When it is required to determine the angular position of rotating shafts then a rotary encoder or optical encoder is used. They usually take the form of a miniature electric motor in appearance, and sometimes get mistaken for stepper motors.
The optical encoder drive shaft is directly coupled to a rotating spindle via a flexible coupling. An electric signal is derived from the unit in a system of digital pulses. This pulse train is produced by a combination of a calibrated disc and light emmitting diode.
The disc, which is attatched to the encoder shaft, rotates, cutting a beam of light produced by the diode. The number of pulses obtained is dependant on the number of graduations set out on the disc. There is a very wide range of encoders available and it is important to select for particular applications. Pulse lines can be as low as 1 pulse/rev to as high as 20000 pulses/rev and can be powered at 5v or 11-30v. Greenbank stock the Italsensor Encoder TK561 with 1024ppr and 11/30V supply.