Apparatus and method for making accurate three-dimensional size measurements of inaccessible objects

• US 6,009,189 A
• Filed: 08/16/1996
• Issued: 12/28/1999
• Est. Priority Date: 08/16/1996
• Status: Expired due to Fees

• Current Assignee: David F. Schaack
• Original Assignee: David F. Schaack
• Inventors: Schaack, David F.
• Primary Examiner(s): Couso, Jose L.
• Assistant Examiner(s): PATEL, KANJIBHAI B
• Application Number:
  US08/689,993
• Time in Patent Office: 1,229 Days
• Field of Search: 382/201, 382/106, 382/154, 382/255, 348/40, 348/42, 348/43, 348/50, 348/51, 348/85, 348/137, 348/142, 348/95, 702/127, 702/150, 702/152, 702/153, 702/158, 356/3
• US Class Current: 382/154
• CPC Class Codes: A61B 1/00147 Holding or positioning arrangements
• CPC Class Codes: A61B 1/00183 for variable viewing angles
• CPC Class Codes: A61B 1/05 characterised by the image sensor, e.g. camera, being in the distal end portion
• CPC Class Codes: A61B 5/065 Determining position of the probe employing exclusively positioning means located on or in the probe, e.g. using position sensors arranged on the probe
• CPC Class Codes: A61B 5/1076 for measuring dimensions inside body cavities, e.g. using catheters A61B3/1005 takes precedence
• CPC Class Codes: G01B 11/02 for measuring length, width or thickness G01B11/08 takes precedence
• CPC Class Codes: G02B 23/24 Instruments or systems for viewing the inside of hollow bodies, e.g. fibrescopes
• CPC Class Codes: G02B 23/2423 of the distal end

Abstract

Spatial locations of individual points on an inaccessible object are determined by measuring two images acquired with one or more cameras which can be moved to a plurality of positions and orientations which are accurately determined relative to the instrument. Once points are located, distances are easily calculated. In distinction to prior art, which uses a fixed separation of camera viewpoints, this new system offers smaller errors, measurement over a larger range of object distances, and measurement of distances which cannot be contained in a single camera view. Random errors are minimized by use of an optimum measurement geometry. Systematic errors are minimized by use of a complete and robust set of calibration procedures. A standard measurement procedure automatically obtains the optimum measurement geometry. A least squares calculation uses all of the image location and calibration data to derive the true three dimensional positions of the selected object points. This calculation is taught explicitly for any camera geometry and motion. In preferred embodiments, the image locations of selected object points are determined by alignment of video cursors with the video images of those points. In certain of the preferred embodiments, the camera is a standard, side-looking rigid borescope, enabling this improved measurement system to be retrofit to existing remote inspection systems at minimum cost. In other preferred embodiments, the measurement system is implemented in either a rigid borescope or in a flexible endoscope.

Claims

1. A method of perspective measurement of the three-dimensional distances between individual user selected points on a remote object using a camera having an angular field of view and an internal coordinate system, said camera being translated along a substantially straight line from a first viewing position to a second viewing position, with there existing a distance between said first and second viewing positions, wherein the improvement comprises the steps of selecting said first and second camera viewing positions such that said distance between said first and second viewing positions is variable, said first and second viewing positions being selected by the user to be appropriate for each individual measurement being made, thereby obtaining the advantages of lower random error in the perspective measurement and ability to make the measurement in a wider range of situations.

2. The method of claim 1 wherein the measurement result is determined with a least squares estimation procedure.

3. The method of claim 1 wherein the orientation of said camera internal coordinate system with respect to said substantially straight line is determined in a calibration process and wherein said orientation is taken into account in the measurement result.

4. The method of claim 3 wherein errors in the motion of the camera are determined in a calibration process and wherein these errors are taken into account in the measurement result.

5. The method of claim 1 wherein the first and second viewing positions are selected so that a single point on the object is viewed at an apparent angular position near one edge of the field of view at the first viewing position, and at substantially the same apparent angle on the other side of the field of view at the second viewing position.

6. The method of claim 5 wherein said apparent angular position is chosen to correspond to an optimum measurement angle, thereby making measurements with the smallest feasible random errors.

7. An electronic measurement borescope apparatus for measuring three-dimensional distances between selected points on an inaccessible object, comprising:

(a) a video camera, including an imaging lens and a solid state imager, for producing video images of the object, and a video monitor, for displaying said video images;

(b) a linear translation means, for moving the video camera with a substantially constant orientation along a substantially straight line, said linear translation means and camera being disposed at the distal end of a rigid probe, and said linear motion means also having a range of travel;

(c) an actuating means, for moving the linear translation means to any position within its range of travel;

(d) a position measurement means, for determining the position of the linear translation means within said range of travel, whereby the position of the video camera is also determined, said position measurement means also producing position measurement data, said position measurement means also having a first data transfer means for supplying the camera position data to a computing means;

(e) a video cursor means, for displaying variable position cursors on said video image, said video cursor means having a second data transfer means for supplying the spatial positions of said variable position cursors to the computing means; and

(f) said computing means having a user interface, said user interface being in communication with said video cursor means and said second data transfer means such that a user can manipulate said video cursor means until said variable position cursors are aligned with the images of said selected points on said inaccessible object, and further such that said spatial positions of said variable position cursors are supplied to the computing means at user command, and further such that said computing means receives the camera position data through said first data transfer means, and further such that said computing means calculates and displays the three-dimensional distances between the selected points on said inaccessible object.

8. The apparatus of claim 7 wherein the actuating means is a motorized micrometer driving a positioning cable, said cable being looped around a pair of idler pulleys and being attached to the linear translation means.

9. The apparatus of claim 7 wherein the actuating means is a motorized micrometer located at the distal end of said rigid probe, said motorized micrometer being attached to the linear translation means.

10. The apparatus of claim 7 wherein said video camera has a field of view, and wherein an illumination means for illuminating said field of view is being carried by the linear translation means, such that the illumination of said field of view remains substantially constant as said camera is moved.

11. An electronic measurement endoscope apparatus for measuring three-dimensional distances between selected points on an inaccessible object, comprising:

(a) a video camera, including an imaging lens and a solid state imager, for producing video images of the object, and a video monitor, for displaying said video images;

(b) a linear translation means, for moving the video camera with a substantially constant orientation along a substantially straight line, said linear translation means also having a range of travel, and said linear translation means and camera being disposed internally into a rigid housing, said rigid housing being disposed at the distal end of a flexible endoscope housing;

(c) an actuating means, for moving the linear translation means to any position within its range of travel;

(d) a position measurement means, for determining the position of the linear translation means within said range of travel, whereby the position of the video camera is also determined, said position measurement means also producing position measurement data, said position measurement means also having a first data transfer means for supplying the position measurement data to a computing means;

(e) a video cursor means, for displaying variable position cursors on said video image, said video cursor means having a second data transfer means for supplying the spatial positions of said variable position cursors to the computing means; and

(f) said computing means having a user interface, said user interface being in communication with said video cursor means and said second data transfer means such that a user can manipulate said video cursor means until said variable position cursors are aligned with the images of said selected points on said inaccessible object, and further such that said spatial positions of said variable position cursors are supplied to the computing means at user command, and further such that said computing means receives the camera position data through said first data transfer means, and further such that said computing means calculates and displays the three dimensional distances between the selected points on said inaccessible object.

12. The apparatus of claim 11 wherein the actuating means is a positioning wire encased in a sheath, which is driven by a motorized micrometer.

13. The apparatus of claim 11 wherein the actuating means is a motorized micrometer located at the distal end of the apparatus, said motorized micrometer being attached to the linear translation means.

14. The apparatus of claim 11 wherein said video camera has a field of view, and wherein an illumination means for illuminating said field of view is being carried by the linear translation means, such that the illumination of said field of view remains substantially constant as said camera is moved.

15. A method of determining a set of three-dimensional coordinates for at least one point on an inaccessible object, thereby determining a location vector for each of said at least one point, comprising the steps of:

(a) providing one or more cameras, each of which has an internal coordinate system and an effective focal length, and providing motion means for moving at least one of said one or more cameras with respect to the inaccessible object, and further providing a plurality of relative camera positions for each of said cameras, wherein each of said cameras has a spatial orientation at each of said relative positions, and wherein said relative positions and said spatial orientations are determined in an external coordinate system;

(b) choosing first and second camera viewing positions, each of which corresponds to one of said relative camera positions, wherein there exists a distance between said first and second camera viewing positions, and wherein there exists a midpoint between said first and second camera viewing positions, and wherein there exists an angle subtended by the distance between said first and second camera viewing positions at any particular one of said at least one point on the inaccessible object, and wherein the distance between said first and second camera viewing positions is chosen to adjust said angle subtended at said particular one point to be substantially the same, independent of the distance between said particular one point on the inaccessible object and said midpoint between said viewing positions, whereby the random errors in the location vectors determined for said at least one point are minimized;

(c) acquiring a set of first images of said at least one point with one of said one or more cameras located at said first viewing position, said camera having a first spatial orientation at said first viewing position, thereby defining a first measurement coordinate system which is coincident with the internal coordinate system of said camera at said first viewing position;

(d) measuring the coordinates of each of said first images of said at least one point in said first measurement coordinate system;

(e) acquiring a set of second images of said at least one point with one of said one or more cameras located at said second viewing position, said camera having a second spatial orientation at said second viewing position, thereby defining a second measurement coordinate system which is coincident with the internal coordinate system of said camera at said second viewing position;

(f) measuring the coordinates of each of said second images of said at least one point in said second measurement coordinate system;

(g) correcting the measured coordinates of each of said first images of said at least one point to adjust for any distortion of the camera located at the first viewing position, and correcting the measured coordinates of each of said second images of said at least one point to adjust for any distortion of the camera located at the second viewing position, thereby producing sets of first and second final point image coordinates for said first and second viewing positions in said first and second measurement coordinate systems; and

(h) computing three dimensional coordinates for each of said at least one point using said first and second final point image coordinates, the effective focal length of the camera located at the first viewing position, and the effective focal length of the camera located at the second viewing position, and also using the relationships between said first and second viewing positions and said first and second spatial orientations determined in said external coordinate system, thereby computing a location vector for each of said at least one point.

16. The method of claim 15 in which step (h) comprises the steps of:

(h) multiplying the first final point image coordinates by the mathematical inverse of the effective focal length of the camera located at the first viewing position and multiplying the second final point image coordinates by the mathematical inverse of the effective focal length of the camera located at the second viewing position, to determine the mathematical tangents of the angles at which each of said at least one point is viewed in said first and second measurement coordinate systems; and

(i) forming least squares estimates of the three dimensional coordinates for each of said at least one point in a third measurement coordinate system using said mathematical tangents of the viewing angles for each of said at least one point in said first and second measurement coordinate systems and the relationships between said first and second camera viewing positions and said first and second camera spatial orientations determined in said external coordinate system, thereby forming a least squares estimate of the location vector for each of said at least one point in said third measurement coordinate system.

17. The method of claim 16 wherein the third measurement coordinate system is the same as the first measurement coordinate system.

18. The method of claim 15 wherein said angle subtended by the distance between said first and second camera viewing positions is calculated during the selection of at least one of said viewing positions using the changes in the coordinates of the image of said particular one point between one or more preliminary first views and one or more preliminary second views, the effective focal length of the camera used for the first views, the effective focal length of the camera used for the second views, and the relationships between the camera viewing positions and orientations.

19. The method of claim 16 for determining the three-dimensional distances between the points of each pair of any set of pairs of points in a plurality of points on an inaccessible object, comprising the steps of:

(i) performing steps (h) through (i) of claim 16 for said plurality of points;

(k) determining a difference vector between the location vectors of a first pair of said set of pairs of points by subtracting the location vector of a first point of said pair from the location vector of the second point of said pair;

(l) determining the length of the difference vector by calculating the square root of the sum of the squares of the components of the difference vector; and

(m) repeating steps (k) and (l) as necessary to determine the distances between the points of all remaining pairs in said set of pairs of points.

20. A method of determining the three-dimensional distance between a pair of points on an object, comprising the steps of:

(a) providing one or more cameras, each of which has an internal coordinate system and an effective focal length, and further providing a plurality of relative camera positions for each of said cameras, wherein each of said cameras has a spatial orientation at each of said relative positions, wherein said relative positions and said spatial orientations are determined in an external coordinate system, such that said camera positions form camera location vectors in said external coordinate system;

(b) acquiring a first image of a first point of said pair of points on the object with one of said one or more cameras located at a first viewing position, said camera having a first spatial orientation at said first viewing position, thereby defining a first measurement coordinate system which is coincident with the internal coordinate system of said camera at said first viewing position;

(c) acquiring a second image of said first point of said pair of points on the object with one of said one or more cameras located at a second viewing position, said camera having a second spatial orientation at said second viewing position, thereby defining a second measurement coordinate system which is coincident with the internal coordinate system of said camera at said second viewing position;

(d) measuring the coordinates of said first image of said first point in said first measurement coordinate system and measuring the coordinates of said second image of said first point in said second measurement coordinate system;

(e) correcting the measured coordinates of the first image of said first point to adjust for any distortion of the camera located at the first viewing position, and correcting the measured coordinates of the second image of said first point to adjust for any distortion of the camera located at the second viewing position, thereby producing first and second final first point image coordinates for said first and second viewing positions in said first and second measurement coordinate systems;

(f) multiplying the first final first point image coordinates by the mathematical inverse of the effective focal length of the camera located at the first viewing position and multiplying the second final first point image coordinates by the mathematical inverse of the effective focal length of the camera located at the second viewing position, to determine the mathematical tangents of the angles at which said first point is viewed in said first and second measurement coordinate systems;

(g) forming a least squares estimate of the three dimensional coordinates of said first point in a first temporary measurement coordinate system, thereby forming an estimate of the vector location of said first point in said first temporary measurement coordinate system, using said mathematical tangents of the viewing angles of said first point in said first and second measurement coordinate systems and the relationships between said first and second camera viewing positions and said first and second camera spatial orientations determined in said external coordinate system, wherein said first temporary coordinate system has an origin and wherein said origin has a vector location in said external coordinate system;

(h) calculating a vector location of said first point in said external coordinate system by adjusting the vector location of said first point in said first temporary measurement coordinate system according to said first and second camera spatial orientations;

(i) acquiring a first image of a second point of said pair of points on the object with one of said one or more cameras located at a third viewing position, said camera having a third spatial orientation at said third viewing position, thereby defining a third measurement coordinate system which is coincident with the internal coordinate system of said camera at said third viewing position;

(j) acquiring a second image of said second point of said pair of points on the object with one of said one or more cameras located at a fourth viewing position, said camera having a fourth spatial orientation at said fourth viewing position, thereby defining a fourth measurement coordinate system which is coincident with the internal coordinate system of said camera at said fourth viewing position, and wherein at least one of said third and fourth viewing positions is different from either of said first and second viewing positions;

(k) measuring the coordinates of said first image of said second point in said third measurement coordinate system and measuring the coordinates of said second image of said second point in said fourth measurement coordinate system;

(l) correcting the measured coordinates of the first image of said second point to adjust for any distortion of the camera located at the third viewing position, and correcting the measured coordinates of the second image of said second point to adjust for any distortion of the camera located at the fourth viewing position, thereby producing first and second final second point image coordinates for said third and fourth viewing positions in said third and fourth measurement coordinate systems;

(m) multiplying the first final second point image coordinates by the mathematical inverse of the effective focal length of the camera located at the third viewing position and multiplying the second final second point image coordinates by the mathematical inverse of the effective focal length of the camera located at the fourth viewing position, to determine the mathematical tangents of the angles at which said second point is viewed in said third and fourth measurement coordinate systems;

(n) forming a least squares estimate of the three dimensional coordinates of said second point in a second temporary measurement coordinate system, thereby forming an estimate of the vector location of said second point in said second temporary measurement coordinate system, using said mathematical tangents of the viewing angles of said second point in said third and fourth measurement coordinate systems and the relationships between said third and fourth camera viewing positions and said third and fourth camera spatial orientations determined in said external coordinate system, wherein said second temporary coordinate system has an origin and wherein said origin has a vector location in said external coordinate system;

(o) calculating a vector location of said second point in said external coordinate system by adjusting the vector location of said second point in said second temporary measurement coordinate system according to said third and fourth camera spatial orientations;

(p) calculating the vector location of the origin of the first temporary coordinate system by forming the average of the camera location vectors for the first and second camera viewing positions;

(q) calculating the vector location of the origin of the second temporary coordinate system by forming the average of the camera location vectors for the third and fourth camera viewing positions;

(r) calculating a vector from the origin of the second temporary coordinate system to the origin of the first temporary coordinate system by subtracting the vector location of the origin of the second temporary coordinate system from the vector location of the origin of the first temporary coordinate system;

(s) calculating the vector from the second point of said pair of points to the first point of said pair of points with the equation

space="preserve" listing-type="equation">r=d.sub.AB +r.sub.AG -r.sub.BG

wherein d_AB is the vector from the origin of the second temporary coordinate system to the origin of the first temporary coordinate system, r_AG is said vector location of said first point in said external coordinate system, and r_BG is said vector location of said second point in said external coordinate system; and

(t) calculating the distance between said pair of points by calculating the length of the vector r.

21. An apparatus for measuring three-dimensional distances between individual user selected points on an inaccessible object, comprising:

(a) a rigid borescope, having a length, being substantially side-looking and having an imaging means for producing images of said points on said object;

(b) linear motion means, for moving the borescope with a substantially constant orientation along a substantially straight line, said linear motion means also having a range of travel;

(c) position measurement means, for determining the position of the linear motion means within said range of travel, said position measurement means also producing position measurement data;

(d) clamping means, for clamping the borescope to the linear motion means at a desired position along said length of said borescope, whereby the position of the borescope is determined by the position of the linear motion means, and whereby said position measurement data also represents the position of the borescope;

(e) image measurement means, for measuring the positions of said images of said points, said image measurement means also producing position data of said images of said points; and

(f) computing means for receiving said borescope position data said position data of said images of said points, said computing means being adapted to calculate the three-dimensional distances between said points on said inaccessible object.

22. The apparatus of claim 21 wherein the position measurement means is a micrometer, and the linear motion means is a linear translation stage being driven by said micrometer.

23. The apparatus of claim 21 wherein the linear motion means is a linear translation stage being driven by an actuator, and the position measurement means is a linear position transducer attached to said translation stage.

24. The apparatus of claim 21 wherein said linear motion means is selected from the group consisting of crossed roller slides and ball slides and air bearing slides and dovetail slides.

25. The apparatus of claim 21 wherein said borescope has a field of view, and wherein said imaging means is comprised in part of a video sensor optically coupled to said borescope, and wherein said video sensor has different spatial resolutions along its two sensing axes, further wherein said video sensor is rotationally oriented with respect to said borescope such that a high spatial resolution axis thereof is aligned parallel to the projection of the linear motion of the borescope as observed in the field of view, thereby obtaining the highest precision in the distance measurement.

34. The apparatus of claim 21 wherein said imaging means is comprised in part of a video sensor and a video monitor, and wherein said image measurement means is comprised in part of an analog video cursor controller connected between said video sensor and said video monitor.

35. The apparatus of claim 23 wherein the actuator is an air cylinder.

26. An apparatus for measuring three-dimensional distances between individual user selected points on an inaccessible object, comprising at least one probe body and additionally comprising:

(a) one or more cameras located near the distal ends of said at least one probe body, said cameras forming images of said selected points on said object;

(b) motion means for moving at least one of said one or more cameras with respect to its probe body, said motion means providing a plurality of relative camera positions for each of said cameras;

(c) orientation means for providing a relative spatial orientation for each of said cameras at each of said relative positions;

(d) position determination means, for determining the relative positions of each of said one or more cameras, said position determination means also producing camera position data;

(e) orientation determination means, for determining the relative orientations of each of said one or more cameras, said orientation determination means also producing camera orientation data;

(f) image measurement means, for measuring the positions of said images of said user selected points on said object, said image measurement means also producing position data of said images of said user selected points; and

(g) computing means for receiving said camera position data and said camera orientation data and said position data of said images of said user selected points, said computing means being adapted to calculate the three-dimensional distances between said user selected points on said inaccessible object.

27. The apparatus of claim 26 wherein the orientation means is combined with the motion means such that said relative orientations are predetermined functions of said relative positions, thereby making it possible to eliminate the use of said orientation determination means except during calibration.

28. The apparatus of claim 26 wherein said plurality of relative camera positions constitutes a set of fixed relative positions, thereby making it possible to eliminate the use of said position determination means except during calibration.

29. The apparatus of claim 26 wherein said relative camera positions all lie along a substantially straight line.

30. The apparatus of claim 26 wherein said relative camera spatial orientations are all substantially the same.

31. The apparatus of claim 26 wherein said relative camera positions all lie along a substantially circular are.

32. The apparatus of claim 31 wherein said circular are has a center of curvature, and wherein each of said cameras has an optical axis, and wherein the orientation of each of said one or more cameras is coupled to its position along the are so that said optical axis is always substantially aligned with said center of curvature of the are.

33. The apparatus of claim 31 wherein said are is contained in a plane, and wherein each of said cameras has an optical axis, wherein the orientation of each of said one or more cameras is such that said optical axis is aligned substantially perpendicular to the plane containing the are.