A transducer is a device that usefully converts energy from one form to another.
Types
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Mechanical transducers convert physical quantities into mechanical outputs or vice versa;
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Electrical transducers convert physical quantities into electrical outputs or signals. Examples of these are:
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a thermocouple that changes temperature differences into a small voltage;
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a linear variable differential transformer (LVDT), used to measure displacement (position) changes by means of electrical signals.
Sensors, actuators and transceivers
Transducers can be categorized by the direction information passes through them:
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A sensor is a transducer that receives and responds to a signal or stimulus from a physical system.
[Fraden J. (2016). Handbook of Modern Sensors: Physics, Designs, and Applications 5th ed. Springer. p.1][Kalantar-zadeh, K. (2013). Sensors: An Introductory Course 2013th Edition. Springer. p.1]
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An actuator is a device that is responsible for moving or controlling a mechanism or system. It is controlled by a signal from a control system or manual control. It is operated by a source of energy, which can be mechanical force, electrical current, hydraulic fluid pressure, or pneumatic pressure, and converts that energy into motion. An actuator is the mechanism by which a control system acts upon an environment. The control system can be simple (a fixed mechanical or electrical system), software-based (e.g. a printer driver, robot control system), a human, or any other input.
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Bidirectional transducers can convert physical phenomena to electrical signals and electrical signals into physical phenomena. An example of an inherently bidirectional transducer is an antenna, which can convert (electromagnetic waves) into an electrical signal to be processed by a radio receiver, or translate an electrical signal from a transmitter into radio waves. Another example is a voice coil, which is used in to translate an electrical audio signal into sound, and in dynamic microphones to translate sound waves into an audio signal.
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Transceivers integrate simultaneous bidirectional functionality. The most ubiquitous example are likely radio (called transponders in aircraft), used in virtually every form of wireless (tele-)communications and network device connections. Another example is ultrasound transceivers that are used for instance in medical ultrasound (echo) scans.
Active vs passive transducers
Passive transducers require an external power source to operate, which is called an excitation signal. The signal is modulated by the sensor to produce an output signal. For example, a
thermistor does not generate any electrical signal, but by passing an electric current through it, its resistance can be measured by detecting variations in the current or
voltage across the thermistor.
[Fraden J. (2016). Handbook of Modern Sensors: Physics, Designs, and Applications 5th ed. Springer. p.7]
Active transducers in contrast, generate electric current in response to an external stimulus which serves as the output signal without the need of an additional energy source. Such examples are a photodiode, and a piezoelectric sensor, photovoltaic, thermocouple.
Characteristics
Some specifications that are used to rate transducers:
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Dynamic range: This is the ratio between the largest amplitude signal and the smallest amplitude signal the transducer can effectively translate.
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Repeatability: This is the ability of the transducer to produce an identical output when stimulated by the same input.
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Noise: All transducers add some random noise to their output. In electrical transducers this may be electrical noise due to thermal motion of charges in circuits. Noise corrupts small signals more than large ones.
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Hysteresis: This is a property in which the output of the transducer depends not only on its current input but its past input. For example, an actuator which uses a gear train may have some backlash, which means that if the direction of motion of the actuator reverses, there will be a dead zone before the output of the actuator reverses, caused by play between the gear teeth.
Applications
Electromagnetic
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Antennae – converts propagating electromagnetic waves to and from conducted electrical signals
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Magnetic cartridges – converts relative physical motion to and from electrical signals
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Tape head, disk read-and-write heads – converts magnetic fields on a magnetic medium to and from electrical signals
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Hall effect sensors – convert a magnetic field level into an electrical signal
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Variable reluctance sensors – the movement of nearby ferrous metal objects induce an alternating current electrical signal
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Pickups – detect movement of metal strings and induce an electrical signal (AC voltage)
Electrochemical
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pH meter
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Electro-galvanic oxygen sensors
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Potentiometric sensor
Electromechanical
Electromechanical
input feeds meters and sensors, while electromechanical
output devices are generically called
):
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Air flow sensors
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Electroactive polymers
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Electric motor,
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Galvanometer
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Linear variable differential transformers or rotary variably differential transformers
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– converts force to mV/V electrical signal using strain gauge
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Microelectromechanical systems
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(when used for measuring position)
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String potentiometers
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Vibration powered generators
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Vibrating structure gyroscopes
Electroacoustic
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, – convert electrical signals into sound (amplifier signal → magnetic field → motion → air pressure)
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– convert sound into an electrical signal (air pressure → motion of conductor/coil → magnetic field → electrical signal)
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Tactile transducers – convert electrical signal into vibration (electrical signal → vibration)
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– convert electrical signals into temperature fluctuations, which become sound (electrical signal → periodic heating of a thin conductor → temperature waves → sound waves)
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Piezoelectricity – convert deformations of solid-state crystals (vibrations) to and from electrical signals
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– convert a ground movement (displacement) into voltage (vibrations → motion of conductor/coil → magnetic field → signal)
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Phonograph – (air pressure → motion → magnetic field → electrical signal)
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– convert changes in water pressure into an electrical signal
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Sonar transponders (water pressure → Motion of conductor/coil → magnetic field → electrical signal)
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Ultrasonic transceivers, transmitting ultrasound (transduced from electricity) as well as receiving it after sound reflection from target objects, availing for imaging of those objects
Electro-optical
Also known as photoelectric:
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– convert electrical power into incoherent light
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Incandescent lamps – convert electrical power into incoherent light
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Light-emitting diodes – convert electrical power into incoherent light
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– convert electrical power into coherent light
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, , , – convert changing light levels into electrical signals
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Photodetector or photoresistor or light dependent resistor (LDR) – convert changes in light levels into changes in electrical resistance
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Cathode-ray tube (CRT) – convert electrical signals into visual signals
Electrostatic
Thermoelectric
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Resistance temperature detectors (RTD) – convert temperature into an electrical resistance signal
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– convert relative temperatures of metallic junctions to electrical voltage
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(includes PTC resistor and NTC resistor)
Radioacoustic
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Geiger-Müller tubes – convert incident ionizing radiation to an electrical impulse signal
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Radio receivers – convert electromagnetic transmissions to electrical signals.
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Radio transmitters – convert electrical signals to electromagnetic transmissions.
See also
External links
