Patent ID: 7372255

Claim:
A piston position sensing circuit for sensing instantaneous piston position of a piston of a free piston Stirling machine or a compressor mechanically connected to a reciprocating member of a linear electric motor or alternator, the linear electric motor or alternator having a winding electrically connected in a winding circuit to a source of electrical driving power or to an electrical load, the reciprocating member being reciprocatable at an operating frequency of reciprocation, the sensing circuit providing an output signal that is substantially a linear function of the piston position and comprising: (a) a source of a high frequency, alternating, electrical signal connected to apply its high frequency signal to the winding; (b) a filter circuit having an input connected to the winding circuit, the filter passing electrical signals at the frequency of the high frequency source and blocking signals at the operating frequency of reciprocation; and (c) a peak detector circuit connected to an output of the filter circuit and detecting the peak of the filter circuit output signal whereby the detected instantaneous peak magnitude is proportional to the instantaneous piston position.