Introduction to Sensors and Transducers. Definition and Principle of Transduction | Sensors and Transducers

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Definition of Sensors

A sensor or transducer is a device that gives a usable output (preferably, an electrical quantity) in response to a specified measurand (physical quantity such as light, sound, temperature…etc)

Daily life examples of sensors include microphones, thermometer, accelerometers, infrared sensors, speakers…etc

Sensor produces a usable output in response to a specified quantity. it uses the sensing principle, that is it senses or detects a physical phenomenon.

A transducer converts one form of energy to another form. The process of conversion of energy from one form to another is called transduction.

Not all sensors are transducers, but all transducers contain sensors. Sensing is the first stage of transduction.

Principle of transduction

The input/output signals (in the form of energy) in a sensor can be divided into six. They are,

Mechanical
Thermal
Electrical
Magnetic
Radiant
Chemical

Some of the physical and chemical transduction principles can be grouped according to the form of energy in which signals are received and generated.

Mechanical input signal

Triggered by a mechanical force like stress or a push/pull

May cause change in temperature (Friction effect, cooling effects)
May cause change in magnetic intensity (Piezomagnetic effect)
May cause change in electricity (piezoelectric effect, resistive, capacitive and inductive changes)
May cause change in radiant energy (Photoelasticity, Doppler effect)

Thermal input signal

May cause change in mechanical energy (Thermal expansion)
May cause change in electricity (Seebeck effect, thermoresistance)
May cause change in radiant energy (Thermo optical effects)
May cause change chemical reaction (Thermal dissociation)

Electrical input signal

May cause change in mechanical energy (Electrokinetic effects)
May cause change in temperature (Peltier effect)
May cause change in electricity (Charge controlled devices)
May cause change in magnetic intensity (Biot-Savart’s electromagnetic law)
May cause change in radiant energy (Kerr effect)
May cause change chemical reaction (Electrolysis)

Magnetic input signal

May cause change in mechanical energy (Magnetometers)
May cause change in temperature (Magnothermal effects)
May cause change in electricity (Galvanomagnetic effect)
May cause change in radiant energy (Magneto-optical effect)

Radiant input signal

May cause change in mechanical energy (Radiation Pressure)
May cause change in temperature (Bolometer)
May cause change in electricity (Photoelectric effect)
May cause change in radiant energy ( Photorefractivity)
May cause change chemical reaction (Photosynthesis)

Chemical input signal

May cause change in mechanical energy (Photoacoustic effect)
May cause change in temperature (Thermal conductivity)
May cause change in electricity (Potentiometry, conductimetry)
May cause change in magnetic intensity (Nuclear magnetic resonance)
May cause change in radiant energy (Spectroscopy)

Some of the measurands corresponding to each energy type are given in the table.

Energy	Measurands
Mechanical	Length, area, volume, force, pressure…etc
Thermal	Temperature, heat flow…etc
Electrical	Charge, current, voltage, resistance, capacitance…etc
Magnetic	Field intensity, flux density, permeability…etc
Radiant	Intensity,phase,refractive index, wavelength…etc
Chemical	Concentration,reaction rate, pH…etc