity source

Dirt Out of adjustment

Defect

Incorrect setting

EMI Voltage variations

Diagram 5.14: Many interference factors effect an optical sensor

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The main source of optical interference are light sources which can be subdivided into constant and variable light sources. The constant light sources include the Sun, which radiates light close to the infrared region and artificial light sources (incandescent lamps). They induce a direct photocurrent in the receiver element, the size of which can be many times greater that the useful signal itself. In addition this direct current produces noise in the receiver component, which has the effect of reducing the signal/noise ratio. Incorrect operation of the optical switch can not be ruled out under very unfavourable conditions. Fast artificial light sources (fluorescent lights), lightning, welding arcs and neighbouring optical switches are examples of interference from variable light-intensity sources. These produce a photocurrent in the receiver with a very small direct current component but with a high alternating component, which as before can be many times larger than the useful signal. The frequency spectrum of these interference sources is unlimited and can lead to incorrect operation of the switch. Diagram 5.15 depicts the induced photocurrent for different interference sources as a function of distance d; as a comparison the useful signal of a direct detection photoelectric sensor is also shown in the diagram.

Incandescent lamp 100W

Sun

fluorescent Tube 40W

Useful Signal

Diagram 5.15: The level of induced photocurrent for different sources.

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In diagram 5.16 the variation in light intensity with time for an incandescent lamp and a fluorescent tube is shown. The incandescent light source shows a relatively large direct current current component and a very small alternating current. For this reason it can be considered in general as a source of constant light interference. The opposite is true in the case of a fluorescent lamp, the intensity curve of which has superimposed phase related impulse spikes, due to fast gas discharge in the in the tube. The frequency spectrum of these impulses is very large and can, as will be seen later, interfere with the sensor sensitivity. Attenuation in the optical path, due to dirty optics and reflectors can also be counted as optical interference. The resulting loss in the receiver power can be so great as to cause the optical switch to drop out. An important non-optical interference variable are temperature changes which above all effect the efficiency of the optoelectronic components. The result of these temperature effects are changes in the sensing range of direct detection sensors and in the case of reflex photoelectric sensors a loss of reserve signal power. Variations in the supply voltage can have a similar effect. In critical applications, for example, a reflex photoelectric sensor is used when a strong background reflection must be taken into account, in these conditions an incorrect sensitivity adjustment, so that the error signal lies close to the decision threshold means that only another very small interference effect is required to cause incorrect switching. External electromagnetic radiation can induce interference in the signal processing circuit which leads to incorrect operation.

Fluorescent Tube Incandescent Lamp

Diagram 5.16:

Graphs of photocurrent against time produced by Incandescent lamp and Fluorescent Tube.

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5.3.2

Stages in Interference Suppression

5.3.2.1

Interference suppression by optical modulation at the Emitter.

As opposed to continuous operation the emitter diode of the optical sensor is supplied with a time variable current iLED and therefore optically modulated. In most cases a rectangular pulse is chosen (Diagram 5.17). This simple measure offers three immediate advantages.

Diagram 5.17: Square-wave modulation of diode current and therefore the light.

One result of the optical modulation is the noticeable difference between interference from extraneous constant light and the alternating voltage pattern of the useful signal. In this way interference from extraneous constant light can be eliminated (Diagram 5.18). First the useful signal is is increased by the constant interference current id. In the subsequent signal processing unit, which has a high-pass characteristic, the continuous current component can be eliminated. The alternating component is (useful signal) remains, this can the be unambiguously interpreted by further signal processing. A further advantage of optical modulation is the possible increase of the emitter power Φs. The receiver power increases by the same amount, all other conditions remaining unchanged, also the signal/noise ratio is similarly increased. In pulse mode operation light emitting diodes can be driven with a high current i LED.

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Using the simple assumption that for the maximum power dissipation given a constant diode current iLED is permissible; then for optical modulation:

iLED =

T - · ILED. ti

The radiated power Φs is increased by the same ratio, for the same operating conditions, also the signal /noise ratio of the receiver signal. Finally the third advantage of the optical modulation should be mentioned, that it is he requirement for so called "blanking", which is described later.

Constant light interference

Without constant light interference

No Object

Object

With constant light interference

No Object

Object

Diagram 5.18:Filtering out the current due to constant light using optical modulation.

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Interference suppression after reception by band-pass

The band-pass characteristic reduces the frequency range of the total system. In the upper frequency range the noise and interference alternating light are attenuated, in the lower frequency region interference constant light (e.g. Day light), low frequency interference light (e.g. 50Hz modulated light from incandescent lamps) and noise are also attenuated. Due to the optical modulation at the emitter the receiver current ir has a pulse width of ti. The pulse shaped receiver current ir is transformed into a voltage U a by the high and low pass characteristic of the circuit. Diagram 5.19 shows a simplified view of the process.

ir

ir

Interference component:

Useful component:

Diagram 5.19: The low frequency interference is filtered out by the band-pass filter.

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Interference suppression using blanking

The processed signal from the receiver amplifier is the digitised by an A/D interface. The interface consists of a comparator with a decision threshold Ues (See diagram 5.20). Interference components greater than Ues pass unimpeded to be also digitised and remain part of the total signal. A useful signal can be expected a very short time after an emitted light pulse. The blanking suppression utilises this fact.

Interference component:

Useful component:

Diagram 5.20: Weak signals are filtered out by the comparator.Interference component Useful component

The symbolic switch in Diagram 5.21 is closed for a very short time only following an emitted pulse, when an useful signal is expected. For this reason the switch is synchronised with the emitter. A number of interference pulses are in this way eliminated during the emitter "off" time. It should be noted, that if by chance an interference pulse occurs at the emitter "ON" time then it will remain part of the total signal as before and without further measures could lead to incorrect operation of the optical switch.

Intergference component:

Useful component:

Diagram 5.21: By clocking the receiver interference pulses outside the time field are filtered out

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Interference suppression by digital filtering

In order to suppress the disturbance a statistical evaluation of the frequency is required. The decisive assumption here is that the previous signal processing has eliminated interference signals to the level that the frequency of these signals is small compared to the useful signals. A simple and very effective method consists of connecting the data stream to a digital up/down counter, which is synchronised with the emitter pulse generator. If immediately after an emitter pulse the receiver data bit logic "1", then the counter increments, the opposite occurs if a logic "0" is present and the counter counts down. When the counter reaches the maximum or minimum value it is reset. The output Q of these Flip-Flops represents the switch output of the optical switch (Diagram 5.22). Interference component

Object

No Object

Object

Counter

Counter State

Flip-Flop

Diagram 5.22: Interference suppression using Digital Filtering

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A delay time ts is associated with this technique between the occurrence (Object/No Object) and the reaction of the output Flip-flop. When no interference is present this time is given by: ts = T·(2n - 1), here n is the "filter depth", this means the maximum counter steps and T the emitter pulse repetition rate. When interference is present the time ts is increased by amount depending on the frequency of the interference. It can be seen that as n increases the noise immunity of the system increases, since more interference pulses can be tolerated before incorrect switching occurs. On the other hand with increase in n the delay time ts is greater and the maximum switching frequency is reduced. 1 fs = —2·ts From the above relationships we can obtain the product

fs·(2n - 1) =

1 —2·T

which leads to the expression : Switching Frequency * Noise Immunity

1 ~ T

The product of switching frequency and noise immunity should be large. The emitter cycle time T must therefore be as small as possible; to achieve this it is indispensable that emitter pulse time ti is as short as possible, because of the pulse loading of the emitter diode. For this reason in high performance optical sensors the relatively slow phototransistor is not used as the receiver (detector), instead a fast PIN diode together with a receiver amplifier is used, which has a higher frequency limit. Some sensor types have the facility to select the "filter depth" depending on the application. A selector switch in the terminal compartment of these sensors enables selection of the switching frequency between 200Hz and 1.5kHz. Internally the "filter depth" is changed correspondingly. At 200Hz the counter reaches it's highest value after 15 pulses, with 1.5kHz the highest value is reached after 3 counter pulses.

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5.3.3

Function Reserve

5.3.3.1

Static Functions Reserve

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For the input amplitude an upper good region (OG) is defined, in which information "sufficient reflection" is available, also a lower good region (UG) is defined related to the information "no reflection"(Diagram 5.23). Between these two regions is the region of the Function Reserve (FB). The switching thresholds also lie in this region. When the amplitude lies in this region incorrect switching may occur due to external factors (temperature, dirt, reflections, etc.). In this situation a special output FRA indicates that the received signal lies in this unfavourable region FB; this enables suitable precautions to be taken in time. The static reserve indicator FRA is unsuitable for dynamic applications, where there is continuous switching of the switched output, since with each switching operation the signal passes twice through the function reserve region and the FRA indicator operates even though the optimal conditions exist. Here the dynamic function reserve indicator is used.

Empfangspegel Receiver Level Switch ON point Einschaltpunkt Schalthysterese Switching hysteresis

Function Reserve Range

Switch OFF point Ausschaltpunkt

Switched output(N.O.) Schaltausgang (Schließer) LED gelb yellow LED DUAL-LED Green - Supply voltage Red - Function Reserve

Function Reserve Output Funktionsreserveausgang

Diagram 5.23: Static Function Reserve

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Dynamic Function Reserve

Following each switching operation the signal amplitude is checked to see if it lies within or without the function reserve region. The interference output is set when the receiver level, before a switching operation, has not left the function reserve range. The report is independent of time, that is it is independent of the speed of the object and the frequency of the switching of the functions reserve indicator due to dirt or maladjustment. The interference indication remains on until the correct switching conditions are again provided. A switching operation can take place even if the ideal level has not been reached (Diagram 5.24). Receiver Level Switch "ON" point Funktionsreserve

Switching Hysteresis Switch "OFF" point

Switched output (N.O.) LED yellow DUAL-LED green-supply voltage red-Functions Reserve

Function reserve Output

Dirt

Poor reflecting object

Background reflection

Diagram 5.24: Dynamic Functions Reserve

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Protection against mutual interaction

A number of optical sensors may be found in the same effective optical field. From a sensor, here called the interference, pulses are emitted and received and processed by a second sensor, the operation of which is then disturbed. The emitter pulse repetition time of the interference is T1 and of the disturbed switch is T2. Under the ideal conditions that T1 and T2, are at all times the same size, if in addition a time shift t0 is present there would be no mutual interference, since the received pulse of the interference arrives in the blanking space of the other sensor. These ideal conditions are highly improbable. In reality the times T 1 and T2 will always be different from one another. This produces a form of beat frequency, with periodic repeating time zones, in which the received interference pulse can pass the interference blanking unhindered. A remedy is to produce different but fixed times for T 1 and T2, as is possible with some sensors switching from frequency 1 to frequency 2 with the selection switch in the terminal compartment. T 2 = T1 + ∆Tv The number S of the interference pulses which occur can be calculated: ti S = —∆Tv By suitable selection of ∆T a good compromise can be found between on the one hand the filter depth n and on the other hand the highest possible switching frequency of the optical sensor.

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5.4

Types

5.4.1

Reflex photoelectric sensor with polarisation filter

5.4.1.1

Polarisation Filter

In addition to the propagation direction or characteristic the oscillating behaviour can be modified. This is achieved using optical filters, which are only translucent to light of a definite wavelength or definite range (e.g. red filter, UV filter). The filtering out of a component of light can be achieved by reflection, absorption or deflection. A part of the energy is always lost with these methods of filtering, since only a portion of the light is allowed through. Linear polarisation filters are of special importance to optical sensors, because they only allow light waves a particular oscillating plane to pass through (Diagram 5.25).

Diagram 5.25: Polarisation filter

With polarisation filters the light is split by either reflection or refraction, during which one of the two beams or both are linearly polarised. This effect, for both reflection and refraction, is dependent on the optical properties of the medium used. These filters also absorb or reflect a part of the light, so that only part of the light energy available can be used. Polarisation filters are used with reflex photoelectric sensors.

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Retro Reflectors

While with the two optical surfaces, a mirror or contact surface, an angle is produced between the incident and the reflected light beam, with a retro reflector the reflected light is parallel to the incident light. This is achieved either by an arrangement of many three sided pyramids (tetrahedron) or by the use of special folia coated with hollow glass balls. Triple reflectors consist of extremely exact cube corners (pyramids) of glass or plexiglass (Diagram 5.26). Here the specular reflection is produced by three total spatial reflections .

Diagram 5.26: Triple Retro Reflector

Retro Reflectors, because of the multiple refractions or reflections, are capable of turning polarised light into depolarised light and/or rotating the polarised plane through 90°. Retro reflectors are used, for example, to improve the recognition of objects in traffic (reflecting signs) and are used with reflex photoelectric sensors.

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Reflex Photoelectric Sensor

With reflecting objects, which are to be detected using a reflex photoelectric sensor, there is only one position in the optical path where the light from the emitter is reflected back to the receiver. Since the reflex sensor is activated by a break in the light beam in this special case it cannot distinguish between the reflecting surface and retro reflector. To prevent this fault occurring polarised light is used. In diagram 5.27 the principle of operation is shown. The emitter and receiver are provided with 90° displaced polarisation filters F1 and F2. The non-polarised light leaving the emitter has only a horizontally oscillating component after passing through the linear polarisation filter F1. The receiver can only receive light polarised in the vertical plane, since the filter F2 is arranged perpendicular to filter F1. Retro reflectors have not only the property of mainly depolarising light, but also to rotate the oscillating plane through 90°. Both effects now enable the vertical component of light to reach the receiver. In this way the reflector is recognised. Ideal reflecting objects however rotate the polarisation plane through 180°, so that the horizontal oscillating plane is maintained. Since the receiver filter F2 blocks horizontally polarised light the light reflecting object is detected with certainty, based on the criteria "receiver beam is interrupted".

Linearpolarised filter

F1 Mineral glasswindow

Diagram 5.27: Reflex photoelectric sensor with polarised light

F2 Retro Reflector

Reflecting Object

Only 90o roated light passes through

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Direct detection sensor with background scattering

As already mentioned optical sensors evaluate the intensity of the reflected diffused light, whatever the source of the light. In unfavourable applications it is possible that the interference reflection from the background scene has an amplitude at the receiver similar to the object itself. In this case it is difficult, if at all possible, to detect the object. A remedy is the so called background scattering, where a clear boundary between the sensing range and the background region is provided. There exists a series of procedures, from which one will be described here: The emitter and receiver optics are arranged so that their optical axes cross (diagram 5.28), because the two cones of light intersect an optical active space R is formed. It can be seen clearly, that an object can only produce a diffused reflection in this space, while the background light is effectively cut out. By turning the optical axis the useful space can be selected for a particular application as required.

Diagram 5.28: Cutting out the background light.

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Direct detection sensor with light guides Light guides Principle of operation

Light guides are fibres of glass or plastic, which are capable of transmitting light fed into it. The light follows the shape of the light guide even when it is bent. This achieved by the use of the total reflection at the boundary surface of two media (diagram 5.29). The light is fed into a fibre of optically "dense" medium (e.g. glass or plastic), the diameter of which is selected so that at the optically rarer surrounding medium the critical angle for total internal reflection is exceeded. The light beam is reflected back from, the boundary between the optically dense fibre and the rarer surrounding medium, into the core and travels in a zig-zag course to the other end of the fibre. Mantle Diagram 5.29: Principle of operation of a Light Guide

Core

5.4.3.1.2 Glass fibre light guides Glass fibre light guides are normally combined together in a bundle of a large number of single fibres (approx. 0.05mm) within a sheath made from PVC, Silicon or stainless steel. In this way the required flexibility is guaranteed. The individual fibres can divided in various ways between emitters and detectors (Diagram 5.30). Depending on the application various arrangements can be chosen. Usually a half circle arrangement is used; for the detection of small objects a concentric or segment shaped arrangement is useful. The diameter of the light beam which can be transmitted depends on the number of glass fibres available; with this the loss increases or decreases and the sensing distance at the end of the light guide which can be achieved. Tables in the catalogue give the relationship between the sensing range determined by the sensor, the glass fibre diameter which should be selected and the distance which can be covered with the light guide. Glass fibre light guides have a lower attenuation with infrared light, therefore they are preferentially combined with sensors operating in the infrared range. Diagram 5.30: Examples of the cross-sectional area of light guides

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Plastic Light guides

Plastic light guides in contrast to glass fibre light guides usually consist of a single 1mm 2mm thick fibre for emitter and receiver. The bending radius possible is less than that of glass fibre light guides, with the same flexibility, because of the soft material. The optical properties are poorer than the glass fibre light guides. In particular the attenuation is considerably larger than that of glass fibre light guides. Optimal use of these light guides is only possible with sensors operating in the red light region. The most important advantages of plastic light guides is the favourable price and hardiness to mechanical wear and tear. Particularly application friendly is easy on-site connections, with a low priced tool (cutter). 5.4.3.2 Sensor with light guide The use of optical sensor together with light guides increases the application spectrum as follows: By the suitable choice of light guide diameter and the use of ancillary optics very small objects can be detected. Applications where temperatures up to 300° C occur can be covered with the use of glass fibre light guides. Light guides can be used in areas where there is the danger of explosion providing certain limiting factors are taken into account. Finally the following layouts enable three basic methods of operation (see diagram 5.31): 1. 2. 3.

The basic direct detection operation is possible with a parallel arrangement. Through beam operation can be achieved when the light guides are opposite each other. When the light guides are arranged at a particular angle to one another, then only two optical axes can be detected. Objects or other reflections, which lie outside of this region are cut out.

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Diagram 5.31: Reflex and through beam detection with light guides.

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Output Stage of Optical Sensors

Optical sensors can operate as normally open (N.O.) or normally closed (N.C.) switches. In the N.O. mode detection of an object is indicated when the switch closes. The opposite switching operation with the N.C. output takes place, that is the switch opens when an object is detected. The method of operation depends on the application, and different sensors have different methods of operation, which can be selected by the user: - by reversing the polarity in the case of a wired operating programme. - by reversing the operating mode, that is reversing the selector switch in the connection compartment. Other features already mentioned are -

Sensitivity adjustment EE Filter depth FT Functions reserve indicator (static and dynamic) FRA Protection against mutual interference (T1/T2) GS Switching time function Switching time adjustment Detection range adjustment

which can be selected either with a DIP-, Rotatory switch or via a bus connection.

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Triangulation Sensors

The dependence of the sensing distance, with basic optical sensors, on the reflecting properties of the object (colour), can be avoided by the use of the triangulation principle. A distance measurement independent of colour can be obtained using this measuring technique. Emitter and receiver are mounted in a common enclosure at a definite separation B and a precise angle to one another (Diagram 5.32). A position sensitive diode (PSD element) is used as the receiver. Depending on the distance of the object the reflected light is incident at the receiver at different angles and therefore at a definite position on the PSD. At this position photocurrents I1 and I2 are produced in the PSD component, from which the position X1 or X2 can be determined; from these values together with the given values B and f the distance of the object can be calculated. This assumes the object produces a diffused reflection and that the intensity of the reflected light is sufficient to enable detection. The possible detection range is determined by the geometrical dimensions ( length of PSD element, focus, distance emitter-receiver, angle of tilt of emitter to receiver optic) and the available emitter power (in the case of poor reflecting objects). Objects directly in front of the sensor cannot be detected, because of the relatively large distance between emitter and receiver light is not reflected back to the receiver. Distances up to 300mm can be achieved with close measurements using triangulation sensors. Depending on the evaluation electronics, sensors using the triangulation principle can be produced with an analogue or digital interface for distance measurement. Due to the presence of a decision threshold or a window range these sensors can also be used as direct detection sensors with adjustable, reflection independent, switching distance or switching range.

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Position Sensitive Photodiode PSD

L Ermitter Diode

Diagram 5.32: Triangulation principle

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Phase Correlation Sensors

Optical distance measurement using phase measurement is based on an indirect measurement of the transit time. The optical emitter radiates a beam of light the intensity of which is modulated by a high frequency sine wave. If the light strikes an object it will be reflected. For non-reflecting objects this will normally be more or less a diffused reflection. After travelling the distance Emitter- Object-Receiver, which is twice the distance object to sensor, the reflected light is received at the receiver. The reflected sinusoidal modulated intensity light is converted into a sinusoidal electrical signal. Due to the final propagation velocity of the light the output signal of the receiver has a phase shift with respect to the emitter modulation. This phase shift is directly proportional to the distance to be measured (diagram 5.33). The sinusoidal reference and receiver signals are converted to square wave pulses, with the appropriate phase shift, by means of "clipper or limiter" amplification. A following Exclusive OR gate serves as a phase comparator. The output is a square wave with a frequency of twice that of the input signal, the mark/space ratio of which depends on the phase shift between the two input signals. A low-pass filter produces an average direct current signal, which is proportional to the phase shift and therefore the distance, this then delivered to an interface for further evaluation. One advantage of this technique is that for the small dimensions of the sensor comparatively large distances can be measured. Further, a low cost construction of the sensor is possible by avoiding the direct measurement of the propagation time. Within certain limits the shape, colour, therefore reflected light intensity, does not effect the measured results. It is only important that a measurable signal is received. That is why this measurement technique is universally applied.

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∆t ~ ϕ

Usend

Uempf

Ud t Ermitter and Receiver Signals

UBegr

t Output Signal of the Limiter Amplifier UEXOR

t Diagram 5.33: Monitoring the flow of material in a Store.

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Applications

Diagram 5.34: Press line with Mechanical Handling

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Diagram 5.35: Scanning Coil Formers

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Diagram 5.36: Monitoring the flow of material in a Store.

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6

MAGNETIC SENSORS

6.1

Fundamental Principles

6.2

Principal of Operation

6.2.1

Hall Effect Sensors

6.2.2

Magnetoresistance Sensors

6.3

Saturated Core Probes

6.3.1

Construction and Mode of Operation

6.3.2 6.3.2.1 6.3.2.2 6.3.2.3

Function and Measurement Circuit Evaluation using an Oscillator Evaluation using Pulsed Current Evaluation using Impedance Measurements

6.4

Applications

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Fundamental Principles

Normally the measurement of magnetic fields has no application in the automation industry. Usually magnetically branded or ferrous objects are detected by magnetic field sensitive sensors. The following are determined in this way: -

Distance (analogue) Numbers of piece goods (digital) Number of revolutions (digital) Turning angle (analogue)

Magnetic fields are produced by electromagnets or permanent magnets. Permanent magnets are predominantly used in sensor technology, because they do not require a power supply. Diagram 6.1 shows the magnetic field of a cylindrical permanent magnet.

Permanent magnet

Diagram 6.1: Magnetic field of a cylindrical permanent magnet

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The lines of magnetic force outside of the magnet run from the north pole to the south pole and close internally. Diagrams 6.2 and 6.3 show the value of the flux density as a function of the axial and radial distance from the magnet.

z/mm

Diagram 6.2: Value of the flux density of the permanent magnet, shown in diagram 6.1, as a function of the radial coordinate z (r=0).

r/mm

Diagram 6.3: Value of the flux density, of the permanent magnet shown in diagram 6.1, as a function of the radial coordinate r(z=0)

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The magnetic lines of force discontinuous at the boundary surface of two materials, with different permeabilities, providing they do not penetrate perpendicular to the surface (Diagram 6.4).

tanß We have:

µ1 =

tanα

. µ2

<

Diagram 6.4: Discontinuity of the magnetic lines of force at a boundary surface.

This effect can be utilised to divert or lead the magnetic lines of force through ferrous materials such as ferrite or steel. Diagram 6.5 shows the magnetic field of the same magnet in diagram 6.1, but in this case the magnetic field is deformed by the presence of a steel plate. This deformation can be measured by a suitable magnetic field sensor, so that the plate is detected. In automatic control technology Hall sensors, magnetic resistive sensors and saturated core sensors are mainly used. They will be described in that order.

Steel Plate

Permanent Magnet

Diagram 6.5: The effect of a steel plate on a Permanent magnet magnetic field.

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6.2

Principle of Operation

6.2.1

Hall-Effect Sensors

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The following phenomenon is understood as the Hall effect (E. Hall, 1879). When a current I = b·d·n·e·v, b,d - Width and thickness of the Hall lamina, n - Concentration of the conducting electrons e, v - drift speed of the electrons, flows through a conducting lamina a Lorentz force is produced at right angles to the current I, providing the magnetic field B passes vertically through the lamina. E = v·B

Diagram 6.6: Principle of a Hall-Effect Sensor

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The open circuit Hall voltage can be obtained from the two equations

1 (1)

B·I

UH =

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The Hall coefficient RH , the dimensions are cm3/As In the case where B is not perpendicular to the lamina but at an angle a to the normal then :

B·I (2)

· cos α.

UH = RH· d

The concentration of conducting electrons by the various materials used is heavily temperature dependent and for pure metals RH is too small to be used for measuring purposes. The semiconductors GaAs, InSb, InAsP and InAs are preferred for Hall Laminae. Hall effect sensors made from GaAs or Si are becoming of increasing importance, due to advances in planar technology it is possible to integrate other functions such as current source, temperature compensation and output amplifier with the Hall-effect element. In the data sheet the so called open circuit sensitivity KH is given instead of the Hall coefficient RH, this can easily be obtained from equation (1): 1 (3)

KH =

UH =

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In the equivalent circuit of a Hall-effect sensor given in diagram 6.6 the following dimensions can be recognised: R1 R2 UH UR

- Bulk resistance in the current path, - Internal resistance of the Hall generator, - Open circuit voltage of the Hall generator, - DC voltage of the Hall electrodes when B=0.

all these parameters are temperature dependent. The numerical value of individual parameters can vary greatly between sensor type to sensor type; this may be due to the material, method of manufacture or the geometry (e.g. thickness d). 119

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Magnetoresistance Sensors

Magnetic field dependent resistances are capable of fulfilling the same functions as Halleffect sensors, because of this they are mainly found in the automation sector as proximity switches or position sensors. Diagram 6.7: A magnetoresistance sensor made from InSb semiconductor material (for the field direction illustrated the maximum R is dependent on B)

Frequently semiconductor materials are used for magnetoresistance sensors. The ground material is for instance InSb. Conducting needle shaped inclusions of NiSb are embedded in the semiconductor, at right angles to the current flow (Diagram 6.7).

When no magnetic field is present the current takes the shortest path through the semiconductor. As in the case of Hall-effect sensors, with a magnetic field present the current is laterally deflected, which increases the length of the current path and a greater resistance has to be overcome. The needles of NiSb have a very high conductivity compared to the base material InSb and therefore function as short circuits; this results in an almost homogeneous electrical field within the semiconductor and a homogeneous distribution of charge carriers is achieved. The current paths run in a zig-zag form through the semiconductor. For low values of magnetic field the resistance increases in proportion to the square of the flux density. The active material is arranged in a twisted form, in order to achieve a resistance of a few hundred ohms (thickness approx. 25µm). The sensors produced are also known as magnetoresistors. Various firms use the ferromagnetic material Permalloy (80% Fe, 20% Ni) for their magnetic sensors. This material is treated during manufacture so that the elementary magnets are mainly in the direction of the thin sensor strip ( x axis in diagram 6.8 ).

Diagram 6.8: A magnetoresistance sensor made from ferromagnetic permalloy (The maximum value of R is dependent on B, for the illustrated direction of the field)

The maximum strip resistance (R=R o) is obtained when no external field is present. The value of the resistance decreases in the presence of a magnetic field, that is proportional to the square of the field. By careful construction of the sensor strip the characteristic can be symmetrically linearised about the point B = 0. For both the sensor types described above care is taken to ensure that effective magnetic fields in various directions are indicated.

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6.3

Saturated core probes

6.3.1

Construction and method of operation

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Instruments for the measurement of magnetic fields, which use saturated core probes, are widely used where weak magnetic fields have to be measured. For example in geophysics for the exact measurement of the earths magnetic field and in space. These have been known, for a long time, under the name Forster Probe or flux-gatemagnetometer method, utilise the non-linearity of the magnetisation curve of high permeability of soft magnetic materials. Here the probe consists of a high permeability rod or ring core (Diagram 6.9).

Core Magnetising winding Sensing Winding

Diagram 6.9: Principle of the saturated core probe

The core material is periodically saturated by means of an alternating current i in the magnetising winding. This induces a voltage u in the sensing winding. Diagram 6.10 illustrates the relationship for the ideal case for a magnetisation curve constructed from the three linear portions. The flux Φ in the core is proportional to the field strength H, and therefore to the current i and the permeability of the core. In saturation Φ is almost independent of i. The induced voltage u in the sensing winding is proportional to the rate of change of flux Φ with time. In the absence of an external magnetic field the changes in H, therefore Φ, are symmetrical about zero. Φ and u contain therefore only the fundamental frequency and odd harmonics. If the probe is placed in an external magnetic field the curve of the field strength is displaced ( dotted line). The flux Φ and the induced voltage u are no longer symmetrical and now contain even harmonics, the amplitudes of which are almost proportional to the DC field.

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A resolution of up to 10-6 A/cm can be achieved with saturated core magnetometers. This is approximately onehundredthousandth of the earths magnetic field strength. These sensors were for a long time limited to expensive measuring instruments, because of their complicated construction extensive evaluation electronic. Advances in electronic integration and most important the development of high permeability materials have made this principle of interest to the automation industry.

Diagram 6.10: Function of a saturated core probe

1) 2) 3) 4)

The magnetising force H produced by the current i in the magnetising winding (superimposed the DC magnetising force H0 shown dotted) Magnetising curve of the core Variation of magnetic flux in the core Induced voltage u in the sensing winding

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6.3.2 Function and Measuring circuit New saturated core probes use a amorphous metal as the core material, which has a number of advantages over the conventional crystallised alloys. Amorphous metals are characterised by a high permeability (up to 500000), small coercive field strength, as well as low eddy current and hysteresis losses. They are manufactured as thin strip (20-50 µm thick), they are very elastic and therefore insensitive to mechanical wear and tear. Diagram 6.11 illustrates the construction principle of a magnetic position sensor. It consists of a strip of amorphous metal encapsulated together with a single coil in a plastic housing.

Coil Amorphous metal

Housing Connecting cable

Diagram 6.11: Principle of a Magnetic Sensor

6.3.2.1 Measurements using an oscillator When a saturated core coil is a frequency determining component of a circuit, similar to an inductive sensor, then the oscillating frequency, that is the change in the amplitude of the LC oscillations is evaluated. The approach of a magnet increases the magnetic field which in turn causes a change in the coil impedance and a change in the Q-factor of the oscillator.

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Measurements using pulsed current

In this simple evaluation the core is driven into saturation by a pulsed current ( e.g. 100kHz) (diagram 6.12).

Sensor element

Diagram 6.12: Basic circuit for pulsed current operation

At each edge of the current pulse a voltage pulse is produced in the coil, the height of which depends on the stored magnetic energy, which in turn depends on the value and direction of the magnetic field to be measured. The induced voltage is rectified and passed through a low pass filter. The signal u which is produced is, to a close approximation, proportional to the magnetic field, providing the sensor core is not already saturated by the external magnetic field. Typical data for these sensors are: measuring range 0.5mT, sensitivity 10V/mT, linearity 1% , frequency limit > 20kHz. 6.3.2.3 Evaluation by impedance measurement A further possible method of evaluation lies in the measurement of the inductance or the Q- factor of the sensor coil. The coil inductance is dependent on the reverse permeability of the core material. This is the alternating field permeability for a small modulation change dH and superimposed ∆C field H0: ∆B

1 µrev =

· µo

∆H

für ∆H → 0.

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For a small change ∆H the hysteresis loop is lancet shaped and with a superimposed DC field moves along the magnetisation curve (Diagram 6.13). The tilt of the lancet axis corresponds to the reversible permeability. Diagram 6.14 Illustrates the relationship between the reversible permeability and the DC field HO.

Diagram 6.13: Definition of the reversible permeability

Diagram 6.14: The reversible permeability of an amorphous metal

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The dependence of the resulting coil Q-Factor on the flux density B is shown in diagram 6.15. A simple evaluation method is to measure the coil impedance; this decreases with increasing field strength as the inductance and the Q-Factor fall in value. When the sensor coil is supplied with an alterating current i, of constant amplitude, the resulting voltage u is a measurement of the field strength (diagram 6.16).

Diagram 6.15: Variation of the coil Q-Factor with changes in flux density, for the magnetic sensor shown in diagram 6.11

Diagram 6.16: Basic circuit for impendance measurement

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6.4 Applications By using amorphous soft magnetic alloys and further development in control technology the saturated core probe principle can find new applications particularly in automation and the automotive industry. Compared to Hall-effect sensors and magnetoresistance elements they are an order of magnitude more sensitive. In comparison to inductive sensors the greater sensing range, with a smaller physical size and the possibility of totally encapsulating the sensor in a metal housing stand out.

Interesting applications could be: - distance and position sensors, - speed of ratation and angle of rotation sensors, - Current sensors, - Sensors for traffic and vehicle counting, - Navigation and earths magnetic field measurements.

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