The Six Most Common PID Configuration Errors and how to find and fix them
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The PID Control Loop is one of the fundamental workhorses of the process industries. Day in and day out, millions of PID loops strive to keep processes safe, stable, and profitable. But there is a secret, known only to control engineers and technicians.
Many of these control loops are not properly configured, and the consequences could be devastating. A newly published white paper from ExperTune identifies the most common PID configuration issues, and provides techniques to help plant-based personnel find and fix these issues.
Surprisingly, as many as 75% of control loops actually increase variability. Many control loops simply do not do their job. Setpoints are not followed, valves swing around, creating oscillations, and many loops are disabled by the operator: placed in MANUAL mode.
Studies of control loops in the process industries give some insight into the root cause of these issues. For example:
- 30% of DCS Control Loops Improperly Configured
- 85% of Control Loops Have Sub-Optimal Tuning
- 15% of Control Valves are Improperly Sized
Derivative action can give you the fastest response, but only if you use it responsibly. Too much derivative will make the loop unstable. Too little and it does little for your response and makes the valve jittery, wearing it out.
The reason to use derivative (or D) is that you can use more Proportional and especially more Integral action. The result can be a much faster reacting control loop. The effect of properly applied D (or derivative) action is most pronounced on second order processes like temperature loops. But, D action can help the response of most loops.
Choosing the best filter
The best PV filter type
In general, the best type of filter is a second order Butterworth, but a simple first order filter can do much to reduce controller output jitter and save the life of your valve or variable frequency drive. Since most systems have first order filters built in, it is usually a good choice.
The best PV filter size
In general, the optimal size PV filter is the largest one that does not degrade the performance of the loop. There are always tradeoffs in process control and choosing the filter type and size are one of them.
How Fast Should I Sample
The short answer to the question of sample interval is to sample between 4 and 10 times faster than the process dead time. 4 times faster being barely adequate and 10 times faster being the best.
Ideal Sample Interval = (Process Dead Time)/10
It is better to err on the side of being too fast. This applies both for what your industrial digital controller should sample at and how fast to collect data for analysis, modeling and PID tuning.