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Tuning a PID controller involves adjusting the proportional band, integral action, and derivative action. Before tuning, the control loop and system must be examined. Different methods are used for tuning, and increasing gain time makes the controller more aggressive. DCS software can automatically adjust control loops, but fine tuning is necessary.
Developing a proportional-integral-derivative controller (PID controller) is a common task for process control engineers. In this case, “tuning” means the modification of the parameters relating to the proportional band of the regulator, integral action and derivative action. There are several methods of manually calculating tuning parameters and several software packages that can be used to automatically tune the controllers in a chemical process. Before starting any tuning, it is imperative that the technician examine the control loop to be tuned and the impact of the control loop on the entire system.
The performance of an automatic controller can be tuned and changed by changing the controller’s tuning parameters. When tuning a PID controller, there are typically three settings that can be changed: the proportional band, the integral action, and the derivative action. These are represented by the first, second and third terms in the classical PID algorithm, respectively u = KP e + KI ∫ and dt + KD de/dt.
The term u represents the return signal; KP is the proportional gain; e is the error term or offset, which represents the difference between the actual value and the controller setpoint; KI is the integral gain, KD is the derivative gain; and is the time. The Laplace transform of this equation can be expressed as KP + KI/s + KDs.
Before tuning a PID controller, a technician must first examine the process to be tuned to determine if an incorrect tune is causing problems or if there is another assignable cause, such as a malfunction or equipment failure. Tuning changes will mean very little if the real cause of the variability turns out to be a stuck control valve, broken tools, or errors in the control system logic. Only when the process has been thoroughly investigated and the functionality of the field instruments has been verified should tuning be considered.
There are several methods used by chemical, electrical, and instrumentation engineers to tune a PID controller. The Ziegler-Nichols method is one such example that uses the final gain and final period of the process to calculate aggressive tuning parameters for P-only, PI-only, and PID control schemes. Other control schemes, such as the Tyreus-Luyben method, are designed to reduce system oscillation. The method used to tune a PID controller can be dictated by the nature of the control loop itself.
In general, increasing the gain time of a controller will cause the controller to act more aggressively. A more integral action will help reduce the offset between the steady state value and the desired setpoint, but can lead to oscillation if too much is used. The term derivative is used to help stop the rapid motion of the controller PV. These are just heuristics that provide a general sense of the effect of each of the classic tuning parameters.
Many Distributed Control System (DCS) packages include software that can be used to automatically adjust control loops. These software packages will often optimize processes by examining past performance or automatically run test methods described by established optimization procedures. As with most procedures, the engineer must make fine tuning and minor adjustments to suit the process after the major tuning procedure has been completed.
