Anisotropic touch screen element
A touch sensitive position sensor for detecting the position of an object in two dimensions is described. The position sensor has first and second resistive bus-bars spaced apart with an anisotropic conductive area between them. Electric currents induced in the anisotropic conductive area by touch or proximity flow preferentially towards the bus-bars to be sensed by detection circuitry. Because induced currents, for example those induced by drive circuitry, flow preferentially along one direction, pin-cushion distortions in position estimates are largely constrained to this single direction. Such one-dimensional distortions can be corrected for very simply by applying scalar correction factors, thereby avoiding the need for complicated vector correction.
1. A touch sensitive position sensor comprising:
a substrate defining a touch sensitive platform:
first and second resistive bus-bars arranged spaced apart on the substrate; and
an anisotropic conductive area having non-zero resistance arranged between the bus-bars such that currents induced in the anisotropic conductive area flow preferentially towards the bus-bars,
a first electrode;
a second electrode, wherein the first resistive bus-bar extends between the first and the second electrode;
a third electrode;
a fourth electrode, wherein the second resistive bus-bar extends between the third and the fourth electrode;
first, second, third and fourth drive channels associated with respective ones of the first, second, third and fourth electrodes, each drive channel being operable to generate an output signal dependent on the resistance between its electrode and the position of the object;
a fifth electrode coupled to the first resistive bus-bar at a location between the first and second electrodes;
a sixth electrode coupled to the second resistive bus-bar at a location between the third and fourth electrodes;
fifth and sixth drive channels respectively associated with the fifth and sixth electrodes, wherein the fifth and six drive channels are operable to generate output signals dependent on the resistance between their respective electrode and the position of the object; and
a processor operable to generate an estimate for the position of the object by comparing the output signals from the fifth and sixth drive channels and the output signals from a selected pair of the output signals from either the first and second drive channels or the third and fourth drive channels.
2. A touch sensitive position sensor according to claim 1 , wherein the processor is configured to estimate the position of the object in a first direction running between the bus-bars from a ratiometric analysis of the sum of the signals associated with the fifth and sixth electrodes and the sum of the signals associated with the selected pair of the other output signals.
3. A touch sensitive position sensor according to claim 1 , wherein the processor is configured to estimate the position of the object in a second direction running along the bus-bars from a ratiometric analysis of the sum of the signals associated with the fifth and the one of the selected pair of the other output signals associated with the first resistive bus bar, and the sum of the signals associated with the sixth and the one of the selected pair of the other output signals associated with the second resistive bus bar.
4. A touch sensitive position sensor comprising:
a substrate defining a touch sensitive platform:
first and second resistive bus-bars arranged spaced apart on the substrate; and
an anisotropic conductive area having non-zero resistance arranged between the bus-bars such that currents induced in the anisotropic conductive area flow preferentially towards the bus-bars guard electrode adjacent to the bus bars and/or the anisotropic conducting area, wherein the guard electrode at least partially surrounds the bus bars and the anisotropic conducting area;
a first electrode;
a second electrode, wherein the first resistive bus-bar extends between the first and the second electrode;
a third electrode;
a fourth electrode, wherein the second resistive bus-bar extends between the third and the fourth electrode;
a guard drive channel associated with the guard electrode, the guard drive channel being operable to generate a guard electrode output signal indicative of a coupling between an object and the guard electrode; and
a processor operable to compare the output signal from the guard electrode with the output signals from at least one of the other electrodes to compensate for changes in the output signal from the at least one of the other electrodes which are not associated with an object whose position is to be determined.
5. A touch sensitive position sensor according to claim 4 , wherein the processor is operable to disregard output signals from the electrodes coupled to the resistive bus bars according to the output signal from the guard electrode.
6. A touch sensitive position sensor comprising:
a substrate defining a touch sensitive platform:
a first resistive bus-bar on the substrate;
a second resistive bus-bar spaced apart from the first resistive bus-bar on the substrate;
an anisotropic conductive area having non-zero resistance arranged between the first and second bus-bars such that currents induced in the anisotropic conductive area flow preferentially towards the first and second bus-bars, wherein the anisotropic conductive area includes a plurality of resistive strips that extend between the first resistive bus-bar and the second resistive bus-bar wherein the resistance of the resistive strips is at least five times greater than the resistance of the first and second resistive bus-bars;
first, second, third and fourth drive channels associated with respective ones of a first and second electrode at ends of the first resistive bus-bar and a third and fourth electrode at ends of the second resistive bus-bar, each drive channel being operable to generate an output signal dependent on the resistance between its electrode and the position of an object touching the sensor; and
a processor operable to compare the output signal from each electrode with the output signals from the other electrodes and compensate for changes in the output signal from the other electrodes which are not associated with an object whose position is to be determined.
7. A touch sensitive position sensor according to claim 6 , wherein the first and second resistive bus-bars have a resistance between 1000 ohms and 50,000 ohms.
8. A touch sensitive position sensor according to claim 6 , wherein there are at least nine resistive strips that extend between the first resistive bus-bar and the second resistive bus-bar.
9. A touch sensitive position sensor comprising:
a substrate defining a touch sensitive platform:
a first resistive bus-bar on the substrate having first and second electrodes at respective ends of the first resistive bus-bar;
a second resistive bus-bar spaced apart from the first resistive bus-bar on the substrate having a third and fourth electrodes at respective ends of the second resistive bus-bar;
a plurality of resistive strips that extend between the first resistive bus-bar and the second resistive bus-bar wherein the resistance of the resistive strips is greater than the resistance of the first and second resistive bus-bars to limit pin cushion effects and form an anisotropic conductive area; and
first, second, third and fourth drive channels associated with respective ones of a first and second electrode at ends of the first resistive bus-bar and a third and fourth electrode at ends of the second resistive bus-bar, each drive channel being operable to generate an output signal dependent on the resistance between its electrode and the position of an object touching the sensor such that non-linear effects are constrained to one dimension.
10. A touch sensitive position sensor according to claim 9 , wherein the resistance of the resistive strips is five to ten times greater than the resistance of the first and second resistive bus-bars.