Autotuning PID – Step Tests

The most basic autotuners simply automate the manual tuning procedures an operator might otherwise perform when commissioning a loop: force a change in the control effort, observe the results, and adjust the tuning parameters accordingly. However, autotuners vary in how they execute those steps.

For example, a basic autotuner can perform a classical “step test” or “bump test” where the control effort is changed in a step-wise manner with feedback disabled. Theoretically, the process’s response to such a bump will provide sufficient information to characterize the process’s dynamic behavior, which in turn will dictate the appropriate tuning parameters. In practice, however, bumping a process just for the purpose of tuning the controller can be impractical in applications where fluctuations in the process variable must be minimized at all times.

Some autotuners can avoid this problem by performing a step test while responding to a setpoint change. Because the process is going to be disturbed anyway, the controller can afford to apply a small bump to the process as it attempts to drive the process variable toward the new setpoint.

For example, refer to the back-to-back step tests shown in Figure 2. When a setpoint change is requested by the operator, the controller applies a 100% control effort (a positive step) then shuts down before the process variable reaches the new setpoint (a negative step). The controller then observes the fluctuations in the process variable to identify a mathematical model of the process’s behavior.

The process time constant for the process can be derived or from the interval between the controller’s shutdown and the point when the process variable begins to drop. The appropriate PID parameters can then be computed from the process deadtime, gain, and time constant using any number of tuning rules plus the operator’s preference for closed-loop performance. After the tuning is complete, the controller can resume normal closed-loop control operations to bring the process variable the rest of the way to the setpoint.

fig 1 1 - Autotuning PID - Step Tests


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