Constant magnification lens for vision system camera

• US 9,715,612 B2
• Filed: 12/23/2013
• Issued: 07/25/2017
• Est. Priority Date: 12/26/2012
• Status: Active Grant

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Nunnink, Laurens
• Primary Examiner(s): Lester, Evelyn A
• Application Number:
  US14/139,867
• Publication Number:
  US 20140183264A1
• Time in Patent Office: 1,310 Days
• Field of Search: 235454, 235455, 23546201, 23546223-46225, 23546232, 23546235, 23546241, 250216, 25022711, 2502272, 25022728, 25022729, 250566, 396 72, 396 79, 396 82, 359440-442, 359642, 359665, 359740, 359794, 359800, 359802, 359809, 359810
• CPC Class Codes: G02B 13/24 for reproducing or copying at short object distances
• CPC Class Codes: G02B 15/14 by axial movement of one or more lenses or groups of lenses relative to the image plane for continuously varying the equivalent focal length of the objective
• CPC Class Codes: G02B 15/1421 the first group being positive
• CPC Class Codes: G02B 15/15 compensation by means of only one movement or by means of only linearly related movements, e.g. optical compensation
• CPC Class Codes: G02B 23/145 Zoom viewfinders
• CPC Class Codes: G02B 26/004 based on a displacement or a deformation of a fluid
• CPC Class Codes: G02B 27/0025 for optical correction, e.g. distorsion, aberration
• CPC Class Codes: G02B 27/0955 Lenses lenses per se G02B3/00
• CPC Class Codes: G02B 3/14 of variable focal length
• CPC Class Codes: G02B 5/005 Diaphragms for cameras G03B9/02
• CPC Class Codes: G02B 7/021 for more than one lens
• CPC Class Codes: G02B 7/10 by relative axial movement of several lenses, e.g. of varifocal objective lens
• CPC Class Codes: G06K 7/10722 Photodetector array or CCD scanning
• CPC Class Codes: G06K 7/10811 Focalisation
• CPC Class Codes: G06K 7/10821 further details of bar or optical code scanning devices
• CPC Class Codes: G06K 7/10831 Arrangement of optical elements, e.g. lenses, mirrors, prisms optical elements per se G02B
• CPC Class Codes: G06K 7/10881 constructional details of hand-held scanners
• CPC Class Codes: G06K 7/1439 including a method step for retrieval of the optical code
• CPC Class Codes: H04N 23/55 Optical parts specially adapted for electronic image sensors; Mounting thereof
• CPC Class Codes: H04N 23/67 Focus control based on electronic image sensor signals

Abstract

This invention provides a lens assembly for a vision system, allowing for a constant magnification at various focal distances. The lens assembly resides movably/adjustably along the optical axis relative to the sensor. In an embodiment, the lens assembly includes a fixed rear lens and a front lens that is moved mechanically to focus the object image on the image sensor. The lens assembly can alternatively include a liquid lens that is controlled to adjust magnification with respect to a fixed front lens so as to maintain a constant system magnification. The liquid lens resides between the (fixed) front lens assembly and the image sensor and can be controlled to focus the image onto the image sensor.

Claims

1. A vision system for acquiring images of objects over a range of focal distances within a field of view comprising:

an image sensor operatively connected to a vision processor;

a constant magnification lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor, the constant magnification lens assembly including a front lens assembly and a rear lens assembly such that an aperture stop of the constant magnification lens assembly is between a back surface of the front lens assembly and a focal point of the front lens assembly; and

wherein the front lens assembly has a first dimension that is smaller than a second dimension associated with the field of view.

2. The vision system as set forth in claim 1 wherein the front lens assembly and the rear lens assembly are arranged in a fixed spatial relationship therebetween.

3. The vision system as set forth in claim 2 wherein the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and a focal point of the rear lens assembly coincide, (b) a magnification of the assembly is constant and equal to a focal length (f2) of the rear lens assembly/a focal length (f1) of the front lens assembly, and (c) the shift in focal position of the assembly is (f1/f2)²*(distance the constant magnification lens assembly moves along the optical axis).

4. The vision system as set forth in claim 3 wherein the front lens assembly and the rear lens assembly are mounted in a barrel that is moved toward and away from the image sensor by an actuator responsive to a focus process.

5. The vision system as set forth in claim 3 wherein the front lens assembly and the rear lens assembly and the aperture stop are each fixed with respect to each other and further comprising an actuator that moves the image sensor toward and away from the constant magnification lens assembly.

6. The vision system as set forth in claim 2 further comprising a liquid lens assembly oriented between the front lens assembly and the image sensor, wherein the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and the front principal plane of the rear lens assembly coincide (d1=f1), (b) the rear lens assembly has a variable optical power, (c) an aperture stop of the constant magnification lens assembly is located between a back surface of the front lens assembly and a focal point of the front lens assembly, the magnification of the assembly is constant and equal to the ratio between the distance d2 from the rear lens group to the sensor and the distance d1 between the two lens groups.

7. A vision system for acquiring images of an object over a range of focal distances within a field of view comprising:

an image sensor operatively connected to a vision processor;

a constant magnification lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor, the constant magnification lens assembly including a liquid lens assembly oriented between the image sensor and a front lens assembly, the front lens assembly comprising one or more fixed lenses, and the liquid lens assembly including an electrical interface that varies a magnification m2 of the liquid lens assembly,

wherein the front lens assembly and liquid lens assembly are arranged in a fixed spatial relationship therebetween; and

a controller that selectively adjusts the magnification m2 of the liquid lens assembly to maintain focus on the object at a constant system magnification M at each focal distance of the range of focal distances, wherein M is defined as a distance between the liquid lens assembly and the image sensor (d2) divided by a distance between the liquid lens assembly and the front lens (d1) such that M=d2/d1.

8. The vision system as set forth in claim 7 wherein the front lens assembly and the liquid lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and a front principal plane of the liquid lens assembly coincide, and (b) the constant magnification lens assembly is constant and equal to a ratio between the distance (d2) from the liquid lens assembly to the image sensor and the distance (d1) between the front lens assembly and the liquid lens assembly.

9. The vision system as set forth in claim 8 wherein the liquid lens assembly comprises a membrane liquid lens assembly.

10. The vision system as set forth in claim 9 wherein the electrical interface comprises an actuator that varies a shape of the membrane.

11. The vision system as set forth in claim 8 wherein the liquid lens assembly comprises at least two iso-density liquids that vary interaction therebetween in accordance with an electro wetting principle in response to a varied electrical input.

12. The vision system as set forth in claim 7 wherein the controller is arranged to iteratively adjust the magnification m2 of the liquid lens assembly until a desired focus at the constant system magnification M is provided.

13. The vision system as set forth in claim 7 wherein the front lens assembly is smaller in area than an area of the field of view.

14. A method for acquiring images of an object over a range of focal distances within a field of view comprising the steps of:

providing an image sensor operatively connected to a vision processor and a constant magnification lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor, the constant magnification lens assembly including a front lens assembly and a rear lens assembly such that an aperture stop of the constant magnification lens assembly is between a back surface of the front lens assembly and a focal point of the front lens assembly; and

iteratively adjusting the constant magnification lens assembly until the object achieves a desired focus at the image sensor.

15. The method as set forth in claim 14 further comprising locating a liquid lens assembly oriented between the front lens assembly and the image sensor, wherein the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and the front principal plane of the rear lens assembly coincide (d1=f1), (b) the rear lens assembly has a variable optical power, (c) an aperture stop of the constant magnification lens assembly is located between a back surface of the front lens assembly and a focal point of the front lens assembly, d the magnification of the assembly is constant and equal to the ratio between the distance d2 from the rear lens group to the sensor and the distance d1 between the two lens groups.