Abstract:
An optical measurement system increases the number of translational and angular measurements made with a single laser beam by combining an optical interferometer with an optical autocollimator. Translational measurements are made with an optical interferometer and yaw and pitch measurements are made with an autocollimator. In a preferred embodiment, angular deviations in the reflected measuring beam are minimized with a reverse telescopic lens assembly, allowing a wider range of angular measurements without significant degradation of interferometer accuracy.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is related to U.S. Pat. No. 4,714,339, entitled “Three and Five Axis Laser Tracking Systems” and issued Dec. 22, 1987; U.S. Pat. No. 6,049,377, entitled “Five-Axis/Six-Axis Laser Measuring System” and issued Apr. 11, 2000; U.S. Pat. App. Pub. No. U.S. 2003/0043362 A1, entitled “Six Dimensional Laser Tracking System and Method” and published Mar. 6, 2003; U.S. Pat. App. Pub. No. U.S. 2003/020685 A1, entitled “Nine Dimensional Laser Tracking System and Method” and published Nov. 6, 2003. The present application claims the benefit of Provisional Application No. 60/601,831, entitled “System and Method for Three-Dimensional Measurement” and filed Aug. 16, 2004. The present application hereby incorporates by reference all above-referenced patents and patent applications in their entirety. 

   BACKGROUND 
   Rapid, precise measurement of the position and orientation of a tool or workpiece is critical to many automated manufacturing processes. Although a variety of different measurement systems have been developed, optical measuring systems have proven precise, adaptable, reliable, and relatively inexpensive. 
   Most optical measuring systems exploit various effects obtained by manipulating the output of low-intensity lasers. For example, highly accurate linear distance measurements can be obtained by counting interference fringes that shift position as a laser beam reflects from a shifting target. Such a system may be initially calibrated by measuring the time of flight of a laser pulse that strikes a target and returns to a source. 
   Orientation measurements have posed more of a challenge, since, for example, a light beam parallel to a rotational axis of a target may register no distance variation. One solution to this problem utilizes the polarizing effects of a Glan-Thompson prism, which resolves an incoming laser beam into two orthogonal vector components that vary in intensity according to the rotational orientation of the prism with respect to the beam. Once such a system is calibrated, a target&#39;s angle of rotation about an axis may be calculated from the measured intensity differential between output vector components. 
   However, obtaining complete positional data for a target using the simplest forms of such measurement techniques may require a separate distance-measuring system for each translational axis and a separate rotation-measuring system for each rotational axis. As the setup and operation of simple multi-dimensional measurement systems may become cumbersome and expensive, it is highly desirable to make as many different but simultaneous measurements as possible with a single light beam. 
   SUMMARY 
   The present invention increases the number of translational and angular measurements made with a single laser beam by combining an optical interferometer with an optical autocollimator. This system and method provides both a precise linear distance measurement on one translational axis and simultaneous yaw and pitch measurements. Utilizing a single laser beam, a translational measurement is made with an optical interferometer and angular measurements are made with an autocollimator. In a preferred embodiment, angular deviations in the reflected measuring beam are minimized with a reverse telescopic lens assembly, allowing a wider range of angular measurements without significant degradation of interferometer accuracy. All of these features and advantages of the present invention, and more, are illustrated below in the drawings and detailed description that follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the present invention&#39;s axes of measurement. 
       FIG. 2  shows a diagram of a typical optical interferometer. 
       FIG. 3  shows a simplified diagram of an autocollimator. 
       FIG. 4  shows a schematic view of a preferred embodiment of the present invention, including an optical interferometer, an autocollimator, and a reverse telescopic lens assembly. 
       FIG. 5  shows a typical measurement setup utilizing two modules of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention combines an optical interferometer with an optical autocollimator to measure a target&#39;s linear translation in one dimension and rotational orientation in two dimensions with a single low-intensity laser beam. As shown in  FIG. 1 , linear translation of an object  130  along the y-axis may be measured by an optical interferometer  110  that directs a light beam  100  against a reflective surface  140 . 
     FIG. 2  shows a diagram of a typical optical interferometer. A stable light source  200  emits a coherent light beam  210  that impinges upon a beam splitter  260  as is known in the art. A reference portion  220  of the light beam  210  is directed to a fixed reference reflector  230  and returned to the beam splitter  260 . A target portion  240  of the light beam  210  passes through the beam splitter  260  to impinge upon a target reflector  250 . The target reflector  250  may be a flat reflector, retroreflector, or other suitable reflector affixed directly or indirectly to a surface or object having its linear translation measured. The target portion  240  of the light beam is returned from the target reflector  250  to the beam splitter  260  to be recombined with the reference portion  220  of the light beam  210 . 
   The position of the reference reflector  230  is fixed with respect to the beam splitter  260 , so linear translation of the target reflector  250  along the axis of the beam causes a phase shift between the target beam  240  and the reference beam  220 . Resulting interference within the recombined beam  270  produces minima and maxima that are sensed by a fringe counter  280  as the target reflector  250  translates on the axis of measurement. The positional change of the target reflector  250  may be calculated from the number of fringes sensed by the fringe counter  280 . 
     FIG. 3  depicts the operation of a generalized autocollimation device utilizing light from a point source. Light rays  310  from a light source  300  are refracted by a lens  320  into a collimated beam  330  comprising parallel rays. The collimated beam  330  is reflected by a flat reflector  340  back through the lens  320 , which focuses the collimated beam  330  to a receiving point  350  on the plane of the light source  300 . If the collimated beam  330  is orthogonal to the flat reflector  340 , the receiving point  350  will coincide with the light source  300 . However, if the flat reflector  340  is angled with respect to the collimated beam  330 , the receiving point  350  will shift with respect to the light source  300  a distance d. For small angles (where tan(2a) is approximately equal to 2a), the slant angle a in radians of the flat reflector  340  may be calculated as a=d/2f where f is the focal length of the lens  320 . 
   The present invention places both an interferometer and an autocollimator in the same beam path, allowing measurement of pitch, yaw, and linear translation with a single beam.  FIG. 4  shows a schematic view of a preferred embodiment of the present invention that combines an interferometer and an autocollimator. An HeNe or other laser  400  as is known in the art emits a beam containing at least two orthogonally polarized components. Output from the laser  400  is conducted by a Polarization Maintaining (PM) fiber  402  to a lens  404  that directs the beam into an interferometer  406 . The PM fiber allows isolation of the laser  400  from the measuring apparatus, reducing extraneous heat and vibration that may degrade measurement accuracy. The preferred interferometer of the present invention comprises a polarizing beam splitter  420 , quarter-wave retardation plates  421 ,  422 , a fixed retroreflector  424 , and a fringe counter  426 , as are all known in the art. The preferred embodiment may also comprise a reverse telescopic lens assembly  428 . 
   As previously described, a light beam directed into the interferometer is divided by the polarizing beam splitter  420  into a reference beam  405  and an outgoing target beam  407 . The outgoing target beam  407  passes through a quarter-wave retardation plate  422  and an autocollimator  408  comprising a beam splitter  430 , a lens  432 , and a detector  434 . The outgoing target beam  407  initially passes through the beam splitter  430  and strikes a flat reflective target surface  410 , from which a return target beam  409  is reflected back through the beam splitter  430 . The target  410  is typically a flat mirror, although corner reflectors and other known specular reflectors may be used. 
   Although  FIG. 4  depicts outgoing and return target beams as traveling separate paths for clarity, both travel the same path when the target surface  410  is orthogonal to the outgoing target beam  407 . The beam splitter  430  directs an autocollimator portion  436  of the return target beam  409  through a lens  432  that focuses the autocollimator portion  436  of the beam onto a detector  434 . The detector  434  generates an output signal corresponding to the location of the focused beam on the detector surface that is communicated via a serial connector  440  or other data connector known in the art to a computer (not shown). A typical detector  434  would utilize a lateral effect photodiode. An alternate embodiment of the present invention may utilize a dual-axis lateral effect photodiode such as an SC/10 from United Detector Technology. A dual-axis photodiode provides two output signals which together measure in two lateral dimensions where on the photodiode focused beam strikes. 
   Since the autocollimator portion  436  of the beam enters the autocollimator  408  as an undiffused laser beam, the preferred autocollimator  408  of  FIG. 4  is simplified in comparison with the generalized autocollimation device of  FIG. 3 . No return reflection path is required within the autocollimator  408  to collimate the autocollimator portion  436  of the beam. No point source is needed to establish a zero-deviation point. Instead, any point on the detector  434  may be arbitrarily designated as a zero-deviation point. 
   When the reflective target surface  410  is orthogonal to the outgoing target beam  407 , the autocollimator portion  436  of the beam is orthogonal to the outgoing target beam  407  and focused on a zero-deviation point on the detector  434 . Reorientations of the reflective target surface  410  corresponding to changes in the pitch or yaw of the surface cause the focal point of the autocollimator portion  436  of the reflected beam to shift across the surface of the detector  434 , allowing measurement of the amount of shift and calculation of the pitch and yaw angles. The output voltage signal from the detector  434  is converted to digital form by an A/D converter for transmission to a computer. 
   The remainder of the return target beam  409  returns to the interferometer beam splitter  420  to be recombined with the reference beam  405  and directed into the fringe counter  426  for measurement of linear translation of the reflective target surface  410 . Fringe counters known in the art typically generate an averaged output signal from an array of detectors (not shown) corresponding to movement of minima and maxima across the detectors. The present invention may utilize any suitable fringe counter known in the art. The fringe counter output signal is communicated via a serial connector  442  or other data connector known in the art to a computer (not shown). The present invention may additionally be equipped with time-of-flight detectors as are known in the art to initially establish the absolute distance between the present invention and the target reflector. 
   Changes in the pitch or yaw of the reflective target surface  410  cause the return target beam  409  to shift across the fringe counter detector arrays, introducing measurement errors and, with a sufficient shift, directing the return target beam  409  away from the array altogether. A preferred embodiment of the present invention introduces a reverse telescopic lens assembly  428  into the return target beam  409  path between the polarizing beam splitter  420  and the fringe counter  426 . The reverse telescopic lens assembly  428 , which is essentially a reversed telescope as is known in the art, reduces the angle of deviation of the return target beam  409  by the reciprocal of assembly&#39;s magnification, so that a 10× telescopic array would reduce a 10 second deviation to a 1 second deviation. This reduction advantageously allows measurement of significantly larger changes in pitch and yaw while still allowing accurate linear translation measurements. Placement of the reverse telescopic lens assembly  428  between the polarizing beam splitter  420  and the fringe counter  426  advantageously allows the reduction of interferometric error without affecting autocollimator  408  operation. 
     FIG. 5  depicts an application of the present invention. A platform  502  with mirrored surfaces  504 A,  504 B moves upon a base  500 . The position of an object (not shown) mounted upon the platform  502  may be measured and calculated as the platform  502  moves. A laser  508  supplies light through PM fibers  510 A,  510 B to measuring devices  506 A,  506 B embodying the present invention. The measuring devices  506 A,  506 B may be mounted on the base  500  or on fixtures within line-of-sight of the platform  502 . Preferentially, the laser beams  512 A,  512 B projected by the measuring devices  506 A,  506 B are mutually orthogonal. Data cables  522 A,  522 B transmit the outputs of both the fringe counters and the autocollimator detectors in each measuring device  506 A,  506 B device to a computer  520 . 
   As the platform  502  moves upon the base  500 , the interferometric components of each measuring device respond to translational movement on the x and y axes, with one measuring device  506 A measuring translation along the x-axis while the other measuring device  506 B measuring translation along the y-axis. Rotation of the platform  502  about the z-axis (yaw) causes the reflected laser beams  512 A,  512 B to shift direction, in turn causing the autocollimator portions of these beams to shift across the autocollimator detector surface within each measuring device  506 A,  506 B. The resulting output signals are processed within a computer  520  utilizing hardware and software disclosed in the applicant&#39;s previous patents and patent applications and/or well-known in the art to calculate, store, display, and/or output changes in platform  502  position and orientation. Both measuring devices can measure yaw, although autocollimator detector output from one would ordinarily be selected. If the platform  502  is rotated out of the plane of the base  500 , one measuring device  506 A can measure roll while the other measuring device  506 B can measure pitch. 
   An additional interferometer (not shown) with a beam parallel to the z-axis could be added to measure translation along the z-axis. With suitable components, the present invention can measure translational movement of one nanometer and angular changes of 1/100 of a second of arc. 
   The principles, embodiments, and modes of operation of the present invention have been set forth in the foregoing specification. The embodiments disclosed herein should be interpreted as illustrating the present invention and not as restricting it. The foregoing disclosure is not intended to limit the range of equivalent structure available to a person of ordinary skill in the art in any way, but rather to expand the range of equivalent structures in ways not previously contemplated. Numerous variations and changes can be made to the foregoing illustrative embodiments without departing from the scope and spirit of the present invention.