Optical sensor device and a method of controlling its exposure time

• US 7,457,476 B2
• Filed: 10/03/2002
• Issued: 11/25/2008
• Est. Priority Date: 10/03/2001
• Status: Active Grant

• Current Assignee: Anoto Group AB
• Original Assignee: Anoto Group AB
• Inventors: Olsson, Andreas
• Primary Examiner(s): Bella; Matthew C.
• Assistant Examiner(s): TUCKER, WESLEY J
• Application Number:
  US10/262,943
• Publication Number:
  US 20030189664A1
• Time in Patent Office: 2,245 Days
• Field of Search: 382/140, 382/274, 382/100, 235/462.01, 348/207.99, 348/296, 348/362
• US Class Current: 382/274
• CPC Class Codes: G06K 7/10752 Exposure time control
• CPC Class Codes: G06K 7/10851 Circuits for pulse shaping, amplifying, eliminating noise signals, checking the function of the sensing device basic electronic circuitry H03
• CPC Class Codes: G06V 10/17 using hand-held instruments
• CPC Class Codes: G06V 10/19 by sensing codes defining pattern positions

Abstract

An optical sensor device has an illuminating light source and an image sensor. The image sensor captures a sequence of images under exposure of light from the light source. A controller in the optical sensor device has a control output and generates an exposure time control signal on the control output. The controller includes an exposure time control loop. Each iteration of the loop includes the steps: calculating an error between a set value and an actual luminance condition value determined in a current image in the sequence of images; calculating a current exposure time as a function of the error and an exposure time calculated during the preceding iteration of the control loop; and generating the exposure time control signal on the control output based on the current exposure time. The exposure time may thereby be determined so as to yield optimum contrast in an image which is to be captured by the image sensor.

Claims

1. A method for exposure time control in an optical sensor device of the type having an illuminating light source and an image sensor for capturing a sequence of images under exposure of light from said light source, wherein said sequence of images contain a position-coding pattern for decoding a sequence of positions of the optical sensor device, said method comprising capturing said sequence of images by repeatedly capturing images as the optical sensor device is moved, and transferring each captured image in said sequence of images to a processor for decoding the position of the optical sensor device, and, said method further comprising forming an exposure time control loop for controlling the exposure time for subsequently captured images in said sequence of images, wherein each iteration of the control loop involves:

capturing a current image in said sequence of images under exposure of light from said light source during an exposure time calculated during the preceding iteration;

determining an actual luminance condition value in said current image;

calculating an error between a set value and said actual luminance condition value; and

calculating a current exposure time as a function of the preceding exposure time and said error, said current exposure time being used for capturing a subsequent image in said sequence of images.

2. The method according to claim 1, wherein determining said actual luminance condition value comprises calculating a mean background luminance in said current image.

3. The method according to claim 2, wherein said actual luminance condition value is determined by:

determining a maximum luminance value in each of a plurality of subsets of said current image; and

calculating the mean background luminance as a mean of said maximum luminance values of said subsets.

4. The method according to claim 2, wherein said current exposure time is determined so as to yield, in a subsequent image which is captured in the following iteration of the control loop, a mean background luminance which is below saturation and approximately matches said set value.

5. The method according to claim 4, wherein said set value for mean background luminance is between about 50% and about 95%, and preferably between about 80% and about 90%, of a saturation level of said image sensor.

6. The method according to claim 1, wherein said current exposure time is calculated so as to yield optimum contrast between the background and at least one object in a subsequent image which is captured in the following iteration of the control loop.

7. The method according to claim 6, wherein said sequence of images contains a position-coding pattern and wherein said position-coding pattern includes said at least one object.

8. The method according to claim 1, wherein said set value is constant over time.

9. The method according to claim 1, wherein the control loop operates with essentially constant poles, independent of operation condition of said optical sensor device.

10. The method according to claim 1, wherein said control loop is based on a model function relating said actual luminance condition value to said preceding exposure time, wherein said method further comprises:

intermittently adjusting said model function to a current operation condition of said optical sensor device, so that the control loop operates with essentially constant poles, independent of operation condition of said optical sensor device.

11. The method according to claim 1, wherein said control loop is based on a model function relating said actual luminance condition value to said preceding exposure time, said model function including at least one model function working point which is essentially common to all operation conditions of said optical sensor device, and wherein calculating said current exposure time comprises parameterizing said model function based on said preceding exposure time and said model function working point, so that the control loop operates with essentially constant poles, independent of operation condition of said optical sensor device.

12. The method according to claim 11, wherein said model function includes at least one parameter which is essentially invariant of operation condition of said optical sensor device, said parameter defining said model function working point, and wherein said the parameterization of said model function is based on said preceding exposure time and said at least one essentially invariant parameter.

13. The method according to claim 11, wherein parameterizing said model function includes setting said actual luminance condition value equal to said set value.

14. The method according to claim 11, wherein the parameterization of said model function is further based on said actual luminance condition value calculated for said current image.

15. The method according to claim 10, wherein said model function is defined around said set value.

16. A method according to claim 1, wherein said exposure time control loop is given by:

$\hspace{1em}<br><br>\{\begin{array}{c}u<br><br>\left(n\right)=u<br><br>\left(n-1\right)+k_1<br><br>\left(n\right)<br><br><br><br>\left(\mathrm{e<br><br>}<br><br>\left(n\right)-\mathrm{e<br><br>}<br><br>\left(n-1\right)\right)+k_2<br><br>\left(n\right)<br><br><br><br>\mathrm{e<br><br>}<br><br>\left(n\right)\\ \mathrm{e<br><br>}<br><br>\left(n\right)=w-y<br><br>\left(n\right)\\ k_1<br><br>\left(n\right)=\frac{l_1}{w-L}<br><br>u<br><br>\left(n-1\right)\\ k_2<br><br>\left(n\right)=\frac{l_2}{w-L}<br><br>u<br><br>\left(n-1\right)\end{array}$

where;

u(n) and u(n−

1) are the current exposure time and the exposure time calculated during the preceding iteration of the control loop, respectively,

w is the set value,

y(n) is the actual luminance condition value in said current image, and

l₁, l₂ and L are constants.

17. The method according to claim 1, wherein determining said actual luminance condition value comprises calculating a mean background luminance as well as a mean object luminance in said current image.

18. The method according to claim 17, wherein determining said actual luminance condition value involves combining said mean background luminance and said mean object luminance such that said mean background luminance has stronger influence than the mean object luminance when calculating said current exposure time.

19. The method according to claim 1, wherein said exposure time is defined as a time period during which said light source is caused to emit light.

20. The method according to claim 1, wherein said exposure time is defined as a time period during which said image sensor is caused to capture one of said images.

21. The method according to claim 1, wherein said exposure time is defined as a time period during which light emitted from said light source is admitted to reach said image sensor.

22. A computer program product directly loadable into a memory of a processor, the computer program product comprising program code for performing the steps of claim 1 when executed by said processor.

23. An optical sensor device comprising an illuminating light source and an image sensor, wherein the image sensor is adapted to capture a sequence of images under exposure of light from said light source, wherein said sequence of images contain a position-coding pattern for decoding a sequence of positions of the optical sensor device, said image sensor being adapted to capture said sequence of images by repeatedly capturing images as the optical sensor device is moved, said optical sensor device further comprising:

a processor which is adapted to receive each captured image in said sequence of images, from the image sensor and to decode the position of the optical sensor device; and

a controller having a control output and being adapted to generate an exposure time control signal on said control output, said controller comprising an exposure time control loop in order to control exposure time for subsequently captured images in said sequence of images, wherein each iteration of the control loop involves;

calculating an error between a set value and an actual luminance condition value determined in a current image in said sequence of images;

calculating a current exposure time as a function of said error and an exposure time calculated during the preceding iteration of the control loop, said current exposure time being used for capturing a subsequent image in said sequence of images; and

generating said exposure time control signal on said control output based on said current exposure time.

24. The sensor device according to claim 23, wherein said actual luminance condition value corresponds to a mean background luminance in said current image.

25. The sensor device according to claim 23, wherein said current exposure time is calculated so as to yield optimum contrast between the background and at least one object in a subsequent image which is captured in the following iteration of the control loop.

26. The sensor device according to claim 23, wherein said controller is adapted to operate with essentially constant poles, independent of operation condition of the sensor device.

27. The sensor device according to claim 23, wherein the control loop is based on a model function relating said actual luminance condition value to said preceding exposure time, wherein said controller is adapted to intermittently adjust said model function to a current operation condition of the optical sensor device, so tat the control loop operates with essentially constant poles, independent of operation condition of the sensor device.

28. The sensor device according to claim 23, wherein the control loop is based on a model function relating said actual luminance condition value to said preceding exposure time, said model function including at least one model function working point which is essentially common to all operation conditions of the optical sensor device, and wherein said controller is adapted to, in calculating said current exposure time, parameterize said model function based on said preceding exposure time and said model function working point, so that the control loop operates with essentially constant poles, independent of operation condition of the optical sensor device.

29. The sensor device according to claim 28, wherein said model function includes at least one parameter which is essentially invariant of operation condition of the optical sensor device, said parameter defining said model function working point, and wherein said controller is adapted to parameterize of said model function based on said preceding exposure time and said at least one essentially invariant parameter.

30. The sensor device according to claim 29, wherein said model function is a linear function of exposure time, said essentially invariant parameter being an offset parameter of said linear function .

31. The sensor device according to claim 28, wherein said controller is designed, in parameterizing said model function, to set said actual luminance condition value equal to said set value.

32. The sensor device according to claim 28, wherein the parameterization of said model function is further based on the actual luminance condition value calculated for said current image.

33. The sensor device according to claim 27, wherein said model function is defined around said set value.

34. A sensor device according to claim 23, wherein the control loop of said controller is given by:

$\hspace{1em}<br><br>\{\begin{array}{c}u<br><br>\left(n\right)=u<br><br>\left(n-1\right)+k_1<br><br>\left(n\right)<br><br><br><br>\left(\mathrm{e<br><br>}<br><br>\left(n\right)-\mathrm{e<br><br>}<br><br>\left(n-1\right)\right)+k_2<br><br>\left(n\right)<br><br><br><br>\mathrm{e<br><br>}<br><br>\left(n\right)\\ \mathrm{e<br><br>}<br><br>\left(n\right)=w-y<br><br>\left(n\right)\\ k_1<br><br>\left(n\right)=\frac{l_1}{w-L}<br><br>u<br><br>\left(n-1\right)\\ k_2<br><br>\left(n\right)=\frac{l_2}{w-L}<br><br>u<br><br>\left(n-1\right)\end{array}$

where;

u(n) and u(n−

1) are the current exposure time and the exposure time calculated during the preceding iteration of the control loop, respectively,

w is the set value for the actual luminance condition value,

y(n) is the actual luminance condition value in said current image, and

l₁, l₂ and L are constants.

35. The sensor device according to claim 23, wherein the control loop is implemented in a PI regulator.

36. The sensor device according to claim 23, wherein the illuminating light source is a light emitting diode.

37. The sensor device according to claim 36, wherein the light emitting diode is adapted to emit infrared light.

38. The sensor device according to claim 23, wherein said image sensor is arranged to capture said sequence of images of a surface which contains a position-coding pattern, said sensor device further comprising means for determining a position from the position-coding pattern in each image.

39. The sensor device according to claim 23, wherein the control output of said controller is operatively coupled to said light source and wherein the exposure time controlled by said control signal is a time period during which said light source is caused to emit light.

40. The sensor device according to claim 23, wherein the control output of said controller is operatively coupled to said image sensor and wherein the exposure time controlled by said control signal is a time period during which said image sensor will produce said image which is to be captured.

41. The sensor device according to claim 23, further comprising an optical member positioned in an optical path between said light source and said image sensor, wherein the control output of said controller is operatively coupled to said optical member and wherein the exposure time controlled by said control signal is a time period during which said optical member admits light emitted from said light source to reach said image sensor.