Electrographic touch sensor and method of reducing bowed equipotential fields therein

• US 4,661,655 A
• Filed: 12/24/1984
• Issued: 04/28/1987
• Est. Priority Date: 12/24/1984
• Status: Expired due to Term

• Current Assignee: Elographics Incorporated (Elo Touch Solutions, Inc.)
• Original Assignee: Elographics Incorporated (Elo Touch Solutions, Inc.)
• Inventors: Gibson, William A., Talmage, John E. Jr.
• Primary Examiner(s): Schreyer, Stafford D.
• Application Number:
  US06/685,348
• Time in Patent Office: 855 Days
• Field of Search: 178/18, 178/19, 178/20, 382/13, 382/59
• US Class Current: 178/18.05
• CPC Class Codes: G06F 2203/04113 Peripheral electrode pattern in resistive digitisers, i.e. electrodes at the periphery of the resistive sheet are shaped in patterns enhancing linearity of induced field
• CPC Class Codes: G06F 3/045 using resistive elements, e.g. a single continuous surface or two parallel surfaces put in contact

Abstract

A resistor electrode type touch sensor having enhanced area of linear response by reducing the bow in perimeters of the sensor and the method of accomplishing the same. A resistive coating is applied to one surface of a substrate. Within this coating orthogonal electrical fields are produced to give coordinates of a selected position. Overlying, but spaced from, the resistive coating is a flexible pick-off sheet having a conductive layer facing the resistive layer which will contact the resistive coating when the pick-off sheet is touched at a selected position. A resistance element is positioned proximate, but insulated from, the perimeter of the resistive coating. Electrodes of a selected effective length and spacing are located along selected paths proximate the edges of the resistive coating. Each electrode is attached to the resistive coating and connected to selected positions along the resistance element to provide selected voltages to the resistive coating. The effective length and spacing of the electrodes, and the positions of connection to the resistance element, are selected to produce an effective voltage gradient at the electrodes that progressively decreases from corners of the sensor to the center line of each edge of the sensor to counteract any voltage drop along the resistance element when potentials are applied thereto so as to substantially eliminate the bow of electrical fields produced in the resistive coating.

Claims

1. A position touch sensor having resistive electrodes which provides a substantially linear response over the total area of said sensor by substantially reducing the bow of equipotential lines along the edges of said sensor, said equipotential lines produced when electric fields are introduced into said sensor, which comprises:

a substrate having an upper and a lower face, said substrate defining a perimeter of a selected configuration;

a uniform resistive coating applied to said upper face of said substrate, said coating covering a substantial portion of said upper face and defining perimeter edges of said coating;

resistance elements positioned proximate each of said perimeter edges of said coating, each of said resistance elements having opposite ends joined to proximate ends of adjoining resistance elements, said resistance elements being insulated from said coating;

a plurality of electrodes positioned along a preselected symmetrical path along each perimeter edge of said coating and electrically attached to said coating, said electrodes each being electrically connected to selected positions along said resistance elements to introduce orthogonal electric fields into said coating, said electrodes having a selected spacing and an effective length along said path;

said spacing and effective length of said electrodes selected to produce a selected voltage gradient at each of said electrodes to compensate for any cummulative voltage drop along said resistance elements perpendicular to current flow through said coating during operation of said sensor when said orthogonal fields are applied to said coating whereby said bow of said equipotential lines is substantially reduced.

2. The sensor of claim 1 wherein said paths are straight lines.

3. The sensor of claim 1 wherein said paths are symmetrical convex curves.

6. The sensor of claim 1 wherein said selected effective length of said electrodes in each of said paths increases from about 0.06 inch at each end to about 0.5 inches at said centerline, with said electrodes having a dimension perpendicular to said path of about 0.06 inch.

7. The sensor of claim 6 wherein said selected spacing of said electrodes in each of said paths decreases from about 1.3 inches at each end to about 0.8 inch at said centerline.

8. The sensor of claim 1 wherein said electrodes are substantially rectangular, and said paths are substantially straight lines.

10. The sensor of claim 1 further comprising a flexible pick-off sheet uniformly spaced from said resistive coating, and proximate thereto, said sheet having a conductive layer facing said resistive coating, and means for preventing inadvertant contact between said conductive layer and said resistive coating but permitting intentional contact therebetween.

11. The sensor of claim 10 wherein said means for preventing inadvertant contact between said conducting layer of said pick-off sheet and said resistive coating comprises a plurality of small insulating islands attached to said resistive coating.

12. The sensor of claim 10 further comprising a flexible pick-off sheet uniformly spaced from said resistive coating, and proximate thereto, said sheet having a conductive layer facing said resistive coating, and means for preventing inadvertant contact between said conductive layer and said resistive coating but permitting intentional contact therebetween.

13. The sensor of claim 10 wherein said means for preventing inadvertant contact between said conducting layer of said pick-off sheet and said resistive coating comprises a plurality of insulating islands attached to said conducting layer.

14. The sensor of claim 1 further comprising circuit means connected to said juncture of ends of adjoining resistance elements, said circuitry providing potentials across appropriate of said resistance elements to produce said orthogonal equipotential field in said resistive coating.

15. The sensor of claim 10 further comprising circuitry connected to said conductive layer of said pick-off sheet for receiving electrical signals generated in said conductive layer when said conductive layer is touched to a specific point on said resistive coating, said signals corresponding to coordinates of said specific point.

16. The sensor of claim 12 further comprising circuitry connected to said conductive layer of said pick-off sheet for receiving electrical signals generated in said conductive layer when said conductive layer is touched to a specific point on said resistive coating, said signals corresponding to coordinates of said specific point.

4. A position touch sensor utilizing resistive electrodes which provides a substantially linear response over the surface area of said sensor by substantially reducing the bow of equipotential field lines produced proximate said electrodes during application of electric fields to said sensor, which comprises:

a substantially rectangular substrate having a top surface and a bottom surface, said substrate defining a first pair of opposite edges of a first dimension and a second pair of opposite edges of a second dimension at substantially right angles to said first pair of edges;

a uniform resistive coating deposited upon a substantial portion of said top surface of said substrate;

a resistance element positioned proximate each one of said first pair and second pair of opposite edges of said substrate and insulated from said resistive coating, said resistance elements joined at ends thereof to form a substantially rectangular array of resistance elements surrounding said coating;

a plurality of electrodes located along a selected path, electrically attached to said resistive coating proximate each of said first and second pair of opposite edges of said substrate, each electrode having a selected effective length along said selected path and a selected spacing from an adjacent electrode in said path;

a plurality of electrical connectors, each one of said connectors electrically connecting one of said electrodes to a selected position on adjacent of said resistance elements to supply orthogonal fields to said coating through said electrodes during sensor operation;

said effective length and spacing of said electrodes selected to produce a selected voltage gradient at each of said electrodes to compensate for any voltage drop along said resistive elements perpendicular to current flow through said coating during operation of said sensor, said selected effective length of said electrodes in said path increasing, and said selected spacing between adjacent electrodes decreasing, from each end of said path toward a centerline of said path to thereby progressively decrease said voltage gradients produced by said electrodes in said coating toward said centerline to compensate for progressively increasing cumulative voltage drop in said resistance elements, whereby said bow of said equipotential lines is reduced.

5. The senosr of claim 4 wherein said first dimension is larger than said second dimension, and wherein each of said first pair of said electrode element groups contains a greater number of electrodes than each of said second pairs of said electrode element groups.

9. The sensor of claim 4 wherein said electrodes are substantially rectangular, and said paths are substantially straight lines.

17. The sensor of claim 4 further comprising circuit means connected to said juncture of ends of adjoining resistance elements, said circuitry providing potentials across appropriate of said resistance elements to produce said orthogonal equipotential fields in said resistive coating.

18. A single sheet resistor type touch sensor which exhibits substantially reduced bow of electric equipotential lines along edges thereof, said equipotential lines produced when electric fields are introduced into said sensor, which comprises:

a substantially rectangular substrate having an upper and a lower face, said substrate defining perimeter edges joined at corners;

a uniform resistive coating applied to a substantial portion of said upper face of said substrate, said resistive coating having a resistivity of between about 10 and about 10,000 ohms per square;

a resistance element proximate each of said perimeter edges of said substrate, said resistance elements insulated from said resistive coating and having a resistance of about 6 to 6,000 ohms per foot, each resistance element joined to ends of adjoining resistance elements proximate said corners of said substrate;

circuitry attached to the junctions of said resistance elements for providing potentials across said resistance elements to produce orthogonal electric fields in said resistive coating;

a plurality of conductive electrodes, aligned in a row proximate said perimeter edges, applied to said resistive coating and electrically joined to said resistive coating, said electrodes having a spacing and an effective length along said row whereby each said electrode produces a voltage gradient in said resistive coating which progressively decrease in value from proximate said corners to centerlines of said row to counteract an increasing voltage drop along said resistance elements due to current flow to said electrodes resulting from said applied potentials from said circuitry and thereby substantially reduce any bow in electrical equipotential lines produced in said resistive coating by said applied potentials to said resistance elements.

19. The sensor of claim 18 further comprising a flexible pick-off sheet spaced uniformly from and proximate said resistive coating, said pick-off sheet having a conductive layer of less than about 1000 ohms per square facing said resistance coating and a plurality of small insulating dots interposed between said resistive coating and said conductive layer to prevent inadvertant contact therebetween but permit contact when pressure is intentionally applied to said pick-off sheet.

20. A method of substantially eliminating any bow in the equipotential lines at the perimeter of a touch sensor of the type having a uniform resistive coating applied to a substantial portion of a substrate and having a resistance element positioned adjacent edges of said resistive coating whereby electric fields can be produced in said resistive coating by connecting selected positions along said resistance element to electrodes on said resistive coating and applying potentials to said resistance element, which comprises:

positioning said electrodes on said resistive coating along selected paths adjacent said edges of said resistive coating, said electrodes electrically connected to said coating;

selecting spacing between adjacent electrodes and effective lengths of said electrodes along said paths such that voltage gradients are produced within said resistive coating at each of said electrodes, which voltage gradients compensate for voltage drops along said resistance element when said potentials are applied to said resistance element, said voltage gradients progressively decreasing in value from each end of said path toward a center point of said path and thereby eliminate said bow in said equipotential lines at said perimeter of said sensor.