Method and system for angle measurement

• US 7,697,127 B2
• Filed: 09/18/2008
• Issued: 04/13/2010
• Est. Priority Date: 02/22/2008
• Status: Active Grant

• Current Assignee: Trimble Jena GmbH (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Vogel, Michael, Glimm, Andreas
• Primary Examiner(s): Punnoose; Roy
• Application Number:
  US12/233,045
• Publication Number:
  US 20090231574A1
• Time in Patent Office: 572 Days
• Field of Search: 356/154
• US Class Current: 356/154
• CPC Class Codes: G01D 5/3473 Circular or rotary encoders

Abstract

An angle measurement device includes a light source configured to emit light along an optical path and a patterned member positioned along the optical path and configured to rotate about an axis of rotation. The patterned member includes a periodic optical variation. Light passing through the patterned member provides a spatially modulated optical waveform. The angle measurement device also includes an imaging device positioned along the optical path and including a plurality of photosensitive elements disposed in an array configuration. The imaging device is configured to receive the spatially modulated optical waveform and provide a signal associated with light intensity at each of the plurality of photosensitive elements. The angle measurement device further includes an angle determination unit coupled to the imaging device and configured to compute a rotation angle of the patterned member based on a reference waveform and the provided signal.

Claims

1. An angle measurement device comprising:

a light source configured to emit light along an optical path;

a patterned member positioned along the optical path and configured to rotate about an axis of rotation, wherein light passing through the patterned member provides a spatially modulated optical waveform;

an imaging device positioned along the optical path and including a plurality of photosensitive elements disposed in an array configuration, wherein the imaging device is configured to receive the spatially modulated optical waveform and provide a signal associated with light intensity at each of the plurality of photosensitive elements; and

an angle determination unit coupled to the imaging device and configured to compute a rotation angle of the patterned member based on a reference waveform and the provided signal.

2. The angle measurement device of claim 1 wherein the patterned member comprises a periodic optical variation.

3. The angle measurement device of claim 1 wherein the patterned member is fixed to a rotating mechanical support and the imaging device is fixed to a stationary mechanical support.

4. The angle measurement device of claim 1 wherein the light source comprises an LED.

5. The angle measurement device of claim 1 wherein the light source is configured to rotate about the axis of rotation.

6. The angle measurement device of claim 1 wherein the patterned member comprises a glass member.

7. The angle measurement device of claim 1 wherein the periodic optical variation comprises a variation in at least one of transmission of light through or reflection of light from the patterned member.

8. The angle measurement device of claim 7 wherein the variation in transmission is associated with an optically opaque material disposed on a surface of the patterned member.

9. The angle measurement device of claim 7 wherein the variation in reflection is associated with an optically opaque material disposed on a surface of the patterned member.

10. The angle measurement device of claim 9 wherein the light source and the imaging device are provided as an integrated component with the light source positioned in a central region of the imaging device.

11. The angle measurement device of claim 8 the optically opaque material comprises at least one of chrome, silver, aluminum, or ink.

12. The angle measurement device of claim 1 wherein the periodic optical variation includes a plurality of rotationally symmetric elements.

13. The angle measurement device of claim 1 wherein the axis of rotation is aligned to a center of rotational symmetry of the patterned member.

14. The angle measurement device of claim 1 wherein the patterned member is disposed perpendicular to the axis of rotation.

15. The angle measurement device of claim 1 wherein the imaging device comprises at least one of a CCD array or a CMOS optical sensor array.

16. The angle measurement device of claim 1 wherein the reference waveform comprises a periodic waveform computed as a function of light intensity captured by each of the plurality of photosensitive elements and associated with a reference position at which the patterned member and the imaging device are in a predetermined mechanical arrangement.

17. The angle measurement device of claim 1 wherein computing the rotation angle is based on a phase shift between the reference waveform and the spatially modulated optical waveform determined after a rotation of the patterned member with respect to the imaging device.

18. The angle measurement system of claim 1 wherein the patterned member comprises a non-periodic pattern.

19. The angle measurement device of claim 1 wherein the patterned member comprises:

a coarse measurement section having a predetermined number of first radially symmetric segments, each of the first radially symmetric segments having different optical properties; and

a fine measurement section having a second predetermined number of second radially symmetric segment pairs, wherein the second predetermined number is greater than the predetermined number.

20. The angle measurement device of claim 19 wherein the first radially symmetric segments comprises an opaque semicircle and a transparent semicircle and the second radially symmetric segment pairs comprise 20 pairs of portions of Siemens Star segments.

21. A method of determining a rotation angle, the method comprising:

illuminating at least a portion of a patterned member with an optical beam;

transmitting at least a portion of the optical beam through the patterned member;

projecting at least a portion of the transmitted beam onto an imaging device having a plurality of photosensitive elements disposed in an array configuration;

determining a first set of element signal values for each of at least a first subset of the plurality of elements;

computing a reference waveform using the determined first set of element signal values;

rotating the patterned member;

determining a second set of element signal values for each of at least a second subset of the plurality of elements;

computing a measurement waveform using the determined second set of element signal values; and

determining the rotation angle as a function of the reference waveform and the measurement waveform.

22. The method of claim 21 wherein the patterned member comprises a glass substrate having a periodic optically opaque layer disposed thereon.

23. The method of claim 21 wherein the patterned member comprises a non-periodic pattern.

24. The method of claim 21 wherein the reference waveform is computed by:

defining a reference position for the patterned member;

illuminating at least a portion of the patterned member with the optical beam;

transmitting at least a portion of the optical beam through the patterned member;

projecting at least a portion of the transmitted beam onto the imaging unit; and

determining element intensity values for each of the plurality of elements.

25. The method of claim 21 wherein computing the measurement waveform comprises:

storing the determined element signal values in a memory;

defining a predetermined angular dimension;

determining a first set of elements characterized by an angle between a reference angle and the predetermined angular dimension;

determining a second set of elements characterized by an angle between the predetermined angular dimension and twice the predetermined angular dimension;

defining a first portion of the measurement waveform based on the determined element signal values for the first set of elements;

defining a second portion of the measurement waveform based on the determined element signal values for the second set of elements.

26. The method of claim 25 wherein the patterned member comprises a plurality of pairs of radially symmetric elements, each of the elements of the plurality of pairs subtending an element angle and wherein the predetermined angular dimension is less than the element angle.

27. The method of claim 26 wherein the predetermined angular dimension is 1/100 of the element angle.

28. The method of claim 21 wherein computing the measurement waveform comprises:

storing the determined element signal values in a memory;

analyzing the determined element signal values associated with a first pair of radially symmetric segments.

29. The method of claim 28 wherein computing the measurement waveform further comprises analyzing the determined element signal values associated with a second pair of radially symmetric segments, the second pair of radially symmetric segments being positioned 180°

with respect to the first pair of radially symmetric segments.

30. The method of claim 29 wherein computing the measurement waveform further comprises analyzing the determined element signal values associated with a third pair of radially symmetric segments, the third pair of radially symmetric segments being positioned between the first pair and the second pair of radially symmetric segments.

31. The method of claim 21 wherein defining the first portion of the measurement waveform comprises averaging the determined element signal values for the first set of elements and defining the second portion of the measurement waveform comprises averaging the determined element signal values for the second set of elements.

32. The method of claim 21 further comprising:

determining sets of elements for multiples of the predetermined angular dimension; and

defining subsequent portions of the measurement waveform based on the determined element signal values for the sets of elements.

33. The method of claim 21 wherein determining the rotation angle comprises computing a phase shift between the reference waveform and the measurement waveform.

34. The method of claim 33 wherein computing the phase shift comprises performing a Fourier Transform of the reference waveform and performing a Fourier Transform of the measurement waveform.

35. The method of claim 34 wherein the Fourier Transform of the reference waveform is an FFT and the Fourier Transform of the measurement waveform is an FFT.

36. The method of claim 33 wherein computing the phase shift comprises using a waveform feature detection method.

37. The method of claim 21 wherein determining the rotation angle comprises:

storing the first set of element signal values in a spreadsheet;

storing the second set of element signal values in a second spreadsheet; and

pattern matching the first set of element signal values and the second set of element signal values.