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A human-machine interface apparatus includes a first, or phase, surface with associated circuitry so devised that an alternating electrical field is created above the surface with its phase, relative to a fixed point on the surface, changing continuously along an axis of the surface; a second, or pi

A human-machine interface apparatus includes a first, or phase, surface with associated circuitry so devised that an alternating electrical field is created above the surface with its phase, relative to a fixed point on the surface, changing continuously along an axis of the surface; a second, or pickup, surface located so that when the operator of the interface apparatus touches the phase surface an electrical signal with phase corresponding to the spot touched is transmitted through the operator's body to the pickup surface; and a phase discriminator with one input connected to the pickup surface and a reference input connected to an alternating voltage source of predetermined phase relationship to the alternating electrical field at a fixed point on the phase surface. The preferred embodiment combines two such apparatuses, sharing common phase and pickup surfaces but operating at different frequencies, to provide outputs corresponding to two orthogonal axes on the phase surface. The phase surface in the preferred embodiment is a square resistive layer on a nonconductive substrate, and the change in phase with position is produced by applying a signal of one frequency to one edge of the square and a signal of the same frequency but different phase to the opposite edge, the remaining two edges similarly being supplied with signals of a second frequency. A level detector provides a third axis output indicating the presence of the operator's finger on the plane. An alternate embodiment uses a sandwich construction of the phase surface to provide the phase shift, with an insulating layer separating a resistive surface layer from a conductive ground plane to provide distributed capacitance and resistance that progressively shift the phase of an applied signal as it travels along the surface. Another embodiment uses multiplexing to measure position in two axes with a single applied frequency and a single phase discriminator.

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