The measurement is an important subsystem in any main system, be it a mechanical system or an electronic system. A measurement system consists of sensors, actuators, transducers and signal processing devices. The use of these elements and devices is not limited to measurement systems. These are also used in systems that perform specific tasks, to communicate with the real world. Communication can be something like reading the status of a signal from a switch or triggering a particular output to turn on an LED.
The words sensor and transducer are widely used in association with measurement systems. The sensor is an element that produces signals related to the quantity that is being measured. According to the Instrument Society of America, "a sensor is a device that provides usable output in response to a specific amount that is measured." The word sensor is derived from the original meaning "perceive". In simple terms, a sensor is a device that detects changes and events in a physical stimulus and provides a corresponding output signal that can be measured and / or recorded. Here, the output signal can be any measurable signal and is generally an electrical quantity.
Sensors are devices that perform an input function in a system when they "detect" changes in a quantity. The best example of a sensor is the mercury thermometer. Here the quantity that is measured is heat or temperature. The measured temperature becomes a readable value in the calibrated glass tube, as a function of the expansion and contraction of the liquid mercury.
Actuators are devices that work in front of the sensors. A sensor converts a physical event into an electrical signal, while an actuator converts the electrical signal into a physical event. When sensors are used at the input of a system, the actuators are used to perform the output function in a system, since they control an external device. Transducers are the devices that convert energy into one form in another form. In general the energy is in the form of a signal. Transducer is a term used collectively for sensors and actuators.