Device that converts energy from one form to another
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and control systems, where electrical signals are converted to and from other physical quantities (energy, force, torque, light, motion, position, etc.). The process of converting one form of energy to another is known as transduction.
Mechanical transducers, so-called as they convert physical quantities into mechanical outputs or vice versa;
Electrical transducers however convert physical quantities into electrical outputs or signals. Examples of these are:
a thermocouple that changes temperature differences into a small voltage;
Transducers can be categorized by which direction information passes through them:
A sensor is a transducer that receives and responds to a signal or stimulus from a physical system. It produces a signal, which represents information about the system, which is used by some type of telemetry, information or control system.
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.
Transceivers integrate simultaneous bidirectional functionality. The most ubiquitous example are likely radio transceivers (in aircraft called transponders), used in virtually every form of wireless (tele-)communications and network device connections. Another example are 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.
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, photovoltic, thermocouple.
Some specifications that are used to rate transducers:
Dynamic range: This is the ratio between the largest amplitude signal and the smallest amplitude signal the transducer can effectively translate. Transducers with larger dynamic range are more "sensitive" and precise.
Repeatability: This is the ability of the transducer to produce an identical output when stimulated by the same input.
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.
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.