Encoders

The Optical Encoders typically consist of a rotating and a stationary member.  The rotor is usually a metal, glass, or a plastic disc mounted on the encoder shaft.  The disc has some kind of optical pattern, which is electronically decoded to generate position information.  The rotor disc in absolute optical encoder uses opaque and transparent segments arranged in a gray-code pattern.  The stator has corresponding pairs of LEDs, and phototransistors arranged so that the LED light shines through the transparent sections of the rotor disc and received by phototransistors on the other side.  See figure below.  Depending upon the shaft position, the phototransistor output is modulated in a gray-code pattern, which can be converted internally to binary or BCD.  Typically CMOS, TTL-, PNP-, and NPN-type outputs with 8- or 10-bit Gray-code, binary, or BCD formats are available.

The incremental optical encoders use a much simpler disc pattern.  This slotted rotor disc alternately interrupts the light beam between the LED transmitter-receiver pair and thus produces a pulse output.  The number of pulses depends on the number of slots on the disc.  The pulses are then fed to a counter, where they are counted to give position information.  The pulse rate indicates shaft speed.  An additional LED pair can also determine the direction of rotation.  Some modules also provide a marker pulse output, which is generated once every revolution at a fixed shaft position and can be used to mark a zero reference point.  Many different pulse configurations are available, but the most commonly known are the quadrature encoders, where two square wave pulses 90 degrees apart from each other are generated.