PID parameter setting
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It is a DN50 control valve; the pressure before the valve is 55 kilograms, while the pressure after the valve is intended to be kept at 1.5 kilograms. The control valve is located 100 meters away from the storage tank (with a capacity of 4 cubic meters), and the pressure transmitter is installed on the tank. If PID control is used, what values would be appropriate for the PID parameters?
The setting of PID parameters should be adjusted gradually based on the actual operating conditions (by observing changes in the control curve), as there is no fixed value.
Nonsense; PID has experienced parameters – go read a book on the topic.
The parameter settings for PID can be determined as follows: Seek the optimal values by checking them in ascending order. Start with the proportional term, then add the integral term, and finally incorporate the derivative term. If the curve oscillates frequently, increase the proportional gain. If the curve wanders around in large loops, decrease the proportional gain. If the curve takes a long time to return to its normal position, reduce the integration time. If the fluctuations in the curve have a long cycle, extend the integration time further. If the oscillation frequency of the curve is high, first reduce the derivative term. Large errors lead to slow fluctuations. The differential time should be increased. The two waves of the ideal curve should be such that the first one is higher and the second one is lower, in a ratio of 4 to 1. It’s necessary to observe carefully, make adjustments, and conduct thorough analysis; this way, the quality of the adjustments will not be poor. Some experimentation is required. My experience suggests the following: 1. As long as there is no oscillation, P should be set as high as possible to ensure good responsiveness. 2. To meet the requirements regarding static error, I should keep I as low as possible, again to ensure good responsiveness. This needs to be used in conjunction with the p-value. 3. The d value is supplementary; depending on the value of pi, it should be set to a somewhat higher value to improve speed
What was said on the second floor is correct: the PID parameters have default values in a DCS system. Based on these default values, it is possible to adjust the P and I settings; generally, the D (derivative) parameter does not need to be set in pressure control applications. As for what someone upstairs said about fixed experience values, yes, there are values shared online, but those values may not be useful in practice as each working condition is different. It’s still better to adjust it gradually according to the operating conditions!
This post was last edited by hfrs on 2017-3-15 at 16:28. You can find this book on \"Process Control\" and take a look for yourself; those are not empirical values shared by someone, but rather scientific principles. The empirical parameter method is generally used for tuning PID parameters, and I’ve never heard anyone say it’s not effective – it is currently the fastest and most efficient method.
Which type of control valve should be chosen? If the pressure difference is too high, be aware that the lifespan will not be very long, and the adjustment performance will also be affected to some extent. Is it possible to increase the bypass around the orifice plate in order to reduce the pressure difference across the control valve?
It’s hard to say; you can only set a set of values based on experience first, and then adjust them by looking at the curve. Different systems result in discrepancies in data. If you study the principles of automatic control, there is a chapter dedicated to PID parameter setting, which outlines three methods – one of which does not involve any calculations.