Sensors are normally used to convert physical variables like pressure to signal variables. Sensors are also often termed to as transducers since they are devices that convert input energy of one form into output energy of another form. Sensors can be classified into two broad classes depending on how they interact with the environment they are measuring i.e. as passive or active sensors.
Passive sensors do not add energy as part of the measurement process but may remove energy in their operation. A typical example of a passive sensor is a thermocouple, which converts a physical temperature into a voltage signal. Another example of a passive sensor is a pressure gauge where the pressure being measured exerts a force on a mechanical system (diaphragm) that converts the pressure force into a displacement, which can employed as a signal variable. For instance, the displacement of the diaphragm can be transmitted through a mechanical gearing system to the displacement of an indicating needle on the display of the gauge.
Also read: Analog vs. Digital Sensors
Active sensors add energy to the measurement environment as part of the measurement process. A case in point of an active sensor is a radar or sonar system, where the distance to some object is measured by actively sending out a radio (radar) or acoustic (sonar) wave to reflect off some object and measure its range from the sensor.
Related: Types of Sensors used in Measurement and Process Control
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