Types of Speed Transducers
Speed transducers are widely used for measuring the output speed of a rotating object. There are many types using different principles and most of them produce an electrical output.
These use a light beam and a light sensitive cell. The beam is either reflected or interrupted so that pulses are produced for each revolution. The pulses are then counted over a fixed time and the speed obtained. Electronic processing is required to time the pulses and turn the result into an analogue or digital signal.
Magnetic Pick ups
These use an inductive coil placed near to the rotating body. A small magnet on the body generates a pulse every time it passes the coil. If the body is made of ferrous material, it will work without a magnet. A discontinuity in the surface such as a notch will cause a change in the magnetic field and generate a pulse. The pulses must be processed to produce an analogue or digital output.
There are two types, A.C. and D.C. The A.C. type generates a sinusoidal output. The frequency of the voltage represents the speed of rotation. The frequency must be counted and processed. The D.C. type generates a voltage directly proportional to the speed. Both types must be coupled to the rotating body. very often the tachometer is built into electric motors to measure their speed.