Abstract: An electro hydraulic servo valve has a pair of oppositely polarized magnets mounted in the surface of the spool of the valve to be controlled. A Hall effect sensor is mounted in the housing adjacent the magnets to precisely measure the position of the spool relative to the housing. The magnets and transducer are mounted on the longitudinal centerline of the spool to eliminate the spurious signals, due to temperature, vibrations, shock, hydraulic supply pressure and back pressure. The strength and spacing of the magnets and air gap are chosen to provide a linear output signal over the range of displacement desired and to provide the accuracy of location necessary for the particular configuration.