86 ◾  Distillation Control, Optimization, and Tuning
The height between the first peak and valley, that is, 50% plus
25%, is 75% of the height of the step change in setpoint. This
response pattern can be recognized as being aggressive. There
is a lot of control action in response to a disturbance to the
control loop.
Generally, when the proportional gain in a controller is less
than the value recommended by the Ziegler–Nichols method,
the integral (reset) time constant can be reduced until a quarter amplitude decay response is developed. However, a low
proportional gain and a short integral time constant can give a
process response that is very sluggish because of the low gain.
In that case, the integral time constant can be increased to be
equal to the peak-to-peak time period, and the proportional
gain can be increased by the same factor that the integral time
constant was increased.
9.5 Pattern Recognition Tuning of
Self-Regulating Control Loops
Robbins 2,3 reported the development of a closed-loop tuning methodology that does not require any of the loops to be
run with sustained cycling. The steps for the Robbins tuning
method for self-regulating control loops are as follows:
1. Record the existing settings in the controller.
2. Turn off the reset, or set the integral time constant at least
four times the response time.
3. Turn off the derivative action, that is, D = 0.
4. Introduce a step change in the controller setpoint, and
observe the height between the first peak and valley in
the process variable response. The optimum proportional
gain will give a height between the first peak and valley
that is 16% of the size of the step change.
5. Record the response time for the process variable to reach
the first peak after introducing a step up change in setpoint.