A Valve positioner is basically a critical link between a plants's controlling element and the control valve of the final control element. This device is basically used to decrease or increase the air pressure that is operating the actuator as per the requirements of the instrument controller it would position the valve stem accordingly. A positioner is used between the valve diaphragm and the controller output, and is a type of "air relay" device. It is very effective in exactly positioning the valve stem in keeping with the requirements of the controller output. There are many reasons to use a positioner including permitting the use of specific cams, increase in fine control of system resolution, permit split ranging, permit for distance between the control valve and controller etc.
Positioner is device witch collect the 4~20mA signal from DCS or controller and simulate the position of control valve accordingly.