User Manual

Connecting Optical Encoders
Advanced Digital Motor Controller User Manual 57
The figure below shows the typical construction of a quadrature encoder. As the disk ro-
tates in front of the stationary mask, it shutters light from the LED. The light that passes
through the mask is received by the photo detectors. Two photo detectors are placed side
by side at so that the light making it through the mask hits one detector after the other to
produces the 90
o
phased pulses.
LED light source
Stationary mask
Photodetector
Rotating
encoder disk
FIGURE 3-18. Typical quadrature encoder construction
Unlike absolute encoders, incremental encoders have no retention of absolute position
upon power loss. When used in positioning applications, the controller must move the
motor until a limit switch is reached. This position is then used as the zero references for
all subsequent moves.
Recommended Encoder Types
The module may be used with most incremental encoder modules as long as they include
the following features:
• Two quadrature outputs (Ch A, Ch B), single ended
• 3.0V minimum swing between 0 Level and 1 Level on quadrature output
• 5VDC operation. 50mA or less current consumption per encoder
More sophisticated incremental encoders with index and other features may be used,
however these additional capabilities will be ignored.
The choice of encoder resolution is very wide and is constrained by the module’s maxi-
mum pulse count at the high end and measurement resolution for speed at the low end.
Specifically, the controller’s encoder interface can process 1 million counts per second,
unless otherwise specified in the product datasheet.
Commercial encoders are rated by their numbers of “Pulses per Revolution” (also some-
times referred to as “Number of Lines” or “Cycles per Revolution”). Carefully read the
manufacturer’s datasheet to understand whether this number represents the number of
pulses that are output by each channel during the course of a 360 degrees revolution rath-
er than the total number of transitions on both channels during a 360 degrees revolution.
The second number is 4 times larger than the first one.
The formula below gives the pulse frequency at a given RPM and encoder resolution in
Pulses per Revolution.
Pulse Frequency in counts per second = RPM / 60 * PPR * 4