GB2174206A - Position coordinate determination device - Google Patents

Abstract

A position coordinate determination device comprising a data tablet having a grid of conductors 2,4 inductively coupled to magnetostrictive elements 8,8' defining an X-Y coordinate system. Current pulses are induced in respective X and Y conductors in response to the generation of flux by the coil 24 of a pointer 22 positioned adjacent the tablet surface. The current pulses induced in the conductors 2,4 in turn induce a pair of oppositely propagating strain waves in each magnetostrictive element 8,8'. Each magnetostrictive element has a pair of sensors 11,13 arranged at the respective ends thereof for outputting a pulsed signal in response to the detection of the arrival of each respective strain wave. A count in four counters is started simultaneously with the generation of flux by the coil of the pointer. Each count is separately stopped upon receipt of the pulsed signal output by the respective sensor. The four counts represent the respective times of travels of the four strain waves. The four counts and two reference data representing the time of travel of strain waves under standard conditions are used to calculate the true X and Y coordinates of the pointer position. <IMAGE>

Description

SPECIFICATION Position coordinate determination device FIELD OF THE INVENTION The invention relates to devices for determining the coordinates of a pointer on the surface of a tablet. More specifically, the invention relates to position coordinate determining devices wherein the position coordinate of a pointer with respect to a coordinate axis is determined by measuring the time of travel of a pair of oppositely traveling magnetostrictively induced strain waves along a magnetostrictive element from a point corresponding to the pointer position to respective reference positions at either end of the coordinate axis.

BACKGROUND OF THE INVENTION It is known in the art of position determining devices, sometimes referred to as digitizers, to provide a construction which employs only a single length of a non-electrical signal propagation medium, i.e., a magnetostrictive medium, for each coordinate dimension and a respective grid comprising a plurality of spaced parallel electrical conductors, each conductor extending transversely from a point adjacent one of the magnetostrictive elements so as to permit measurement of strain wave travel time along each magnetostrictive element between a reference position and a position corresponding to the location of a pointer on the grid.Such a digitizer is disclosed in U.S. patent application Serial No. 162,311, which teaches a construction for an automatic coordinate determining device having a tablet with a grid of first and second sets of parallel spaced electrical conductors, which sets of conductors are mutually orthogonal. Each respective magnetostrictive element (i.e., wire) is situated with its axis transverse to the corresponding set of electrical conductors. A pointer is movably arranged adjacent the conductors.

This pointer includes a flux-producing element which is inductively coupled to at least one conductor from each set of parallel conductors. When the flux-producing element is energized, an electrical current is induced in the adjacent conductors, which current in turn induces a strain wave in the respective magnetostrictive element at a region adjacent the respective conductor.

The magnetostrictive elements correspond respectively to the x and y coordinate axes. The strain wave induced in the respective magnetostrictive element travels along the axis to a region of the magnetostrictive element where sensor means are provided. The respective sensor means act as sensors for producing a signal in response to detection of the traveling strain wave which was produced in response to energization of the pointer. In addition, a fiducial signal induction coil is provided at each end of both magnetostrictive elements. These fiducial signal induction coils are energized to produce strain waves at two different times. First, the fiducial signal induction coils are both energized to produce a pair of traveling strain waves along the magnetostrictive element, which strain waves are received by the sensor means at the end of the magnetostrictive element.The signals output by the sensor means upon detection of the respective strain waves are clocked to determine the time of travel separating the two strain waves.

This detected time of travel is then compared with a reference value corresponding to a standard length of the magnetostrictive element. The difference between the detected time of travel and the reference value constitutes an error which must be compensated for. It should be noted that this calibration process takes place without energization of the flux-producing element.

After storage of the value of error compensation required, the logic and control network again energizes the fiducial signal induction coils. At this time the flux-producing element of the pointer is also energized. The time of travel separating the strain was induced by the pointer and the strain wave induced by one of the fiducial signal induction coils is then determined. The latter time of travel for each magnetostrictive elements represents the corresponding uncompensated coordinates of the pointer position. Following compensation, the true coordinates of the pointer position are output to data storage or display.

This prior art digitizer has the disadvantage that the calibration of the magnetostrictive element and digitization of the pointer position must be carried out in separate steps. The separate step of pulsing the fiducials for the purpose of calibrating the digitizer takes up time which could be otherwise used for the digitization of pointer coordinates.

Furthermore, it is important to note that in this prior art digitizer the strain waves induced in the magnetostrictive element following the pulsing of the fiducials and following the simultaneous pulsing of one fiducial and the pointer traverse the same portion of the magnetostrictive element at different times. Thus, although the time separating the arrival of the strain waves induced by the fiducial pulses is processed for the purpose of calibrating the subsequent digitized signals, this calibration takes into account the conditions affecting the magnetostrictive element as of the time when the fiducials are pulsed, not as of the time of digitization of the pointer position.

Later when one of the fiducials and the pointer are pulsed, the conditions affecting the performance of the magnetostrictive element may have changed. In other words, the strain waves induced by one of the fiducials and the pointer traverse the magnetostrictive element at a time subsequent to the calibration, which calibration may no longer be accurate in view of changed conditions experienced by the magnetostrictive element. The change in conditions occurring between the time of calibration of the system and the time of digitization of pointer position is a source of imprecision.

SUMMARY OF THE INVENTION In order to overcome the aforementioned disadvantages of prior art magnetostrictive digitizers, the present invention teaches a digitizer construction wherein the fiducial signal induction coils are eliminated and each magnetostrictive element is provided with a sensor induction coil at both ends thereof. The pair of orthogonal magnetostrictive elements are inductively coupled respectively to a first and second plurality of spaced parallel electrical conductors, said first plurality being orthogonal to said second plurality. The conductors of each plurality extend transversely from points adjacent the respective magnetostrictive elements.Current in any one conductor induces a pair of strain waves in the respective magnetostrictive element which travel along the length of the magnetostrictive element in opposite axial directions until the respective strain waves arrive at the corresponding sensor coils arranged at the respective ends of the magnetostrictive element. The current in the conductor in turn is induced by the energization of the pointer, which energization takes place in response to a pulse which simultaneously starts a plurality of counters. Each sensor coil is operatively connected to a counter for outputting a pulse in response to detection of the arrival of a strain wave, which output pulse stops the respective counter.Thus, the pointer pulse induces a current in a conductor extending transverse to the x coordinate magnetostrictive element, which conductor induces strain waves that travel in opposite axial directions until they arrive at the respective sensor coils arranged at first and second reference positions, the respective times of travel of the propagating strain waves representing first and second x dimensions measured with respect to first and second reference positions.In the same way, the pointer pulse induces a current in a conductor extending transverse to the y coordinate magnetostrictive element, which current induces strain waves which travel in both axial directions until arrival at the respective sensor coils arranged at first and second reference positions at the ends of the magnetostrictive element, the respective times of travel of the propagating strain waves representing first and second y dimensions measured with respect to first and second reference positions, The first and second x dimensions and the first and second y dimensions are output to a processing unit, which uses the input data to perform a correction to compensate for non-uniformity in the magnetostrictive elements. The correction is a ratiometric proportionality determined by a specific constant which relates to the size of the data surface.

It is therefore an object of the present invention to provide an automatic digitizer for determining position coordinates and compensating for nonuniformities and fluctuations in conditions which does not require the use of fiducial signal induction coils at the respective ends of each magnetostrictive element. In contrast to the prior art digitizer discussed above, in the present invention the separate calibration step is eliminated, thereby speeding up the digitization process and enhancing the digitization precision.

Another object of the invention is to provide an automatic digitizer in which a sensor induction coil is arranged at reference positions on both ends of each magnetostrictive element. Since the present invention contemplates the provision of only two orthogonal magnetostrictive elements, a total of four sensor induction coils must be provided. The prior art digitizer utilizing magnetostrictive elements did not contemplate the measurement of the time of travel for each oppositeiy traveling strain wave induced in the magnetostrictive element by a current in a conductor. The measurement of the separate times of travel provides two x coordinate data and two y coordinate data relating to the position of the pointer in relation to the coordinate system defined on the tablet.A correction factor for the x dimension can be derived from the two x coordinate data using a ratiometric proportionality determined by a specific constant which relates to the size of the data tablet surface. Likewise, a correction factor for the y dimension can be derived from the two y coordinate data.

It is a further object of the invention to carry out the calibration and digitization operations in response to a single pulsing of the pointer or cursor. In response to this single pulsing of the pointer, all of the necessary x coordinate data and y coordinate data are output to a central processing unit which performs the necessary calibration before calculating the true coordinates.

By avoiding the two-step process of the prior are digitizer, the present invention enables faster digitization and enhanced precision.

BRIEF DESCRIPTION OF THE DRA WINGS The preferred embodiment of the present invention will be described in detail with reference to the following drawings, in which like reference numerals are used to indicate like parts: Figure 1 is a plan view of the preferred embodiment of the improved digitizer according to the present invention with the logic and control network indicated in schematic form.

Figure 2 is a partial sectional elevation view of the preferred embodiment according to the present invention.

Figure 3 is a plan view of the conductor grid and the magnetostrictive elements of the preferred embodiment with the paths of travel of the respective strain waves depicted diagramatically in relation to point (X,Y) at which flux is produced in the pointer.

Figure 4 is a schematic block diagram of the electrical circuitry of the preferred embodiment of the invention.

Figure 5 is a top view of another preferred embodiment of the present invention wherein the sensor coils on the magnetostrictive elements are replaced by piezoelectric transducers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The digitizer of the present invention comprises a tablet 1 (see Fig. 2) having embedded beneath its surface a grid of electrical conductors 2 which are in parallel equidistantly spaced relationship. Referring to Fig. 1, it can be seen that the parallel electrical conductors 2 run vertically and are spaced from one another in the horizontal direction. For convenience the horizontal direction will be termed the X direction while the vertical direction will be termed the Y direction. A second grid of parallel equidistantly spaced conductors 4 running in the X direction and spaced along the Y direction is disposed slightly beneath the grid formed by the conductors 2 so that none of the conductors 2 make electrical contact with any of the conductors 4.The electrical conductors 2 and 4 extend over the entire area of the tablet surface in which coordinate measurement or digitizing is to take place.

Supported at either of its ends in damping mounts 6 is an elongated magnetostrictive element 8 in the form of a wire which has an axis transverse to and which is supported to close proximity or in physical contact with but does not make electrical contact with the electrical conductors 2. The magnetostrictive element 8 is formed from a composition which exhibits magnetostrictive properties such as a nickel-chromium-vanadium or iron-cobalt-vanadium alloy.

Upon passage of an electrical current through any of the electrical conductors 2, the resulting electromagnetic field in the region where the energized conductor is proximate to the magnetostrictive element 8 results in the formation of a vibration or strain wave in the magnetostrictive element 8. This strain wave propagates along the axis of the magnetostrictive element 8 in opposite directions from the region of proximity between the magnetostrictive element 8 and the energized conductor 2. The magnetostrictive element 8 is sheathed in an elongated tubular jacket or sleeve 10 which is preferably made of a low friction material. Teflon has been found to be a material suitable for sheathing the magnetostrictive element 8 without substantial damping of the magnetostrictively induced strain waves.

At predetermined positions at each end of the magnetostrictive element 8 there are provided inductive sensor coils 11 and 13 which circumscribe the magnetstrictive element 8 and which are connected to the inputs of respective preamplifiers 12 and 14. Permanent magnets 15 and 16 are disposed in predetermined spaced relationship to and with their major axis parallel to the common axis of the respective coils 11 and 13 and the magnetostrictive element 8. Each coupled sensor coil 11 and 13 and preamplifiers 12 and 14 forms a circuit for outputting an electrical signal in response to the detection of a change in the magnetic field of the respective permanent magnets 15 and 16 resulting from the propagation of the strain wave through this portion of the magnetostrictive element.The permanent magnets 15 and 16 at each end serve to magnetically bias the respective portions of the magnetostrictive element 8 within the sensor coils 11 and 13 so that the output of the respective preamplifiers 12 and 14 in response to the arrival of a propagated magnetostrictive strain wave has a predetermined polarity and amplitude range.

The entire length of the sheath or jacket 10 is wrapped within a conducting helical bias coil 18 preferably formed from a good conductor such as copper. One end of the bias coil 18 is connected to the output of a bias signal generator 20. The opposite end of the bias coil 18 can be connected to ground. Application of a bias signal from the generator 20 to the coil 18 sets up an electromagnetic field around the magnetostrictive element 8 which restores the magnetostrictive element 8 to an initial operating state and compensates for hysteresis or other external effects which can cause the magnetostrictive element 8 to vary in its strain wave propagation response to electric current induced in the conductors 2. Since in the instant invention only one magnetostrictive element is needed for each coordinate direction, only one biasing coil need be employed for each coordinate direction.Biasing of the magnetostrictive element 8 takes place before the pointer measurement cycle is initiated. The biasing operation may be performed before each measurement cycle or periodically between groups of several measurement cycles.

The biasing can be done independently of the timing of the measurement signals.

Freely movable over the surface of the digitizer tablet 1 (see Fig. 2) is a pointer device 22 which can be either a stylus or a cursor. The pointer 22 includes a circular coil 24 having an axis normal to the plane of the grid conductors 2 and 4. The coil 24, as a result of its closely spaced proximity to the surface of the tablet, is inductively coupled to individual ones of the conductors 2 and 4 which are adjacent to the coil 24. The pointer coil 24 acts as the primary of a transformer with each of the grid wires 2 and 4 serving as secondaries.

The coil 24 of the pointer 22 is energized by a firing circuit 26 in order to induce signals in the secondaries. The firing circuit 26 includes capacitor 28 which is connected to the anode of a silicon controlled rectifier (SCR) 30. The cathode of the SCR 30 is connected to the pointer coil 24. The gate terminal of the SCR 30 is connected to a logic and control network 32, which can produce pulses at a fixed frequency equal to 100 Hz in the preferred embodiment. At a predetermined point in each cycle of the signal output of the control circuit 32, a pulse applied to the gate electrode which is lower than the cathode potential of the SCR 30 causes the SCR 30 to conduct, thereby permitting the capacitor 28 to discharge through the pointer coil 24.

When the SCR 30 conducts, the resultant electrical flux produced in the pointer coil 24 induces current in the adjacent conductors. The current induced in conductors 2 in turn produces strain waves in the adjacent region of the magnetostrictive element 8. The strain waves translate in both axial directions toward the sensor coils 11 and 13. Upon arrival at the reference positions on the magnetostrictive element 8 which are circumscribed by the respective sensor coils 11 and 13, the strain waves cause electrical pulse signals to be generated at the outputs of the respective sensor coils 11 and 13, which pulse signals are amplified by the associated preamplifiers 12 and 14. Each amplified pulse signal can be used to stop a counter, as will be subsequently explained in detail.

In order to prevent the strain waves from being reflected back after they reach the ends of the magnetostrictive element 8, the damping elements 6, which support the ends of the magnetostrictive element 8, can be formed from a combination of felt material and a hard rubber. This combination of materials has been found suitable for sufficiently damping pulses arriving at the ends of the magnetostrictive element 8 so that the amplitude of any reflections is well below the threshold amplitude in response to which the sensor coils 11 and 13 and the preamplifiers 12 and 14 will be able to provide output pulses of sufficient amplitude to activate the circuitry of the control network 32.

The digitizer illustrated in Fig. 1 provides for measurement of two orthogonal coordinates, one of which will be termed the X coordinate and the other of which is termed the Y coordinate. To determine the Y coordinate, a second magnetostrictive element 8' is positioned orthogonally to the magnetostrictive element 8 and supported in dampers 6'. The magnetostrictive element 8' is sheathed in a Teflon jacket 10' and surrounded by a helical biasing coil 18' which is preferably, although not necessarily, connected in series with the helical biasing coil 18 so that a single biasing current serves to clear and restore both magnetostrictive elements 8 and 8'.

A pair of Y coordinate sensor coils 11' and 13' and a pair of preamplifiers 12' and 14' are provided for sensing the magnetostrictive strain waves that are propagated along magnetostrictive element 8', to determine a Y coordinate.

In Fig. 3 the sequence of inductive steps is diagrammatically depicted with reference to an X-Y coordinate system. The grid of orthogonal sets of parallel conductors arranged on the data tablet define a two-dimensional coordinate plane over the surface of which the pointer 24 travels. Although the first and last conductors of each plurality are depicted in Fig. 1 as being separated from the adjacent sensor coils by a finite distance, in practice this separation distance should be as small as possible. The distance separating the sensor coils 11 and 13 define a reference dimension X"". X,, represents the maximum distance which can be measured in the X direction along the data tablet.Likewise, the dimension Y"., separating the sensor coils 11' and 13' represents the maximum measurable distance of the data tablet in the Y direction. The values X"., and Y,, are determined under standard conditions and are subsequently used as reference values in the calibration process.

As noted earlier, the coil 24 produces a flux in response to a pulse received from a firing circuit 26. This flux in the coil induces current in adjacent conductors, which current is conducted along conductors extending transverse to the X axis and conductors extending transverse the Y axis in both directions. The current which is conducted toward the respective magnetostrictive elements will induce oppositely traveling strain waves in the region of the magnetostrictive element which is adjacent to the current-carrying conductor. The points at which strain waves are induced in the respective magnetostrictive elements are indicated by the letters A and B in Fig. 3. Following the induction of strain waves at point A in magnetostrictive element 8, the strain waves propagate in both axial directions toward the respective ends of the magnetostrictive element. The leftward-propagating strain wave will travel a distance X1 before reaching the sensor coil 11 arranged on the left end of the magnetostrictive element 8. The rightwardpropagating strain wave will travel a distance X2 before reaching the sensor coil 13 arranged at the other end of the magnetostrictive element 8. The strain wave arriving at sensor coil 11 induces a pulse which is amplified by the preamplifier 12. The oppositely moving strain wave induces a pulse in sensor coil 13 which is amplified by preamplifier 14.

As will be described in detail with reference to Fig. 4. the pulses output by preamplifiers 12 and 14 associated with magnetostrictive element 8 each stop a counting sequence in an associated counter. These counting sequences were started simultaneously by the output from the firing circuit 26 of a pulse to the coil 24. These counts represent the times of travel of the respective oppositely propagating strain waves in the magnetostrictive element 8. In a similar way, counts are produced in counters which represent the times of travel of the respective oppositely propagating strain waves in the magnetostrictive element 8'. Because the speed of propagation of a strain wave in the particular magnetostrictive material used is known, these counts (i.e., times of travel) represent the distance traveled by the strain waves.These distances are respectively denoted by X1, X2, Y1, and Y2 in Fig. 3. These quantities represent the uncalibrated coordinate dimensions of the pointer position. For example, with respect to the X-Y coordinate system substantially defined by the leftmost vertical conductor and the lowest horizontal conductor in Fig. 3, the coordinates of coil 24 would be (X1, Y1). However, if the magnetostrictive elements are subjected to varying conditions, the coordinates (X1, Y1) will not be identical with the true coordinates of pointer 22 and a correction will be necessary. This correction takes place by calibrating the distance X,+X2 with respect to the distance Xref Similarly, the distance Y1+Y2 is calibrated with respect to the distance Y,,,.In this way a selfcorrection for the X1 and Y dimensions can be generated by employing the X2 and Y2 dimensions. The correction is a ratiometric proportionality determined by using the specific constant X,0,, which is related to the size of the data surface. This ratiometric proportionality may be expressed as follows: Xco,= (X,,f/(X +X2))X1 wherein Xco, represents the corrected X coordinate. The Y correction is computed in precisely the same manner, i.e., Y=(Y,,/(Y +Y2))Y1 wherein Y,,, represents the corrected Y coordinate. These computations are effected by means of a calculation device which will be described in detail in connection with Fig. 4.

Thus, in contrast to the prior art digitizer already discussed wherein the distances X1 and X1+X2 are digitized in separate pulsing steps, the present invention has the advantage that the distances X1 and X2 are simultaneously measured in response to a single pulsing step, from which the calibrating quantity X1+X2 can be computed. This ratiometric proportionality calculation for calculating a corrected coordinate digitization value for each measured coordinate is disclosed in detail in U.S. Patent No. 4,018,989, which patent is assigned to the assignee of the present application. However, the digitizer disclosed in U.S. Patent No. 4,018,989 requires the use of sequential pulsing whereby the digitization of the respective dimensions X1, X2, Yl, and Y2 are carried out in sequence.The advantageous construction of the present invention enables the simultaneous measurement of the dimensions X1, X2, Y and Y2, as a result of which the speed of the digitization process is considerably enhanced. As a result of these corrections, variations in linearity for large data tablets caused by transmission media anomalies such as wire conditions or nonuniformities in the wire, which cause changes in the wire's character, and other variations due to light, temperature, processing defects and so forth are compensated for. The correction system would, in addition, have absolute accuracy and zero temperature dependence.

The digitization techniques for corrected X and Y values are carried out continuously. The ratios are therefore calculated successively upon each successive reading of X and Y values and variations in both long-term and short-term characteristics are completely cancelled.

Fig. 4 shows the electronic circuitry of the invention with many of the components being represented by blocks.

As was explained above, the pointer 22 may be in the form of a stylus or cursor having a coil 24 or other flux-generating means adjacent the lower end or surface thereof and coupled by means of a conductor 34 to a firing circuit 26 which includes the SCR 30 and capacitor 28. The firing circuit 26 is in turn actuated by means of triggering pulses derived by any suitable means from an external source 36. The manner of introduction of trigger pulses may be controlled by means of a multiple position switch (not shown), as described and depicted in U.S. patent application Serial No. 162,311.For example, a computer or like remote control source (not shown) may be employed to provide triggering pulses, a continuous trigger circuit (not shown) may be provided in cooperation with a rate control (not shown) for varying the frequency of the actuating pulses, or a manually operated signal pulse control circuit such as a one-short monostable multivibrator (not shown) may be provided with a manualy operated switch (not shown) for providing a manually controlled pulse rate from the one-short multivibrator. The continuous trigger circuit and one-short multivibrator may be of conventional form, and the computercontrolled signal may be derived from a computer or from any external source of triggering signals. These features are well-known to one of ordinary skill in the art and are disclosed in the aforementioned patent application.

As already described, the X and Y generated magnetostrictive disturbances are picked up by the sensor coils 11, 11', 13, 13' and then amplified by the respective amplifiers 12, 12', 14, 14'. The amplified pulses are then applied to respective threshold discriminators 38, 40, 42 and 44. The threshold discriminators operate to sense the first zero crossing after the achievement of a minimal threshold and provide an output pulse corresponding to the occurrence of the zero crossing signa. The outputs of the threshold discriminators 38, 40, 42, and 44 are coupled to the respective inputs of conventional bistable flip-flops 46, 48, 50, and 52. One output port of each flip-flop is connected to an input port of respective AND gates 54, 56, 58 and 60. The output port of each AND gate 54, 56, 58 and 60 is respectively connected to an input port of respective counters 62, 64, 66, 68.The other input port of each AND gate 54, 56, 58, and 60 is connected to receive a clocking signal from a clock pulse generator 70. Each of the counters 62, 64, 66, and 68 is coupled to a central processing unit 72. The CPU receives digital data from each counter 62, 64, 66 and 68 representing the measured dimensions X1, X2, Y and Y2.

The CPU 72 is also connected to read only memory (ROM) 74, in which a program is stored.

The operation of the preferred embodiment of the present invention will now be described with reference to Fig. 4. The signal which causes SCR 36 in firing circuit 26 to conduct is received along conducting line 74. This signal takes the form of a triggering pulse output by the trigger source 36. This trigger pulse is also conducted simultaneously along a line 76 to a reset terminal R, whereat the leading edge of the trigger pulse is employed to reset the counters 62, 64, 66 and 68 in a conventional manner. At the same time, the trigger signal is inverted and conducted to further AND gate 78, which will be described in detail below. In addition, the trigger signal is simultaneously conducted to each of the flip-flops 46, 48, 50 and 52 through a phase lock circuit 80.The phase lock circuit 80 delays the triggering of the flip-flops 46, 48, 50 and 52 for time periods sufficient to insure that a full-width clock pulse is provided from the clock pulse generator 70 to the AND gates 54, 56, 58, and 60. The effect of the trigger signal on the flip-flops 46, 48, 50 and 52 is to set each flip-flop in a state permitting the respective AND gates 54, 56, 58, and 60 coupled thereto, to pass clock pulses from the clock 70. In the preferred embodiment, the clock has a frequency of 20 MHz. As a result, the counters each begin to accumulate a digital count. The count in each counter continues to accumulate until a signal is received from the respective threshold discriminator 38, 40, 42, or 44 corresponding to the first zero crossing after passage of the minimum threshold level set in the threshold discriminator circuits.Detection of a pulse at this point by the respective threshold discriminator serves to reset the associated flip-flop 46, 48, 50 or 52, thereby blocking the action of the associated AND gates 54, 56, 58, or 60 and causing a cessation of the count accumulation in the respective counter. The period between the trigger pulses is sufficient to allow the X and Y coordinate strain waves propagated in the respective magnetostrictive elements 8 and 8' to damp out prior to initiation of the next successive trigger pulse. The counter reset operation already described is enabled by the leading edge of the trigger pulse and the unblocking of the AND gates, i.e., the start of the counting, is enabled by the trailing edge of the trigger pulse.

The complementary outputs of the flip-flop 46, 48, 50, and 52 are respectively coupled to a separate input of the aforementioned AND gate 78. The AND gate 78 is coincidentally enabled only during the period of time after the counter accumulation is complete but before the counters have been reset by the trigger pulse. The output of the enabled AND gate 78 provides a "data ready" signal which may be utilized for transferring the accumulated count to an appropriate output device by enabling the AND gates 82 and 84. For purposes of illustration, the gate 78 may be employed in conjunction with signals output by the CPU 72 to enable either gate 82 or gate 84 when it is desired to make specific use of the information.For example, the enabling of AND gate 78 will permit the output of data from the CPU 72 to a digital-to-analog conversion circuit 86 for conversion of the digital data to an analog signal suitable for display on a display device 88. The display device may be a conventional cathode ray tube. Alternatively, it may be desired to store the information in a computer or other form of permanent data store, in which event the gate 84 would be enabled to allow the output of data from the CPU 72 to a data storage device 90.

As noted above, the CPU 72 receives the binary counts in digital form, which counts represent the dimensions X, X2, Y and Y2. The CPU 72 also receives the dimensions X,,f and Y,,f, previously discussed, in digital form from ROM 74, these reference dimensions being permanently stored therein. A program is also stored in the ROM comprising instructions for carrying out the correction based on the ratiometric proportionality calculation previously described. In accordance with these instructions, the CPU 72 calculates X,,, and YCO, from Xl, X2, X,,f and from Y1, Y2 Yre respectively.

In an alternative embodiment (not depicted), the CPU and ROM may be replaced by a pair of adder circuits both connected to an error compensation circuit and readout circuit. The counts of the X1 and X2 counters are output to the first adder to form the quantity X,+X2, which is in turn sent to the error compensator, where Xl+X2 is compared with X,,. The error compensator produces an error signal having a magnitude proportional to the difference between X1+X2 and X,,,, and outputs the error signal to the readout where it is applied to the X, datum to obtain XcOr. Y,or can be computed in a similar manner.

In a still further embodiment, the parallel digitizing depicted in Fig. 4 can be replaced by sequential digitizing. In the latter case, two of the counters are removed, along with the associated threshold discriminators, flip-flops and AND gates. The preamplifiers 12 and 14 and the preamplifiers 12' and 14' will be respectively alternately connected to the remaining pair of threshold discriminators during alternating pulses from the firing circuit 26. For example, preamplifiers 12 and 12' could be connected to threshold discriminator 38 by way of appropriate circuitry. Switching circuitry could be provided for connecting the input of the threshold discriminator 38 to the output of preamplifier 12 during a first trigger pulse and to the output of preamplifier 12' during the next trigger pulse.The provision of such switching circuitry is within the competence of one having ordinary skill in the art. Thus, first the X1 and X2 dimensions will be determined and stored, and then the Y, and Y2 dimensions will be determined. These X and Y determinations will continue in alternating sequence.

In a still further embodiment depicted in Fig. 5, piezoelectric transducers 91 and 92 can be substituted for sensor coils 11 and 13 arranged on magnetostrictive element 8 and for sensor coils 11' and 13' arranged on magnetostrictive element 8'. In the magnetostrictive element depicted in Fig. 5, the ends of the piezoelectric transducers 91 and 92 are respectively connected to the inputs of preamplifiers 12 and 14. The other ends of the piezoelectric transducers are joined to the magnetostrictive element by means of epoxy. In particular, the piezoelectric transducers may be strain gage piezoelectric transistors. The use of piezoelectric transducers to sense the propagated strain waves is advantageous because biasing of the piezoelectric transducers is not necessary. Thus, the permanent magnets 15, 15', 16 and 16' (shown in Fig. 1) can be eliminated.Furthermore, the piezoelectric transducers do not need to be magnetically shielded from the pointer coil. In contrast, the sensor coils 11 and 13 must be magnetically shielded so that when the pointer coil 24 is in the vicinity of the sensor coil, no extraneous pulse is produced.

Resolution of the system is a function of the clocking frequency and not of the spacing between adjacent parallel electrical conductors 2 and 4. However it is desirable to have the spacing between parallel electrical conductors small so as to insure that at least one and preferably several conductors are always closely proximate the pointer coil 24 to insure reliable signals and to minimize signal amplitude bounce as the pointer is moved across the surface of the tablet. Hence the conductor spacing should be a function of the diameter of the coil 24. In the case of a cursor which employs a relatively large coil 24, greater spacing can be used than when a stylus is to be used as the pointer, in which case the coil is apt to be substantially smaller.

The velocity of the strain wave propagating along the magnetostrictive elements 8 and 8' is approximately 5,000 metres per second. Hence the strain wave travels approximately .01 inches in 50 nanoseconds. Since 50 nanoseconds is the period of the 20 MHz clock 70, it is possible to resolve up to 100 lines per inch. Increasing the frequency of the clock 94 increases the resolution of the system. For example, a 200 MHz clock would allow resolution on the order of 1 ,000 lines per inch. A low-cost embodiment with a medium resolution of 200 lines per inch can be achieved with a construction wherein counting takes place on both the leading and trailing edges of the pulses from a 20 MHz clock.As a result of the use of a non-electrical signal propagation medium, i.e., a magnetostrictive element, the signal is propagated with a continuous, uniform velocity and, hence, the resolution of the system is limited only by the frequency of the clock 70 and is independent of the characteristics of the signal propagation medium.

As a result of the built-in calibration function which is enabled by the use of four sensor coils and four counters, it is possible to automatically determine the size of the tablet in use, i.e., by measuring X,+X2 and Y1+Y2. Hence a single logic control circuit can be employed in the system irrespective of the tablet size. The programmed central processing unit can calculate scale factors to correct for temperature-related and other errors which may occur, for example, errors due to stretching of the paper on which coordinates are to be measured or errors of scale.

Errors attributed to temperature effects and incorrect scaling of the paper can be expected to be within predetermined limits. The logic circuit and control network 32 can be adapted to determine ranges of (Xt+X2) and (Y1+Y2) values and, depending upon the range into which these values fall, the tablet can be deemed to be of a predetermined dimension corresponding to the detected range. Separate range determinations can be, made for the X and Y directions so that, for example, a 14-inch by 14-inch tablet can be distinguished from a 14-inch by 17-inch tablet.

After the dimensions of the tablet have been automatically determined, appropriate scaling can be applied to correct for smaller deviations which would be attributable to temperature effects and scale errors.

The foregoing description of the preferred embodiment is presented for illustrative purposes only and is not intended to limit the scope of the invention as defined in the appended ciaims.

Modifications may be readily effected by one having ordinary skill in the art without departing from the spirit and scope of the inventive concept herein disclosed.

Claims (24)

1. An automatic position coordinate determining device comprising: (a) a tablet having a surface and a first plurality of parallel spaced electrical conductors arranged therein; (b) a first magnetostrictive element arranged on said tablet with an axis transverse to said first plurality of electrical conductors, saif first magnetostrictive element being inductively coupled to said first plurality of electrical conductors such that oppositely propagating first and second strain waves are produced in said first magnetostrictive element in response to a current pulse in any one of said first plurality of electrical conductors;; (c) a pointer movable over said surface of said tablet and including a flux-producing element which is inductively coupled to at least one of said first plurality of electrical conductors when said pointer is adjacent said tablet surface such that an induced current pulse is produced in said inductively coupled electrical conductor in response to a current pulse in said flux-producing element; (d) a current pulse source electrically connected to output a current pulse to said fluxproducing element; and (e) first and second sensor means respectively arranged at first and second reference positions with respect to said first magnetostrictive element, each of said sensor means being capable of outputting a pulsed signal in response to detection of the arrival of a propagating strain wave at said respective reference position.

2. The automatic position coordinate determining device as defined in claim 1, further comprising first and second counting means operatively connected to begin counting in response to the output of a current pulse by said current pulse source, wherein said first and second counting means are respectively operatively connected to stop counting in response to the output of a pulsed signal by said first and second sensor means.

3. The automatic position coordinate determining deviced as defined in claim 2, further comprising calculating means for performing a ratiometric proportionality calculation in accordance with a program, and memory means for storing said program and a first reference datum representing the time of travel of a strain wave propagating from said first reference position to said second reference position along said first magnetostrictive element under standard conditions, wherein said calculating means is connected to receive first and second sets of data signals representing the stopped counts in said first and second counting means respectively, and a third set of data signals representing said first reference datum stored in said memory means, said calculating means producing a corrected datum, representing the true position coordinate of said pointer with respect to said first reference position, from said counts and said reference datum by means of said ratiometric proportionality calculation.

4. The automatic position coordinate determining device as defined in claim 1, wherein each of said sensor means comprises an inductive element inductively coupled to said first magnetostrictive element for producing a voltage in response to the arrival of a propagating strain wave at said respective reference position.

5. The automatic position coordinate determining device as defined in claim 1, wherein each of said sensor means comprises a piezoelectric element coupled to said first magnetostrictive element to allow the propagation of a strain wave from said first magnetostrictive element to said respective piezoelectric element for producing a voltage in response to the arrival of a propagating strain wave at said respective reference position.

6. The automatic position coordinate determining device as defined in claim 4, further comprising means for applying a constant magnetic field at said first and second reference positions.

7. The automatic position coordinate determining device as defined in claim 6, wherein said means for applying a constant magnetic field comprises a pair of magnets arranged in the region of said first and second reference positions respectively.

8. The automatic position coordinate determining device as defined in claim 1, wherein said current pulse source comprises a firing circuit including a silicon-controlled rectifier.

9. The automatic position coordinate determining device as defined in claim 1, further comprising means for applying magnetic bias to the portion of said first magnetostrictive element extending between said first and second reference positions prior to the output of a current pulse to said flux-producing element.

10. The automatic position coordinate determining device as defined in claim 9, wherein said biasing means comprises a coil circumscribing said first magnetostrictive element and means for energizing said coil.

11. The automatic position coordinate determining device as defined in claim 1, further comprising first and second means for amplifying the pulsed signals output by said first and second sensor means respectively, and first and second means respectively connected to said first and second amplifying means for detecting an amplified pulsed signal having a characteristic value greater than a predetermined threshold value and outputting a disabling signal in response to said detection.

12. The automatic position coordinate determining device as defined in claim 11, further comprising first and second counting means operatively connected to begin counting in response to the output of a current pulse by said current pulse source, wherein said first and second counting means are operatively connected to stop counting in response to the output of a disabling signal by said first and second threshold detection means respectively.

13. The automatic position coordinate determining device as defined in claim 3, wherein said calculating means comprises means for calculating a first ratio formed by said first reference datum over the sum of said counts in said first and second counters and means for calculating the product of the count in said first counter and said first ratio, said product representing said true position coordinate of said pointer with respect to said first reference position.

14. The automatic position coordinate determining device as defined in claim 3, further comprising digital-to-analog conversion means connected to said calculating means for receiving a set of data signals representing said corrected datum in digital form and outputting an analog signal, and display means connected to said digital-to-analog conversion means for displaying numeric characters representing said analog signal.

15. The automatic position coordinate determining device as defined in claim 3, further comprising data storage means connected to said calculating means for receiving and storing a set of data signals representing said corrected datum.

16. The automatic position coordinate determining device as defined in claim 1, further comprising: a) a second plurality of parallel spaced electrical conductors arranged in said tablet, said second plurality of electrical conductors lying transverse to and being electrically insulated from said first plurality of electrical conductors; b) a second magnetostrictive element arranged on said tablet with an axis transverse to said second plurality of electrical conductors, said second magnetostrictive element being inductively coupled to said second plurality of electrical conductors such that oppositely propagating third and fourth strain waves are produced in said second magnetostrictive element in response to a current pulse in any one of said second plurality of electrical conductors; and c) third and fourth sensor means respectively arranged at first and second reference positions with respect to said second magnetostrictive element, each of said sensor means being capable of outputting a pulsed signal in response to detection of the arrival of a propagating strain wave at said respective reference position, wherein said flux-producing element is inductively coupled to at least one of said second plurality of electrical conductors when said pointer is adjacent said tablet surface such that an induced current pulse is produced in said inductively coupled electrical conductor in response to said current pulse in said flux-producing element.

17. The automatic position coordinate determining device as defined in claim 16, further comprising first, second, third, and fourth counting means operatively connected to begin counting in response to the output of a current pulse by said current pulse source, wherein said first, second, third, and fourth counting means are operatively connected to stop counting in response to the respective output of a pulsed signal by said first, second, third, and fourth sensor means.

18. The automatic position coordinate determining device as defined in claim 16, further comprising first and second counting means operatively connected to simultaneously begin a first counting cycle in response to the output of a first current pulse by said current pulse source, to respectively stop said first counting cycle in response to the respective output of a pulsed signal by said first and second sensor means, to simultaneously begin a second counting cycle in response to the output of a second current pulse by said current pulse source, and to respectively stop said counting cycle in response to the respective output of a pulsed signal by said third and fourth sensor means, wherein said first and second current pulses are output at different times, while said pointer is in the same position with respect to said tablet surface.

19. The automatic position coordinate determining device as defined in claim 17, further comprising calculating means for performing first and second ratiometric proportionality calculations in accordance with a program, and a memory means for storing said program and first and second reference data representing the respective times of travel of a strain wave propagating from said first reference position to said second reference position along said first and second magnetostrictive elements under standard conditions, wherein said calculating means is connected to receive first, second, third, and fourth sets of data signals representing the stopped counts in said first, second, third and fourth counting means respectively, and fifth and sixth sets of data signals representing said first and second reference data respectively stored in said memory means, said calculating means producing a first corrected datum, representing the true position coordinate of said pointer with respect to the first reference position of said first magnetostrictive element, from said first and second counts and said first reference datum by means of said first ratiometric proportionality calculation and producing a second corrected datum, representing the true position coordinate of said pointer with respect to the first reference position of said second magnetostrictive element, from said third and fourth counts and said second reference datum by means of said second ratiometric proportionality calculation.

20. An automatic position coordinate determining device comprising: a) a tablet having a planar coordinate surface defined by mutually perpendicular X and Y coordinate axes and a grid of electrical conductors arranged therein, said grid comprising a first and a second plurality of parallel spaced electrical conductors, said second plurality of electrical conductors being arranged substantially perpendicular to said first plurality of electrical conductors; b) first and second magnetostrictive elements arranged on said tablet with axes respectively arranged transverse to said respective first and second pluralities of electrical conductors; c) means for simultaneously inducing current pulses in at least one of said first plurality of electrical conductors and in at least oe of said second plurality of electrical conductors at a point on said tablet surface; and d) first and second sensor means respectively arranged at first and second reference positions with respect to said first magnetostrictive element, and third and fourth sensor means respectively arranged at first and second reference positions with respect to said second magnetostrictive element, each of said sensor means being capable of outputting a pulsed signal in response to detection of the arrival of a propagating strain wave at said respective reference position, wherein said magnetostrictive elements and said first and second pluralities of electrical conductors are arranged with inductive coupling such that oppositely propagating strain waves are produced in one of said magnetostrictive elements in response to a current pulse in one of said electrical conductors.

21. The automatic position coordinate determining device as defined in claim 20, further comprising first, second, third, and fourth counting means operatively connected to begin counting in response to the inducement of said current pulses and to stop counting in response to the respective output of a pulse signal by said first, second, third, and fourth sensor means.

22. The automatic position coordinate determining device as defined in claim 21, further comprising calculating means for performing first and second ratiometric proportionality calculations in accordance with a program, and memory means for storing said program and first and second reference data representing the respective times of travel of a strain wave propagating from said first reference position to said second reference position along said respective first and second magnetostrictive elements under standard conditions, wherein said calculating means is connected to receive first, second, third, and fourth set of data signals representing the stopped counts in said first, second, third, and fourth counting means respectively, and fifth and sixth sets of data signals reprsenting said first and second reference data respectively stored in said memory means, said calculating means producing a first corrected datum, representing the true X coordinates of said pointer, from said first and second counts and said first reference datum by means of said first ratiometric proportionality calculation, and producing a second corrected datum, representing the true Y coordinate of said pointer, from said third and fourth counts and said second reference datum by means of said second ratiometric proportionality calculation.

23. A method of determining the position of a pointer with respect to an X-Y coordinate system defined on a data tablet, wherein said data tablet has a grid of electrical conductors formed therein, said grid comprising a first and a second plurality of parallel conductors, said first plurality being substantially perpendicular to said second plurality, said electrical conductors of said first and second pluralities being respectively inductively coupled to first and second magnetostrictive elements said magnetostrictive elements, respectively having a pair of sensor means inductively coupled to the respective ends thereof at respective first and second reference positions, said sensor means being connected to output a pulse signal to respective counting means in response to the detection of the arrival of a propagating strain wave, said method comprising the steps of:: a) positioning said pointer adjacent said tablet surface: b) generating a flux at said pointer sufficient to induce current pulses in at least one of said first plurality of electrical conductors and at least one of said second plurality of electrical conductors; c) simultaneously starting four separate counts in said respective counting means at the instant when said flux is generated; d) inducing oppositely propagating strain waves in said first magnetostrictive element adjacent the conductor of said first plurality which carries an induced current pulse and simultaneously inducing oppositely propagating strain waves in said second magnetostrictive element adjacent the conductor of said second plurality which carries an induced current pulse; e) detecting the respective arrival of each of said propagating strain waves by means of said respective sensing means;; f) outputting a pulsed signal in response to detection of said respective arrivals by said sensing means to a respective counting means for stopping said respective count, said respective stopped count representing the respective times of travel of said propagating strain waves.

24. The method as defined in claim 23, further comprising the steps of: g) performing a first ratiometric proportionality calculation using the counts in said first and second counting means and a first reference datum corresponding to the time of travel of a strain wave propagating from said first reference position to said second reference position along said first magnetostrictive element under standard conditions to obtain a true X coordinate of said pointer position, and h) performing a second ratiometric proportionality calculation using the counts in said third and fourth counting means and a second reference datum corresponding to the time of travel of a strain wave propagating from such first reference position to said second reference position along said second magnetostrictive element under standard conditions to obtain a true Y coordinate of said pointer position.