Optical encoders
Incremental encoder
An incremental encoder is a pulse generator that provides square–wave signals and a zero index.

The disc has 2 channels divided into “n” equal angular intervals; alternately opaque and clear. “n” is called resolution or number of counts.

Pulses counted by the processing electronics give the angular position of the rotor. For this purpose 2 squarewaves A and B are used with a third channel, Z, synchronous with signals A and B giving only one signal per revolution. It allows the user to initialize or reset the count.
Incremental encoder
Incremental encoder chart
Absolute encoder
Why another concept ?
This concept has been developed to compensate for the performance limitations of incremental encoders :
  • need for initialization
  • must be re–zeroed after a power interruption: zero reset to obtain the mechanism angular position
  • sensitivity to the interference
The absolute encoder supplies the shaft position as a binary code.
The output code is unique for each position. The resolution is equal to 2n (n= number of bit).
The shaft position information is given instantly when power is supplied.


There are two different types of absolute optical encoders :
  • Single turn absolute optical encoder –> gives the absolute angular position over 360° rotation.
  • Multiturn absolute encoder –> a system of additional shafts and discs which allows the number of revolutions to be counted.
The complete cycle is realized by « m » shaft rotations. The total resolution is equal to m x n (m=number of revolutions, n=number of counts per revolution).


Absolute encoder
Multiturn Absolute Encoder Principle