Calibration of capacitance probe

• US 5,583,443 A
• Filed: 06/07/1995
• Issued: 12/10/1996
• Est. Priority Date: 10/03/1990
• Status: Expired due to Fees

• Current Assignee: Renishaw PLC
• Original Assignee: Renishaw PLC
• Inventors: McMurtry, David R., Bound, David C., Thomas, David K.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): BROWN, GLENN WARNER
• Application Number:
  US08/481,161
• Time in Patent Office: 552 Days
• Field of Search: 324/149, 324/601, 324/662, 324/663, 324/684, 324/690, 324/754, 324/758
• US Class Current: 324/601
• CPC Class Codes: G01B 7/008 using coordinate measuring machines
• CPC Class Codes: G01B 7/023 for measuring distance between sensor and object G01B7/082 and G01B7/102 take precedence
• CPC Class Codes: G01B 7/28 for measuring contours or curvatures
• CPC Class Codes: G01B 7/34 for measuring roughness or irregularity of surfaces

Abstract

A capacitance probe is used by a coordinate measuring machine or machine tool to determine distances from the probe to the workpiece surface at various points over the surface. The probe is calibrated by moving it along a line, which is skewed to the surface. During this movement, a plurality of values of the capacitance and the corresponding values of the actual distance moved along the line are recorded. A datum value for the movement along the skewed line is also determined, which corresponds to a position at which the probe would touch the surface. The probe is calibrated without needing separate independent measurements of the distance from the probe to the surface. The workpiece surface is scanned using the thus-calibrate probe, in which different calibration values are used at different points on the surface, in order to account for the local shape of the surface.

Claims

1. A method of calibrating a capacitance probe of a position determining machine, said machine determining a displacement of the probe relative an origin of the machine, the probe measuring a capacitance between the probe and a surface, said surface located at an arbitrary and unknown position relative to the probe, the calibration method comprising the steps of:

moving the probe along a line which intersects the surface;

using only the position determining machine to measure actual displacements of the probe relative to the origin of the machine, as the probe is moved;

determining a touch point on the line at which the probe would touch the surface;

determining a probe displacement relative to the origin of the machine if said probe were at said touch point at which the probe would touch the surface; and

determining a relationship between measured capacitance between the probe and the surface and distance between the probe and the surface based on the determined probe displacement relative to the origin of the machine if said probe were at said touch point to enable the probe to be used to measure said distance.

2. A method according to claim 1, further comprising the step of monitoring a variation of the capacitance between the probe and the surface as a function of the actual position of the probe relative to the machine.

3. A method according to claim 2, wherein said touch point determining step comprises:

causing the probe to touch the surface; and

detecting a discontinuity in said variation of the capacitance at the position at which the probe touches the surface.

4. A method according to claim 3, wherein said discontinuity is detected by an electronic detector circuit.

5. A method according to claim 3, wherein said discontinuity determining step comprises:

recording a plurality of values of the capacitance as the probe is moved along said line; and

performing a computer analysis of the recorded values.

6. A method according to claim 1, wherein the probe includes a deflectable stylus for contacting the surface.

7. A method according to claim 6, wherein the probe comprises sensing means for providing a trigger signal upon contact with the surface, said touch point being determined upon an occurrence of said trigger signal.

8. A method according to claim 1, wherein the probe comprises sensing means for providing a trigger signal upon contact with the surface, said touch point being determined upon an occurrence of said trigger signal.

9. A method according to claim 1, wherein said touch point is determined by detecting a threshold value of capacitance as the probe moves towards the surface, corresponding to a negligibly small distance between the probe and the surface.

10. A method according to claim 1, wherein the skewed line is normal to the surface.

11. A method of calibrating a capacitance probe for measuring a distance between the probe and a surface by measuring a capacitance between the probe and the surface, the probe being mounted in a machine for producing relative movement between the probe and the surface, the machine having means for measuring the actual distance moved by the probe, the calibration method comprising:

moving the probe along a line which is skewed to the surface;

recording a plurality of values of the capacitance and the corresponding values of the actual distance moved by the probe along said line at points spaced at unknown distances from the surface; and

producing, from the recorded capacitance and distance values, calibration values for use in determining a distance between the probe and the surface from a measured value of capacitance.

12. A method according to claim 11, further comprising the step of calculating values of constants in an equation which relate the capacitance to the distance.

13. A method according to claim 11, wherein the skewed line is normal to the surface.

14. A method of scanning a workpiece surface, comprising passing a capacitance probe over points on the surface in spaced relationship therefrom, measuring a value of a capacitance between the probe and the surface at each point, and determining from the measured values of the capacitance the distances from the probe to the surface, wherein said probe is separately calibrated for each of a plurality of said points.

15. A method according to claim 14, further comprising the step of determining a local shape of the surface at at least one of said points.

16. A method according to claim 15, further comprising the step of predicting another point to be scanned from the determined local shape, and moving the probe in accordance with the prediction.

17. A method according to claim 14, wherein the calibration at each of said plurality of points is performed by moving the probe along a line which is skewed to the surface, and monitoring the variation of a capacitance between the probe and the surface as a function of the actual distance moved along the line.

18. A method according to claim 17, wherein said movement along said line is effected by superimposing on the probe an oscillating relative motion in a direction skewed to the surface, while simultaneously scanning the probe over said points on the surface in a spaced relationship.

19. A method according to claim 17, further comprising the step of determining a datum value for said movement along the line, the datum value corresponding to a position at which the probe would touch the surface.

20. A method according to claim 17, wherein the skewed line is normal to the surface.

21. A method according to claim 14, wherein the calibration at each of said plurality of points is performed by moving the probe along a line which is skewed to the surface, and recording a plurality of capacitance values and corresponding values of the actual distance moved along the line.

22. A method according to claim 21, further comprising the step of calculating from the recorded values the values of constants in an equation which relates the capacitance to the actual distance moved by the probe.

23. A method for calibrating a capacitive probe without using a direct measurement of distance between a surface of the capacitive probe and a surface to be analyzed, comprising:

(a) placing the surface of the capacitive probe at a first point, the first point spaced a first unknown distance from the surface to be analyzed;

(b) measuring a first capacitance between the surface of the capacitive probe and the surface to be analyzed when the surface of the capacitive probe is at the first point;

(c) placing the surface of the capacitive probe at a second point, the first and second points defining a line intersecting the surface to be analyzed at an intersection point, the second point spaced a second unknown distance from the surface to be analyzed and spaced along the line a first known distance from the first point;

(d) measuring a second capacitance between the surface of the capacitive probe and the surface to be analyzed when the surface of the capacitive probe is at the second point; and

(e) repeating steps (c) and (d) at at least one additional point, each additional point at an unknown distance from the surface to be analyzed and on the line;

(f) determining, based on at least the measured capacitances and the known distances between the first, second and each additional point, an estimated distance between the surface of the capacitive probe and the surface to be analyzed; and

(g) calibrating the capacitive probe based on the measured capacitances and the estimated distance between the surface of the capacitive probe and the surface to be analyzed.

24. The method of claim 23, wherein the second and each additional point is closer to the surface to be analyzed than any previous point.

25. The method of claim 23, wherein the estimated distance between the surface of the capacitive probe and the surface to be analyzed is an estimated distance between one of the first, second and each additional point and the intersection point.

26. The method of claim 23, further comprising:

(h) moving the surface of the capacitive probe to a current additional point closer to the surface to be analyzed than any preceding point, the current additional point being on the line and being a known distance from the first point;

(i) measuring an additional capacitance between the surface of the capacitive probe and the surface to be analyzed when the surface of the capacitive probe is at the current additional point; and

(j) updating the estimated distance based on the measured additional capacitance and the known distance corresponding to the current additional point.

27. The method of claim 26, further comprising:

(k) repeating steps (h)-(j) until the estimated distance is zero.

28. The method of claim 23, further comprising:

(h) determining a relationship between measured capacitance between the surface of the capacitive probe and the surface to be analyzed and distance between the surface of the capacitive probe and the surface to be analyzed based on the estimated distance, and the measured capacitances and the measured distances for the first, second and each additional point, to enable the capacitive probe to be used to measure the distance between the surface to the capacitance probe and the surface to be analyzed.

29. The method of claim 23, wherein the capacitive probe is mounted on a position determining machine, and each measured distance is measured using the position determining machine.

30. A method of scanning a surface to be analyzed with a capacitive probe at various points on the surface to be analyzed and for calibrating the capacitive probe at at least one of said points without using a direct measurement of distance between a surface of the capacitive probe and the surface to be analyzed, comprising:

(a) determining a first distance between the capacitive probe and the surface to be analyzed at a first point on the surface to be analyzed;

(b) moving the capacitive probe from the first point across the surface to be analyzed in a first direction and at a first speed, the capacitive probe spaced from the surface to be analyzed;

(c) as the capacitive probe is moved across the surface to be analyzed, moving the capacitive probe in a second direction which is at an angle to the first direction and at a second speed which is greater than the first speed, wherein the probe is alternately moved closer to and farther from the surface to be analyzed;

(d) measuring a capacitance between the surface of the capacitive probe and the surface to be analyzed at a plurality of points around each point as the probe is moved in the first and second directions;

(e) measuring a distance in the second direction from the first point for each of the plurality of points for each at least one of said points; and

(f) determining, for each point, based on at least the measured capacitances and the measured distances for the corresponding plurality of points, an estimated distance between the surface of the capacitive probe and the surface to be analyzed; and

(g) calibrating the capacitive probe for each point based on the measured capacitances and the estimated distance between the surface of the capacitive probe and the surface to be analyzed.

31. The method of claim 30, further comprising determining a local shape of the surface to be analyzed at at least one of said points.

32. The method of claim 31, further comprising:

predicting another one of the various points to be scanned from the determined local shape, and

moving the probe in accordance with the prediction.

33. The method of claim 30, wherein the calibration at each of said various points is performed by moving the probe along a line which is skewed to the surface to be analyzed, and monitoring the variation of a capacitance between the probe and the surface to be analyzed as a function of the actual distance moved along the line.

34. The method of claim 33, wherein said movement along said line is effected by superimposing on the probe an oscillating relative motion in a direction skewed to the surface to be analyzed, while simultaneously scanning the probe over said various points on the surface to be analyzed in a spaced relationship.

35. The method of claim 33, further comprising determining a touch point on the line at which the probe would touch the surface.

36. The method of claim 33, wherein the skewed line is normal to the surface to be analyzed.

37. The method of claim 30, wherein the calibration at each of said various points is performed by moving the probe along a line which is skewed to the surface to be analyzed, and recording a plurality of a capacitance values and corresponding values of the actual distance moved along the line.

38. The method of claim 37, further comprising calculating from the recorded values the values of constants in an equation which relates the capacitance to the actual distance moved by the probe.