Machine vision methods using feedback to determine calibration locations of multiple cameras that image a common object

• US 5,978,521 A
• Filed: 09/25/1997
• Issued: 11/02/1999
• Est. Priority Date: 09/25/1997
• Status: Expired due to Term

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Michael, David J., Wallack, Aaron S.
• Primary Examiner(s): Au, Amelia
• Assistant Examiner(s): FREDERICK, GILBERTO
• Application Number:
  US08/937,384
• Time in Patent Office: 768 Days
• Field of Search: 382/287, 382/291, 382/294, 382/295, 382/103, 382/130
• US Class Current: 382/294
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01B 11/275 for testing wheel alignment
• CPC Class Codes: G01B 21/042 Calibration or calibration artifacts G01B3/30, G01B9/02072 take precedence
• CPC Class Codes: G01B 2210/12 Method or fixture for calibrating the wheel aligner

Abstract

The invention provides improved machine vision methods for determining a calibration relationship among the imaging reference frames of multiple cameras (or other image acquisition devices) that acquire images of a common moveable object. The calibration relationship includes parameters for transforming positional coordinates among the camera'"'"'s respective imaging reference frames. The method includes placing on the common object, stage, or belt a plurality of fiducials, e.g., calibration marks, that are at known positions relative to each other with respect to a frame of reference referred to as the "alignment reference frame"; aligning an axis of the alignment reference frame with an axis of the object'"'"'s reference frame of motion, i.e., the reference frame in which the object, frame or belt moves; placing a respective fiducial in the field of view of each camera; determining a calibration relationship among the imaging reference frames of the respective image acquisition devices as a function of (i) the known relative locations of the fiducials (vis-a-vis the alignment reference frame), (ii) the fiducial calibrating positions determined for each camera (vis-a-vis that camera'"'"'s acquisition reference frame), and (iii) the object calibrating positions determined for each respective image acquisition device (vis-a-vis the motion reference frame).

Claims

1. A machine vision method of determining a calibration relationship between imaging reference frames of multiple respective image acquisition devices that acquire images of a common object, wherein the object has thereon at least two fiducials, each being at a known location relative to the other with respect to an alignment reference frame associated with at least a selected one of the fiducials, the method comprising the steps of:

A. aligning at least a selected axis of the alignment reference frame with at least a selected axis of the motion reference frame of the object;

B. for each image acquisition device,

i. placing a respective one of the fiducials in a field of view of that device;

ii. determining a position of that fiducial with respect to the imaging reference frame of that device, that position being referred to as a "fiducial calibrating position";

iii. determining a position of the object with respect to the motion reference frame, that position hereinafter referred to as an "object calibrating position"; and

C. determining a calibration relationship between the imaging reference frames of the respective image acquisition devices as a function of the known relative locations of the fiducials with respect to the alignment reference frame, as a function of fiducial calibrating position determined for each image acquisition device, and as a function of the object calibrating position of each respective image acquisition device.

2. A method according to claim 1, wherein step (A) comprises the steps of

i. placing the selected fiducial in a field of view of a selected image acquisition device;

ii. determining a location of the selected fiducial with respect to an imaging reference frame of the selected image acquisition device, that location being referred to as a "first fiducial position";

iii. moving the object along an axis of the motion reference frame and placing it in the field of view of the selected image acquisition device;

iv. determining a location of the selected fiducial with respect to the imaging reference frame of the selected image acquisition device, that location being referred to as a "second fiducial position";

v. determining an orientation of the motion reference frame with respect to the imaging reference frame of the selected image acquisition device as a function of the first and second fiducial positions; and

vi. rotating the object to substantially align the selected fiducial with the motion reference frame with respect to the imaging reference frame of the selected image acquisition device.

3. A method according to claim 2, wherein step (v) comprises determining the orientation of a line defined by the first and second fiducial positions.

4. A method according to claim 3, wherein step (v) comprises determining an orientation τ

of the motion reference frame by as a function of the relation;

space="preserve" listing-type="equation">τ

=-a tan 2(i.sub.y1 -i.sub.y0, i.sub.x1 -i.sub.x0)

where,

(i_x0, i_y0) are imaging reference frame coordinates of the first image position;

(i_x1, i_y1) are imaging reference frame coordinates of the second image position;

a tan 2() is an arctangent function.

5. A method according to claim 3, wherein step (vi) comprises rotating the object to bring the selected fiducial in substantial alignment with the line defined by the first and second fiducial positions.

6. A method according to claim 3, wherein the selected fiducial has an alignment feature and wherein step (vi) comprises rotating the object to bring the alignment feature in substantial alignment with the line defined by the first and second fiducial positions.

7. A method according to claim 6, wherein the alignment feature is substantially aligned with the selected axis of the alignment reference frame.

8. A method according to claim 1, wherein at least the selected fiducial has a linear feature in substantial alignment with the selected axis of the alignment reference frame and wherein step (A) comprises the steps of

(A)(i) placing the selected fiducial in a field of view of a selected image acquisition device;

(A)(ii) determining parameters of a line characterizing the linear feature with respect to the imaging reference frame of the selected image acquisition device, those parameters being to as "first linear parameters";

(A)(iii) moving the object along an axis of the motion reference frame;

(A)(iv) determining parameters of a line characterizing the linear feature with respect to the imaging reference frame of the selected image acquisition device, those parameters being to as "second linear parameters";

(A)(v) comparing the first and second linear parameters to determine whether the linear feature is in substantial alignment with the selected axis of the motion reference frame.

9. A method according to claim 8, wherein step (A)(v) comprises responding to a determination that the first and second linear parameters describe lines that do not have a identical axial intercept for rotating the object and repeating the steps (A)(i)-(A)(v).

10. A method according to claim 1, wherein step (C) comprises determining a calibration relationship between the imaging reference frames of the respective image acquisition devices in accord with the relation:

##EQU3## where, O_ix, O_iy is a location, in the motion reference frame, corresponding to an upper lefthand corner of a field of view of camera i;

F_ix, F_iy represent the known relative locations of a fiducial i vis-a-vis the alignment reference frame;

w_i is the unit height of the field of view of camera i vis-a-vis the motion reference frame;

h_i is the unit height of the imaging reference frame of camera i vis-a-vis the motion reference frame;

τ

is the orientation of the imaging reference frame of camera i vis-a-vis the motion reference frame; and

I_ix, I_iy represent a position of fiducial i with respect to the corresponding imaging reference frame.

11. A machine vision apparatus for determining a calibration relationship between imaging reference frames of multiple respective image acquisition devices that acquire images of a common object, wherein the object has thereon at least two fiducials, each being at a known location relative to the other with respect to an alignment reference frame associated with at least a selected one of the fiducials, the apparatus comprising:

A. alignment means for aligning at least a selected axis of the alignment reference frame with at least a selected axis of the motion reference frame of the object;

B. fiducial position-finding means for effecting the following operations with respect to each image acquisition device;

i. placing a respective one of the fiducials in a field of view of that device;

ii. determining a position of that fiducial with respect to the imaging reference frame of that device, that position being referred to as a "fiducial calibrating position";

iii. determining a position of the object with respect to the motion reference frame, that position being referred to as an "object calibrating position"; and

C. relative position-determining means for determining a calibration relationship between the imaging reference frames of the respective image acquisition devices as a function of the known relative locations of the fiducials with respect to the alignment reference frame, as a function of fiducial calibrating position determined for each image acquisition device, and as a function of the object calibrating position determined for each respective image acquisition device.

12. An apparatus according to claim 11, wherein the alignment means effects the following operations:

i. placing the selected fiducial in a field of view of a selected image acquisition device;

ii. determining a location of the selected fiducial with respect to an imaging reference frame of the selected image acquisition device, that location being referred to as a "first fiducial position";

iii. moving the object along an axis of the motion reference frame and placing it in the field of view of the selected image acquisition device;

iv. determining a location of the selected fiducial with respect to the imaging reference frame of the selected image acquisition device, that location being referred to as a "second fiducial position";

v. determining an orientation of the motion reference frame with respect to the imaging reference frame of the selected image acquisition device; and

vi. rotating the object to substantially align the selected fiducial with the motion reference frame with respect to the imaging reference frame of the selected image acquisition device.

13. An apparatus according to claim 12, wherein the alignment means determines the orientation of a line defined by the first and second fiducial positions.

14. An apparatus according to claim 13, wherein the alignment means determines an orientation τ

of the motion reference frame by as a function of the relation;

space="preserve" listing-type="equation">τ

=-a tan 2(i.sub.y1 -i.sub.y0, i.sub.x1 -i.sub.x0)

where,

(i_x0, i_y0) are imaging reference frame coordinates of the first image position;

(i_x1, i_y1) are imaging reference frame coordinates of the second image position;

a tan 2() is an arctangent function.

15. An apparatus according to claim 13, wherein the alignment means rotates the object to bring the selected fiducial in substantial alignment with the line defined by the first and second fiducial positions.

16. An apparatus according to claim 13, wherein the selected fiducial has an alignment feature and wherein the alignment means rotates the object to bring the alignment feature in substantial alignment with the line defined by the first and second fiducial positions.

17. An apparatus according to claim 16, wherein the alignment feature is substantially aligned with the alignment reference frame.

18. An apparatus according to claim 11, wherein at least the selected fiducial has a linear feature in substantial alignment with the selected axis of the alignment reference frame and wherein step (A) comprises the steps of

(A)(i) placing the selected fiducial in a field of view of a selected image acquisition device;

(A)(ii) determining parameters of a line characterizing the linear feature with respect to the imaging reference frame of the selected image acquisition device, those parameters being to as "first linear parameters";

(A)(iii) moving the object along an axis of the motion reference frame;

(A)(iv) determining parameters of a line characterizing the linear feature with respect to the imaging reference frame of the selected image acquisition device, those parameters being to as "second linear parameters";

(A)(v) comparing the first and second linear parameters to determine whether the linear feature is in substantial alignment with the selected axis of the motion reference frame.

19. An apparatus according to claim 18, wherein the alignment means responds to a determination that the first and second linear parameters describe lines that do not have a identical axial intercept for rotating the object and repeating the steps (A)(i)-(A)(v).

20. An apparatus according to claim 11, wherein the relative position-determining means comprises determining a calibration relationship between the imaging reference frames of the respective image acquisition devices in accord with the relation:

##EQU4## where, O_ix, O_iy is a location, in the motion reference frame, corresponding to an upper lefthand corner of a field of view of camera i;

F_ix, F_iy represent the known relative locations of a fiducial i vis-a-vis the alignment reference frame;

w_i is the unit height of the field of view of camera i vis-a-vis the motion reference frame;

h_i is the unit height of the imaging reference frame of camera i vis-a-vis the motion reference frame;

τ

is the orientation of the imaging reference frame of camera i vis-a-vis the motion reference frame; and

I_ix, I_iy represent a position of fiducial i with respect to the corresponding imaging reference frame.

21. An article of manufacture comprising a computer usable medium embodying program code for causing a digital data processor to carry out a method of determining a calibration relationship between imaging reference frames of multiple respective image acquisition devices that acquire images of a common object, wherein the object has thereon at least two fiducials, each being at a known location relative to the other with respect to an alignment reference frame associated with at least a selected one of the fiducials, the method comprising the steps of:

A. aligning at least a selected axis of the alignment reference frame with at least a selected axis of the motion reference frame of the object;

B. for each image acquisition device,

i. placing a respective one of the fiducials in a field of view of that device;

ii. determining a position of that fiducial with respect to the imaging reference frame of that device, that position being referred to as a "fiducial calibrating position";

iii. determining a position of the object with respect to the motion reference frame, hereinafter referred to as an "object calibrating position"; and

C. determining a calibration relationship between the imaging reference frames of the respective image acquisition devices as a function of the known relative locations of the fiducials with respect to the alignment reference frame, as a function of fiducial calibrating position determined for each image acquisition device, and as a function of the object calibrating position determined for each respective image acquisition device.

22. An article of manufacture according to claim 21, wherein step (A) comprises the steps of

i. placing the selected fiducial in a field of view of a selected image acquisition device;

ii. determining a location of the selected fiducial with respect to an imaging reference frame of the selected image acquisition device, that location being referred to as a "first fiducial position";

iii. moving the object along an axis of the motion reference frame and placing it in the field of view of the selected image acquisition device;

iv. determining a location of the selected fiducial with respect to the imaging reference frame of the selected image acquisition device, that location being referred to as a "second fiducial position";

v. determining an orientation of the motion reference frame with respect to the imaging reference frame of the selected image acquisition device; and

vi. rotating the object to substantially align the selected fiducial with the motion reference frame with respect to the imaging reference frame of the selected image acquisition device.

23. An article of manufacture according to claim 22, wherein step (v) comprises determining the orientation of a line defined by the first and second fiducial positions.

24. An article of manufacture according to claim 23, wherein step (v) comprises determining an orientation τ

of the motion reference frame by as a function of the relation;

space="preserve" listing-type="equation">τ

=-a tan 2(i.sub.y1 -i.sub.y0, i.sub.x1 -i.sub.x0)

where,

(i_x0, i_y0) are imaging reference frame coordinates of the first image position;

(i_x1, i_y1) are imaging reference frame coordinates of the second image position;

a tan 2() is an arctangent function.

25. An article of manufacture according to claim 23, wherein step (vi) comprises rotating the object to bring the selected fiducial in substantial alignment with the line defined by the first and second fiducial positions.

26. An article of manufacture according to claim 23, wherein the selected fiducial has an alignment feature and wherein step (vi) comprises rotating the object to bring the alignment feature in substantial alignment with the line defined by the first and second fiducial positions.

27. An article of manufacture according to claim 26, wherein the alignment feature is substantially aligned with the selected axis of the alignment reference frame.

28. An article of manufacture according to claim 21, wherein at least the selected fiducial has a linear feature in substantial alignment with the selected axis of the alignment reference frame and wherein step (A) comprises the steps of

(A)(i) placing the selected fiducial in a field of view of a selected image acquisition device;

(A)(ii) determining parameters of a line characterizing the linear feature with respect to the imaging reference frame of the selected image acquisition device, those parameters being to as "first linear parameters";

(A)(iii) moving the object along an axis of the motion reference frame;

(A)(iv) determining parameters of a line characterizing the linear feature with respect to the imaging reference frame of the selected image acquisition device, those parameters being to as "second linear parameters";

(A)(v) comparing the first and second linear parameters to determine whether the linear feature is in substantial alignment with the selected axis of the motion reference frame.

29. An article of manufacture according to claim 28, wherein step (v) comprises responding to a determination that the first and second linear parameters describe lines that do not have a identical axial intercept for rotating the object and repeating the steps (A)(i)-(A)(v).