Impact of Changing Process Gain

From a control perspective, it is highly desirable that the process gain is constant. If the process gain is constant, then the same proportional gain can be used over the entire operating range of the control loop. Using a control valve as an example, if the valve characteristic was not properly selected based on the process requirements, the valve installed characteristic could be non-linear.  In Chapter 3 of Advanced Control Foundation – Tools, Techniques, and Applications the following example is provided that shown the impact of a non-linear valve installed characteristic. In this example the process gain varies from 0.5 to 4; that is, the process gain changes by a factor of eight.

For such non-linear installed characteristics, the impact of the changing process gain with valve position can often be seen by trending the controlled parameter (PV), setpoint (SP) and controller output (OUT) to the value. If valve is found to have non-linear installed characteristic then stable operation can be achieved by tuning the control loop in the high gain region, but doing this will result in slow response when the process is operating in the low gain region. To compensate for the changes in process gain, a characterizer block (SGCR) can be installed between the PID and Analog Output blocks as illustrated below.

Because of the time and understanding that are necessary to install, commission, and maintain a control loop that includes a characterizer, this approach is not commonly found in the process industry. To improve control performance, the valve installation may be modified to provide linear installed characteristics. For example, the valve trim can be modified or a digital valve positioner installed that supports characterization.