IB-11B012 INDUSTRIAL INDEXING SYSTEMS, INC.
USER'S GUIDE MOTION CONTROL SYSTEM, MSC-250
1 - 6 DESCRIPTION MARCH 2003
1.2.1.1 Position Feedback
Instructions from the Macroprogram provide the parameters for indexing and positioning.
This motion information is processed and converted into voltage information which is sent
to the drive to cause the motor to rotate to a desired position. At the same time, the
encoder, which is mechanically connected to the motor shaft, sends information to the
processor which indicates the actual position of the motor shaft. The difference between
the actual position of the motor shaft, as indicated by the encoder, and the commanded
position is called the "following error".
The feedback device is an A⋅B⋅M shaft encoder or electronic equivalent. The MSC-250 will
accept inputs from a variety of encoders, but each drive system and its corresponding
Macroprogram must be designed around a specific encoder line count.
WARNING
WHEN A SHAFT ENCODER IS REPLACED, ALWAYS USE AN
IDENTICAL ENCODER. IF A DIFFERENT ENCODER LINE COUNT IS
USED, THE MACROPROGRAM MUST BE REVIEWED AND POSSIBLY
REVISED BEFORE THE SYSTEM IS OPERATED. FAILURE TO
CORRECTLY MATCH THE MACROPROGRAM TO THE ENCODER
BEING USED MAY RESULT IN IMPROPER SYSTEM MOVEMENT
WHICH COULD CAUSE EQUIPMENT DAMAGE AND POSSIBLE
PERSONAL INJURY.
The encoder produces current loop pulses of approximately 10 mA per channel. There are
three channels. Channels A and B are quadrature encoded with channel A leading
channel B by 90° for clockwise shaft rotation. Channel M provides a marker once per
encoder revolution. The pulses are square-wave signals (refer to Figure 1.4).
NOTE
On some encoders, channels A and B are designated channels 1 and
2. Always connect encoders so the leading channel for clockwise
rotation is connected to channel A while the second quadrature
channel is connected to channel B. The marker channel is always
channel M (or "3" or "Z").
Each edge (change in state) of the quadrature-encoded square waves is counted by an
interface circuit. Thus, the change from low to high is one edge and the change from high
to low of the same pulse is a second edge. The counting of edges plus the 90° offset of
the two pulse streams provide position and direction sensing with a resolution equal to four
times the encoder line count. Other line counts can be used with software scaling of
parameters.
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