PLCs often also include logic for single-variable generic industrial feedback loop, a "proportional, integral, derivative" loop, or "[[PID loopcontroller]]." A PID loop is the standard solution to many industrial control processes that require proportional control. The basic idea is that the controller reads a sensor. Then it subtracts the read value from a "setpoint" to determine an "error".
A PID loop is the standard solution to many industrial control processes that require proportional control.
The error is then treated in three different ways:
* To handle the present, the error is multiplied by a proportion, the "P" of the pid loop.
* To handle the past, the error is averaged over a period of time, where that time is the "I" of the PID loop.
* To handle the future, the rate of change of the variable is multipled by another constant.
All these are added to the last output of the PID loop.
To tune a PID loop, first set the I and D values to zero. Increase the P until the output of the loop oscillates. Then increase I until oscillation stops. FInally, adjust D until the loop is acceptably quick to reach its setpoint.
