INCREMENTAL ENCODERS

Answer 2
Incremental encoders
Incremental encoder produce an output which is a pulse for each increment of resolution but these make no distinction between increments.An incremental encoder typically has four parts:
A light source(LED)
A rotary(or translator )disc
A stationary mask
A sensor (photodiode










The disc has alternate opaque and transparent sectors of equal width which is etched by means of a photographic process on to a plastic disc(slots are cut out if it is a metal disc).As the disc rotates during half of the increment cycle the transparent sectors of rotating and stationary discs come in alignment permitting the light from the LED to reach the sensor and thereby generating an electrical pulse.For fine resolution encoders ,multi-slit mask is often used to maximize the reception of shutter light.
The waveform of the sensor output of an encoder is generally triangular or sinusoidal depending upon the resolution required.Square wave signal compatible with digital logic are obtained from it by means of linear OPAMP and comparator.Alternate transparent/opaque sectors of the disc and the square wave pulse form (obtained after signal processing) in synchronous with the disc is shown in figure.The resolution of such an incremental encoder is given as:
Basic resolution=360/N
N:number of sectors of disc;each sector is half transparent and half opaque.
In a dual channel encoder two optoelectronic channels are employed.These are installed in the same rotating disc and the mask but displaced at 900 to each other such that the two pulse output signals have a relative times phase displacement of 900electrical.A circuit that senses the relative time phase of the outputs of the two channels determines the direction of rotation of the disc or the encoder shaft.
The output of the encoder is fed to a counter which counts the number of pulses;the count being the measure of angle(or translation)through which the encoder shaft has rotated.By sampling the counter at regular intervals by means of clock pulses it is possible to compute the speed of encoder shaft.

Reference:
Control System And Engineering(Nagarath And Gopal).
www.wikipedia.com