Steps for PID Tuning
(1) Determine the Proportional Gain (Kp) or Proportional Band (PB)
When determining the proportional gain Kp, first remove the integral and derivative terms by setting Ti = 0 and Td = 0, making it a purely proportional control. Set the input to 60%–70% of the system's maximum allowable output. Gradually increase Kp from 0 until the system begins to oscillate; then, gradually decrease Kp from this point until the oscillation disappears. Record the Kp value at this point, and set the PID proportional gain Kp to 60%–70% of this recorded value.
(2) Determine the Integral Time Constant (Ti)
After the proportional gain Kp is determined, set a relatively large integral time constant Ti. Gradually decrease Ti until the system begins to oscillate, then gradually increase Ti until the oscillation disappears. Record the Ti value at this point, and set the PID integral time constant Ti to 150%–180% of this recorded value.
(3) Determine the Derivative Time Constant (Td)
Generally, the derivative time constant Td does not need to be set and can be left at 0, converting the PID control into PI control. If it needs to be set, use the same method as determining Kp, taking 30% of the value at which no oscillation occurs.
Properly and correctly mastering the commissioning of the control system, ensuring the system enters closed-loop control smoothly and rapidly without disturbance, is a prerequisite for the stable operation of the process. The only requirement for control system commissioning is a smooth transition from manual to automatic operation-i.e., achieving bumpless transfer (meaning the signal to the valve remains essentially unchanged during manual/auto switching). System commissioning should be closely coordinated with the plant startup process, during which the control and detection systems are inspected and debugged through static and dynamic testing. The general commissioning sequence for the components of the control system is as follows:
(1) Precautions for Commissioning the Detection System
Commissioning the temperature and pressure detection systems is relatively simple; instruments and transmitter instruments can be turned on individually to verify the accuracy of their displayed values. For flow and level detection systems, follow the requirements of the transmitter instruments: slowly open the relevant root valves and isolation valves one by one, starting from the root of the detection element, to prevent pressure shocks to the transmitter, until the display is normal. If all parts of the transmitter are functioning normally after startup, it can be put into operation. If a fault occurs during operation requiring an emergency shutdown, the shutdown sequence should be the exact reverse of the startup steps mentioned above.
(2) Precautions for Commissioning the Control System
Start with manual DCS control and gradually transition each control loop to automatic operation, ensuring bumpless transfer.
① Commissioning of manual DCS control valves: Manual DCS valve control is essentially manual operation from the control room. Adjust the valve opening manually and observe the changes in the controlled variable to roughly determine the proportional relationship between the opening and the controlled variable. Generally, after manual operation is stable, set the PID parameters and switch the controller from manual to automatic mode when the controlled variable is close to or equal to the setpoint. At this point, the preliminary commissioning process is complete.
The controller automatically calculates and provides the output based on the error. Since automatic output requires bumpless transfer, do not switch to automatic mode immediately after adjusting the valve opening. Let the system run for a period until the PV (Process Variable) oscillation stabilizes, and switch to automatic mode when the value is in the middle position (avoiding peaks and troughs). When modifying the setpoint (SV), try to modify it when SV ≈ PV to minimize interference and fluctuations. Never artificially offset the setpoint to increase the rate at which PV approaches SV when the deviation between PV and SV is large, as this will cause greater fluctuations.
Contrary to commissioning, when the production process experiences significant disturbances and the controlled variable becomes unstable, the control system must be taken out of automatic operation and switched to manual DCS operation (auto-to-manual transfer). This process also requires bumpless transfer, but in emergencies, there is no need to wait for the perfect timing; it should be executed immediately.
(3) Precautions for Control System Parameter Adjustment
After the control system is commissioned, the transient process may not meet the requirements. In this case, the three parameters PB, Ti, and Td need to be further adjusted according to the characteristics of the process until the process and control quality requirements are met. PB, Ti, and Td are not fixed; they must be adjusted as process conditions and related equipment change. After each plant restart, the operation of the control system should be further checked, and any issues should be analyzed and resolved promptly. For example, check whether the control valve sizing is correct, whether the valve flow characteristic is appropriate, and whether the transmitter range is suitable. When changing a parameter in the control system, consider its impact on the system. For instance, if the control valve sizing or transmitter range is changed, the controller's proportional band should be adjusted accordingly.
Based on the above analysis, we summarize the tuning steps as follows:
① Understand the Process: Familiarize yourself with the current process operation to select an appropriate, stable time for tuning that will not disrupt the process.
② Output Settings: Based on process requirements, set the upper and lower limits of the MV (Manipulated Variable) within a certain range to prevent the valve position or inverter from jumping erratically due to unreasonable initial parameter settings.
③ Curve Response: Create a trend group for MV, SV, and PV. Observe the PV changes for a period to find PID parameters that have performed well in the same process, same craft, same location, and same system (DCS or PLC model).
④ Adjustment Range: Based on the required precision and stability of the process, optimize the display range for PV, SV, and MV, focusing on the PV value. For example, to pursue higher precision, I once modified the display range of a PV with a 0–200 kPa scale to a 10 kPa window (120–135 kPa), and modified the display range of a PV with a 0–450°C scale to a 300–340°C window.
Suzhou Pharma Machinery Co.,Ltd.
2026/07/22
Gino
