The functionality of a touchscreen system (typically including a touchscreen and an electronic controller) requires that there exist a relationship between the physical location of a touch, e.g. by a person's finger, and some coordinate schema. In general, the coordinate system of choice is a two-dimensional Cartesian system with orthogonal horizontal (X) and vertical (Y) axes. The system accuracy is defined as the error between the physical location of the touch and the location reported by the touchscreen/controller. Typically, system accuracy is expressed as a percentage of the touchscreen dimensions.
A touchscreen system may be considered to have two classes of error, (i) those resulting from the design and implementation of the coordinate transformation method (systematic error), and (ii) those resulting from random unit to unit errors within a given class of sensors (manufacturing variance).
Known conductive touchscreen systems have a transparent substrate with a conductive film, e.g., indium tin oxide (ITO) deposited thereon, which is subject to variation in surface conductivity, i.e., .+-.5% or .+-.10%. A particular additional source of errors in systems employing such substrates is the non-linear variation in sensed probe injection current inherent in the configuration of a generally rectangular substrate with electrodes at the corners. This results in a non-uniform current density at various portions of the substrate, especially near the electrodes. Because of the gross non-linearities, it is generally considered undesirable to attempt to perform a piecewise linear compensation, i.e., directly compensate for repositionable electrode position based on a lookup table calibration procedure. Prior methods have therefore sought to include physical linearization structures, such as complex current injection electrodes, in order to reduce the non-uniformity in surface current density, and to linearize the potentials on the substrate. These complex linearizing structures often include complex conductive patterns, diodes or transistors to redistribute or control the redistribution of currents. Still other methods have sought to apply a mathematical algorithm to compensate for the expected distortions due to the rectangular physical configuration.
The coordinate transformation methods employed in prior systems may be categorized into two basic technologies, herein called electromechanical and modeling, each based on a ratiometric approach, whereby there is an assumed mathematical relationship between measured data and a physical location on the surface of the sensor. Typical distortion of the coordinate values in X and Y of an uncompensated rectangular conductive substrate is shown in FIG. 1.
Lookup tables provide an addressable storage for correction coefficients, and have been proposed for use in correcting the output of touch position sensors based on a number of technologies. These systems receive an address, i.e., a pair of X and Y values, which corresponds to an uncorrected coordinate, and output data which is used to compensate for an expected error and produce a corrected coordinate, generally in the same coordinate space as the uncorrected coordinate. Proposals for such schemes range from zero order to polynomial corrections. See, U.S. Pat. No. 4,678,869, incorporated herein by reference. In general, the uncorrected coordinate input to the proposed lookup table is initially linearized, i.e., by physical means or by algorithmic means, as discussed below, so that the lookup table operates in a linearized space. Lookup table data values derived from a calibration procedure thus directly correspond to the calibration data coordinate values, and define calibration regions.
Electromechanical Methods
There is a class of systematic error compensating methods comprising electromechanical modifications to the touchscreen system, seeking to approximate an orthogonal grid of electrical potentials from the characteristics shown in FIG. 1. There are four basic methods (summarized below) in this category. The design of such electromechanical methods addresses the systematic error, described above, for a given class of touchscreen. The nature of these methods often results in a significant current drain on the system and the multiplicity of electrodes and/or resistance patterns leads to a high sensor cost. Further, the management of the corrective methods, e.g. excitation switching, sensing plane selection, electrode selection, etc., mandates an interactive control mechanism that adds to the system cost. To correct unacceptable errors occasioned by manufacturing variances within the given class of touchscreens, additional error correcting methods, such as table lookup, may be employed for each individual touchscreen.
Bus-Bar Methods
This, the most elementary form of correcting the fundamental distortion characteristics is by creating highly conductive bus bars 3 on opposing axes of the substrate 1 (FIG. 2). Excitation is applied to the bus bars 4 and a conductive coversheet 2 provides for the relocatable electrode. Measurement is made as if the touchscreen were a potentiometer, the position of the "wiper" being the location of the touch, in that plane of excitation. The excitation is then switched to a second set of bus bars in an orthogonal plane (in some cases located on the cover sheet 2) to define the second coordinate. This technique is exemplified by U.S. Pat. No. 3,622,105. The principal drawback to this technology is its current drain. Further, in those cases where the cover sheet is employed for the second excitation plane, any coversheet damage will result in positional location errors.
Multi-Feed Methods
Multi-feed technology, typified by U.S. Pat. No. 5,438,168, employs active control of multiple electrodes 10 located around the periphery of the resistive substrate 11, as shown in FIG. 3. The operation of these systems are generally functionally equivalent to that of bus-bar technology, in that linear voltage gradients are generated for sampling by a cover sheet relocatable position sensor. Since all electrodes 10 are located on one substrate 11, it is unaffected by cover sheet damage. However, it is a high current drain system, and requires a large number of interconnections. Failure or degradation of any of its switching elements 12 results in system errors.
Resistive Pattern Methods
Many known of corrective methods include use of resistive patterns 21 on, or external to, the touchscreen 20, in such a sequence that the resistive gradient of the touchscreen 20 is approximately the same across its surface, as shown in FIG. 4. U.S. Pat. Nos. 3,798,370, 4,293,734, and 4,661,655 typify this technique. These systems have the high current consumption associated with electromechanical methods, and, because of the complexity of the resistive patterns 21, are prone to errors resulting from manufacturing variances.
Modeling
A second category of coordinate generation technique is based on mathematical functions, chosen because of assumed mathematical relationships for a given class of touchscreens. These methods result in X and Y values that require further adjustments or corrections either because of inadequacies in the assumptions or because of manufacturing variations, or both.
One method, described in U.S. Pat. No. 4,631,355 and Federico et al., "17.2: Current Distribution Electrograph" SID 86 Digest. p. 307, relies on an a priori assumption concerning the mathematical distribution for points on a touchscreen. Each plane is extracted by ratiometric methods, and the axial "astigmatism" of each plane, as exemplified in FIG. 1, is then linearized by the use of a second order polynomial equation whose coefficients are empirically derived. U.S. Pat. No. 4,631,355 notes that manufacturing variation errors on the order of 5% are usual, but does not compensate for them, and therefore would need to be corrected for by additional techniques in order to provide an accurate touch position sensing method. Therefore, Federico et al., "17.2: Current Distribution Electrograph" SID 86 Digest, proposes storing calibration data in a lookup table, for operation separately from the algorithmic compensation system and as a subsequent step to correct the sensor output.
U.S. Pat. No. 4,806,709 is predicated on the assumption of a linear relationship between signals at an electrode located on the conductive surface and the distance between that electrode and the touch location. Using this assumption, the signal from each electrode is employed in an equation that describes the arc of a circle with its origin at the electrode, with a second equation that defines the touch location as the intersection of two or more of such arcs. An implementation of such an approach would have two principal sources of error, (a) non-linearities in the assumed signal/distance relationship, measured data confirming such non-linearities, and/or manufacturing variances which would lead to an error in the calculation of each arc radius, and (b) the classic problem of positional error caused by the difficulty in resolving the angles of intercept as the arcs approach tangency.