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 of electrically conductive or resistive material held in position across the surface of a fixed member, the said surface comprising a plurality of parallel conductors spaced apart from each other and supported on a resistive layer by means of insulating supports, resistive means for so connecting with the parallel conductors that when a current is passed between two terminals of the resistive means a voltage gradient is set up from conductor to conductor in the direction at right angles to the conductors, the sheet being held in position adjacent to the said surface, and the sheet being of such material and/or so mounted that when pressed towards the surface by means indicating the position of a point it bridges any of the parallel conductors in the locality of the point to the resistive layer, changeover means for causing current supplied to the apparatus to flow alternately in two paths, a first path in which the current flowing in the resistive means sets up a voltage gradient across the parallel conductors and at right angles thereto, and a second path in which the current flowing in the resistive layer sets up a voltage gradient in a direction parallel to the conductors, and means for deriving first and second signals when the sheet is pressed at a relatively small area to bridge at least one of the parallel conductors to the resistive layer, the first signal being dependent on the voltage of the parallel conductors when current flows in the resistive layer, and the second signal being dependent on the voltage of the resistive layer when current flows in the resistive means, and the first and second signals being representative of first and second co-ordinates of the said small area, respectively.