Page 383 - The Mechatronics Handbook

A
shaft

B
rotating stationary
codewheel mask

FIGURE 19.8 Schematic of an incremental encoder.

use of triangulation to determine the distance to the target. Such distance measuring sensors are man-
ufactured by Sharp and Hamamatsu.
Optical Encoders
An optical encoder uses photointerrupters to convert motion into an electrical pulse train. These electrical
pulses “encode” the motion, and the pulses are counted or “decoded” by circuitry to produce the displace-
ment measurement. The motion may be either linear or rotational, but we focus on more common rotary
optical encoders.
There are two basic configurations for rotary optical encoders, the incremental encoder and the absolute
encoder. In an incremental encoder, a disk (or codewheel) attached to a rotating shaft spins between two
photointerrupters (Fig. 19.8). The disk has a radial pattern of lines, deposited on a clear plastic or glass
disk or cut out of an opaque disk, so that as the disk spins, the radial lines alternately pass and block the
infrared light to the photodetectors. (Typically there is also a stationary mask, with the same pattern as
the rotating codewheel, in the light path from the emitters to the detectors.) This results in pulse trains
from each of the photodetectors at a frequency proportional to the angular velocity of the disk. These
signals are labeled A and B, and they are 1/4 cycle out of phase with each other. The signals may come
from photointerrupters aligned with two separate tracks of lines at different radii on the disk, or they
may be generated by the same track, with the photointerrupters placed relative to each other to give out
of phase pulse trains.
By counting the number of pulses and knowing the number of radial lines in the disk, the rotation of
the shaft can be measured. The direction of rotation is determined by the phase relationship of the A
and B pulse trains, i.e., which signal leads the other. For example, a rising edge of A while B = 1 may
indicate counterclockwise rotation, while a rising edge of A while B = 0 indicates clockwise rotation. The
two out-of-phase signals are known as quadrature signals.
Incremental encoders commonly have a third output signal called the index signal, labeled I or Z. The
index signal is derived from a separate track yielding a single pulse per revolution of the disk, providing
a home signal for absolute orientation. In practice, multiple photointerrupters can be replaced by a single
source and a single array detecting device.
IC decoder chips are available to decode the pulse trains. The inputs to the chip are the A and B signals,
and the outputs are one or more pulse trains to be fed into a counter chip. For example, the US Digital
LS7083 outputs two pulse trains, one each for clockwise and counterclockwise rotation, which can be
sent to the inputs of a 74193 counter chip (Fig. 19.9). Standard decoding methods for the quadrature
input are 1X, 2X, and 4X resolution. In 1X resolution, a single count is generated for the rising or falling
edge of just one of the pulse trains, so that the total number of encoder counts for a single revolution
of the disk is equal to the number of lines in the disk. In 4X resolution, a count is generated for each
rising and falling edge of both pulse trains, resulting in four times the angular resolution. An encoder

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