Process characterization is the identification of the dynamic responses of a process to changes in its inputs. As part of the chapter on process characterization in Chapter 9 of Control Loop Foundation, we address inverse process response. A few processes exhibit a behavior where for a step change in input, the output initially changes in a direction that is opposite to the final direction it will have taken when the process has fully responded to the input change. Such a process response is shown below and is characterized as an inverse response.
Often a process will only exhibit an inverse response to a step change in one direction of change but not in the opposite direction. The reason for this behavior may be understood when the source of the inverse response is better understood. For example, the level in the bottom of the distillation column illustrated below may show an inverse response to a step change increase in heat input.
As a result of a large step change increase in heat input to the column, a lot of bubbling and froth may temporarily result in a false level indication. However, long term, the increased energy input will boil away more material at a faster rate and the actual level will drop at a faster rate than previously. An inverse response has been observed on an increase in energy input. On a decrease in energy input, the same disruption of the level measurement will not be observed. A similar inverse response may be seen in boiler drum level control and other applications. The size and direction of an input change will often determine whether an inverse response is
observed for a change in the process input.
The second workshop for the chapter on process characterization is designed to give the user experience working with a process that may characterized as exhibiting an inverse process. By accessing the second exercise on characterization using your web browser. The viewer below may be used to see the solution to this exercise.