GLOBAL CALIBRATION FOR STEREO VISION PROBE

• US 20080243416A1
• Filed: 03/30/2007
• Published: 10/02/2008
• Est. Priority Date: 03/30/2007
• Status: Abandoned Application

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Bryll, Robert Kamil
• Application Number:
  US11/694,837
• Publication Number:
  US 20080243416A1
• Time in Patent Office: Days
• US Class Current: 702/95
• CPC Class Codes: G01B 21/042 Calibration or calibration artifacts G01B3/30, G01B9/02072 take precedence
• CPC Class Codes: G01C 11/06 by comparison of two or more pictures of the same area
• CPC Class Codes: G01C 25/00 Manufacturing, calibrating, cleaning, or repairing instruments or devices referred to in the other groups of this subclass testing, calibrating or compensating compasses G01C17/38

Abstract

A method for global calibration of a multi-view vision-based touch probe measurement system is provided which encompasses calibrating camera frame distortion errors as well as probe form errors. The only required features in the calibration images are the markers on the touch probe. The camera frame distortion calibration comprises an iterative process that depends on a portable calibration jig and the touch probe, but that process is unaffected by probe form distortion errors in the touch probe and/or tip. The probe tip position calibration depends on applying the results of the camera frame distortion calibration. When the same probe tip is used throughout the global calibration, the probe tip position calibration uses images from the set of images used by the camera frame distortion calibration. The global calibration method is particularly advantageous for low cost portable versions of multi-view vision-based touch probe measurement systems.

Claims

1. A method for calibrating a multi-view vision-based touch probe system, the method comprising:

(A) providing a manual touch probe comprising a marker pattern including at least three probe markers and a probe tip that is fixed relative to the marker pattern;

(B) providing a multi-view triangulation system comprising at least two imaging viewpoints having intersecting fields of view, each viewpoint having a camera operable to provide an image of a probe marker located in the intersecting fields of view and the triangulation system operable to determine first-level 3D coordinates for the probe marker based on at least two respective images from at least two respective viewpoints;

(C) providing a reference object comprising a plurality of probe tip positioning reference features, wherein each probe tip positioning reference feature has at least one of a known geometric relationship and a known coordinate relationship in relation to other probe tip positioning reference features;

(D) estimating first-level 3D coordinates for each of a selected plurality of probe tip positioning reference features, the estimating comprising for each selected probe tip positioning reference feature;

(D-1) constraining the probe tip against translation at that probe tip positioning reference feature, and providing at least four orientations of the manual touch probe and the marker pattern, and for each of the at least four of the orientations;

(D-1-i) determining first-level 3D coordinates of each of the probe markers in the marker pattern for that orientation, and

(D-1 -ii) analyzing the first-level 3D coordinates of each of the probe markers in the marker pattern to determine first-level 3D coordinates for a marker pattern reference point of the marker pattern for that orientation,

(D-2) estimating the first-level 3D coordinates for that probe tip positioning reference feature based on the first-level 3D coordinates of at least four marker pattern reference points corresponding to the at least four orientations, such that the first-level 3D coordinate position of the probe tip positioning reference feature is estimated to be approximately equidistant to each of the first-level 3D coordinate positions of the at least four marker pattern reference points;

(E) determining a first-phase camera frame distortion characterization for distortions included in first-level 3D coordinates, based on comparing at least one of the known geometric relationships and the known coordinate relationships between the selected probe tip positioning reference features to corresponding relationships that are based on the estimated first-level 3D coordinates of the selected probe tip positioning reference features; and

performing operations comprising at least one of (F) and (G), wherein;

(F) comprises;

applying the first phase camera frame distortion characterization to estimate improved 3D coordinates for at least some of the selected probe tip positioning reference features, and determining a next-phase camera frame distortion characterization, based on comparing at least one of the known geometric relationships and the known coordinate relationships between the at least some of the selected probe tip positioning reference features to corresponding relationships that are based on the estimate improved 3D coordinates of the at least some of the probe tip positioning reference features, and

(G) comprises;

(G-1) corresponding to a selected probe tip positioning reference feature, applying one of the first-phase camera frame distortion characterization and a next-phase camera frame distortion characterization, to determine calibrated 3D coordinates of the probe markers in the marker patterns for at least four orientations of the manual touch probe and the marker pattern at that selected probe tip positioning reference feature;

(G-2) for each of the at least four orientations of the manual touch probe and the marker pattern in (G-1), determining a respective local coordinate system (LCS) based on the calibrated 3D coordinates of the probe markers in the marker pattern for that respective orientation;

(G-3) estimating calibrated 3D coordinates for the selected probe tip positioning reference feature of (G-1), based on calibrated 3D coordinates for respective marker pattern reference points identified in each of the respective LCSs determined in (G-2), such that the calibrated 3D coordinates for the selected probe tip positioning reference feature are approximately equidistant to the calibrated 3D coordinates of the respective marker pattern reference points;

(G-4) determining a plurality of respective probe tip position vectors in terms of the respective LCSs determined in (G-2), each respective probe tip position vector extending from a respective marker pattern reference points identified in a respective LCS to the location of the calibrated 3D coordinates for the selected probe tip positioning reference feature as expressed in terms of that respective LCS; and

(G-5) determining a probe tip position calibration based at least partially on the plurality of respective probe tip position vectors determined in (G-4).

2. A method for calibrating a multi-view vision-based touch probe system, the method comprising:

(A) providing a manual touch probe comprising a marker pattern including at least three probe markers and a probe tip that is fixed relative to the marker pattern;

(B) providing a multi-view triangulation system comprising at least two imaging viewpoints having intersecting fields of view, each viewpoint having a camera operable to provide an image of a probe marker located in the intersecting fields of view and the triangulation system operable to determine first-level 3D coordinates for the probe marker based on at least two respective images from at least two respective viewpoints;

(C) providing a reference object comprising a plurality of probe tip positioning reference features, wherein each probe tip positioning reference feature has at least one of a known geometric relationship and a known coordinate relationship in relation to other probe tip positioning reference features and is configured such that when the probe tip is constrained against translation at that reference feature an effective location of the center of the probe tip is the same for a plurality of angular orientations of the touch probe relative to the reference object;

(D) estimating first-level 3-D coordinates for each of a selected plurality of the probe tip positioning reference features, the estimating comprising for each selected probe tip positioning reference feature;

(D-1) constraining the probe tip against translation at that probe tip positioning reference feature and providing a plurality of orientations relative to the reference object, and for each of at least four of the orientations;

(D-1-i) determining the first-level 3D coordinates of each of the probe markers in the marker pattern for that orientation, and

(D-1-ii) analyzing the first-level 3D coordinates of each of the probe markers in the marker pattern to determine first-level 3D coordinates for a marker pattern reference point of the marker pattern for that orientation,

(D-2) estimating the first-level 3-D coordinate location for that probe tip positioning reference feature based on the first-level 3D coordinates of the marker pattern reference points corresponding to the at least four orientations, such that the first-level 3-D coordinate location for that probe tip positioning reference feature is estimated to be approximately equidistant to each of those marker pattern reference points as indicated by their first-level 3D coordinates;

(E) determining a first-phase camera frame distortion characterization for distortions included in first-level 3D coordinates, based on comparing at least one of the known geometric relationships and the known coordinate relationships of at least some of the selected plurality of the probe tip positioning reference features to corresponding relationships that are based on the estimated first-level 3D coordinates of the at least some of the selected plurality of probe tip positioning reference features; and

(F) estimating improved 3-D coordinates for at least one probe tip positioning reference feature, the estimating comprising for each at least one probe tip positioning reference feature;

(F1) applying the first-phase camera frame distortion characterization to determine improved 3D coordinates for a marker pattern reference point of a marker pattern associated with that probe tip positioning reference feature for at least four orientations of the manual touch probe and the marker pattern at that selected probe tip positioning reference feature, the at least four orientations provided while the probe tip is constrained against translation at that probe tip positioning reference feature; and

(F-2) estimating the improved 3-D coordinates for that probe tip positioning reference feature based on the improved 3D coordinates for the marker pattern reference points of the marker patterns for the at least four orientations, such that the 3-D coordinate location for that probe tip positioning reference feature is estimated to be approximately equidistant to each of those marker pattern reference points as indicated by their improved 3D coordinates.

3. The method of claim 2, further comprising:

(G) determining, for each of a plurality of the marker patterns of step (F-i), next-level 3D coordinates for each of their probe markers, and defining a marker pattern frame of reference for each of those marker patterns based on the next-level 3D coordinates for each of their probe markers; and

(H) determining, for each of the plurality of marker patterns having defined frames of reference in step (G), a probe tip position vector that extends from a marker pattern reference point in the defined frame of reference of that marker pattern to the improved 3-D coordinates of the nearest probe tip positioning reference feature if it has improved 3-D coordinates, and determining a probe tip position calibration based at least partially on a plurality of the probe tip position vectors.

4. The method of claim 3, wherein in step (G) the next-level 3D coordinates for the probe markers are determined based on applying the first-level distortion characterization to adjust and replace the first-level 3D coordinates of each of the probe markers in the marker pattern.

5. The method of claim 2, wherein in step (F-1) determining improved 3D coordinates for the marker pattern reference point of the marker pattern comprises, for each of the orientations:

determining next-level 3D coordinates for each of the probe markers in the associated marker pattern, based on applying the first-level distortion characterization to adjust and replace the first-level 3D coordinates of each of the probe markers in that marker pattern, and

determining the improved marker pattern reference point corresponding to the associated marker pattern, based on the next-level 3D coordinates for each of the probe markers for that marker pattern.

6. The method of claim 2, wherein the plurality of probe tip positioning reference features comprises at least four reference features.

7. The method of claim 2, wherein at least part of the method is iterated a plurality of times to improve the accuracy.

8. The method of claim 7, wherein the part of the method that is iterated a plurality of times includes at least step (B), wherein the first-phase camera frame distortion characterization becomes a next phase camera frame distortion characterization that is based on estimated next-level 3D coordinates of the at least some of the selected plurality of probe tip positioning reference features.

9. The method of claim 7, wherein the plurality of iterations are ceased when a selected criteria is reached.

10. The method of claim 9, wherein the selected criteria comprises one or more of:

a specified maximum number of iterations being reached, an error value being reached that is smaller than a selected threshold, or the selected computed estimated locations no longer changing above a selected threshold amount within a selected number of additional iterations.

11. The method of claim 2, wherein in step (D-1 -ii) the first-level marker pattern reference point of the marker pattern comprises a calculated centroid of the marker pattern.

12. The method of claim 11, wherein step (D-2) comprises fitting a sphere to the centroids of the marker patterns, and the center of the sphere determines the first-level 3-D coordinate location for that probe tip positioning reference feature.

13. The method of claim 2, further comprising determining one or more next-level camera frame distortion characterizations in addition to the first-level camera frame distortion characterization.

14. The method of claim 2, wherein the plurality of probe tip positioning reference feature on the reference object have 3-D coordinate locations which vary over approximately equal ranges in all three dimensions.

15. The method of claim 2, wherein the reference object further comprises markers which are separate from the markers on the probe and which may be utilized as part of the distortion calibration process.

16. The method of claim 3, wherein the probe tip is removable and when a new probe tip is attached to the probe at least the steps G and H are repeated to determine a new probe tip position calibration for that new probe tip.

17. A calibration device for use with a machine vision system, the vision system generally including a manual touch probe comprising a marker pattern and a probe tip that is fixed relative to the marker pattern, the vision system being operable to determine first-level three dimensional coordinates for the marker pattern, the calibration device comprising:

a reference object comprising a plurality of probe tip positioning reference feature configured such that for each probe tip positioning reference feature a location for an effective center of the probe tip is the same for a plurality of angular orientations of the touch probe relative to the reference object when the probe tip is constrained against translation in the reference feature and that effective center is defined a location of that probe tip positioning reference feature, arid the locations of the probe tip positioning reference features on the reference object relative to one another are known.

18. The device of claim 17, wherein the reference object is usable in combination with the machine vision system for calibration by:

(A) estimating a first-level reference feature location for each of a plurality of the probe tip positioning reference feature s on the reference object; and

(B) determining a first-phase camera frame distortion characterization of scale distortions included in the first- phase estimated reference feature locations, the first-phase camera frame distortion characterization determined based on comparing a configuration of known locations of at least some of the reference features to a corresponding configuration of the first-level reference feature locations.

19. The device of claim 17, wherein the reference object further comprises its own markers which are detectable by the machine vision system such that their first-level 3-D coordinates may be established and used for camera frame distortion calibration.