Sensor classification schemes range from very simple to the complex. Depending on the classification purpose, different classification criteria may be selected. Here, we offer several practical ways to look at the sensors.

All sensors may be of two kinds: passive and active. A passive sensor does not need any additional energy source and directly generates an electric signal in response to an external stimulus; that is, the input stimulus energy is converted by the sensor into the output signal. The examples are a thermocouple, a photodiode, and a piezoelectric sensor. Most of passive sensors are direct sensors .

The active sensors require external power for their operation, which is called an excitation signal. That signal is modified by the sensor to produce the output signal. The active sensors sometimes are called parametric because their own properties change in response to an external effect and these properties can be subsequently converted into electric signals.

It can be stated that a sensor’s parameter modulates the excitation signal and that modulation carries information of the measured value. For example, a thermistor is a temperature-sensitive resistor. It does not generate any electric signal, but by passing an electric current through it (excitation signal), its resistance can be measured by detecting variations in current and/or voltage across the thermistor.

These variations (presented in ohms) directly relate to temperature through a known function. Another example of an active sensor is a resistive strain gauge in which electrical resistance relates to a strain.

To measure the resistance of a sensor, electric current must be applied to it from an external power source. Depending on the selected reference, sensors can be classified into absolute and relative.

An absolute sensor detects a stimulus in reference to an absolute physical scale that is independent on the measurement conditions, whereas a relative sensor produces a signal that relates to some special case. An example of an absolute sensor is a thermistor: a temperature-sensitive resistor. Its electrical resistance directly relates to the absolute temperature scale of Kelvin.

Another very popular temperature sensor—a thermocouple—is a relative sensor. It produces an electric voltage that is function of a temperature gradient across the thermocouple wires. Thus, a thermocouple output signal cannot be related to any particular temperature without referencing to a known baseline.

Another example of the absolute and relative sensors is a pressure sensor. An absolute-pressure sensor produces signal in reference to vacuum—an absolute zero on a pressure scale. A relative-pressure sensor produces signal with respect to a selected baseline that is not zero pressure (e.g., to the atmospheric pressure).

Another way to look at a sensor is to consider all of its properties, such as what it measures (stimulus), what its specifications are, what physical phenomenon it is sensitive to, what conversion mechanism is employed, what material it is fabricated from, and what its field of application is.

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