Industrial controllers

In industrial processes, controllers are used by comparing a measured value with a set point value and controlling a corrective device based on that value. An example is a controller that receives the actual value from a flow meter and compares it to the set desired flow. The controller then controls a control valve that further opens or closes the passage in the pipe to achieve the desired flow. There are simple on-off controllers, as well as sophisticated PID controllers. PID is short for Propotional, Integral and Differential Control.

Unlike on-off controllers, a proportional controller controls harder when you're further away from the desired value and less hard when you're close to it. So this makes the control a lot less harsh, but a proportional control can still overshoot slightly over the desired value. To counteract that, you can set an integration time on some controllers so that the control is averaged a bit over a longer time. The longer the integration time, the quieter, but also the slower, the control becomes. So it is important to find the right balance in this. The differential function of controllers prevents overshoot in particularly slow processes, such as a temperature control. Based on the rate of change in the measured value, a differential controller actually predicts when the desired value is reached.