Patent ID: 7560920
Filing Date: 2009-07-14
Classification: G01N,G01R

Abstract:
1. An apparatus comprising: a sensor unit having a plurality of electromagnetic sensing coils; an excitation/sensing circuit that generates an AC excitation signal, wherein the excitation/sensing circuit includes a differential amplifier; a plurality of reference coils including a first reference coil and a second reference coil, wherein the first reference coil is configured for a first type of metal and the second reference coil is configured for a second type of metal different from the first type; a first circuit that includes a multiplexer that at some first period of time electrically connects a first sensing coil selected from the plurality of sensing coils to a first input of the differential amplifier and through a predetermined impedance to the AC excitation signal; and a second circuit that at the first period of time selectively connects one of the plurality of reference coils to a second input of the differential amplifier, wherein the excitation/sensing circuit is configured to demodulate a first signal from the first sensing coil and the first reference coil in order to detect a phase change, an amplitude change, or both phase and amplitude changes due to a magnetic anomaly of an object being scanned.