A controller in control theory is a device that attempts to control states or outputs of a dynamic system. Generally, it accomplishes this using feedback to reject disturbances to the system: known as closed-loop control. Open-loop control also occurs: in systems sufficiently well-characterized to predict what inputs will necessarily achieve the desired states or outputs. For example, the velocity of a motor may be well-characterized for the voltage fed into it, in which case feedback may prove unnecessary. PID controller is commolny used controller type.