US20120050752A1

US20120050752A1 - Large scale metrology apparatus and method

Info

Publication number: US20120050752A1
Application number: US13/214,684
Authority: US
Inventors: W. Thomas Novak; Daniel G. Smith
Original assignee: Nikon Corp
Current assignee: Nikon Corp
Priority date: 2010-08-24
Filing date: 2011-08-22
Publication date: 2012-03-01
Also published as: US20120050726A1; US8582119B2

Abstract

A metrology system that uses multiple rotating receiving heads and either (i) photo-emitting targets or (ii) passive retro-reflector targets. In either case, each receiver head includes a pair of slit-shaped field view collectors, at opposing degrees, relative to the axis of rotation of the head. As each head rotates, radiation either generated by or reflected off the targets, passes through slit-shaped field view collectors and onto photo-detectors. A signal processor couple to each of the rotating heads determines the relative position and height of each of the targets based on the signals generated by the photo-detectors.

Description

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application 61/376,477 entitled “Large Scale Metrology System Using Passive Targets” filed Aug. 24, 2010, incorporated herein for all purposes.

BACKGROUND

1. Field of the Invention
This invention relates to large-scale metrology systems used for robotic control of position tracking, alignment, and assembly of large-scale industrial equipment, and more particularly, to a metrology system that uses (i) heads that receive radiation from a plurality of targets attached to the objects whose positions are to be determined and (ii) optionally either photo-emitter targets or passive reflective targets.
2. Description of Related Art
Large-scale metrology systems used for the factory assembly of industrial equipment are known. These metrology systems typically include both target-sensors and laser-transmitters. During operation in an aircraft assembly facility for example, a number of the target-sensors are placed on the major components of an airplane to be assembled, such as the fuselage, wings, tail, etc. The laser-transmitters are positioned at various locations across the assembly floor. In response to the laser-transmitters, each target-sensor generates pulsed signals that are indicative of its relative azimuth and height with respect to the transmitters. A signal processor then determines the relative azimuth and height of each target-sensor. Once the azimuth and height of each target-sensor is determined, a robotic control system can be used to move the components to be assembled with a high degree of accuracy. As each component is moved, the azimuth and height information of the targets is updated in real-time, allowing the robot system to make adjustments on the fly. As a result, the components can be assembled easier and with greater precision and accuracy.
With one type of known metrology system, each target-sensor is typically cylindrical in shape and includes at least two cylindrical shaped light detectors, each including a multitude of faceted photovoltaic detectors. When radiation from the laser-transmitters contacts the photovoltaic detectors, electrical pulses are generated. By performing signal processing on the electrical pulses, the height and azimuth of each target-sensor relative to the laser transmitters can be calculated. With the height and azimuth information, the precise position and height of each target-sensor may be determined.
Although advantageous, the above-described metrology system has a number of issues that are less than ideal. The multi-faceted photovoltaic detectors on the target-sensors are very complicated and expensive to make. As a result, the number of sensors that may practically be used is limited by cost. In addition, the computers, electronics and power supplies needed to perform the signal processing are either contained in or connected to the target-sensors via electronic cabling. Since the target-sensors are positioned on the objects to be assembled, the cabling, computers, electronics and power supplies are unwieldy and often interfere with the assembly process. These shortcomings limit the usability of current metrology systems.

SUMMARY OF THE INVENTION

The above-described problems are solved by a metrology system that uses multiple rotating receiving heads and either (i) photo-emitting targets or (ii) passive retro-reflector targets. In either case, each receiver head includes a pair of slit-shaped field view collectors, at opposing degrees, relative to the axis of rotation of the head. As each head rotates, radiation either generated by or reflected off the targets, passes through the slit-shaped field view collectors and onto a corresponding pair of photo-detectors. A signal processor coupled to each of the rotating heads determines the relative position and height of each of the targets based on the signals generated by the photo-detectors. With the signals to be processed generated at the receiving heads, as opposed to the targets, the problems associated with computers, electronics and cabling connected to the targets is eliminated. Instead, this equipment may be conveniently positioned away from and not interfering with, the objects to be assembled. In addition, both the photo-emitting and passive targets are much simpler devices relative to the photovoltaic detecting targets of the prior art. The photo-emitting and passive targets are therefore each less expensive to manufacture, allowing the targets to be pervasively used, possibly as a consumable or disposable items. As a result, the overall cost, accuracy, and speed of the assembly processes is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings, which illustrate specific embodiments of the invention.

FIG. 1 illustrates a metrology system in accordance with the principles of the present invention.

FIG. 2 is a non-exclusive embodiment of a receiver head used in the metrology system of the present invention.

FIGS. 3A and 3B are various views of a non-exclusive embodiment of the photo-emissive targets used in one embodiment of the metrology system of the present invention.

FIG. 4 illustrates the metrology system of the present invention during operation.

FIG. 5 is a diagram illustrating a second embodiment of the metrology system of the present invention.

FIGS. 6A-6B and 6C-6D are various views of two different types of passive targets used in the second embodiment of the metrology system of the present invention.

FIG. 7 is a diagram of the second embodiment of the metrology system during operation.

It should be noted that like reference numbers refer to like elements in the figures.
The above-listed figures are illustrative and are provided as merely examples of embodiments for implementing the various principles and features of the present invention. It should be understood that the features and principles of the present invention may be implemented in a variety of other embodiments and the specific embodiments as illustrated in the Figures should in no way be construed as limiting the scope of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The invention will now be described in detail with reference to various embodiments thereof as illustrated in the accompanying drawings. In the following description, specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art, that the invention may be practiced without using some of the implementation details set forth herein. It should also be understood that well known operations have not been described in detail in order to not unnecessarily obscure the invention.
Referring to FIG. 1, a non-exclusive embodiment of a metrology system in accordance with the principles of the present invention is shown. The metrology system 10 includes a plurality of photo-emissive targets 12, a plurality of rotating receiver heads 14, and a master controller/signal processor 16. The photo-emissive targets 12 are each positioned at first locations, typically located on the various objects (not illustrated) to be assembled, within a volume defining an assembly area. The receiver heads 14 are each positioned at second locations within the volume. The master controller/signal processor 16 is also provided in or near the assembly area, typically away from the photo-emissive targets 12 and heads 14. The master controller/signal processor 16 optionally communicates with the photo-emissive targets 12 and receiver heads 14 either wirelessly or through a wired network (not illustrated).
Referring to FIG. 2, a non-exclusive embodiment of one of the receiver heads 14 is shown. Each receiver head 14 includes two slit-shaped field view collector pair 22A and 22B arranged at opposing degrees with respect to one another in a V-shaped pattern. Photo- detectors 24A and 24B are provided behind the slit-shaped field view collector pair 22A and 22B and internal to the receiver head 14. In one non-exclusive embodiment, the field of view of the slit-shaped pair 24A and 24B is +/−45 degrees. In other embodiments, the field of view may range from +/−5 degrees to +/−75 degrees.
Referring to FIG. 3A, a non-exclusive embodiment of a photo-emissive target 12 is shown. Each target 12 includes a pair of transparent spheres 26 laterally spaced apart from one another by an elongated structure 28. As best illustrated in FIG. 3B, each sphere 26 includes a photo-emitter 30, such as an LED, that is arranged to emit radiation through a lens 32 onto a reflective convex surface 36. The surface 36 is designed to create a radiation pattern of parallel rays 34, ranging in elevation from +/−10 degrees to +/−60 degrees and 360 degrees around the azimuth. In one non-exclusive embodiment, the elevation is +/−45 degrees.
Prior to operation, it is necessary to determine the location of each of the receiver heads 14 within the volume defining the assembly area. In accordance with various embodiments, this can be accomplished in a number of different ways. In one embodiment, each the receiver heads 14 are placed at a known location, which is provided to the master controller/signal processor 16. Alternatively, as illustrated in FIG. 2, a target 12 may be placed on top of each of the heads 14, which are then placed at different locations within the volume defining the assembly area. Under the control of the master controller/signal processer 16, a calibration sequence is performed to determine the location of each of the heads 14. This calibration sequence involves turning off all of the photo-emissive targets 12 in the volume defining the assembly area, except those placed on top each of the heads 14. As described in more detail below, the master controller/signal processor 16 then determines the location of each of the heads 14 based on signal derived from the radiation received from the activated photo-emissive targets 12 on top of the heads 14. Once the location of the heads 14 is known, the system 10 may be used for the assembly of components.
Referring to FIG. 4, the metrology system 10 during operation is illustrated. In this example, the photo-emissive targets 12 have been placed on the components to be assembled (not illustrated), while the position of the heads 14 is known using either of the methods described above. The heads 14 are then rotated at a constant velocity, for example 10 revolutions per second. As each head 14 rotates, the azimuth around each head 14 is scanned. With each revolution, radiation from each target 12 sweeps across the opposing slit-shaped field view collector pair 22A and 22B. For each revolution, the photo- detectors 24A and 24B generate a pulsed pair of signals per target 12 in the volume defining the assembly area. All of the pulsed pair of signals, generated by each head 14 for each target 12 per revolution, is provided to the master controller/signal processor 16. For any two targets 14, each photo- detector 24A and 24B generates two pulses per revolution, resulting in an ambiguity of four possible target directions. With three targets 14, nine possible target directions are defined. This relationship exists regardless of the number of targets 14. Thus, the number of possible target directions is determined by the equation N², where N is the number of targets 14.
The master controller/signal processor 16 determines the azimuth and height of each of the targets 12 from all the pulsed pair signals. For each pulsed pair, the timing between the signals determines the height of the corresponding target 12, while the timing of each pulse relative to the rotational position of the head 14 determines the azimuth of the corresponding target 12. Each pulsed pair of signals thus determines a potential direction and height of the corresponding target 12.
Depending on the angular position of the head 14, there may be multiple targets 12 viewed at any point in time. Consequently, for N targets 12 sensed at any point in time, there are (i) N pulsed pair signals and (ii) N²possible directional lines for the N targets per head 14.
The location of a given target 12 is determined when at least two heads 14 detect the target 12 along an intersection of directional lines 40, as illustrated in FIG. 4. Each directional line 40 is representative of a small spheroidal angle. The probability of multiple targets 12 along a single directional line 40 is relatively low. This probability is further reduced when multiple heads 14 are used. Consequently, the location of a target 12 is determined when two (or more) heads 14 identify the target 12 along intersecting directional lines 40. The height of the target 12 is calculated from the elevation calculation determined by the timing difference between each pulsed pair of signals generated by the two (or more) heads 14. Due to the ambiguity described above, there is N²−N false directions for each rotating head 14. These false directions are recognized when there are no intersections with other lines 40 from any of the other heads 14.
Based on the algorithms described above, the master controller/signal processor 16 determines the instantaneous location and height for each of the targets 12. As the objects to be assembled are moved, the instantaneous location information of the targets 12 is calculated and updated on the fly. As a result, the objects can be assembled with a high degree of accuracy and efficiency.
As noted above, if the location of the heads 14 is not previously known, a calibration process is first needed before the metrology system 10 may be used for assembly. One possible calibration process involves the master controller/signal processor 16 turning off all the targets 12, except those positioned at the heads 14. The above-described process is then performed using the same algorithms. The intersecting directional lines 40 define the location of the active targets 12, and hence the heads 14. Once the location of the heads 14 is known, the system 10 may be used for assembly as described above.
Referring to FIG. 5, a diagram illustrating a second embodiment of the metrology system of the present invention is shown. In this embodiment, the metrology system 50 includes laser-transmitting rotating heads 52 and passive targets 54. During operation, each of the heads 52 rotates at a constant velocity, generating two infrared fan-out beams 56A and 56B. In one embodiment, the fan-out beams 56A and 56B define an elevation range of +/−45 degrees respectively. The fan beams 56A and 56B also rotate at a constant velocity scanning 360 degrees around each head 52. The heads 52 are otherwise essentially the same as the heads 14 and include the same V-shaped slit-shaped field view collectors 22A and 22B and photo- detectors 24A and 24B. In various other embodiments, the fan beams 56A and 56B may range from +/−5 degrees to 75 degrees.
Referring to FIGS. 6A and 6B, two diagrams of the passive targets 54 are shown. As illustrated in FIG. 6A, each passive target 54 includes two targets 56, laterally spaced apart from one another.
As best illustrated in FIG. 6B, each target 56 includes a transparent window 57, first reflective surface 58, a second reflective surface 60, and a lens 62. The first reflective surface 58 defines a rotationally symmetric reflective surface. In various embodiments, the first reflective surface can be either concave or convex. In another non-exclusive embodiment, the second reflective surface 60 is flat mirror. In alternative embodiments, the reflective surface 60 can be either a flat surface that is painted or otherwise coated with a retro-reflective material or a corner-cube, or contain an array of retro-reflecting elements such as corner cubes or half silvered spheres. During operation, radiation from the fan beams 56A and 56B pass through the window 57, reflect off the first reflective surface 58, through the lens 62, and onto the second reflective surface 60, which then reflects the radiation through the lens 62, off of the first reflective surface 58 and then out of the window 57. The radiation rays entering and exiting the target 56 are parallel with respect to one another, as depicted by the arrows 64. In one non-exclusive embodiment, the radiation pattern of incoming and outgoing rays is 360 degrees around the target 56 and with an elevation of approximately +/−45 degrees.
Referring to FIGS. 6C and 6D, diagrams of another passive target 54 is shown. As illustrated in FIG. 6C, each passive target 54 includes two targets 56A laterally spaced apart from one another. Each target 56A is painted or otherwise coated with a retro-reflective surface material, such as glass beads that reflect light in the same direction as the incoming light.
Referring to FIG. 7, the metrology system 50 is shown during operation. Like the previous embodiment, the passive targets 54 are placed on the objects to be assembled. The laser-transmitting heads 52 are either positioned in known locations or are first calibrated as described above. During assembly operations, each of the heads 52 rotates at a constant velocity, generating the two fan-out beams 56A and 56B rotating 360 degrees with an elevation of +/−45 degrees. In response, each target 54 reflects two radiation pulses per fan- out beam 56A and 56B pair, which are each sensed by the photo-detectors in each of the rotating heads 52. As a result, the photo- detectors 24A and 24B of each head 52 generates a pulsed pair of signals for each target 12 per revolution. The master controller/signal processor 16 computes the location and height of the passive targets 54 using the same algorithms as described above from all the pulsed pair signals.
In the embodiment described above, the two slit-shaped field view collectors 22A and 22B and the elevation of the radiation pattern created around each photo-emissive target 12 or passive target 54 are all at +/−45 degrees. It should be noted that 45 degrees is exemplary and in no way should be construed as limiting the invention. In various embodiments, the slit-shaped field view collectors 22A and 22B and the elevation of the radiation pattern created by the targets each may vary within a predetermined range of range of +/−10 to +/−60 degrees. Although many of the components and processes are described above in the singular for convenience, it will be appreciated by one of skill in the art that multiple components and repeated processes can also be used to practice the techniques of the system and method described herein. Several slit-shaped field view field collectors may be used per head 14 or 52 to cover even larger fields. For example, with a slit-shaped field view field collector with a 30 degrees full field of view, eight collectors on a single head 14 or 52 to cover +/−60 degrees for both inclinations. Further, while the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that changes in the form and details of the disclosed embodiments may be made without departing from the spirit or scope of the invention. For example, embodiments of the invention may be employed with a variety of components and should not be restricted to the ones mentioned above. It is therefore intended that the invention be interpreted to include all variations and equivalents that fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A system, comprising:

a plurality of targets configured to each be positioned at first locations within a predetermined volume;

a plurality of photo-detecting heads configured to each be positioned at second locations within the predetermined volume, each of the photo-detecting heads configured to rotate within the predetermined volume and receive radiation from the plurality of targets; and

a signal processor configured to determine the relative position and height of the plurality of targets within the predetermined area based on signals generated by the plurality of rotating photo-detecting heads in response to radiation received from the plurality of targets respectively.

2. The system of claim 1, wherein each of the plurality of photo-detecting heads comprises:

a pair of slit-shaped field view collectors; and

a pair of photo-detectors positioned to receive light passing through the pair of slit-shaped field view collectors.

3. The system of claim 2, wherein each pair of the photo-detectors is configured to generate a plurality of pulsed pair of signals.

4. The system of claim 3, wherein each of the plurality of pulsed pair signals is generated, per each revolution of the rotating heads, for each of the plurality of targets in the predetermined volume respectively.

5. The system of claim 4, wherein the signal processor is further configured to generate, for each of the plurality of pulsed pair signals, the following:

(i) an elevation for the corresponding target based on the elapsed time between the pulsed pair signals; and

(ii) an azimuth of the corresponding target from the timing of the pulsed pair signals relative to the rotational position of the photo-detecting head that generated the pulsed pair signal.

6. The system of claim 5, wherein the signal processor is further configured to generate a plurality of direction lines from the plurality of pulsed pair signals.

7. The system of claim 6, wherein the signal processor is further configured to:

(i) ascertain the direction lines that intersect; and

(ii) determine the relative position of the plurality of targets based on the ascertained intersections.

8. The system of claim 7, wherein determining the relative position further comprises one of the following:

(i) determining the relative position of the plurality of targets in the volume in two dimensions; and

(ii) determining the relative position of the plurality of targets in the volume in three dimensions.

9. The system of claim 1, wherein the targets are photo-emitting targets.

10. The system of claim 9, wherein the photo-emitting targets include first and second photo-emitters laterally spaced apart.

11. The system of claim 10, wherein the first and the second photo-emitters use radiation from LEDs.

12. The system of claim 9, wherein each of the photo-emitting targets generate radiation 360 degrees around each target respectively.

13. The system of claim 9, wherein each of the photo-emitting targets generate radiation having an elevation ranging from +/−10 to +/−60 degrees.

14. The system of claim 1, wherein the targets are passive-reflective targets.

15. The system of claim 14, wherein each of the passive-reflective targets includes a first reflective surface and a second reflective surface.

16. The system of claim 15, wherein the second reflective surface consists of one of the following: a mirror, a corner cube or a surface coated with a reflective material, or an array of retro-reflecting elements.

17. The system of claim 15, wherein the first and the second reflective surfaces are contained within a transparent spherical window.

18. The system of claim 14, wherein each of the passive reflective targets comprise at least one sphere covered with a retro-reflective material.

19. The system of claim 1, wherein the photo-detecting heads are each configured to generate two fan beams to illuminate the predetermined volume.

20. The system of claim 19, wherein the fan beams rotate 360 degrees around each photo-detecting head respectively.

21. The system of claim 19, wherein the fan-out beams have an elevation ranging from +/−5 to +/−75 degrees respectively.

22. The system of claim 10, further comprising a master controller for selectively activating or deactivating the photo-emitting targets.

23. The system of claim 1, further comprising a master controller configured to calibrate the system by determining the location of the plurality of photo-detecting heads within the predetermined area.