Compensation for nonlinear variation of gap capacitance with displacement: United States Patent 9990087

A capacitive force sensor characterization system for calibrating a capacitive force sensor included in a personal electronic device. The capacitive force sensor includes a first capacitor plate coupled to a flexible element of the personal electronic device, which is coupled to the device housing, and a second capacitor plate coupled to an internal structural member of the personal electronic device. The internal structural member is not coupled to the housing during the characterization. The capacitive force sensor characterization system includes: a housing fixture adapted to hold the housing of the personal electronic device; a member fixture adapted to hold the internal structural member of the personal electronic device; a positioner coupled to at least one of the housing fixture or the member fixture; a position sensor; control circuitry electrically coupled to the positioner to provide a drive signal to the positioner; and a processor coupled to the first and second capacitor plates and the position sensor. The positioner is adapted to vary the gap width between the first and second capacitor plates in response to the drive signal. The position sensor is adapted to measure differences in the gap width between the capacitor plates relative to an initial gap width. The control circuitry is adapted to generate the drive signal following a test procedure such that, in response to the drive signal, the positioner varies the gap width between the capacitor plates from the initial gap width to at least two test gap widths. The processor adapted to determine: a capacitive sensor gain; a capacitive sensor offset; and an initial effective separation between the capacitor plates.