Commissioning Control Systems – Control Response

In many processes, it is desirable to have an overdamped response. To achieve that response, the proportional gain used in the PID is typically much less than that required for an underdamped or unstable response. When selecting the PID gain, it is important to consider that the process gain may change with the operating conditions. For example, the installed characteristic of a valve may provide a non-linear response to valve position in the valve operating range. The PID gain should be based on the operating conditions that provide for maximum process gain. If the processgain changes, it has the same effect on the control loop response as changes made in the PID gain.

A mistake that is often made in tuning control loops is to examine loop control performance at one operating point. For example, when a valve is operating at approximately 50% open, tuning may be adjusted to achieve an underdamped response. When the setpoint is changed to where thevalve is operating at 30% open an entirely different response may be observed because the process gain has changed. As has been emphasized in Chapter 12 of Control Loop Foundation – Batch and Continuous Processes, it is wise to select a conservative PID gain setting which will provide stable control even if the process gain changes with the setpoint. As a consequence, the “best” PID gain often gives a slower response to changes in setpoint and load disturbances. This conservative tuning will help ensure stable operation even if the process gain changes with increases or decreases in the operating setpoint.

To ensure stable operation over the entire operating range of a control loop, the normal practice is to establish the PID tuning when the process is operating in the region with the highest process gain. If the loop operating point changes to a low-gain region, the worst that happens is the response to a setpoint change is sluggish, but the response is stable. If a slower response does not meet process requirements, then it may be necessary to use a characterizer to provide linear installed characteristics as discussed in my August 8th blog. When the process gain changes with operating conditions, the best control performance over the entire operating range can be achieved by automatically modifying PID tuning based on valve position or some other measured or calculated parameter that is related to changes in the process dynamics or gain. Such adaptive control capability may be used to achieve best performance for all operating conditions as is illustrated below.