Transducers, Sensors and Actuators

Measurement is an important subsystem of a mechatronics system, which is used to collect the information on the system statuses such as temperature, humidity, pressure, or proximity and feed it to the microprocessor(s) for controlling the system.

Transducer

A transducer is an essential part of any information processing system that operates in more than one physical domain such as optical, electrical, magnetic, thermal, and mechanical domains. A transducer in general is defined as a device that converts one form of energy into another using the transduction process. The transduction process or function represents the characteristics or properties of the device used for energy conversion. 

For example, the diaphragm in a microphone converts vibrations generated by sound waves into electrical signals. This property of the diaphragm is represented as a transduction function or process.

A transducer has at least one input and one output. In the measurement system, information processing is performed by electrical signals, with either the input or the output is of electrical such as voltage, current, resistance, capacitance, and so on, whereas the other is a non-electrical signal such as displacement, temperature, elasticity, and so on.


Sensor


A sensor is an input transducer with a non-electrical input, intended to convert a non-electrical quantity into an electrical signal in order to measure that quantity. For example, a thermocouple which is a temperature sensor converts non-electrical input that is the temperature, into an electrical signal that is a voltage.

Actuator

An actuator, on the contrary, is an output transducer with an electrical input, intended to convert an electrical quantity into a non-electrical signal in order to control that quantity. For example, an electric pump or motor converts electrical input that is the voltage into a non-electrical output that is rotational motion


Bidirectional Transducer


Bidirectional transducers are capable of converting physical quantities into electrical signals and vice-versa. For example, antennas in the transmitting mode convert the electrical signals into radio waves, while in the receiving mode converts radio waves into electrical signals.


It should be noted that this terminology is not standardized. According to Vocabulaire International de Metrologie (VIM), transducer is a device used in measurement, that provides an output quantity having a specific relation to the input quantity. The same document defines a sensor as the element of a measuring system that is directly affected by a phenomenon, body, or substance carrying a quantity to be measured.


All sensors are transducers but not all transducers are sensors


The terms sensors and transducers are used interchangeably often, however, there is a slight difference in them. A common statement which is largely used in engineering states that: “All sensors are transducers but not all transducers are sensor”. 


Essentially, all sensors are transducers as they convert physical quantities into electrical quantities (conversion of one form of energy into another form) but not all transducers are sensors, as in case of actuators, where there is also the conversion of one form of energy into another form, but are not capable of sensing, rather are used for controlling. 

Difference between sensors and transducers when considering a measurement system

Let us now understand the difference between sensor and transducer when used as a measuring system. In a measurement system, a sensor is defined as a device that converts the physical quantity (usually non-electrical input) to be measured into directly an electrical output. For example, a piezoelectric pressure sensor will sense the pressure (physical quantity) into voltage (electrical output) through the transduction process. No conversion of the output of the transduction process is required in this case as the output signal is already in a usable format. 

On the contrary, a transducer in a measurement system is defined as a device that converts the physical quantity (the non-electrical input) into an electrical signal such as resistance, capacitance, and so on. This electrical signal obtained from the transduction process needs conversion to make the signal usable through the use of a signal conditioning circuit. For example, an RTD is a temperature transducer that senses temperature (the physical quantity) and converts it into resistance. A signal conditioning circuit such as Wheatstone bridge (usually termed as - signal conditioning circuit) is required to convert resistance into voltage. This is a small difference between sensor and transducer when considering a measurement system.


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