Allen Bradley HSCE2 Counter glitches?

J

Thread Starter

Jack Sprocket

We have some SLC500 systems controlling speed using the AB HSCE2 counter modules. We also read position from these. We've added a Red Lion Datastation+ to log PLC internal values (at 1 second rate) for diagnostics and quality control. Speed is typically 2000 counts/ sec. That corresponds to about 32 counts per module sample period.

The problem:

Some of the systems (not all) show the speed reading for one channel intermittently falling by 25-75% for one log period. The PID output shows a corresponding blip.

The mystery:

The corresponding position reading doesn't show any step that would indicate a temporary loss of input. The control performance seems generally satisfactory.

The PLC loop time is less than 20ms, so it could well be that the glitch lasts for only one cycle, just as the logger happens to sample it, and the effects are ironed out by the load characteristics.

The encoders, supplies, and wiring all check out OK. So has anyone experience of similar effects, or any credible explanation of this?
 
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