CA1256180A - Electrographic apparatus - Google Patents

Electrographic apparatus

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Publication number
CA1256180A
CA1256180A CA000509172A CA509172A CA1256180A CA 1256180 A CA1256180 A CA 1256180A CA 000509172 A CA000509172 A CA 000509172A CA 509172 A CA509172 A CA 509172A CA 1256180 A CA1256180 A CA 1256180A
Authority
CA
Canada
Prior art keywords
electrodes
row
rows
resistive layer
active area
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
CA000509172A
Other languages
French (fr)
Inventor
Shoichiro Nakamura
Robert G. Kable
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Scriptel Corp
Original Assignee
Scriptel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Scriptel Corp filed Critical Scriptel Corp
Application granted granted Critical
Publication of CA1256180A publication Critical patent/CA1256180A/en
Expired legal-status Critical Current

Links

Classifications

    • GPHYSICS
    • G06COMPUTING; CALCULATING OR COUNTING
    • G06FELECTRIC DIGITAL DATA PROCESSING
    • G06F3/00Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
    • G06F3/01Input arrangements or combined input and output arrangements for interaction between user and computer
    • G06F3/03Arrangements for converting the position or the displacement of a member into a coded form
    • G06F3/041Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
    • G06F3/045Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means using resistive elements, e.g. a single continuous surface or two parallel surfaces put in contact
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01BMEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B7/00Measuring arrangements characterised by the use of electric or magnetic techniques
    • G01B7/004Measuring arrangements characterised by the use of electric or magnetic techniques for measuring coordinates of points
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01DMEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/14Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
    • G01D5/16Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying resistance
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01DMEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/14Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
    • G01D5/16Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying resistance
    • G01D5/165Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying resistance by relative movement of a point of contact or actuation and a resistive track
    • GPHYSICS
    • G06COMPUTING; CALCULATING OR COUNTING
    • G06FELECTRIC DIGITAL DATA PROCESSING
    • G06F2203/00Indexing scheme relating to G06F3/00 - G06F3/048
    • G06F2203/041Indexing scheme relating to G06F3/041 - G06F3/045
    • G06F2203/04113Peripheral electrode pattern in resistive digitisers, i.e. electrodes at the periphery of the resistive sheet are shaped in patterns enhancing linearity of induced field

Abstract

ELECTROGRAPHIC APPARATUS
Abstract of the Disclosure An electrographic apparatus and system utilizing a resistive layer having an active area which operates in conjunction with a stylus or tracer or other suitable position locator. An a.c. source or alternatively a signal treatment means is connected to the resistive layer through rows of electrodes. Improved accuracy is achieved through the use of electrodes at the ends of the electrode rows and through the use of an enhanced conductivity region which also allows the boundary of theactive area to be placed in close proximity to the electrodes.

Description

1256~80 ~;CR 2-019 ELECT~OGRAPHIC APPARATUS

Background of the Invention The generation of electrical signals representing the coordinates of locations on electrographic devices ha~ 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 utlization, however, the digitizers 5or graphics tablets representing such devices are called upon to perform with a high degree of accuracy. Additionally, in some applications the size of these devicesmust be limited. As an example, a digitizer may be mounted upon the surface of acathode ray tube which displays inputs from the device.
The opcration of a digitizer or graphics tablet generally involves the same 10manual procedures as are employed in conventional graphics design, a stylus ortracer 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 position of the stylus to generate analog paired coordinate signals which are digitized and conveyed to a host computer 15facility.
For the most part, 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 20signals. 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, 25linear 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 highly transparent composite sheet.Grid formations within composite structures generally preclude such a transparency feature.
30Early investigators have observed the advantage of developing graphics tabletshaving writing surfaces formed as a continuous resistive coating. I~n immediately recognized advantage for this approach to tablet design resides in the inherent simplicity of merely providing a resistive surface upon a supportive insulative ,,. ~

~256~80 .

sul)strate such ns glass or plnstic. I;urtller, thc substrates and nssocinted rcsistive coatings may be trnnspnrent to pcrmit nn expanded rnnge Or industrial applications.
rhe history of the dcvclopment ot such rcsistive coating type devices shows tllat investiglltors havc cncountercd a variety ot technicnl problcms, one of whicl being the non-unirorm nnture of tlle coordinate readouts acllieved with the coatings.
Gcner~llly, precise one-to-onc corrcspondcncc or lincarity is requircd betwcen the actual styllls or tracer position nnd tlle rcsultant measured coordinnte signals.
13ccnuse lhe resistive contillgs cam~ot be developed prnctically without local resislance vnrinlions, for exnmple of abollt plus or minus ten per cent, the non-lincar nspccls Or the otherwise promising design npproach have impedcd tl-e dcvetopmcnt ol prnctical devices unlil rccenlly. Ilowever, certnin hnportarit tcchnicnl npprouchcs to ulilizing the rcsistive surtaces have bcen achicved. Forexnmple, Tllrner discloses n border treatment or switching tecllnique in U.S. Pat.
No. 3,699,~39 entitled "Eleclrical-rrobe Position nesponsive l~ppnralus nnd Metllod"
issued October 17, 1972, nssigned in common herewitl). This approach utilizes ~
Jircct currcnt form of input to the rcsistive surface from a hnnd-llcld slylus, the tip of whicll is physically npplied to the resistive surface. Schlosscr, et nl. describes anolller improvement wherein nn a.c. input signal is utilized in conjunction with the devices nnd signal treatment of the resulting coordinate pair output signal is considernbly improved. See U.S. Pat. No. 4,~56,787 entitled "Electrographic System nnd Method", issued June 26, 1984, also nssigned in common herewith. Position responsive performance of the resistive layer devices further has been improved by a Yoltnge wnve form crossing npproncll and an arrangement wherein a.c. signals are npplied to the rcsistive layer itself lo be detected by n stylus or tracer as described in U.S. Pat. No. 4,055,726 by Turner, et al. entitled "Electrical Position Resolving by %ero-Crossing Delay" issucd October 25, 1977, nnd also assigned in common l~erewith. Kable descril)es still anotller improvement in position responsive perlor-mnnce whcrein an ~.c. input is utilized in conjunction Witll a solid state switching arrangement and Q computer program. ~ description of tllis improvement may l)e found in U.S. Pat. No. 4,600,807, entitlcd "Electrograpllic f~ppnrntus" also assigned in common herewith. Still anotllcr improvement is disclosed in Naknmurn, et nl. U.S. Pat. No. 4,650,926, entitled "Electrogrnphic System and Method" nnd nlso nssigned in common llcrcwith.l ln this approacll, position responsive performance is enhanced through utilization of a computcr controlled interpolutive error correction procedure.
As tllè designs of resistive layer digitizers now reach a level of technical devclopment permitting their practical implementation in precision computer 12S6~0 graphics, computer aided design and computer aided manufacturing systems, further need has been exhibited for their additional refinement with respect to improve-ments in linearity, i.e. with respect to the accuracy of their performance. Suchimprovements are most necessary at the edge regions and corner regions of the active or working area of the tablet where non-linearity has been most prominent.
The approach to date for accommodating edge region phenomcnona which occurs adjacent electrodes placed at the edges of the resistive layer on the tablet has been to established a non-usable buffer region between the active area and the elec-trodes. Such a buffer region may have to be relatively wide, to obtain an acceptable level of accuracy within the active area of a graphics tablet. For applications having sufficient space the resistive layer in the tablet may be sized to provide the desired active area and the necessary buffer region. However, some applications cannot accommodate a wide buffer region. For example, in npplica-tions where a digitizer is applied to the surface of a display device such as a cathode ray tube (CRT), the digitizer may be required to fit within the boundaries of the CRT and the active area of the digitizer may have to be the same as the active area of the CRT display. In these applications the non-usable region of the digitizer can be no greater than the space between the active area and the outside edges of the CRT.
Summary of the Invention The present invention is addressed to an electrographic apparatus for generating coordinate data wherein coordinate positions upon the active area of a resistive layer are identified through the use of signals generated from an a.c.source. The a.c. source or alternately a signal treatment means is connected to the resistive layer through rows of electrodes. To improve the accuracy of the apparatus electrodes are placed at the ends of each of the electrode rows. In this regard, these electrodes may be single electrodes placed at each end of the electrode rows or corner electrodes placed at the intersection of adjacent rows of electrodes.
Further improvement in the accuracy of the apparatus is achieved by interposing an enhanced conductivity region between the active area and the electrodes of the device. Utilization of the enhanced conductivity region allows the boundary of the active area to be located in close proximity to the electrodes.
It is a further feature of the invention to provide an electrographic system comprising an electrically insulative substrate, a resistive layer supported upon said insulative substrate and having an active area extending in an x- coordinate direction between first and second spaced-apart edges and extending in a y coordinate direction between third and fourth spaced-apart edges. A first row ofdiscrete spaced-apart electrodes is electrically coupled with said resistive layer intermediate said active area and said first edge and a second row of discrete, spaced-apart electrodes is electrically coupled with said resistive layer intermediate 5 said active area and said second edge. Similarly, a third row of discrete, spaced apart electrodes is electrically coupled with said resistive layer intermediate said active area and said third edge and a fourth row of discrete, spaced-apart electrodes is electrically coupled with said resistive layer intermediate said active area and said fourth edge. A first corner electrode means is coupled with the resistive layer 10 at the juncture of said first and third rows and a second corner electrode means is coupled with said resistive layer at the juncture of said first and fourth rows.Likewise, a third corner means is coupled with said resistive layer at the juncture of said second and third rows and a fourth corner electrode means is coupled with said resistive layer at the juncture of said second and fourth rows. A position locator is 15 provided adjacent said active area, and a time varying excitation source of select frequency, a ground reference, and switching means are further included. The switching means applies said ground reference to said first row of electrodes and to - said first and second corner electrodes and simultaneously applies said source to said second row of electrodes and to said third and fourth corner electrodes and 20 electrically isolates said third and fourth rows of electrodes during a firstoperational mode. During a second operational mode said switching means reversessaid applications of said ground reference and said source to said first and second rows of electrodes and to said first, second, third, and fourth corner electrodes and maintains said electrical isolation of said third and fourth rows of electrodes. The 25 switching means applies said ground reference to said third row of electrodes and to said first and third corner electrodes and simultaneously applies said source to said fourth row of electrodes and to said second and fourth corner electrodes and electrically isolates said first and second rows of electrodes during a third operational mode. Said switching means reverses said applications of said ground30 reference and said source to said third and fourth rows of electrodes and to said first, second, third, and fourth corner electrodes and maintains said electricalisolation of said first and second rows of electrodes during a fourth operational moae. Control means are provided for collecting a signal from said position collector.

~, ...,. `

256i8~

Another aspect of the invention concerns an electrographic system having the electrically insulated substrate, resistive layer and active area as above-described.
Additionally, the system incorporates the first, second, third, and fourth discrete spaced apart electrodes as above discussed in addition to the first, second, third, and fourth corner electrodes as set forth above. The system additionally incorporates a position locator adjacent the active area, a time varying excitation source of select frequency, signal treatment means and a ground reference. A switching arrangement is provided for applying the ground reference to the first row of electrodes and to the first and second corner electrodes, and simultaneously applying the signal treatment means to the second row of electrodes and to the third and fourth corner electrodes, and electrically isolating the third and fourth rows of electrodes during a first operational mode. The switching arrangement serves to reverse the applications of the ground reference and the signal treatment means to the first and second rows of electrodes and to the first, second, third, and fourth corner electrodes and maintaining the electrical isolation of the third and fourth rows of electrodes during a second operational mode. The signal switching means additionally applies the ground reference to the third row of electrodes and to the first and third corner electrodes and simultaneously applies the signal treatment means to the fourth row of electrodes and to the second and fourth corner electrodes and electrically isolates the first and second rows of electrodes during a third operational mode and reverses the applications of the ground reference and the signal treatment means to the third and fourth rows of electrodes and to the first, second, third, and fourth corner electrodes and maintains the electrical isolation of the first and second rows of electrodes during a fourth operational mode.
Another aspect of the invention provides an electrographic system which comprises an electrically insulative substrate and resistive layer supported upon the substrate which has an active area extending in an x-coordinate direction between first and second spaced apart edges and extends in a y-coordinate direction between third and fourth spaced-apart edges. A first row of discrete, spaced apart electrodes electrically coupled with the resistive layer intermediate the active area and the first edge are provided as well as a second row of discrete, spaced apart electrodes electrically coupled with the resistive layer intermediate the active area on the second edge. A third row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the third edge, while a fourth row of discrete, spaced-apart electrodes is electrically coupled with .

~Z56~80 the resistive layer intermediate the active area and the fourth edge. One electrode in the first row of electrodes is positioned at the end of the first row adjacent the third row and another electrode in the first row is positioned at the opposite end of the first row adjacent the fourth row. One electrode in the second row of 5 electrodes is positioned at the end of the second row adjacent the third row and another electrode in the second row is positioned at the end of the second row adjacent the fourth row. One electrode in the third row of electrodes is positioned at the end of the third row adjacent the first row and another electrode in the third row is positioned at the opposite end of the third row adjacent the second row. One 10 electrode in the fourth row of electrodes is positioned at the end of the fourth row adjacent the first row and another electrode in the fourth row is positioned at the opposite end of the fourth row adjacent the second row. A position locator is provided adjacent the active area and a time varying excitation source of selectfrequency and a ground reference further are provided. A switching arrangement 15 applies the ground reference to the first row of electrodes and sirnultaneously applies the source to the second row of electrodes and electrically isolates the third and fourth rows of electrodes during a first operational mode. The switching arrangement reverses the applications of ground reference and the source to the first and second rows of electrodes and maintains the electrical isolation of the 20 third and fourth rows of electrodes during a second operational mode. The switching arrangement applies the ground reference to the third row of electrodes and sirnultaneously applies the source to the fourth row of electrodes and electrically isolates the first and second rows of electrodes during a third operational mode and serves to reverse the applications of ground reference and source to the third and 25 fourth rows of electrodes and maintains the electrical isolation of the first and second rows of electrodes during a fourth operational mode. A control arrangement is provided for collecting a signal from the position locator.
As a further aspect, an electrographic system is provided which comprises an electrically insulated substrate and a resistive layer supported upon the substrate 30 having an active area extending in an x-coordinate direction between first and second spaced-apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges. A first row of discrete, spaced-apart electrodes are electrically coupled with the resistive layer intermediate the active area and the first edge, and a second row of discrete, spaced-apart electrodes is electrically 35 coupled with the resistive layer intermediate the active area and the second edge.

f``~ ~

~2s6~s~

-5b-A third row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the third edge, while a fourth row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the fourth edge. One electrode in the first row of 5 electrodes is positioned at the end of the first row adjacent the third row and another electrode in the first row is positioned at the opposite end of the first row adjacent the fourth row. One electrode in the second row of electrodes is positioned at the end of the second row adjacent the third row and another electrode in thesecond row is positioned at the end of the second row adjacent the fourth row. One 10 electrode in the third row of electrodes is positioned at the end of the third row adjacent the first row and another electrode in the third row is positioned at the opposite end of the third row adjacent the second row, while one electrode in the fourth row of electrodes is positioned at the end of the fourth row adjacent the first row and another electrode in the fourth row is positioned at the opposite end of the 15 fourth row adjacent the second row. A time varying excitation source of select frequency is provided, as well as a position locator which is adjacent the active area for inputting the source at a select location in the active area. Signal treatment means and a ground reference are provided. A switching arrangement is provided for applying the ground reference to the first row of electrodes and simultaneously 20 applying the signal treatment means to the second row of electrodes and electrically isolating the third and fourth rows of electrodes during a first operational mode.
The switching arrangement reverses the applications of the ground reference and the signal treatment means to the first and second rows of electrodes and maintains the electrical isolation of the third and fourth rows of electrodes during a second 25 operational mode. The switching arrangment applies the ground reference to the third row of electrodes and simultaneously applies the signal treatment means to the fourth row of electrodes and electrically isolates the first and second rows of electrodes during a third operational mode and reverses the applications of the ground reference and the signal treatment means to the third and fourth rows of 30 electrodes and maintains the electrical isolation of the first and second rows of electrodes during a fourth operational mode.
Another aspect of the invention looks to an improved graphics tablet for an electrographic system which comrpises an electrically insulative substrate, and a resistive layer supported upon the insulative substrate defining an active area 35 extending in an x-coordinate sense between and inwardly of first and second spaced-~, .
r ~256~8~
--sc--apart edges and extending in a y-coordinate sense between and inwardly of third and fourth spaced-apart edges. A plurality of electrodes are positioned intermediate the active area and the edges and enhanced conductivity region is provided intermediate the active area and the electrodes.
The instant invention also provides an improved graphics tablet for an electrographic system comprising an electrically insulative substrate and a resistive layer supported upon said insulative substrate and having an active area extending in an x- coordinate direction between first and second spaced-apart edges and extending in a y- coordinate direction between third and fourth spaced-apart edges.
A first row of discrete, spaced-apart electrodes are electrically coupled with said resistive layer intermediate said active area and said first edge and a second row of discrete spaced-apart electrodes are electrically coupled with said resistive layer intermediate said active area and said second edge. Similarly, a third row of discrete spaced-apart electrodes are electrically coupled with said resistive layer intermediate said active area and said third edge and a fourth row of discrete, spaced-apart electrodes are electrically coupled with said resistive layer inter-mediate said active area and said fourth edge. First corner electrode means are coupled with the resistive layer at the juncture of said first and third rows and second corner electrode means are coupled with said resistive layer at the juncture of said first and fourth rows. Likewise, third corner electrode means are coupled with said resistive layer at the juncture of said second and third rows and fourth corner electrode means are coupled with said resistive layer at the juncture of said second and fourth rows. An enhanced conductivity region is provided intermediatesaid active area and said electrodes.
Another aspect of the invention provides an electrographic system which comprises an electrically insulated substrate and a resistive layer supported upon the substrate which has an active area extending in an x-coordinate direction between first and second spaced-apart edges and extends in a y-coordinate direction between third and fourth spaced-apart edges. A first row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the first edge, while a second row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the second edge. A third row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the third edge, while a fourth row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the fourth edge. An enhanced ~; conductivity region is positioned intermediate the active area and the electrodes. A
i 1256~80 -5d-first corner electrode is coupled with the resistive layer at the juncture of the first and third rows, while a second corner electrode is coupled with the resistive layer at the juncture of the first and fourth rows. A third corner electrode is coupled with the resistive layer at the juncture of the second and third rows, while a fourth5 corner electrode is coupled with the resistive layer at the juncture of the second and fourth rows. A position locator is provided adjacent the active area and a time varying excitation source of select fre~uency is provided in addition to a ground reference. A switching arrangement is provided for applying the ground referenceto the first row of electrodes and to the first and second corner electrodes and10 simultaneously applying the source to the second row of electrodes and to the third and fourth corner electrodes and electrically isolating the third and fourth rows of electrodes during a first operational mode. The switching arrangement reverses the applications of the ground reference and source to the first and second rows of electrodes and to the first, second, third, and fourth corner electrodes and maintains 15 the electrical isolation of the third and fourth rows of electrodes during a second operational mode. The switching arrangement applies the ground reference to the third row of electrodes and to the first and third corner electrodes and simultaneously applies the source to the fourth row of electrodes and to the second and fourth corner electrodes and electrically isolates the first and second rows of 20 electrodes during a third operational mode. Additionally, the switching arrangement reverses the applications of the ground reference and source to the third and fourth rows of electrodes and to the first, second, third, and fourth corner electrodes and maintans the electrical isolation of the first and second rows of electrodes during a fourth operational mode. A control arrangement is provided for collecting a signal 25 from the position locator.
Another aspect of the invention is the provision of an electrographic system which comprises an electrically insulative substrate, and a resistive layer supported upon the substrate which has an active area extending in an x-coordinate direction between first and second spaced apart edges and extends in a y-coordinate direction 30 between third and fourth spaced-apart edges. A first row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the first edge, while a second row of discrete, spaced-apart electrodes is electrically coupled with the resistive layer intermediate the active area and the second edge. A third row of discrete, spaced-apart electrodes is electrically 35 coupled with the resistive layer intermediate the active area and third edge, while a fourth row of discrete, spaced-apart electrodes is electrically coupled with theresistive layer intermediate the active area and the fourth edge. An enhanced ':

`~--1256~80 -Se-conductivity region is located intermediate the active area and the electrodes. A
first corner electrode is coupled with the resistive layer at the juncture of the first and third rows, while a second corner electrode is coupled with the resistive layer at the juncture of the first and fourth rows. A third corner electrode is coupled with 5 the resistive layer at the juncture of the second and third rows, while a fourth corner electrode is coupled with the resistive layer at the juncture of the second and fourth rows. A time varying excitation source of select frequency is provided and a position locator is provided adjacent the active area for inputting the source at a select position in the active area. Signal treatment means are provided, as well as a 10 ground reference. A switching arrangement is provided for applying the groundreference to the first row of electrodes and to the first and second corner electrodes and simultaneously applying the signal treatment means to the second row of electrodes and to the third and fourth corner electrodes and electricallyisolating the third and fourth rows of electrodes during a first operational mode.
15 The switching arrangement reverses the applications of the ground reference and the signal treatment means to the first and second rows of electrodes and to thefirst, second, third, and fourth corner electrodes and maintains the electrical isolation of the third and fourth rows of electrodes during a second operationalmode. The switching arrangement applies the ground reference to the third row of20 electrodes and to the first and third corner electrodes and simultaneously applies the signal treatment means to the fourth row of electrodes and to the second andfourth corner electrodes and electrically isolates the first and second rows of electrodes during a third operational mode and serves to reverse the applications of the ground reference and the signal treatment means to the third and fourth rows of 25 electrodes and to the first, second, third, and fourth corner electrodes and maintains the electrical isolation of the first and second rows of electrodes during a fourth operational mode.

Other features of the invention will, in part, be obvious and will, in part, appear hereinafter.
The invention, accordingly, comprises the apparatus and system possessing the construction, combination of elements and arrangement of parts which are exem-plified in the following detailed description.
For a fuller understanding of the nature and features of the invention, reference should be had to the following detailed description taken in conjunction with the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a schematic representation of a one-dimensional model of an electrographic ~pparatus;
Fig. 2 is a schematic equivalent circuit model of Fig. l;
Fig. 3 is a schematic ideali~ed curve showing voltage distribution across one lS axis of the resistive layer represented in Fig. 1;
Fig. 4 is a schematic representation showing the circuit and switching components of an electrographic apparatus;
Fig. 5 is a schematic representation of timing diagrams and control sequence curves for sequential operational or data modes of the apparatus of Fig. 4;
Fig. 6 is an enlarged schematic representation of the lower right quadrant o~ a traditional graphics tablet as depicted in Fig. 4 which does not employ the improvements of the instant invention;
Fig. 7 is a representation generated using a computer model of the voltage gradients on both axes across the entire physical area of the graphics tablet quadrant depicted in Fig. 6;
Fig. 8 is a view similar to Fig. 7 of the voltage gradients on both axes across the active area shown bounded by a dashed line in Figs. 6 and 7;
Fig. 9 is a plan view of a graphics tablet employing the corner electrodes and the enhanced conductivity region of the invention described in the instant application;
Fig. 10 is a schematic representation of the lower right guadrant of the graphics tablet illustrated in Fig. 9 showing only the corner electrodes of the instant invention;
Fig. 11 is a representation derived from a computer model of the voltage gradients on both axes of the active area of the graphics tablet quadrant illustrated in Fig. 10;

~Z56~80 Fig. 12 is n schemnlic represcntntion of thc lower right qundrnnt o~ the graphics tablet sllown in Fig. 9 dcpicting botl) the corner electrodcs nnd the enhanccd cond~lctivity region of the instant invention; and Fig. 13 is a representation generated utilizing a computer model of the voltage S gradients on both axes of the entire physical area of the graphics tablet quadrant pictured in Fig. 12.

l~etailed Description In the discourse to follow, an electrographic device is described wherein the resistive surface of a digitizer or graphics tablet is excited by an n.c. source as opposed to the application of such source through a stylus or tracer or other suitable position locator. I~owever, it should be understood that, Witll the exception of the selection of excitation frequencies, the same structure and circuit as is described herein may be utilized with the latter configuration.
As a preliminary consideration of the instant apparatus, reference is mnde to Figs. 1 througll 3 wherein an idealized, one-dimensionRI model of the graphics tablet is revealed. In Fig. 1, a resistive surface or sheet, for example, formed of a layer of indium-tin-oxide is represented at 10. This surface has been deposited onto a dielectric material such QS glQSS represented at 12. ~lectrodes ~re shown coupled to the resistive layer 10 at 14 and 16. Electrode 14 is coupled with an a.c. source designated V0 from line 18, while electrode 16 is coupled to ground through line 20.
A pickup 22 which may be a stylus, tracer, or other suitable position locator ispositioned on material 12 adjacent resistive layer 10 at any given location and through capacitive coupling serves to pickup a voltage output at line 24, such voltage being labled l'Vsense". The equivalent circuit for this idealized one-dimensional model is represented in Fig. 2 where the resistive layer 10 is shown as a resister and the distance of the stylus 22 from the edge of the resistor closest to the source Vo is represented as "X" and the distance between electrodes 14 and 16 isrepresented as "D". The fraction of resistance "R" of layer 10 extending from the source of voltage excitation to the stylus at location X,may be represented by XR/D, while the distance from the location of the stylus 22 to the opposite electrode 16 may be represented as the labeled (1 - X~D~R. me corresponding idealized value for VSenSe is shown in Fig. 3 as beirlg linear, as represented by line 26. For various reasons, some of which will be discussed hereinafter such linearity is not readily achieved. To derive si~nals representing coordinate pairs representing the position of stylus 22 on the resistive layer 10, measurements of the voltageVSense are made along orthogonally disposed axes designated x and y. Through the ~ ~25618~

utilization of switching, the application of the voltage source QS through line 18 and the connection of ground as through line 20 as shown in Fig. 1 are alternately reversed for each of the x and y coordinates. With the values thus obtained for each designated x and y coordinate, a difference/sum voltage ratio is determined to 5 obtain a coordinate position signal.
Referring to ~ig. 4, an electrographic apparatus is shown generally in schematic fashion at 28. Apparatus 28 incorporates a switching device for carrying out the difference/sum ratio coordinate determining technique. In the figure, a resistive layer is shown at 30 having a rectangular shape and accessed by a position 10 locator present as a stylus or tracer 32 at some point (x,y~. The resistive layer 30 is shown having designated x+ and x- axes as well as y+ and y- axes, the intersection therebetween being essentially at the center of the rectangularly configured sheet 30.
Designating the coordinate system shown to range from +1 to -1 in both the x 15 and y directions, a signal representing any given coordinate (x, y) pair can be determined by measuring the voltage value picked up by stylus 32 under a procedure where the alternating current source or time varying excitation source initially is applied to one edge of the resistive layer 30 in one coordinate direction while ground reference to applied to the oppositely disposed edge. This procedure then is 20 reversed for the first coordinate direction and the combined readings may be used to determine one coordinate. The procedure then is carried out in the opposite coordinate sense. During the data collection procedure, one set of coordinate border regions or edges of the resistive layer 30, for example, the y+ and y- border regions are permitted to "float" in electrical isolation while the oppositely disposed 25 or, for example, the x+ and x- coordinate border regions are operated upon byalternately applying ground and the a.c. source thereto. Permitting the set of non-operating coordinate border regions to float enables the voltage gradients perpen-dicular to these regions to become substantially linear at the border regions. If the inactive border regions were held to a set potential the voltage gradients adjacent 30 these regions would be distorted.
l~pplication of the difference/sum voltage ratio to derive paired coordinate signals may be seen from the following example. In this example, the output of stylus 32 will be arbitrarily designate XPLUS when an a.c. current source is applied along the x+ coordinate border region of sheet 30 while simultQneously ground is35 applied to the opposite x- border region and the stylus output will be arbitrarily designated XMINUS when the opposite condition obtains wherein the a.c. source isapplied along the x- coordinate border region of sheet 30 while simultaneously ;

-` ~L25618~:) ground is applied to the oppositely disposed ~+ region. Similarly, YPLUS will be the arbitrarily designated output of stylus 32 when the a.c. source is applied to resistivc layer 30 at the y+ border region and ground is applied to the opposite y-region and the output signal at stylus 32 will be designated YMINUS when the a.c. current S source is applied to the y- border region of resistive layer 3û and ground is applied to the opposite y+ region. Utilizing the aforementioned difference/sum voltage ratio in conjunction with the designated signal values, paired coordinate signals for any position of the stylus 32 on surface may be derived as follows:

10 Position x= (XPLUS) - (XMINUS) (XPLUS) + (XMINUS) Position y= (YPLUS) - (YMINUS) (YPLUS) + (YMINUS) The imposition of the a.c. signals as well as application of the ground couplings to the resistive layer 30 are carried out through contacts or electrodes provided as elongate pads positioned in rows in the border regions. Conventionally such electrodes are spaced equal distance from each other and from the ends of the rows 20 and have a length approximately equal to the distance they are spaced-apart. Fig. 4 shows an array of four such electrodes along the x+ border region at 34, while an oppositely disposed array of such electrodes for the x- region is represented at 36.
Correspondingly, an array of four spaced-apart electrodes along the y+ designated border region is shown at 38 while an oppositely positioned array of electrodes along 25 the region designated y- is shown at 40.
Each of the electrodes within the array 34 thereof at the x+ border region is connected to one side of a single-pull, single-throw analog switch of an array thereof shown at 42. Similarly, each electrode of the array 36 at the x- region is connected to a corresponding single-pull, single-throw analog switch of an array30 thereof shown generally at 44. Correspondingly, each electrode within the array 38 at the y+ border region is connected to a corresponding single-pull, single-throw analog switch of an array thereof shown generally at 46, while the oppositely disposed y-border region electrodes of array 40 are each coupled to a corresponding single-pull, single-throw analog switch of an array 48 thereof. An a.c. source for 35 exciting the resistive layer 30 is represented at 50 having an output at line 52 extending through line 54 to the inputs of two single-pull, single-throw analog switches 56 and 58. The output side of switch 56 is coupled to a bus 60a which, in turn, extends in common to the inputs of each analog switch within arrays 46 and 42.
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~256~80 , ~

Correspondingly, the output of ~switch 58 extends ViQ bus componcnts 62a nnd 62b to the common inputs of the analog swilches witllin arrays 44 and 48. t~ ground which must be established tor operating tlle electrographic apparatus 28 incorporating the rcsistive layer 30 is derived from line 64 which extends through line 66 to the inputs 5 of two single-pull, single-throw analog switchcs 68 nnd 70. The output of switch 68 is coupled to bus componcnt 60b leading through bus 60a in common to the hlputs of the annlog switches within arrays 42 nnd 46. Similarly, the output of unnlog switch 70 is coupled to bus component 62b which is connected in common to the inputs ofthe ~nalog switches within arrays 44 and 48. All of the above-described analog 10 switches are actuated by logic compatible voltflge signals developed at the output of a central control including a microprocessor as represented at block 72. Thus, by approprinte signnl actuation through lines 74 and 76 labeled "X Control" (first coordinatc logic signal), all of tlle signals nlong tlle x nxis as represcnted at arrays 42 and 44 mfly be closed or opcned simultAneously. In similar fashion, the control 72 15 m~y assert nn actuating signal along lines 78 and 80, labeled "Y Control" (second coordinate logic signal), to simultnneollsly open or close all of the switches along the y axis as reprcscnted at arrays 46 and 48. Control 72 also may nssert simultaneous actuating and deactuating signals from along lines 82 nnd 84, Iflbeled "Plus Control"
(first directional logic signal) to switches 56 and 70. By such actUQtion, the a.c.
20 source may be npplied to bus 60a and thence to the inputs of the y+ switch array 46 and to the inputs of the x~ swltch nrrQy 42 while the oppositely disposed y- switch nrray 48 and x- switch arrQy 44 rnay be coupled to ground simultaneously throughbus 62b. In a similflr fnshion, the control 72 may assert an actuating signnl along lines 86 flnd 88 labeled "Minus Control" (second directional logic signal) to effect tlle 25 actuntion Or switclles 58 and 68 nnd passnge of signals from a.c. source 50 through buscs G2n nnd 62b to the inputs of tlle x- switch array 44 nnd the y-switch array 48, whi1e, simultnneously, connccting the inpllts of tlle y~ switch array 46 and the x~
switch array 42 with ground througll bus 60b. l~y sequentially nctunting the "X
Control" and "Y Control" and alternnting the "Plus Control" and the "Minus Control"
30 during the time the x and y controls nre actuated, the n.c. source will be applied scqucntially first to the switch arrays 42 and 44 in the x coordinate direction and subscqucntly to tlle switch arrays 46 and 48 in ihe y coordinate direction. ln this regard, reference is madc to Fig. 5 wherein the moqcs of opcrntion for cnrrying out one coordinate mcasurement cycle are illustratcd in timing dingram form. Thus, it 35 may be observed thQt x control lincs 74 and 76 provide an "on" or actunting signal reprcsented nt 90 simultaneously with a corrcsponding "on" signal at plus control lines 82 and 84 as reprcsented at 92 to develop to an XPLUS signal as reprcsented at , ~2s6lsa curve 94 during a first quarter intervnl of one measurement cycle. Similarly, an"on" condition for the x control signal as represented at 90 generated in combination with A corresponding "on" actuating signal at minus control lines 86 and 88 as represented at 96 causes an XMINUS signal to be developed. This signal is 5 represented at curve 98 nnd represents the second quarter of the measurement cycle. The third quarter of the measurement cycle is shown developing the YPl.USsignal shown at curve 100 by the assumption of an "on" or actuating status of the y control lines 78 and 80 as represented at 102. Simultaneously, with this "on" status as represented at 102, the plus control lines 82 and 84 carry an actuating signal as represented by the "on" status shown at 92. Finally, the YMINUS signal 104 is developed for the fourth quarter of the measurement cycle with the assertion of an "on" status at the y control lines 78 and 80 as represented at 102 in combination with a corresponding "on" condition at the minus control lines 86 and 88 as represented at 96. With the arrangement thus depicted, the a.c. source 50 is applied first to one border region and then to the opposite region in the x coordinate direction. Subsequently, the same arrangement is provided for the y coordinate direction. The switches in the border regions of the coordinate direction not being operated remain open to permit that pair of border regions to "float" in electrical isolation and thereby reduce distortion of voltage gradients in those regions.
Utilizing the coordinate determining procedure described in conjunction with Fig. 4 the measured position of the stylus 32 (signal domain) on the resistive layer 30 would correspond substantially with the actual position of the stylus 32 (physical domain) if the voltage gradients (change of voltage per unit of distance) across the resistive layer 30 were uniform. It has been found that for the graphics tablet as configured as shown in Fig. 4 the voltage gradients are substantially uniform in the central region of the resistive layer 30. Whatever non-linearity of the voltage gradients that exists within that region can be accommodated through the imposi-tion of an error correction procedure which employs an algorithm. With this procedure a relatively high degree of accuracy or correspondence between the measured position and the physical position of the stylus 32 may be obtained.
However, a significant degree of voltage gradient distortion occurs at the border regions and corner regions of the electrographic apparatus 28. This distortion reduces the accuracy of the device and requires the algorithm in the error correction technique to be more complex which reduces the computational speed ofthe device. For those applications where the active or usable area of the resistive layer 30 on the device can be spaced a relatively large distance from the electrodes in the border regions and the corners of the border region, voltage gradient ``1256~80 distortion in these rcgions does-not prescnt u serious problem. A bufîer region or zone having sutficient widlh to provide an active area with a minimal amount of voltage gradient distortion can be intcrposed between the electrodes and the active area. llowever, for those npplications of a grnphics tablet which require that the Sactive area extend closely to the outer edge of the device the imposition of a relatively wide buffer region to avoid the effects of voltage grndient dislortion in the nctive aren becomes unncceptnble.
The distortion or non-uniformity of the voltnge gradients in the corner and border regions of the electrogrnphic device or graphics tablet 28 depicted in ~ig. 4 10may be seen by reterring to Figs. 6-8. Looking to Fig. 6 which is an cnlarged schemntic representation of the lower right quadrant of graphics tablet 28, it may be observed Ihat tlle qundrnnt includes two electrodes xl and x2 in the x+ elcctrode array 34 at the x~ border region and two clcctrodcs yl nnd y2 in the y-elcctrodearray 40 at the y- border region. Thcse elcctrodcs have been identified by the same 15alphanumeric designntions in Fig. 4. Returning to Fig. 6 an active area bounded by line 126 is sllown on resistive layer 30 at 120. A relatively wide buffer zone or region 122 detined on layer 30 surrounds active area 120 and spaces its from theelectrodcs xl, x2, yl, nnd y2.
A plot of the voltage gradients across one qundrant of the graphics tablet 20configured ns described in connection witll Fig. 4 and reprcsented in Fig. 6 which has been generated by A computer model may bc seen in Fig. 7. It shollld be noted that a plot of the nctual mensurements of voltage gradicnts across botll axes of the graphics tablet configured as described in connection with Fig. 4 corresponds with the plot derived from the computer model. It should also be noted 25tl~at the voltage gradients illustrated in Fig. 7 for one quadrant of a graphics tablet nre representative of these gradients at corresponding locations in the other quadrnnts of the tablet.
Each of the solid horizontnl and vertical lines shown on the grid in Fig. 7 represents the voltage along a line parallel to nn edge of the tablet. lf the voltage 30grndients across the rcsistive sheet 30 of graphics tnblet 28 were uniform or distortion free each of thcse lines would be straight and would be pnrnllcl to the otller lines extending in the same dircction. /~dditionally, the distance betwcen cnch ot the lines would be equal. It is apparent that theivoltages on resistive layer 30 undergo scvere distortion in thc x~ and y- bordcr rcgions nround and between the35electrodes xl, x2, yl, and y2 and at the corner region 130. Additionally, in the corner region 130 between electrodes x1 and yl the voltnge lines tend to merge.
This means that diflerent locations in the corner region 130 of tablet 28 will have , ~

~Z56180 the same or nearly the same voltage v~lue. ~s is apparent discrete voltage va]ues on the surface Or resistive layer 30 are required to achieve an effective readout of tlle coordinates Or the location on tablet 28.
As previously mentioned a non-usable buffer region may be nrbitrarily dcsignated between the electrodes in the border regions and an active area to provide an active area in whicll voltage gradient uniîormity is adequaie to generate rcndouls having n dcsircd systeln accuracy by employing error correction techniqucs sucll as described in U.S. Pat. No. 4,650,926, noted hereinbefore. In a convcntional eraphics tablet it has bcen dctcrmined that a bufrer region having a width of about 1.5 inchcs is necessary to obtain tl~e reguircd accuracy Or the system.
Turrling to l; ig. 8 wl~ich is a vicw similnr to Fig. 7 but reprcsents only thosc voltage grndicnt lincs within the active area 12~ bounded by line 126 nnd spnccd npproxi-mately t.5 inches from the electrodes xl, x2, yl, nnd y2, it may be observed that thc voltage grndient lines within area 126 are relatively uniform. l he greatestdistortion of the voltnge gradient lincs for this area occurs in corner region 130. It has been found tl~at voltage distortion in the corners Or the active arca limit the nccuracy of a graphics tablct. Conscquently, the accuracy of the tablet will be irnproved if the distortion Or voltages in tlle corner regions can be reduced to where it is no greater than the distortion of voltages in the center portion of the active aren.
It has been determined that the rnore severe distortion of the voltage gradicnts in thc corner regions of a grnphics tablet can be reduced to nn important extcnt by placing an electrode ut the end of each of the rows of electrodes in the border rcgions. Rererring to ~ig. 9, a graphics tablet 132 having a rcsistive layer 142 is depicted in which rows of electrodes 134, 136, 138 and 140 are located in tl~e respective x~, x-, y~, and y-border regions.l,ayer 142 is formcd by a process wherein an indium-tin-oxide (ITO) Inyer is formed upon a dielectric supporting substrate, for example glass. The outer border regions of the cornposite of glass and Iro then is etcl~ed to provide an Iro region having a rectullgular peripllcral extent larger than tlle nnticipated active area by about .5 inches. Tllis leaves a gluss region suited for supporting printcd circuit Icnds rcpresented by an array 144. TheselcAds may be silk screened over the glass portion using, for example, a silver ink. Note that one each of the leads exten(3s to tlle electrodes within tlle IrO re6ion.
I~n electrode 15û is positioned in tlle lower right corner 152 of tablet 132 at the intersection of electrode row 134 in the x+ border region and electrode row 14~
in tlle y- border region. Similarly, an electrode 154 is located in the lower left corner 156 of tablet 132 at the intersection of electrode rows t4~ and 136 in the , ~

rcspcctive y- and x- border rcgiPns. I.ikewise, nn eleclrode 158 is positioncd at the upper right corner 160 of tflblet 132 nt lhe intersection of the row of electrodcs 134 in the x~ border region nnd the row of electrodes 138 in the y+ border region and an electrode 162 is locnted in the upper left corner 164 of tablet 132 at the intersection of tlle electrode rows 136 and 138 in the respective y- nnd y~ border regions. I~nch of the corner electrodes 150, 154, 158, and 162 l1aS a pnir Or perpendicular legs which are joined toget1-er, which nre of equal length nnd which extend into ndjacent rows of elcctrodes. I:or exnmple, one leg 150n of electrodé 150 extends into electrode row 134 while the other leg 150b extends into electrode row 140. Similarly, electrode 154 has one leg 154a which extends into electrode row 136 and another leg 154b whicll extends into electrode row 140. Likewise, one leg 158a of elcctrode 158 extcnds into electrode row 134 and the other leg 158b extends into electrode row 138. ~dditionally, clectrode 162 has one leg 162b which extcnds into electrode row 138 and fl second leg 162a which extends into clcctrode row 136.
In connection with the dcscription of tl-e operation of electrogrnphic appnrntus28 shown in Fig. 4 it may be recalled that when ~he pairs of electrodes in one of the x or y border regions were activated the opposite pair of electrodcs were allowed to float in electrical isolation. The description of operation of electrographic apparatus 28 nlso applies to tl-e operation of graphics tablet 132 witll one exception.
! ao 13ecnuse the corner electrodes 150, 154, 158, and 162 extend into adjncent x and y bordcr regions these electrodes are not permitted to float. One of the two clectrode rows into which the legs of the corner electrodcs extend is always active nnd these electrodes must assume the state of the active or on electrode row.
ConscglJently, corner electrodes 150, 154, 158, and 162 will alwuys have either an n.c. source, signnl trentment menns or a ground reference applied to them. For this rcason, tlle corner electrodes 150, 154, 158, and 162 must be connected to SWitcllCS which are opcrnted by n control reprcsentcd nt 72 in Fig. 4 indcpendently of tl!e operation of any of the rows of .electrodes 134,136,138,and 140. Because the corner electrodes do not flont it has been found desirnble to make thc legs of these electrodes shorter thnn the lengtll of other electrodes in tl-e rows. For instance, if each leg of a corner electrode 150, 154, 158j and 162 is three units long, it ispreferred to hnve the remaining electrodes in the rows 134, 136, 138, nnd 140 six units long. Further, it is preferred to hnve each of tljlC electrodcs in a row including the corner elcctrodcs spaccd bctwccn five nnd six units npnrt. ~eturning , ~4 125618~

-14a-to Fig. 4, the corner electrodes within the y-region 40 are shown addressed by switches 200 and 202 which are controlled via lines 204 and 208 from control function 72. The switches 200 and 202 are seen coupled with line 62b for appropriate assertion of ground or a.c. source.
In similar fashion, the corner electrodes within the y+
region 38 are controlled from switches 210 and 212 which, in turn, are controlled via lines 214 and 216 from control function 72. Switches 210 and 212 are, in turn, connected to the line 60a for assertion of appropriate ground or source through them to the corner electrodes.
A schematic representation of the lower right quadrant of the graphics tablet 132 of the instant invention may be seen in Fig. 10. In this diagram, a region 172 has been designated as a buffer region between an active area 170 having a boundary .~ l ~ .

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~256~80 174 shown in dashed lines, and the rows of electrodes 134 and 140. For comparison purposes, the width of buffer region 172 has been made equal to the width of buffer region 122 described in connections with Figs. 6-8. A computer model of the voltage gradients across both axes of the active area 170 of tablet 132 may be seen in Fig. 11. lt should be observed that with the addition of electrodes at the ends of the electrode rows the level of voltage gradient distortion in the corner region of 152 has been reduced to where that distortion is no greater than the distortion of voltage gradients in the central portion of the resistive layer 142. When the plot of the voltage gradients in the active area 170 of the graphics tablet 132 of the instant invention as shown in Fig. 11 is compared with a similar plot of the active ar~a 120 of the conventional tablet 28 which does not employ corner electrodes as " shown in Fig. 8, it may be seen that these electrodes significantly reduce the level of distortion of voltage gradients in the corner regions.
Although the electrodes 150, 154, 158, and 162 at the ends of the electrode rows 134, 136, 138, and 140 are illustrated as having pairs of joined prependicular legs, which extend into adjacent rows of electrodes, the advantages accruing from placing electrodes at the ends of the rows may be obtained from an alternate configuration of the electrodes. For example, the perpendicular legs of the corner electrodes 150, 154, 158, and 162 could be detached and spaced a short distance from each other. Consequently, in the second or alternate configuration of electrodes, there is a single electrode at each end of the electrode rows 134, 136, 138 and 140 and the electrodes at the ends of one row do not en~age the electrodes at the ends of an adjacent row. In the second configuration the electrodes at each end of the rows assume the same state as the other electrodes in the row.
Consequently, the electrodes at the ends of the rows assume a floating state as well as an operating state. It has been found- preferable to provide the L-shaped corner electrodes described in connection with Figs. 9 and 10 because fewer leads and fewer switches are required for this configuration.
In those applications where the active area of the graphic tablets must extend closely to the outer edge of the device, such as where the resistive layer is applied to the surface of a CRT, the conventional practice of having a relatively wide, non-usable buffer region on the surface of the resistive layer between the electrodes and the active area is unacceptable. It has been discovered that the active area of a graphics tablet may be positioned in close proximity with the rows of electrodes in the border regions and also have a voltage gradient uniformity sufficient to generate positional readouts of a desired system accuracy by carrying out a controlled alteration of the resistivity of the material in the region between the 1256~8Q

electrodes and thc active area. Looking ngain to Fig. 9, the active area boundary 174 of graphics tablet 132 is shown separated from the electrode rows 134, 136, 138, and 140 by a very narrow enhanced conductivity zone or region 180. In conventional graphics tablets the bulk resistivity, i.e. the resistance per square unit 5 of the resistive layer is uniform throughout the entire surface of the tablet.However, in the graphics tablet 132 of the instant invention the bulk resistivity of that portion of resistive layer t42 which is designated the ~ctive area 170 is different from the bulk resistivity of that portion of resistive layer 142 which is designated the enhanced conductivity region 180.
In tablet 132, active area 170 has a bulk resistivity of approximately 600 ohms per square, i.e., any square unit of active area 170 such as one inch by one inch has a resistance of 600 ohms. In contrast thereto, the enhanced conductivity region 180 has a bulk resistivity of Qpproximately 60 ohms per square, i.e., the resistance of region 180 is one tenth the resistance of active area 170. Stated another way the conductivity of region 180 is ten times greater than the conductivity of active area 170. It has been found that the voltage gradients at the boundary 174 of active area become more uniform as the conductivity of enhanced conductivity region 180 is increased. Thus, the active area 170 may be moved closer to the rows of electrodes 134, 136, 138, and 140 and the voltage gradients at the boundary 174 can be madeao sufficiently uniform to enable a desired system accuracy to be obtained by increasing the conductivity of region 180. It appears that if the width of the conductivity region 180 is changed by the same percentage and in inverse proportion to the change in conductivity of the conductivity region 180 of the active area, the voltage gradients at the boundary 174 of the active area 170 remain unchanged. In 2S graphics tablet 132 the conductivity region 180 has a width of approximately .2 inches.
The effect of increasing the conductivity of region 180 by a factor of ten on voltage gradient uniformity at the boundary 174 of active area 170 may be seen by referring to Figs. 12 and 13. Fig. 12 is a schematic representation of the lower right quadrant of graphics tablet 132 and Fig. 13 is a representation derived from a computer model of the voltage gradients on both axes of the entire physical area of the quadrant pictured in Fig. 12. From Fig. 13 it is apparent that the voltage gradients at the boundary 174 of active area 170 are remarkably uniform especially when it is recalled that boundary 174 is spaced approximately .2 inches from therows of electrodes 134 and 140. When Fig. 13 is compared with Fig. 8 which illustrates the voltage gradients along the boundary 126 of an active area 120 spaced approximately 1.5 inches from the rows of electrodes in a conventional tablet it may 1256~80 be seen tllat the enhanced conductivity region has permitted the active area to be placed in close proximity to tlle rows of electrodes and substantifllly increased the uniformity of the voltage gradients at the boundary of the Hctive area. In fact, the uniformity of the voltage gradients at boundary 174 of active area 170 has become substantially equivalent to lhe uniformity of the voltage gradients in the center of active area 170.
It seems reasonable to assume it would be desirable to further increase the conductivity of the conductivity region. Ilowever, the adv~ntages otherwise accruing tend to lessen when the conductivity of the enhanced conductivity region becomes grenter tllan about ten times that of the active area. It has been learned that aæ the conductivity of this region increascs, the powcr consumption of the grQphics tnblet also increases. It is theorized that the buffer region of a conventional graphics tablet can be considered as comprised of a large number ofdiscrete resistors. Such a region having a certain width would have a given amount of inter electrode resistance. I~s the region becomes wider, tlle nmount Or inter clcctrode resistance becomes less, somewhat similar to adding resistors in parallel.
By nnalogy lowering the bulî< resistivity or increasing the conductivity of tlleenhanced conductivity region also reduces the inter electrode resistance and hasthe same effect ns making tlle buffer region of a conventional tablet wider. Of course, if an infinite number of resistors are udded in pnrallel to Q circuit the resistance becomes very sm~ll and a short circuit condition results. Similarly, ir the bulk resistivity of the conductivity region is made too low, power consumption becomes excessive and ultimately a short circuit will occur. Thus, logical design tradeoffs will occur to the investigator.
In graphics tablet 132 the conductivity of region 180 was increaied by depositing a greater amount of indium-tin-oxide on the resistive layer 142. Conse-quently, the enhanced conductivity region was formed from the same material as the resistive layer o~f the active area. Of course, the conductivity region could be manufactured from a different material than that of the resistive layer for the active area.
From the above it may be seen that graphics tablets having increased accuracy and having a diminished distance betwcen the electrodes in the active areas but exhibiting most acceptable voltnge grudient unif(ormities may be achieved in accordance with the teachings of the instant invention.
Since certain changes may be made to the above-described apparatus without departing from the scope of the invention herein, it is intended that all mattercontained in the description thereof or shown in the accompanying drawings shall be interpreted QS illustrative and not in a limiting sense.

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Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. An electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate and having an active area extending in an x-coordinate direction between first and second spaced apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges;
a first row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said first edge;
a second row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said second edge;
a third row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said third edge;
a fourth row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said fourth edge;
first corner electrode means coupled with said resistive layer at the juncture of said first and third rows;
second corner electrode means coupled with said resistive layer at the juncture of said first and fourth rows;
third corner electrode means coupled with said resistive layer at the juncture of said second and third rows;

fourth corner electrode means coupled with said resistive layer at the juncture of said second and fourth rows;
a position locator adjacent said active area;
a time varying excitation source of select frequency;
a ground reference;
switching means for applying said ground reference to said first row of electrodes and to said first and second corner electrodes and simultaneously applying said source to said second row of electrodes and to said third and fourth corner electrodes and electrically isolating said third and fourth rows of electrodes during a first operational mode, for reversing said applications of said ground reference and said source to said first and second rows of electrodes and to said first, second, third, and fourth corner electrodes and maintaining said electrical isolation of said third and fourth rows of electrodes during a second operational mode, for applying said ground reference to said third row of electrodes and to said first and third corner electrodes and simultaneously applying said source to said fourth row of electrodes and to said second and fourth corner electrodes and electrically isolating said first and second rows of electrodes during a third operational mode, and for reversing said applications of said ground reference and said source to said third and fourth rows of electrodes and to said first, second, third and fourth corner electrodes and maintaining said electrical isolation of said first and second rows of electrodes during a fourth operational mode; and control means for collecting a signal from said position locator.
2. An electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate and having an active area extending in an x-coordinate direction between first and second spaced apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges;
a first row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said first edge;
a second row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said second edge;
a third row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said third edge;
a fourth row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said fourth edge;
first corner electrode means coupled with said resistive layer at the juncture of said first and third rows;
second corner electrode means coupled with said resistive layer at the juncture of said first and fourth rows;
third corner electrode means coupled with said resistive layer at the juncture of said second and third rows;
fourth corner electrode means coupled with said resistive layer at the juncture of said second and fourth rows;

a time varying excitation source of select frequency;
a position locator adjacent said active area for inputting said source at a select position in said active area;
signal treatment means;
a ground reference;
switching means for applying said ground reference to said first row of electrodes and to said first and second corner electrodes and simultaneously applying said signal treatment means to said second row of electrodes and to said third and fourth corner electrodes and electrically isolating said third and fourth rows of electrodes during a first operational mode, for reversing said applications of said ground reference and said signal treatment means to said first and second rows of electrodes and to said first, second, third, and fourth corner electrodes and maintaining said electrical isolation of said third and fourth rows of electrodes during a second operational mode, for applying said ground reference to said third row of electrodes and to said first and third corner electrodes and simultaneously applying said signal treatment means to said fourth row of electrodes and to said second and fourth corner electrodes and electrically isolating said first and second rows of electrodes during a third operational mode, and for reversing said applications of said ground reference and said signal treatment means to said third and fourth rows of electrodes and to said first, second, third and fourth corner electrodes and maintaining said electrical isolation of said first and second rows of electrodes during a fourth operational mode.
3. An electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate and having an active area extending in an x-coordinate direction between first and second spaced apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges;
a first row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said first edge;
a second row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said second edge a third row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said third edge;
a fourth row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said fourth edge;
one electrode in said first row of electrodes being positioned at the end of said first row adjacent said third row and another electrode in said first row being positioned at the opposite end of said first row adjacent said fourth row;
one electrode in said second row of electrodes being positioned at the end of said second row adjacent said third row and another electrode in said second row being positioned at the end of said second row adjacent said fourth row;
one electrode in said third row of electrodes being positioned at the end of said third row adjacent said first row and another electrode in said third row being positioned at the opposite end of said third row adjacent said second row;

one electrode in said fourth row of electrodes being positioned at the end of said fourth row adjacent said first row and another electrode in said fourth row being positioned at the opposite end of said fourth row adjacent said second row;
a position locator adjacent said active area;
a time varying excitation source of select frequency;
a ground reference;
switching means for applying said ground reference to said first row of electrodes and simultaneously applying said source to said second row of electrodes and electrically isolating said third and fourth rows of electrodes during a first operational mode, for reversing said applications of said ground reference and said source to said first and second rows of electrodes and maintaining said electrical isolation of said third and fourth rows of electrodes during a second operational mode, for applying said ground reference to said third row of electrodes and simultaneously applying said source to said fourth row of electrodes and electrically isolating said first and second rows of electrodes during a third operational mode, and for reversing said applications of said ground reference and said source to said third and fourth rows of electrodes and maintaining said electrical isolation of said first and second rows of electrodes during a fourth operational mode; and control means for collecting a signal from said position locator.
4. An electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate and having an active area extending in an x-coordinate direction between first and second spaced apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges;
a first row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said first edge;
a second row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said second edge;
a third row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said third edge;
a fourth row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said fourth edge;
one electrode in said first row of electrodes being positioned at the end of said first row adjacent said third row and another electrode in said first row being positioned at the opposite end of said first row adjacent said fourth row;
one electrode in said second row of electrodes being positioned at the end of said second row adjacent said third row and another electrode in said second row being positioned at the end of said second row adjacent said fourth row;
one electrode in said third row of electrodes being positioned at the end of said third row adjacent said first row and another electrode in said third row being positioned at the opposite end of said third row adjacent said second row; and one electrode in said fourth row of electrodes being positioned at the end of said fourth row adjacent said first row and another electrode in said fourth row being positioned at the opposite end of said fourth row adjacent said second row;
a time varying excitation source of select frequency;
a position locator adjacent said active area for inputting said source at a select location in said active area;
signal treatment means;
a ground reference; and switching means for applying said ground reference to said first row of electrodes and simultaneously applying said signal treatment means to said second row of electrodes and electrically isolating said third and fourth rows of electrodes during a first operational mode, for reversing said applications of said ground reference and said signal treatment means to said first and second rows of electrodes and maintaining said electrical isolation of said third and fourth rows of electrodes during a second operational mode, for applying said ground reference to said third row of electrodes and simultaneously applying said signal treatment means to said fourth row of electrodes and electrically isolating said first and second rows of electrodes during a third operational mode, and for reversing said applications of said ground reference and said signal treatment means to said third and fourth rows of electrodes and maintaining said electrical isolation of said first and second rows of electrodes during a fourth operational mode.
5. An improved graphics tablet for an electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate defining an active area extending in an x-coordinate sense between and inwardly of first and second spaced-apart edges and extending in a y-coordinate sense between and inwardly of third and fourth spaced-apart edges;
a plurality of electrodes positioned intermediate said active area and said edges; and an enhanced conductivity region intermediate said active area and said electrodes.
6. The improved graphics tablet of claim 5 in which the bulk resistivity of said enhanced conductivity region is different from the bulk resistivity of said active area.
7. The improved graphics tablet of claim 6 in which the bulk resistivity of said enhanced conductivity region is less than the bulk resistivity of said active area.
8. The improved graphics tablet of claim 7 in which the bulk resistivity of said active area is at least five times greater than the bulk resistivity of said enhanced conductivity region.
9. The improved graphics tablet of claim 5 in which said resistive layer and said enhanced conductivity region are formed from the same material.
10. The improved graphics tablet of claim 5 in which changing the width of said enhanced conductivity region in direct proportion to and by the same percentage as the bulk resistivity of said enhanced conductivity region is changed with respect to the bulk resistivity of said resistive layer causes the level of signal distortion at the interface of said resistive layer and said enhanced conductivity region to remain substantially constant.
11. An improved graphics tablet for an electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate and having an active area extending in an x-coordinate direction between first and second spaced apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges;
a first row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said first edge;
a second row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said second edge;
a third row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said third edge;
a fourth row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said fourth edge;
first corner electrode means coupled with said resistive layer at the juncture of said first and third rows;

second corner electrode means coupled with said resistive layer at the juncture of said first and fourth rows;
third corner electrode means coupled with said resistive layer at the juncture of said second and third rows fourth corner electrode means coupled with said resistive layer at the juncture of said second and fourth rows; and an enhanced conductivity region intermediate said active area and said electrodes.
12. The improved graphics tablet of claim 11 wherein the bulk resistivity of said enhanced conductivity region is different from the bulk resistivity of said active region.
13. An electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate and having an active area extending in an x-coordinate direction between first and second spaced apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges;
a first row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said first edge;
a second row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said second edge;
a third row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said third edge;

a fourth row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said fourth edge;
an enhanced conductivity region intermediate said active area and said electrodes;
first corner electrode means coupled with said resistive layer at the juncture of said first and third rows;
second corner electrode means coupled with said resistive layer at the juncture of said first and fourth rows;
third corner electrode means coupled with said resistive layer at the juncture of said second and third rows;
fourth corner electrode means coupled with said resistive layer at the juncture of said second and fourth rows;
a position locator adjacent said active area;
a time varying excitation source of select frequency;
a ground reference;
switching means for applying said ground reference to said first row of electrodes and to said first and second corner electrodes and simultaneously applying said source to said second row of electrodes and to said third and fourth corner electrodes and electrically isolating said third and fourth rows of electrodes during a first operational mode, for reversing said applications of said ground reference and said source to said first and second rows of electrodes and to said first, second, third, and fourth corner electrodes and maintaining said electrical isolation of said third and fourth rows of electrodes during a second operational mode, for applying said ground reference to said third row of electrodes and to said first and third corner electrodes and simultaneously applying said source to said fourth row of electrodes and to said second and fourth corner electrodes and electrically isolating said first and second rows of electrodes during a third operational mode, and for reversing said applications of said ground reference and said source to said third and fourth rows of electrodes and to said first, second, third and fourth corner electrodes and maintaining said electrical isolation of said first and second rows of electrodes during a fourth operational mode; and control means for collecting a signal from said position locator.
14. An electrographic system comprising:
an electrically insulative substrate;
a resistive layer supported upon said insulative substrate and having an active area extending in an x-coordinate direction between first and second spaced apart edges and extending in a y-coordinate direction between third and fourth spaced-apart edges;
a first row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said first edge;
a second row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said second edge;
a third row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said third edge;

a fourth row of discrete, spaced-apart electrodes electrically coupled with said resistive layer intermediate said active area and said fourth edge;
an enhanced conductivity region intermediate said active area and said electrodes;
first corner electrode means coupled with said resistive layer at the juncture of said first and third rows;
second corner electrode means coupled with said resistive layer at the juncture of said first and fourth rows;
third corner electrode means coupled with said resistive layer at the juncture of said second and third rows;
fourth corner electrode means coupled with said resistive layer at the juncture of said second and fourth rows;
a time varying excitation source of select frequency;
a position locator adjacent said active area for inputting said source at a select position in said active area;
signal treatment means;
a ground reference; and switching means for applying said ground reference to said first row of electrodes and to said first and second corner electrodes and simultaneously applying said signal treatment means to said second row of electrodes and to said third and fourth corner electrodes and electrically isolating said third and fourth rows of electrodes during a first operational mode, for reversing said applications of said ground reference and said signal treatment means to said first and second rows of electrodes and to said first, second, third, and fourth corner electrodes and maintaining said electrical isolation of said third and fourth rows of electrodes during a second operational mode, for applying said ground reference to said third row of electrodes and to said first and third corner electrodes and simultaneously applying said signal treatment means to said fourth row of electrodes and to said second and fourth corner electrodes and electrically isolating said first and second rows of electrodes during a third operational mode, and for reversing said applications of said ground reference and said signal treatment means to said third and fourth rows of electrodes and to said first, second, third and fourth corner electrodes and maintaining said electrical isolation of said first and second rows of electrodes during a fourth operational mode.
15. An electrographic system as defined in claim 3, 4 or 5, wherein said resistive layer is provided as a coating of indium-tin-oxide having a thickness selected for exhibiting a resistivity of at least about 600 ohms per square, said system further including an enhanced conductivity region intermediate said active area and said electrodes, said region being formed of a layer of indium-tin-oxide supported by said substrate and having a thickness selected for exhibiting a resistivity of about 60 ohms per square.
CA000509172A 1985-06-07 1986-05-14 Electrographic apparatus Expired CA1256180A (en)

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US06/742,734 US4649232A (en) 1985-06-07 1985-06-07 Electrographic apparatus
US742,734 1985-06-07

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US4649232A (en) 1987-03-10
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