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When
New Position
Controlling:
Throughput Time is:
8 outputs
200 us
6 outputs
154 us
4 outputs
111 us
2 outputs
71 us
You must take into account the fixed throughput time, the number of
outputs per module, and the number of increments between the preset
values when determining the appropriate machine preset values for a
design shaft speed. Due to the effects shown in the first example, you may
want to adjust the preset values to account for the throughput time. This
is important if the module is used near its maximum design speed.
If the maximum encoder shaft speed, determined from the above
equations, is too slow for your application, you should consider the
following.
If you increase the input speed slightly, you can still maintain control to a
one-count resolution. However, the encoder position value and the output
status read by the PC may not correspond. If, for example, you request an
output to turn on at position 100, for one PC scan the PC might see a
position value of 099 while the output-on bit is set. The comparisons will
be performed correctly, but the status of the outputs read by the PC may
not correspond to the encoder position value.
This may not matter in your application if you do not use the read-data in
your PC application program. However, if this is not acceptable, you may
be able to trade resolution for speed. Remember that the maximum shaft
speed depends on the number of encoder positions. A 0 to 4,095-count
encoder has a lower maximum rpm rating than a 0 to 359-count encoder.
Similarly, a 0 to 99-count encoder turns at an even higher rotational speed
to control within a one-count resolution.
You can also trade accuracy for speed. Suppose your application can
tolerate having an output come on anywhere between position 030 and
035 and go off between 045 and 050. The encoder shaft may be turning
fast enough to go through several positions during the module comparison
processing time:
The module reads position 028 during the first module scan and leaves
the output off.
Appendix B
B 5
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