US Patent 11002874 Foreign object detection circuit using mutual impedance sensing

The present disclosure describes techniques for detecting foreign objects. In some aspects, an apparatus for detecting objects is provided. In particular, the apparatus includes a circuit for measuring changes of a mutual impedance between sense coils of a plurality of sense circuits. The described implementations relate to configurations for driving and measurement circuitry for one or more sense circuits that allow for higher accuracy, sensitivity, and temperature stability as needed for detecting smaller objects. Implementations based on mutual impedance sensing allow for improved detection sensitivity e.g., in a presence of strong temperature variations such as may be experienced in an outdoor environment. The apparatus includes a driver circuit, a measurement circuit, and a plurality of sense circuits, each of the plurality of sense circuits including a primary sense coil, a secondary sense coil, and a capacitor connected to each of the primary sense coil and the secondary sense coil.