Lab 1 Full Report

1.0

OBJECTIVE 1. To observe the inductive displacement sensor characteristic. 2. To observe the capacitive displacement sensor characteristic.

2.0

INTRODUCTION

Displacement measurement is basic of measuring position, velocity, pressure, stress, force and etc. There are several types of displacement sensors such as inductive displacement sensors and capacitive displacement sensors. An inductive displacement sensors consist of a magnetic circuit made from ferromagnetic core with a coil would on it. The coil act as a source of magnetomotive force that drives the flux through magnetic circuit and the air gap. The presence of the air gap causes a large increase in circuit reluctance and a corresponding decrease in a flux. Hence, a small variation in the air gap results in measurable change in inductance.

Linear variable differential transformer (LVDT) The linear variable differential transformer (LVDT) (also called just a differential transformer, linear variable displacement transformer, or linear variable displacement transducer) is a type of electrical transformer used for measuring linear displacement (position). A counterpart to this device that is used for measuring rotary displacement is called a rotary variable differential transformer (RVDT).

Cutaway view of an LVDT is driven through the primary coul at A, causing an induction current to be generated through the secondary coils B

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LVDTs are robust, absolute linear position/displacement transducers; inherently frictionless, they have a virtually infinite cycle life when properly used. As AC operated LVDTs do not contain any electronics, they can be designed to operate at cryogenic temperatures or up to 1200 °F (650 °C), in harsh environments, under high vibration and shock levels. LVDTs have been widely used in applications such as power turbines, hydraulics, automation, aircraft, satellites, nuclear reactors, and many others. These transducers have low hysteresis and excellent repeatability. The LVDT converts a position or linear displacement from a mechanical reference (zero, or null position) into a proportional electrical signal containing phase (for direction) and amplitude (for distance) information. The LVDT operation does not require an electrical contact between the moving part (probe or core assembly) and the coil assembly, but instead relies on electromagnetic coupling. Capacitive displacement sensors “are non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target” They are also able to measure the thickness or density of non-conductive materials. Capacitive displacement

sensors

are

used

in

a

wide

variety

of

applications

including semiconductor processing, assembly of precision equipment such as disk drives, precision thickness measurements, machine tool metrology and assembly line testing. These types of sensors can be found in machining and manufacturing facilities around the world. Capacitance is an electrical property which is created by applying an electrical charge to two conductive objects with a gap between them. A simple demonstration is two parallel conductive plates of the same profile with a gap between them and a charge applied to them. In this situation, the Capacitance can be expressed by the equation:

[3]

-Where C is the capacitance, ε0 is the permittivity of free space constant, K is the dielectric constant of the material in the gap, A is the area of the plates, and d is the distance between the plates. There are two general types of capacitive displacement sensing systems. One type is used to measure thicknesses of conductive materials. The other type measures thicknesses of non-conductive materials or the level of a fluid. 2

A capacitive sensing system for conductive materials uses a model similar to the one described above, but in place of one of the conductive plates, is the sensor, and in place of the other, is the conductive target to be measured. Since the area of the probe and target remain constant, and the dielectric of the material in the gap (usually air) also remains constant, "any change in capacitance is a result of a change in the distance between the probe and the target." [4] Therefore, the equation above can be simplified to:

-Where α indicates a proportional relationship. Due to this proportional relationship, a capacitive sensing system is able to measure changes in capacitance and translate these changes into distance measurements. -The operation of the sensor for measuring thickness of non-conductive materials can be thought of as two capacitors in series, with each having a different dielectric (and dielectric constant). The sum of the thicknesses of the two dielectric materials remains constant but the thickness of each can vary. The thickness of the material to be measured displaces the other dielectric. The gap is often an air gap, (dielectric constant = 1) and the material has a higher dielectric. As the material gets thicker, the capacitance increases and is sensed by the system. -A sensor for measuring fluid levels works as two capacitors in parallel with constant total area. Again the difference in the dielectric constant of the fluid and the dielectric constant of air results in detectable changes in the capacitance between the conductive probes or plates.

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3.0

EQUIPMENTS

3.1 Inductive displacement sensor card SO4203-5U 3.2 Capacitive displacement sensor card SO4203-5W

4.0

PROCEDURE

4.1

The experiments of Inductive displacement measurement. 4.1.1) The sensor characteristic was determine. 4.1.2) Then, the experiment circuit was assemble. 4.1.3) The function generator was open to setting the parameters. 4.1.4) The power was on firstly to setting the function generator. 4.1.5) After that, the signal form sine was adjust and the amplitude was achieve to 100%. 4.1.6) Next, the factor was 1:1 and the frequency was 5kHz. 4.1.7) The Volmeter A was open to setting. For setting the voltmeter, the mode DC was operating then display P.

After that, voltmeter was

measuring until range 5V. 4.1.8) The core of coil was move to its lower limit and to measure the corresponding output voltage (uOut). 4.1.9) The core was move upwards and the output was measuring for each case. 4.1.10) The value was taken to plot the chart and see the characteristic of the sensor. 4

4.2

The experiments of Capacitive displacement measurement II 4.2.1) The sensor characteristic was determine. 4.2.3) After that, circuit need to assemble. 4.2.4) The voltmeter was open to setting the parameters. For setting, the mode DC was operating. Then, AV was also display and the measuring range need 10V. 4.2.5) The capacitor was move to its lower limit. The corresponding output voltage was measured. 4.2.6) Then, the core was move upwards to measure the output voltage on each occasion. 4.2.7) The reading value was taken to plot the value to see the characteristic of the sensor.

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5.0

RESULTS

Table 1.1 X 0.00 2.50 5.00 7.50 10.00 12.50 15.00 17.50 20.00 22.50 25.00 27.50 30.00 32.50 35.00 37.50 40.00

u Out (V) 5.00 5.00 5.00 4.69 4.29 3.82 3.39 2.83 2.36 1.81 1.38 0.87 0.47 0.04 -0.23 -0.51 -0.55

u Out (V) 6 5 4 3 2 1 0

0

2.5

5

7.5

10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 35 37.5 40

-1 u Out (V)

6

Table 1.2

X 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.00

u Out (V) 2.0 2.2 2.4 3.0 3.3 3.7 4.3 5.2 5.7 7.2 7.2 8.1 9.1

u Out (V) 10 9 8 7 6 5 4 3 2 1 0

0

5

10

15

20

25

30

35

u Out (V)

6.0

DISCUSSION 7

40

45

50

55

60

6.1 In experiment (A), what is the gain of the measurement amplifier in volts per scale division for a displacement in the middle of the range? Amplifiers have the ability to increase the magnitude of an input signal. It is useful to be able to rate an amplifier’s amplifying ability in terms of an output/input ratio. The technical term for an amplifier’s output/input magnitude ratio is gain. As a ratio of equal gain is naturally a unitless measurement. Mathematically, gain is symbolized by the capital letter “A”. A=

V output 2.36−2.83 = =0.094 V input 5

6.2 Determine the linear equation for Experiment A. (Explain your calculations). Linearity is a relationship between input and output. It normally represented by a straight line equation which show the relation between measured variable and measurement output. Linear equation,

y=mx +c

Figure 1: best fit line graph

5.0=m ( 5.5 ) +c

--1

0.75=m ( 30 ) +c

--2

8

4.25=−24.5m

1 – 2, m=

4.25 −24.5

m=−0.173

When m=-0.173, c = 5.954 Hence, y= -0.173x + 5.954 The linear equation is

y=−0.173 x+5.954 .

6.3 Determine the sensitivity for Experiment A. (Explain your calculations), Sensitivity indicates how much the output of an instrument system or system element changes when the quality being measured changes by a given amount. Sensitivity =

=

output input

or the slope of the graph.

4.69−3.39 7.50−15.00

= -0.1733 The sensor have a sensitivity of -0.1733V/coil and so give an output of 0.1733 for each 1 change in coil.

6.4 In Experiment (B), what is the shape of the characteristic? How do you explain the results? The sensor characteristic is calibration. As u refer to graph at page 7, the graph will show almost same shape as picture below. A sensor or instrument is calibrated by applying a number of known physical inputs and recording the response of the system.

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6.5 Determine the linear equation for Experiment B. (Explain your calculations).

2.4=m ( 10 ) + c

--1

7.8=m ( 52 ) +c

--2

2–1, m=

5.4=42 m

5.4 42

m=0.129 When m=0.129, c = 1.11 Hence, y= 0.129x +1.11 10

The linear equation is

y=0.129 x +1.11 .

6.6 Determine the sensitivity for Experiment B. (Explain your calculations). Sensitivity =

=

output input 5.2−3.0 35−15

or the slope of the graph.

= 0.11V/coil

The sensor have a sensitivity of 0.11V/coil and so give an output of 0.11V for each 1 change in coil.

6.7 Explain the errors occur during the experiment. The sensor is big, bulky and heavy. The bulk of material and the requirement for carefully wound coils makes them expensive to produce, especially high accuracy devices that require precision winding. Each sensor often requires the associated AC generation and signal processing circuitry to be separately specified and purchased. This often requires a significant amount of skill and knowledge of analogue electronics.

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7.0

CONCLUSION In the conclusion, the experiment is related to the displacement sensor. Displacement

sensors are concerned with the measurement of amount by which some object has moved. Inductive proximity sensor consists of a coil wound a core. It basically comprises an oscillator whose windings constitute the sensing face. An alternating magnetic field is generated in front of these windings. These changes can be monitored by its effect on a resonant circuit to trigger a switch. It can only be used for detection of metal objects and is best with ferrous metals. In our experiment, when we move the core upwards the output voltage will decrease slightly. Inductive proximity sensors enable the detection, without physical contact of the metal objects. It also has high operating rates and fast response. The inductive detection has an excellent resistance to industrial environments. Capacitive proximity sensors are used for non-contact detection of metallic objects & non-metallic objects (liquid, plastic, wooden materials and so on). Capacitive proximity sensors use the variation of capacitance between the sensor and the object being detected. When the object is at a present distance from the sensitive side of the sensor, an electronic circuit inside the sensor begins to oscillate. The rise or fall of such oscillation is identified by a threshold circuit that drives an amplifier for the operation of an external load. A screw placed on the backside of the sensor allows regulation of the operating distance. This sensitivity regulation is useful in applications, such as detection of full containers and nondetection of empty containers. The capacitive sensor is move at lower limit to upper limit which can result us to measure the corresponding voltage output. After the measurement, we get the result is the value of output voltage is increase on each occasion. It shows that there is a linear displacement graph is form by the result. Nonetheless, the characteristic of modern sensors and applications presented in this paper prove that inductive sensor and capacitive sensor still play very important role in the measuring technology.

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8.0

REFERENCE

1. http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041227 /print.html 2. http://www.fargocontrols.com/inductive_sensors.html 3. http://www.fargocontrols.com/sensors/inductive_op.html 4. G. Dehmel , “Magnetic field sensors: Induction coil (search coil) sensors”, Chapter

6

in

Sensors

–

a

comprehensive

survey,

vol.

5,

VCH

Publishers,

1989, pp. 205-254 5.

P.

Ripka

,

“Induction

sensors”,

Chapter

magnetometers, Artech House, 2001, pp. 47-74

13

2

in

Magnetic

sensors

and

9.0

APPENDIX

The Apparatus And Software of Displacement Measurement 14

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