US Patent 7514919 Sensing apparatus and method

A sensor for sensing a parameter such as position, comprises: (i) an excitation winding for example coils (7, 9) in quadrature; (ii) a signal generator (41, 42, 43) operable to generate an excitation signals and arranged to apply the generated excitation signal to the excitation winding; (iii) a sense coil (11) that can be electromagnetically coupled to the excitation winding such that, in response to the excitation signal being applied to the excitation winding by the signal generator, a periodic electric signal is generated in the sense coil that is indicative of the value of the parameter to be measured by the sensor; and (iv) a signal processor (108) operable to process the periodic electric signal generated in the sensor winding to determine a value representative of the parameter being measured. The signal processor may be arranged to generate a second signal (f1) at a frequency slightly different from that of the excitation signal, and to mix the second signal with the signal received from the sense coil to generate a third signal having a frequency component (f0−f1) equal to the difference between the frequency of the excitation signal and that of the second signal. The processing unit then determines the parameter from the phase of the third signal.