Method for improving measurement accuracy of a portable coordinate measurement machine

• US 6,996,912 B2
• Filed: 09/09/2004
• Issued: 02/14/2006
• Est. Priority Date: 02/14/2002
• Status: Expired

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Hasloecher, Kenneth J., Raab, Simon, Barber, Marc, Sajedi, Seyed Ali
• Primary Examiner(s): Fulton, Christopher W.
• Assistant Examiner(s): Reis, Travis
• Application Number:
  US10/937,129
• Publication Number:
  US 20050028393A1
• Time in Patent Office: 523 Days
• Field of Search: 335/03, 33/1.PT, 33/1.N, 335/34
• US Class Current: 33/503
• CPC Class Codes: B25J 18/002 comprising beam bending compensation means
• CPC Class Codes: G01B 11/005 coordinate measuring machines
• CPC Class Codes: G01B 11/03 by measuring coordinates of points
• CPC Class Codes: G01B 11/25 by projecting a pattern, e.g. one or more lines, moiré fringes on the object G01B11/255 takes precedence ; image analysis for depth or shape recovery G06T7/50
• CPC Class Codes: G01B 21/045 Correction of measurements G01B9/02055 takes precedence
• CPC Class Codes: G01B 5/008 using coordinate measuring machines

Abstract

A method for improving the measurement accuracy of a portable coordinate measurement machine which comprises an articulated arm having jointed arm segments is presented. This method includes sensing deformation of a portion of the articulated arm when the arm is placed under a load, this deformation causing an error in the measurement made by the CMM, and correcting such error in response to the sensed deformation.

Claims

1. A portable coordinate measurement machine (CMM) which measures the position of an object in a selected volume, the CMM including a manually positionable articulated arm having opposed first and second ends, said arm including at least five rotary joints such that said articulated arm has at least five degrees of freedom, a measurement probe attached to a first end of said articulated arm and an electronic circuit which receives the position signals from transducers in said arm and which provides a digital coordinate corresponding to the position of the probe in a selected volume comprising:

means for sensing deformation of a portion of said articulated arm when said arm is placed under a load, said deformation causing an error in the measurement made by the CMM; and

means for correcting said error in response to said deformation.

2. The CMM of claim 1 wherein said deformation occurs in bearing structure associated with at least one of said joints.

3. The CMM of claim 1 wherein said deformation occurs in tubing associated with said articulated arm.

4. The CMM of claim 1 wherein said deformation causes an error in the angular measurement of at least one transducer associated with a joint and including:

means for correcting the error in the angular measurement.

5. The CMM of claim 1 wherein at least one of said rotary joints includes a periodic pattern of a measurable characteristic, at least two read heads spaced from and in communication with said pattern, said pattern and said at least two read heads being positioned within said joint so as to be rotatable with respect to each other, and wherein:

said at least two read heads sense deformation of the articulated arm.

6. The CMM of claim 5 wherein said two read heads are positioned 180 degrees apart.

7. The CMM of claim 5 wherein:

said pattern comprises an optical fringe pattern; and

said at least one read head comprises an optical read head.

8. The CMM of claim 7 wherein:

said optical fringe pattern is disposed on an optical encoder disk.

9. The CMM of claim 8 wherein said communication comprises:

said read head detecting the interference between diffraction orders to produce sinusoidal signals from said read head inserted in said fringe pattern, said sinusoidal signals being electronically interpolated to detect displacement.

10. The CMM of claim 5 wherein said at least two read heads cause cancellation effects that can be averaged.

11. The CMM of claim 5 wherein:

said pattern of a measurable characteristic is at least one of the characteristics selected from the group consisting of reflectivity, opacity, magnetic field, capacitance, inductance and surface roughness.

12. The CMM of claim 5 wherein:

said pattern is rotatable with respect to said at least two read heads; and

said two read heads are stationary with respect to said pattern.

13. The CMM of claim 5 wherein:

said pattern is stationary with respect to said at least two read heads; and

said at least two reads heads are rotatable with respect to said pattern.

14. The CMM of claim 5 wherein said joint further comprises:

a first and second housing, and a rotatable shaft extending from said second housing into said first housing;

a bearing disposed between said shaft and said first housing permitting said rotatable shaft to rotate within said first housing;

said pattern being attached to said rotatable shaft;

said at least two read heads being fixed within said first housing such that rotation of the first housing with respect to the second housing causes said at least two read heads to move relative to said pattern.

15. The CMM of claim 5 wherein said at least one joint comprises:

a first housing;

a second housing;

a rotatable shaft fixed to said second housing and extending into said first housing;

at least one bearing supported within said first housing and supporting said rotatable shaft for rotation about its axis;

wherein one of said pattern and said at least two read heads are fixed to an end of said shaft and the other of said pattern and said at least two read heads are fixed within said first housing.

16. The CMM of claim 5 wherein said at least one of said rotary joints comprises at least two of said rotary joints.

17. The CMM of claim 5 wherein said at least one of said rotary joints comprises at least three of said rotary joints.

18. The CMM of claim 5 wherein said at least one of said rotary joints comprises at least four of said rotary joints.

19. The CMM of claim 5 wherein said at least one of said rotary joints comprises at least five of said rotary joints.

20. The CMM of claim 5 including at least six rotary joints such that said articulated arm has at least six degrees of freedom and wherein said at least one of said rotary joints comprises at least six rotary joints.

21. The CMM of claim 5 including at least seven rotary joints such that said articulated arm has at least seven degrees of freedom and wherein said at least one of said rotary joints comprises at least seven rotary joints.

22. The CMM of claim 5 wherein said at least one of said rotary joints comprises all of said rotary joints.

23. The CMM of claim 1 wherein said joints comprise long joints for swiveling motion and short joints for hinged motion.

24. The CMM of claim 23 including three joint pairs, each joint pair comprising a long joint and a short joint.

25. The CMM of claim 1 wherein said joints are arranged in the joint configurations selected from the group consisting of 2-2-2, 2-1-2, 2-2-3, and 2-1-3.

26. The CMM of claim 1 wherein at least one of said rotary joints includes a periodic pattern of a measurable characteristic, at least one read head spaced from and in communication with said pattern, said pattern and said read head being positioned within said joint so as to be rotatable with respect to each other, and at least one sensor which measures relative movement in said periodic pattern with respect to said at least one read head and including:

means for using said at least one sensor to sense deformation of the articulated arm.

27. The CMM of claim 26 wherein said at least one sensor comprises a plurality of spaced sensors which measure displacement.

28. The CMM of claim 26 wherein said at least one sensor comprises a sensor which measures displacement.

29. The CMM of claim 28 including at least one sensor for measuring X-axis displacement of said pattern.

30. The CMM of claim 29 including at least one sensor for measuring Y-axis displacement of said pattern.

31. The CMM of claim 28 including at least one sensor for measuring Y-axis displacement of said pattern.

32. The CMM of claim 26 wherein said at least one joint includes a shaft surrounded, at least in part, by a housing, said shaft and said housing being adapted to rotate relative to one another, and wherein said at least one sensor includes at least one sensor for measuring relative movement between said shaft and said housing.

33. The CMM of claim 32 including a plurality of sensors for measuring relative movement between said shaft and said housing.

34. The CMM of claim 33 wherein said shaft is rotatable.

35. The CMM of claim 34 wherein said at least one sensor includes:

at least two sensors for measuring relative movement of said shaft including a first sensor for measuring X axis displacement and a second sensor for measuring Y axis displacement.

36. The CMM of claim 35 wherein said plurality of sensors for measuring relative movement of said shaft includes a third sensor for measuring X axis rotation, a fourth sensor for measuring Y axis rotation and a fifth sensor for measuring Z axis displacement.

37. The CMM of claim 36 wherein said third, fourth and fifth sensors are positioned at about 120 degrees with respect to each other.

38. The CMM of claim 35 wherein said at least one read head measures Z axis rotation of said shaft.

39. The CMM of claim 38 wherein said at least one read head measures Z axis rotation of said shaft.

40. The CMM of claim 32 wherein said shaft is rotatable.

41. The CMM of claim 32 including at least five sensors which, together with said read head, measure at least six degrees of freedom of said shaft.

42. The CMM of claim 26 wherein said at least one sensor includes:

at least two sensors which measure movement in said periodic pattern with respect to said at least one read head.

43. The CMM of claim 42 wherein:

said at least two sensors are positioned at about 90 degrees to each other.

44. The CMM of claim 26 wherein said at least one of said rotary joints comprises at least two of said rotary joints.

45. The CMM of claim 26 wherein said at least one of said rotary joints comprises at least three of said rotary joints.

46. The CMM of claim 26 wherein said at least one of said rotary joints comprises at least four of said rotary joints.

47. The CMM of claim 26 wherein said at least one of said rotary joints comprises at least five of said rotary joints.

48. The CMM of claim 26 including at least six rotary joints such that said articulated arm has at least six degrees of freedom and wherein said at least one of said rotary joints comprises at least six rotary joints.

49. The CMM of claim 26 including at least seven rotary joints such that said articulated arm has at least seven degrees of freedom and wherein said at least one of said rotary joints comprises at least seven rotary joints.

50. The CMM of claim 26 wherein said at least one of said rotary joints comprises all of said rotary joints.

51. A portable coordinate measurement machine (CMM) which measures the position of an object in a selected volume, the CMM including a manually positionable articulated arm having opposed first and second ends, said arm including at least one rotary joint, a measurement probe attached to a first end of said articulated arm and an electronic circuit which receives the position signals from transducers in said arm and which provides a digital coordinate corresponding to the position of the probe in a selected volume wherein at least one of said rotary joints includes a periodic pattern of a measurable characteristic, at least one read head spaced from and in communication with said pattern, said pattern and said read head being positioned within said joint so as to be rotatable with respect to each other, and at least one sensor which measures relative movement in said periodic pattern with respect to said at least one read head, comprising:

means for using said at least one sensor for sensing deformation of a portion of said articulated arm when said arm is placed under a load, said deformation causing an error in the measurement made by the CMM; and

means for correcting said error in response to said deformation.

52. The CMM of claim 51 wherein said deformation occurs in bearing structure associated with at least one of said joints.

53. The CMM of claim 51 wherein said deformation occurs in tubing associated with said articulated arm.

54. The CMM of claim 51 wherein said deformation causes an error in the angular measurement of at least one transducer associated with a joint and including:

means for correcting the error in the angular measurement.

55. The CMM of claim 51 wherein:

said pattern comprises an optical fringe pattern; and

said at least one read head comprises an optical read head.

56. The CMM of claim 55 wherein:

said optical fringe pattern is disposed on an optical encoder disk.

57. The CMM of claim 51 wherein:

said pattern of a measurable characteristic is at least one of the characteristics selected from the group consisting of reflectivity, opacity, magnetic field, capacitance, inductance and surface roughness.

58. The CMM of claim 51 wherein said joints comprise long joints for swiveling motion and short joints for hinged motion.

59. The CMM of claim 58 including three joint pairs, each joint pair comprising a long joint and a short joint.

60. The CMM of claim 51 wherein said joints are arranged in the joint configurations selected from the group consisting of 2-2-2, 2-1-2, 2-2-3, and 2-1-3.

61. The CMM of claim 51 wherein said at least one sensor comprises a plurality of spaced sensors which measure displacement.

62. The CMM of claim 51 wherein said at least one sensor comprises a sensor which measures displacement.

63. The CMM of claim 62 including at least one sensor for measuring X-axis displacement of said pattern.

64. The CMM of claim 63 including at least one sensor for measuring Y-axis displacement of said pattern.

65. The CMM of claim 62 including at least one sensor for measuring Y-axis displacement of said pattern.

66. The CMM of claim 51 wherein said at least one joint includes a shaft surrounded, at least in part, by a housing, said shaft and said housing being adapted to rotate relative to one another, and wherein said at least one sensor includes at least one sensor for measuring relative movement between said shaft and said housing.

67. The CMM of claim 66 including a plurality of sensors for measuring relative movement between said shaft and said housing.

68. The CMM of claim 67 wherein said shaft is rotatable.

69. The CMM of claim 68 wherein said at least one sensor includes:

at least two sensors for measuring relative movement of said shaft including a first sensor for measuring X axis displacement and a second sensor for measuring Y axis displacement.

70. The CMM of claim 69 wherein said plurality of sensors for measuring relative movement of said shaft includes a third sensor for measuring X axis rotation, a fourth sensor for measuring Y axis rotation and a fifth sensor for measuring Z axis displacement.

71. The CMM of claim 70 wherein said third, fourth and fifth sensors are positioned at about 120 degrees with respect to each other.

72. The CMM of claim 69 wherein said at least one read head measures Z axis rotation of said shaft.

73. The CMM of claim 72 wherein said at least one read head measures Z axis rotation of said shaft.

74. The CMM of claim 66 wherein said shaft is rotatable.

75. The CMM of claim 66 including at least five sensors which, together with said read head, measure at least six degrees of freedom of said shaft.

76. The CMM of claim 51 wherein said at least one sensor includes:

at least two sensors which measure movement in said periodic pattern with respect to said at least one read head.

77. The CMM of claim 76 wherein:

said at least two sensors are positioned at about 90 degrees to each other.

78. The CMM of claim 51 wherein said at least one of said rotary joints comprises at least two of said rotary joints.

79. The CMM of claim 51 wherein said at least one of said rotary joints comprises at least three of said rotary joints.

80. The CMM of claim 51 wherein said at least one of said rotary joints comprises at least four of said rotary joints.

81. The CMM of claim 51 wherein said at least one of said rotary joints comprises at least five of said rotary joints.

82. The CMM of claim 51 including at least six rotary joints such that said articulated arm has at least six degrees of freedom and wherein said at least one of said rotary joints comprises at least six rotary joints.

83. The CMM of claim 51 including at least seven rotary joints such that said articulated arm has at least seven degrees of freedom and wherein said at least one of said rotary joints comprises at least seven rotary joints.

84. The CMM of claim 51 wherein said at least one of said rotary joints comprises all of said rotary joints.

85. A method for improving the measurement accuracy of a portable coordinate measurement machine (CMM) which measures the position of an object in a selected volume, the CMM including a manually positionable articulated arm having opposed first and second ends, said arm including a plurality of rotary joints, a measurement probe attached to a first end of said articulated arm and an electronic circuit which receives the position signals from transducers in said arm and which provides a digital coordinate corresponding to the position of the probe in a selected volume wherein at least two of said plurality of rotary joints each includes a periodic pattern of a measurable characteristic, at least two read heads spaced from and in communication with said pattern, said pattern and said at least two read heads being positioned within said joint so as to be rotatable with respect to each other, comprising:

using said at least two read heads in each of said at least two rotary joints to sense deformation of a portion of said articulated arm when said arm is placed under a load, said deformation causing an error in the measurement made by the CMM; and

correcting said error in response to said deformation.

86. The method of claim 85 wherein said at least two of said rotary joints comprise at least three of said plurality of rotary joints.

87. The method of claim 85 wherein said at least two of said rotary joints comprise at least four of said plurality of rotary joints.

88. The method of claim 85 wherein said at least two of said rotary joints comprise at least five of said plurality of rotary joints.

89. The method of claim 85 wherein said at least two of said rotary joints comprise at least six of said plurality of rotary joints.

90. The method of claim 85 wherein said at least two of said rotary joints comprise at least seven of said plurality of rotary joints.

91. The method of claim 85 wherein said at least two of said rotary joints comprises all of said plurality of rotary joints.

92. A method for improving the measurement accuracy of a portable coordinate measurement machine (CMM) which measures the position of an object in a selected volume, the CMM including a manually positionable articulated arm having opposed first and second ends, said arm including a plurality of rotary joints, a measurement probe attached to a first end of said articulated arm and an electronic circuit which receives the position signals from transducers in said arm and which provides a digital coordinate corresponding to the position of the probe in a selected volume wherein at least one of said rotary joints includes a periodic pattern of a measurable characteristic, at least one read head spaced from and in communication with said pattern, said pattern and said read head being positioned within said joint so as to be rotatable with respect to each other, and at least one sensor which measures relative movement in said periodic pattern with respect to said at least one read head comprising:

using said at least one sensor for sensing deformation of a portion of said articulated arm when said arm is placed under a load, said deformation causing an error in the measurement made by the CMM; and

correcting said error in response to said deformation.

93. The method of claim 92 wherein said deformation occurs in bearing structure associated with at least one of said joints.

94. The method of claim 92 wherein said deformation occurs in tubing associated with said articulated arm.

95. The method of claim 92 wherein said deformation causes an error in the angular measurement of at least one transducer associated with a joint and including:

correcting the error in the angular measurement.

96. The method of claim 92 wherein:

said pattern comprises an optical fringe pattern; and

said at least one read head comprises an optical read head.

97. The method of claim 96 wherein:

said optical fringe pattern is disposed on an optical encoder disk.

98. The method of claim 92 wherein:

said pattern of a measurable characteristic is at least one of the characteristics selected from the group consisting of reflectivity, opacity, magnetic field, capacitance, inductance and surface roughness.

99. The method of claim 92 wherein said joints comprise long joints for swiveling motion and short joints for hinged motion.

100. The method of claim 99 including three joint pairs, each joint pair comprising a long joint and a short joint.

101. The method of claim 92 wherein said joints are arranged in the joint configurations selected from the group consisting of 2-2-2, 2-1-2, 2-2-3, and 2-1-3.

102. The method of claim 92 wherein said at least one sensor comprises a plurality of spaced sensors which measure displacement.

103. The method of claim 92 wherein said at least one sensor comprises a sensor which measures displacement.

104. The method of claim 103 including at least one sensor for measuring X-axis displacement of said pattern.

105. The method of claim 104 including at least one sensor for measuring Y-axis displacement of said pattern.

106. The method of claim 103 including at least one sensor for measuring Y-axis displacement of said pattern.

107. The method of claim 92 wherein said at least one joint includes a shaft surrounded, at least in part, by a housing, said shaft and said housing being adapted to rotate relative to one another, and wherein said at least one sensor includes at least one sensor for measuring relative movement between said shaft and said housing.

108. The method of claim 107 including a plurality of sensors for measuring relative movement between said shaft and said housing.

109. The method of claim 108 wherein said shaft is rotatable.

110. The method of claim 109 wherein said at least one sensor includes:

at least two sensors for measuring relative movement of said shaft including a first sensor for measuring X axis displacement and a second sensor for measuring Y axis displacement.

111. The method of claim 110 wherein said plurality of sensors for measuring relative movement of said shaft fastener includes a third sensor for measuring X axis rotation, a fourth sensor for measuring Y axis rotation and a fifth sensor for measuring Z axis displacement.

112. The method of claim 111 wherein said third, fourth and fifth sensors are positioned at about 120 degrees with respect to each other.

113. The method of claim 111 including at least five sensors which, together with said read head, measure at least six degrees of freedom of said shaft.

114. The method of claim 110 wherein said at least one read head measures Z axis rotation of said shaft.

115. The method of claim 114 wherein said at least one read head measures Z axis rotation of said shaft.

116. The method of claim 107 wherein said shaft is rotatable.

117. The method of claim 92 wherein said at least one sensor includes:

at least two sensors which measure movement in said periodic pattern with respect to said at least one read head.

118. The method of claim 117 wherein:

said at least two sensors are positioned at about 90 degrees to each other.

119. The method of claim 92 wherein said at least one of said rotary joints comprises at least two of said rotary joints.

120. The method of claim 92 wherein said at least one of said rotary joints comprises at least three of said rotary joints.

121. The method of claim 92 wherein said at least one of said rotary joints comprises at least four of said rotary joints.

122. The method of claim 92 wherein said at least one of said rotary joints comprises at least five of said rotary joints.

123. The method of claim 92 wherein said at least one of said rotary joints comprises at least six rotary joints.

124. The method of claim 92 wherein said at least one of said rotary joints comprises at least seven rotary joints.

125. The method of claim 92 wherein said at last one of said rotary joints comprises all of said rotary joints.