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The factory has an old Allen-Bradley MACSV system, and a temperature control circuit was installed recently. I encountered a problem while tuning the PID parameters; I hope experts can help me solve it. This temperature control system is used for both heating and temperature regulation, so pure proportional control is considered first when setting the parameters. Therefore, set the proportion value to a relatively high value, the integration time to 100000 (integration has no effect), and the differentiation time to 0. The principle for setting the proportional value is to set it at 10% per 1-degree temperature difference; therefore, when a 10-degree temperature difference is specified in the system, the controller outputs 100%. Since heating is required, the set value exceeds the process value by 50 degrees; therefore, the system’s PID controller outputs 100%. However, it was found that as the system temperature increased, the output of the PID controller gradually decreased, starting from 100% at a deviation of 50 degrees; that is, at a deviation of 49 degrees, the opening degree was around 90%. This is completely different from my understanding of the PID formula. The following are the PID calculation formulas from the system manual. U(s) 1 1 Si——— = —————— * —— ( 1 + ——— + TDs ) E(s) 1+(TD/KD)s BD TIs Could some expert help to solve this difficult problem?