Sensors and Signal Conditioning, 2nd Edition

4.3   ELECTROMAGNETIC SENSORS

Sensors discussed so far in this chapter, other than Wiegand sensors and SQUIDs, can be described by one or two variable capacitors or by one or two variable inductances or mutual inductances. There are other devices where a physical quantity can result in a change in a magnetic or electric field without implying a change in inductance or capacitance. This section describes some of the more frequently used sensors.

4.3.1   Sensors Based on Faraday's Law

In 1831 Michael Faraday reported that in any circuit or coil consisting of N turns linking a magnetic flux Φ, whenever Φ changes with time, a voltage or electromotive force e is induced that is proportional to the rate of change of Φ according to

The flux Φ can be variable in itself (e.g., when it is produced by an alternating current), or the position of the circuit can be made to change with respect to a constant magnetic flux. Ac tachometers are devices of the first kind, whereas dc tachometers, linear velocity meters, search coil magnetometers, and electromagnetic flowmeters are of the second kind.

4.3.1.1 Generating Tachometers. A generating tachometer (ac tachometer) is similar, in its working principle, to an electric power generator. The voltage induced in a circuit with N turns moving with an angular speed n (r/s) with respect to a constant magnetic field with a flux density B is

Because