U.S. patent application number 13/287513 was filed with the patent office on 2012-03-01 for laser-based coordinate measuring device and laser-based method for measuring coordinates.
This patent application is currently assigned to FARO TECHNOLOGIES INC.. Invention is credited to D. Scott Ackerson, Robert E. Bridges, Lawrence B. Brown, James K. West.
Application Number | 20120050712 13/287513 |
Document ID | / |
Family ID | 42733924 |
Filed Date | 2012-03-01 |
United States Patent
Application |
20120050712 |
Kind Code |
A1 |
Bridges; Robert E. ; et
al. |
March 1, 2012 |
LASER-BASED COORDINATE MEASURING DEVICE AND LASER-BASED METHOD FOR
MEASURING COORDINATES
Abstract
A laser based coordinate measuring device measures a position of
a remote target. The laser based coordinate measuring device
includes a stationary portion, a rotatable portion, and at least a
first optical fiber. The stationary portion has at least a first
laser radiation source and at least a first optical detector, and
the rotatable portion is rotatable with respect to the stationary
portion. The first optical fiber system, which optically
interconnects the first laser radiation source and the first
optical detector with an emission end of the first optical fiber
system, has the emission end disposed on the rotatable portion. The
emission end emits laser radiation to the remote target and
receives laser radiation reflected from the remote target with the
emission direction of the laser radiation being controlled
according to the rotation of the rotatable portion.
Inventors: |
Bridges; Robert E.; (Kennett
Square, PA) ; Brown; Lawrence B.; (Cochranville,
PA) ; West; James K.; (Landenberg, PA) ;
Ackerson; D. Scott; (Easton, MD) |
Assignee: |
FARO TECHNOLOGIES INC.
Lake Mary
FL
|
Family ID: |
42733924 |
Appl. No.: |
13/287513 |
Filed: |
November 2, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12882444 |
Sep 15, 2010 |
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13287513 |
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09621645 |
Jul 24, 2000 |
7800758 |
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12882444 |
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60145686 |
Jul 26, 1999 |
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60145315 |
Jul 23, 1999 |
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Current U.S.
Class: |
356/3.1 |
Current CPC
Class: |
G01B 11/03 20130101;
G01C 3/08 20130101; G01S 17/66 20130101; G01S 5/163 20130101; G01B
9/02012 20130101; G01B 11/02 20130101; G01B 11/002 20130101; G01B
11/026 20130101; G01S 17/42 20130101 |
Class at
Publication: |
356/3.1 |
International
Class: |
G01C 3/08 20060101
G01C003/08 |
Claims
1. A distance measuring apparatus for measuring a distance from the
apparatus to an object, comprising: a first lens system including
at least one lens; a first source of light and a second source of
light, a first light from the first source and a second light from
the second source transmitted toward the object, the first light
having a first wavelength and the second light having a second
wavelength different from the first wavelength, a first portion of
the first light reflected by the object and a second portion of the
second light reflected by the object, the first portion and the
second portion passing through at least a portion of the first lens
system; a first optical detector that detects the first portion and
a second optical detector that detects the second portion; and a
first dichroic beam splitter and a second dichroic beam splitter,
each dichroic beam splitter adapted to reflect at least one of the
first light and the second light toward the first optical detector
and the second optical detector, respectively.
2. The distance measuring apparatus according to claim 1 wherein
the first dichroic beam splitter and the second dichroic beam
splitter are plates.
3. The distance measuring apparatus according to claim 1 wherein a
first mirror redirects at least one of the first portion and the
second portion toward one of the detectors.
4. The distance measuring apparatus according to claim 1 further
comprising: a structure rotatable about a first axis and a second
axis, the structure including the first lens system and at least a
portion of a distance meter, wherein the distance meter includes
the first light source, a first beam of first light launched
through a portion of the first optical system to the object, the
distance meter configured to measure a first distance from the
apparatus to the target based at least in part on the speed of
light; a first encoder that measures a first angle of rotation
about the first axis; and a second encoder that measures a second
angle of rotation about the second axis.
5. The distance measuring apparatus according to claim 4, wherein
the object is a retroreflector target.
6. The distance measuring apparatus according to claim 5, further
comprising: a position detector that collects from a portion of the
first optical system a portion of the beam reflected by the target;
and a first electronics system that causes the beam to track the
target based at least in part on a signal from the position
detector, wherein the tracking is obtained by rotating the
structure about the first axis and the second axis.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a Continuation Application of
U.S. Ser. No. 12/882,444 filed Sep. 15, 2010, which claims the
benefit of U.S. patent application Ser. No. 09/621,645 filed on
Jul. 24, 2000, 60/171,474 filed Dec. 22, 1999, 60/145,686 filed
Jul. 26, 1999 and 60/145,315 filed Jul. 23, 1999, which are hereby
incorporated by reference. The present application also hereby
incorporates by reference U.S. patent application Ser. No.
09/285,654 filed Apr. 5, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a coordinate measuring
device and, more particularly, to a laser based coordinate
measuring device.
[0004] 2. Discussion of the Related Art
[0005] There is a class of instrument that measures the coordinates
of a point by sending a laser beam to a retroreflector target that
is in contact with the point. The instrument determines coordinates
by measuring the distance and the two angles to the retroreflector
target. There is another class of instrument that is capable of
measuring the three orientation angles (pitch, yaw, and roll) of a
retroreflector target. If such an instrument can also measure the
three coordinates of a point in space, it is said to measure six
degrees of freedom. However, such six degree-of-freedom systems,
whether or not they are employing laser techmques, are generally
inaccurate, slow, limited in radial or angular range, and/or
expensive. Exemplary systems for determining position (three to six
degrees of freedom) are described by U.S. Pat. No. 4,790,651 to
Brown et al.; U.S. Pat. No. 4,714,339 to Lau et al.; U.S. Pat. No.
5,5059,789 to Salcudean; U.S. Pat. No. 5,367,373 to Busch-Vishniac
et al.; U.S. Pat. No. 5,973,788 to Pettersen et al.; and U.S. Pat.
No. 5,267,014 to Prenninger, et al. (the disclosures of which are
hereby incorporated by reference).
[0006] The laser tracker is a particular type of
coordinate-measuring device that tracks the retroreflector target
with one or more laser beams it emits. To provide a beam-steering
mechanism for this tracking function, laser trackers conventionally
include a stationary base onto which a rotating stage or platform
is mounted. Until now, most laser trackers have used optical
elements, such as mirrors or prisms, to steer the laser beam from
its source in the base to optics in the rotating stage and through
or off those optics toward the retroreflector. These optical
elements and their mounts are costly. Also, they are subject to
tilting and bending as a result of thermal and/or mechanical
stresses that are usually present in tracker work environments. The
consequence of these stresses is reduced accuracy and stability.
Examples of beam-steering laser trackers are described by Lucy, et
al., Applied Optics, pp. 517-524, 1966; Bernard and Fencil, Applied
Optics, pp. 497-505, 1966; Sullivan, SPIE, Vol. 227, pp. 148-161,
1980; U.S. Pat. No. 4,020,340 to Cooke; U.S. Pat. No. 4,025,193 to
Pond; U.S. Pat. No. 4,386,848 to Clendenin et al.; U.S. Pat. No.
4,436,417 to Hutchin; U.S. Pat. No. 4,457,625 to Greenleaf et al.;
U.S. Pat. No. 4,714,339 to Lau et al.; U.S. Pat. No. 4,721,385 to
Jelalian et al.; Gennan Patent DE 3205362 A1 to Pfeifer et al.
(which are hereby incorporated by reference). An example of a
beam-steering mechanism that uses prismatic optical elements is
described by U.S. Pat. No. 4,790,651 Brown et al. (which is hereby
incorporated by reference).
[0007] A device that is closely related to a laser tracker is the
laser scanner. The laser scanner steps one or more laser beams to
points on a diffuse surface. The laser tracker and laser scanner
are both coordinate-measuring devices. It is common practice today
to use the term laser tracker to also refer to laser scanner
devices having distance- and angle-measuring capability. This broad
definition of laser tracker, which includes laser scanners, is used
throughout this application.
[0008] An alternative to steering the laser beam with a mirror or
prism is to launch the laser beam from an optical fiber mounted on
a rigid platform. Although such devices have been built, none has
taken full advantage of the simplicity, stability, and flexibility
possible with such an approach. For example, such systems usually
require separate optical fibers for transmitting and receiving the
laser light. An exemplary system that tracks a laser beam launched
from an optical fiber is described in Nakamura, et al., Review of
Scientific Instruments, pp. 1006-1011, 1994; Takatsuji et al.,
easurement Science & Technology, pp. 38-41, 1998; Takatsuji, et
al., Measurement Science & Technology, pp. 1357-1359, 1998; and
Takatsuji, et al., Dimensional Metrology in the 21.sup.st Century,
International Dimensional Metrology Workshop sponsored by Oak Ridge
Metrology Center, May 10-13, 1999 (which are hereby incorporated by
reference). Non-tracking systems that launch laser beams from
optical fibers are numerous in the prior art and include U.S. Pat.
No. 4,459,022 to Morey; U.S. Pat. No. 5,095,472 to Uchino, et al.;
U.S. Pat. No. 5,198,874 to Bell et al.; U.S. Pat. No. 5,200,838 to
Nudelman; U.S. Pat. No. 5,402,230 to Tian, et al.; U.S. Pat. No.
5,508,804 to Furstenau; and U.S. Pat. No. 5,557,406 to Taylor
(which are hereby incorporated by reference).
SUMMARY OF THE INVENTION
[0009] Accordingly, the present invention is directed to a
laser-based coordinate measuring device that substantially obviates
one or more of the problems due to limitations and disadvantages of
the related art.
[0010] An object of the present invention is to provide a
laser-based coordinate measuring device with improved laser beam
steering, six degree of freedom measurements, and capability to
locate multiple retroreflectors distributed throughout large
volumes.
[0011] Another object of the present invention is to provide a
reliable laser-based coordinate measuring device that is easily
manufactured at a low cost without complex beam-steering
optics.
[0012] Additional features and advantages of the invention will be
set forth in the description that follows, and in part will be
apparent from the description, or may be learned by practice of the
invention. The objectives and other advantages of the invention
will be realized and attained by the structure particularly pointed
out in the written description and claims hereof as well as the
appended drawings.
[0013] To achieve these and other advantages and in accordance with
the purpose of the present invention, as embodied and broadly
described, a laser based coordinate measuring device for measuring
a position of a remote target comprising a stationary portion
having at least a first laser radiation source and at least a first
optical detector; a rotatable portion that is rotatable with
respect to the stationary portion; and at least a first optical
fiber system for optically interconnecting the first laser
radiation source and the first optical detector with an emission
end of the first optical fiber system, the emission end disposed on
the rotatable portion for emitting laser radiation to the remote
target and for receiving laser radiation reflected from the remote
target, wherein an emission direction of the laser radiation is
controlled according to the rotation of the rotatable portion.
[0014] In another aspect, a laser based coordinate measuring device
comprises a rigid structure rotatable about two substantially
orthogonal axes; a laser radiation source disposed off the rigid
structure to provide laser radiation; an optical detector disposed
off the rigid structure; a retroreflective target disposed remote
from the rigid structure; a first optical fiber path optically
coupled with the laser radiation source to transmit laser radiation
from the laser radiation source to the rigid structure, the first
optical fiber path having an end disposed on the rigid structure
for emitting the laser radiation to the retroreflective target
according to an orientation of the rigid structure and for
receiving retroreflected radiation reflected by the retroreflective
target; and an optical coupler optically connecting the optical
detector with the first optical fiber path to receive the
retroreflected radiation.
[0015] In another aspect, a laser based coordinate measuring device
for measuring a position of a remote target comprises a stationary
portion having at least a first laser radiation source; a rotatable
portion that is rotatable about first and second axes of rotation
with respect to the stationary portion; an optical fiber path for
optically interconnecting the first laser radiation source with the
rotatable portion, wherein a first portion of the optical fiber
path is disposed along the first axis and a second portion of the
optical fiber path is disposed along the second axis.
[0016] In another aspect, a laser based coordinate measuring device
comprises a structure rotatable about two substantially orthogonal
axes; a laser radiation source disposed off the rotatable structure
to provide laser radiation; a retroreflective target disposed
remote from the rotatable structure, the retroreflective target
having a pattern thereon; an optical system for directing the laser
radiation from the laser radiation source to the rotatable
structure and then to the retroreflective target in accordance with
the rotation of the rotatable structure, the retroreflective target
reflecting the laser radiation to the rotatable structure; and an
orientation camera optically coupled with the reflected laser
radiation to determine an orientation of the retroreflective
target, the orientation camera including a detector and a lens
system that forms an image of the pattern on the detector.
[0017] In another aspect, a laser based coordinate measuring device
comprises a structure rotatable about two substantially orthogonal
axes; a laser radiation source disposed off the rotatable structure
to provide laser radiation; a retroreflective target disposed
remote from the rotatable structure; an optical system for
directing the laser radiation from the laser radiation source to
the rotatable structure and then to the retroreflective target in
accordance with the rotation of the rotatable structure, the
retroreflective target reflecting the laser radiation to the
rotatable structure; and an orientation camera disposed on the
rotatable structure and optically coupled with the reflected laser
radiation to determine a three dimensional orientation of the
retroreflective target.
[0018] In another aspect, a laser based coordinate measuring system
comprises a structure rotatable about two substantially orthogonal
axes; a target disposed remote from the rotatable structure; a
locator camera disposed on the rotatable structure for determining
an approximate location of the target; and an actuator system to
orient the rotatable structure in accordance with the location
determined by the locator camera.
[0019] In another aspect, a laser based method for measuring
coordinates of a remote retroreflective target comprises the steps
of coupling laser radiation into a first end of an optical fiber
path, the optical fiber path having a second end disposed on a
rotatable structure; controlling the rotation of the rotatable
structure to direct the laser radiation to the remote
retroreflective target; coupling a first portion of retroreflected
laser radiation with an orientation camera; coupling a second
portion of the retroreflected laser radiation with a distance
meter; and calculating three positional and three orientational
degrees of freedom of the remote retroreflective target.
[0020] In another aspect, a distance measuring apparatus for
measuring a distance from the apparatus to an object includes a
first lens system including at least one lens and a first source of
light and a second source of light. In this aspect, the first light
from the first source and a second light from the second source is
transmitted toward the object and the first light has a first
wavelength and the second light has a second wavelength different
from the first wavelength. A first portion of the first light is
reflected by the object and a second portion of the second light is
reflected by the object and the first and second portions pass
through at least a portion of the first lens system. The apparatus
also includes a first optical detector that detects the first
portion and a second optical detector that detects the second
portion and a first dichroic beam splitter and a second dichroic
beam splitter, each dichroic beam splitter adapted to reflect at
least one of the first light and the second light toward the first
optical detector and the second optical detector, respectively.
[0021] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute apart of this specification, illustrate embodiments of
the invention. In the drawings:
[0023] FIG. 1 depicts an embodiment of a laser tracker according to
the present invention with a beam-steering mechanism and six
degree-of-freedom measurement capability;
[0024] FIG. 2 depicts in block form the major components of a rigid
structure of the laser tracker of FIG. 1;
[0025] FIG. 3 depicts in block form components of the beam combiner
of FIG. 2;
[0026] FIG. 4 depicts the components of the coupler assembly of
FIG. 3;
[0027] FIG. 5 depicts the components of the interferometer assembly
of FIG. 3;
[0028] FIG. 6 depicts in block form components of the beam expander
of FIG. 2;
[0029] FIG. 7 depicts in block form components of the orientation
camera of FIG. 2 showing the locations of the intermediate and
final images;
[0030] FIGS. 8a and 8b define the coordinate system for an
unrotated cube-corner retroreflector;
[0031] FIGS. 9a and 9b show the effect of pitch angle on the
retroreflector;
[0032] FIGS. 10a and 10b show the effect of yaw angle on the
retroreflector;
[0033] FIGS. 11a and 11b show the effect of roll angle on the
retroreflector;
[0034] FIG. 12 illustrates the appearance of the image on the
photosensitive array within the orientation camera;
[0035] FIG. 13 depicts the laser tracker of FIG. 1 where the rigid
structure is rotated to enable a wide-field locator camera to
simultaneously view plural retroreflector targets;
[0036] FIG. 14a is a front view of the locator camera on the rigid
structure;
[0037] FIG. 14b is a cross sectional view of the locator camera of
FIG. 14a taken along line 14b-14b;
[0038] FIGS. 15a-15c depict the formation of an image on the
wide-field locator camera;
[0039] FIG. 16 depicts a method of routing optical fibers near the
two mechanical axes;
[0040] FIG. 17 depicts a probe assembly of the preferred second
embodiment; and
[0041] FIG. 18 depicts a conventional laser tracker to which an
orientation camera has been added for measuring six degrees of
freedom.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings.
[0043] The present invention may be implemented as a laser-based
coordinate measurement machine, laser tracker, or other suitable
system. The present inventon provides a new type of beam-steering
mechanism; the ability to measure six degrees of freedom rather
than just three degrees of freedom; and the ability to locate a
plurality of retroreflector targets over a relatively wide field of
view.
[0044] The invention does not require beam-steering optics because
the laser light is routed through the laser tracker with optical
fibers rather than with beam-steering mirrors or prisms. Laser
light is processed, detected, and analyzed by optical and
electrical components located for the most part away from the
rotating elements within the tracker. One advantage of this
approach is that it reduces the size and cost of the laser tracker
system. Another advantage is that it improves accuracy and
stability of the laser tracker system. The architecture is flexible
enough to allow any number of laser beams to be launched without
the use of optical beam-steering components.
[0045] The invention also provides the laser tracker with the
ability to measure the six degrees of freedom of a target object
which, in an exemplary embodiment, may be a cube-corner
retroreflector. A hidden-point probe (capable of measuring points
that are obscured from view) can be formed by attaching the target
object to one end of a shaft and a probe tip to the other end of
the shaft. The target object may also be attached directly to a
machine tool or to the end-effector of a robot to more precisely
control the movement of the tool or robot.
[0046] The invention also provides the laser tracker with the
ability to determine the location of a plurality of retroreflector
targets over a relatively large volume surrounding the tracker. To
activate the target-locator feature of the tracker, the rotating
portion of the tracker is turned to bring a ring of LED's
surrounding a lens and photosensitive array to face the
retroreflector targets. Flashes of light from the LED's travel to
the retroreflectors then return to the tracker, where they pass
through the lens onto the photosensitive array. The locations of
the spots on the array indicate the angular directions of the
targets.
[0047] Because the invention has the capability of launching
multiple laser beams of different types, several modes of distance
measurement are possible. One mode of distance measurement uses a
laser beam that tracks a retroreflector to indicate either absolute
or incremental distance. Another mode of distance measurement uses
a laser beam to scan a diffuse surface. Either or both modes of
distance measurement may be included in a given
coordinate-measuring device.
[0048] FIG. 1 shows a perspective, block-diagram view of a laser
tracking system according to an embodiment of the invention. The
laser tracking system comprises a laser tracker 100 and a probe
assembly 180. The probe assembly comprises target object 185,
adjustable stage 181, probe shaft 170, and probe tip 171. The
target object 185 comprises retroreflector 107 and housing 109
comprise target object 185. Laser tracker 100 emits a laser beam
153 toward cube-corner retroreflector 107 mounted on housing 109.
Housing 109 is attached to adjustable stage 181 that is designed to
pivot about axis 182 and lock into place. Adjustable stage 181 is
attached to probe shaft 170, which is attached on the opposing end
to probe tip 171. Probe tip 171 is held in contact with the object
175 under evaluation. With the combination of laser tracker 100,
computer 25, and probe assembly 180, it is possible to measure the
coordinates of the object 175 under evaluation, even if object 175
is not directly accessible to the laser beam emitted from the
tracker.
[0049] The configuration of the laser tracker of FIG. 1 will now be
described. Laser 102 on stationary base 101 of the laser tracker
100 injects laser light (at least essentially coherent light having
one wavelength) into a first end of an optical fiber 111. The laser
of the absolute-distance meter (ADM) 103 injects laser light into a
first end of an optical fiber 115 (as shown in FIG. 3) that is
contained within optical fiber assembly 112. Laser 104 injects
laser light into a first end of an optical fiber 110. These optical
fibers are routed to rigid structure 190, at which location laser
light is launched out of the second end of optical fibers 110, 111,
115 (as shown in FIG. 3). If desired, the combined laser light may
be conditioned by optical elements within rigid structure 190 and
emitted from rigid structure 190 as laser beam 153. Laser light 153
travels to retroreflector 107, where it is reflected parallel to
laser beam 153. If laser beam 153 is centered on the vertex of the
cube-corner retroreflector, then laser beam 163 will coincide with
laser beam 153. That is, laser beam 163 will retrace the path of
laser beam 153. The laser beam 163 enters rigid structure 190,
where it is conditioned, if desired, and injected back into optical
fibers 110, 111, and/or 115, or otherwise detected and processed as
explained below. Rigid structure 190 is rotated by motor 81 with
the angle of rotation indicated by angular encoder 91. Steering
platform 195 includes rigid structure 190 and the elements mounted
thereto, i.e., motor 81 and angular encoder 91. Steering platform
195 is turned on base 101 by motor 80, with the angle of rotation
indicated by angular encoder 90. Rigid structure 190 is therefore
supported for rotation about two orthogonal axes on base 101.
[0050] ADM 103 measures the absolute distance from laser tracker
100 to retroreflector 107. This device is capable of measuring the
distance to retroreflector 107 in a single shot. Consequently, it
can be used to perform rapid point-and-shoot measurements of
multiple retroreflector targets. Laser 104 is used in conjunction
with optical and electrical elements to measure the incremental
distance moved by retroreflector 107. An example of a device that
measures incremental-distance movement is the laser interferometer
which measures the number of interference fringes that occur as the
retroreflector is moved from a starting position. In a laser
interferometer, if an obstruction is placed in the path of the
interferometer's laser beam, all displacement information will be
lost. In this circumstance, if a laser-tracker system has only an
incremental-distance measurement system and not an
absolute-distance measurement system, then the retroreflector must
be returned to a reference position and the measurement started
anew. Laser 102 is a stand-alone laser and will be discussed in
more detail with reference to fiber launch assembly 310 of FIG. 3.
Any number of laser beams may be sent over optical fibers into
rigid structure 190.
[0051] Electronics box 140 provides electrical power to motors 80
and 81, angular encoders 90 and 91, lasers 102 and 104, ADM 103, as
well as other electrical components within rigid structure 190.
Electronics box 140 analyzes signals from angular encoders 90 and
91, from ADM 103, and from other electrical components to calculate
angles and distances from tracker 100 to retroreflector 107.
Electronics box 140 is attached to computer 25, which provides
application software for the advanced analysis of coordinate
data.
[0052] The preferred optical elements within rigid structure 190
are shown in block diagram form in FIG. 2. The main functional
blocks within rigid structure 190 include beam-combiner block 200,
orientation-camera block 210, beam-expander block 220, and
locator-camera block 230.
[0053] The optical fiber assemblies 110, 111, 112, etc. are routed
into beam-combiner block 200, which combines the laser beams and
sends them out as a single composite laser beam 250 that includes
coherent light at a plurality of separate, discrete wavelengths.
The composite laser beam 250 passes through orientation-camera
block 210 to become laser beam 251. Laser beam 251 is beam expanded
by beam-expander block 220, thereby exiting rigid structure 190 as
expanded laser beam 153. The laser beam 153 travels to
retroreflector 107 as shown in FIG. 1 and returns as laser beam
163. The laser beam 163 retraces the path of the outgoing laser
beams 153, 251, 250 back through beam-expander block 220 and
orientation-camera block 210 into beam-combiner block 200.
Electrical lines 41 provide power from electronics box 140 to
mechanical and electro-optical devices. Electrical lines 41 also
route electrical signals from electro-optical devices to
electronics box 140 for analysis. Rigid structure 190 rotates
around the center of shaft 270 which is attached to motor 81 as
shown in FIG. 1. In a typical mode of operation, motor 81 rotates
rigid structure 190 so that laser beam 153 points toward
retroreflector 107, thereby causing laser beam 163 to retrace the
path of laser beam 153. In another mode of operation, motor 81
rotates rigid structure 190 until aperture 231 of locator camera
230 is aimed in the general direction of one or more
retroreflectors in the surrounding environment. Locator camera 230
determines the approximate location of the retroreflector targets
within a wide field of view.
Beam-Combiner Block
[0054] FIG. 3 shows diagrammatically the optical and
electro-optical components within a preferred beam-combiner block
200. The main assemblies within beam-combiner block 200 are first
laser-beam fiber launch and pickup assembly 300, second laser-beam
fiber launch and pickup assembly 320, laser-beam fiber launch
assembly 310, and position-detector assembly 340.
[0055] First laser-beam fiber launch and pickup assembly 300
receives light an end of optical fiber 115 which is attached at its
opposing end to a laser in the laser and ADM 103 shown in FIG. 1.
Laser light (at least one essentially coherent light at a first
discrete frequency) travels in optical fiber 115 until it reaches
coupler assembly 305. Part of the laser light emerging from coupler
assembly 305 is in optical fiber 306. It travels to fiber
termination 301, at which point it diverges as cone of light 360.
Lens 302 collimates this light as laser beam 361 which passes
through beam splitter 314 to become laser beam 365. Another part of
the laser light emerging from coupler assembly 305 travels through
an optical fiber 307 to fiber retroreflector 303 and returns
through optical fiber 307 into coupler assembly 305 thereby forming
a reference path as will be discussed with reference to FIG. 4.
[0056] Laser-beam fiber launch assembly 310 receives light from an
end of optical fiber 111 which is attached at its opposing end to
laser 102 shown in FIG. 1. Laser light travels in optical fiber 111
until it reaches fiber termination 311 where it diverges as cone of
light 362. Lens 312 collimates the light as laser beam 363 which
then reflects off mirror 313 to become laser beam 364. Laser beam
364 reflects off beam splitter 314 to join laser beam 365 from
first laser-beam fiber launch and pick-up assembly 300. Laser beam
365 passes through beam splitter 324 to become laser beam 369.
[0057] Second fiber launch and pickup assembly 320 receives light
from an end of optical fiber 110 which is attached at its opposing
end to laser 104 shown in FIG. 1. Laser light travels in
polarization-maintaining (PM) optical fiber 110 until it reaches
fiber termination 321 where it diverges as cone of light 366. Lens
322 collimates the laser light as laser beam 370. Laser beam 370
passes into interferometer assembly 325 and emerges as laser beam
367. The laser beam reflects off mirror 323 as laser beam 368 and
off beam splitter 324 as a part of laser beam 369. Laser beam 369
passes through beam splitter 342 to become laser beam 250.
[0058] FIG. 2 shows that laser beam 250 passes out of beam-combiner
block 200 and continues through the rest of the elements in rigid
structure 190, then travels as laser beam 153 to retroreflector 107
of FIG. 1 and returns as laser beam 163 to rigid structure 190. The
laser light of beam 163 retraces the path of laser beams 251, 250
through the optical elements 220, 210 within rigid structure 190.
As shown in FIG. 3, when the returning laser beam enters
beam-combiner block 200, some of the returning light reflects off
beam splitter 342 as laser beam 374. Beam splitter 342 reflects a
portion of all of the wavelengths of laser light within laser beam
250. Optical filter 343 blocks all but one wavelength of the light
within laser beam 374, which it transmits as laser beam 373.
Position detector 341 is aligned so that laser beam 373 strikes the
center of position detector 341 when laser beam 153 of FIG. 1 is
centered on retroreflector 107. If laser beam 373 does not strike
the center of position detector 341, an error signal is generated
at detector 341, thereby causing motors 80 and 81 to turn rigid
structure 190 to center laser beam 153 on retroreflector 107. In
this way, position detector 341 enables the outgoing laser beam 153
of FIG. 1 to automatically track a moving retroreflector 107.
Position detector 341 can be any device capable of giving an
electrical signal in response to the position of light on a
two-dimensional surface. Such a device may include, but is not
limited to, a quadrant detector, a lateral-effect detector, a
charge-coupled-device (CCD) array, a charge-injection-device (CID)
array, or a complementary-metal-oxide-semiconductor (CMOS)
array.
[0059] The number of laser beams launched out of beam-combiner
block 200 can be increased or decreased as desired by adding more
or fewer beam splitters within beam-combiner block 200. One way to
combine and separate different types of laser beams is on the basis
of wavelength. A dichroic beam splitter is a type of beam splitter
that can pass particular wavelengths while reflecting other
wavelengths. In a specific implementation using dichroic beam
splitters, optical fiber 115 may emit laser light at a wavelength
of 1550 nm, optical fiber 111 may emit laser light at 690 nm, and
optical fiber 110 may emit laser light at 633 nm. Thus, beam
splitters 314 and 324 may be dichroic beam splitters with the
following characteristics. Beam splitter 314 transmits laser
wavelengths longer than 1400 nm, but reflects wavelengths shorter
than 1400 nm. Beam splitter 324 transmits wavelengths longer than
660 nm, but reflects wavelengths shorter than 660 nm. In this way,
the laser beams are combined as they pass through beam-combiner
block 200 on the way out of rigid structure 190. Similarly, the
laser beams are separated on the reverse path through beam-combiner
block 200. Combining and separating the wavelengths with dichroic
beam splitters reduces the interaction among the laser beams,
thereby preventing measurement errors. Furthermore, the use of
dichroic beam splitters reduces power loss that would result from
the use of wavelength-insensitive beam splitters.
[0060] The laser beam sent out of fiber launch assembly 310 may
serve a number of purposes. In the specific example shown in FIGS.
1 and 3, the laser beam launched from second laser-beam fiber
launch and pickup assembly 320 is red (633 nm), thereby providing a
visible indication of the direction to which the laser beam is
pointing. In the event that the laser beam from second laser-beam
fiber launch and pickup assembly 320 is turned off or, not visible,
is otherwise not available, the laser beam emitted by laser-beam
fiber launch assembly 310 can serve as a visible pointer beam to
assist the operator in locating retroreflector targets with the
tracker. This same laser beam may be used as a part of a complex
system for other purposes such as determining the orientation of a
retroreflector target. There are occasions in which it is very
useful to have the laser tracker emit multiple laser beams. As
noted above, the flexible architecture of the invention allows as
few or as many laser beams as desired to be launched.
[0061] FIG. 4 shows a detailed view of the coupler assembly 305.
Laser light enters coupler assembly 305 on optical fiber 115 and
travels to Faraday isolator 420 which allows light to travel in
only one direction. Faraday isolator 420 is included to prevent
back-reflected laser light from entering and destabilizing the
laser found in the ADM 103. The laser light passes through Faraday
isolator 420 and enters optical coupler 401 which, in an exemplary
configuration, sends 85% of the optical power to optical coupler
402 and 15% of the optical power to optical coupler 403. Of the
optical power entering coupler 402, a portion such as one half is
sent to low-reflectance termination 412 and the remaining half
travels to optical fiber 306. As shown in FIGS. 1 and 3, the laser
light in optical fiber 306 is launched from the fiber and travels
to retroreflector 107. The light from the retroreflector retraces
its path through the laser tracker and re-enters optical fiber 306.
When light is received via optical coupler 402, half of the optical
power is sent to the Faraday isolator 402 where it is blocked. The
remainder is sent to optical fiber 309 and continues to ADM 103 via
optical fiber assembly 112. Of the optical power that is sent from
optical coupler 401 to optical coupler 403, half travels to
low-reflectance termination 413, and the other half travels along
optical fiber 307 to fiber retroreflector 303. The light retraces
its path back along optical fiber 307 into coupler 403. Half of the
optical power is sent to coupler 401 where it is sent in equal
parts to the low-reflectance termination 411 and the Faraday
isolator 420. The other half of the optical power is sent into
optical fiber 308 and continues to ADM 103 via optical fiber
assembly 112.
[0062] The optical couplers shown in FIG. 4 split light into two
paths in the forward direction and two paths in the reverse
direction. A low-reflectance termination is used to absorb the
light in one of the four possible paths (two forward paths plus two
reverse paths). Another term for a possible path is a "port," so
the couplers shown in FIG. 4 are examples of four-port couplers
having a low-reflectance termination on one of the four ports. An
alternative to the type of coupler shown in FIG. 4 is the optical
circulator, which has three ports, rather than four, ports. In
other words, in an optical circulator, the laser light travels
along one optical-fiber path in the forward direction and branches
to a different optical fiber path in the reverse direction. For the
purposes of this invention, the term optical coupler is used to
encompass both four-port and three-port light splitting devices. In
other words, a term fiber-optic coupler (or simply coupler) is
understood to include any type of device that splits light in an
optical fiber and therefore can be either a four-port coupler or a
three-port circulator.
[0063] For absolute-distance measurement, two paths are used: a
measurement path and a reference path. Both paths begin at the
laser of the ADM 103 and include the optical fiber 115 and the
Faraday isolator 420. In the measurement path, the laser light
travels through optical fiber 306, through rigid structure 190, to
the retroreflector 107 and back, into fibers 306 and 309, and then
into a measurement detector (not shown) in the ADM 103. In the
reference path, the laser light travels through optical fiber 307,
to the fiber retroreflector 303 and back, into fibers 307 and 308,
and then into a reference detector (not shown) in the ADM 103. The
optical fibers 308 and 309 are in the reference and measurement
channels, respectively, and are matched in length. They are routed
in close proximity to one another so that the local temperatures
experienced by each are nearly equal. This commonality of length
and temperature has the effect of minimizing the errors caused by
temperature-induced changes in the index of refraction of the
optical fibers. Without this commonality, a changing temperature
might be mistaken for a changing distance to the
retroreflector.
[0064] Many types of ADM are compatible with the fiber delivery
beam-steering mechanism depicted in FIG. 1. While any suitable type
of ADM can be employed, an exemplary type of ADM operates by
measuring the phase shift of laser light that is intensity
modulated by a sine wave. Thus, the particular type of laser might
be a distributed feedback (DFB) semiconductor laser whose optical
power is modulated by the direct application of a radio-frequency
(RF) electrical signal at a single (sinusoidal) frequency of 3 GHz.
For any given distance to the retroreflector 107, there will be a
corresponding difference in the phase of the reference and
measurement channels. If a is a constant, f is the frequency of
modulation (3 GHz), c is the speed of light
(.apprxeq.3.times.10.sup.8 m/s), n is the group index of refraction
of the air through which the laser light travels (.apprxeq.1), m is
an integer, and .phi. is the phase difference measured by the ADM,
then the distance d from laser tracker 100 to retroreflector 107 is
given by the following formula:
d = a + c 2 fn ( m + .phi. 2 .pi. ) . ( 1 ) ##EQU00001##
The constant a sets the distance scale so that a distance of zero
is set at the pivot point through which the laser beam appears to
emanate as the laser tracker is turned to different angles. The
pivot point is located approximately at the intersection of the
laser beam and the center of shaft 270. The integer m is equal to
the number of complete multiples of 2.pi. radians in the phase
difference (measurement phase minus reference phase) measured by
the ADM. For example, if the frequency of modulation f is 3 GHz,
then from Eq. (1) the distance corresponding to a phase difference
of 2.pi. radians is approximately
3.times.10.sup.8/2(3.times.10.sup.9)(1)m=0.05 m. This distance is
sometimes referred to as the unambiguous range. If the distance d-a
is 1.22 meters, then the number of complete multiples of 2.pi.
radians in the phase difference is int(1.22/0.05)=24 and the
residual phase shift is approximately
.phi..apprxeq.2.pi.(1.22-0.0524)/0.05=0.8.pi. radians. The most
convenient way to determine the integer m is to temporarily reduce
the frequency f to a value that is small enough to cover the entire
range of interest, but with an accuracy that is large enough to
determine the value of m. For example, suppose that the frequency
is temporarily reduced to 2.5 MHz. In this case, the unambiguous
range is 3.times.10.sup.8/2(2.5.times.10.sup.6)(1) m=60 m. If the
accuracy of the phase measurement is one part in 10.sup.5, then the
position of retroreflector 107 is known to an accuracy of
6010.sup.-5 m=0.6 mm at any distance up to 60 meters from the
tracker. This value is much smaller than the unambiguous range of
50 mm for the higher modulation frequency of 3 GHz. This means that
a single measurement of phase difference with the lower modulation
frequency is sufficient to determine the integer m in Eq. (1). This
technique of reducing the frequency to determine the value of m is
of greatest value if it is needed only at the start of a
measurement or after the laser beam has stopped tracking the
retroreflector 107. For this to be the case, the phase measurements
must be taken rapidly enough to ensure that the retroreflector has
not moved over a complete unambiguous range between measurements.
For example, if measurements are made 1000 times per second, then
the radial speed must not exceed (0.05)(1000)/2=25 meters per
second. The human arm is not capable of moving a retroreflector
target at a radial speed of greater than about 4 meters per second,
so this technique of determining m is feasible under the conditions
given above.
[0065] The modulated laser light that travels on optical fibers 308
and 309 within optical fiber assembly 112 arrives at optical
detectors located within ADM 103. These optical detectors convert
the laser light to electrical signals. For the particular type of
ADM described above, electrical components within ADM 103 process
the electrical signal to determine the phase of the signal for the
measurement and reference paths.
[0066] As shown in FIG. 3, laser light that is launched from
optical fiber 110 is collimated by lens 322 to become laser beam
370. This laser beam travels to interferometer assembly 325, a
detailed view of which is shown in FIG. 5. The laser light of beam
370 is linearly polarized at 45 degrees to the plane of the paper
in FIG. 5. In other words, half of the laser light is polarized in
the plane of the paper and half of the light is polarized
perpendicular to the plane of the paper, with both polarizations
having the same phase. The arrow that is perpendicular to laser
beam 370 in FIG. 5 represents the laser light that is polarized in
the plane. The small circle that is centered on laser beam 370 in
FIG. 5 represents the laser light that is polarized perpendicular
to the plane of the paper. Laser beam 370 travels to polarizing
beam splitter 501. The portion of laser beam 370 that is polarized
perpendicular to the plane of the paper in FIG. 5 reflects off
polarizing beam splitter 501 to become laser beam 510. The light
travels to quarter waveplate 502 having a fast axis oriented at 45
degrees to the plane of the paper in FIG. 5. The waveplate converts
the polarization state of laser beam 510 from linear to circular.
Lens 503 focuses the light onto mirror 504, which retroreflects the
laser beam 510 back on itself. Alternatively, a retroreflector
(such as a cube-corner retroreflector) may be substituted for lens
503 and mirror 504. Lens 503 collimates the retroreflected light,
sending it back through quarter waveplate 502, changing the
polarization state of the light from circular to linear, with the
direction of the linearly polarized light now in the plane of the
paper. This light, which is now p-polarized with respect to the
polarizing beam splitter 501, passes through the beam splitter to
become part of laser beam 511. That portion of laser beam 370 that
is in the plane of the paper in FIG. 5 travels straight through
polarizing beam splitter 501 to become laser beam 367. This light
passes through quarter waveplate 367, whose fast axis is oriented
at 45 degrees with respect to the plane of the paper. When laser
beam 367 passes through the waveplate, its polarization state
changes from linear to circular. The resulting laser beam travels
through the optical elements in rigid structure 190, travels to
retroreflector 107, and travels back through the optical elements
in rigid structure 190 to arrive at quarter waveplate 505. As laser
beam 367 travels in the reverse direction through quarter waveplate
505, its polarization state changes from circular to linear, with
the direction of the linearly polarized laser light now in
perpendicular to the plane of the paper in FIG. 5. (As an
alternative to placing quarter waveplate 505 inside interferometer
assembly 325, the waveplate may be placed at some later point along
the path of the laser beam.) Laser beam 367, which is now
s-polarized with respect to polarizing beam splitter 501, reflects
off the beam splitter to become part of laser beam 511. Laser beam
511 comprises of two portions: a reference portion that is
polarized in the plane of the paper and a measurement portion that
is polarized perpendicular to the plane of the paper. As the
retroreflector 107 is moved in a radial direction with respect to
laser tracker 100, the phase difference between these two linearly
polarized components will vary. There will be a phase change of
2.pi. radians for each change of one-half wavelength in the radial
distance to the retroreflector. Here, the wavelength is that of the
laser light in laser beam 370 as seen in the local medium (air)
through which the laser light travels. Laser beam 511 travels to
processing optics 506, which uses optical elements such as
beamsplitters, waveplates, and optical detectors to provide two
electrical signals. One electrical signal is proportional to cos p,
and the other electrical signal is proportional to sin p, where p
is the phase difference between the two linearly polarized portions
of laser beam 511. The electrical signals are sent to a counter
circuit 507 that counts the number of half wavelengths traveled by
retroreflector 107. The product of the wavelength of the light and
the number of wavelengths traveled gives the total displacement of
retroreflector 107 relative to some starting position. Counter 507
sends electrical signals over electrical line 41 to electronics box
140 for conversion from counts to a radial distance. If laser beam
153 is obstructed from reaching retroreflector 107, even for a
moment, then information on the correct number of counts is lost,
and the measurement must be started anew from some reference
position whose distance to the tracker has been previously
established. The type of interferometer shown in FIG. 5 is known as
a homodyne interferometer because the reference portion and
measurement portion that are combined to form laser beam 511 are
both at the same wavelength. Alternatively, a heterodyne
interferometer in which two different laser wavelengths are mixed
together prior to optical detection or other suitable system could
be used.
Beam-Expander Block
[0067] The optical components within beam-expander block 220 of
FIG. 2 are shown in FIG. 6. The beam-expander block 220 expands the
laser beam as it travels in the forward direction and to contract
the laser beam as it travels in the reverse direction. Lens 601
converts collimated laser beam 251 into cone of light 651. Lens 602
converts cone of light 651 into collimated laser beam 153.
[0068] The reason for expanding the laser beam before it leaves
rigid structure 190 is to reduce the divergence of the laser beam
during propagation. This makes it possible to place retroreflector
107 farther from laser tracker 100 than would otherwise be the
case. Alternatively, the beam-expander block 220 could be
eliminated by increasing the distance in FIG. 3 between the fiber
terminations 301, 311, and 321 and the corresponding lenses 302,
312, and 322 while increasing the focal lengths of lenses 302,312,
and 322 by a corresponding amount. Accordingly, the diameters of
laser beams 361,363, and 370 would be increased, thereby
eliminating the need for beam-expander block 220. The disadvantage
of this approach is that it requires that many optical elements
(lenses, mirrors, beam expanders, and position detector) be made
larger to accommodate the larger beam diameters. By adding
beam-expander block 220, the overall size of beam-combiner block
200 is reduced.
Orientation-Camera Block
[0069] The main elements of orientation-camera block 210 of FIG. 2
are shown in FIG. 7. On the return path from retroreflector 107,
laser beam 251 travels along optical axis 741. Beam splitter 701
reflects a portion of the beam to a path along optical-axis
segments 750, 742, 743, and 744. Eventually, this reflected light
arrives at photosensitive array 753. The complete lens system,
which comprises the beam-expander block 220, afocal lens block 710,
and relay lenses, 721 and 723, produces an image on the
photosensitive array of the pattern of light in the vicinity of the
vertex of retroreflector 107. The beam-expander block 220 and the
afocal lens block 710 work together to produce a first intermediate
image 751 of this pattern of light. The location of first
intermediate image 751 will depend on the distance of
retroreflector 107 from the laser tracker. Motorized stage 728 is
activated to move lens 721 to an appropriate distance from first
intermediate image 751. Lens 721 forms second intermediate image
752 located past negative lens 723 but inside the back focal point
of negative lens 723. Negative lens 723 converts the second
intermediate image into a real image 753 on photosensitive array
725.
[0070] The orientation-camera block 210 allows the distance between
the tracker and the retroreflector target to be large. For example,
a distance of more than thirty meters is possible. The lens systems
of the orientation-camera block 210 and beam-expander block 220
have two main functions. First, a magnification that is
approximately constant is maintained so that the image will nearly
fill the photosensitive array, thereby maintaining high accuracy
for large and small distances alike. Second, the adverse effects of
diffraction, which may result in lines or other features changing
shape or direction during propagation over large distances, are
minimized. To maintain constant magnification, afocal lens systems
220 and 710 are used. An afocal lens system is one that converts an
incoming ray of light that is parallel to the optical axis into an
outgoing ray of light that is also parallel to the optical axis. A
succession of afocal lens systems, as represented by the
combination of lens systems 220 and 710, has the property of
constant magnification. In other words, the size of first
intermediate image 751 is constant, regardless of the distance from
retroreflector 107 to the tracker. If first intermediate image 751
is located between lenses 711 and 714, then it is not possible to
place photosensitive array 725 at the location of this intermediate
image. Relay lenses 721 and 723 eliminate this problem by
converting first intermediate image 751 into image 753 on array
725. Motorized stage 728 places lens 721 an appropriate distance
from first intermediate image 751. Knowledge of the distance to
retroreflector 107, which is a quantity measured by the tracker,
along with knowledge of the focal lengths and positions of the lens
elements, is sufficient to determine the correct placement of lens
721. As is explained below, it is not necessary for the lens system
to obtain an exactly prescribed magnification, so motorized stage
728 can be relatively inexpensive. The distance that motorized
stage 728 must move will depend on the range of distances to be
covered, as well as on the magnification of the lens system.
Longitudinal magnification of a lens system varies in proportion to
the square of the transverse magnification. As an example, suppose
that a 12.times.12 millimeter area of target object 185 is imaged
onto a photosensitive array having an area of 3.times.3 mm. The
required (transverse) magnification for the system will then be
3/12=1/4. This could be achieved by making the combined
magnification of the afocal lens systems equal to 1/4 and the
combined magnification of the relay lenses 721 and 723 equal to 1.
In this case, however, to cover distances of 1 to 33 meters from
the tracker, it would be necessary for motorized stage 728 to have
a range of movement of (33-1) m/4.sup.2=2 m. Such a large range of
movement is impractical for most real systems. To solve this
problem, the magnification of the afocal lens systems could be
reduced, and the reduced magnification could be compensated with
the relay lenses. For example, suppose that the afocal lens pairs
have a combined magnification of 1/32, while the relay lenses have
a combined magnification of 8. In this case, the net magnification
is still 1/4, but the motorized stage 728 needs to have a range of
movement of only (33-1) m/32.sup.2=31.25 mm.
[0071] The photosensitive array 725 can be any device capable of
returning detailed electrical information about the pattern of
light incident on the array. Exemplary photosensitive arrays
include the charged-coupled-detector (CCD) array, the
charge-injection-device (CID) array, and the
complementary-metal-oxide-semiconductor (CMOS) array. Among these,
CCD arrays have high performance and small size, but CMOS arrays
are often capable of providing high-speed readout with simpler
electrical circuitry. CMOS and CID arrays often have the
advantageous feature of random-access readout of pixel data.
[0072] We will now discuss how the image on the orientation camera
can be used to determine the pitch, yaw, and roll angles of
retroreflector 107. FIGS. 8a and 8b show an unrotated cube-corner
retroreflector. In other words, in FIGS. 8a and 8b, the roll angle
is zero, the yaw angle is zero, and the pitch angle is zero. By
definition, the x direction shown in FIGS. 8a and 8b is opposite
the direction of the laser beam that is sent into the
retroreflector. The three perpendicular reflecting surfaces of the
cube-corner retroreflector form three lines of intersection. As
shown in FIG. 8a, the x-y plane contains the x axis and one of the
lines of intersection. The x-y plane also contains the y axis,
which is perpendicular to the x axis and passes through the vertex
of the cube corner. The dashed line in FIG. 8a is parallel to the y
axis and has been included for clarity. In the front view of FIG.
8b, they and z axes lie in the plane of the paper, while the x axis
points out of the paper. FIGS. 9a and 9b show the effect of
rotating the retroreflector about the -y axis by the pitch angle P,
which is 15 degrees in this example. This rotation operation
results in new coordinate system: x', y', z', with y=y'. FIGS. 10a
and 10b show the effect of rotating the retroreflector about the z'
axis by the yaw angle Y, which is 10 degrees in this example. This
rotation results in a new coordinate system: x'', y'', z'', with
z''=z'. FIGS. 11a and 11b show the effect of rotating the
retroreflector about the x'' axis by the roll angle R, which is 40
degrees in this example. Note that in FIG. 11b, the x axis
(direction opposite that of the laser beam) still points straight
out of the paper. The roll, yaw, and pitch angles are found from a
measurement of the three lines of intersection by the orientation
camera 210. The camera detects the three lines of intersection of
cube-corner retroreflector 107. The vertex of cube-corner
retroreflector 107, which is defined as the common point of the
three reflecting surfaces, remains centered on the photosensitive
array 725 of FIG. 7. The electrical signals from photosensitive
array 725 may be sent to a local digital-signal processing chip or
sent over electrical wires 41 to electronics box 140 of FIG. 1.
These electrical components determine the slopes of the three lines
of intersection. By definition, the y and z axes on the surface of
photosensitive array 725 point in the horizontal and vertical
directions, respectively. The slope of the first (reference) line
of intersection is defined as
m.sub.1=.DELTA.z.sub.1/.DELTA.y.sub.1, where .DELTA.y.sub.1 and
.DELTA.z.sub.1 are the horizontal and vertical distances on the
surface of photosensitive array 725 from the image of the
cube-corner vertex to the image of an arbitrary point on the first
line of intersection The slopes of the second and third lines of
intersection are defined in a similar manner as
m.sub.2=.DELTA.z.sub.2/.DELTA.y.sub.2 and
m.sub.3=.DELTA.z.sub.3/.DELTA.y.sub.3. The three unknown angles,
the roll angle R, the yaw angle Y, and the pitch angle P, are found
by simultaneously solving the following three equations:
m 1 = sin P cos Y / 2 - sin P sin Y cos R + cos P sin R sin Y / 2 +
cos Y cos R , ( 2 ) m 2 = sin P cos Y / 2 - sin P sin Y cos ( R +
120 .degree. ) + cos P sin ( R + 120 .degree. ) sin Y / 2 + cos Y
cos ( R + 120 .degree. ) , ( 3 ) m 3 = sin P cos Y / 2 - sin P sin
Y cos ( R + 240 .degree. ) + cos P sin ( R + 240 .degree. ) sin Y /
2 + cos Y cos ( R + 240 .degree. ) . ( 4 ) ##EQU00002##
For the example considered here in which R is 40 degrees, Y is 10
degrees, and P is 15 degrees, Eqs. (2)-(4) yield m.sub.1=0.874,
m.sub.2=-0.689, and m.sub.3=-2.651. As a check of these results,
the slope values can also be calculated directly from the y and z
values of the lines of FIG. 11b. These calculations yield
m.sub.1=0.7667/0.8772=0.874, m.sub.2=0.5528/-0.8026=-0.689, and
m.sub.3=-0.7788/0.2938=-2.651, which match exactly the results
obtained from Eqs. (2)-(4).
[0073] The visibility of the lines on photosensitive array 725 of
FIG. 7 may be improved by increasing the thickness of the lines of
intersection of retroreflector 107 or by coating the lines with a
non-reflective material. Thicker lines of intersection on
retroreflector 107 will cause the images of the lines seen on the
photosensitive array to have higher contrast. However, thicker
lines of intersection on retroreflector 107 will not usually result
in thicker image lines on photosensitive array 725. Usually, the
thickness of the lines as seen on photosensitive array 725 is
determined by the effects of diffraction of the laser light that
passes through the clear aperture of laser tracker 100. The larger
the clear aperture (the opening through which the light passes into
the tracker), the smaller will be the deleterious effects of
diffraction, which included broadening, smearing, and chopping of
the image on photosensitive array 725. The deleterious effects of
diffraction are also smaller when the retroreflector 107 is moved
closer to the laser tracker 100. Fortunately, for the system
considered here, the smearing effects of diffraction are
symmetrical about the lines of intersection, so there is no bias in
measuring the slopes of the lines.
[0074] The appearance of the lines on the image of photosensitive
array is shown in FIG. 12. The lines on this image that pass
through vertex V appear on both sides of the vertex. By comparison,
FIG. 11b shows that the lines of intersection of the physical
cube-corner retroreflector appear on only one side of the vertex.
This difference is the result of the symmetry in the paths that
light can take when the laser beam is centered on vertex V. For
example, if a pencil of light reflects off mirror 1, then mirror 2,
then mirror 3 of the cube corner, then another pencil of light can
reflect off mirror 3, then mirror 2, then mirror 1. If, instead of
striking a mirror, the pencil of light strikes a line of
intersection, then the light will not reflect back to the tracker
and a dark spot will appear on the image of photosensitive array
725. However, this dark spot would also appear if the light had
traveled in the reverse direction before encountering the line of
intersection. Hence light entering on either side of vertex V is
blocked. It is impossible to tell from the image of the
photosensitive array 725 alone which of the three line segments
corresponds to which of the three lines of intersection of a
cube-corner retroreflector. There are several ways around this
problem. The simplest, but least convenient, method for assigning
the lines of the image (FIG. 12) to the lines of intersection (FIG.
11b) is to have the operator indicate the approximate orientation
of probe assembly 180 at the start of a measurement sequence. An
approximate orientation is sufficient to determine which of the
image line segments corresponds to each of the lines of
intersection. Another simple but effective method is to temporarily
turn off or reduce the power of the laser beam 153 emitted by laser
tracker 100 and, at the same time, to increase the exposure time of
photosensitive array 725. Under these conditions, the
photosensitive array will ordinarily be able to make out the
features of housing 109 and hence obtain information on the
orientation of retroreflector 107. A third method for assigning the
line segments to the corresponding lines of intersection is
described in a second embodiment that is discussed later.
[0075] The method for determining the pitch, roll, and yaw angles
as described above provides has two main advantages. First, an
essentially constant-magnification camera maintains the accuracy of
the measurement for a probe located either near the tracker or far
from it. Second, elimination of spurious diffraction effects
improves accuracy, which may otherwise change the angles of the
lines or dramatically change the appearance of the lines,
especially at large distances.
[0076] FIG. 1 shows that housing 109 can be pivoted about axis 182
mounted on adjustable stage 181 and then locked in place. This
allows cube-corner retroreflector 107 to be oriented in any desired
direction. This flexibility in the orientation of retroreflector
107 is desirable because it allows probe tip 171 to be placed in
slots, holes, and so forth at any given angle. Preferably shaft 182
is aligned with the vertex of retroreflector 107 to simplify
calculations to determine the location of probe tip 171. If the
locking mechanism allows a limited number of angular adjustments,
each known to a sufficient angular accuracy (perhaps a few arc
seconds), then measurement may resume as soon as the lock down is
complete. If the locking mechanism is not sufficiently precise,
then an alternative approach involves adjusting housing 109 to any
given orientation and performing a simple compensation routine to
determine the angle between retroreflector 107 and probe shaft 170.
Such a compensation routine might include measuring the location of
a reference point with a spherically mounted retroreflector target
and then measuring the same location with probe assembly 180 tilted
to cover a range of pitch, yaw, and roll angles.
[0077] It is possible to replace the described cube-corner
retroreflector 107, which is made of three reflecting mirrors, with
a cube-corner retroreflector prism formed of solid glass. Each type
of retroreflector has advantages. For example, the cube-corner
retroreflector that uses mirrors (also known as a hollow-core
cube-corner retroreflector) is more accurate because it is not
prone to transverse and radial offset errors and because it has no
glass/air interface to cause unwanted optical reflections. The
solid glass cube-corner retroreflector has a wider field of view
and is usually less expensive. Equations (2), (3), and (4) can be
readily modified to account for a solid-glass, rather than a
hollow-core, cube-corner retroreflector.
Locator-Camera Block
[0078] The locator-camera block 230 of FIG. 2 allows laser tracker
100 to quickly determine the approximate location of multiple
retroreflectors within a wide field of view. The locator camera is
shown in greater detail in FIGS. 14a and 14b. As shown in FIG. 2,
locator-camera block 230 is placed to one side of rigid structure
190 and has an aperture at 231, which might be considered the "top"
side of rigid structure 190. When rigid structure 190 is rotated
about the center of shaft 270, locator-camerablock 230 faces the
retroreflectors in the region of interest. Locator-camera block 230
then emits cone of light 1320 as shown in FIG. 13. This light
reflects off retroreflectors 107, 1311, 1312, and 1313 shown in
FIG. 13. Here, retroreflector 107 represents a target of interest
and retroreflectors 1311, 1312, and 1313 represent a number of
other targets. The corresponding reflected light bundles 1357,
1351, 1352, and 1353 enter rigid structure 190. The light entering
locator-camera block 230 falls onto a photosensitive array 1404 in
FIG. 14b, and the pattern is analyzed to determine the approximate
location of the targets in the region of interest.
[0079] FIGS. 14a and 14b depict an example locator-camera
arrangement. A plurality of identical light sources 1401 is
provided in a ring surrounding a lens 1402. The individual light
sources emit overlapping cones of essentially incoherent light 1440
that collectively constitute the cone of light 1320 in FIG. 13.
Each of the retroreflectors 107, 1311-1313 reflects some of the
light from the cone of light 1320 back to the locator-camera block
230 as the bundles of light 1351-1353 or 1357. The bundle of light
1357 is shown in FIG. 14b. Lens 1402 focuses the bundle 1357 down
to a spot on the surface of photosensitive array 1404. The
photosensitive array 1404 is separated from the front principal
plane, 1403, of lens 1402 by the focal length f of the lens.
[0080] Electrical wires 41 provide power from electronics box 140
to light emitters 1401 and photosensitive array 1404. Electrical
wires 41 also transmit the pixel data from photosensitive array
1404 to electronics box 140 for analysis. Electronics box 41
analyzes the pattern of light on photosensitive array 1404 to
determine the location of central point 1452 on photosensitive
array 1404. Electronics box 140 also performs this analysis of the
pattern formed by the other bundles of light returned by the
retroreflectors. In other words, reflected light bundles 1357,
1351, 1352, and 1353 are focused by lens 1402 into patterns on
photosensitive array 1404. Electronics box analyzes these patterns
to determine the central point of each pattern. From the location
of the central points, the approximate angular direction to each of
the retroreflectors can be determined.
[0081] Suppose that the retroreflector of interest is
retroreflector 107. Once the information from the locator camera
has been used to determine the approximate direction to
retroreflector 107, motors 80 and 81 are activated to turn rigid
structure 190 until laser beam 153 points in the approximate
direction of retroreflector 107. The tracker then begins a search
pattern, in which the direction of laser beam 153 is changed in a
systematic fashion. For example, the laser beam might be steered
along a spiral pattern. When the laser beam intersects the target,
position detector 341 of FIG. 3 senses the reflected light. The
signals from position detector 341 provide enough information to
enable motors 80 and 81 to point rigid structure 190 directly to
the center of retroreflector 107.
[0082] FIG. 15a shows rays of light emitted by light emitter 1401
located above lens 1402. Ray of light 1520 travels to vertex V of
retroreflector 107. Reflected light 1521 is sent directly back to
light emitter 1401. It does not enter lens 1402 or appear as a spot
of light on photosensitive array 1404. Ray of light 1530 is sent to
the bottom of retroreflector 107 and emerges as reflected light
1532. It also misses lens 1402 and photosensitive array 1404.
[0083] FIG. 15b shows additional rays of light from light emitter
1401 located above lens 1402. Light emitter 1401 sends ray of light
1540 to a location above vertex V on retroreflector 107. This ray
emerges as reflected ray 1541, which passes near the top of lens
1402, is bent into ray 1542, and arrives at photosensitive array
1404 near central point 1563. Light emitter 1401 sends ray of light
1550 to the top of retroreflector 107. This ray emerges as
reflected ray 1552, which travels to lens 1402, is bent into ray
1553, and arrives at photosensitive array 1404 near central point
1563. As the distance from light emitter 1401 to retroreflector 107
increases, rays 1541 and 1552 become nearly parallel, and the spot
of light about point 1563 gets smaller and smaller.
[0084] FIG. 15c shows rays of light from light emitter 1401 located
below lens 1402. The rays of light in the bottom diagram are mirror
images of the rays in the middle diagram. If N is the number of
pixels in photosensitive array 1404, W is the width of
photosensitive array 1404, D is the diameter of lens 1402, and h is
the distance from the edge of lens 1402 to light emitters 1401, the
number of pixels between central points 1563 and 1593 will, in most
cases, be less than [2 N(D+h)/L] arctan(W/2 f). For example, if N=5
12, D=25 mm, h=5 mm, L=3 m, W=13 mm, and f=10 mm, the number of
pixels between central points 1563 and 1593 will be less than six.
Since light emitters 1401 are arranged in a circle, the image will
be symmetrical, somewhat blurry, and about six pixels across. For
retroreflectors further than 3 meters away, as most will be, the
pattern of dots will be smaller. Electrical signals are sent from
photosensitive array 1404 through electrical wire 41 to electronics
box 140. Electronics box 140 analyzes the intensity of light in the
pixels to obtain the best estimate of the center of the pattern
produced by each retroreflector.
Routing of Optical Fibers
[0085] FIG. 16 shows a configuration of a fiber launch laser
tracker 1600 for providing advantageous routing of optical fibers.
Here, optical fibers are preferably routed close to the two
mechanical axes 1670 and 1671. The routing of the optical fibers in
the system of FIG. 16 has many advantages. For example, a large
angular field of view of the tracker can be obtained. Also, bending
or kinking of the optical fibers is prevented, thereby preserving
measurement accuracy. Laser beam 1653, which is launched from rigid
plate 1690, travels to retroreflector 107 as shown in FIG. 1 and
returns as laser beam 1663. Laser light from optical fiber 1611 is
collimated by lens 1613, reflected by mirror 1615, and transmitted
through beam splitter 1614. Laser light from optical fiber 1612 is
collimated by lens 1612 and reflected off beamsplitter 1614. Any
number of laser beams can be combined into a common path to form
outgoing laser beam 1653. Returning laser light may pass through a
number of elements as previously discussed and omitted in FIG. 16
for clarity. For example, beam splitters for the position detector
and the orientation camera may be employed in accordance with the
specific application. The returning laser light may reflect off
beam splitter 1614 and pass through lens 1612 to be coupled into
optical fiber 1610. Alternatively, the laser light may pass into
another device located on rigid plate 1690 for processing as
previously discussed, for example, with reference to the absolute
distance meter and the interferometer. Similarly, the returning
laser light may reflect off mirror 1615 and couple back into
optical fiber 1611. Alternatively, this laser light may travel to
another device on rigid plate 1690 for processing.
[0086] The direction of laser beam 1653 is determined by the
orientation of rigid plate 1690, which in turn is determined by the
angle of rotation of the zenith mechanical axis 1671 and the
azimuth mechanical axis 1670. The zenith motor 1681 rotates the
zenith axis 1671, and the azimuth motor 1680 rotates the azimuth
axis 1670. Zenith angular encoder 1691 and the azimuth angular
encoder 1690 measure the zenith and azimuth angles. Bearings 1681
and 1680 are also attached to the zenith and azimuth axes. The
outside of zenith bearings 162 1, zenith angular encoder 1691, and
zenith motor 1681 are attached to the azimuth structural frame (not
shown). The azimuth structural frame turns with the azimuth axis.
Consequently, the zenith axis rotates within the azimuth structural
frame. The outside of azimuth bearings 1620, azimuth angular
encoder 1690, and azimuth motor 1680 are attached to stationary
structural frame (not shown). The stationary structural frame is
stationary with respect to the surroundings to which the tracker is
mounted. Consequently, the azimuth axis rotates within the
stationary structural frame.
[0087] Optical fibers 1610 and 1611 are incorporated into optical
fiber assembly 1605. Optical fiber assembly 1605 passes through
zenith axis 1671 and azimuth axis 1670. Lasers within
optoelectronic module 1606 (which, like the azimuth motor 1680, is
stationary) inject laser light into optical fibers 1610 and 1611.
Optoelectronic module 1606 may also contain optical detectors and
electronics to determine the distance to retroreflector 107 or to a
diffuse surface under investigation. The optical fiber assembly
1605 travels from optoelectronic module 1606 to the underside of
azimuth axis 1670. It is attached to the stationary structural
frame near point A shown in FIG. 16. At point A, the fiber is
stationary with respect to the rotating azimuth axis. At the other
end of the azimuth axis, optical fiber assembly 1605 is attached to
the azimuth structural frame near point B, which rotates along with
azimuth axis 1670. Since one end of the fiber is fixed and the
other end of the fiber is rotating with respect to the rotation of
the azimuth axis, the optical fiber will experience a torsional
twist. In most cases, a gentle twist of this sort will not degrade
measurement accuracy. Optical fiber assembly 1605 is routed to the
zenith axis, where it is attached to the azimuth structural frame
near point C. At point C, the fiber is stationary with respect to
the rotating zenith axis. At the other end of the zenith axis,
fiber assembly 1605 is attached near point D, which rotates along
with the zenith axis 1671.
[0088] Optical fiber assembly 1605 is routed through the two
mechanical axes. The fiber assembly is stationary at one end of
each axis. At the other end, the fiber assembly rotates along with
the axis. This produces a torsional twist, which is acceptable in
most situations. A slightly different method of routing optical
fiber assembly 1605 near the two mechanical axes may be preferable
in some cases. In this method, the optical fibers are placed in
coils to the outside of the mechanical axes, with the end of the
optical fiber attached at one end to a point that is stationary
relative to the mechanical axis and at the other end attached to a
point that moves with the mechanical axis. Here, the diameter of
the coils will change slightly as the axis is rotated. In most
cases, this small change in the radius of the coiled fiber assembly
will not adversely affect measurement accuracy. By heat treating
fiber assemblies, it is possible to make low-cost cables that
naturally coil into the desired geometry, thereby simplifying
production and increasing reliability.
Second Embodiment
[0089] The second embodiment of the invention is generally similar
to that shown in FIG. 1 except for the probe assembly 180 is
replaced by probe assembly 1780, as shown in FIG. 17. Probe
assembly 1780 contains a single small retroreflector 1708 to the
side of retroreflector 107. Retroreflector 1708 is approximately
aligned with the first line of intersection. At the start of the
measurement, the photosensitive array 725 of FIG. 7 displays a
pattern similar to that of FIG. 12, with the details of the pattern
dependant on the pitch, yaw, and roll angles of retroreflector 107.
As explained previously, at the start of the measurement, it is not
possible to tell which line segments correspond to each of the
three lines of intersection. To resolve this ambiguity, the tracker
performs a search in which it directs laser beam 153 in succession
to each of the six possible locations of retroreflector 1708. A
flash of light on position detector 341 of FIG. 3 indicates that
the first line of intersection has been identified. Also on probe
assembly 1780, two thin wires 1711 and 1712 have been stretched
across the top of retroreflector 107. Additional thin wires or
alternative shapes may also be used. These wires provide redundant
information for determining the pitch, yaw, and roll angles for
those cases in which accuracy is more important than measurement
speed.
[0090] Probe assemblies 180 and 1780 can be used in either a
scanning mode or a trigger mode. In the scanning mode, probe tip
171, shown in FIGS. 1 and 17, is moved across the surface of the
object under evaluation 175 while data is continually collected at
a high rate. In the trigger mode, probe 180 or 1780 is moved
successively to the points of interest. When the probe is properly
positioned, the operator triggers the measurement by performing an
action such as pressing a button or issuing a voice command.
[0091] Either target object 185 in the first preferred embodiment
or target object 1785 in the second preferred embodiment can be
detached from adjustable stage 181 and probe shaft 170, then
attached to the end effector of a robot arm. Alternatively, the
target object can be attached to a machine tool such as a drilling
or milling machine. The tracker sends a laser beam to the target
object to determine the six degrees of freedom of the drill or
mill. The information provided by the tracker on the six degrees of
freedom of target object 185 or 1785 can be used in a control loop
to precisely direct the machine tool or robot end effector to the
desired locations. If the tracker measures the six degrees of
freedom fast enough, real-time control of machine tools and robots
is possible.
Third Embodiment
[0092] The third embodiment of the invention provides a laser
tracker 1800 as shown in FIG. 18 that uses a steering reflector
1804 within gimbal mount 95 to direct laser beam 1853 to
retroreflector 107. Laser 1802 emits laser light that is sent to
retroreflector 107. Optical block 1806 contains beam expander 220
and any other optical beam-conditioning elements that may be
required. Laser light returning from retroreflector 107 is sent to
distance-measuring device 1814, which may be either an
absolute-distance meter or an incremental-distance meter. Part of
the returning laser light is also reflected off beam splitter 1809
to position detector 341. The beam splitter 701 reflects a portion
of laser beam 54 into orientation-camera subsystem 1810.
Orientation camera subsystem 1810 comprises afocal lens block 710
and relay/array block 720, also shown in FIG. 7. The optical
elements within blocks 1806 and 1810 of FIG. 18 are substantially
equivalent to the optical elements within blocks 220 and 210 of
FIGS. 6 and 7. In effect, an orientation camera comprising elements
701 and 1810 is embedded within laser tracker 1800. This
orientation camera is equivalent to the orientation camera 210 of
FIG. 2 and can therefore be used to measure the six degrees of
freedom of target object 185.
[0093] It will be apparent to those skilled in the art that various
modifications and variations can be made in the laser-based
coordinate measuring device of the present invention without
departing from the spirit or scope of the invention. Thus, it is
intended that the present invention covers the modifications and
variations of this invention provided they come within the scope of
the appended claims and their equivalents.
* * * * *