The quantity under measurement in an instrumentation system is called ‘Measurand’ or ‘Information’. The primary sensing elements, called ‘Sensors’, sense the condition, state, or value of the measurand and convert the energy to an output of some other type of energy. The sensors are called ‘Primary Trunsducers’ or ‘Detectors’. Sometimes a ‘Secondary Transducer’ may be used to convert the output of the primary sensor to another type of energy. In some cases, the primary transducers are not necessary. Then the secondary transducers are called Transducers only.
Major elements of an instrumentation system :
An instrumentation system generally consists of the following major elements :
- An input device,
- a signal-conditioning or processing device and
- an output device.
The input device receives the quantity under measurement and delivers a proportional electrical signal to the signal-conditioning device) Transducers are used as input devices. The signal-conditioning device is used to amplify, filter, or otherwise modify the signal to a format acceptable to the output device, e.g. different types of amplifiers, modulators, filters, A/D or D/A converters, etc. The output device may be a simple indicating meter, a magnetic tape recorder, a digital computer, or a CRO, etc.
Classification of Transducers:
A transducer is defined as the device used to convert a nonelectrical quantity into an electrical signal. A transducer is a device used to convert position displacement, thermal and optical signals into electrical quantities, that may be amplified, recorded, and otherwise processed in the instrumentation system.
Characteristics required from transducers :
- Small size and weight,
- low cost,
- high sensitivity,
- accuracy for the measurement of fast-transient pressures,
- ability to withstand changes in environmental conditions and
- a linear relationship between input and output
The electrical transducers are classified as :
- Active transducers and
- Passive transducers
These transducers are of the ‘ self-generating type’. They do not require external power. An analog voltage or current is produced when stimulated by some physical form of energy. e.g. thermocouple, piezoelectric pick up, photovoltaic cell, etc.
- Thermocouple: An emf is generated across the junction of two
dissimilar metals or semiconductors when the junction is heated. Application: Conversion of temperature.
- Piezoelectric pick-up: An emf is generated when an external
force is applied to certain crystals such as quartz, tourmaline.
Application: Conversion of sound, displacement, or pressure changes.
These transducers require external power. They produce a variation in some electrical parameter, such as resistance, capacitance etc. which can be measured as a voltage or current variation. These transducers absorb some energy from the physical phenomenon under study. e.g. thermistor, photoconductive cell, resistance strain gage, capacitor microphone, etc.
(A) Resistance transducers :
- Resistance strain gage: Resistance of a wire is changed by elongation or compression due to externally applied stress.
Application: Conversion of force, torque, and displacement.
- Thermistor: Resistance of certain metal oxides with a negative temperature coefficient of resistance varies with temperature.
Application: Conversion of temperature.
- Pirani gage or hot wire meter: Resistance of a heating element is changed by convection cooling of a stream of gas.
Application: Gas flow or gas pressure measurement.
(B) Capacitance transducers :
- Capacitor microphone: Sound pressure changes the capacitance between a fixed plate and a movable diaphragm.
Application: Conversion of speech or music.
- Variable capacitance pressure gage: Distance between two parallel plates is changed by an externally applied force.
Application: Conversion of displacement or pressure.
(C) Inductance transducers : Linear Variable:
Differential Transformer (LVDT): The differential voltage of two secondary windings of a transformer is changed by positioning the magnetic core through an externally applied force.
Application: Conversion of pressure or displacement.
(D) Voltage and current transducers :
Photo-emissive cell: Electrons are emitted due to radiation incident
on the photoemissive surface.
Application: Conversion of light