theory of optical encoder?

Answer 1

A light source (usually an LED) shines through some sort of grating onto a phototransistor so that, as the grating moved, the phototransistor is turned on and off. The resultant pulse stream can be used to tell how far something has moved, how far a shaft has turned, etc.

HTH

Doug

Answer 2

There are two basic types of optical shaft encoder. One type gives the absolute position of the shaft and the other type indicates how fast and in what direction the shaft is being turned.

The absolute position type consists of a transparent disc carrying a pattern of binary codes produced by making some parts of the disc opaque. To indicate 16 positions, for example, you would use 4-bit codes, a different one for each of the 16 segments of the disc. There would be one LED / photoransistor pair for each of the 4 bits.

The codes are arranged in what is known as Gray code, in which rotating the disc between adjacent segments always causes only one bit to change. This is to prevent the jitter that would otherwise occur if more than one bit had to change and the changes were not exactly simultaneous. See reference below.

The other type of shaft encoder is simpler and more commonly used. If the encoder can be initialised to a known position in some way, absolute position can subsequently be determined by dead reckoning. This encoder uses a transparent disc with a number of radial slots (or gaps in an otherwise opaque background).

Two LED / phototransistor pairs are positioned almost diametrically opposite each other, but not quite: one is displaced by half the angle between adjacent slots. This creates a phase difference between the pulses from the two phototransistors, which is reversed when the direction of rotation is reversed, thus enabling this direction to be determined.

Delightful littel gadgets. Have fun!