Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

46 Chapter 1 Motor and Motion Control Systems
Absolute Encoders
An absolute shaft-angle optical encoder contains multiple light sources
and photodetectors, and a code disk with up to 20 tracks of segmented
patterns arranged as annular rings, as shown in Figure 1-37. The code
disk provides a binary output that uniquely defines each shaft angle, thus
providing an absolute measurement. This type of encoder is organized in
essentially the same way as the incremental encoder shown in Figure 1-
35, but the code disk rotates between linear arrays of LEDs and photodetectors
arranged radially, and a LED opposes a photodetector for each
track or annular ring.
The arc lengths of the opaque and transparent sectors decrease with
respect to the radial distance from the shaft. These disks, also made of
glass or plastic, produce either the natural binary or Gray code. Shaft
position accuracy is proportional to the number of annular rings or tracks
on the disk. When the code disk rotates, light passing through each track
or annular ring generates a continuous stream of signals from the detector
array. The electronics board converts that output into a binary word.
The value of the output code word is read radially from the most significant
bit (MSB) on the inner ring of the disk to the least significant bit
(LSB) on the outer ring of the disk.
The principal reason for selecting an absolute encoder over an incremental
encoder is that its code disk retains the last angular position of the
encoder shaft whenever it stops moving, whether the system is shut
down deliberately or as a result of power failure. This means that the last
readout is preserved, an important feature for many applications.
Figure 1-37 Binary-code disk for
an absolute optical rotary
encoder. Opaque sectors represent
a binary value of 1, and the
transparent sectors represent
binary 0. This four-bit binary-code
disk can count from 1 to 15.