Sensaphone SCADA 3000 Users manual - Manual (Page 212)

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SCADA 3000 User’s Manual
If the lag time is brief, however, and the process reacts quickly to the controller’s efforts, then
the integral term plays less of a role in the controller’s output, as the errors will be too short-
lived for it to work. Instead, the derivative term will be larger due to the rapid changes.
Each term will share some importance in any controller’s output; how much is determined by
the behavior of the process being controlled. Determining the best mix for your application is
the essence of tuning. For a slow process, the Kd constant might need to be large in order to
accelerate the controller’s reaction to a setpoint change. But that same Kd setting for a fast-act-
ing process might cause wild fluctuation and potential damage.
Kp, Ki, and Kd tuning is not an exact science. Experienced control engineers develop an intui-
tive sense of how to mix these factors. They’ve most likely achieved this through repeated trial
and error. You can do the same. Tweak your parameters and then watch how the controller
handles the next error. If it eliminates the error expeditiously, then leave your settings alone. If
it’s either too conservative or too aggressive, then adjust one or more of the tuning constants to
compensate, and watch how it handles the next error, and so forth. This is not easy, as changing
one tuning constant will likely affect one or both of the others as the rate and degree of error
and compensation change.