Control Notes

Some control loops have two seemingly conflicting objectives of keeping the process variable under control, but also minimizing controller output movement. Although such a loop will have a set point, it is more important to keep its process variable within predefined bounds than to keep it exactly at set point. A typical application of gap control is averaging level control. (Averaging control is somewhat like surge tank control, but the process variable is controlled around its set point. True surge tank level control rarely controls around its set point.)

Controller manufacturers have designed a modification of the standard PID control algorithm for use on processes with these conflicting objectives. This modification is called gap control and it works on the principle of two user-definable control regions, one for each of the two control objective.

The first region is far from set point (outside the gap) and requires a strong control action to turn the process around and bring it back to set point. The normal controller settings are used outside the gap. The second region is close to set point (inside the gap) within which the controller detunes itself based on a configurable gain multiplier M (between 0.0 and 1.0). The detuning helps to minimize controller output movements.

Gap Control (click to enlarge)

Tuning a Gap Controller

To use gap control effectively, you should set the size of the gap around the set point according to the typical variation of the process variable so that the process variable does not frequently and unnecessarily venture outside the gap. Tune the controller for fast response  outside the gap to provide quick recovery from disturbances. Use a gain multiplier (M) of 0.5 or less to minimize the controller output movement inside the gap.

For integrating processes divide the calculated integral time (Ti) by M (i.e. make the integral time longer in proportion to decreasing the controller gain). This is a requirement only for integrating processes and is done to ensure a stable integral term when the controller gain is reduced inside the gap. Do not use a gain multiplier of zero when controlling integrating processes because this dead band will cause the process variable to continuously cycle through the gap.

Gap control should not be used in control loops of which the objective is to keep the process variable as close to set point as possible. Use regular PI or PID control for those loops.

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners