System and method for determining and controlling focal distance in a vision system camera

• US 8,576,390 B1
• Filed: 07/31/2012
• Issued: 11/05/2013
• Est. Priority Date: 07/31/2012
• Status: Active Grant

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Nunnink, Laurens
• Primary Examiner(s): NGUYEN, TU T
• Application Number:
  US13/563,499
• Time in Patent Office: 462 Days
• Field of Search: 356/126
• US Class Current: 356/126
• CPC Class Codes: G01M 11/0228 by measuring refractive power
• CPC Class Codes: G02B 3/14 of variable focal length
• CPC Class Codes: G03B 3/10 Power-operated focusing
• CPC Class Codes: G03B 43/00 Testing correct operation of photographic apparatus or parts thereof
• CPC Class Codes: G06T 7/80 Analysis of captured images to determine intrinsic or extrinsic camera parameters, i.e. camera calibration
• CPC Class Codes: H04N 17/002 for television cameras
• CPC Class Codes: H04N 23/67 Focus control based on electronic image sensor signals

Abstract

This invention provides a system and method for determining and controlling focal distance in a lens assembly of a vision system camera using an integral calibration assembly that provides the camera'"'"'s image sensor with optical information that is relative to focal distance while enabling runtime images of a scene to be acquired along the image axis. The lens assembly includes a variable lens located along an optical axis that provides a variable focus setting. The calibration assembly generates a projected pattern of light that variably projects upon the camera sensor based upon the focus setting of the variable lens. That is, the appearance and/or position of the pattern varies based upon the focus setting of the variable lens. This enables a focus process to determine the current focal length of the lens assembly based upon predetermined calibration information stored in association with a vision system processor running the focus process.

Claims

1. A system for determining focal distance of a lens assembly comprising:

a variable lens located along an optical axis of the lens assembly constructed and arranged to provide a variable focus setting;

a calibration assembly integral with the lens assembly, the calibration assembly including a projected pattern of light that variably projects upon a camera sensor receiving light from the lens assembly based upon the focus setting of the variable lens; and

a focus process that determines the focal distance of the lens assembly at the focus setting based upon predetermined calibration information with respect to the pattern.

2. The system as set forth in claim 1 wherein the calibration assembly is located so as to project the pattern an axis approximately perpendicular to the optical axis through a reflecting surface onto the optical axis.

3. The system as set forth in claim 1 wherein the reflecting surface is at least one of a prism and a mirror.

4. The system as set forth in claim 3 wherein the reflecting surface covers a portion of a field of view of the lens assembly and is constructed and arranged to project the pattern onto a portion of the camera sensor.

5. The system as set forth in claim 4 wherein the pattern is located on a calibration target located along an axis orthogonal to the optical axis and remote from the reflecting surface.

6. The system as set forth in claim 3 wherein the reflecting surface is a beam splitter located in front of the variable lens that allows light from an object along the optical axis to pass therethrough.

7. The system as set forth in claim 6 wherein the pattern is provided on a calibration target located along an axis orthogonal to the optical axis and remote from the reflecting surface, the calibration target defining transparent and opaque regions and being illuminated, the calibration target further being oriented along an acute slant angle with respect to a plane perpendicular to the orthogonal axis.

8. The system as set forth in claim 7 further comprising a calibration assembly lens located between the beam splitter and the target, the opaque regions being respectively associated with a plurality of focal distances.

9. The system as set forth in claim 8 wherein the opaque regions include parallel lines.

10. The system as set forth in claim 9 wherein the target is illuminated by light of a predetermined wavelength and the focus process is constructed and arranged to distinguish the predetermined wavelength.

11. The system as set forth in claim 7 wherein the pattern defines a light beam having a spherical wave front and further comprising at least one micro lens defining a portion of a wave front sensor that is oriented to project the pattern onto a predetermined portion of the camera sensor.

12. The system as set forth in claim 1 wherein the focus process is constructed and arranged to control the focus setting of the variable lens based upon the determined focal distance and a desired focal distance for the lens assembly.

13. The system as set forth in claim 1 wherein the variable lens comprises a membrane-based liquid lens.

14. A method for determining focal distance of a lens assembly including a variable focus lens comprising the steps of:

providing a focus setting to the variable lens;

projecting, with a calibration assembly, a pattern of light that variably projects upon a camera sensor receiving light from the lens assembly based upon the focus setting of the variable lens; and

determining the focal distance of the lens assembly at the focus setting based upon predetermined calibration information with respect to the pattern.

15. The method as set forth in claim 14 further comprising projecting, with the calibration assembly, the pattern from an axis approximately perpendicular to the optical axis through a reflecting surface onto the optical axis.

16. The method as set forth in claim 15 wherein the step of projecting includes projecting the pattern onto a portion of the camera sensor.

17. The method as set forth in claim 16 wherein the step of projecting includes providing a pattern defining a light beam having a spherical wave front and transmitting the light beam through at least one micro lens defining a portion of a wave front sensor that is oriented to project the pattern onto a predetermined portion of the camera sensor.

18. The method as set forth in claim 15 wherein the step of projecting includes passing the pattern through a beam splitter located in front of the variable lens that allows light from an object along the optical axis to pass therethrough.

19. The method as set forth in claim 18 wherein the step of projecting includes (a) providing a calibration target located along an axis orthogonal to the optical axis and remote from the reflecting surface, (b) illuminating the calibration target to project transparent and opaque regions, and (c) orienting the calibration target along an acute slant angle with respect to a plane perpendicular to the orthogonal axis.

20. The method as set forth in claim 19 further comprising locating a calibration assembly lens between the beam splitter and the target, and associating respective of the opaque regions with each of a plurality of focal distances.

21. The method as set forth in claim 19 wherein the step of illuminating the calibration target includes transmitting light of a predetermined wavelength and the step of determining includes distinguishing the predetermined wavelength received by the sensor.

22. The method as set forth in claim 14 wherein further comprising controlling the focus setting of the variable lens based upon the determined focal distance and a desired focal distance for the lens assembly.