Travelling wave digitizer

• US 4,255,617 A
• Filed: 08/27/1979
• Issued: 03/10/1981
• Est. Priority Date: 08/27/1979
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Company (HP Inc.)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Tremblay, Michael A., Carau, Frank P. Sr., Hetzel, Henry T.
• Primary Examiner(s): Robinson, Thomas A.
• Application Number:
  US06/069,956
• Time in Patent Office: 561 Days
• Field of Search: 364/520, 340/709, 340/146.3 SY, 340/347 AD, 340/347 CC, 178/18, 178/19, 178/20, 33/1 M, 33/1 P, 33/1 HH, 346/139 C, 371/1
• US Class Current: 341/5
• CPC Class Codes: G06F 3/0442 using active external devices, e.g. active pens, for transmitting changes in electrical potential to be received by the digitiser

Abstract

A digitizer shifts long and short wavelength square waves across x and y grids in a platen to perform upon a cursor signal time measurements that represent coarse and fine cursor position data. The periodic staircase cursor signal is filtered before use to permit time measurements that resolve cursor positions to locations between lines in the grids. The contribution of the filter to the coarse and fine time measurements is obtained in a special reference measurement, so that it may be removed. A processor combines these data into Cartesian coordinates, and implements procedures for error reduction. These include averaging of consecutive time measurements, dynamic adjustment of a timing reference to remove ambiguities inherent in measuring very small or very large cyclic quantities, and correction of coordinates for the effects of cursor motion during measurement and for any non-perpendicularity of the x and y grids.

Claims

1. Apparatus for digitizing the coordinate of a point along a measurement axis of a platen, the apparatus comprising:

a platen having an ordered plurality of uniformly spaced separate parallel conductors each generally perpendicular to the measurement axis;

clock signal means for generating a clock signal;

reference signal means for producing a reference signal having half-wave symmetry and whose frequency is a submultiple of that of the clock signal;

propagation circuit means coupled to receive both the clock signal and the reference signal and having an ordered plurality of outputs coupled in one-to-one correspondence to the separate parallel conductors of the platen for propagating across the platen at a rate of one conductor per cycle of the clock signal a half-wave symmetric electric field having transitions in intensity corresponding to the half-wave symmetry of the reference signal;

a capacitive pickup positionable over the point whose coordinate is to be digitized, capacitively coupled thereat to the parallel conductors in the platen and providing a half-wave symmetric pickup signal of frequency equal to that of the reference signal as the half-wave symmetric electric field propagates across the platen and past the capacitive pickup;

a filter tuned to reject harmonics of the reference signal and having an input coupled to the capacitive pickup for providing a filtered pickup signal at an output of the filter; and

phase comparison means coupled to the output of the filter and to the reference signal means, for producing data corresponding to the phase difference between the filtered pickup signal and the reference signal.

2. Apparatus as in claim 1 wherein the wavelength of the propagated half-wave symmetric electric field is at least as long as the center-to-center distance between the two parallel conductors of the platen that are respectively connected to the first and last outputs in the ordered plurality of outputs of the propagation circuit means.

3. Apparatus as in claim 1 wherein the wavelength of the propagated half-wave symmetric electric field is shorter than the center-to-center distance between the two parallel conductors of the platen that are respectively connected to the first and last outputs in the ordered plurality of outputs of the propagation circuit means.

4. Apparatus as in claim 1 wherein the propagation circuit means further comprises a reference drive means for suspending the propagation of the half-wave symmetric electric field across the platen, and for causing instead of each of the outputs of the ordered plurality identical and simultaneous variations of voltage at the frequency of the reference signal.

5. Apparatus as in claim 1 further comprising reference measurement means coupled between the capacitive pickup and the input of the filter and also coupled to the reference signal means, the reference measurement means for selectively applying to the input of the filter a signal whose frequency equals that of the reference signal and whose phase is independent of the position of the capacitive pickup along the measurement axis.

6. Apparatus as in claim 1 further comprising phase reversal means coupled between the reference signal means and the phase comparison means and responsive to the data produced by the phase comparison means, for introducing a 180 degree phase shift into the reference signal supplied to the phase comparison means whenever the phase difference between the filtered pickup signal and the supplied reference signal is outside a preselected range.

7. Apparatus as in claim 1 further comprising timing signal means for producing a timing signal, and wherein:

the timing signal has a frequency higher than the frequency of the clock signal; and

the phase comparison means comprises a counter configured to count cycles of the timing signal between a transition in the reference signal and a corresponding transition in the filtered pickup signal.

8. Apparatus for digitizing the coordinate of a point along a measurement axis of a platen, the apparatus comprising:

a platen having an ordered plurality of uniformly spaced separate parallel conductors each generally perpendicular to the measurement axis;

coarse clock signal means for generating a coarse clock signal;

fine clock signal means for producing a fine clock signal whose frequency is a submultiple of the frequency of the coarse clock signal;

reference signal means for producing a reference signal having half-wave symmetry and whose frequency is a submultiple of the frequency of the fine clock signal;

selection means coupled to the coarse clock signal means and to the fine clock signal means for producing a selected clock signal corresponding at separate times to each of the coarse and fine clock signals by coupling at separate times one of the coarse and fine clock signals to an output of the selection means;

propagation circuit means coupled to receive both the selected clock signal and the reference signal and having an ordered plurality of outputs coupled in one-to-one correspondence to the separate parallel conductors of the platen for propagating across the platen at a rate of one conductor per cycle of the selected clock signal a half-wave symmetric electric field having transitions in intensity corresponding to the half-wave symmetry of the reference signal and having at separate times coarse and fine wavelengths as the selected clock signal separately corresponds to the coarse and fine clock signals, respectively;

a capacitive pickup positionable over the point whose coordinate is to be digitized, capacitively coupled thereat to the parallel conductors in the platen and providing at separate times half-wave symmetric coarse and fine pickup signals each of frequency equal to that of the reference signal as the electric field having half-wave symmetric coarse and fine wavelengths propagates across the platen and past the capacitive pickup;

a filter tuned to reject harmonics of the reference signal and having an input coupled to the capacitive pickup for providing filtered coarse and fine pickup signals at an output of the filter; and

phase comparison means coupled to the output of the filter and to the reference signal means, for producing at separate times data corresponding to the phase differences between the filtered coarse pickup signal and the reference signal and between the filtered fine pickup signal and the reference signal.

9. Apparatus as in claim 8 wherein the coarse wavelength of the propagated electric field is at least as long as the center-to-center distance between the two parallel conductors of the platen that are respectively connected to the first and last outputs in the ordered plurality of outputs of the propagation circuit means, and also wherein the fine wavelength of the propagated electric field is shorter than the center-to-center distance.

10. Apparatus as in claim 8 wherein the propagation circuit means further comprises a reference drive means for suspending the propagation of the half-wave symmetric electric field across the platen, and for causing instead at each of the outputs of the ordered plurality identical and simultaneous variations of voltage at the frequency of the reference signal.

11. Apparatus as in claim 8 further comprising reference measurement means coupled between the capacitive pickup and the input of the filter and also coupled to the reference signal means, the reference measurement means for selectively applying to the input of the filter a signal whose frequency equals that of the reference signal and whose phase is independent of the position of the capacitive pickup along the measurement axis.

12. Apparatus as in claim 8 further comprising phase reversal means coupled between the reference signal means and the phase comparison means and responsive to the data produced by the phase comparison means, for introducing a 180 degree phase shift into the reference signal supplied to the phase comparison means whenever the phase difference between the filtered pickup signal and the supplied reference signal is outside a preselected range.

13. Apparatus as in claim 8 further comprising timing signal means for producing a timing signal, and wherein:

the timing signal has a frequency that is higher than the frequency of the coarse clock signal; and

the phase comparison means comprises a counter configured to count cycles of the timing signal between a transition in the reference signal and a correspondng transition in the filtered pickup signal.

14. A circuit for propagating differing wavelength portions of an electric field across a surface containing an ordered plurality of conductors, the circuit comprising:

first clock signal means for generating a first clock signal;

second clock signal means for generating a second clock signal having a frequency different than the frequency of the first clock signal;

third clock signal means for generating a third clock signal having a frequency less than the frequency of the first clock signal and also less than the frequency of the second clock signal;

selection means coupled to the first clock signal means and to the second clock signal means, for producing a selected clock signal corresponding at separate times to each of the first and second clock signals by coupling at separate times one of the first and second clock signals to an output of the selection means;

propagation circuit means coupled to receive both the selected clock signal and the third clock signal and having an ordered plurality of outputs coupled in one-to-one correspondence to the ordered plurality of conductors for propagating across the surface at a rate of one conductor per cycle of the selected clock signal an electric field having transitions in intensity corresponding to transitions in the third clock signal and having at separate times first and second wavelengths as the selected clock signal separately corresponds to the first and second clock signals, respectively.

15. A method of propagating a half-wave symmetric electric field across a surface containing an ordered plurality of uniformly spaced conductors, comprising the steps of:

driving each conductor with a voltage corresponding to the logical value of a bit in an ordered sequence of bits in one-to-one correspondence with the ordered plurality of conductors; and

shifting by one bit at a time along the ordered sequence of bits an integral number of consecutive logical ones in alternation with an equal number of consecutive logical zeros, with equal intervening time intervals between each shift.

16. A method of propagating differing wavelength half-wave symmetric portions of an electric field across a surface containing an ordered plurality of uniformly spaced conductors, comprising the steps of:

driving each conductor with a voltage corresponding to the logical value of a bit in an ordered sequence of bits in one-to-one correspondence with the ordered plurality of conductors;

shifting by one bit at a time along the ordered sequence of bits a first integral number of consecutive logical ones in alternation with an equal number of consecutive logical zeros, with equal intervening first time intervals between each such shift; and

shifting by one bit at a time along the ordered sequence of bits a second integral number of consecutive logical ones in alternation with an equal number of consecutive logical zeros, with equal intervening second time intervals between each such shift.

17. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. propagating along the measurement axis a plurality of cycles of a half-wave symmetric electric field;

b. coupling the cursor to the propagating half-wave symmetric electric field to produce an AC signal having half-wave symmetry;

c. filtering the AC signal;

d. measuring the phase difference between a reference signal and the filtered AC signal; and

e. diminishing the measured phase difference by an amount corresponding to the fractional portion of the delay in excess of an integral number of cycles of the AC signal through the filter.

18. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. propagating along the measurement axis a plurality of cycles of a half-wave symmetric electric field;

b. coupling the cursor to the propagating half-wave symmetric electric field to produce an AC signal having half-wave symmetry;

c. filtering the AC signal;

d. measuring the phase difference between a reference signal and the filtered AC signal;

e. measuring the fractional portion of the delay in excess of an integral number of cycles of the AC signal through the filter; and

f. diminishing the measured phase difference by an amount corresponding to the measured fractional portion of the delay through the filter.

19. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. in alternation with step b below, propagating along the measurement axis at a first rate a plurality of half-wave symmetric cycles of a coarse electric field having a first wavelength longer than the longest distance to be determined;

b. in alternation with step a above, propagating along the measurement axis at a second rate a plurality of half-wave symmetric cycles of a fine electric field having a second wavelength that is an aliquot portion of the first wavelength, the ratio of the first rate to the first wavelength equalling the ratio of the second rate to the second wavelength;

c. coupling the cursor to the propagating electric fields to produce coarse and fine AC signals each of the same frequency and each having half-wave symmetry;

d. filtering the coarse and fine AC signals;

e. measuring the respective coarse and fine phase differences between a reference signal and each of the filtered coarse and fine AC signals;

f. diminishing each of the measured coarse and fine phase differences by an amount corresponding to the fractional portion of the delay in excess of an integral number of cycles of the coarse and fine AC signals through the filter; and

g. combining the diminished coarse and fine phase differences.

20. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. in alternation with step b below, propagating along the measurement axis at a first rate a plurality of half-wave symmetric cycles of a coarse electric field of a first wavelength longer than the longest distance to be determined;

b. in alternation with step a above, propagating along the measurement axis at a second rate a plurality of half-wave symmetric cycles of a fine electric field of a second wavelength that is an aliquot portion of the first wavelength, the ratio of the first rate to the first wavelength equalling the ratio of the second rate to the second wavelength;

c. coupling the cursor to the propagating electric fields to produce coarse and fine AC signals each of the same frequency and having half-wave symmetry;

d. filtering the coarse and fine AC signals;

e. measuring the respective coarse and fine phase differences between a reference signal and each of the filtered coarse and fine AC signals;

f. measuring the fractional portion of the delay in excess of an integral number of cycles of the coarse and fine AC signals through the filter;

g. diminishing each of the measured coarse and fine phase differences by an amount corresponding to the measured fractional portion of the delay through the filter; and

h. combining the diminished coarse and fine phase differences.

21. In a digitizer having a filter responsive to an output signal from a cursor coupled to one or more conductors in a platen, a method of determining for a given frequency the remainder of the filter delay in excess of n periods of the given frequency, n=0, 1,2,3, . . . , the method comprising the steps of:

simultaneously energizing in the platen all conductors associated with measurement in at least one dimension with a reference signal of the given frequency, to produce from the cursor an output signal whose phase is independent of the cursor location upon the platen;

filtering the output signal; and

measuring the phase difference between the filtered output signal and the reference signal.

22. Apparatus for digitizing the coordinate of a point along a measurement axis of a platen, the apparatus comprising:

a platen having an ordered plurality of uniformly spaced separate parallel conductors each generally perpendicular to the measurement axis;

clock signal means for generating a clock signal;

reference signal means for producing a reference signal having half-wave symmetry and whose frequency is a submultiple of that of the clock signal;

propagation circuit means coupled to receive both the clock signal and the reference signal and having an ordered plurality of outputs coupled in one-to-one correspondence to the separate parallel conductors of the platen for propagating across the platen at a rate of one conductor per cycle of the clock signal a half-wave symmetric magnetic field having transitions in intensity corresponding to the half-wave symmetry of the reference signal;

an inductive pickup positionable over the point whose coordinate is to be digitized, inductively coupled thereat to the parallel conductors in the platen and providing a half-wave symmetric pickup signal of frequency equal to that of the reference signal as the half-wave symmetric magnetic field propagates across the platen and past the inductive pickup;

a filter tuned to reject harmonics of the reference signal and having an input coupled to the inductive pickup for providing a filtered pickup signal at an output of the filter; and

phase comparison means coupled to the output of the filter and to the reference signal means, for producing data corresponding to the phase difference between the filtered pickup signal and the reference signal.

23. Apparatus as in claim 22 wherein the wavelength of the propagated half-wave symmetric magnetic field is at least as long as the center-to-center distance between the two parallel conductors of the platen that are respectively connected to the first and last outputs in the ordered plurality of outputs of the propagation circuit means.

24. Apparatus as in claim 22 wherein the wavelength of the propagated half-wave symmetric magnetic field is shorter than the center-to-center distance between the two parallel conductors of the platen that are respectively connected to the first and last outputs in the ordered plurality of outputs of the propagation circuit means.

25. Apparatus as in claim 22 further comprising reference measurement means coupled between the magnetic pickup and the input of the filter and also coupled to the reference signal means, the reference measurement means for selectively applying to the input of the filter a signal whose frequency equals that of the reference signal and whose phase is independent of the position of the inductive pickup along the measurement axis.

26. Apparatus as in claim 22 further comprising phase reversal means coupled between the reference signal means and the phase comparison means and responsive to the data produced by the phase comparison means, for introducing a 180 degree phase shift into the reference signal supplied to the phase comparison means whenever the phase difference between the filtered pickup signal and the supplied reference signal is outside a preselected range.

27. Apparatus as in claim 22 further comprising timing signal means for producing a timing signal, and wherein:

the timing signal has a frequency higher than the frequency of the clock signal; and

the phase comparison means comprises a counter configured to count cycles of the timing signal between a transition in the reference signal and a corresponding transition in the filtered pickup signal.

28. Apparatus for digitizing the coordinate of a point along a measurement axis of a platen, the apparatus comprising:

a platen having an ordered plurality of uniformly spaced separate parallel conductors each generally perpendicular to the measurement axis;

coarse clock signal means for generating a coarse clock signal;

fine clock signal means for producing a fine clock signal whose frequency is a submultiple of the frequency of the coarse clock signal;

reference signal means for producing a reference signal having half-wave symmetry and whose frequency is a submultiple of the frequency of the fine clock signal;

selection means coupled to the coarse clock signal means and to the fine clock signal means for producing a selected clock signal corresponding at separate times to each of the coarse and fine clock signals by coupling at separate times one of the coarse and fine clock signals to an output of the selection means;

propagation circuit means coupled to receive both the selected clock signal and the reference signal and having an ordered plurality of outputs coupled in one-to-one correspondence to the separate parallel conductors of the platen for propagating across the platen at a rate of one conductor per cycle of the selected clock signal a half-wave symmetric magnetic field having transitions in intensity corresponding to the half-wave symmetry of the reference signal and having at separate times coarse and fine wavelengths as the selected clock signal separately corresponds to the coarse and fine clock signals, respectively;

an inductive pickup positionable over the point whose coordinate is to be digitized, inductively coupled thereat to the parallel conductors in the platen and providing at separate times half-wave symmetric coarse and fine pickup signals each of frequency equal to that of the reference signal as the magnetic field having half-wave symmetric coarse and fine wavelengths propagates across the platen and past the inductive pickup;

a filter tuned to reject harmonics of the reference signal and having an input coupled to the inductive pickup for providing filtered coarse and fine pickup signals at an output of the filter; and

phase comparison means coupled to the output of the filter and to the reference signal means, for producing at separate times data corresponding to the phase differences between the filtered coarse pickup signal and the reference signal and between the filtered fine pickup signal and the reference signal.

29. Apparatus as in claim 28 wherein the coarse wavelength of the propagated magnetic field is at least as long as the center-to-center distance between the two parallel conductors of the platen that are respectively connected to the first and last outputs in the ordered plurality of outputs of the propagation circuit means, and also wherein the fine wavelength of the propagated magnetic field is shorter than the center-to-center distance.

30. Apparatus as in claim 28 further comprising reference measurement means coupled between the inductive pickup and the input of the filter and also coupled to the reference signal means, the reference measurement means for selectively applying to the input of the filter a signal whose frequency equals that of the reference signal and whose phase is independent of the position of the inductive pickup along the measurement axis.

31. Apparatus as in claim 28 further comprising phase reversal means coupled between the reference signal means and the phase comparison means and responsive to the data produced by the phase comparison means, for introducing a 180 degree phase shift into the reference signal supplied to the phase comparison means whenever the phase difference between the filtered pickup signal and the supplied reference signal is outside a preselected range.

32. Apparatus as in claim 28 further comprising timing signal means for producing a timing signal, and wherein:

the timing signal has a frequency that is higher than the frequency of the coarse clock signal; and

the phase comparison means comprises a counter configured to count cycles of the timing signal between a transition in the reference signal and a corresponding transition in the filtered pickup signal.

33. A circuit for propagating differing wavelength portions of a magnetic field across a surface containing an ordered plurality of conductors, the circuit comprising:

first clock signal means for generating a first clock signal;

second clock signal means for generating a second clock signal having a frequency different than the frequency of the first clock signal;

third clock signal means for generating a third clock signal having a frequency less than the frequency of the first clock signal and also less than the frequency of the second clock signal;

selection means coupled to the first clock signal means and to the second clock signal means, for producing a selected clock signal corresponding at separate times to each of the first and second clock signals by coupling at separate times one of the first and second clock signals to an output of the selection means;

propagation circuit means coupled to receive both the selected clock signal and the third clock signal and having an ordered plurality of outputs coupled in one-to-one correspondence to the ordered plurality of conductors for propagating across the surface at a rate of one conductor per cycle of the selected clock signal a magnetic field having transitions in intensity corresponding to transitions in the third clock signal and having at separate times first and second wavelengths as the selected clock signal separately corresponds to the first and second clock signals, respectively.

34. A method of propagating a half-wave symmetric magnetic field across a surface containing an ordered plurality of uniformly spaced conductors, comprising the steps of:

driving each conductor with a current corresponding to the logical value of a bit in an ordered sequence of bits in one-to-one correspondence with the ordered plurality of conductors; and

shifting by one bit at a time along the ordered sequence of bits an integral number of consecutive logical ones in alternation with an equal number of consecutive logical zeros, with equal intervening time intervals between each shift.

35. A method of propagating differing wavelength half-wave symmetric portions of a magnetic field across a surface containing an ordered plurality of uniformly spaced conductors, comprising the steps of:

driving each conductor with a current corresponding to the logical value of a bit in an ordered sequence of bits in one-to-one correspondence with the ordered plurality of conductors.

shifting by one bit at a time along the ordered sequence of bits a first integral number of consecutive logical ones in alternation with an equal number of consecutive logical zeros, with equal intervening first time intervals between each such shift; and

shifting by one bit at a time along the ordered sequence of bits a second integral number of consecutive logical ones in alternation with an equal number of consecutive logical zeros, with equal intervening second time intervals between each such shift.

36. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. propagating along the measurement axis a plurality of cycles of a half-wave symmetric magnetic field;

b. coupling the cursor to the propagating half-wave symmetric magnetic field to produce an AC signal having half-wave symmetry;

c. filtering the AC signal;

d. measuring the phase difference between a reference signal and the filtered AC signal; and

e. diminishing the measured phase difference by an amount corresponding to the fractional portion of the delay in excess of an integral number of cycles of the AC signal through the filter.

37. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. propagating along the measurement axis a plurality of cycles of a half-wave symmetric magnetic field;

b. coupling the cursor to the propagating half-wave symmetric magnetic field to produce an AC signal having half-wave symmetry;

c. filtering the AC signal;

d. measuring the phase difference between a reference signal and the filtered AC signal;

e. measuring the fractional portion of the delay in excess of an integral number of cycles of the AC signal through the filter; and

f. diminishing the measured phase difference by an amount corresponding to the measured fractional portion of the delay through the filter.

38. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. in alternation with step b below, propagating along the measurement axis at a first rate a plurality of half-wave symmetric cycles of a coarse magnetic field having a first wavelength longer than the longest distance to be determined;

b. in alternation with step a above, propagating along the measurement axis at a second rate a plurality of half-wave symmetric cycles of a fine magnetic field having a second wavelength that is an aliquot portion of the first wavelength, the ratio of the first rate to the first wavelength equalling the ratio of the second rate to the second wavelength;

c. coupling the cursor to the propagating magnetic fields to produce coarse and fine AC signals each of the same frequency and each having half-wave symmetry;

d. filtering the coarse and fine AC signals;

e. measuring the respective coarse and fine phase differences between a reference signal and each of the filtered coarse and fine AC signals;

f. diminishing each of the measured coarse and fine phase differences by an amount corresponding to the fractional portion of the delay in excess of an integral number of cycles of the coarse and fine AC signals through the filter; and

g. combining the diminished coarse and fine phase differences.

39. A method of determining the distance of a cursor along a measurement axis comprising the steps of:

a. in alternation with step b below, propagating along the measurement axis at a first rate a plurality of half-wave symmetric cycles of a coarse magnetic field of a first wavelength longer than the longest distance to be determined;

b. in alternation with step a above, propagating along the measurement axis at a second rate a plurality of half-wave symmetric cycles of a fine magnetic field of a second wavelength that is an aliquot portion of the first wavelength, the ratio of the first rate to the first wavelength equalling the ratio of the second rate to the second wavelength;

c. coupling the cursor to the propagating magnetic fields to produce coarse and fine AC signals each of the same frequency and having half-wave symmetry;

d. filtering the coarse and fine AC signals;

e. measuring the respective coarse and fine phase differences between a reference signal and each of the filtered coarse and fine AC signals;

f. measuring the fractional portion of the delay in excess of an integral number of cycles of the coarse and fine AC signals through the filter;

g. diminishing each of the measured coarse and fine phase differences by an amount corresponding to the measured fractional portion of the delay through the filter; and

h. combining the diminished coarse and fine phase differences.

40. In a digitizer having a filter responsive to an output signal from a cursor coupled to one or more conductors in a platen, a method of determining for a given frequency the remainder of the filter delay in excess of n periods of the given frequency, n=0, 1,2,3, . . . , the method comprising the steps of:

coupling a reference signal of the given frequency to the input of the filter in place of the signal from the cursor; and

measuring the phase difference between the reference signal and the output of the filter.

41. A method of error reduction while combining measured coarse and fine components of the distance to a cursor along a measurement axis comprising the steps of:

a. measuring the coarse component with sufficient resolution to allow prediction of the fine component;

b. measuring the fine component;

c. converting the coarse component into an equivalent integral number n of maximum length fine distance units plus a remainder portion r corresponding to a fractional amount of a fine distance unit;

d. incrementing the integral number n by one whenever both the measured fine component is less than a preselected first lower limit and the remainder portion r is greater than a preselected first upper limit;

e. decrementing the integral number n by one whenever both the measured fine component is greater than a preselected second upper limit and the remainder portion r is less than a preselected second lower limit; and

f. after steps a through e, combining the measured fine component with the product of a weighting factor times the value of the integral number n.

42. A digitizer providing cartesian coordinates corrected for motion of a cursor traveling along a path upon a platen, the digitizer comprising:

coordinate measurement means for measuring abscissas and ordinates in alternate succession, and determining consecutive abscissa and ordinate pairs representing points dissociated from the path traveled by the cursor;

velocity determination means, responsive to the consecutive abscissa and ordinate pairs, for determining a quantity indicative of the velocity of the cursor; and

coordinate correction means, responsive to the consecutive abscissa and ordinate pairs and to the quantity indicative of the velocity of the cursor, for correcting consecutive abscissa and ordinate pairs to correspond to points located on the path traveled by the cursor.

43. A digitizer providing cartesian coordinate sets, each set including first and second elements defining magnitude, the digitizer comprising:

a digitizing surface upon which coordinate sets may be digitized;

cursor means movable along a path, for selecting points on the digitizing surface;

measurement control means for repetitively indicating sequences of first and second periods of time;

first axis measurement means responsive to the position of the cursor during the first period of time for determining the first element of a coordinate set;

second axis measurement means responsive to the position of the cursor during the second period of time for determining the second element of said last named coordinate set;

coordinate means repetitively responsive to the first and second axis measurement means for providing a sequence of coordinate sets uncorrected for cursor motion;

offset determination means responsive to the sequence of coordinate sets from the coordinate means for determining the amount by which one element of a coordinate set must be corrected in a direction parallel to the axis of said one element to provide the coordinate set of a point on the path described by the motion of the cursor; and

offset correction means responsive to the offset determination means and to the coordinate means for correcting said one element by the amount provided by the offset determination means, thereby providing a coordinate set corrected for cursor motion.

44. A method by which digitized cartesian coordinates of a cursor location having ordinate and abscissa dimensions are corrected for errors produced by motion of the cursor during the digitization of the coordinates, comprising the steps of:

digitizing ordinate and abscissa dimensions in alternate succession;

determining the velocity of the cursor along one of said ordinate and abscissa dimensions; and

correcting said one digitized dimension by an amount equal to the product of said cursor velocity by the time required to digitize the other of said dimensions.

45. A method as in claim 44 wherein the step of digitizing the ordinate and abscissa dimensions includes the step of averaging a series of consecutively obtained values to produce the value of the dimension prior to its use in the steps for the correction of errors due to cursor motion.

46. A method by which digitized cartesian coordinates of a cursor location having ordinate and abscissa dimensions, each including coarse and fine components, are corrected for errors produced by motion of the cursor during the digitization of the coordinates, comprising the steps of:

digitizing ordinate and abscissa dimensions in alternate succession;

determining the velocity of the cursor along one of said ordinate and abscissa dimensions; and

correcting said one digitized dimension by an amount equal to the product of said cursor velocity by the time required to digitize the fine component of the other of said dimensions.

47. A method as in claim 46 wherein the step of digitizing the ordinate and abscissa dimensions includes the step of averaging a series of consecutively obtained values for at least one of the coarse and fine components.

48. Apparatus for digitizing cartesian coordinates of a point located on a platen and correcting them for error produced by non-orthogonality of grids within said platen, the apparatus comprising:

first coordinate measurement means for determining a first element of a coordinate set for a point located on said platen;

second coordinate measurement means for determining a second element of said coordinate set for said point;

skew angle encoder means for storing a predetermined quantity indicative of the degree of non-orthogonality between grids within said platen; and

coordinate correction means, responsive to the first and second coordinate measurement means and to the skew angle encoder means, for correcting one of said first and second elements of said coordinate set to obtain a cartesian coordinate set corrected for grid non-orthogonality.

49. A method of correcting digitized coordinates for errors produced by non-orthogonality of grids in a platen, comprising the steps of:

ascertaining the error angle by which the grids are non-orthogonal;

digitizing an uncorrected abscissa and ordinate; and

calculating a corrected abscissa equal to the sum obtained by finding the product of the uncorrected ordinate multiplied by the tangent of the error angle, finding the quotient of the uncorrected abscissa divided by the cosine of the error angle, and then adding the product to the quotient.

50. A method of correcting digitized coordinates for errors produced by non-orthogonality of grids in a platen, comprising the steps of:

ascertaining the error angle by which the grids are non-orthogonal;

digitizing an uncorrected abscissa and ordinate; and

calculating a corrected ordinate equal to the sum obtained by finding the product of the uncorrected abscissa multiplied by the tangent of the error angle, finding the quotient of the uncorrected ordinate divided by the cosine of the error angle, and then adding the product to the quotient.

51. A digitizer platen comprising:

a series of parallel active conductors having a first active conductor at one extreme of the series and a last active conductor at the other extreme of the series;

a first section of at least one conductor, the first section being outside the series of parallel active conductors and adjacent to the first active conductor, each conductor of the first section being parallel with and electrically connected to the first active conductor; and

a second section of at least one conductor, the second section being outside the series of parallel active conductors and adjacent to the last active conductor, each conductor of the second section being parallel with and electrically connected to the last active conductor.

52. A method of reducing error in a coordinate digitized at an extremity of a digitizing platen incorporating a series of selectively energized parallel active conductors, the method comprising the step of energizing at least one additional conductor, located at said extremity and parallel to said collection, whenever the active conductor nearest said extremity is energized.

53. A method of reducing error due to cursor motion while sequentially digitizing coarse and fine components for first and second elements of cartesian coordinates, comprising the steps of:

digitizing a coarse component of a first element of a cartesian coordinate;

digitizing a fine component of said first element;

digitizing without delay a fine component of a second element of said cartesian coordinate; and

digitizing a coarse component of said second element.

54. A method of sequentially digitizing coarse, fine, and reference components for first and second elements of cartesian coordinates, comprising the steps of:

digitizing a coarse component of a first element of a cartesian coordinate;

digitizing a reference component of said first element;

digitizing a fine component of said first element;

digitizing a fine component of a second element of said cartesian coordinate;

digitizing a reference component of said second element; and

digitizing a coarse component of said second element.