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Sep
19

Commissioning Control Systems – Selection of Control Structure

Most control systems allow the user to pick the components, also known as the structure of the PID algorithm, that will be used in a control application. In some cases, this is done by providing different control blocks for P, PI, or PID. However, some systems such as DeltaV provide a single PID block and the selection of PID components is made through a “structure” parameter. Through the structure parameter it is possible to select all combinations of the different components that may be used in PID control applications, that is, PID, PI, proportional-only, integral-only. In the implementation of integral-only control, the proportional term is removed.

Given this degree of flexibility, the natural question is, “When should I use PI versus I –only versus PID versus proportional-only?” It has been shown by Corripio and Smith that based on the characterized process response, it is possible to determine the control structure that will provide the best control performance. In Control Loop Foundation – Batch and Continuous Processes, the results of their work is summarized in the guidelines shown below.

Most processes contain a mixture of process dynamics. In applying the guideline for selecting control structure, it is important to consider the dominant process dynamics of the control application. For example, few processes are characterized as a pure gain process. However, many processes, such as liquid flow and liquid pressure, are dominated by process gain and often exhibit negligible delay or lag in response. The response of this type of process is very close to that of a pure gain process. In most cases, the tuning of this type of process is dominated by integral action with a small proportional gain.

If the process response can be characterized as first-order, the PI structure should be selected. When the process is known to be made up of at least two dominant lags (second-order response), then PID may be more appropriate than PI control if the measurement is noise-free. For example, temperature measurement is often characterized by a second-order response. When addressing an integrating, non-self-regulating process, then selecting proportional-only or PI in which the integral term is minimized through tuning may provide the best response. These rules of thumb may be used in selecting PID structure, as well as in determining which tuning parameters should primarily be used in setting up the control system.