PID control problem
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In a PID-regulated sub-circuit, under CAS control mode, significant oscillations occur when the valve opening is around 40%. The operation is normal at other opening levels, but this problem persists with unchanged PID parameters. How can this be resolved? I would appreciate some advice from everyone
My personal suggestion is to first identify the source of interference: whether it is the main circuit or the auxiliary circuit. Try to switch out of cascade mode, and then manually adjust the setpoint of the secondary loop until the valve opening is around 40%, to check for any oscillation. It could also be a problem with the flow meter; try switching to manual mode and adjust the valve opening manually to around 40%, then observe the fluctuations in the values read by the flow meter.
It might be a problem with the flow meter or the valve, resulting in poor repeatability of the valve.
Thank you all. I’ll give it a try when I get the chance; this phenomenon occurs only occasionally, so it’s difficult to conduct experiments during the production process
It’s likely there’s a problem with the valve; it doesn’t adjust automatically. Try setting the output to 40% manually to test the valve.
First do proportioning, then integration, and finally add differentiation. If the curve oscillates frequently, increase the scale of the proportioning dial; if the curve wanders around in large loops, reduce the scale of the proportioning dial. If the curve takes a long time to return to its normal position, decrease the integration time. If the fluctuation period of the curve is long, extend the integration time further. If the oscillation frequency of the curve is high, first reduce the value of differentiation. Large errors result in slow fluctuations. The differential time should be increased. For the ideal curve, there are two waves – higher at the beginning and lower at the end, in a 4-to-1 ratio. Observe carefully, make adjustments as needed, and conduct thorough analysis; this way, the quality of the adjustments will remain high
It’s likely there’s a problem with the valve; you can first set the control to manual mode and try operating the valve at 40% output.
It’s almost certain that it’s a valve issue. Possible reasons are: 1. The valve gets stuck at an opening degree of 40%; this is usually due to rust on the valve stem or foreign objects in the valve seat. 2. The screws of the valve actuator are loose, especially those related to the feedback lever, which causes the valve to be unstable at a certain position