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speakerman

Old Quantum 984 PID2 Performance

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Hello again Forum! Been a while since I stopped by. Busy as all heck. Ran into an interesting problem with an old Quantum PLC, using ProWORX NXT. I am relatively new to PID tuning, and am not sure where to begin to correct this. We have a PID2 to run a pump command for a metered pump that targets a setpoint generated by a fixed recipe value. The setpoint is then modulated to mach the flow of raw material through the process, and the PID2 would respond to the setpoint changes by modulating the pump command, as expected. There were four different PID loops for four pumps, and all were tuned beautifully, all trends showing the PV line almost invisible beneath the setpoint, with the MV following the production changes well. Over the past two years I've been here, there have been several pump changes and this has never caused a ripple in this performance, however, suddenly one loop changed. It tracks the setpoint, but is off by about 7 percent, reading high but not changing the pump command to bring the PV down to SP. This seems very strange, as the error shows the difference, the pump flow is verified correct, and the loop will modulate the pump and flow to exactly track the changes in SP, just always off by 7 percent high. I can understand how the new pump would perhaps have a different flow characteristic, and would guess that all the changed pumps did vary a bit, but I cannot understand how the PID would leave the flow offset by such an obvious margin. Does anyone know how a lop could allow the PV to sit apart from the SP, and yet follow the curve so precisely? Any light shed in this direction is much appreciated. Cheers, Speakerman.

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Have you checked the raw data value coming from your analog input card? I have seen these cards fail with an "offset". Another possibility is that someone or something has overwritten a parameter in the PID2 block.

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had a similar problem after a customer had installed a vfd and new equipment onsite. Once I arrived I saw they had used all non metallic conduits and the existing motor wiring had no grounds!! the drive freqs had about 38 volts on the casing of the motor, which in turn energized all the attached remote equipment. All the sensors were going crazy! What fun getting rid of noise and loops. Something as simple as a corroded ground can offset your reference, especially when you are dealing with the resonant effects introduced by drives.

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Thanks for the leads. I would bet there's some noise in the system, and the reason the PID looks like it's not tracking the number is that it is oscillating with the noise and creating the offset. The 4-20 mA source is a micromotion flow meter, and there was no work done to the drives or motors, just the physical pump itself. We'll look into the micromotion connections at the pump and I'll post the results when we sort this out. I did check for register overwrites, and there is none. I also verified the analog input value, and that checks out. The rest of the card is also fine. The flow on the micromotion display also matches the analog value in the PLC, so if it is noise, it's right at the source. Should make it easier to find. Good to be back. I'll try to do a little quid pro quo and help someone before disappearing again. Cheers, Speakerman.

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Hello All; Here's the latest update on the PID MV modulating with, but not following, the set point: There was a calculated bias used in the PID registers, scaled to follow the setpoint, but off by a scaled amount. This offset number was set differently for three pumps - based on the density of the material, it would appear. There was no offset in one of the PID loops. By tuning the offset, I was able to bring the flow in line with the setpoint. We did check the system for a loose wire, missing shield, or anything of that kind, and found nothing wrong. I also put traps in to see if the value was oscillating, and found nothing. With the offset retuned, the performance is accurate at all speeds, and the global backchecks for weight of product transfered do match the totalized flow. Unless something new arises, it would appear that the offset performance changed with this new pump. Must be a reason for the offset register to be there, after all. Any thoughts on this development? Has anyone used these offset registers in the PID before? Happy with the results, anyway. Cheers, Speakerman.

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