84 ◾  Distillation Control, Optimization, and Tuning
a cycle. These concerns provided the motivation for developing the pattern recognition methods for closed-loop tuning in
Section 9.5.
9.3 Troubleshooting an Oscillating Control Loop
Sometimes an existing process control loop is already running
with sustained cycling, and tuning of the control loop can be
approached as a troubleshooting activity. There can be a number of possible causes for the process variable oscillating up
and down, and the problem is not always due to poor tuning.
If the controller is put in manual output mode so that it does
not respond to feedback from the process variable and the
oscillations continue, the problem is most likely due to causes
other than the tuning of that one controller. One possibility is
interaction from another control loop that causes the process
variable to cycle. If a controller is put in manual mode and
the oscillations stop, the problem may possibly be solved with
controller tuning.
One of the first patterns to observe in a cycling control
loop is the timing of the peaks and valleys in the process variable (controller input) and the manipulated variable (controller
output) and whether or not the cycles are in phase with each
other. Observe the point in time when the controller output is
at a peak or a valley. If the process variable is also at a peak
or a valley at the same time, the problem may be due to the
proportional gain being too high, that is, too narrow a proportional band. However, if the process variable reaches a peak
or a valley at a different point in time than the controller output, the problem may be due to the integral (reset) time constant being too short relative to the peak-to-peak time period.
The integral time constant needs to be about the same as the
peak-to-peak time period. Reducing the integral (reset) time
constant in the controller to be shorter than the peak-to-peak
time period usually does not speed up the control response.