[0013] The sensor elements are preferably Hall-sensors. Hall-sensors have the advantage that changes in the magnetic field of the permanent magnetcarried by a piston can be directly converted into voltage when a currentflows though the Hall-sensor. The voltage decreases both perpendicular tothe current direction as well as the magnetic field direction relative tothe Hall-sensor. Hall-sensors have the advantage that the produced voltages are linearly dependent on the magnetic field. Different magnetic fields can be reached without a saturation barrier. By using two Hall-sensors, two voltage values are generated which form an unambiguous group of signal values that are readily processed in the evaluation unit.