Laser tracker with position-sensitive detectors for searching for a target

• US 9,696,140 B2
• Filed: 12/06/2012
• Issued: 07/04/2017
• Est. Priority Date: 12/06/2011
• Status: Active Grant

• Current Assignee: Leica Geosystems AG (Hexagon AB)
• Original Assignee: Leica Geosystems AG (Hexagon AB)
• Inventors: Markendorf, Albert
• Primary Examiner(s): Vaughn, Jr., William C
• Assistant Examiner(s): Towe, Joseph A
• Application Number:
  US14/363,280
• Publication Number:
  US 20140320643A1
• Time in Patent Office: 1,671 Days
• Field of Search: None
• CPC Class Codes: G01B 11/14 for measuring distance or clearance between spaced objects or spaced apertures G01B11/26 takes precedence; rangefinders G01C3/00
• CPC Class Codes: G01S 17/66 Tracking systems using electromagnetic waves other than radio waves

Abstract

Some embodiments of the invention relate to a laser tracker for continuously pursuing a reflective target and for determining the position of the target. The method may include a base which defines a vertical axis, a beam guiding unit for emitting a measurement beam, the beam guiding unit being pivotable by motor relative to the base about the vertical axis and an inclination axis, and a measurement axis being defined by an emission direction of the measurement beam. The laser tracker may include a precision distance measurement unit and an angle measurement function. The target search unit may have illumination source for illuminating the target, a first camera with a first position-sensitive detector and at least one part of the illumination beam reflected on the target being determinable as a first target position, and a control and evaluation unit.

Claims

1. A laser tracker for progressive tracking of a reflective target and for determining a position of the target having:

a base which defines a standing axis,

a beam guiding unit for emitting measuring radiation and for receiving at least a part of the measuring radiation reflected on the target, wherein

the beam guiding unit is pivotable by a motor about the standing axis and an inclination axis in relation to the base and

a measuring axis is defined by an emission direction of the measuring radiation

a fine distance meter for precisely determining the distance to the target using the measuring radiation,

an angle sensor for determining an alignment of the beam guiding unit in relation to the base, and

a target search unit having

a light source for illuminating the target using illumination light,

a first camera, which defines a first field of vision, having a first position-sensitive detector, wherein

a first image can be acquired using the first camera and at least a part of the illumination light reflected on the target can be determined in the first image as a first target position and

the first camera is arranged having its optical axis offset to the measuring axis, and

a second camera, which defines a second field of vision, having a second position-sensitive detector, wherein

a second image can be acquired using the second camera and at least a part of the illumination light reflected on the target can be determined in the second image as a second target position,

the second camera is arranged having its optical axis offset to the measuring axis, and

the second camera is arranged having fixed positioning and alignment, which is known in relation to the first camera and to the measuring axis, and the first and second cameras are arranged such that the fields of vision of the first and second cameras at least partially overlap,

wherein

in the scope of the target search unit a computer is present configured such that upon execution of a search functionality, a finding of the target is performed as a function of an integrated consideration of the first target position of the first image and the second target position of the second image and based thereon the measuring radiation is aligned automatically on the target by control of the beam guiding unit, so that the measuring radiation is incident on the target and the distance to the target can be precisely determined by means of the fine distance meter,

whereby the computer is configured such that upon execution of the search functionality ambiguities, which are present upon a use of only the first target position for finding the target and by way of a parallax provided by the measuring axis and the optical axes of the first and second cameras, are resolved by a mutual use of both the first and also the second target positions for finding the target, and

the laser tracker has a calibration function, upon the execution of which the fixed positioning and alignment of the second camera in relation to the first camera and in relation to the measuring axis are determined, in particular wherein in the scope of the calibration function;

the target is measured, by means of the measuring radiation emitted by the beam guiding unit, in at least two different measuring positions offset to another,

for each of said two measuring positions of the target a first image is acquired with the first camera and a second image is acquired with the second camera,

the first target position of each first image and second target position of each second image are determined, and

the fixed positioning and alignment of the second camera in relation to the first camera and in relation to the measuring axis are derived from said measuring of the target in the at least two measuring positions and the first and second target positions determined thereto.

2. The laser tracker as claimed in claim 1, wherein the computer is configured such that upon execution of the search functionality, a coarse distance to the target is determined as a function of the first and second target positions.

3. The laser tracker as claimed in claim 2, wherein the computer is configured such that upon execution of the search functionality, respectively a target direction to the target is derived as a function of the first and second target positions, and a coarse position is determined as a function of the target directions, using photogrammetry.

4. The laser tracker as claimed in claim 3, wherein the first and second cameras are arranged such that the measuring axis lies at least partially in an overlap region defined by the overlapping fields of vision of the first and second cameras.

5. The laser tracker as claimed in claim 4, wherein the first and second cameras are arranged such that their optical axes are respectively arranged:

offset in parallel to the measuring axis, or

at a defined angle in relation to the measuring axis, and/or

the first and second cameras are arranged axially-symmetrical in relation to the measuring axis.

6. The laser tracker as claimed in claim 1, wherein:

the light source is implemented such that the illumination light can be emitted divergently having a wavelength in the infrared range;

or

the first and second cameras are implemented such that essentially only infrared illumination light can be acquired, wherein the first and second cameras each have a light filter for essentially exclusive transmission of the infrared light to the respective position-sensitive detector.

7. The laser tracker as claimed in claim 1, wherein the laser tracker has a third or multiple cameras having a third or multiple position-sensitive detectors for acquiring a third or multiple target positions, wherein the third or multiple cameras are arranged such that a third or multiple optical axes of the third or multiple cameras are arranged offset to the measuring axis, and the computer is configured such that upon execution of the search functionality, the searching for the target is performed as a function of the three or more target positions, wherein at least one of the cameras is implemented to acquire light in a defined range around an illumination wavelength of the illumination light.

8. The laser tracker as claimed in claim 1, wherein the laser tracker has:

a support, which is pivotable by a motor about the standing axis in relation to the base and defines the inclination axis, and

a targeting unit, which is implemented as a beam guiding unit and is pivotable by a motor about the inclination axis in relation to the support, having a telescope unit for emitting the measuring radiation and for receiving at least a part of the measuring radiation reflected on the target.

9. A method for searching for a target using a laser tracker, the laser tracker comprising:

a base which defines a standing axis, and

a beam guiding unit for emitting measuring radiation and for receiving at least a part of the measuring radiation reflected on the target, wherein

the beam guiding unit is pivotable by a motor about the standing axis and an inclination axis in relation to the base, and

a measuring axis is defined by an emission direction of the measuring radiation, wherein the method is performed by;

an illumination of the target is performed using illumination light,

a position-sensitive acquisition of a first image is performed using a first camera, which is arranged on the laser tracker and defines a first field of vision, wherein the first camera is arranged having its optical axis offset to the measuring axis, and

a first determination of at least a part of the illumination light reflected on the target is performed as a first target position in the first image,

a position-sensitive acquisition of a second image using a second camera, which is arranged on the laser tracker and defines a second field of vision, wherein the second camera;

is arranged having its optical axis offset to the measuring axis, and

is arranged having fixed positioning and alignment, which is known in relation to the first camera and to the measuring axis, and the first and second cameras are arranged such that the fields of vision of the first and second cameras at least partially overlap,

a second determination of at least a part of the illumination light reflected on the target as a second target position in the second image, and

finding of the target as a function of an integrated consideration of the first target position of the first image and second target position of the second image with a thereon based automatic alignment of the measuring radiation on the target by control of the beam guiding unit,

whereby ambiguities, which are present upon a use of only the first target position for finding the target and by way of a parallax provided by the measuring axis and the optical axes of the first and second cameras, are resolved by a mutual use of both the first and also the second target positions for finding the target, and

wherein

the fixed positioning and alignment of the first camera in relation to the first camera and in relation to the measuring axis are calibrated by;

measuring the target, by means of the measuring radiation, in at least two different measuring positions offset to another,

for each of said two measuring positions of the target a first image is acquired with the first camera and a second image is acquired with the second camera, determining the first target position of each first image and second target position of each second image, and

deriving the fixed positioning and alignment of the second camera in relation to the first camera and in relation to the measuring axis from the said measuring of the target in the at least two measuring positions and the first and second target positions determined thereto.

10. The method as claimed in claim 9, wherein ambiguities, which are present upon a use of only the first target position for finding the target and by way of a parallax provided by the measuring axis and the optical axes of the first and second cameras, are resolved by a mutual use of both the first and also the second target positions.

11. The method as claimed in claim 9, wherein a coarse distance to the target is determined as a function of the first and second target positions.

12. The method as claimed in claim 11, wherein respectively a target direction to the target is derived as a function of the first and second target positions, and a coarse position of the target is determined as a function of the target directions using photogrammetry.

13. A computer program product having program code which is stored on a non-transitory computer readable medium, for controlling or carrying out the method as claimed in claim 9.

14. A laser tracker for progressive tracking of a reflective target and for determining a position of the target having:

a base which defines a standing axis,

a beam guiding unit for emitting measuring radiation and for receiving at least a part of the measuring radiation reflected on the target, wherein

the beam guiding unit is pivotable by a motor about the standing axis and an inclination axis in relation to the base, and

a measuring axis is defined by an emission direction of the measuring radiation,

a fine distance meter for precisely determining the distance to the target by means of the measuring radiation,

an angle sensor for determining an alignment of the beam guiding unit in relation to the base, and

a target search unit having;

a light source for illuminating the target using illumination light,

a first camera, which defines a first field of vision, having a first position-sensitive detector, wherein;

a first image can be acquired using the first camera and at least a part of the illumination light reflected on the target can be determined in the first image as a first target position, and

the first camera is arranged having its optical axis offset to the measuring axis, and

a second camera, which defines a second field of vision, having a second position-sensitive detector, wherein;

a second image can be acquired using the second camera and at least a part of the illumination light reflected on the target can be determined in the second image as a second target position,

the second camera is arranged having its optical axis offset to the measuring axis, and

the second camera is arranged having fixed positioning and alignment, which is known in relation to the first camera and to the measuring axis, and the first and second cameras are arranged such that the fields of vision of the first and second cameras at least partially overlap,

wherein;

in the scope of the target search unit a computer is present configured such that upon execution of a search functionality, a finding of the target is performed as a function of an integrated consideration of the first target position of the first image and second target position of the second image and based thereon the measuring radiation is aligned automatically on the target by control of the beam guiding unit, so that the measuring radiation is incident on the target and the distance to the target can be precisely determined by means of the fine distance meter,

whereby the computer is configured such that upon execution of the search functionality ambiguities, which are present upon a use of only the first target position for finding the target and by way of a parallax provided by the measuring axis and the optical axes of the first and second cameras, are resolved by a mutual use of both the first and also the second target positions for finding the target, and

the computer has a target position determination functionality, upon the execution of which the first target position of the first image and the second target position of the second image are determined by means of image processing such that in the respective image a shape of the light cross-section of the illumination light reflected on the target and acquired in the respective image is determined, and thereby the first respectively the second target position is determined, which represent the respective location of the illumination light acquired in the image in the respective image.

15. The laser tracker as claimed in claim 14, wherein:

the target position determination functionality is configured such that the first and second target positions are determined;

by means of focal point calculation based on the shape acquired in the respective image, in particular by means of brightness and/or contrast analysis, and/or

by means of matching, in particular bringing into correspondence, of the shape acquired in the respective image with a stored pattern on the basis of a best fit method, in particular with subpixel precision, wherein the respective target position of the illumination light acquired in the image is determined on the basis of the location of the pattern in the image which is brought into correspondence, in particular with subpixel precision.

18. The laser tracker as claimed in claim 14, wherein the computer is configured such that upon execution of the search functionality, a coarse distance to the target is determined as a function of the first and second target positions.

19. The laser tracker as claimed in claim 18, wherein the computer is configured such that upon execution of the search functionality, respectively a target direction to the target is derived as a function of the first and second target positions, and a coarse position is determined as a function of the target directions, using photogrammetry.

20. The laser tracker as claimed in claim 19, wherein the first and second cameras are arranged such that the measuring axis lies at least partially in an overlap region defined by the overlapping fields of vision of the first and second cameras.

16. A method for searching for a target using a laser tracker, wherein the laser tracker has:

a base which defines a standing axis,

a beam guiding unit for emitting measuring radiation and for receiving at least a part of the measuring radiation reflected on the target, wherein;

the beam guiding unit is pivotable by a motor about the standing axis and an inclination axis in relation to the base, and

a measuring axis is defined by an emission direction of the measuring radiation,

wherein the method is performed by;

an illumination of the target using illumination light,

a position-sensitive acquisition of a first image using a first camera, which is arranged on the laser tracker and defines a first field of vision, wherein the first camera is arranged having its optical axis offset to the measuring axis,

a first determination of at least a part of the illumination light reflected on the target as a first target position in the first image,

a position-sensitive acquisition of a second image by using a second camera, which is arranged on the laser tracker and defines a second field of vision, wherein the second camera;

is arranged having its optical axis offset to the measuring axis, and

is arranged having fixed positioning and alignment, which is known in relation to the first camera and to the measuring axis, and the first and second cameras are arranged such that the fields of vision of the first and second cameras at least partially overlap,

a second determination of at least a part of the illumination light reflected on the target as a second target position in the second image, and

finding of the target as a function of an integrated consideration of the first target position of the first image and second target position of the second image with a thereon based automatic alignment of the measuring radiation on the target by control of the beam guiding unit,

whereby ambiguities, which are present upon a use of only the first target position for finding the target and by way of a parallax provided by the measuring axis and the optical axes of the first and second cameras, are resolved by a mutual use of both the first and also the second target positions for finding the target, and

wherein;

the first target position of the first image and the second target position of the second image are determined by means of image processing such that in the respective image a shape of the light cross-section of the illumination light reflected on the target and acquired in the respective image is determined, and thereby the first respectively second target position is determined, which represent the respective location of the illumination light acquired in the image in the respective image.

24. A computer program product having program code which is stored on a non-transitory computer readable medium, for controlling or carrying out the method as claimed in claim 16.

17. The method as claimed in 16, wherein the determination of the first and second target positions is performed respectively:

by means of a focal point calculation based on the shape acquired in the respective image, in particular by means of brightness and/or contrast analysis, and/or

by means of matching, in particular bringing into correspondence, of the shape acquired in the respective image with a stored pattern on the basis of a best fit method, in particular with subpixel precision, wherein the respective target position of the illumination light acquired in the image is determined on the basis of the location of the pattern in the image which is brought into correspondence, in particular with subpixel precision.

21. The laser tracker as claimed in 20, wherein the first and second cameras are arranged such that their optical axes are respectively arranged:

offset in parallel to the measuring axis, or

at a defined angle in relation to the measuring axis, and/or

the first and second cameras are arranged axially-symmetrical in relation to the measuring axis.

22. The method as claimed in 16, wherein a coarse distance to the target is determined as a function of the first and second target positions.

23. The method as claimed in 22, wherein respectively a target direction to the target is derived as a function of the first and second target positions, and a coarse position of the target is determined as a function of the target directions using photogrammetry.