As a manufacturing professional PID tuning is probably one of your many responsibilities. It may also be one of the most frustrating aspects of your job if you tune control loops either manually or using software that requires a steady-state condition. Regardless of the repeatability of your approach, the end objective probably changes with every tuning session as each PID controller fulfills a different function.
Knowing how to tune a PID loop while satisfying its unique control objective is critical. Fortunately there is a group of descriptive statistics that characterizes a controller’s core performance attributes and that helps you to get the tunings right each time. Below are four (4) KPIs that process control experts frequently use when determining if their new PID tuning parameters are right for the job:
One aspect of control loop performance that’s always worth considering is Settling Time. Essentially Settling Time is a measure of the time needed by a process to return to its target – or Set Point – in response to a disturbance. For some PIDs it’s essential for the loop to settle within a fixed period of time as longer periods of variability could have a negative impact on one or more processes. A shorter Settling Time is generally a good thing as it is indicative of a control loop that is performing more efficiently.
Another aspect worth considering when tuning PID controllers is Percent Overshoot. This KPI quantifies the degree to which the Process Variable exceeds the associated Set Point in response to a Set Point change. Percent Overshoot can be an especially important performance assessment tool when tuning loops that regulate highly sensitive processes. Practitioners in the biotech sector in particular can appreciate the need to limit overshoot as microorganisms respond quite unfavorably to significant changes in temperature. Even a modest spike in temperature over a Set Point change can upset a bioreactor process and potentially kill the microorganisms.
For some practitioners a key consideration when tuning is to minimize maintenance costs by minimizing wear and tear on common control assets such as valves and dampers. For this group Output Travel is a metric that merits attention. Output Travel measures the movement of the Controller Output which corresponds directly with the amount of work performed by the loop’s Final Control Element (FCE). More specifically, PID controllers that are tuned too aggressively can accelerate the mean time to failure of the valve. The unplanned downtime that results from failure can be costly. Knowing the amount of Output Travel can help practitioners to tune loops so as to limit wear and tear and avoid unexpected downtime.
Stability – or Robustness – is one more performance measure that many practitioners consider when tuning PIDs. It’s well understood that instability cripples the control of a given loop. What’s more, the performance of one unstable loop can cascade downstream and negatively impact other production processes. In its simplest sense Robustness is a measure of a control loop’s ability to accommodate change without becoming unstable. Change can come in the form of a different feedstock, equipment modifications, or even seasonal variability. A PID loop with a high Stability factor can accommodate those changes and still provide effective control.
There are a variety of things to consider when tuning PID control loops. These different performance attributes should be taken into consideration and they should align with the loop’s unique control objective. These four KPIs and others are included in select tuning software solutions such as Loop-Prouner as they help practitioners to make well-informed decisions.