US Patent 9857928 Force sensitive device

A display with a force sensing function has two layers of a reference voltage, for example a ground voltage. One reference voltage layer is inside the display stack. A sensor layer between the two reference voltage layers senses capacitance from the distance between the sensor layer and the reference voltage layer. The capacitance changes according to the force applied to the display, as the display moves slightly when the user presses the display. A thin foam layer allows the distance between the sensor layer and the reference voltage to change. This change is reflected in the measured capacitance. Multiple force sensors also enable detection of the position of the touch.