Problems with cascade control
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I would like to ask the experts: when using cascade control with a flow meter and a level gauge, both the meter and the valve are in good condition, and they function properly manually. However, once cascade control is activated, the flow meter experiences severe fluctuations. Is this a PID-related issue? If so, how should it be adjusted? Thank you!
First adjust the flow rate of the secondary circuit; once it is stable, enable automatic operation of the secondary circuit. Adjust the liquid level again, and once it is stable, switch to automatic mode. The setting of the PID parameters for the main and auxiliary circuits requires careful attention.
The sensitivity of the flow meter may be too high.
If the sensitivity to flow rate is high, should we increase both p and i, while reducing the valve position amplitude and extending the time?
An overly sensitive flow secondary circuit can easily cause large fluctuations. The solution is to reduce the sensitivity of the flow rate feedback loop, that is, by decreasing the proportional gain Kp or increasing the proportional range δ; it is also possible to add some integral action by reducing the integral time constant Ti or increasing the integral gain Ki. Note that the flow control loop must not have a differential action, as differential action introduces noise; and since there is often flow noise in flow loops, noise is present in their signals as well.