Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to U.S. provisional application, 60/614,778, filed Sep. 30, 2004, the entire contents of which are hereby incorporated by reference. 

   BACKGROUND 
   The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or may impinge on a retroreflector target that is in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. Exemplary systems for determining coordinates of a point are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 to Lau et al. 
   The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. A device that is closely related to the 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. 
   One type of laser tracker contains only an interferometer without an absolute distance meter. If an object blocks the path of the laser beam from one of these trackers, the interferometer loses its distance reference. The operator must then track the retroreflector to a known location before continuing the measurement. A way around this limitation is to put an absolute distance meter (ADM) in the tracker. The ADM can measure distance in a point-and-shoot manner. Some laser trackers contain only an ADM without an interferometer. An exemplary laser tracker of this type is described in U.S. Pat. No. 5,455,670 to Payne, et al. Other laser trackers typically contain both an ADM and an interferometer. An exemplary laser tracker of this type is described in U.S. Pat. No. 5,764,360 to Meier, et al. 
   One of the main applications for laser trackers is to scan the surface features of objects to determine their geometrical characteristics. For example, an operator can determine the angle between two surfaces by scanning each of the surfaces and then fitting a geometrical plane to each. As another example, an operator can determine the center and radius of a sphere by scanning the sphere surface. Up until this time, an interferometer, rather than an ADM, has been required for the laser tracker to scan. The reason for this is that absolute distance measurements have only been possible on stationary targets. Consequently, to get full functionality with both scanning and point-and-shoot capability, laser trackers have required both an interferometer and an ADM. What is needed is an ADM that has the ability to accurately and quickly scan a moving target. This permits tracker cost to be reduced because the interferometer is no longer needed. 
   SUMMARY 
   The above and other problems and disadvantages of the prior art are overcome and alleviated by embodiments the present laser device, which utilizes an absolute distance meter to determine the distance to a moving retroreflector. 
   A laser device and method is disclosed capable of one or more dimensional absolute distance measurements and/or surface scanning and/or coordinate measurements of a moving external retroreflector or other moving target surfaces without using an incremental interferometer depending upon what the application requires. 
   The above-discussed and other features and advantages of the present apparatus and method will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES: 
       FIG. 1  is a perspective view of an exemplary laser tracker sending a laser beam to an external retroreflector; and 
       FIG. 2  is a block diagram of some of the main elements within the exemplary laser tracker of  FIG. 1 ; and 
       FIG. 3  is a block diagram of the elements within the exemplary fiber-coupling network of  FIG. 2 ; and 
       FIG. 4  is a block diagram of the elements within the exemplary ADM electronics of  FIG. 2 ; and 
       FIG. 5  is a block diagram of the elements within an exemplary ADM data-processing system for computing the distance to a moving retroreflector. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. 
   An exemplary laser tracker  10  is illustrated in  FIG. 1 . An exemplary gimbaled beam-steering mechanism  12  of the laser tracker comprises zenith carriage  14  that is mounted on azimuth base  16 . The zenith and azimuth mechanical axes internal to the tracker (not shown) are turned to point the laser beam  46  in the desired direction. The laser beam may comprise one or more laser wavelengths, as will be described in the discussion that follows. The zenith and azimuth angular encoders internal to the tracker (not shown) are attached to the zenith and azimuth mechanical axes and indicate, to high accuracy, the angles of rotation. For the sake of clarity and simplicity, this sort of gimbal mechanism  12  is assumed in the following discussion. However, other types of gimbal mechanisms are possible, and the techniques described here are also applicable to these other types. 
   Laser beam  46  travels to external retroreflector  26 . The most common type of retroreflector is a spherically mounted retroreflector (SMR), which comprises a metal sphere into which a cube-corner retroreflector (not shown) is embedded. The cube-corner retroreflector comprises three perpendicular mirrors that come together at a common apex point. The apex point is placed at the center of the metal sphere. Instead of an SMR, a retrosphere or any other device that sends the return laser beam back on itself may be used as the external retroreflector  26 . 
   Elements of the Laser Tracker 
   Some of the main elements within the laser tracker are shown in  FIG. 2 . ADM electronics  300  modulates the optical power of ADM laser  102 , which sends light through fiber-optic cable  104  and fiber-coupling network  200 . Some of the light from the fiber-coupling network  200  travels to ADM beam launch  140 . Another part of the light travels through fiber loop  106  and then back into fiber-coupling network  200 . ADM beam launch  140  comprises stable ferrule  142  and positive lens  144 . Collimated light  108  emerges from the fiber launch  140 . 
   In the event that the ADM laser operates at an infrared wavelength, it is convenient to provide a visible laser beam to help make the ADM beam easier to find. Visible-light laser  110  sends visible light into beam launch  150 , which comprises stable ferrule  152  and positive lens  154 . The visible laser beam  112  that emerges to the beam launch  150  is collimated. Dichroic beam splitter  114  transmits ADM beam  108  but reflects visible beam  112 . To the right of beam splitter  114 , composite laser beam  116  comprises the visible laser beam and ADM laser beam, which are substantially collinear. Laser beam  116  passes through beam splitter  118  and beam expander  160 , emerging as a larger collimated laser beam  46 . The beam expander comprises negative lens  162  and positive lens  164 . 
   The laser beam  46  travels to external retroreflector  26 , as shown in  FIG. 1 . The beam reflects off retroreflector  26  and returns to the laser tracker. If the laser beam strikes the center of the retroreflector, the reflected laser beam retraces the path of the incident laser beam. If the laser beam strikes the retroreflector off the center, the reflected laser beam returns parallel to the incident beam but offset from it. The returning laser beam re-enters the tracker and retraces the path back through the optical system. Some of the returning laser light reflects off beam splitter  118 . Reflected laser light  126  passes through optical filter  128  and strikes position detector  130 . The optical filter  128  blocks either the ADM light or the visible light in the beam  126 . The position detector  130  responds to the light that passes through the optical filter  128  by indicating the position of the laser beam on its surface. The retrace point of the position detector is defined as the point that the laser beam  126  strikes if the beam  46  strikes the center of retroreflector  26 . When the laser beam  46  moves off the center of retroreflector  26 , the laser beam  126  moves off the retrace point and causes the position detector  130  to generate an electrical error signal. A servo system processes this error signal to activate motors that turn the laser tracker toward the center of the external retroreflector  26 . 
   The dichroic beam splitter  114  reflects the returning visible laser beam but transmits the returning ADM laser beam. The returning ADM laser beam travels through the beam launch and is coupled into the optical fiber within the stable ferrule  142 . This light travels through the fiber-coupling network  200  and emerges from optical fiber  230 . That portion of the laser light that traveled through fiber loop  106  emerges from optical fiber  232 . Both fibers  230  and  232  continue into the ADM electronics section  300 , where their modulated powers are converted into electrical signals. These signals are processed by the ADM electronics to provide the result, which is the distance from the tracker to the retroreflector target. 
   Fiber-coupling Network 
   Exemplary fiber-coupling network  200  of  FIG. 3  comprises first fiber-optic coupler  204 , second fiber-optic coupler  206 , and low-reflection terminations  208  and  210 . Light from ADM laser  102  travels through fiber-optic cable  104  and enters first fiber-optic coupler  204 . Fiber-optic coupler  204  sends 10% of the laser light through fiber-loop  106  and into optical fiber  232 , which travels to ADM electronics  300 . Fiber-optic coupler  204  sends the other 90% of the laser light through fiber-optic coupler  206 , which sends half of the laser light to low-reflection termination  208  and the other half of the laser light to stable ferrule  142 . Light from stable ferrule  142  propagates to external retroreflector  26  and back into ferrule  142 , as described above. Half of the laser light returning through ferrule  142  travels back through second fiber-optic coupler  206 , through fiber-optic cable  230 , and into ADM electronics  300 . The other half of the laser light travels through second fiber-coupler  206 , first fiber-optic coupler  204 , and into ADM laser  102 , where it is blocked by an internal Faraday isolator (not shown). 
   ADM Electronics 
   ADM electronics  300  of  FIG. 4  comprises frequency reference  302 , synthesizer  304 , measure detector  306 , reference detector  308 , mixers  310 ,  312 , amplifiers  314 ,  316 ,  318 ,  320 , frequency divider  324 , and analog-to-digital converter (ADC)  322 . Frequency reference  302  provides the time base for the ADM and should have low phase noise and low frequency drift. The frequency reference may be an oven-controlled crystal oscillator (OCXO), rubidium oscillator, or any other highly stable frequency reference. Preferably the oscillator frequency should be accurate and stable to within a small fraction of a part per million. The signal from the frequency reference is put into the synthesizer, which generates three signals. The first signal is at frequency f RF  and modulates the optical power of ADM laser  102 . This type of modulation is called intensity modulation (IM). Alternatively, it is possible for the first signal at frequency f RF  to modulate the electric field amplitude, rather than the optical power, of the laser light from ADM laser  102 . This type of modulation is called amplitude modulation (AM). The second and third signals, both at the frequency f LO , go to the local-oscillator ports of mixers  310  and  312 . 
   Fiber-optic cables  230  and  232  carry laser light. The light in these fiber-optic cables is converted into electrical signals by measure detector  306  and reference detector  308 . These optical detectors send the modulation frequency f RF  to amplifiers  314 ,  316  and then to mixers  310 ,  312 . Each mixer produces two frequencies, one at |f LO -f RF | and one at |f LO +f RF |. These signals travel to low-frequency amplifiers  318 ,  320 . These amplifiers block the high-frequency signals so that only the signals at the intermediate frequency (IF), f IF =|f LO −f RF | pass through to the analog-to-digital converter (ADC)  322 . The frequency reference  302  sends a signal into frequency divider  324 , which divides the frequency of the reference  302  by an integer N to produce a sampling clock. In general, the ADC may decimate the sampled signals by an integer factor M, so that the effective sampling rate is f REF /NM. This effective sampling rate should be an integer multiple of the intermediate frequency f IF . 
   Here are frequencies for an exemplary ADM: The frequency reference is f REF =20 MHz. The synthesizer RF frequency that drives the laser is f RF =2800 MHz. The synthesizer LO frequency that is applied to the mixers is f LO =2800.01 MHz. The difference between the LO and RF frequencies is the intermediate frequency of f IF =10 kHz. The frequency reference is divided by N=10, to produce a 2-MHz frequency that is applied to the ADC as a sampling clock. The ADC has a decimation factor of M=8, which produces an effective sampling rate of 250 kHz. Since the IF is 10 kHz, the ADC takes 25 samples per cycle. 
   The ADC sends the sampled data for the measure and reference channels to data processors  400  for analysis. Data processors include digital signal processor (DSP) chips and general-purpose microprocessor chips. The processing performed by these processors is described below. 
   Data Processor 
   Data processor  400  of  FIG. 5  takes the digitized data from ADC  322  and derives from it the distance from the tracker to external retroreflector  26 .  FIG. 5  refers to this distance as the RESULT. Data processor  400  comprises digital signal processor  410 , microprocessor  450 , and crystal oscillators  402 ,  404 . 
   Analog-to-digital converter  322  sends sampled data to DSP  410 . This data is routed to a program that runs within the DSP. This program contains three main functions: phase-extractor function  420 , compensator function  422 , and Kalman-filter function  424 . The purpose of the phase-extractor function is to determine the phases of the signals in the reference and measure channels, that is, the phases of the signals that pass through the measure detector  306  and reference detector  308 . To determine these phases, the modulation range must first be calculated. Modulation range is defined as the round-trip distance traveled by the ADM laser light in air for the phase of the laser modulation to change by 2 pi radians. The modulation range R MOD  is given by
 
 R   MOD   =c/ (2  n f   RF ),  (1)
 
where c is the speed of light in vacuum, n is the group index of refraction of the ADM laser light in air, and f RF  is the RF frequency generated by synthesizer  304  and applied to ADM laser  102 . In an exemplary ADM having an RF frequency of 2860 MHz, the modulation range is approximately 52 millimeters.
 
   As discussed previously, the sample clock applied to ADC  322  has an effective frequency Of f SAMP =f REF NM and the number of ADC samples per cycle is V=f SAMP f IF . In an exemplary tracker, f REF =20 MHz, N=10, M=8, and f IF =10 kHz. The sample frequency is then 250 kHz and the number of ADC samples per cycle is N ADC =25 samples per cycle. 
   Let x k  be the k th  sampled data value from the ADC for the measure channel and let v be the corresponding speed of external retroreflector  26  during the measurement. Phase-extractor function  420  calculates the phase p M  of the measure channel for moving external retroreflector  26  as follows: 
                   a   =       ∑     k   =   0       V   -   1       ⁢       x   k     ⁢     sin   ⁡     (     2   ⁢   π   ⁢           ⁢   k   ⁢           ⁢         f   IF     -     v   /     R   MOD           f   SAMP         )             ,           (   2   )                 b   =       ∑     k   =   0       V   -   1       ⁢       x   k     ⁢     cos   ⁡     (     2   ⁢   π   ⁢           ⁢   k   ⁢           ⁢         f   IF     -     v   /     R   MOD           f   SAMP         )             ,           (   3   )                 p   M     =         tan     -   1       ⁡     (     a   /   b     )       .             (   4   )               
Let y k  be the k th  sampled data values from the ADC for the reference channel. Phase-extractor function  420  calculates the phase p R  of the reference channel for moving external retroreflector  26  as follows:
 
   
     
       
         
           
             
               
                 
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   Significantly, the phase-extractor function  420  is dependent on the speed or velocity v, for example the radial speed, of the target as show in equation (2), (3), (5), and (6). The phase-extractor function  420  also delivers the measure phase p M  and the reference phase p R  to the compensator function, which uses these phases to calculate a distance value:
 
 d=d   0   +R   MOD   [W +( p   M   −p   R )/2π].  (8)
 
The quantity W is an integer that accounts for the number of whole modulation intervals to the target. The method for finding this integer is discussed below. In some systems, there may be additional systematic errors that can be removed by appending additional terms to equation (8). For example, some systems experience an error that varies with distance as a sinusoid with a period equal to the modulation range R MOD . To remove this type of error, it is necessary to use the ADM to measure targets at accurately known distances and observe the sinusoidal error pattern.
 
   The compensator  422  sends the distance values to Kalman filter  424 . The Kalman filter is a numerical algorithm applied to the distance data to give the best estimate of distance and speed of external retroreflector  26  as a function of time and in the presence of noise. The ADM distance data is collected at high speed and has some level of random noise in the distance readings. This small error is greatly amplified in calculating speed, since small differences in distance are divided by a small increment in time. The Kalman filter can be thought of as an intelligent smoothing function that optimizes accuracy based on the noise of the system and the speed of the target. 
   The Kalman filter also serves to synchronize the ADM readings with the readings of the angular encoders and the position detector. The angular encoders and position detector latch their readings whenever they receive the sync pulse, which occurs at frequency f SYNC . The frequency of the sync pulse is in general different than the frequency of calculation of the ADM. In an exemplary tracker, the ADM calculates at a rate of f IF =10 kHz, while the sync pulse has a frequency of 1.024 kHz. The Kalman filter provides synchronization of the ADM with the angular encoders and position detector by extrapolating the position forward in time to the next sync pulse. 
   There are five general equations that govern the behavior of the Kalman filter. In general, the quantities in these equations are represented by matrices, whose dimensions are determined by the complexity of the implementation of the Kalman filter. The five general equations are
 
x m =Φx p ,  (9)
 
 P   m   =ΦP   p Φ T   +Q,   (10)
 
 K=P   m   H   T ( HP   m   H   T   +R ) −1 ,  (11)
 
 x   p   =x   m   +K ( z−Hx   m ),  (12)
 
 P   p =( P   m   −1   +H   T   R   −1   H ) − .  (13)
 
   In these equations, the subscript m represents an a priori estimate and the subscript p represents an a posteriori estimate. The quantity x is the state variable that may take a variety of forms. Because the exemplary ADM system measures at a high rate, a relatively simple state vector containing only two components—the position d and radial speed v—are needed: 
                 x   =       (         d           v         )     .             (   14   )               
The corresponding time propagation matrix, assuming unit time steps, is
 
                 Φ   =       (         1       1           0       1         )     .             (   15   )               
Equation (9) then corresponds to the equations d m =d p +v p , which means that the estimated distance at the present point in time (d m ) is equal to the estimated distance at the last point in time (d p ) times the estimated speed at the last point in time (v p ) times the time interval between the current and last points in time, which is assumed to equal one. The matrix Q is the process noise covariance. In the simple Kalman filter employed here, the acceleration is not explicitly calculated. Instead the acceleration is assumed to have a dispersion characterized by the variance σ A   2 . The process-noise variance σ A   2  is selected so as to minimize the error in the position of a moving target. The resulting covariance for the process noise is
 
                 Q   =         σ   A   2     ⁡     (           1   /   4           1   /   2               1   /   2         1         )       .             (   16   )               
P m  is the state covariance matrix at the present point in time. It is found from the state covariance matrix at the last point in time and the process noise covariance. The state covariance matrix and the measurement noise covariance R are used to determine the filter gain K. In the simple case considered here, the measurement noise covariance is just the variance σ M   2  in readings caused by noise in the measurement device. In this case, the measurement noise in the ADM system is determined by simply calculating the variance σ ADM   2  in the distances reported while the ADM is measuring a stationary target. H is the measurement matrix, which is defined such that H times the state estimate x is equal to the estimated output, against which measured output, is compared. In the case considered here the measurements are of the distance d and so H=(1 0).
 
   Equation (12) is interpreted as follows. x m  is the initial guess for the state vector (distance and radial speed) based on the distance and radial speed for the previous point in time. The quantity z is the measured distance d and Hx m  is the estimated distance. The quantity z−Hx m  is the difference between the measured and estimated distances. This difference is multiplied by the gain matrix K to provide an adjustment to the initial estimate x m  for the state matrix. In other words, the best estimate for the distance is a value between the measured distance and the estimated distance. Equation (12) provides a mathematically sound method of selecting the best (a posteriori) estimate of the distance and radial speed. Equation (13) provides a new estimate for the state covariance P p  at the next point in time. Equations (9)-(13) are solved each time compensator function  422  sends a new measured value to the Kalman filter. 
   To synchronize the ADM measurement to the measurements of the angular encoders and position detector, counter  414  determines the difference in time between the sync pulse and the last state distance. It does this in the following way. Crystal oscillator  404  sends a low-frequency sine wave to frequency divider  452 , located within microprocessor  450 . This clock frequency is divided down to f SYNC , the frequency of the sync pulse. The sync pulse is sent over device bus  72  to DSP  410 , angular encoder electronics  74 , and position-detector electronics  76 . In an exemplary system, the oscillator sends a 32.768 kHz signal through frequency divider  452 , which divides by 32 to produce a sync-pulse frequency f SYNC =1.024 kHz. The sync pulse is sent to counter  414 , which resides within DSP  410 . The counter is clocked by crystal  402 , which drives a phase-locked loop (PLL) device  412  within the DSP. In the exemplary system, oscillator  402  has a frequency of 30 MHz and PLL  412  doubles this to produce a clock signal of 60 MHz to counter  414 . The counter  414  determines the arrival of the sync pulse to a resolution of 1/60 MHz =16.7 nanoseconds. The phase-extractor function  420  sends a signal to the counter when the ADC  322  has sent all the samples for one cycle. This resets counter  414  and begins a new count. The sync pulse stops the counting of counter  412 . The total number of counts is divided by the frequency to determine the elapsed time. Since the time interval in the above equations was set to one, the normalized time interval t NORM  is the elapsed time divided by the time interval. The state distance x EXT  extrapolated to the sync pulse event is
 
 x   EXT   =x   k   +v   k   t   NORM .  (17)
 
The Kalman-filter function  424  provides the result, which is the distance from the tracker to external retroreflector  26 . The Kalman filter also provides the velocity to phase-extractor function  420  to apply in equations (2), (3), (5), and (6).
 
   Previously the quantity W was introduced in equation (8) as an integer that accounts for the number of whole modulation intervals to the target. This integer value W is found by first measuring the distance to the external retroreflector  26 . The frequencies f RF  and f LO  are changed by a fixed amount and the distances are again measured. If the RF frequencies before and after the change are f 1  and f 2  and the phase difference between the two measurements is p then the integer W is equal to the integer portion of (p/2π)(f 1 /|f 2 −f 1 |). This technique will work out to a range of (c/2n)/(f 2 −f 1 ). For example, if f 1  and f 2  differ by 2.5 MHz, and if they f 1  is 2800 MHz, then the technique will work out to about 60 meters. If desired, a third frequency can be added to assist in determining the value of the integer W. Once W has been determined, it is not necessary to switch the frequencies again unless the beam is broken. If the ADM continues to measure the external retroreflector  26  without interruption, then it can easily keep track of the changes in the integer W. 
   It will be apparent to those skilled in the art that, while exemplary embodiments have been shown and described, various modifications and variations can be made to the apparatus and method of measuring a moving retroreflector with an absolute distance meter disclosed herein without departing from the spirit or scope of the invention. Accordingly, it is to be understood that the various embodiments have been described by way of illustration and not limitation.

Technology Category: 3