Method for setting parameters of a vision detector using production line information

• US 20050275728A1
• Filed: 11/02/2004
• Published: 12/15/2005
• Est. Priority Date: 06/09/2004
• Status: Active Grant

• Current Assignee: Cognex Technology & Investment Corporation (Cognex Corporation)
• Original Assignee: Cognex Technology & Investment Corporation (Cognex Corporation)
• Inventors: Silver, William M., Mirtich, Brian V.
• Granted Patent: US 7,545,949 B2
• Time in Patent Office: Days
• US Class Current: 348/211.990
• CPC Class Codes: G03B 7/087 for control of both exposure time and aperture
• CPC Class Codes: G03B 7/097 for control of both exposure time and aperture
• CPC Class Codes: H04N 23/62 Control of parameters via user interfaces

Abstract

Disclosed are systems and methods for setting various operating parameters of a vision detector from production line information that can be supplied by a manufacturing technician who is not skilled in the art of the invention. These operating parameters include shutter time, video gain, idle time, frame count, and locator search range. The production line information includes line speed, field of view size, direction of motion, and object spacing.

Claims

1. A method for setting a shutter time parameter and a gain parameter of an imager, the gain parameter having a minimum allowable gain value and a maximum allowable gain value, comprising:

configuring the imager to capture images of a field of view of a production line in relative motion with respect to the field of view in a direction and at a nominal line speed, the field of view having a size in the direction of motion of the production line;

choosing a motion parameter that specifies a desired relative motion of the production line, in units related to pixels, during an interval specified by the shutter time parameter;

estimating, responsive to the nominal line speed, an apparent line speed in units related to pixels per unit of time;

computing, responsive to the apparent line speed and the motion parameter, a motion shutter time;

inputting a desired image brightness parameter;

computing, responsive to the desired image brightness parameter, a brightness shutter time;

computing a desired shutter time and a desired gain, such that

the brightness shutter time is approximately equal to the product of the desired shutter time and the desired gain,

the desired gain is not less than the minimum allowable gain value and not greater than the maximum allowable gain value, and

at least one of a first condition, a second condition, and a third condition hold, wherein

the first condition holds when the desired shutter time is approximately equal to the motion shutter time,

the second condition holds when the desired shutter time is less than the motion shutter time and the desired gain is approximately equal to the minimum allowable gain value, and

the third condition holds when the desired shutter time is greater than the motion shutter time and the desired gain is approximately equal to the maximum allowable gain value; and

setting the shutter time parameter of the imager to the desired shutter time, and the gain parameter of the imager to the desired gain.

2. The method of claim 1, wherein the step of estimating the apparent line speed comprises inputting estimates of at least one production line value comprising information about physical attributes of the production line;

inputting an estimate of the size of the field of view in the direction of motion of the production line; and

computing, responsive to the estimates of the at least one production line value and the estimate of the size of the field of view, the apparent line speed.

3. The method of claim 2, wherein the at least one production line value comprises the nominal line speed.

4. The method of claim 2, wherein

the production line contains a sequence of discrete objects at a nominal object spacing, such that the objects move passed the imager at a nominal object frequency;

and

the at least one production line value comprises the nominal object frequency and the nominal object spacing.

5. A method for setting an idle time parameter of a vision detector comprising:

configuring the vision detector to detect a sequence of discrete objects in continuous motion at a nominal object speed relative to a field of view of the vision detector, including detecting each object of the sequence of discrete objects responsive to an analysis of at least one frame captured by the vision detector, each frame comprising an image of the field of view;

choosing a motion parameter that specifies a desired motion of an object, in units related to pixels, during an idle step of the vision detector, the idle step comprising an interval of time during which objects are not detected;

estimating, responsive to the nominal object speed, an apparent object speed in units related to pixels per unit of time;

computing, responsive to the apparent object speed and the motion parameter, a desired idle time; and

setting the idle time parameter of the vision detector to the desired idle time.

6. The method of claim 5, wherein the step of choosing the motion parameter includes choosing the motion parameter so as to avoid detecting an object more than once, and so as to avoid missing an object.

7. The method of claim 5, wherein the idle time parameter comprises a minimum idle time parameter.

8. The method of claim 5, wherein the idle time parameter comprises a maximum idle time parameter.

9. The method of claim 5, wherein the step of computing the desired idle time comprises computing a ratio of the motion parameter to the apparent line speed.

10. A method for setting an idle time parameter of a vision detector comprising:

configuring the vision detector to detect a sequence of discrete objects in continuous motion relative to a field of view of the vision detector, the sequence of discrete objects having a nominal object period between successive objects, wherein configuring the vision detector includes detecting each object of the sequence of discrete objects responsive to an analysis of at least one frame captured by the vision detector, each frame comprising an image of the field of view;

choosing a motion parameter that specifies a desired motion of an object in the sequence of discrete objects, as a fraction of the nominal object period, during an idle step of the vision detector, the idle step comprising an interval of time during which objects are not detected;

computing, responsive to the nominal object period and the motion parameter, a desired idle time; and

setting the idle time parameter of the vision detector to the desired idle time.

11. The method of claim 10, wherein the step of choosing the motion parameter includes choosing the motion parameter so as to avoid detecting an object more than once, and so as to avoid missing an object.

12. The method of claim 10, wherein the idle time parameter comprises a minimum idle time parameter.

13. The method of claim 10, wherein the idle time parameter comprises a maximum idle time parameter.

14. A method for setting a frame count parameter of a vision detector comprising:

configuring the vision detector to inspect a sequence of discrete objects in continuous motion in a direction and at a nominal object speed relative to a field of view of the vision detector, including

inspecting each object of the sequence of discrete objects responsive to a first analysis of a first plurality of frames captured by the vision detector, the frames each respectively comprising images of the field of view, and wherein

the vision detector operates at a nominal frame rate, and

the vision detector has a detection range within which an object can be detected, the detection range having a length in the direction of motion;

estimating, responsive to the nominal object speed and the nominal frame rate, a motion value that specifies a nominal movement of each object between successive frames;

computing, responsive to the length in the direction of motion of the detection range and the motion value, an expected frame count value that specifies a nominal count of frames for which each object of the sequence of discrete objects can be located;

computing, responsive to the expected frame count value, a desired frame count; and

setting the frame count parameter of the vision detector to the desired frame count.

15. The method of claim 14, further comprising

further configuring the vision detector to detect each object in the sequence of discrete objects, wherein detecting each object is responsive to a second analysis of a second plurality of frames captured by the vision detector;

and wherein

the frame count parameter specifies the minimum number of frames that must be found to contain sufficient evidence that each object is present in the field of view in order for each object to be detected.

16. The method of claim 15, wherein the step of computing the desired frame count comprises setting the desired frame count to a predetermined fraction of the expected frame count.

17. The method of claim 14, further comprising

further configuring the vision detector to detect each object, wherein detecting each object is responsive to a second analysis of a second plurality of frames captured by the vision detector;

and wherein

the frame count parameter specifies, for each object, the maximum number of frames in the first plurality of frames that will be subject to the first analysis.

18. The method of claim 17, wherein the step of computing a desired frame count comprises setting the desired frame count to a predetermined fraction of the expected frame count.

19. The method of claim 14, wherein

the vision detector is further configured to detect each object, wherein detecting each object is responsive to an external trigger signal;

and

the frame count parameter specifies, for each object, the number of frames in the first plurality of frames that will be subject to the first analysis.

20. The method of claim 19, wherein the step of computing the desired frame count comprises setting the desired frame count to a predetermined fraction of the expected frame count.

21. A method for setting a search range parameter of a locator of a vision detector comprising:

configuring the vision detector to inspect a sequence of discrete objects in continuous motion in a direction and at a nominal object speed relative to a field of view of the vision detector, wherein

inspecting each object of the sequence of discrete objects is responsive to an analysis of a plurality of frames captured by the vision detector, the frames comprising images of the field of view,

the vision detector operates at a nominal frame rate, and

configuring the vision detector comprises configuring the locator to locate objects in the direction of motion;

choosing a target frame count parameter that specifies, for each object, a desired count of frames in the plurality of frames to be subject to the analysis;

estimating, responsive to the nominal object speed and the nominal frame rate, a motion value that specifies a nominal movement of each object between successive frames;

computing, responsive to the target frame count parameter and the motion value, a desired search range value; and

setting the search range parameter of the locator to the desired search range value.