Sensors, Signals, and Systems

-A sensor is often defined as a “device that receives and responds to a signal or stimulus”
-This definition is broad. In fact, it is so broad that it covers almost everything from the human eye to the trigger in a pistol.
-A sensor is a device that receives a stimulus and responds with an electrical signal.
-The stimulus is the quantity, property, or condition that is received and converted into an electrical signal.
-Examples of stimuli are light intensity and wavelength, sound, force, acceleration, distance, rate of motion, and chemical composition.
-Measurand, which has the same meaning as a stimulus, however with the stress on the quantitative characteristic of sense.
-We may say that a sensor is a translator of a generally nonelectrical value into an electrical value.
-The sensor’s output signal may be in the form of voltage, current, or charge. These may be further described in terms of amplitude, polarity, frequency, phase, or digital code.
-The set of output characteristics is called the output signal format. Therefore, a sensor has input properties (of any kind) and electrical output properties.

(Ref: Handbook of Modern Sensors, Jacob Fraden)

Sensor Vs Detector

-The terms sensor and term detector are synonyms, used interchangeably, and have the same meaning.
-However, the detector is more often used to stress the qualitative rather than quantitative nature of measurement.
-For example, a PIR (passive infrared) detector is employed to indicate just the existence of human movement but generally cannot measure direction, speed, or acceleration.

(Ref: Handbook of Modern Sensors, Jacob Fraden)

Sensor, Actuators, and Transducer

-The term sensor should be distinguished from the transducer. The latter is a converter of any one type of energy or property into another type of energy or property, whereas the former converts it into an electrical signal.
-An example of a transducer is a loudspeaker that converts an electrical signal into a variable magnetic field and, subsequently, into acoustic waves.
-This is nothing to do with perception or sense.
-Transducers may be used as actuators in various systems.
-An actuator may be described as opposite to a sensor—it converts an electrical signal into generally nonelectrical energy.
-For example, an electric motor is an actuator—it converts electric energy into mechanical action.
-Another example is a pneumatic actuator that is enabled by an electric signal and converts air pressure into force.

(Ref: Handbook of Modern Sensors, Jacob Fraden)

Direct and Hybrid Sensor

-A direct sensor converts a stimulus into an electrical signal or modifies an externally supplied electrical signal.
-A hybrid sensor (or simply — a sensor) in addition needs one or more transducers before a direct sensor can be employed to generate an electrical output.

(Ref: Handbook of Modern Sensors, Jacob Fraden)

Sensor’s Classifications

-All sensors may be of two kinds: passive and active.
-A passive sensor does not need any additional energy source. It 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. Examples are a thermocouple, a photodiode, and a piezoelectric sensor.
-Many passive sensors are direct sensors as we defined them earlier.
-The active sensors require external power for their operation, which is called an excitation signal. Eg. Thermistor (temperature sensitive resistor)-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 of 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.

(Ref: Handbook of Modern Sensors, Jacob Fraden)