Abstract:
Methods and systems for fabricating a composite structure are provided. The method includes receiving data representing a portion of a surface of the structure, measuring a surface of the structure, the measured surface corresponding to the received surface wherein the measuring is performed with the structure in a flexed condition, and determining a difference in a first and a second direction between the measured surface and the received surface at areas that correspond to the measured surface. The method also includes determining a difference in a third direction, transmitting to a morphing algorithm the determined differences in the first, second, and third directions, and determining a position in the first, second, and third directions of a point on the surface in the flexed condition that corresponds to a respective point on the received surface when the structure is placed in the nominal condition.

Description:
BACKGROUND 
     Embodiments of the disclosure relate generally to methods and systems for manufacturing large components and more particularly, to methods and systems for forming composite structures to close tolerances. 
     Manufacturing techniques for assembling large structures from composite material use massive tooling to provide a stable platform to form the structure. Determinant assembly (DA) holes or other indexing features are use to fix locations of the structure relative of points of reference. The DA holes permit accurately ascertaining the location of any point on the structure surface. Such accurate location may be needed when further manufacturing processes are performed on the structure after the initial forming process. Such further processes may include cutting apertures at predetermined locations or milling certain features into the surface of the structure. However, the massive tooling may introduce an amount of sag into the structure that will be removed from the structure when the tooling is disassembled and the structure becomes free standing. For example, an airplane fuselage barrel may be formed of composite material wound around tooling. The tooling may be massive enough to sag between the points of rotation at either end of the tooling. As the structure is formed, it acquires the sag from the tooling. Before the formed barrel is removed from the tooling, typically window, and door apertures and attachment fixtures are cut into the barrel. If the apertures are cut into the barrel in a sag state, the apertures will be mispositioned when the tooling is removed and the barrel conforms to its nominal or unflexed shape. 
     What are needed are methods and systems for providing accurate placement of apertures and fixtures to manufactured parts that may flex during manufacturing. 
     SUMMARY 
     In one embodiment, a computer-implemented method of fabricating a composite structure includes receiving data representing at least a portion of a surface of a model of the structure in a nominal condition, measuring at least a portion of a surface of the structure at a plurality of areas along the surface, the measured portion corresponding to a portion of the surface of the model wherein said measuring is performed with the structure in a flexed condition, determining a difference in a first and a second direction between the measured portion of the structure surface at each of the plurality of areas and the portion of the surface of the model at areas in the model that correspond to the measured areas relative to one or more fixed points observable during the measurement, and determining a difference in a third direction between the measured portion of the structure surface at each of the plurality of points and the portion of the surface of the model at points in the model that correspond to the measured points. The method transmitting to a morphing algorithm the determined differences in the first, second, and third directions, determining a position in the first, second, and third directions of a point on the structure in the flexed condition that corresponds to a respective point on the structure when the structure is placed in the nominal condition; and outputting the determined position. 
     In yet another embodiment, a system for compensating for variations of a part between an as-built shape relative to a nominal configuration of the part wherein the nominal configuration is predetermined using a model of the part wherein the system includes a database embodied on a computer readable media, said database comprising data relating to the three dimensional shape of a nominal configuration of the part, a non-contact metrology system configured to survey an as-built configuration of the part, the survey comprising measured points corresponding to at least a portion of the data relating to the three dimensional shape of a nominal configuration of the part, and a processor configured to receive data from said database and at least a portion of the as-built configuration from said metrology system. The processor is further configured to define a plurality of nominal plane patches using the received three dimensional shape data from said database, locate points in the survey data that correspond to the three dimensional shape data, determine a positional error between at least one of the plurality of nominal plane patches and the corresponding measured points in two dimensions, determine a positional error in a third dimensions using the positional error is two dimensions, and output the positional errors in three dimensions. 
     In another embodiment, a computer implemented method of directing automatic tooling includes receiving data representing a shape of a part in a nominal condition, measuring at least a portion of the part surface in an as built configuration using non contact metrology, and determining a positional difference in three dimensions between the measured portion of the part surface and a corresponding surface of the represented shape of the part in the nominal condition. The method further includes transmitting the determined positional difference in three dimensions of each portion to a morphing algorithm, determining a position in the first, second, and third directions of a point on the structure in the flexed condition that corresponds to a respective point on the structure when the structure is placed in the nominal condition, and outputting the determined position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side perspective view of an exemplary work-piece that may be fabricated using the methods described herein; 
         FIG. 2  is a side perspective view of workpiece shown in  FIG. 1  in a nominal condition; 
         FIG. 3A  is a side view of a portion of workpiece that includes a feature to be measured; 
         FIG. 3B  is an end view of feature shown in  FIG. 3A ; 
         FIG. 4  is a flow chart of an exemplary method of determining position corrections for a workpiece flexed from a nominal condition; 
         FIG. 5  is a flow chart of an exemplary method of determining position corrections for a workpiece flexed from a nominal condition; 
         FIG. 6  is a flow chart of another exemplary method of determining position corrections for a workpiece flexed from a nominal condition; 
         FIG. 7A  is an end view of an exemplary workpiece feature illustrating a method of determining the surface deviations at each surface patch section; 
         FIG. 7B  is an end view of workpiece feature shown in  FIG. 7A  illustrating a method of determining the Y and Z errors from the surface deviations; and 
         FIG. 8  is a simplified block diagram of a Fabrication Alignment System (FAS) including a server system and a plurality of client sub-systems. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description illustrates the disclosure by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the disclosure, describes several embodiments, adaptations, variations, alternatives, and uses of the disclosure, including what is presently believed to be the best mode of carrying out the disclosure. The disclosure is described as applied to a preferred embodiment, namely, a process of forming airplane fuselage barrels. However, it is contemplated that this disclosure has general application to manufacturing major components and assemblies where adherence to a specified set of dimensional tolerances is desired, particularly where the weight of the component and/or manufacturing tooling generates a deviation from manufacturing tolerances. 
       FIG. 1  is a side perspective view of an exemplary work-piece  100  that may be fabricated using the methods described herein. In the exemplary embodiment, workpiece  100  is mounted on massive tooling  102 . Tooling  102  is supported at a first end  104  and a second end  106  and is rotatable about a longitudinal axis  108  during the fabrication process. Workpiece  100  is formed by rotating tooling  102  about axis  108  and winding a composite material in the form of for example, a strand, rope, or web about the rotating tooling  102 . In an alternative embodiment, workpiece  100  is fabricated by holding tooling  102  stationary and winding the composite material about tooling  102  by traversing a winding assembly circumferentially about an outer periphery of tooling  102 . However, because tooling  102  is massive and is supported at ends  104  and  106 , tooling  102  and workpiece  100  tend to sag in between ends  104  and  106 . The sag illustrated in  FIG. 1  is exaggerated for explanation purposes. During a cutting portion of the fabrication process, apertures and/or holes may be cut through workpiece  100 . In the exemplary embodiment, workpiece  100  is an aircraft fuselage barrel that will include window apertures  110 , door apertures (not shown), and/or mounting holes (not shown) when the fabrication process is completed. 
     Typically, window apertures  110  in an aircraft fuselage are cut such that the edges of window apertures  110  are aligned along a line on the surface of the fuselage that is substantially parallel to the longitudinal axis of the fuselage. If window apertures  110  are cut while workpiece  100  is in a flexed condition due to sag, window apertures  110  will not be aligned along a line parallel with longitudinal axis  108  when workpiece  100  is in a nominal or unflexed condition. 
     Determinant assembly (DA) holes  112  are features that are fixed on tooling  102  and are viewable during a measuring process wherein workpiece  100  is measured to determine the positions of selectable predetermined points on workpiece  100  with respect to DA holes  112 . Workpiece  100  may be measured using a non-contact metrology system such as an optical metrology system  114 . For example, a plurality of light point may be projected onto workpiece  100  and the surface of workpiece  100  may then be measured using photogrammetry techniques. Features associated with the surface of workpiece  100  may also be used to measure the surface of workpiece  100  with respect to DA holes  112  or other component or feature that would provide a reference for measuring workpiece  100 . Other surface measurements techniques may be used such as but not limited to ultrasonic, laser light, and radio frequency. In the exemplary embodiment, portions of the surface of workpiece  100  are measured, however in other embodiments the entire surface of workpiece  100  may be measured. 
       FIG. 2  is a side perspective view of workpiece  100  (shown in  FIG. 1 ) in a nominal condition. In the embodiment illustrated in  FIG. 2 , apertures  110  are not aligned along a line on the surface of workpiece  100  that is parallel to longitudinal axis  108 . Rather, with massive tooling  102  removed, workpiece  100  returns to a nominal condition wherein workpiece  100  is supporting its own weight and is substantially not in a flexed condition. Edges of window apertures  110  are aligned an arcuate line  202  on workpiece  100  that is not parallel to longitudinal axis  108 . To avoid such misalignment the method of locating positions on workpiece  100  when in the flexed or sagging condition that correspond to positions on a predetermined model of workpiece  100  in a nominal condition is described herein. 
       FIG. 3A  is a side view of a portion of workpiece  100  that includes a feature  302  to be measured.  FIG. 3B  is an end view of feature  300  (shown in  FIG. 3A ). In the exemplary embodiment, feature  302  comprises a non-constant dimension stringer trough that extends along workpiece  100  between a first end  304  and a second end  306 . Trough  302  includes a first sidewall  308  and a second sidewall  310  that extends between a surface  312  of workpiece  100  and a base  314  of trough  302 . Selectable portions or patches of surface  312  and/or trough  302  may be marked for measurement using light or a physical marking of the surfaces such as an edge of a feature. For example, a station cut  316  and sidewall  308  or  310  may bound a patch  318  to be measured. In addition, patch  318  may be bounded by light projections delineating bounding edges of patch  318 . 
       FIG. 4  is a flow chart of an exemplary method  400  of determining position corrections for a workpiece flexed from a nominal condition. In the exemplary embodiment, method  400  generates a nominal surface to which a measured surface is compared. Method  400  includes defining  402  boundaries in a first direction of an area to be measured and defining  404  boundaries in a second direction, for example, a station cut, of an area to be measured. A nominal plane patch is created  406  from the four points defined by the intersection of the boundaries in the first direction and the boundaries in the second direction wherein the plane patches are defined as surface patches. The nominal surface patch is transferred  408  to a job template file. The structure is measured  410  in its flexed condition and the measurement results stored. 
       FIG. 5  is a flow chart of an exemplary method  500  of determining position corrections for a workpiece flexed from a nominal condition. In the exemplary patch-to-patch based embodiment, method  500  includes extracting  502  points from measured data that corresponds to each nominal surface patch. Points common to each surface patch are analyzed  504 . A standard deviation of each relative surface patch is compared  506  to a predetermined threshold. Deviations that exceed the predetermined threshold are reported  508 . Average deviations relative to each surface patch at each station cut are outputted  510 . The premeasured level state error at x and the measured sag state error are combined  512 . The angle of the feature to the relative axis is determined  514  to calculate the hypotenuse, z error. The angle of the feature to the relative axis is determined  516  for the y-axis if not on the axis plane to calculate the hypotenuse, z error. x error is determined  518  as a percentage of y error. The determined errors are then exported  520  into the using/manufacturing system for morph or positional correction. 
       FIG. 6  is a flow chart of another exemplary method  600  of determining position corrections for a workpiece flexed from a nominal condition. In the exemplary point to patch based embodiment, method  600  includes generating  602  surface patches such as polygonized patches from the measured data. The patch boundary intersection points are extracted  603 . The points are projected  604  to patches using xy plane projection. Deviations in the x and y directions are determined  606 . The premeasured stat error at X location and the measured sag state error in X and Y are combined  608 . Z errors are determined  610  using the determined X and Y errors. The determined errors are then exported  612  into the using/ manufacturing system for morph or positional correction. 
       FIG. 7A  is an end view of an exemplary workpiece feature  700  illustrating a method of determining the surface deviations at each surface patch section.  FIG. 7B  is an end view of workpiece feature  700  (shown in  FIG. 7A ) illustrating a method of determining the Y and Z errors from the surface deviations. In the exemplary embodiment, workpiece feature  700  is a stringer channel extending along a surface  702  of a workpiece  704  such as a fuselage barrel for an aircraft. A level state measured point cloud  706  of points from workpiece  704  is plotted. The level state points are measured in a level state such as when midsections of workpiece  704  are supported. A sag state measured point cloud  706  of points from workpiece  704  is plotted. The sag state points are measured in a sag state such as when midsections of workpiece  704  are not supported. Differences between points in point cloud  706  and a nominal surface  710  of workpiece  704 , and points in point cloud  708  and nominal surface  710  are determined. In the exemplary embodiment, points in point cloud  706  deviate from nominal surface  710  by +0.006 and points in point cloud  708  deviate from nominal surface  710  by −0.017 on average. Accordingly, total plane deviation  712  is 0.023. 
     In the exemplary calculation, a triangle is formed using the known total plane deviation as a known leg. A known angle is a complementary angle of the feature angle. In the exemplary embodiment, angle  714  is approximately 49.74 degrees. In the exemplary embodiment, the Y error is equal to the Y-plane error of 0.002. Z error is determined using total plane deviation  712  divided by the sine of angle  714 . In the exemplary embodiment, this Z error equals 0.023/sine(49.74°) or 0.0301. X error is determined as a percentage of the Y plane movement. In the exemplary embodiment, movement along the longitudinal axis (x-direction) is negligible for workpiece  704  between the level state and the sag state. Using the above described error calculation, error values in three dimensions are output to a morphing algorithm to determine cutting locations in a flexed workpiece in a sag state that result in a proper placement of for example, but not limited to windows, doors, and attachment fixtures when the workpiece is in the not flexed or nominal state. 
       FIG. 8  is a simplified block diagram of a Fabrication Alignment System (FAS)  800  including a server system  812 , and a plurality of client sub-systems, also referred to as client systems  814 , connected to server system  812 . Computerized modeling and measurement tools, as described above, are stored in server  812 , and can be accessed by a requester at any one of computers  814 . In one embodiment, client systems  814  are computers including a web browser, such that server system  812  is accessible to client systems  814  using the Internet. Client systems  814  are interconnected to the Internet through many interfaces including a network, such as a local area network (LAN) or a wide area network (WAN), dial-in-connections, cable modems, and special high-speed ISDN lines. Client systems  814  could be any device capable of interconnecting to the Internet including a web-based phone, personal digital assistant (PDA), or other web-based connectable equipment. A database server  816  is connected to a database  820  containing information on a variety of matters, as described above. In one embodiment, centralized database  820  is stored on server system  812  and can be accessed by potential users at one of client systems  814  by logging onto server system  812  through one of client systems  814 . In an alternative embodiment, database  820  is stored remotely from server system  812  and may be non-centralized. 
     The above-described methods of forming composite structural members and composite structures formed thereby are cost-effective and highly reliable. The methods and structures include composite material formed using massive tooling that causing a deflection of the structure from its nominal condition during forming. Such deflection is measured and the amount of deflection between the deflected state and the nominal state is determined in three dimensions. The errors are input into morphing algorithms that orient cutting tools to a position that will be the correct position for windows and doors after the structure returns to its nominal state after the removal of the tooling. Accordingly, the methods and structures facilitate proper location of apertures and attachment members of the structural member in a cost-effective and reliable manner. 
     While embodiments of the disclosure have been described in terms of various specific embodiments, those skilled in the art will recognize that the embodiments of the disclosure can be practiced with modification within the spirit and scope of the claims.