Abstract:
A measurement apparatus calibrated to measure an absolute diameter of a part in a shop floor environment. The measurement apparatus includes a calibration that includes compensation factors for thermal expansion, shifting of measurement parts (arm, support tower, and related laser), and variances of these parts. The resulting measurements report an absolute diameter of a part to a higher degree of accuracy than previously possible. Also, the calculated compensation factor eliminate the need for an isolated, climate-controlled measurement room.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/148,857, filed on Jan. 30, 2009. The entire teachings of the above application is 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. Current measurement apparatuses are only capable of performing relative measurements, such as eccentricity and roundness. 
       SUMMARY OF THE INVENTION 
       [0003]    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 account for error caused by temperature changes, movements of measurement parts, and unavoidable alignment imprecision between parts of the measurement apparatus. 
         [0004]    In one embodiment, a system includes a rotary table on a base, a support tower on the base that carries a horizontal measurement arm, and a laser device configured to indicate change in orientation of the horizontal measurement arm with respect to a centerline of rotation of the rotary table. The system is calibrated at multiple heights to determine (i) a measurement error factor of the horizontal measurement arm, (ii) a measurement error factor caused by displacement of the horizontal measurement arm, which is caused by variation of the support tower, and (iii) a measurement error factor caused by displacement of the laser path. The system measures multiple points around a circumference of a subject object on the rotary table and a temperature is measured for each point. Each measurement point is corrected based on the three factors described above and also based on a thermal expansion correction factor based on the measured temperature for the point. An absolute diameter and radius of the subject object are determined from the corrected multiple points. 
         [0005]    In some embodiments, the measurement error factor caused by displacement of the horizontal measurement arm is determined as a function of height above a reference height on the support tower. In some embodiments, the measurement error factor caused by displacement of the laser path is determined as a function of height above a reference height on the support tower. In some embodiments, the measurement error factor of the horizontal measurement arm is determined by comparing a measured radius of a test object to the known radius of the test object, and the measurement error factor being the difference between the two. In some embodiments, the measurement error factor caused by displacement of the horizontal measurement arm, determined as a function of height on the support tower, is determined by determining the error factor at two heights on the support tower and interpolating between the two measurement errors. In some embodiments, the measurement error factor caused by the displacement of the laser path, determined as a function of height on the support tower, is determined by measuring the measurement error factor at two heights on the support tower and interpolating between those two heights. 
         [0006]    In some embodiments, the absolute diameter and radius of a subject object are determined by applying the corrected multiple points in a multi-point polygon mathematical model. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    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. 
           [0008]      FIG. 1  is a side view of an absolute diameter measurement apparatus embodying the present invention; 
           [0009]      FIG. 2A  is a conceptual drawing showing support tower lean; 
           [0010]      FIG. 2B  is a conceptual drawing showing support tower variance; 
           [0011]      FIG. 3  is a conceptual drawing showing laser beam variance from an ideal laser optical path; 
           [0012]      FIG. 4A  is a conceptual drawing showing calibration measurements at a low (or reference) height employed by embodiments of the present invention; 
           [0013]      FIG. 4B  is a conceptual drawing showing calibration measurements at a second height employed by embodiments of the present invention; 
           [0014]      FIG. 5  is a conceptual drawing showing calibration calculations performed from calibration measurements shown in  FIGS. 4A and 4B ; 
           [0015]      FIG. 6  is a conceptual drawing showing measurements performed on a calibrated absolute diameter measurement apparatus of the present invention, such as the apparatus shown in  FIG. 1 , on a part to measure the part&#39;s absolute diameter; 
           [0016]      FIG. 7  is a schematic view of a computer network in which embodiments are deployed; and 
           [0017]      FIG. 8  is a block diagram of a computer node in the network of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    A description of example embodiments of the invention follows. 
         [0019]      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. 
         [0020]    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 or a spring suspension. 
         [0021]    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 end 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 . 
         [0022]    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 where 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. 
         [0023]    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). A laser  114  is also mounted at the granite base  102  and is aligned so that its centerline beam is nearly-perfectly parallel to the centerline of rotation  106  of the high-precision rotary table  104 . The laser  114  measures a displacement of the housing  122  and high-stiffness horizontal arm  118  perpendicular to the laser  114  centerline beam. This perpendicular displacement also corresponds to an equivalent radial displacement of the housing  122  and high-stiffness horizontal arm  118  with the centerline of rotation  106  of the high-precision rotary table  104 . The housing  122  and high-stiffness horizontal arm  118  may 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 . 
         [0024]      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 (not shown) and high-stiffness horizontal arm (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 ′. 
         [0025]      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. 
         [0026]    The laser ( 114  in  FIG. 1 ) measures variations in perpendicular displacement of the housing ( 122  in  FIG. 1 ) and high-stiffness horizontal arm ( 118  in  FIG. 1 ) with respect to the longitudinal direction of the support tower  108 . However, the laser ( 114  in  FIG. 1 ) also has an error component because its optical path is not at all times perfectly parallel to the centerline of rotation ( 106  in  FIG. 1 ) of the high-precision rotary table ( 104  in  FIG. 1 ).  FIG. 3  illustrates the laser misalignment.  FIG. 3  shows a base  302  and a centerline of rotation  306  of a high-precision rotary table (not shown in  FIG. 3 , but  104  in  FIG. 1 , for example).  FIG. 3  also shows an ideal laser optical path  316  (laser not shown) that is perfectly parallel to the centerline of rotation  306 . However, the actual laser optical path  317  is not perfectly parallel. Note that the actual laser path  317  misalignment is greatly exaggerated for illustrative purposes. Also note that the laser misalignment may be in different directions, such as line  317 ′ for example. 
         [0027]      FIG. 4A  illustrates the measurements involved in calibrating an absolute diameter measurement arm according to an embodiment of the present invention.  FIG. 4A  shows a line  404  representing a tower (such as  108  in  FIG. 1 ) mounted to a base  402 . The high-stiffness support tower  404  is shown leaning (greatly exaggerated for illustration purposes) away (or off) from perpendicular relative to the upper planar surface (such as  101  in  FIG. 1 ) of the base  402 .  FIG. 4A  also shows a centerline of rotation (COT)  406  of a high-precision rotary table (not shown, but see  104  in  FIG. 1 ) and a horizontal measurement arm  408  (i.e.,  118  in  FIG. 1 ). In this first step, the horizontal measurement arm  408  is set at a low height H YL  on the tower  404  and the horizontal measurement arm  408  is set at zero with respect to the COT  406 . Thus, motions of the horizontal measurement arm  408  away from the COT  406  result in an increasing radius measurement of a subject on the high-precision rotary table  104 . After the horizontal measurement arm  408  is set at zero with respect to the COT  406 , a master ring having known radii R M  is placed on the high-precision rotary table (not shown, but  104  in  FIG. 1 ) at the low point H YL . For each radius on the master ring (not shown), the horizontal measurement arm  408  measures 2,000 points H XL  around the master ring&#39;s circumference. The 2,000 points H XL  are entered into a multi-point polygon model, which calculates a radius R from the points. Examples of multi-point polygon models that may be used or employed include known least squares best fit algorithms or other known mathematical fit models. The calculated radius R is compared to the known radius R M  of the master ring, and the difference is a calibration difference H CAL  for the horizontal measurement arm  408  at low height H YL . These steps are performed for different known radii on the master ring to gather several H CAL  values. 
         [0028]    Before the horizontal measurement arm  408  is moved from the low height H YL , an laser offset value L L  is also read, which represents misalignment between the laser beam  410  and the tower  404  at that height H YL . The laser offset value L L  is the distance between (i) the intersection between the horizontal arm  408  axis and the laser beam  410  at the low height H YL  and (ii) the intersection between the horizontal arm and the support tower  404  at the low height H YL . Also, a thermocouple measures the temperature of the high-stiffness horizontal measurement arm  408  during the measurement of the 2,000 points H XL . A length correction can be applied to each of the 2,000 points H XL  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  408  is made. 
         [0029]    In a second step, shown in  FIG. 4B , the high-stiffness horizontal measurement arm is moved to a high position  408 ′. At the high position, the high position height H YH  is measured and the laser offset value L H  at this height is measured. The laser offset value L H  is the distance between (i) L L  (from the intersection between the horizontal arm axis and the laser beam  410  at the low height H LH ) and (ii) the intersection between the horizontal arm axis and the laser beam  410  at the high height H YH , less the calibration difference (see below). The master ring (not shown) is again measured at 2,000 circumferential points around each of its different radii. At each radius, the horizontal measurement arm&#39;s  408 ′ 2,000 measurements H XH  are combined with the H CAL  value for the radius that was calculated at the low point H YL . Also, temperature measurements are taken at each of the 2,000 points H XH  and a temperature correction, as described above, is incorporated into the measurements. The 2,000 combined H XH +H CAL  values are again entered into the least squares best fit model, which calculates a radius R′. The calculated radius R′ is compared to the known radius R M  of the master ring, and the difference is the calibration difference T C  for the horizontal measurement arm  408 ′ at the high position H YH . The calibration difference T C  is applied to the laser offset value L H  to remove from the laser offset value L H  any affect caused by the tower lean angle being different from the laser misalignment angle. 
         [0030]      FIG. 5  illustrates calculations that are performed based on the above-described measurements. The calibration difference T C  is a distance measure of the amount of tower lean. The tower lean can be described by an angle b CAL  by the equation: b CAL =tan −1 (T C /(H YH −H YL )). The laser lean angle (relative to the ideal tower) α CAL  can also be calculated by the equation: α CAL =tan −1 ((L H −L L )/(H YH −H YL )). Note that the sum of angles α CAL  and b CAL  results in a constant value. Any local imperfections in the tower, i.e., differences from the ideal tower position  502  will cause an increase in one of the two angles and an equal decrease in the other angle such that the summed angle value remains constant. 
         [0031]      FIG. 6  illustrates measurement of a subject part  602  on the high-precision rotary table (not shown, but see  104  in  FIG. 1 ) using the now-calibrated measurement arm  118 . The horizontal measurement arm  118  is moved to a (current) height H CUR  at the height of the part radius to be measured. The height H CUR  is translated into H Y  by the equation: H Y =H CUR −H YL . Next, a current laser measurement L CUR  is read and is translated to an actual reading by the equation: L ACT =L CUR −L L . The actual laser angle α ACT  can be calculated by the equation: α ACT =tan −1 (L ACT /H Y ). As stated above, the sum of α CAL  and b CAL  results in a constant value, which translates into a known height at a given H Y . The change in the tower offset due to variations T Cchange  can be calculated according to the equation: T Cchange =−H Y (tan(α CAL −α ACT )). After determining the tower variation offset T Cchange , the high-precision gauge head  120  on the horizontal measurement arm  118  is brought into contact with the subject part  602  being measured and H X  (the reading on the horizontal linear scale) is read and s temperature correction is applied. The offset for tower lean partT C  is then calculated based on the equation: partT C =tan(b CAL )×H Y . All of the above-calculated variables and corrections are combined to form an actual radius measurement R ACT  according to the equation: R ACT =H X +H CAL +partT C +T Cchange . H ACT (=R ACT ) is calculated for 2,000 points around the circumference of the subject part  602  and entered into a multi-point polygon model, such as a 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  602 . 
         [0032]      FIG. 7  illustrates a computer network or similar digital processing environment in which the present invention may be implemented. 
         [0033]    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. 
         [0034]      FIG. 8  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. 7 . 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. 7 ). 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. 
         [0035]    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 . 
         [0036]    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. 
         [0037]    Generally speaking, the term “carrier medium” or transient carrier encompasses the foregoing transient signals, propagated signals, propagated medium, storage medium and the like. 
         [0038]    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.