Lecture Notes

DAR ES SALAAM INSTITUTE OF TECHNOLOGY  LECTURE NOTES TRANSDUCER AS INPUT ELEMENT TO THE INSTRUMENTATION SYSTEM

Prepared by Lihakanga.R

Objective Upon completion of this lecture you will be able to Understand basic idea on instrumentation system Know what is transducer. Know when transducer is considered as sensor. Know when transducer is considered as an actuator. Know how to select suitable transducer when designing . Know basic operation of widely used mechanical transducers. 

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Content  

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Basic instrumentation system. Classification of transducers. How to select transducer. Mechanical transducers.

Basic instrumentation system

Basic instrumentation cont 

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Transducer-Is the device that convert one form of  energy to another form. Sensor (Input transducer)-Is the device that converts the physical/non physical quantity to be measured into electrical signal so it can be processed or transmitted electronically. Signal conditioning circuit- this include amplifier, signal converter circuit, comparator, filters and circuit  that carry out mathematical operation.  Actuator (output transducer)-A device that converts the modified electrical signal into non electrical signal.

Classification of transducers Passive transducer (externally powered).  Active transducer (internally powered).

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Passive transducers Resistive transducers Potentiometric device-Positioning of slide by an external force varies the resistance in potentiometer or bridge. -used to measure pressure, displacement. Resistance strain gage-resistance of wire/semiconductor is changed by elongation or compression due to externally applied force. -used to measure force, torque, displacement. Thermistor-resistance of certain metal oxide with negative temperature coefficient of resistance varies with temperature. -used to measure temperature. Photoconductive-resistance of a cell change as circuit element varies with incident light. -used as photosensitive relay. Resistive hygrometer-resistance of conductive strip change awith moisture content  -used to measure relative humidity. 

Passive transducer cont. Capacitive transducers Dielectric gage-variation in capacitance by change in dielectric. -used to measure thickness, liquid level. Capacitor microphone-Sound pressure varies the capacitance btn fixed plate and movable diaphram. -used I measuring speech, music, noise.  Variable capacitance pressure gage-distance btn two parallel plates is varied by externally applied force. Used to measure displacement, pressure. Inductive transducers Reluctance pick up-reluctance of magnetic circuit is varied by changing the position of iron core of the circuit. -used to measure pressure, displacement, vibration, position. Differential transformer-the differential voltage of two secondary windings of a transformer is varied by positioning the magnetic core through an externally applied force. -used to measure pressure, force, displacement, position 

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Passive transducer cont..  Voltage/current transducer Photoemmisive cell-electron emission due to incident radiation on photo emissive surface. -used to measure light, radiation. Hall effect pick up-potential defference is generated across semiconductor plate (germanium) when magnetic flux interacts with an applied current. -used to measure magnetic flux, current. 

 Active transducer Photovoltaic cell-voltage is generated in a semiconductor junction device when radiant energy stimulates the cell. -used in light meter, solar cell. Piezo electric pick up-an emf is generated when an external force is applied to a certain crystalline material such as quartz. -used to measure sound, vibration, acceleration, pressure change. Moving coil generator-motion of of a coil in a magnetic field generates voltage. -used to measure velocity, vibration.

How to select transducer 

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What is the physical quantity to be measured? Which transducer principle can best be used to measure this quantity? What accuracy is required for this measurement? -Fundamental transducer parameters. -Physical condition. -Ambient condition. -Environment condition. -Compatibility of the associated equipment. Reducing the total measurement error -Artificially controlling environment to minimize error. -Simultaneously monitoring the environment and correcting data accordingly.

Mechanical Transducers 

Potentiometric

Mechanical Transducers cont 

Capacitive

Mechanical Transducers cont 

Inductive (LVDT)

Mechanical Transducers cont 

Strain gauge

END OF THE LECTURE

THANK YOU.