What is transducers? what are its types? how transducers are selected?

Basic Instrumentation system and transducers

Transducers are the heart of an electronic instrumentation system. The transducer is an electromechanical device used in an electronic instrument system and in other processing systems. To understand the working and need of transducer we must know the basic instrument system.

An electronics system used for the process of measurement of any physical quantity is called an instrument system. The heart of the electronic instrument system is the transducer.

The basic measurement system consists of following circuits as shown in the following block diagram

1. Input circuit– it receives the physical quantity under measurement. It then produces proportional electrical signals to the processing unit.
2. Processing circuit – it receives proportional electrical signals from the input circuit. It amplifies, filter or modifies the signals in such a form that it is compatible with output circuit or device.
3. Output circuit – it receives the processed signals from the processing circuit and connects it to an output device that may be simple indicating meter, CRO, magnetic tape recorder or even computer.

Necessities of transducer

transducers are used in the instrumentation system to convert physical quantity into proportional electrical signals. It provides a medium to interface with electronic circuits with physical or electrical quantities.

“Transducer is a device that converts one form of energy into another form. It is also defined as a device that converts a non-electrical quantity into proportional electrical quantity”.

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

Here are some examples of transducers used in our daily life.

• A solar cell used in calculators converts light energy into equivalent electrical energy.
• A pencil cell converts chemical energy into equivalent electrical energy.
• A LED converts electrical energy into light energy.

Types of transducer

Active transducer: The transducer which does not require an external electrical power supply to convert a physical parameter into an electrical quantity. It is also called a self-generating transducer it generates its own voltage or current.

• Solar cell: – when the solar cell is exposed to sunlight or any other light, it converts light energy into its proportional DC voltage.
• Piezoelectric crystal: – when the pressure is applied to it a proportional potential difference is produced across its opposite ends.

Passive transducer: The transducer requires an external electrical power supply to convert a physical parameter into an electrical quantity.  It cannot generate its own voltage or current it only produces a change in its electrical parameter like resistance or capacitance etc.

• Light Dependent Resistor: –The resistance of LDR decreases due to an increase in light intensity.
• Thermistor: –The resistance of thermistor changes due to a change in temperature.

Types of transducers according to the working principle

Mechanical transducers –LVDT, strain gauge

Thermal transducers -thermistor, thermocouple

Magnetic transducers -LVDT

Radiation transducers – solar cell, LED, photodiode, photo transducer

Sound transducers – Microphone, Loudspeaker

Types of transducers according to Physical Quantity

 Temperature transducers – this transducer converts heat energy into its equivalent electrical energy.

• Active temperature transducers – thermo-couple which converts heat energy into equivalent electrical voltage.
• Passive temperature transducers – thermistor or resistance thermometer is a passive transducer. It only changes its resistance due to a change in temperature.

Pressure transducers – these are of two types: the stress and strain types. When either stress or strain is applied, they produce a proportional electrical voltage.

• Active pressure transducers –piezo electric crystal is a good example of active pressure transducer. It produces proportional electrical voltage when pressure is applied on it.
• Passive pressure transducers – strain gauge, capacitive transducer. When either stress or strain is applied, their passive parameter like resistance or capacitance proportionally changes.

Light transducers – it converts light energy into equivalent electrical energy.

• Active light transducers – in this photovoltaic cell, photomultiplier tubes, and solar cells are used. It converts light into electrical energy.
• Passive light transducer – in this contains a light-dependent resistor. Its resistance changes as the light on it change.

Sound transducers – it converts sound energy into equivalent electrical energy and vice versa.

• Active sound transducer – the microphone is good examples of the active transducer. It converts sound into proportional AC voltage.
• Passive sound transducers – the capacitive microphone is a passive transducer. Its capacity (C) changes proportionally due to the change in sound intensity.

Selection of Transducers

Following factors must be considered while selecting a transducer for a particular measuring system

1. Physical quantity to be measured must be considered for –

• The type of physical quantity whether it is an electrical quantity or non-electrical quantity like pressure, intensity, displacement, speed, heat, etc.
• Range and required unit of the physical quantity to be measured.

2. The principle of the transducer must be considered for –

• The system and transducer must be compatible i.e. the output characteristics of transducer and input characteristics of the system and must be compatible with each other.
• This means that the principle of maximum power transfer theorem must be satisfied.

3. The measurement accuracy of the transducer must be considered which depends on –

• Type and range of quantity under measurement.
• Physical conditions like mechanical and electrical connections and mounting of the transducer.
• Surrounding conditions like nonlinearity effect and frequency response etc.
• Environmental conditions like temperature effects, shocks or vibrations, etc.
• Compatibility of some associated types of equipment like zero balancing provision, sensitivity tolerance, impedance matching, etc.