while "defrost" quoted wikipedia, a PID controller is used to prevent oscillations in equipment that have changing load. Say a milling machine that slows a bit

while "defrost" quoted wikipedia, a PID controller is used to prevent oscillations in equipment that have changing load. Say a milling machine that slows a bit when the tool interfaces the workpiece, so a "dumb" controller would just increase spindle RPM, but now the spindle is overspeed when the tool is lifted from the surface - say to move to a different area - and so the dumb controller slow the spindle down. Either the spindle is moving too slow or too fast per the work's spec sheet, and you get a piece of metal launched into the innards of the machine somewhere. The proportional part is similar to a "dumb" controller - spindle slow, speed up spindle. The Integral part will increase the rate at which the spindle changes speed the longer the spindle remains the wrong speed, and the derivative part of the controller attempts to prevent oscillations of spindle speed.