Offset error in proportional controllers is a problem when trying to maintain a process variable at an exact value. As we saw in the proportional control module, one way to eliminate offset error is to manually reset the controller. The problem with manual reset is just that — it’s manual. If we could automatically perform a reset every time a process supply or demand change occurred, the issue of offset error would be eliminated. We can do just that by adding integral to a controller. Integral, sometimes called reset, is used in conjunction with proportional control to bring the process back to set point without having an offset error. While integral eliminates the offset error, it brings its own issues, which are dealt with by adding derivative to the mix. What results is a control mode known as proportional, integral, and derivative control, commonly known as PID control.
