This was invented by Christiaan Huygens in the 17th century to regulate the gap between millstones in windmills depending on the speed of rotation, and thereby compensate for the variable speed of grain feed. Origins Ĭontinuous control, before PID controllers were fully understood and implemented, has one of its origins in the centrifugal governor, which uses rotating weights to control a process. The magnitudes of the three terms (P, I and D) are adjusted by the dials at the top. But the PID controller is broadly applicable since it relies only on the response of the measured process variable, not on knowledge or a model of the underlying process.
The response of the controller can be described in terms of its responsiveness to an error, the degree to which the system overshoots a setpoint, and the degree of any system oscillation. In these cases lead–lag compensation is required to be effective. Situations may occur where there are excessive delays: the measurement of the process value is delayed, or the control action does not apply quickly enough. The use of the PID algorithm does not guarantee optimal control of the system or its control stability (see § Limitations, below). PI controllers are fairly common in applications where derivative action would be sensitive to measurement noise, but the integral term is often needed for the system to reach its target value. This is achieved by setting the unused parameters to zero and is called a PI, PD, P or I controller in the absence of the other control actions.
A PID controller continuously calculates an error value e ( t ) Selective use of control terms Īlthough a PID controller has three control terms, some applications need only one or two terms to provide appropriate control. A proportional–integral–derivative controller ( PID controller or three-term controller) is a control loop mechanism employing feedback that is widely used in industrial control systems and a variety of other applications requiring continuously modulated control.
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