Abstract:
A measurement apparatus and corresponding method can be used to measure an absolute diameter of a part in a shop floor environment. A tracker such as a laser tracker monitors a position of a probe end of a measurement arm of the apparatus. The position measured by the laser tracker can be used directly account for errors in the apparatus such as, for example, positioning errors of the measurement arm. The position monitoring of the tracking device eliminates complex apparatus calibrations and calculations used for previous devices.

Description:
RELATED APPLICATION(S) 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 14/010,197, filed Aug. 26, 2013 which is a continuation of U.S. application Ser. No. 13/491,035, filed Jun. 7, 2012, now U.S. Pat. No. 8,538,725, which is a continuation of U.S. application Ser. No. 12/695,304, filed Jan. 28, 2010, now U.S. Pat. No. 8,219,353, which claims the benefit of U.S. Provisional Application No. 61/148,857, filed on Jan. 30, 2009. The entire teachings of the above applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In assembly of rotary machines, such as gas turbine engines, many measurements of parts are taken to determine assembly orientation to minimize vibration and run-out. Typical measurement apparatuses are capable of performing only relative measurements, such as eccentricity and roundness. U.S. Pat. Nos. 8,219,353 and 8,538,725 (both by assignee and herein incorporated by reference) provide apparatuses and methods for performing absolute diameter measurements. 
       SUMMARY OF THE INVENTION 
       [0003]    In apparatus such as described in U.S. Pat. Nos. 8,219,353 and 8,538,725, significant calibrations of the apparatus are required to take into account any imperfections in a horizontal measurement arm and a tower to which the arm is attached. Thus, certain physical aspects/behavior of the horizontal measurement arm and the tower on which it is mounted are measured and calibrated. Based on the calibration, the effect of movements of the measurement arm on a probe end of the measurement arm are predicted. 
         [0004]    In contrast to the &#39;353 and &#39;725 patents, embodiments of the present invention locate and/or measure a probe end of the measurement arm directly. Thus, the effect of tower and measurement arm imperfections on the probe end of the measurement arm are measured directly instead of being predicted based on calibration. Therefore, embodiments of the present invention can greatly simplify any necessary calibration, and can perform more accurate measurements of subject parts. 
         [0005]    Embodiments of the present invention perform calibration steps that improve the accuracy of measurements and then use the higher-accuracy measurements of a part to compute the part&#39;s absolute diameter. Embodiments of the present invention can account for error caused by temperature changes, movements of measurement apparatus parts, and unavoidable alignment imprecision between parts of the measurement apparatus. 
         [0006]    In one embodiment, an apparatus and corresponding method for measuring absolute diameter of an object on a rotary table includes a stable base with a substantially flat surface, the base having vibration-isolating mounts to isolate the stable base from ambient vibrations. The apparatus also includes a precision rotary table mounted to the flat surface of the stable base and a support tower mounted to the flat surface of the stable base, the tower being substantially perpendicular to the flat surface. A horizontal measurement arm is mounted to the support tower substantially parallel to the flat surface and configured to move towards and away from a center line of rotation of the precision rotary table. The horizontal measurement arm is configured to measure a distance of a precision gauge head on a probe end of the horizontal measurement arm away from the center line of rotation. A tracker is also mounted to the stable base and is configured to obtain a position in three-dimensional space of the probe end of the horizontal measurement arm. A controller is configured to determine a measurement arm correction based on the tracked and obtained position of the probe end of the measurement arm; determine a plurality of corrected points by applying a measurement arm correction to each of a plurality of measured points, the measured points being measured by the horizontal measurement arm (probe) around a circumference of a subject object on the rotary table, and the measurement arm correction being based upon the position in three-dimensional space of the probe end of the measurement arm; and determine an absolute radius of the object by applying the plurality of corrected points in a multi-point polygon model. 
         [0007]    In some embodiments, the tracker is a laser tracker. The vibration-isolating mounts can be rubber bushings, air suspension mounts, or a spring suspension. The controller can be further configured to determine the plurality of corrected points by applying a thermal expansion correction to each of the plurality of measured points, and the thermal expansion correction can be based on one or more measured temperatures of the horizontal measurement arm. The controller can further be configured to determine the plurality of corrected points by applying a calibration correction to each of the plurality of measured points, the calibration correction being based on a difference between a known radius of a test object and a measured radius of the test object. The multi-point polygon model used by the computer controller can be a least-squares best-fit model. 
         [0008]    In another embodiment, a method of determining absolute diameter of an object on a rotary table includes having an assembly formed of (i) a rotary table on a base, and (ii) a support tower on the base and carrying a horizontal measurement arm with a measurement probe end. The method also includes tracking the measurement probe end of the horizontal measurement arm to obtain a position in three-dimensional space of the measurement probe end of the horizontal measurement arm. The method further includes measuring a plurality of points around a circumference of a subject object on the rotary table using the horizontal measurement arm. For each measured point, a respective corrected point is determined by applying a measurement arm correction to the measured point, the measurement arm correction being based on the tracked and obtained position of the probe end of the horizontal-measurement arm. The determined corrected points are applied in a multi-point polygon model and absolute diameter of the subject object is determined. 
         [0009]    Tracking the measurement probe end can include tracking with a laser tracker. The method can further include measuring a temperature of the horizontal measurement arm at each of the plurality of points around the circumference of the subject object, and determining the respective corrected point can include applying a thermal expansion correction based on the measured temperature at each respective point. The horizontal measurement arm can be calibrated by measuring a test object of known radius on the rotary table using the horizontal measurement arm. Determining the respective corrected point can further include applying an arm calibration correction equal in magnitude to a difference between a measured radius of the test object and the known radius. Determining the absolute diameter can include employing the multi-point polygon mathematical model to provide a radius. 
         [0010]    In yet another embodiment, a method and corresponding apparatus for calibrating measurements by a horizontal measurement arm includes using a high precision rotary table on a base, wherein a support tower on the base carries the horizontal measurement arm, and a tracker is configured to track a measurement probe end of the horizontal measurement arm to obtain a position in three-dimensional space of the measurement probe end of the horizontal measurement arm. The method also includes determining a measurement arm error based on the tracked and obtained position of the measurement probe end of the horizontal measurement arm. The determined measurement arm error is used to effectively measure a plurality of corrected points around a circumference of a subject object on the rotary table with the horizontal measurement arm such that an absolute radius of the subject object is able to be determined by applying the plurality of corrected points in a multi-point polygon model. 
         [0011]    Effectively measuring the plurality of corrected points can include applying a thermal expansion correction based on temperature of the measurement arm measured at each of the plurality of points around the circumference of the subject object. Effectively measuring the plurality of corrected points can also include applying the calibration correction by correcting the measured points by an amount equal in magnitude to an absolute value of the difference between the measured radius and the known radius of the test object. The method can further include determining a calibration correction by measuring the radius of a test object with a known radius and calculating a difference between the measured radius and the known radius of the test object. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0013]      FIG. 1  is a side view of an absolute diameter measurement apparatus embodying the present invention. 
           [0014]      FIG. 2A  is a conceptual drawing showing support tower lean. 
           [0015]      FIG. 2B  is a conceptual drawing showing support tower variance. 
           [0016]      FIG. 3  is a conceptual drawing showing various radial measurements performed on a part and an equation for a best estimate of measured radius. 
           [0017]      FIG. 4  includes a conceptual drawing and equation showing how a y-direction error in radial measurement can be corrected. 
           [0018]      FIG. 5  is a schematic view of a computer network in which embodiments are deployed. 
           [0019]      FIG. 6  is a block diagram of a computer node in the network of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    A description of example embodiments of the invention follows. 
         [0021]      FIG. 1  shows a measurement apparatus  100  according to an embodiment of the present invention. The measurement apparatus  100  includes a base  102  preferably made out of granite. Granite is heavy and insensitive to thermal changes, thereby providing a stable platform on which measurements may be performed. Granite is also easy to precisely mill to provide a nearly-perfect flat and level upper surface  101  on which to perform the measurements. A person having ordinary skill in the art understands that other materials also may provide an acceptable base  102  with level upper surface  101  for the measurement apparatus. 
         [0022]    The granite base  102  is mounted to a vibration-isolating mount  124  to isolate the base  102  from ambient shop vibrations. The vibration-isolating mounts  124  are shown in conceptual form in  FIG. 1 . A person having ordinary skill in the art understands that there are many ways to incorporate vibration-isolating mounts  124  in the installation of the measurement apparatus  100 , and that the vibration-isolating mounts  124  may take many different forms, such as rubber pads, air suspension, or a spring suspension. 
         [0023]    A high-precision rotary table  104  and high-stiffness support tower  108  are mounted to the level upper surface  101  of the granite base  102 . The high-precision rotary table  104  supports parts being measured (not shown). The high-stiffness support tower  108  carries a precision horizontal linear scale (PHLS)  110  and a high-stiffness horizontal arm  118 . The high-stiffness horizontal arm  118  has a known length L, which is known to a high degree of precision. The PHLS  110  and high-stiffness horizontal arm  118  positionally move along a vertical (or along a longitudinal) axis of the high-stiffness support tower  108 . The PHLS  110  measures the horizontal position of high-stiffness horizontal arm  118 , which moves laterally or horizontally, i.e., at a right angle, to the high-stiffness support tower  108 . The PHLS  110  is typically measuring the distance from a gauge head  120 , mounted to the distal (or measurement) end  119  of the high-stiffness horizontal arm  118 , from the centerline of rotation  106  of the high-precision rotary table  104 . The gauge head  120  may be configured to measure either an interior surface diameter or an exterior surface diameter of a subject part positioned on rotary table  104 . A person having ordinary skill in the art understands that the precision horizontal scale  110  may measure a different distance, e.g., a distance of the gage head  120  from a surface of the housing  122 . 
         [0024]    Gage heads, such as gage head  120 , typically make contact with an object, e.g., subject part, being measured. The gage heads are typically capable of deflection to avoid transmitting forces to the object being measured. Such gage heads are usually high precision, and the position of the gage head and any deflection are known to a very high degree of accuracy. There are many types of precision gage heads available that are known to persons having ordinary skill in the art, any of which are suitable for use in the measurement arm  118  described herein. For the purposes of the measurement arm  118  described herein, the gage head  120  is assumed to be a part of the horizontal measurement arm  118  and to have no deflection. 
         [0025]    The high-stiffness support tower  108  also carries a precision vertical linear scale (PVLS)  112 , which measures the height of housing  122  and high-stiffness horizontal arm  118  above the upper planar surface  101  of the granite base  102  (or above the surface of a base made of a different material). 
         [0026]    The device shown in  FIG. 1  differs from the devices shown in previous U.S. Pat. Nos. 8,219,353 and 8,538,725 in the method for correcting measurement errors. In the previous patents, a laser was mounted at the granite base  102  to measure displacement of the horizontal arm housing  122  and high-stiffness horizontal arm  118 . The housing  122  and high-stiffness horizontal arm  118  can displace, i.e., shift, perpendicular to the longitudinal axis of the high-stiffness support tower  108  as they move vertically on the high-stiffness support tower  108  for two reasons: displacement of the high-stiffness support tower  108  away from the parallel axis, i.e., tower sway, and imperfections in the surface of the high-stiffness support tower  108 . The previous patents used laser measurements of tower sway and imperfections, and through a series of calibrations, it was possible to predict the effect of sway and imperfections on the measurements performed at the probe end  119  of the measurement arm  118 . 
         [0027]    In contrast to the previous U.S. Pat. Nos. 8,219,353 and 8,538,725, embodiments of the present invention measure the effect of tower and arm imperfections directly at the probe end  119  of the measurement arm  118 .  FIG. 1 , for example, shows a laser tracker  122  mounted to the granite base  102 . The laser tracker  122  measures one or more coordinate positions in three-dimensional space of the probe end  119  of the measurement arm. The laser tracker  122  has an angular range  124  in which it can direct a laser beam to a laser tracker sensor  125  mounted at or near the probe end  119  of the measurement arm. The laser tracker sensor  125  outputs a signal to sensor electronics (not shown), which can provide to a processor one or more coordinate positions in three-dimensional space of the probe end  119 . Of primary importance is the x (or in-plane radial) position, which can be used to correct measurements of radius of a subject object on the rotary table  104 . However, the laser tracker  122  can also measure coordinates in three dimensions, and the additional coordinate positions can also be used to correct radial measurements, as illustrated later in conjunction with  FIG. 4 . 
         [0028]    An optional temperature sensor  126  is also shown in  FIG. 1 . The temperature sensor  126  is embedded into the measurement arm  118  and can measure a temperature of the horizontal measurement arm  118  for each radius measurement obtained by the horizontal measurement arm  118 . These temperature measurements can be used then to correct the measured radii for thermal expansion of the measurement r i  arm, for example. The temperature sensor  126  is optional because the laser tracker  122  measurements of the probe end  119  of the measurement arm will include much of the effect of thermal expansion of the measurement arm  118 . Thus, correction for thermal expansion may be sufficiently included in the effect of the laser tracker  122  measurements and corresponding corrections. 
         [0029]      FIGS. 2A and 2B  illustrate these two reasons for perpendicular displacement of the housing  122  and high-stiffness horizontal arm  118  from  FIG. 1 .  FIG. 2A  shows a base  202  and a centerline of rotation  206  of a high-precision rotary table (not shown) and an ideal tower position  208 . The ideal tower position  208  is perfectly parallel to the centerline of rotation  206 . However, the tower (such as tower  108 ) will deflect by a small amount due in part to the tower  108  not being perfectly perpendicular to the base  202  and due to the weight of the housing  122  (not shown) and high-stiffness horizontal arm  118  (not shown) exerting a bending moment on the tower. Thus, the actual tower is not perfectly parallel to the centerline of rotation  206  and is a displaced tower (generally position referenced displacement  209 ). Generally, the higher the housing (not shown) and high-stiffness horizontal arm (not shown) move up (away from base surface  101 ) along the high-stiffness support tower  108 , the greater the high-stiffness support tower  108  will deflect from ideal position  208 . Note that the actual tower displacement  209  is shown greatly exaggerated for illustration purposes. Further note that the actual tower displacement  209  may be in a different direction, such as displacement  209 ′. 
         [0030]      FIG. 2B  shows a base  202  and a centerline of rotation  206  of a high-precision rotary table (not shown) and an ideal tower position  208 . Again, the ideal tower position  208  is perfectly parallel to the centerline of rotation  206 . However, the tower will have small variances caused by manufacturing imperfections.  FIG. 2B  illustrates an actual tower position  211  that is different from the ideal tower position  208 . The tower variance  211  is shown greatly exaggerated for illustration purposes. 
         [0031]    The errors illustrated in  FIGS. 2A and 2B  are the types of errors whose effect the laser tracker  122  in  FIG. 1  can measure directly, greatly simplifying any calibration required for the system. 
         [0032]      FIG. 3  illustrates radius measurements performed on a subject part  328  with unknown radius R. The probe  130  mounted to the probe end  119  of the measurement arm shown in  FIG. 1  contacts the part  328  to obtain measurements r i  of the radius of the part  328  as the part is rotated on the rotary table  104  shown in  FIG. 1 . Several of these measurements are shown in  FIG. 3 , including the current measurement r i , the previous measurement r i−1 , and the next measurement r i+1 . Many of these radial measurements can be obtained to produce a best estimate for radius R or diameter of the part  328 , and typically thousands of these measurements are obtained to calculate the best estimate for radius R (and hence diameter, which is twice the radius). 
         [0033]      FIG. 3  also shows equations that can be used to obtain a best estimate radius R for the part  328 : 
         [0000]    
       
         
           
             
               
                 
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         [0000]    The individual radius measurements r i  are combined into a multi-point polygon model, such as the least squares best fit model. The least squares best fit model outputs the absolute radius (or absolute diameter, which is the absolute radius multiplied by two) of the subject part  328 . Equation (1) shown in  FIG. 3  is an example of such an equation based on a least squares best fit model. N is the total number of individual radius measurements taken. 
         [0034]    Once all radius measurements r i  are obtained, the measurements can be input into Equation (1) to obtain the best estimate for measured radius R. The apparatus of  FIG. 1  obtains the individual measurements r i  according to Equation (2) shown in  FIG. 3 . The value r Ai  is the radius measurement obtained by the horizontal measurement arm  118  based on the PHLS  110 , shown in  FIG. 1 . Each value Δr Gi  is a change in radius obtained from the gauge head probe  130 . Each value Δr Ti  is a change in horizontal measurement arm position based on the tracking laser  122  and for laser tracker sensor  125  shown in  FIG. 1 . The values e i  are corrections that can be applied to the radius if the probe  130  is not perfectly directed toward the center of the subject part  328 . In other embodiments, a temperature correction can be applied to the radius measurements r i  shown in Equation (2). The temperature corrections can be based on temperatures of the horizontal measurement arm measured for each radius r i  using the temperature sensor  126  shown in  FIG. 1 . Temperature corrections can also be based on a coefficient of thermal expansion of the horizontal measurement arm  118 . Further, in other embodiments, the temperature of the horizontal measurement arm  118  may be measured using non-contact temperature measurement. 
         [0035]    The arm radius measurement r Ai  can be based on a calibrated horizontal measurement arm  118 . The horizontal measurement arm  118  in  FIG. 1  is calibrated using a part of known radius (not shown) to produce correct values for r Ai , Δr Gi , and Δr Ti . A part of known radius (not shown) is placed on the rotary table  104 , and the horizontal measurement arm  118  is extended so that the probe  130  contacts the part of known radius. Upon contact, positions of the horizontal measurement arm  118 , the probe  130 , and the laser tracker sensor  125  are set appropriately. Namely, the horizontal measurement arm  118  position is set to the known radius for the known part, and the positions for the probe  130  and for the laser tracker sensor  125  are set to zero. 
         [0036]    After measurement arm calibration, the part of known radius is removed from the rotary table  104 , and the subject part  328  of unknown radius R is placed on the rotary table  104 . Thereafter, when radius measurements on the unknown subject part  328  are taken, the horizontal measurement arm  118  reports an absolute radius measurement r Ai  that is calibrated based on the known part. The probe  130  reports a value Δr Gi  that represents any change in the value of the probe position since coming into contact with the known part during calibration. Similarly, the laser tracker sensor  125  reports a position Δr Ti  that represents any difference between the laser tracker position measured at the known part during calibration and the unknown part during measurements of r i . Error measurements e i  as seen in Equation (2) can be obtained as shown in  FIG. 4 . 
         [0037]      FIG. 4  illustrates the effect that can occur if the probe  130  is not directed perfectly toward the center of the known part  328 . The difference between the positions of the on center probe  130  and the off-center probe  130 ′ is exaggerated for illustration purposes. The probe  130  is on center or directed radially toward the part  328 , and it measures a correct value r i  for radius. In contrast, the probe  130 ′ is off-center, and it reports an incorrect value r i′  for radius. The difference between r i ′ and r i  is the error e i . The value y i  is the distance between the radius r i ′ and the position where the probe  130 ′ contacts the edge of the part  328  along a line perpendicular to the radius r i ′. The value y i  can be obtained from the laser tracker sensor  125  in a way similar to how the radial measurement Δr Gi  is obtained from the laser tracker sensor  125 . It can be shown that: 
         [0000]        r   i =√{square root over ( y   i   2   +r′   i   2 )}(3)
 
         [0000]        e   i =√{square root over ( r′   i   2   +y   i   2 )}−r′ i   (4)
 
         [0038]    The error correction e i  shown in Equation (4) can be calculated and applied to Equation (2) in  FIG. 3  to correct for any horizontal displacement along the y-axis of the probe  130 . 
         [0039]    Corrections to the radial measurements r i  are not limited to the x and y axes. The laser tracker  122  can obtain height measurements for the laser tracker sensor  125  in the z direction shown in  FIG. 1 . Thus, corrections to the radial measurements r i  due to any z direction displacement of the probe end  119  of the measurement arm can be made to the radial measurements r i  in a similar way to that in which y-direction corrections are obtained as shown in  FIG. 4 . 
         [0040]    In addition to the corrections shown in  FIG. 4 , a thermocouple measures the temperature of the high-stiffness horizontal measurement arm  118  during the measurement of the 2,000 radii measurements r i . A length correction can be applied to each of the 2,000 points r i  by calculating the change in temperature from a starting temperature and multiplying the change in temperature by a known coefficient of expansion of the material from which the high-stiffness horizontal measurement arm  118  is made. However, in some embodiments, temperature corrections are not necessary because the effect of thermal expansion of the horizontal measurement arm  118  is adequately accounted for by the laser tracer sensor  125 . 
         [0041]      FIG. 5  illustrates a computer network or similar digital processing environment in which the present invention may be implemented. Client computer(s)/devices  50  and server computer(s)  60  provide processing, storage, and input/output devices executing application programs and the like. Client computer(s)/devices  50  can also be linked through communications network  70  to other computing devices, including other client devices/processes  50  and server computer(s)  60 . Communications network  70  can be part of a remote access network, a global network (e.g., the Internet), a worldwide collection of computers, Local area or Wide area networks, and gateways that currently use respective protocols (TCP/IP, Bluetooth, etc.) to communicate with one another. Other electronic device/computer network architectures are suitable. 
         [0042]      FIG. 6  is a diagram of the internal structure of a computer (e.g., client processor/device  50  or server computers  60 ) in the computer system of  FIG. 5 . Each computer  50 ,  60  contains system bus  79 , where a bus is a set of hardware lines used for data transfer among the components of a computer or processing system. Bus  79  is essentially a shared conduit that connects different elements of a computer system (e.g., processor, disk storage, memory, input/output ports, network ports, etc.) that enables the transfer of information between the elements. Attached to system bus  79  is I/O device interface  82  for connecting various input and output devices (e.g., keyboard, mouse, displays, printers, speakers, etc.) to the computer  50 ,  60 . Network interface  86  allows the computer to connect to various other devices attached to a network (e.g., network  70  of  FIG. 5 ). Memory  90  provides volatile storage for computer software instructions  92  and data  94  used to implement an embodiment of the present invention (e.g., error measurement code detailed above). Disk storage  95  provides non-volatile storage for computer software instructions  92  and data  94  used to implement an embodiment of the present invention. Central processor unit  84  is also attached to system bus  79  and provides for the execution of computer instructions. 
         [0043]    In one embodiment, the processor routines  92  and data  94  are a computer program product (generally referenced  92 ), including a computer readable medium (e.g., a removable storage medium such as one or more DVD-ROM&#39;s, CD-ROM&#39;s, diskettes, tapes, etc.) that provides at least a portion of the software instructions for the invention system. Computer program product  92  can be installed by any suitable software installation procedure, as is well known in the art. In another embodiment, at least a portion of the software instructions may also be downloaded over a cable, communication and/or wireless connection. In other embodiments, the invention programs are a computer program propagated signal product  107  embodied on a propagated signal on a propagation medium (e.g., a radio wave, an infrared wave, a laser wave, a sound wave, or an electrical wave propagated over a global network such as the Internet, or other network(s)). Such carrier medium or signals provide at least a portion of the software instructions for the present invention routines/program  92 . 
         [0044]    In alternate embodiments, the propagated signal is an analog carrier wave or digital signal carried on the propagated medium. For example, the propagated signal may be a digitized signal propagated over a global network (e.g., the Internet), a telecommunications network, or other network. In one embodiment, the propagated signal is a signal that is transmitted over the propagation medium over a period of time, such as the instructions for a software application sent in packets over a network over a period of milliseconds, seconds, minutes, or longer. In another embodiment, the computer readable medium of computer program product  92  is a propagation medium that the computer system  50  may receive and read, such as by receiving the propagation medium and identifying a propagated signal embodied in the propagation medium, as described above for computer program propagated signal product. 
         [0045]    Generally speaking, the term “carrier medium” or transient carrier encompasses the foregoing transient signals, propagated signals, propagated medium, storage medium and the like. 
         [0046]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.