Control Systems
PID–The Basic Technique for Feedback Control
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.
Multivariable Controllers Balance Performance with Cost
Controllers that juggle multiple process variables are neither simple nor common, but they can handle some of the most complex control problems.
The Smith Predictor: A Process Engineer’s Crystal Ball
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.)