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4. CONTROL OF VIX DRIVES 89
At the end of the main move the controller waits until the in position time delay and the settle
time (if programmed) have timed out, at this point the encoder count is read. A calculation is
performed which compares the encoder count with the target position, if the difference
between these two readings is less than the defined dead-band then the move is complete
and the next user program command is executed. If the two readings differ by an amount
greater than the dead-band then position maintenance is used to correct the move error.
Assuming the motor has not quite reached the target position and that position maintenance
is required, the difference between the target position and the encoder count will be the
number of steps yet to be moved. The indexer will automatically apply a correction move,
based upon the number of steps yet to be moved, and will then, after the appropriate delays,
re-read the encoder count. Once again a comparison is made between the encoder count
and target position and the whole process is repeated, depending upon the result of the
comparison.
Dead Band Range
With Position Maintenance enabled, if you command the motor to move one revolution, at
the end of the move you would expect the encoder count to read the encoder resolution. In
practice, mechanical alignment errors and lost motion within the system will usually result in
a small offset existing between commanded motor steps and the encoder reading. To take
account of this offset an error band is defined, known as the Dead Band Range. It has a
range of 0 to 32767 encoder counts and a default value of 10. The number of counts
entered must be positive, but the range will check the number of counts on both positive and
negative sides of the target position. Position maintenance will have deemed to be
successful if the final correction move positions the motor within the dead band range.
Note: the value entered must be in load-mounted encoder steps if LOADENC is enabled,
otherwise it is entered in motor-mounted encoder steps.
Output
An optional output can be used to signal when position maintenance is enabled and the
motor is in position. In position is defined as not moving whilst positioned within the dead
band range. See the note at the end of the example.