The generation of electrical signals representing the coordinates of locations on electrographic devices has been the subject of investigation and study for many years. Such devices are found in computer graphics, computer-aided design, and computer-aided manufacturing systems. For such utilization, however, the digitizers or graphics tablets representing such devices are called upon to perform with a high degree of accuracy. The operation of a digitizer or graphics tablet generally involves the same manual procedures as are employed in conventional graphics design, a stylus or tracer or other suitable position locator representing a writing instrument being drawn across or selectively positioned upon the tablet surface. in turn, the electrographic device responds to the stylus to generate analog paired coordinate signals which are digitized and conveyed to a host computer facility.
Traditionally, graphics tablets have been fashioned as composite structures wherein a grid formed of two spaced arrays of mutually orthogonally disposed fine wires is embedded in an insulative carrier. One surface of this structure serves to yieldably receive a stylus input which is converted to coordinate signals. Various methods have been evolved for generating coordinate defining signals as a stylus-grid interaction, for example a magnetostrictive effect may be established between stylus and grid or a capacitive coupling effect may be evoked between these components.
Graphics tablets utilizing composite structures, while providing accurate, linear output coordinate signals necessarily are expensive to fabricate and are prone to damage in the normal course of use. Further, for many applications, it is desirable that the tablet be fabricated as a transparent sheet. Grid formations within composite structures generally preclude such a transparency feature.
Early investigators have observed the advantage of developing graphics tablets having writing surfaces formed as a continuous resistive layer. An immediately recognized advantage for this approach to tablet design resides in the inherent simplicity of merely providing a resistive surface upon a supportive insulative substrate such as glass or plastic. Further, the substrates and associated resistive layers may be transparent to permit an expanded range of industrial applications.
As resistive layer type electrographic systems or digitizers have evolved, technical improvements have been achieved which have enhanced the accuracy, i.e. the correspondence between the actual stylus or tracer position and the measured or indicated position, of the devices. As an example, Schlosser, et al. describes an improvement wherein an a.c. input signal is utilized in conjunction with the devices and signal treatment of the resulting coordinate pair output signal is improved considerably. See U.S. Pat. No. 4,456,787 entitled "Electrographic System and Method", issued June 26, 1984, and assigned in common herewith. Position responsive performance of the resistive layer devices has been improved further by a voltage waveform crossing approach in an arrangement wherein a.c. signals are applied to the resistive layer itself to be detected by a stylus or tracer as illustrated in U.S. Pat. No. 4,055,726 by Turner, et al. entitled "Electrical Position Resolving by Zero-Crossing Delay", issued Oct. 25, 1977, and also assigned in common herewith. Kable describes still another improvement in position responsive performance wherein an a.c. input is utilized in conjunction with a solid-state switching arrangement and a computer program. A description of this improvement may be found in U.S. Pat. No. 4,600,807, also assigned in common herewith. Still another improvement is disclosed in Nakamura, et al., U.S. patent application Ser. No. 664,980, abandoned, entitled "Electrographic System and Method" and assigned in commn herewith. In Nakamura, et al., position responsive performance is enhanced through utilization of a computer controlled interpolative error correction procedure.
A variety of technical problems have been encountered in the development of such resistive layer type devices for applications which demand a very high degree of accuracy or correspondence between the actual stylus or tracer position and the measured or indicated position, e.g. accuracy on the order of 0.010 inch. Applications that require highly accurate devices include computer graphics, computer-aided design, and computer-aided manufacturing systems.
One technical problem which has been encountered is that of erroneous position readings caused by stray capacitance. An example of the effect of stray capacitance may be observed when hands or arms or other objects are placed upon the surface of the digitizer and the digital readout changes while the stylus remains stationary. This effect of stray capacitance is called "hand effect". Another problem which has been encountered in resistive layer devices is that of obtaining the same resistance between the x-coordinate and y-coordinate, i.e. horizontal and the vertical pairs of edges of rectangular non-square devices. In a resistive layer device of uniform thickness, the edge-to-edge resistance is a function of the length of the material which forms the resistive layer. When the device is rectangular but not square, the lengthwise extent of the resistive layer is different in the horizontal and vertical directions and the edge-to-edge resistances are different in these directions. If the edge-to-edge resistance is not the same in both of these directions, the resolution of the system is not the same in both directions.
A further problem which has arisen in the development of resistive layer type digitizers is that of "drift" or change of system accuracy with time. Investigators have discovered that for digitizers utilizing some types of resistive layer materials, the accuracy of the digitizer may change adversely over a period of months or years and that ultimately the accuracy of the device may become unacceptable. Still another problem which has been encountered in the development of highly accurate resistive layer type devices is that of electrical interference, or "noise" in the operating environment of the devices.
It is desirable to provide a resistive layer type digitizer which will be relatively immune to stray capacitances and which will have the same edge-to-edge resistances in the horizontal and vertical directions when the device is shaped in irregular fashion. Additionally, it is desirable to provide a resistive layer type which will not undergo a change of accuracy over a reasonable period of time and which will have an optimum signal-to-noise ratio.