fluid flow through the valve 44 or a position sensor employed to sense the position of the rod 8 a microprocessor 30 is included within the controller 24 and is adapted to receive a signal indicative of the degree of opening of the valve 44 which in the present example is a pressure signal from the pressure meter 26 the microprocessor 30 is further adapted to provide a control signal to the current regulator 28 which acts to modify a peak amplitude a w so called u201c working amplitude u201d of the superimposed current the microprocessor 30 is programmed to modify this working amplitude dependent on a calculated magnitude of a temporal deviation of a monitored parameter in this example pressure indicative of the degree of opening of the valve the microprocessor 30 operates as is described in detail below to set the working amplitude to the amplitude at which the monitored variations are minimized according to the present example the microprocessor 30 operates during a measurement period to provide a control signal to the current regulator 28 to change the peak amplitude of the superimposed oscillatory current from the present working amplitude a wold to a new peak amplitude a 1 during this measurement period a number m determinations of a variance var j is made by the microprocessor 30 each variance var j is determined for a number n which may be different for each m samples of the pressure signal p i according to var ue8a0 j 1 n 1 ue89e u2211 i 1 n ue89e p ue89e ue89e i p dt ue8a0 i 2 j 1 m 1 the signal p dt.