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This post was last edited by Feng Shaohui on 2020-11-12 07:10. Figure 42-1 shows the typical closed-loop step response curve when optimizing control loops. The step response curve of this setpoint contains a lot of information, which serves as the basis and key to PID tuning. Before continuing to read, you can take some time to think about what you can come up with. Perhaps you have thought of more than I have. Figure 42-1 Closed-loop step response curve of the control loop: distinguishes between the measured value, setpoint, and controller output. In the above diagram, the green thick lines that change linearly are definitely the set values. Of the remaining two lines, the yellow line that changes rapidly when the set value changes is the controller output, while the other line that changes smoothly is the measured value. Error: The two lines adjacent to each other represent the set value and the measured value; the straight line is the set value, while the smooth line is the measured value. Since each curve in a typical DCS can have its own independent display range, two lines that are close to each other may be due to the settings for displaying the curves. When the form of the PID control algorithm varies, the controller output may not change rapidly in response to changes in the setpoint. This loop is in closed-loop control mode, and the controller’s output adjusts accordingly when the setpoint changes. The setpoint changes smoothly; this circuit should be the main circuit or a single circuit. The controller output decreases, and the measured value also decreases. The controller uses a standard PID form and incorporates differentiation. When the setpoint changes, the controller output experiences a sudden change in deviation; under the combined effect of proportional and differential control, the controller output changes rapidly before returning to its normal level. The controlled object is a self-balancing object. Because once the setpoint changed stably, the steady-state value output by the controller underwent a significant change. The controlled object has a delay of about 1 minute: it takes approximately 1 minute for the measured value to change after the controller output changes. Looking at the label, it’s a temperature control loop, but the response speed of the controlled object is very fast. Since there is a significant difference in the response curves between the two setpoint changes, the PID parameters of the control must have been adjusted between those changes. The modified PID parameters are more reasonable. Because when the setpoint changed for the last time, the controller exhibited overshoot in its output without oscillation, while the measured value showed no overshoot. And with the previous setting change, there was oscillation in the controller output as well as in the measured values. Based on the response curve to the previous setpoint change, it can be seen that due to oscillations in the controller’s output and their being in the same phase, the proportional action of the controller is too strong ； Although the measurement values oscillate, there seems to be a \"tailing\" phenomenon, and the integrating effect remains weak ； The jump caused by the change in the controller output setpoint did not reach a steady-state value; however, the controller output continued to oscillate, indicating that the pure time lag of the controlled system is greater than its time constant. The ratio of pure lag to time constant is a key criterion for determining whether an object is a highly delayed object. The controlled object is also a large pure lagging object.