Control Notes

Many tuning rules are designed to produce a super-fast control loop response. These rules include Ziegler-Nichols, Cohen-Coon, and Minimum IAE tuning rules. When tuned with one of these fast-response tuning rules, the controller responds so aggressively to disturbances and setpoint changes that the process variable overshoots its setpoint and oscillates around the setpoint a few times before it finally settles out. You might have heard of the Quarter-Amplitude-Damping response that Ziegler and Nichols and others aimed for (Figure 1).

Figure 1. Simulated response of a temperature control loop tuned for Quarter-Amplitude Damping.

Perhaps even more concerning than the oscillatory response, is the fact that these aggressive tuning methods push a control loop very close to instability. This could become a huge problem because a small change in the process gain or dynamics can cause a control loop tuned in this way to become completely unstable. We say the control loop is not robust.

Detuning a Controller

Since oscillatory response and low robustness can both be detrimental to control loop performance, it is therefore good practice to tune controllers in a way that will produce a more stable control loop. Starting from tuning settings calculated with an aggressive tuning rule, we can detune the controller to obtain a more stable response and increased robustness. In this sense, detuning is a good thing and it simply means that we are slightly backing away from the aggressive, oscillatory, and semi-unstable tuning, to obtain a loop that is less oscillatory and more tolerant to changes in process characteristics.

On controllers with interactive and noninteractive algorithms, detuning is done by simply reducing the controller gain (Kc). Detuning the controller by a factor of two simply means dividing the controller gain by two. For simplicity, I call this detuning factor the stability margin (SM):

Kc to use = (calculated Kc) / SM.

On controllers with the parallel algorithm, the proportional, integral, and derivative gains need to be lowered in equal proportions.

Detuning a control loop by too little leaves it oscillatory and close to instability. Detuning a control loop by too much makes it very stable and robust, but slow to respond to disturbances and setpoint changes. Detuning the controller by a factor of 2 to 3 (using a stability margin of 2 to 3) is normally sufficient for eliminating oscillations and improving loop robustness.

Figure 2. Simulated response of a control loop tuned for quarter-amplitude-damping response (SM = 1), compared to detuning it by factors of 2 and 4 (SM = 2 and SM = 4).

To summarize, if you use one of the aggressive tuning rules to calculate controller settings, you should detune the controller by dividing the calculated controller gain by a stability margin of 2 or more to reduce overshoot, eliminate oscillations, and improve control loop robustness.

Stay tuned!
Jacques Smuts

Author of Process Control for Practitioners, available from Amazon.com: http://www.amazon.com/gp/product/0983843813