The control piston in a valve controls the oil flow to the hydraulic cylinder. As the piston moves back and forth, drilled holes in the valve body are covered and uncovered. The piston is moved by a linear motor. For this measurement, the sensor induNCDT series LVDT from Micro-Epsilon is used. The sensor is mounted on a pressure pipe located on the valve. In this pressure line a freely moving plunger is guided, which is permanently fixed to the spool. The sensor acquires the plunger through the metal of the pressure line and in this way determines the exact position of the spool. The plunger forms a passive position transmitter having a number of advantages. The measurement occurs without contact and is not subject to pressure, because the sensor is mounted outside of the pressurised region.