Transducer and Instrumentation

Any instrument consists of three basic elements. They are

The input device

The signal conditioning or processing device

The output device

Considering the input device it may receive an electrical quantity as its input or a non electrical quantity as an input.

When a non electrical quantity is the input to an instrument, we require a device that converts the non electrical quantity to an electrical signal. This electrical signal must be proportional to the non electrical quantity.

The name transducer is familiar to us. The one that covers one form of energy to the other form is a transducer. An example of transducer is the microphone, which converts the sound energy to corresponding electrical signal. Yet another example is the loud speaker which converts the electrical signal to sound.

In industrial Instrumentation we come across the physical quantities that are to be converted into proportionate electrical signals. The displacement, strain, vibration, pressure, flow, temperature, force, etc., are some of the quantities that are to be converted into proportionate electrical signals.

Therefore the input device for an instrumentation system will be a transducer. Sometimes we call the transducer as a 'Sensor'. The following is the block diagram showing the elements of a basic instrumentation system.

Block Diagram of an Instrumentation System

In this blog post, we will study important transducers that are used in the industrial environment. Ultrasonic is the name given to a band of frequencies beyond the audio range i.e. above 20 kHz. These frequencies find their application in many areas. They are used in modern manufacturing processes. They find their use in oceanography, and medical electronics.

Transducer:

A transducer is a sensing device that converts the physical, mechanical or opt quantities into an electrical signals. Transducer is essentially a device which converts process variables in one form into a variable in another form. In many cases the output is a signal in the form of a voltage wave which is quantitatively related to some variable of process.

Example: A potentiometer converting angular position of the shaft into an output voltage.

In order that the transducers are of any practical use in a control system or any instrumentation scheme, it should desirably produce an output signal or some unvarying single valued function of a given variable expressed in terms of its characteristics, static and dynamic.

Transducers - A Classification:

A transducer is a device which, when actuated by energy in one transmission system, supplies energy in the same form or in another to a second transmission system.

This energy transmission may be electrical, mechanical, optical (radiant) or thermal.

Classification of transducers is difficult. Broad classification on the electrical principle involved is as

1. Active transducers

2. Passive transducers

Few examples of active and passive transducers are given here under:

(a) Active/Self Generating Type:

These are transducers producing an analog voltage or current when stimulated by some physical form of energy.

The examples under Active/Self generating type are

1. Thermocouple and thermopile

2. Piezoelectric pickup

3. Photocell

4. Moving coil generator

5. Magnetostrictive

6. Electrokinetic

7. Electromagnetic

8. Pyroelectric

9. Galvanic

(b) Passive Transducers

These are transducers producing a variation in some electrical parameter such as resistance capacitance and so on.

The examples under passive transducers are

1. Potentiometers

2. Resistance strain gauge

3. Pirani gauge or hot wire meter

4. Resistance thermometer

5. Thermistor

6. Photo conductive cell

7. Reluctance pickup

8. Inductance transducer

9. Differential transformer

10. Eddy current gauge

11. Magnetostriction gauge

12. Hall effect pick up

13. Synchro

14. Gyro

15. Radioactive absorption

16. Ionic Conduction