Eddy current microsensor for use in micromachine applications

In this paper we investigate an eddy current microsensor which uses a low input impedance trans-impedance amplifier to minimise the influence of stray capacitances on the sensor operation when being used for micromachine applications. A technique using a two transistor feedback pair for realising very low input impedance over a wide bandwidth in these amplifiers is studied. An improved circuit configuration, which further reduces the amplifier input impedance at higher frequencies, is investigated by computer simulation. At the same time as minimising the influence of the stray capacitances, the trans-impedance amplifier improves the signal conditioner sensitivity and output drift performance. The operation of the sensor signal conditioner has been investigated experimentally using target materials suitable for micromachine applications. Results of these experiments are presented which show that acceptable sensitivities are achieved.