Position feedback control system

• US 6,519,860 B1
• Filed: 10/19/2000
• Issued: 02/18/2003
• Est. Priority Date: 10/19/2000
• Status: Expired due to Fees

• Current Assignee: Sandia National Laboratories (Government of the United States of America)
• Original Assignee: Sandia Corporation (Martin Marietta Corporation)
• Inventors: Ensz, Mark T., Watson, Robert D., Jokiel, Bernhard Jr., Bieg, Lothar F.
• Primary Examiner(s): Gutierrez, Diego
• Assistant Examiner(s): Gonzalez, Madeline
• Application Number:
  US09/692,024
• Time in Patent Office: 852 Days
• Field of Search: 33/1.PT, 335/02, 335/03, 335/49, 335/55, 335/56, 335/59, 33D/IG.1, 73/17.9, 700/254
• US Class Current: 33/503
• CPC Class Codes: B23H 7/26 Apparatus for moving or positioning electrode relatively to workpiece; Mounting of electrode
• CPC Class Codes: G01B 5/008 using coordinate measuring machines

Abstract

Disclosed is a system and method for independently evaluating the spatial positional performance of a machine having a movable member, comprising an articulated coordinate measuring machine comprising: a first revolute joint; a probe arm, having a proximal end rigidly attached to the first joint, and having a distal end with a probe tip attached thereto, wherein the probe tip is pivotally mounted to the movable machine member; a second revolute joint; a first support arm serially connecting the first joint to the second joint; and coordinate processing means, operatively connected to the first and second revolute joints, for calculating the spatial coordinates of the probe tip; means for kinematically constraining the articulated coordinate measuring machine to a working surface; and comparator means, in operative association with the coordinate processing means and with the movable machine, for comparing the true position of the movable machine member, as measured by the true position of the probe tip, with the desired position of the movable machine member.

Claims

1. A metrology system for independently evaluating the spatial positional performance of a working machine having a movable member and a position controller responsive to a command to move said movable member to a desired position, the system comprising:

an articulated coordinate measuring machine (ACMM) having at least two degrees of freedom, comprising;

a first revolute joint;

a probe arm, having a proximal end rigidly attached to the first joint, and having a distal end with a probe tip attached thereto, wherein the probe tip is rotatably coupled to the movable machine member with rotatable coupling means;

a second revolute joint;

a first support arm serially connecting the first joint to the second joint; and

coordinate processing means, operatively connected to the first and second revolute joints, for calculating the spatial coordinates of the probe tip;

rotatable coupling means for rotatably coupling the probe tip to the movable machine member;

means for kinematically constraining the ACMM to a working surface; and

electronic comparator means, in operative association with the ACMM'"'"'s coordinate processing means and with the working machine'"'"'s position controller, for simultaneously comparing the true position of the movable machine member, as measured by the true position of the probe tip, with the desired position of the movable machine member to create a position error.

2. The system of claim 1, further comprising means for measuring changes in the length of the probe arm.

3. The system of claim 2, wherein the means for measuring changes in the length of the probe arm comprises a displacement sensing transducer or a laser interferometer.

4. The system of claim 1, wherein at least one of the revolute joints comprises:

a circular encoder wheel;

bearing means for supporting the encoder wheel, while permitting free rotation of the encoder wheel about the wheel'"'"'s axis of rotation;

a plurality of marks disposed around at least a portion of the circumference of the encoder wheel; and

a sensor, rigidly attached to the bearing means, for detecting the motion of at least some of the marks as the encoder wheel rotates; and

wherein the coordinate processing means further comprises means, operatively connected to the sensor, for converting the output of the sensor into an angle measurement.

5. The system of claim 4, further comprising a flexible encoder tape wrapped at least partially around the circumference of the encoder wheel, wherein the plurality of marks are disposed on said encoder tape.

6. The system of claim 1, further comprising:

a third revolute joint; and

a second support arm serially connecting the second joint to the third joint;

wherein the coordinate processing means is additionally operatively connected to the third revolute joint.

7. The system of claim 1, further comprising magnetic means for urging the probe tip towards the rotatable coupling means.

8. The system of claim 1, wherein the movable machine member comprises a six-axis or seven-axis articulated coordinate measuring machine.

9. The system of claim 1, further comprising protective means, surrounding at least some of the articulated coordinate measuring machine, for providing a nearly constant thermal environment, and for preventing debris from contaminating the coordinate measuring machine.

10. The system of claim 1, wherein the coordinate processor means further comprises:

means for storing a set of best-fit kinematic parameters; and

means for using the set of best-fit kinematic parameters for computing the probe tip'"'"'s position.

11. The system of claim 10, wherein the coordinate processor means further comprises:

means for storing a residual error map; and

means for correcting, in real-time, the probe tip'"'"'s apparent position by using the residual error map to adjust the probe tip'"'"'s reported position so that the actual error is reduced below an acceptable limit.

12. The system of claim 1, further comprising a second articulated coordinate measuring machine, having a second probe tip pivotally mounted to a second position on the movable machine member, whereby at least two additional degrees of freedom, representing the orientation of the movable machine member, can be measured.

13. The system of claim 12, further comprising a third articulated coordinate measuring machine, having a third probe tip pivotally mounted to a third position on the movable machine member, whereby all six degrees of freedom for the position and orientation of the movable machine member can be measured.

14. The system of claim 1, wherein the articulated coordinate measuring machine is portable.

15. The system of claim 1, wherein the probe tip comprises a precision spherical ball;

and further wherein the rotatable coupling means constrains all three translational degrees of freedom of the ball, but allows for unconstrained rotation of the ball about all three orthogonal axes.

16. The system of claim 1, wherein the rotatable coupling means is selected from the group consisting of a low-friction three-point kinematic pivot mount for rigidly holding a spherical ball in a stable tripod-like geometry, a gimbaled geometry, a universal joint geometry, and a ball-and-socket geometry.

17. The system of claim 1, wherein the three-point kinematic pivot mount comprises three precision-machined tooling balls rigidly mounted to a body and arranged approximately 120 degree apart.

18. The system of claim 1, wherein the rotatable coupling means is inserted directly into a tool holder or end effector position on the movable machine member.

19. The system of claim 1, further comprising air bearing means for supporting at least one revolute joint.

20. The metrology system of claim 19, wherein the acceptable position error limit is less than or equal to about ten microns.

21. The system of claim 1, further comprising feedback means for feeding back the position error to the working machine'"'"'s position controller, which adjusts the position of the movable machine member in a manner that reduces the position error below an acceptable limit.

22. The system of claim 21, wherein the acceptable position error limit is less than or equal to about ten microns.

23. A metrology system for independently evaluating the spatial positional performance of a working machine having a movable member and a position controller responsive to a command to move said movable member to a desired position, the system comprising:

an articulated coordinate measuring machine (ACMM) comprising;

a revolute joint, comprising;

a circular encoder wheel having an outside cylindrical surface;

bearing means for rotatably supporting the encoder wheel, while permitting free rotation of the encoder wheel about the wheel'"'"'s axis of rotation;

a plurality of uniformly spaced marks disposed around at least a portion of the outside cylindrical surface of the encoder wheel; and

a sensor, rigidly attached to the bearing means, for detecting the motion of at least some of the marks as the encoder wheel rotates; and

a probe arm, having a proximal end rigidly attached to the encoder wheel; and

having a distal end with a probe Up attached thereto;

wherein the probe tip is rotatably coupled to the movable machine member with rotatable coupling means; and

coordinate processing means, operatively connected to the sensor, for converting the output of the sensor into an angle measurement, and for calculating the spatial coordinates of the probe tip based on said angle measurement;

rotatable coupling means for rotatably coupling the probe tip to the movable machine member;

means for kinematically constraining the ACMM to a working surface; and

electronic comparator means, in operative association with the ACMM'"'"'s coordinate processing means and with the working machine'"'"'s position controller, for simultaneously comparing the true position of the movable machine member, as measured by the true position of the probe tip, with the desired position of the movable machine member to create a position error.

24. The system of claim 23, further comprising means for measuring changes in the length of the probe arm.

25. The system of claim 24, wherein the means for measuring changes in the length of the probe arm comprises a displacement sensing transducer or a laser interferometer.

26. The system of claim 23, wherein the plurality of marks comprise parallel lines spaced apart a distance equal to or greater than about 20 microns.

27. The system of claim 23, further comprising an interpolator module operatively associated with the sensor for electronically interpolating in-between two adjacent marks.

28. The system of claim 27, wherein the plurality of marks comprise parallel lines spaced equal to or greater than about 20 microns apart, and further wherein the interpolator module provides an effective line spacing that is greater than or equal to about 50 nm (0.05 microns).

29. The system of claim 23, wherein the ACCM comprises a structural metal alloy having essentially zero thermal expansion coefficient, selected from the group consisting of INVAR-36, SUPER INVAR, and KOVAR.

30. The system of claim 23, wherein the circular encoder wheel has a diameter greater than or equal to about 4.5 inches.

31. The system of claim 23, wherein the circular encoder wheel comprises a plurality of weight-reducing penetrations.

32. The apparatus of claim 23, wherein the outside cylindrical surface of the circular encoder wheel comprises a precisely ground or lapped surface.

33. A method of independently evaluating the spatial positional performance of a working machine having a movable machine member and a position controller responsive to a command to move said movable member to a desired position, the method comprising:

a) kinematically mounting the base of an articulated coordinate measuring machine (ACMM) having at least two degrees of freedom to a working surface, wherein the ACMM comprises a probe tip, coordinate processing means, electronic comparator means, and rotatable coupling means for rotatably coupling the probe tip to the movable machine member;

b) rotatably coupling the probe tip to the movable machine member with the rotatable coupling means;

c) operatively connecting the electronic comparator means to the working machine'"'"'s position controller and to the ACMM'"'"'s coordinate processing means;

d) commanding the working machine'"'"'s position controller to move the movable machine member to a home position;

e) initializing the ACMM'"'"'s coordinate processing means while the movable machine member is at the home position;

f) commanding the working machine'"'"'s position controller to move the movable machine member to the desired position;

g) measuring the position of the movable machine member with the ACMM to create a true position;

h) using the electronic comparator means to simultaneously compare the true position with the desired position to create a position error; and

i) repeating steps f) through h) as often needed to evaluate the spatial positional performance of the working machine.

34. The method of claim 33, wherein the step g) of measuring the position of the movable machine member is performed dynamically while the movable machine member is moving.

35. The method of claim 33, wherein the step g) of measuring the position of the movable machine member is performed statically after the movable machine member has completed the previous step f) of moving to a desired position and has stopped moving.

36. The method of claim 33, wherein at least one of the revolute joints comprises:

a circular encoder wheel;

bearing means for supporting the encoder wheel, while permitting free rotation of the encoder wheel about the wheel'"'"'s axis of rotation;

a plurality of marks disposed around at least a portion of the circumference of the encoder wheel;

a sensor, rigidly attached to the bearing means, for detecting the motion of at least some of the marks as the encoder wheel rotates; and

coordinate processing means, operatively connected to the sensor, for converting the output of the sensor into an angle measurement.

37. The method of claim 33, wherein the articulated coordinate measuring machine has been calibrated prior to performing steps d) through i);

wherein the calibration process comprises;

j) storing an initial estimate of kinematic parameters in the ACMM'"'"'s coordinate processing means;

k) measuring a plurality of ultra-accurate probe tip positions using an ultra-accurate reference metrology tool that is more accurate than the ACMM;

l) using a least-squares minimization algorithm to generate a set of best-fit kinematic parameters, utilizing the ultra-accurate coordinates of the plurality of probe tip positions measured in step k), whereby the average global position error within the workspace of the calibrated ACMM is minimized; and

m) storing the generated set of best-fit kinematic parameters within the ACCM'"'"'s coordinate processor means, whereby the ACMM becomes calibrated.

38. The method of claim 37, wherein measuring the position of the movable machine member in step g) further comprises using a residual error map stored in the coordinate processor means of the calibrated ACMM to adjust the probe tip'"'"'s reported position so that the real error in the probe tip'"'"'s coordinates is reduced below an acceptable error limit;

wherein the process of generating the residual error map comprises;

n) holding the probe tip in a first position;

o) generating a set of apparent coordinates of the first position of the probe tip with the calibrated ACMM;

p) independently measuring the first position of the probe tip with an ultra-accurate reference metrology tool that is more accurate than the ACMM, thereby generating a set of ultra-accurate coordinates;

q) calculating a first set of residual coordinate errors by comparing set of apparent coordinates of the first position of the probe tip with the set of ultra-accurate coordinates;

r) creating a residual error map by repeatedly performing steps n) through r) for a plurality of different probe tip positions; and

s) storing the residual error map within the ACMM'"'"'s coordinate processor means.

39. The method of claim 38, wherein the residual error map is represented mathematically by multi-variable polynomial functions.

40. The method of claim 38, wherein said acceptable error limit is better than about three microns.

41. The method of claim 33, further comprising after step h), and before step i), feeding back the position error to the working machine'"'"'s position controller, and adjusting the position of the movable machine member in a manner that reduces the position error below an acceptable limit.