Positioning the control valve is a primary accessories It will stem displacement signal as a feedback measurement signal input to the controller output signal as a setting signal, to compare, when a deviation between the two, changes its output signal to the actuator , the executing agency action, the establishment of a correspondence between the stem displacement times and the controller output signal. Therefore, the positioning displacement is composed of a measurement signal to the valve stem, the controller output is set to a signal feedback control system. Manipulated variable of the control system is to perform agency locator output signal.