SYSTEM FOR ADJUSTING FOCUS OF A LIQUID LENS IN A MACHINE VISION SYSTEM

• US 20100243862A1
• Filed: 03/31/2009
• Published: 09/30/2010
• Est. Priority Date: 03/31/2009
• Status: Active Grant

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Nunnink, Laurens
• Granted Patent: US 8,487,228 B2
• Time in Patent Office: Days
• US Class Current: 250/201.200
• CPC Class Codes: G02B 3/14 of variable focal length

Abstract

A method and apparatus for recalibrating a liquid lens. In one embodiment, a lens holder is provided to adjust the focal length of the lens as a function of temperature. In another embodiment, a recalibration circuit including a second lens of similar characteristics to the imaging lens is used to determine an appropriate focus. In other embodiments, an open loop calibration process is used.

Claims

1. A system for performing a vision process using a camera, the system comprising:

a liquid lens;

an imaging sensor; and

a mounting device for mounting the liquid lens a distance from the imaging sensor, wherein the mounting device is adapted to adjust a distance between the lens and the imaging sensor to adjust the focus of the lens.

2. The system as defined in claim 1, wherein at least a portion of the mounting device comprises a material that is selected to have a thermal expansion coefficient that varies the distance between the mounting device and the lens an amount selected to correspond to a focal length adjustment to focus the liquid lens in a selected range of temperatures.

3. A system for performing a vision process using a camera, the system comprising:

an imaging liquid lens having an imaging lens field of view in which the focus of the imaging liquid lens is adjustable;

a calibration liquid lens having a calibration lens field of view that is focused on a target;

an imaging sensor for imaging the target in the calibration lens field of view;

a processor configured to adjust a focus of the calibration liquid lens, the processor being programmed to;

receive imaging data from the calibration imaging sensor;

evaluate the focus of the calibration liquid lens on the target and adjust the focus until the target is in focus;

extract focus information from the calibration imaging sensor when the target is in focus; and

provide a signal to adjust the focus of the imaging liquid lens based on the focus information.

4. The system as recited in claim 3, wherein the processor is programmed to provide the signal to adjust the focus of the imaging lens directly to the imaging lens.

5. The system as recited in claim 3, wherein the processor is programmed to provide the signal to adjust the focus of the imaging lens to a processor associated with the imaging liquid lens for processing image data from the imaging field of view.

6. The system as recited in claim 3, wherein the optical characteristics of the calibration liquid lens are selected to be substantially equivalent to the optical characteristics of the imaging liquid lens.

7. The system as recited in claim 3, wherein a thermal affect on the behavior of the calibration liquid lens is selected to be substantially equivalent to a thermal affect on the behavior of the imaging liquid lens.

8. The system as recited in claim 3, wherein an aging affect on the behavior of the calibration liquid lens is selected to be substantially equivalent to an aging affect on the behavior of the imaging liquid lens.

9. The system as recited in claim 3, wherein the processor is programmed to adjust the focus of the calibration liquid lens by applying a driving voltage to the calibration liquid lens.

10. The system as recited in claim 9, wherein the processor is programmed to remove the driving voltage to the calibration liquid lens after acquiring an image, and before acquiring another image.

11. A system for performing a vision process using a camera, the system comprising:

a liquid lens having an imaging lens field of view where the focus of the liquid lens is adjustable;

an imaging sensor for imaging the field of view;

a target positioned at a predetermined location within the field of view;

a processor receiving imaging data from the imaging sensor, the processor being programmed to;

receive imaging data from the imaging sensor at the predetermined location in the field of view;

evaluate the focus of the liquid lens on the target; and

adjust the focus of the liquid lens until the target is in focus.

12. The system as recited in claim 11, wherein the liquid lens, the imaging sensor, and the processor are located in a housing, and the predetermined position of the target is located within the housing.

13. The system as recited in claim 12, wherein the target is positioned within a field of view at the distal end of the housing.

14. The system as recited in claim 13, wherein a window is provided at the distal end of the housing, and wherein the target is positioned on the window.

15. The system as recited in claim 11, further comprising at least one aiming illuminator for providing feedback to an operator as to the position of the field of view, and wherein the processor is further programmed to determine a distance to an external target based on feedback from the aiming illuminator, and to evaluate a focus of the liquid lens based on the focus of the external target.

16. The system as recited in claim 15, further comprising a second aiming illuminator, and wherein the processor is further programmed to:

calculate a distance between the aiming illuminator and the second aiming illuminator;

focus the lens to obtain a sharp image of the calibration target and determine a size of the calibration target;

calculate a ratio of the distance to the size of the calibration target; and

use the ratio to determine a distance to the calibration target.

17. The system as recited in claim 11, further comprising a mirror reflecting an image of the target on the imaging sensor.

18. The system as recited in claim 11, wherein the processor is programmed to image the target when the system is powered up.

19. The system as recited in claim 11, wherein the processor is programmed to image the target when a trigger is pulled.

20. The system as recited in claim 16, wherein the processor is further programmed to store the distance in a memory.