PID tuning issues

Empirical values for tuning the PID parameters: Control object, characteristics, KP, Ti (s), Td (s). For flow control, when the value is small and there is noise, use 1-2.5 and 0.1-1 respectively. For temperature control, in systems with large time lags, use 1.6-5 and 3-10; for pressure control, in systems with smaller time lags, use 1.4-3.5 and 0.4-3. For level control, integration can be omitted if a certain static error is acceptable, using values of 1.25-5. To find the optimal parameters, start with smaller values and increase them gradually; first use the proportional term, then the integral term, and finally add the derivative term. If the curve oscillates frequently, increase the proportional gain. If the curve wanders around in large fluctuations, decrease the proportional gain. If the curve takes a long time to return to its normal state, reduce the integral time. If the fluctuation cycle is long, further increase the integral time. If the oscillation frequency is high, first reduce the derivative term. Large dynamic errors result in slow fluctuations. The derivative time should be increased. For the ideal curve, there are two waves: the first one is higher than the second, with a ratio of 4:1. Observe and make adjustments while conducting thorough analysis; this way, the quality of the adjustments will be satisfactory. Adjust the proportional coefficient; generally, based on experience, it should be set between 0.8 and 1.2. Try adjusting it—it’s best to do so manually first before switching to automatic operation. The value you’ve set for SV is too high for the current flow rate. You can set a value that is not too different from the current flow rate. It’s best to readjust all the parameters. For the engineering tuning of PID controller parameters, the following empirical data for PID parameters in various control systems can be referred to: Temperature T: P = “20”~60%, T = 180~600s, D = 3–180s; Pressure P: P = “30”~70%, T = 24~180s; Liquid level L: P = “20”~80%, T = 60~300s; Flow rate L: P = “40”~100%, T = 6~60s. General steps for parameter adjustment: a. Determine the proportional gain P. When determining the proportional gain P, first remove the integral and derivative terms from the PID control mechanism; generally, Ti is set to 0 and Td is set to 0 (see the instructions on PID parameter setting for details), thereby turning the PID control into a pure proportional control system. Set the input to 60%–70% of the maximum value allowed by the system, and gradually increase the proportional gain P from 0 until oscillations occur in the system ； Conversely, as the proportional gain P at this point gradually decreases until the system stops oscillating, record the value of P at that time, and set the proportional gain P in the PID controller to 60%–70% of this value. The tuning of proportional gain P is completed. b. Determining the integral time constant Ti: After determining the proportional gain P, set an initial value for the integral time constant Ti that is relatively large. Then, gradually decrease Ti until the system begins to oscillate. Afterwards, do the opposite—gradually increase Ti until the oscillations cease. Record the current value of Ti; set the integral time constant Ti of the PID controller to 150%–180% of this value. The tuning of the integration time constant Ti is completed. c. Determine the derivative time constant Td. Generally, there is no need to set the derivative time constant Td; setting it to 0 is sufficient. To set it, use the same method as determining P and Ti, taking 30% of the value without oscillation. d. Conduct joint testing of the system under no-load and loaded conditions, then fine-tune the PID parameters until the requirements are met.

This post was last edited by jlshnlhj on 2022-1-1 at 10:14. Similar to decoupling control, the simplest current method is to appropriately adjust the P and I parameters of the PID controllers for the liquid levels in the two tanks: gradually increase the proportional gain (i.e., reduce the proportional action) and decrease the integral time. The goal is to make the valve positions change more slowly, not too quickly, so that the other tank’s level can also be adjusted automatically.

The description is a bit vague; since the level valve operates automatically, which level is it referring to? Is it still the signal distribution module being used to control both valves simultaneously? It depends on how the control is implemented

Alternatively, the outputs of the two valves can be processed in this way: the opening degree of valve a = output of the PID module + opening degree of valve b × 0.2; a similar treatment is applied to valve b as well.