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Jan
17

Tuning and Loop Performance

During the commissioning of feedback control based on the PID algorithm, the tuning parameters associated with the PID (that is, the proportional, integral and derivative gain) must be set to specific values to achieve the best controller response to setpoint and disturbance input changes. The PID tuning parameters are normally specified as positive, floating-point values including zero and can take on many combinations of values. The procedure used to set the PID tuning parameters is commonly referred to as loop tuning. In this chapter 12 of Control Loop Foundation – Batch and Continuous Process, insight is provided into techniques that may be used to set the PID tuning parameters for best control performance.

Older control systems and single loop controllers may not include tools that allow the PID tuning to be automatically established. In such cases, it is necessary to introduce a step change in a manipulated input with the PID controller or PID block in Manual and then observe the response of the controlled parameter. Also, even when using a modern control system that support autotuning, the manual establishment of PID block tuning can be useful when initially commissioning process areas since this allows the tuning to be quickly established and control to be placed in Automatic during plant startup.

When commissioning PID control associated with a self-regulating process, the procedure outlined below may be quickly applied to both old and new control systems to determine the tuning for PI control.

TuningProcedure.jpg

The size of the step change introduced in the process input depends on the process gain. In general, the step size should be just large enough to easily distinguish the resulting change in the controlled process output. Based on the process response and the size of the step change, the process gain, deadtime, and time constant may be determined as described in a previous block on process characterization. The reset should be set to a value equal to the sum of the process deadtime and the time constant.

For example, if the total response time (time constant + deadtime) is15 seconds and the RESET units are seconds/repeat then the Reset would be set to 15. With the Reset being set, for PI control it is only necessary to determine the proportional gain setting of the PID. A very conservative gain, for example, 0.2 should initially be selected and the PID mode then changed to automatic control. Once the PID is placed in automatic control, the setpoint may be changed. If the response to the setpoint change is slower than desired, then the proportional gain should be increased by a small amount (e.g., 20%) and the setpoint changed again. If the response is still too slow then the proportional gain should be increased again. This process of changing the proportional gain and observing the response to a setpoint change should continue until the desired response is achieved.

The PID Tuning workshop included in Chapter 12 of Control Loop Foundation provides several exercises that may be used to further explore loop tuning. A copy of heater process described in the PID Control workshop is used in this PID Tuning workshop. By accessing the book’s web site , you may complete this PID Tuning workshop using your web browser. The viewer below may be used to see the solution to this exercise.