Patent ID: 8243033

Claim:
A position detector comprising: first and second relatively movable members, the first member comprising an electromagnetic resonator which, when energized, is operable to generate a resonator field which varies with time at a resonant frequency of the resonator; the second member comprising a sensor winding operable to sense the resonator field and operable to output a sensed signal in response, wherein the sensed signal varies with time at said resonator frequency and varies with the relative position of said first and second members; excitation circuitry operable to generate a time varying excitation electromagnetic field comprising a sequence of excitation pulses, for energizing said resonator; processing circuitry operable to process said sensed signal to determine the relative position of said first and second members and to estimate the resonant frequency of said resonator; and control circuitry operable to control said excitation circuitry and said processing circuitry to operate: (i) in a first stage in which said excitation circuitry is operable to generate an excitation field comprising a first number of excitation pulses at a predetermined excitation frequency and in which said processing circuitry is operable to process the resulting sensed signal to estimate the resonant frequency of said resonator; and (ii) in a second stage in which said excitation circuitry is operable to generate an excitation field comprising a second, greater number of excitation pulses substantially at the resonant frequency estimated in said first stage, and in which said processing circuitry is operable to process the resulting sensed signal to determine the relative position of said first and second members and/or to determine a more accurate estimate of the resonant frequency of said resonator.