Method of determining the pen velocity along a graphic tablet, and graphic tablet suitable for carrying out the method

• US 6,271,836 B1
• Filed: 10/10/1997
• Issued: 08/07/2001
• Est. Priority Date: 10/11/1996
• Status: Expired due to Fees

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Makinwa, Kofi
• Primary Examiner(s): Hjerpe, Richard
• Assistant Examiner(s): DINH, DUC Q
• Application Number:
  US08/948,474
• Time in Patent Office: 1,397 Days
• Field of Search: 345/173-174, 345/104, 345/177, 345/179, 178/18-1-, 178/19-1-
• US Class Current: 345/173
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment

Abstract

The invention relates to the determination of the velocity of a conductive pen 102 along a graphic tablet 100. Generation of successive pulses in successive conductors, arranged in a regular pattern in the tablet, produces an electromagnetic signal in a conductive pen situated in the vicinity of the tablet. When the pen is near the writing surface of the tablet, the pen signal exhibits a local maximum which is indicative of the location of the pen. The distance between the edges to both sides of this local maximum constitutes an indication of the velocity of the pen.

Claims

1. A method of determining a velocity of a pen in a direction along a graphic tablet, provided with a regular pattern of conductors, comprising:

measuring an electromagnetic signal caused in the pen by a series, being a succession of elements, each element consisting of successive electric pulses generated in respective conductors and each element of the series causing in the pen a pen signal which is a combination of responses to the pulses of this element, and deriving the pen velocity in said direction from the shape of the pen signal to both sides of a local maximum in the pen signal.

2. A method as claimed in claim 1, further comprising calculating the pen velocity in said direction from the distance between the two edges to both sides of the local maximum in the pen signal.

3. A method as claimed in claim 1, further comprising deriving a position of the pen in said direction from the location of the local maximum of the pen signal.

4. A method as claimed in claim 3, further comprising successively determining pen positions in a first and in a second direction, together with associated pen velocities, the second direction extending transversely of the first direction, replacing the determined pen position in one direction by an estimate of the pen position in that direction at the instant of measurement of the other pen position, said estimate being made on the basis of the determined pen positions and pen velocities, so that two pen positions are obtained in two different directions associated with the same instant.

5. A graphic tablet provided with a pen, comprising:

a regular pattern of conductors and a first drive unit for generating a first series, being a succession of elements, each element consisting of successive electric pulses in respective conductors, each element of the first series causing in the pen a respective pen signal which is a combination of responses to the pulses of this element, wherein the graphic tablet is provided with a detection unit for deriving a pen velocity in a first direction along the tablet from the shape of the pen signal to both sides of a local maximum in the pen signal.

6. A graphic tablet as claimed in claim 5, wherein the detection unit is arranged to determine the pen velocity in the first direction from the distance between the two edges to both sides of the local maximum in the pen signal.

7. A graphic tablet as claimed in claim 6, wherein the graphic tablet and the pen are suitable for capacitive transfer of electric voltages in said pattern of conductors to the pen, resulting in said pen signal.

8. A graphic tablet as claimed in claim 7, wherein said pattern of conductors comprises a first set of parallel, straight conductors in a plane extending parallel to the tablet, the direction of the conductors extending transversely of the first direction, and the first drive unit is arranged to generate the first series in the first set of conductors.

9. A graphic tablet as claimed in claim 8, wherein said pattern of conductors comprises a second set of parallel conductors, extending transversely of the first set, the graphic tablet comprises a second drive unit for generating a second series in the second set of conductors, and the detection unit is arranged to determine a pen velocity in a second direction along the tablet in a manner analogous to the determination of the pen velocity in the first direction.

10. A graphic tablet as claimed in claim 5, wherein the detection unit is arranged to derive a position of the pen in the first direction along the tablet from the location of a local maximum of the pen signal caused by an element of the first series, and the detection unit is arranged to derive a position of the pen in the second direction along the tablet from the location of a local maximum of a pen signal caused by an element of the second series.

11. A graphic tablet as claimed in claim 10, wherein the detection unit is arranged to determine the pen positions and the pen velocities successively in the first and in the second direction, and to replace subsequently the determined pen position in one direction by an estimate of the pen position in that direction at the instant of measurement of the pen position in the other direction, so that two pen positions are obtained in two different directions associated with one and the same instant.

12. A graphic tablet as claimed in claim 5, further comprising an adaptive low-pass filter for filtering the pen signal, the degree of filtering of which being a function of the pen velocity.

13. A method of determining a velocity of a pen in a direction along a graphic tablet having a regular pattern of conductors, comprising:

applying successive electrical pulses in respective conductors;

sampling a pen signal arising from capacitive coupling of the pen to the conductors, wherein samples of the pen signal define a signal shape; and

deriving the pen velocity from the signal shape ranging from before to after a local maximum sample value of the samples of the pen signal.

14. The method of claim 13, wherein the maximum sample value has a sample time and further comprising:

determining respective times before and after the time of the maximum sample value at which respective sample values are less than the maximum sample value by a quantity that is a function of the maximum sample value; and

deriving the pen velocity from a difference between the respective times before and after the time of the maximum.

15. The method of claim 14, wherein the sample values at the respective times before and after the time of the maximum sample value are approximately one-half the maximum sample value.

16. The method of claim 13, further comprising deriving a position of the pen from the time of the maximum sample value.

17. The method of claim 13, wherein the signal shape is generally a bell curve and the velocity of the pen is derived from the width of the bell curve.

18. The method of claim 16, further comprising:

detecting velocity in a first direction if the width is less than a threshold width; and

detecting velocity in a second direction if the width is greater than the threshold width.

19. An apparatus for determining a velocity of a pen in a direction along a graphic tablet having a regular pattern of conductors, comprising:

means for applying successive electrical pulses in respective conductors;

means for sampling a pen signal arising from capacitive coupling of the pen to the conductors, wherein samples of the pen signal define a signal shape; and

means for deriving the pen velocity from the signal shape ranging from before to after a local maximum of the samples of the pen signal.

20. A system for determining a velocity of a pen in a direction along a graphic tablet, comprising:

a regular pattern of conductors;

a first drive unit coupled to the conductors and arranged to generate successive electrical pulses to the conductors;

a detection unit coupled to the pen, the detection unit configured and arranged to sample a signal from the pen arising from capacitive coupling of the pen to the conductors, wherein samples of the pen signal define a signal shape, the detection unit further configured to derive the pen velocity from the signal shape ranging from before to after a local maximum sample value of the samples of the pen signal.

21. The system of claim 20, wherein the maximum sample value has a sample time and the detection unit is further configured and arranged to determine respective times before and after the time of the maximum sample value at which respective sample values are less than the maximum sample value by a quantity that is a function of the maximum sample value, and derive the pen velocity from a difference between the respective times before and after the time of the maximum.

22. The system of claim 21, wherein the sample values at the respective times before and after the time of the maximum sample value are approximately one-half the maximum sample value.

23. The system of claim 20, wherein the detection unit is further configured and arranged to derive a position of the pen from the time of the maximum sample value.

24. The system of claim 20, wherein the signal shape is generally a bell curve and the velocity of the pen is derived from the width of the bell curve.

25. The system of claim 24, wherein the detection unit is further configured and arranged to detecting velocity in a first direction if the width is less than a threshold width, and detect velocity in a second direction if the width is greater than the threshold width.