Graphical input apparatus for electrical equipment

Claim 1

Apparatus for obtaining signals representative of the co-ordinates of a point indicated in a working area, including a sheet having a first surface and a fixed member having a second surface facing the first surface, one of the surfaces being partially formed by a plurality of parallel conductors spaced apart from one another, the other surface being a substantially homogeneous resistive surface or being formed by parallel conductors at right angles to those in the said one surface, the sheet being held in tension across, and adjacent to, the fixed member with the first and second surface out of contact, and the sheet being of such material and/or so mounted that the first and second surfaces are able to make contact temporarily with one another when the sheet is pressed towards the fixed member by means for indicating the position of a point, the apparatus also including resistive means for each surface which has parallel conductors so connected to the conductors that when a current is passed between two terminals of the resistive means a voltage gradient is set up from conductor to conductor of that surface in a direction at right angles to the conductors, changeover means for causing current supplied to the apparatus to flow alternately in two paths, a first path in which the current sets up a voltage gradient in one direction in the first surface, and a second path in which the current sets up a voltage gradient in a direction at right angles to the said one direction in the second surface, and means for deriving first and second signals when the sheet is pressed at a relatively small area towards the fixed member so that the first and second surfaces make contact, the first signal being dependent on the voltage of the first surface when current flows in the second surface and the second signal being dependent on the voltage of the second surface when the current flows in the first surface, and the first and second signals being representative of first and second co-ordinates of the said small area, respectively.