Systems and methods for controlling strobe illumination

• US 9,234,852 B2
• Filed: 08/23/2011
• Issued: 01/12/2016
• Est. Priority Date: 07/29/2005
• Status: Active Grant

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Gladnick, Paul G., Delaney, Mark
• Primary Examiner(s): Perungavoor, Sath V
• Assistant Examiner(s): PONTIUS, JAMES M
• Application Number:
  US13/137,514
• Publication Number:
  US 20110310270A1
• Time in Patent Office: 1,603 Days
• US Class Current: 1/1
• CPC Class Codes: G01N 2021/8838 Stroboscopic illumination; synchronised illumination
• CPC Class Codes: G01N 21/8806 Specially adapted optical and illumination features
• CPC Class Codes: H04N 23/74 by influencing the scene brightness using illuminating means

Abstract

The repeatability of strobe illumination for image exposure is improved over a wide dynamic range in a machine vision system wherein a relationship between a camera integration period and a pulse duration of a strobe light generator control the effective exposure of a camera image during a timing overlap between a beginning of the camera integration period and an end of the pulse duration. To avoid noise in the illumination, the illumination pulse duration may start, the camera integration period may begin after a delay relative to that start and not later than the end of the pulse duration, the pulse duration may end, and the camera integration period may end not earlier than the end of the pulse duration. The timing overlap may also be synchronized with a periodic ripple component in the strobe illumination, to provide improved repeatability.

Claims

1. A machine vision system that controls image exposure strobe illumination used to illuminate a workpiece, such that the workpiece may be imaged by a camera in the machine vision system while moving at high speed, the system comprising:

a camera providing a camera image of the workpiece;

a strobe light generator that emits radiation used by the camera for imaging; and

a controller configured to control a timing relationship between an image integration period of the camera and a pulse duration of the radiation emitted by the strobe light generator to control an image exposure of a camera image during a timing overlap which occurs between a beginning of the image integration period and an end of the pulse duration, wherein the timing relationship is controlled such that the pulse duration starts, the image integration begins after a delay relative to a start of the pulse duration and not later than the end of the pulse duration, the pulse duration ends, and the image integration period ends not earlier that the end of the pulse duration.

2. The machine vision system of claim 1, wherein the timing relationship is further controlled such that the image integration period ends after the end of the pulse duration.

3. The machine vision system of claim 1, wherein the controller is further configured to control the delay to be at least 25 μ

s.

4. The machine vision system of claim 1, wherein the controller is further configured to variably control a length of the timing overlap in order to variably control the image exposure of the camera image.

5. The machine vision system of claim 4, wherein the controller is further configured to set the pulse duration to a constant value and to control the delay in order to control the length of the timing overlap.

6. The machine vision system of claim 5, wherein the controller is further configured to control the pulse duration to be in a range of least 5 μ

sand at most 150 μ

s.

7. The machine vision system of claim 4, wherein the controller is further configured to set the delay to a constant value and to control the pulse duration in order to control the length of the timing overlap.

8. The machine vision system of claim 4,

wherein the controller is further configured to control the length of the timing overlap based on an image brightness control setting.

9. The machine vision system of claim 8, wherein the image brightness control setting includes a lowest setting which corresponds to a state in which the length of the timing overlap is approximately zero.

10. The machine vision system of claim 9, wherein the controller is configured (1) to define a timing offset d₀ that is a fixed timing offset that the controller consistently adds to or subtracts from one or more timings that determine the length of the timing overlap, and (2) to make the timing offset d₀ cause the length of the timing overlap to be approximately zero when the image brightness control setting is at the lowest setting.

11. The machine vision system of claim 8, further comprises a clock used to control the timing relationship, and the image brightness control setting includes a lowest setting which corresponds to a state in which the length of the timing overlap is at most 5 clock cycles.

12. The machine vision system of claim 8, further comprises a clock used to control the timing relationship, and a range of settings of the image brightness control setting corresponds to a range of settings for a number of clock cycles that define the length of the timing overlap.

13. The machine vision system of claim 1, wherein

the strobe light generator comprises an illumination source and a drive circuit, the drive circuit providing a drive signal to power the illumination source, the drive signal exhibiting a periodic ripple component, and

the controller is further configured to consistently synchronize the end of the timing overlap relative to the periodic ripple component.

14. The machine vision system of claim 13, wherein the controller is configured to set the pulse duration to a constant value, and to consistently synchronize the start of the pulse duration relative to the periodic ripple component, and control the delay in order to control the length of the timing overlap.

15. The machine vision system of claim 1, wherein

the strobe light generator comprises an illumination source and a drive circuit, the drive circuit providing a drive signal to power the illumination source, the drive signal exhibiting a periodic ripple component, and

the controller is further configured to consistently synchronize the start of the timing overlap relative to the periodic ripple component.

16. The machine vision system of claim 15, further comprises a clock, wherein the controller is further configured to consistently synchronize the beginning of the image integration period relative to the periodic ripple component and the end of the pulse duration based on a number of clock cycles relative to the beginning of the image integration period, in order to control the length of the timing overlap.

17. The machine vision system of claim 1, further comprising a controllable spatial light modulator configured to provide a light channel aperture positioned to controllably attenuate the radiation emitted by the strobe light generator,

wherein the controller is further configured to control the image exposure of the camera image based on controlling the timing overlap which occurs between a beginning of the image integration period and an end of the pulse duration, in combination with controlling the light channel aperture to attenuate the radiation emitted by the strobe light generator.

18. A machine vision system that controls image exposure strobe illumination used to illuminate a workpiece, such that the workpiece may be imaged by a camera in the machine vision system while moving at high speed, the system comprising:

a camera that provides a camera image of the workpiece;

a strobe light generator that emits radiation used by the camera for imaging; and

a controller configured to control a timing relationship between an image integration period of the camera and a pulse duration of the radiation emitted by the strobe light generator to control an image exposure of the camera image during a timing overlap which occurs between the image integration period and the pulse duration,

wherein the machine vision system is configured according

the timing relationship being controlled such that the pulse duration starts, the camera integration begins after a delay relative to the start of the pulse duration and not later than an end of the pulse duration, the pulse duration ends, and the image integration period ends after the end of the pulse duration, and

the strobe light generator comprises an illumination source and a drive circuit, the drive circuit providing a drive signal to power the illumination source, the drive signal exhibiting a periodic ripple component, and the controller being configured to consistently synchronize one of the start and the end of the timing overlap relative to the periodic ripple component.

19. A method for controlling image exposure strobe illumination used to illuminate a workpiece in a machine vision inspection system, such that the workpiece may be imaged by a camera in the machine vision system while moving at high speed, the method comprising:

providing a machine vision system comprising a camera that provides a camera image of the workpiece, a strobe light generator that emits radiation used by the camera for imaging, and a controller; and

operating the controller to control a timing relationship between an image integration period of the camera and a pulse duration of the radiation emitted by the strobe light generator to control an image exposure of the camera image during a timing overlap which occurs between a beginning of the image integration period and an end of the pulse duration, wherein the operating the controller to control the timing relationship comprises;

starting the pulse duration;

beginning the image integration period after a delay relative to a start of the pulse duration and not later than the end of the pulse duration, in order to start the image exposure;

ending the pulse duration in order to stop the image exposure; and

ending the image integration period not earlier than the ending of the pulse duration.