The present invention relates to an apparatus and method for determining the coordinates of a location in a two-dimensional system, such as a touch sensitive screen for producing output signals related to a touch position. More particularly, the present invention relates to an apparatus and method for generating signals representing a touch position in which non-linear corrections are applied.
Touchscreens are becoming the computer input device of choice for an increasing variety of applications. A touchscreen is a transparent input device that is able to sense the position of the touch of a finger or other electronically passive stylus relative to the touchscreen. Typically, touchscreens are placed over display devices such as cathode-ray-tube monitors and liquid crystal displays. Touchscreen input is often preferred for applications such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, lap-top computers, and other such applications.
Many schemes have been proposed for touchscreen construction, some of which have met with commercial acceptance. One important aspect of touchscreen performance is a close correspondence between actual and measured touch positions at all locations within an active touch area. There are many types of touchscreens available including five-wire resistive touchscreens, four-wire resistive touchscreens, capacitive touchscreens, ultrasonic touchscreens, and infrared touchscreens. All of these types of touchscreen have attempted to deliver high standards of performance at cost-competitive prices.
Five-wire resistive touchscreens, such as the AccuTouch® product line of touchscreens from Elo TouchSystems, Inc. of Fremont, Calif., have been widely accepted for many touchscreen applications. In five-wire resistive touchscreens, mechanical pressure from a finger or stylus causes a flexible sheet, such as a plastic coversheet, to flex and make physical contact with an underlying rigid substrate, such as a glass substrate. The rigid substrate is coated with a resistive coating upon which voltage gradients are generated. Through electrical connections to the four corners of the rigid substrate, associated electronics can sequentially generate voltage gradients in the X and Y directions. The underside of the flexible sheet has a conductive coating that provides an electrical connection at the touch location between the resistive coating and the conductive coating. It should be noted that in this type of touchscreen system there are a total of five electrical connections, i.e., “five wires”, between the touchscreen and the controller electronics. Further details regarding five-wire resistive touchscreens are found in the following U.S. Patents: U.S. Pat. No. 4,220,815 to Gibson; U.S. Pat. Nos. 4,661,655 and 4,731,508 to Gibson et al.; U.S. Pat. No. 4,822,957 to Talmadge et al.; U.S. Pat. No. 5,045,644 to Dunthorn; and U.S. Pat. No. 5,220,136 to Kent, the specifications of which are all herein incorporated by reference.
Four-wire resistive touchscreens dominate the low-end of the touchscreen market since the manufacturing costs for four-wire resistive touchscreens are generally less than the manufacturing costs for five-wire resistive touchscreens. However, in applications demanding reliable performance in the face of heavy use, the five-wire resistive technology has generally proven superior. To measure both X and Y coordinates, four-wire resistive touchscreens alternate between generating a voltage gradient on the substrate resistive coating and generating an orthogonal voltage gradient on the conductive coating of the flexible sheet. Performance of four-wire touchscreens degrades as the uniform resistivity of the conductive coating is lost as a result of mechanical flexing of the flexible sheet. This is not a problem for five-wire touchscreens, where both X and Y voltage gradients are generated on the rigid substrate's resistive coating, and the conductive coating on the flexible sheet need only provide electrical continuity. However, in a five-wire touchscreen, a peripheral electrode pattern of some complexity is required to enable sequential generation of both X and Y voltage gradients on the same resistive coating. A key design feature that distinguishes five-wire touchscreens from four-wire touchscreens is the presence of four corner connection points on the substrate of the five-wire touchscreen at which voltages are applied to a peripheral electrode pattern.
The controller electronics can obtain touch information from a five-wire resistive touchscreen through current injection, as well as voltage gradient generation as described above. In order to obtain touch information through current injection, a current source injects current though the flexible sheet and the current arriving at each of the four corner connection points is then measured. From the sums and ratios of these corner currents, touch positions are reconstructed. The choice between current injection and voltage generation is an electronics design choice and is largely independent of touchscreen design. Peripheral electrode pattern designs for touchscreen systems with voltage generation electronics are equally applicable to touchscreen systems using current injection.
In a capacitive touchscreen, the flexible sheet is replaced by a thin transparent dielectric coating that then forms an exterior layer over the ITO or ATO coated substrate. In one approach to electronic readout, an oscillating voltage is applied to the four corner connection points. A finger touch provides an AC shunt to ground and hence serves as an AC current source at the location of the touch. The division of this AC current between the four corner connection points is measured and used to determine the touch coordinates. An AC variant of current-injection electronics is used. Capacitive touchscreens often require peripheral electrode patterns that serve the same basic function as in five-wire resistive touchscreens. For example, 3M Touch Systems, Inc. offers both capacitive touchscreens (ClearTek™) and five-wire resistive touchscreens (TouchTek™) with peripheral electrode patterns similar to those illustrated in FIG. 1b of U.S. Pat. No. 4,371,746 to Pepper, the specification of which is herein incorporated by reference. It is widely known that peripheral electrode patterns can be used in both five-wire resistive and capacitive systems.
It is sometimes advantageous to have both a drive line and a sense line connection between the electronics and each of the four corner connection points. With appropriate feedback loops in the electronics, the combination of drive and sense lines gives the controller electronics better control over the voltages applied to the corner connection points. This leads to a variant of “five-wire” touchscreens that includes nine wire connections between the electronics and the touchscreen, otherwise known as a nine-wire touchscreen. The design of the peripheral electrode pattern is largely unaffected by the choice between five-wire and nine-wire connection schemes. Both involve four corner connection points on the substrate at which voltages are applied to a peripheral electrode pattern.
The use of separate drive and sense lines also leads to a variant of 4-wire touchscreens, namely the “8-wire” resistive touchscreen such as those sold by Gunze USA of Austin, Tex. and 3M Touch Systems, Inc. For example, if zero and 5 volts are applied to a pair of drive lines that excite a voltage gradient on a resistive coating, voltage drops on the drive lines may lead to a reduced total voltage drop across the resistive coating, say from 0.2 to 4.8 volts. Furthermore, if the drive line voltage drops vary with aging or environmental conditions, the relationship between touch position and measured voltages will also vary. However, by monitoring voltages on sense lines, such variations can be tracked and accounted for with linear corrections to the raw measured touch coordinates.
Rather than manufacturing touchscreens to exacting standards, corrections may be applied to touchscreen data in order to compensate for manufacturing variations and non-ideal material properties. Two types of corrections can be used: linear and non-linear. Resistive and capacitive touchscreens, in which X and Y voltage gradients are alternately applied to a common resistive coating, are often designed to be “linear.” That is, during the measurement of a voltage in the horizontal or X direction, equipotential lines on the resistive coating are substantially straight, vertical, and uniformly spaced. During the measurement of a voltage gradient in the vertical or Y direction, the equipotential lines are also substantially straight and uniformly spaced, but are horizontal. If the equipotential lines are not straight and uniformly spaced in either the X direction or the Y direction, then the touchscreen is deemed to be non-linear. The design and manufacture of linear touchscreens involves satisfying these linearity conditions to a good approximation despite manufacturing variations. While linear touchscreens minimize the computational burden on the controller electronics, significant constraints are placed on the design and manufacture of linear touchscreens.
It is possible to have a non-linear touchscreen, and yet have a linear touchscreen system. In this case, the controller electronics or driver software on a host computer must apply non-linear corrections to the raw touchscreen measurements. As the cost of electronics and information processing software continues to drop, it becomes attractive to move the burden of linear system-level performance more towards the electronics and software.
A key issue of non-linear touchscreen systems is the determination of non-linear parameters. One can calibrate a non-linear touchscreen by mechanically touching an appropriate grid of points at known positions. However, this is a significant addition to the touchscreen manufacturing process or the touchscreen system installation process which inevitably adds cost. Alternatively, one can use a fixed set of non-linear correction parameters and ensure that each touchscreen is manufactured with the same non-linear distortions. However, this leads to similar tolerance requirements as in the manufacturing process for linear touchscreens and therefore inevitably adds cost. Thus, there is a need for an improved method for determining non-linear correction parameters for touchscreen systems.
Non-linear corrections are generally fixed constants in prior systems. As a consequence, non-linear distortions that vary with time or with changes in environmental conditions are problematic and limit the choice of materials and manufacturing processes for touchscreens. Thus, there is also a need for a convenient and automatic means to update the non-linear parameters and to track changes in non-linear distortions that occur with time and with changes in environmental conditions.