Method of error compensation in a coordinate measuring machine

• US 20060266100A1
• Filed: 09/22/2004
• Published: 11/30/2006
• Est. Priority Date: 09/22/2003
• Status: Active Grant

• Current Assignee: Renishaw PLC
• Original Assignee: Renishaw PLC
• Inventors: Weston, Nicholas John, Trull, Stephen James, McMurtry, David Roberts, McFarland, Geoff, Nai, Kenneth Cheng-Hoe
• Granted Patent: US 7,568,373 B2
• Time in Patent Office: Days
• US Class Current: 73/1.790
• CPC Class Codes: G01B 21/042 Calibration or calibration artifacts G01B3/30, G01B9/02072 take precedence
• CPC Class Codes: G01B 21/045 Correction of measurements G01B9/02055 takes precedence
• CPC Class Codes: G01B 3/30 Bars, blocks, or strips in which the distance between a pair of faces is fixed, although it may be preadjustable, e.g. end measure, feeler strip
• CPC Class Codes: G01B 5/012 Contact-making feeler heads therefor

Abstract

A method of calibrating an articulating probe head comprising the steps of measuring an artefact of known dimensions with the workpiece sensing probe mounted on the articulating probe head, in which the articulating probe head is unlocked. An error functional map is generated corresponding to the difference between the measured and known dimensions of the artefact. Subsequent workpieces are measured with the articulating probe head unlocked and the corresponding correction applied. The true dimensions of the artefact may be determined by measuring it with a probe mounted on an articulating probe head in which the axes of the articulating probe head are locked. A mechanical lock is provided to lock the axes of the articulating probe head.

Claims

1. A method of calibrating an articulating probe head, the articulating probe head being mounted on an arm of a coordinate positioning apparatus, in which a surface sensing device mounted on the articulating probe head is moved into a position-sensing relationship with an artefact and a position reading taken, the method comprising the following steps, in any suitable order:

a) measuring an artefact whose true measurements have been determined, wherein there is relative movement between the surface sensing device and the arm of the coordinate positioning apparatus;

b) generating an error function or map corresponding to the difference between the measurements obtained in step a and the true measurements of the artefact;

c) measuring subsequent workpieces wherein there is relative movement between the surface sensing device and the arm of the coordinate measuring apparatus; and

d) correcting measurements of subsequent workpieces obtained in step c using the error function or map generated in step b.

2. A method according to claim 1 wherein the true measurement of the artefact is determined by using a calibrated artefact.

3. A method according to claim 2 wherein the calibrated artefact comprises at least one circular profile.

4. A method according to claim 1 wherein the true measurements of the artefact are determined by measuring the artefact when there is no relative movement between the surface sensing device and the arm of the coordinate positioning apparatus.

5. A method according to claim 4 wherein the step of measuring the artefact when there is no relative movement between the surface sensing device and the arm of the coordinate positioning apparatus is carried out at a slow speed.

6. A method according to claim 4 wherein the artefact comprises a workpiece in the series of workpieces.

7. A method according to claim 4 wherein the artefact has features the size and location of which approximate the workpiece.

8. A method according to claim 7 wherein where the features of the artefact differ from the features of the workpiece, the correction in step d) is achieved by interpolation.

9. A method according to claim 1 wherein in step a the arm of the coordinate measuring apparatus is stationary.

10. A method according to claim 1 wherein in step a) the arm of the coordinate measuring apparatus is moving at a constant velocity.

11. A method according to claim 1 wherein the measurements comprise continuous measurements.

12. A method according to claim 1 wherein the measurements comprise discrete measurements.

13. A method according to claim 1 wherein the surface sensing device is a contact probe.

14. A method according to claim 1 wherein the surface sensing device is a non-contact probe.

15. A method according to claim 4 wherein a locking mechanism prevents relative movement between the surface sensing device and the arm of the coordinate positioning apparatus.

16. A method according to claim 4 wherein the articulating probe head is provided with at least one motor to rotate the surface sensing device relative to the arm of the coordinate positioning apparatus, and wherein said at least one motor is used to prevent relative movement between the surface sensing device and the arm of the coordinate positioning apparatus.

17. A method according to claim 1 wherein steps (a) and(c) are carried out at a fast speed.

18. A method according to claim 4 wherein substantially the same measurement path is used in step (c) and when determining the true measurement of the artefact.

19. An articulating probe head comprising a first mount for mounting on a support and a second mount onto which a surface sensing device may be mounted, the second mount being rotatable relative to the first mount about one or more axis, characterised in that the articulated probe head is provided with at least one mechanical brake to lock the position of the second mount relative to the first mount about at least one axis and whereby at least one position measuring device is provided to determine the position of the second mount relative to the first mount about said at least one axis in the locked position.

20. An articulating probe head according to claim 19 wherein the articulating probe head includes a rotary member rotatable about at least one axis, and wherein the lock has a lock member which moves between first and second positions, in its first position it engages the rotarv member to lock the rotary member in position and in its second position it disengages the rotary member, allowing the rotary member to rotate.

21. An articulating probe head according to claim 20 wherein the rotary member comprises a drive belt.

22. An articulating probe head according to claim 20 wherein the rotary member comprises a driven wheel.

23. An articulating probe head according to claim 20 wherein the rotary member is provided with a toothed profile and wherein a toothed lock assembly is provided on the lock, such that in the first position the teeth of the brake assembly and the rotary member interlock.

24. An articulating probe head according to claim 19 wherein the lock has an actuator to move the lock member between its first and second positions.

25. An articulating probe head according to claim 24 wherein the point of contact between the actuator and the lock member is separated transversely from the point of contact between the lock member and the rotary member.

26. An articulating probe head according to claim 25, wherein the lock member comprises a lever assembly pivotable about a pivot point and wherein the point of contact between the lock member and the rotary member is located between the pivot joint and the point of contact between the actuator and the lock member.

27. An articulating probe head according to claim 19 wherein the lock member comprises a lever assembly.

28. An articulating probe head according to claim 20 wherein biasing means are provided to bias the lock member against the rotary member when the lock member is in its first position.

29. An articulating probe head according to claim 19 wherein at least one actuator is provided for fine adjustment of the position of the second mount relative to the first mount about said at least one axis.

30. An articulating probe head according to claim 29 wherein feedback from the position measuring device is used to determine the amount of adjustment required.

31. An articulating probe head according to claim 29 wherein the articulating probe head includes a rotary member rotatable about at least one axis, and wherein the lock has a lock member which moves between first and second positions, in its first position it engages the rotary member to lock the rotary member in position and in its second position it disengages the rotary member, allowing the rotary member to rotate and whereby the at least one actuator acts to change the position of the lock member whilst it is engaged with the rotary member.

32. An articulating probe head according to claim 29 in which the at least one actuator comprises at least one piezo stack.