CA1118072A - Graphic digitizer - Google Patents

Abstract

Abstract The invention relates generally to apparatus for translating the position of a writing instrument into electrical signals for transmission to a remote location such that the position and corresponding movements of the writing instrument may be recreated. Thus, the apparatus includes a grid drive multiplexer coupled between a clock and X and Y grid lines upon a surface. A
system control is coupled between the clock and the grid drive multiplexer for controlling the energization of the grid lines by the grid drive multiplexer means. Sample and filter detector circuits are coupled between a cursor including a coil movable on the grid and system control for relating cursor position to time in a linear mode. Means are provided for coupling X and Y
counters to the system control whereby gated clock pulses are accumulated in the X and Y counters to measure the position of the cursor on the grid. By comparison with prior art devices, the apparatus has a number of desirable features and advantages, including the facility for determining the position of the cursor in a continuous linear fashion by using accurate electrical interpolation techniques to determine position between grids.

Description

This invention relates generally to the determination of the physical position or coordinate determination on a surface by employing a cursor embodying a coil with relation to a grid of parallel conductors and more specifically to determining the position of the cursor in a continuous linear fashion.
Apparatus for translating the position of a writing instrument into electrical signals Eor transmission to a remote location such that the position, and corresponding movements, of the writing instrument may be recreated, are well known in the art. Thus, drawings, manuscripts, or the 10 like, may be reproduced at remote locations. .~mong the more sophisticated prior art devices, are those in which movement of the writing instrument in the X and Y coorclinates are sensed by electromagnetic means, or the like, ancl eacll sensed dimension is translated into a s;gnal capable oE transmissLon. X ancl Y
coordinate positlonal information clerived in the tra-litional manller may provicle inputs to data processing apparatus such as computers, remote data terminals and special systems for processing coordinate data.
; Some ob~jectlons to ~some oE the l~nown nrt apl)aratus are lLmited resolvillg I power, detrimental environmental effects, sensitivity to adjustment and instability and lack of accuracy to the degree ~hich would be desirable. A
20 number of other problems exist in these ~no~m systems including the need Eor a high density of grid wires for comparable performance and more complex circuits. As an example, most of these systems are both amplitude sensitive and phase sensitive, ~hich places strict limitations on the inputs to the system. Another problem is that the spacing of the grid lines is extreme]y critical and very little variation is allowable. ~ccordingly, manufacture of the grid tablet is relatively expensive. A further problem relates to the criticality of a coil diameter and the necessity of the position of the sensor being substantially parallel to the grid. Yet another problem Witll the known . ~

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sensors is the ~act that the cursor cannot be removed and replaced during a single operation, but must be initiated from the start if it is so removed.
Accordingly, lt is an object oE the present invention to provide apparatus which, at least to a substantial degree, avoids the disadvantages of prior art apparatus discussed above.
Thus, according to the present invention there is provided apparatus for determining the position of a selected point on a grid having X and Y grid lines comprising:
clock means;
grid drive multiplexer means coupled between said clock means and said grid lines;
control means coupled between said clock means and said grid drive multiplexer means for controlling the energization of said grid lines by said grid drive multiplexer means;
cursor means including a coil moveable on said grid;
sample and filter detector means coupled between said cursor and said control means for relatLng cursor pOSitiOIl to tLme in a lLnear mode;
X and Y counter means; and means for coupling said X and Y counter means to said control means whereby gated clock pulses are accumulated in said X and Y counter means to measure the position of said cursor on said grid.
The invention will now be described further by way of example only and with reference to the accompanying drawings, wherein:
Figs. 1 and 2 are diagrams which clarify the mathematics use to derive a representation of the complex cursor signal;
Fig. 3 is a graph of the functions H(x,t) and f(t) derived in the cursor signal analysis;
Fig. 4 is a block diagram of the basic components of the present ~.

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i invention;
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Fig. 5 is a schematic diagram of a preEerred embodiment of the present invention;
Fig. 6 is a schematic diagram of the constant current grid drive multiplexer and grid tablet;
1 Fig. 7 is an exploded sectional view of the grid board; and Fig. 8 are graphical representations of the signal outputs at various points in the system of Fig. 5.
The basic principles of the invention can be broadly described in the ~ 10 context of a coordinate digitizing system in which a positioning cursor is ¦ moved over a surface with parallel wires for each axis with the wires perpendicular to the axis. The wires may be hand or machine layed, printed or etche.d on fiberglass printed circuit board, glass or other suitclble stable substrate.
The main feature of this invention .is that an electromagnetic Eield or wave front is generated by sequentially scanning or stepping down the grid in incremental steps, is made to appear to be travelling down the grid in cremental steps and is made to appear do~m the ~rid at a uniformed controlled rate as it passes the cursor coil. Because this wave can be made to appear to be travelling at a highly uniform rate down the grid, a simple time measurement can be made to determine the position of the cursor over that grid.
This invention uses several well known principles to accomplish this task. They are (1) that when a coil is placed near a conductor conducting an AC signal, the closer the coil is to the conductor, the greater the energy transfer; (2) that when a conductor is excited first on one side of a coil in a given phase and then on the other side, the respective signals piclced up by thecoil will be 180 out of phase.
A less obvious principle involves detecting or interpolating a reference ' ~ 3 -~ ~, :
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~Li8~'~2 level signal or "STOP" signal, which is linearly related to time, ~rom the ! cursor coil signal envelope when a timed or controlled wave, generated by successively activating grid lines, is made to pass from one end to the other i end of a grid network and, therefore, from one side to the other side of the cursor coil. In this invention, this is accomplished in a unique and linear fashion by detecting the null (see Fig. 8B) in the envelope of the cursor signal each time the grid is scanned. Many conventional methods could be utilized to detect the null in the cursor envelope signal. The current embodiment employs a filter which responds uniquely to the complex cursor waveform to predict or interpolate the cursor coil electrical center with a ' resolution, accuracy and stability not obtainable with other techniques using I comparable wire spacing and component parts count, and without adjustments.
To determine the electrical makeup of this filter, the complex cursor waveform was defined mathematically as follows:
, Referring to Fig. 1, the ~iclc~lp coil (cursor) is talcell at a heigllt h l above the successively excited parallel grid wires. As is described later, ~here is a steel shield a distance d below these wires. The resultLng magnetic I field is the same as that produced by a wire at a distance hl below the coil and another wire with opposite current at a distance h2 = hl + 2d from the ~O pickup coil.
The flux passing through the coil was calculated by considering an arbitrary point on the pickup plane (tablet surface). Referring to Fig. 2, the distance to the wire along the horizontal plane is denoted by x. Therefore, the normal component of flux is given by:
i x - cos ô = _ (eq 1) r x2 + h2 Because of the shield, the total component is xJ(x2 + h2) - x/(x2 + h22) = U (eq 2) . .

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To determine the total flux linking the coil, the integral of U over the area of the circle bounding tlle coil is computed. /~s seen in Fig. 2, Xo denotes the horizontal distance of the center o~ the coil to tlle grid wire.
scale factor is taken so that the radius of the coil is 1.
In the plane of the coil, y denotes the axis parallel to the grid J wires. Integrating once in the x direction, the total flux U is +l (Xo-X)2 + hl2 (Xo+X)2 + hl2 U = - \ log X I dy (eq 3)

2 J (Xo+x)2 ~ h22 (Xo_x)2 + 1122 where x2 + y2 = 1 Substituting y = cos~ and x = sin~, dy = -sinOdO
The total f lux U is given by r~ r(sin O - x)2 -i- h12 (sin ~3 -~ x)2 -~ hl2l ~ sin ~d~3 2U = -\ log ~ X _ _ _ _ _ ___ J ~ (sin 0 + x)2 + h22 (sin ~3 - x)2 + h22 J

Having defined the compLex c~lrsor waveform, a Eilter fullction E(t), which is practical and accurate, was chosen to operate on U(x) such that the resultant zeros produce a highly accurate and linear output as a function of time and distance.
The output of the system is defined as H(x,t) = nu (x-k)f(t-k) (eq 5) o The zeros of H were calculated, using a computer, with various sample functions for f(t). Extremely accurate and linear results were achieved using f(t)=e --5T sin4(.5t), t > 0 (eq 6) f(t)=0 t < 0 (eq 73 Using many sample points, and using a least squares linear fit to evaluate the deviation, a theoretical system error of .0014 was achieved.

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i Fig. 3 sho~rs plots of H(l,t), H(3,t)1 and f(t).
~ s can be seen in Fig. 3, the amplitudes of the maximums of ~I(x,t) are diTfferent but the zeros of lI(x,t) are exactly two divisions apart on the time axis in this example.
Sample and filter detector 23, shown in Fig. 4, was synthesized from the mathematical filter function f(t)-e --5t sin4(.5t).
The electrical implementation of f(t) is extremely simple Erom a component count and assembly point of view and utilizes only inexpensive, commercially available devices.
; lO Detection of the cursor coil position by the above means provides a level of performance not obtainable through instantaneous or peak amplitude, or phase meas-lring techniqlles using the same n-lmber of gricl wires and componellts.
As shown by the above equations, the detecl:ion schemt! is mathematicaLLy predictable and shows that an exceptionally higll level o~ l)erEormance is obtainable. The filter characteristics are thereEore ~mique and are critically related to the generact(l cursor coil si~nalL.
Other varlations oE stimuLatiTlg the gricl ~rires coulcl be utilizetl to obtain similar results providing that the predicting or interpolating filtering circuit is altered to provide an output which accurately and linearly relates the distance of the electrical center of phase re~ersal point of the cursor from a reference to time.
This circuit then operates on the complex cursor signal, which is induced in the cursor coil from the sequelltial actuation of grid lines hy passing current through them, to afEord a means of measuring cursor position relative to an arbitrary reference point by relating distance to time in an accurate linear fashion. The current system enables a precision clock to counters when the electromagnetic wavefront passes the arbitrary reference point and inhibits the precision clock when the detection circuit; discussed ._ , .

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above detects a phase reversal in the complex cursor signal. The contents of the counter then contain a count which is precisely related to cursor position. ~n ~Y scanning system is used to thoroughly define the cursor position, that is, the horizontal position of the cursor is initially determined by scanning the X axis and then the vertical position is determined similarly by scannlng the Y axis. Adding to the efficiency of the system is the fact that X axls and Y axis detection and counting circuitry can be common, thereby further reducing assembly and parts costs.
The invention will be explained by first describing the schematic illustrations of a preferred embodiment thereof witll a subsequent description of tile operating characteristics and signal outputs within the system.
Referring to the bloclc diagram of ~ig. 4, a precision crystaL oscillator and divider ll provides the basic system clock alld subdivisions thereoE
required by the digitiæing system. Connections are made to grid drive multiplexer 13 and system control circuits 15. Grid drive multiplexer 13 utilizes subdivisions of the basic system cloclc and inputs Erom system contro~
15 to sequentially energize the ~ and then the Y grid lines oE the grid tnblet 19. Grid drive multiplexer 13 is unique in that it minimizes the number of interconnections between grid tablet 19 and controller digitizing system lO, and eliminates the need for active components in grid tablet 19 which is very advantageous from a maintenance and assembly point oE view. Crid drive multiplexer l3 thus establishes the electromagnetic field which induces an electrical signal in the cursor 21. The cursor inputs this signal to sample and filter detector 23, where it is processed to provide an input to system control 15 which relates the cursor position to time in a precise, linear fashion. System control 15 oversees system operation and provides gated clock inputs to the ~+Y counter 17 where these gated clock pulses are accumulated in counters to precisely represent the physical position of the cursor on the grid , ~

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Fig. 5 is a schematic diagram of a preferred embodiment o~ the present invention. The input to the system is provided by a crystal oscillator 11 having a fixed frequency. The output of the oscillator is coupled to a frequency divider and scaler 35 whose division parameters determine both the rate of scan and the system resolution.
A first output from divider 35 is supplied to the scan control counter 59. Scan control counter 59 also receives inputs from the start/stop/control counter 53. Under control from these inputs, the scan control counter 59 provides inputs to the constant current grid drive multiplexer 13 to enable current to one grid line at a time in the proper order (sequentially left to right-X axis, followed sequentially bottom to top-Y axis.
A further output from the divider is coupled to the constant current grid drive multiplexer 13. This input enables a constant current source in the constant current grid drive multiplexer 13 which passes A controlled, ~ixed current to the selected grid wire.
Constant current grid drive multiplexer 13 provides outputs to grid tablet 19 to generate the moving electromngnletic field which is sensed by cursor 21 as the complex cursor waveform which was previously discussed.
Constant current grid drive multiplexer 13 is critical to this invention in that it eliminates the need for active switching elements in grid tablet 19 by minimizing the number of interconnections required to control a large number of grid lines. This circuitry is shown in more detail in Fig. 6. As can be seen, the circuit is divided into sink and source elements. A sink element provides a ground to one group of grid lines while the source element provides a constant current signal to one grid line at a time. Other grid lines are connected to the activated source line but no current flows in these grid lines - since the sink elements at the other end of those grid lines are inactive.

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Cursor 21 inputs the complex cursor waveform to filter 31, which is one stage of the synthesized circuit which represents the mathematical model required for optimum interpolation of the cursor signal, as discussed previously. The output of filter 31 is coupled to a sample and hold circuit 33. The output of filter 31 is sampled under control of another output from the divider and scaler circuit 35, thus synchronizing the sample to the grid scan. The sampled signals are lleld capacitively and input to Eilter 37 wllich completes the synthesized circuit representing the previously discussed mathematical model required to linearly relate cursor position to time.
The output of filter 37 is coupled to two level detectors, lock detect 39 and stop detect 41. The output o~ filter 37 is a voltage envelope approximately one sinusoidal cycle, as will be discussed later in conJunction with Fig. 8. Lock detector 39 detects an arbitrary voltage level on this envelope which indicates that the cursor is coupled electrically to the grid tablet sufficiently to provide accurate results. The output oE lock detector 39 clocks F/F 43 to remove the inhibit signal Erom gate 45.
The disclosed embodiment oE this invelltion detècts a transLtion across O volts to activate stop detector 41. Therefore, the Eirst transition across OV of the output oE filter 37, following the removal of the inhibit output of F/F 43 will pass through gate 45 as a STOP signal to clock, F/F ~7 thereby removing the count window enable Erom count gates 49 and 51.
Count gates 49 and 51 also have inputs Erom divider and scaler 35.
These inputs are a high frequency clock (count clock) which are passed through count gate 49 or 51 to become the ~ CQUNT or Y COUNT signals. The frequency of count clock relative to the grid scan rate determines the resolution of the system.
Count gates 49 and 51 also receive inputs (~ Axis and Y Axis) from the start/stop/control counter 53. These signals indicate which axis is being _ g _ B~7~

scanned and, along with the count window signal (discussed below) enable the count clock through the proper count gate 49 or 5I to the X counter 55 or the Y
counter 57.
Start/stop/control counter 53 receives an input Erom divider and scaler 35 which is a clock signal with a frequency of two times the basic grid scan rate. Counter 53 generates a START signal which sets F/F 47 to enable the COUNT WINDOW signal h and clears F/F 43. This signal, ST~RT, indicates the arbitrary reference point discussed previously from whicll time is measured to the STOP signal to give an accurate representation of cursor position. The time from START to STOP is represented by the duration of the COUNT WINDO~
signal.
Start/stop/control counter 53 also has an OUtp~lt to scan controL counter 59 which synchrollizes the grid scan to the remainder of the system circuitry.
Other outputs from start/stop/control counter 53 are the '`counter clear"
and "register load" signals. ~le counter clear signal clears X counter 55 and Y counter 57 following the completion of an X and Y scan and ~ust prior to the start of a new X and Y scan. The register llad sign.ll loads the contents oE X
counter 55 and Y counter 57 into X and Y OUtpllt registers 6l and 63 ~ollowing complete X and Y scan but prior to the counter clear signal.
X counter 55 and Y counter 57 receive the X count and Y count signals, respectively, as inputs. The contents of these counters, at the time the register load signal occurs, represents the position of the cursor on the grid tablet relative to an arbitrary reference point. X counter 55 and Y counter 57 have outputs to X output register 61 and Y output register respectively. These outputs are stored in the registers when the register load signal from counter 53 goes active. The outputs of these registers are available to external interface equipment such as computers, terminals, etc., for Eurther processing or storage.

It should be noted that X counter 55 and Y counter 57 can be combined into a single counter, to further optimize the circuitry, with the outputs multiplexed to an external device. Also, the system could have the ~ count and Y count signals as outputs to eliminate the need for counters and registers in this invention. In this case, the external interface equipment would provide the counting circuitry required to determine cursor position.
Fig. 7 is a cross-sectional view of a preferred digitizing table. As can be seen, the construction is very simple, consisting of only four parts, thus minimizing both material and labor costs. Reliability of tlle digitizing table is excellent since there are no active electronic components in the table.
The digitizing table is enclosed by a protective top cover 71 whicll has a smooth top surface made of durable, abrasion resistant material. The present embodiment of this invention utilizes a printed circllit board 73, with conductors forming an XY grid array (shown scllematically in Fig. 6) with parallel X conductors on the top sur~ace of the board and parallel Y conductors on the bottom surface of the board, to generate the moving electromagnetic field discussed previously. The printed circuit board also routes individual grid lines to the anodes of diodes 75, or to source bus 81 as shown in Fig. 6.
Source bus 81 and the cathodes of diodes 75 are then routed to a card edge connector ~not shown) for connection via a cable to the constant current grid drive multiplexer electronics. Other techniques for manufacturing the XY grld network would work equally well. Among these are hand or machine strung insulated wires bonded to virtually any non-ferrous substrate, etch and fill, and deposition. A non-conductive spacer 77 (Fig. 7) serves two purposes. It insulates the Y grid conductors on grid board 73 from shield 79 and it establishes the distance d between the shield and the grid wires as shown in Fig. 1 and discussed previously in conjunction with the derivation o~ the f~
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~B~72 mathematical model of the electromagnetic field generated by the grid array. A
cutout on one edge 76 of spacer 77 is cut so as to provide space for sink diodes 75. Shield 7~ serves a protective bottom cover for the digitizing tables. More importantly, it is an integral component in the generation of the electromagnetic field. As can be seen in equations 2, 3 and ~, the shield serves as a non-linear attenuator to the generated electromagnetic field. It virtually cancels the field generated by wires not in close proximity to the cursor pickup coil. This is beneficial in that it minimizes un~anted edge effects caused b~j the discontinuity of the XY grid network at the edges of the table and by the fields generated by the routing conductors from the edge connector. Also, it modifies the generated field such that the complex cursor signal is more readily linearized (distance to time) between the discrete grid lines. The shield additionally minimizes the eflect oE unwanted externally generated electronic noise. Finally, it adcls rigidity to the structure. The current embodiment of this invention utilizes cold rolled steel as a shielding material.
The digitizing table has been made trallslucent Eor back lightlng application. This is accomplished by utilizing a clear or translucent material for protective top cover 71 and spacer 77. Good results are achieved using standard PC board material for the printed circuit board 73. ~]owever, best light transmission results from an XY grid net-~or~ of conductors bonded to a clear glass or plastic substrate. A perforated shield may be utilized to allow for light transmission while still retaining the beneficial effects of the solid shield discussed previously.
A transparent tablet, for rear projection applications, has been realized by manufacturing a precisely registered 2 layer XY grid network with grid currents flowing in opposite directions in each plane as suggested by the - mathematical model. This technique eliminates the need for a shield, but is ; - 12 -.,.. : ~............................................... .

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~8072 more costly to manufacture and is more susceptible to externally generated electrical noise.
Figs. 8 a) through i) show outputs of the system at various points as identified in Fig. 5.
The input to the scan control counter 59 from divider and scaler 35 is the scan clock and is sho-m in Fig. 8 a). It is a constant clock which drives scan control counter 59 to enable current to one grid line at a time in the proper sequence through the constant current grid drive multiplexer 13.
Fig. 8 b) is the complex cursor waveform after it has been filtered and amplified by filter 31. As can be seen, when cursor waveform is present, there is one cycle in the cursor waveform for each cycle of the clock 8 a). Also, a 180 phase reversal is shown. This occurs as the Inoving electromagnetic field passes the exact electrical center of the cursor coil.
Fig. 8 c) is a step fullction which represents the output of sample and hold circuit 33. This signal is input to filter 37. The output of filter 37 is shown in Fig. 8 d). This signal corresponds to the function H (x,t), discussed previously and shown in Fig. 3, the zero crossing of ~lich linearly relates cursor position to time.
The output o~ lock detector 39 is shown in 8 e). This circuit is a level detector which monitors the negative transition of the output of filter 37. When signal 8 e) is not present, it indicates that the cursor coil is not sufficiently electrically coupled to the grid tablet to provide accurate results.
` Fig. 8 f) is the output of stop detector 41. The first positive `~ transition of this signal following the lock detect signal, 8 e) clocks F/F 47 to remove the count window signal, Fig. 4 h). This positive transition indicates the zero of the function H (x,t) shown in Fig. 3. F/F 43 is set by the start pulse, Fig. 3 g) which indicates the arbitrary reference point from `~ :

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which time is measured to the stop signal to represent cursor position. The count window signal Fig. 4 h) is a signal which is true for the duration of this period, from start pulse to stop pulse. It is used to gate the high frequency count clock through count gates 49 and 5l. Fig. 4 i) is the gated count signal at the output of either count gate 49 or 51. It should be noted that the frequency of the count clock relative to the scan clock, Fig. 4 a) determines the system resolution. By varying this ratio, virtually any scale or resolution is possible.
As will be appreciated for the foregoing, the invention provides a number of desirable features and advantages, the more important of which are:
the position of the cursor can be determined in a continuous linear fashion by using accurate electrical interpolation techniques to determine position between grids;
the rate of counting is variable, thus providing any resolution desired;
the accuracy of the output is not wholly dependent upon the scan rate;
the apparatus is substantially insensitive to amplitude and phase variation;
the diameter of the cursor coil is not critical and some tilt of the cursor coil is permissible;
absolute coordinate determination is obtainable while permitting removal and replacement of the cursor Erom the grid tablet without re-initializing.
Furthermore, the system requires a relatively small number of parts with consequent reduction in assembly cost; ic may readily be implemented by use of interchangeable subassemblies; it is relatively immune to temperature, humidity, noise, dielectric variations, magnetization and electrical noise and is also relatively immune to source (hard copy) material and thickness (except ferrous metals); it is relatively stable and requires little or no adjustment and requires only a small number of wires per inch. ~lso, the grid wires may 7'~-be energized by shclri.ng or m~lltiple~ing wires at di~erent positiors in the tablet to minimize the feed wires required.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for determining the position of a selected point on a grid having X and Y grid lines comprising:
clock means;
grid drive multiplexer means coupled between said clock means and said grid lines;
control means coupled between said clock means and said grid drive multiplexer means for controlling the energization of said grid lines by said grid drive multiplexer means;
cursor means including a coil moveable on said grid;
sample and filter detector means coupled between said cursor and said control means for relating cursor position to time in a linear mode;
X and Y counter means; and means for coupling said X and Y counter means to said control means whereby gated clock pulses are accumulated in said X and Y counter means to measure the position of said cursor on said grid.

2. The apparatus of Claim 1 wherein said clock means comprises a crystal oscillator; and a frequency divider and scaler coupled to the output of said oscillator.

3. The apparatus of Claim 1 further comprising scan control counter means coupled between said clock means, said control means and said grid drive means for sequentially controlling the energization of said grid lines.

4. The apparatus of Claim 1 wherein said grid drive means comprises a constant current grid drive multiplexer;
a sink element for providing a ground to one group of grid lines; and a source element for providing a constant current signal to one grid line at a time.

5. The apparatus of Claim 1 wherein said sample and filter detector means comprises a sample and hold circuit;
a first filter coupled between said cursor means and said sample and hold circuit;
a lock detector;
a stop detector;
a second filter coupled between the output of said sample and hold circuit for providing a predetermined voltage envelope to said detectors; and said means for coupling said X and Y counter means to said control means couple said lock and stop detectors to said X and Y counter means.

6. The apparatus of Claim 1 wherein said grid drive means comprises a multiplexer.

7. The apparatus of Claim 1 further comprising a non-conductive substantially flat plate for supporting said X and Y
grid lines;
a translucent cover for said flat plate having a substantially smooth top surface;
a conductive shield bottom cover; and a non-conductive spacer between said flat plate and said shield.

8. The apparatus of Claim 7 wherein said shield is perforated for light passage.

9. A digitizing table for use with a cursor comprising a non-conductive substantially flat plate;
X grid lines on one side of said plate;
Y grid lines on the other side of said plate;
a translucent cover for said flat plate having a substantially smooth top surface;

a conductive shield bottom cover; and a non-conductive spacer between said flat plate and said shield.

10. The table of Claim 9 wherein said shield is perforated for light passage.