Tuning a PID controller is conceptually simple–observe the behavior of the controlled process and fine tune the controller’s proportional (P), integral (I), and derivative (D) parameters until the closed-loop system performs as desired. However, PID tuning is often more of an art than a science. The best choice of tuning parameters depends upon a variety of factors including the dynamic behavior of the controlled process, the controller’s objectives, and the operator’s understanding of the tuning procedures. Self-tuning PID controllers simplify matters by executing the necessary tuning procedures automatically.
Conveying understandable process information to an operator requires more than dazzling HMI graphics in real time.
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A feedback controller is designed to generate an output that causes some corrective effort to be applied to a process so as to drive a measurable process variable towards a desired value known as the setpoint. Shown is a typical feedback control loop with blocks representing the dynamic elements of the system and arrows representing the flow of information, generally in the form of electrical signals. Virtually all feedback controllers determine their output by observing the error between the setpoint and the actual process variable measurement. PID control A proportional-integral-derivative or ‘PID’ controller looks at the current value of the error, the integral of the error over a recent time interval, and the current derivative of the error signal to determine not only how much of a correction to apply, but for how long.
Controllers that juggle multiple process variables are neither simple nor common, but they can handle some of the most complex control problems.
Control Engineering classic: In this article, the consequences of performing feedback control with sampled rather than continuous data are examined.
Arguably the trickiest problem to overcome with a feedback controller is process deadtime -- the delay between the application of a control effort and its first effect on the process variable. During that interval, the process does not respond to the controller's activity at all, and any attempt to manipulate the process variable before the deadtime has elapsed inevitably fails. This classic article is among the most-read on the Control Engineering site. (See diagrams.)
KEY WORDSMotors, drives, & motion controlAC variable-speed drivesAC induction motorsAdaptive controlControversial right from their name, "sensorless" ac drives actually use current and voltage sensors to achieve motor control. Even beyond terminology, it's not obvious what sensorless ac drives can or can't do, and where they fit into the big picture.