We learned in control theory courses that too high a PID gain causes oscillations and can lead to instability. Operators do not like the large sudden changes in PID output from a high PID gain. Operators may see what they think is the wrong valve open in split range control as the setpoint is approached when PID gain dominates the response. Most tuning tests and studies use a setpoint response rather than a load response for judging tuning. A high PID gain that is set to give maximum disturbance rejection in load response will show overshoot and some oscillation for a setpoint response. Internal Model Control or any control algorithm or tuning method that considers disturbances on the process output will see a concern similar to what is observed for the setpoint response because the load and setpoint change are immediately appearing at the PID algorithm as inputs. There are many reasons why PID gain is unfavorably viewed. Here I try to show you that PID gain is undervalued and underutilized.
First, let’s realize that the immediate feedback correction based on the change in process variable being controlled may be beneficial in some important cases. The immediate action reduces the deadtime and oscillations from deadband, resolution and sensitivity limits in the measurement, control valve and variable frequency drive. The preliminary draft of ISA-PCS-2017-Presentation-Solutions-to-Stop-Most-Oscillations.pdf for the ISA 2017 Process Control and Safety Symposium show how important PID gain is for stopping oscillations from non-ideal measurements and valves and also from integrating processes.
PID gain does not play as important a role in balanced self-regulating process often shown in control theory courses and publications. The primary process time constant is not very large compared to the dead time. Consequently, there is more negative feedback action seen in these self-regulating processes. When the time constant becomes more than 4 times the dead time, we consider these processes to be near-integrating in that in the time frame of the PID they appear to ramp losing self-regulation. For these processes and true integrating processes, PID gain provides the negative feedback action missing in the process to halt or arrest the ramp. Integrating process tuning rules are used where lambda is an arrest time. Not readily understood is that there is a window of allowable gains, where too low of a PID gain cause larger and slower oscillations than too high of a PID gain. The problem is even more serious and potentially dangerous for runaway processes (highly exothermic reactors). Most loops on integrating and runaway processes have a reset time that is orders of magnitude too small and a PID gain that is an order of magnitude too low. A PID gain greater than 50 may be needed for a highly back mixed polymerization reactor. Many users are uncomfortable with such high gain settings.
For integrating and runaway processes, the PID output must exceed the load disturbance to return the process variable to setpoint. This is more immediately and effectively done by PID gain action. We can often see an immediate improvement in control by greatly increasing the reset time and then the gain. The gain must be greater than twice the inverse of the product of the open loop integrating process gain (%/sec/%) and reset time (sec) to prevent the start of slow oscillations from violation of the low gain limit.
As the process variable approaches setpoint, there is an immediate reduction in the PID contribution to the output from the PID gain. Reset has no sense of direction and will continue to change the output in the same direction not reversing direction till the process variable crosses setpoint reversing the sign of the error. Operators looking at digital displays waiting for a temperature to rise to setpoint will think a heating valve should still be open if the temperature is just below setpoint when in fact the cooling valve should be open to prevent overshoot. If the loop waits till the PV crosses setpoint, the correction is too late due to deadtime. PID gain provides the anticipatory action missing in reset action.
A setpoint filter equal to the reset time or a PID structure of proportional action on process variable instead of error will eliminate overshoot of the setpoint for PID tuning that maximizes disturbance rejection where the peak error and the integrated error are both inversely proportional to PID gain.
Finally, let’s realize that the use of external-reset feedback allows us to use setpoint rate of change limits on the valve signal or secondary loop signal that will prevent the large abrupt jump in PID output form a high PID gain that upsets operators and some related loops. External-reset feedback will also prevent the PID output from changing faster than a valve or secondary loop can respond. No retuning needed.
These days we personally don’t have time to wait for taking corrective action in our lives and need to seek more anticipation based on where we are going. We also benefit from external-reset feedback. We need to realize the same for PID loops. There is a lot to be gained from PID gain.