This is controlling a centrifuge. Recently the centrifuge faulted on overspeed. The controller claimed the DC motor for the backdrive (DCS800) exceeded set speed. When I run in standby to test. The encoder input to the DP820 is accurate for the speed. I measure 61 khz at DP820 input. Encoder speed is encoder pulse freq / 25. 61 khz is 2,440 rpm. This agrees with the DCS800 keypad display. In the ControlBuilder software for the DP820 channel status, I see approx. 66.5 khz (2,660 rpm). This error is proportional on speed. I can set DCS800 to 750 rpm. PM856 program & DP820 hardware channel status reads ~30 rpm high.
Tried reloading boot flash. Replaced DP820.
Any advise would be appreciated. I'm out of ideas.
What sort of encoder electrical interface are you using with DP820 ? RS422 is more resistant to EMI than 5/12/24 voltage interface. Can you check with oscilloscope if there is any flickering at encoder signal edges? I assume DP820 counts extra pulses from flickering. Check grounding of encoder cable. Should be grounded at one side of cable only. I hope it helps