A device used to regulate a process variable in a controlled process. The purpose is to receive an input signal of the difference between a measured value and the desired value, and provide an output signal that is then used to control the process. The controller has a reference input or set point. This is the desired value for the process measurement signal and is transmitted to the controller. The controller measures the difference or error between the set point and the measurement signal. The error is manipulated by the controller to provide the controller output, which corrects a valve position to reduce the error towards zero. In a block diagram, the controller is represented by the summing junctions and the control modes block. The main forms of control are proportional, integral, and derivative control. The simplest type is on/off control. To reduce the frequency of switching on and off, which can lead to excessive wear and tear, a neutral zone is used. For example, if temperature is being measured and is rising within the neutral zone, the controller stays on, whereas if the temperature is falling within the neutral zone then the controller stays off. See pid control.