September 1997, Issue 86

PC/104 Quarter:
Motion Control with PC/104

ENCODER BASICS

Optical encoders fall into two basic categories--absolute and incremental.

Output from an absolute encoder represents the absolute position of the encoder rotor to within one shaft rotation. Each bit of resolution requires additional coding on the optical disc.

The primary advantage of an absolute encoder is its ability to give absolute position information at powerup. Its disadvantages are the larger code disc size and higher cost associated with increased resolution.

Incremental encoders fall into several subcategories--pulse (tachometer), pulse and direction, up/down, and quadrature.

When designing in encoder feedback, consider both system and encoder resolution, maximum encoder output frequency (pulses or lines per second), maximum acceleration rate needed for the encoder, and the interface (voltage and current) requirement.