I am using a PLC 5 PID to control material (think powder) flow. Setpoint is kg/minute, which is converted for the PID function. The PID output controls a VFD which controls RPM of an auger. The feedback comes from a flow transmitter directly after the auger. I am having a problem where the setpoint of the PID doesn’t change, but the output (and consequently the feedback) go into wild oscillations for no apparent reason. These oscillations eventually cause us to shut down the process and bring it back up to speed slowly, which equals downtime. The oscillations show no relation to the frequency that the PID enabled (every second). Also, I can see the trends of all this data on our historian and there is no apparent cause, usually the PID output just drops to almost nothing, then spikes , then levels out for a few seconds, then repeats.
What could cause this?
What could cause this?