Investigators have developed a variety of technical approaches to the generation of coordinate pairs of signals from electrographic devices. Industrial requirements for these devices are increasing concomitantly with the evolution of computer graphics, computer-aided design, and computer-aided manufacturing systems. Thus, a considerable degree of accuracy in pinpointing physical positions upon the surfaces of the digitizers is required for many applications. Other applications of the electrographic services include touch screen devices wherein the operator's finger or a stylus or the like is used to touch a portion of the accessing surface such that it emulates a key of a keyboard.
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 representing a writing instrument being drawn across or selectively positioned upon the position responsive surface of the digitizer. In turn, the electrographic device responds to the position of the stylus to generate paired analog coordinate signals which are digitized and conveyed to a host computer facility.
Early approaches to digitizer structures, for example, have looked to employing compostie 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 yielding causes the grid to read out coordinate signals. More recent and improved approaches to achieving readouts have been accomplished through resort to a capacitive coupling of the stylus or locating instrument with the position responsive surface to generate the paired analog coordinate signals. Such capacitive coupling can be carried out either with a grid layer which is formed of spaced linear arrays of conductors or through resort to the use of an electrically resistive material layer or coating.
An immediately apparent advantage of developing position responsive surfaces or digitizers having writing surfaces formed of a continuous resistive material resides in the inherent simplicity of merely providing a resistive surface upon a supportive substrate such as glass or plastic. Further, unlike conventionally encountered grid structures, the resistive coatings as well as their supportive substrates may be transparent to considerably broaden the industrial applications for the devices. For example, the digitizers may be placed over graphics or photographic material for the purpose of tracing various profiles.
A variety of technical problems have been encountered in the development of resistive coating type digitizer devices, one of which concerns the non-uniform nature of the coordinate readouts achieved with the surfaces. Generally, precise one-to-one coorespondence or linearity is required between the actual stylus or tracer position and the resultant coordinate signals. Because the resistive coatings cannot be practically developed without local resistance (thickness) variations, for example of about .+-.10%, the non-linear aspects of the otherwise promising approach has required a considerable amount of investigation and development. Exemplary of such development is the border treatment or switching technique of Turner in U.S. Pat. No. 3,699,439 entitled "Electrical Probe-Position Responsive Apparatus and Method" issued Oct. 17, 1972, and assigned in common herewith. This approach uses a direct current form of input to the resistive surface from a hand-held stylus, the tip of which is physically applied to the resistive surface. Schlosser et al describes still another 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 considerably improved. See U.S. Pat. No. 4,456,787 entitled "Electrographic System and Method", issued June 26, 1984, also assigned in common herewith. Position responsive performance of the resistive layer devices further has been improved by a voltage waveform zero crossing approach and an arrangement wherein a.c. signals are applied to the resistive layer itself to be detected by a stylus or tracer as described 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. Substantially improved accuracies for the resistive surface type digitizer devices have been achieved through a correction procedure wherein memory retained correction data are employed with the digitizer such that any given pair of coordinate signals are corrected in accordance with data collected with respect to each digitizer resistive surface unit during the manufacture of the digitizers themselves. With such an arrangement, the speed of correction is made practical and the accuracy of the devices is significantly improved. The correction table improvements for these services is described, for example, in application for United States patent, Ser. No. 664,980, filed Oct. 26, 1984, by Nakamura et al. and assigned in common herewith as well as in application for United States Pat. Ser. No. 742,733, entitled "Electrographic System and Method," filed June 7, 1985, by Nakamura et al. and assigned in common herewith.
Capacitive coupling using a stylus or locating device has been employed with grid layers which are formed as adjacent but spaced-apart arrays of elongate thin conductors. For example, these grid conductors may be provided as lines of silver ink deposited in orthogonally disposed relationships upon the opposite faces of a sheet of insulative material such as Mylar. As described in Rodgers et al., U.S. Pat. No. 4,492,819, issued Jan. 8, 1985, this grid surface may be employed with a stylus which injects an a.c. signal capacitively at the surface thereof. To detect this signal, a ladder form of resistance network is employed with each of the conductor arrays such that a predetermined resistance is coupled between each conductor from first to last and a discrete resistor is coupled from the union of two successive resistors to ground. Generally these devices operate in a current mode such that current values are determined at the peripherally disposed resistor strings. As is apparent, because of the necessity of employing a conductive form of grid line, these devices are limited to opaque position responsive surface applications. While discrete matched resistors are required to couple the grid line nodes, a conductive or carbon loaded ink advantageously may be employed to provide the grid-to-grid resistive components of the unit, however, at the expense of a resistance value deviation for each discrete increment of resistance between adjacent parallel grid lines.
Investigations have determined that there are operational trade-offs occasioned with the various design approaches to grid-type digitizers. For example, where a.c. signals are injected from the stylus into a passive orthogonal grid, the grid structured surface electrically appears as a high impedance to the stylus or pick-up. Thus, the presence of moisture at the tablet or digitizer surface will cause severe inaccuracies. Further, this type device is prone to react adversely in terms of readout accuracy to slightly conductive materials including certain forms of paper.
On the other hand where the grid-type digitizers are excited by a.c. signals emanating from the peripheral resistor strings or multi-nodal stripes, a potential gradient is established for coordinate identification. The stylus or pick-up, in turn, receives analog coordinate signals in an arrangement desirably immune from hand effects, moisture effects and the like. However, the linearity of these devices becomes severely impaired to essentially negate the advantages otherwise sought.
Where it is desired to provide the above-noted grid structures in a transparent embodiment for a digitizer, limitations may be observed. To achieve transparency of the grid lines themselves, a transparent material must be employed, for example an indium tin oxide. These materials, however, may exhibit an impedance or resistivity which establishes a finite resistance between the oppositely disposed ends of a tablet or position responsive surface. Such finite resistance, unless accommodated for, will impose debilitating error unless somehow corrected. The finite resistance further may impose ambiguities within the system such that a voltage at one position along one trace or grid line will occur in equal value at a different location in another grid line. A practical implementation, therefore, of grid type tablets requires a minimization of the chance of occurrence of such ambiguities.