When talking aerospace sensors, ‘analogue’ or ‘digital’ become hardly necessary, being simply a matter of method of operation, and that it is the underlying physical principles that are all-important.
Classification of sensors
In discussing sensing devices one has to decide whether to classify them according to the physical property they use (such as piezoelectric, photovoltaic, etc.) or according to the function they perform (such as measurement of length, temperature, etc.). In the former case one can present a reasonably integrated view of the sensing process, but it is a little disconcerting when one wishes to compare the merits of, say, two types of digital temperature sensor, if one has to look through separate sections on resistive, thermoelectric and semiconductor devices to make the comparison.
Alternatively, to try and differentiate devices by function often tends to be a rather boring catalogue of numerous unrelated devices. The important thing about them is signals are transformed from one form to another. It is also possible to discuss sensors from the functional viewpoint, under headings such as length, temperature, etc., suitable for someone who actually wants to select or use a sensor for a particular application rather than just read around the subject.
The words ‘sensors’ and ‘transducers’ are both widely used in the description of measurement systems. The former is popular in the USA whereas the latter is more often used in Europe. The choice of words in science is rather important. In recent years there has been a tendency to coin new words or to misuse (or misspell) existing words, and this can lead to considerable ambiguity and misunderstanding, and tends to diminish the preciseness of the language. The matter has been very apparent in the computer and microprocessor areas, where preciseness is particularly important, and can seriously confuse persons entering the subject.
The word ‘sensor’ is derived from sentire, meaning ‘to perceive’ and ‘transducer’ is from transducere meaning ‘to lead across’. A dictionary definition Chambers Twentieth Century) of ‘sensor’ is ‘a device that detects a change in a physical stimulus and turns it into a signal which can be measured or recorded’; a corresponding definition of ‘transducer’ is ‘a device that transfers power from one system to another in the same or in different form’.
A sensible distinction is to use ‘sensor’ for the sensing element itself, and ‘transducer’ for the sensing element plus any associated circuitry. For example, thermistors are sensors, since they respond to a stimulus (changes its resistance with temperature), but only become transducers when connected in a bridge circuit to convert change in resistance to change in voltage, since the complete circuit then transduces from the thermal to the electrical domain. A solar cell is both a sensor and a transducer, since it responds to a stimulus (produces a current or voltage in response to radiation) and also transducer from the radiant to the electrical domain. It does not require any associated circuitry, though in practice an amplifier would usually be used. All transducers thus contain a sensor, and many (though not all) sensors are also transducers.
The distinction is rather small and as soon as one actually uses a sensor (by applying power to it) it becomes a transducer. An interesting classification of devices can be achieved by considering the various forms of energy or signal transfer.
The word ‘actuate’ means ‘to put into, or incite to, action’ and actuators are devices that produce the display or observable output in a measurement system such as a light-emitting diode (LED) or moving coil meter. They are of course transducers used for output purposes, since they transduce from one domain to another (ie. electrical to radiant for LEDs).