Abstract:
Methods and systems for numerically predicting surface imperfections on stamped sheet metal parts are disclosed. FEM mesh includes a plurality of shell elements and a plurality of nodes that represents a stamped sheet metal part. At least one surface of the part needs to be examined for imperfection, which can be used for adjusting the die for forming the sheet metal part. Each surface is created by fitting all of nodes of a portion of the FEM mesh in a group-to-group scheme. A group is defined to include a center element and its neighbors. Neighbor elements share a side with the center element are always included in the group. Each group includes at least three neighbors in additional to the center element. Node-sharing elements are added into the group such that the criterion of at least three neighbors is met.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to numerical simulation of stamped metal parts, more particularly to methods and systems for numerically predicting surface imperfections of an engineering part (e.g., components of car, airplane, etc.) created by sheet metal stamping. 
     BACKGROUND OF THE INVENTION 
     Computer aided engineering (CAE) has been used for supporting engineers in many tasks. For example, in a structure or product design procedure, CAE analysis, in particular finite element analysis (FEA) or finite element method (FEM), has often been employed to evaluate responses (e.g., stresses, displacements, etc.) under various loading conditions (e.g., static or dynamic). 
     FEA is a computerized method widely used in industry to simulate (i.e., model and solve) engineering problems relating to complex products or systems (e.g., cars, airplanes, etc.) such as three-dimensional non-linear structural design and analysis. FEA derives its name from the manner in which the geometry of the object under consideration is specified. The geometry is defined by elements and nodes referred to as FEM mesh. There are many types of elements, solid elements for volumes or continua, shell or plate elements for surfaces and beam or truss elements for one-dimensional structure objects. 
     To numerically simulate sheet metal forming or stamping process, a FEM mesh of shell elements is used for representing sheet metal (or blank) at the outset of a stamping operation. Blank is then formed or stamped into a part by hydraulically or mechanically pressing a punch through it into a die. After the part is formed including springback, certain areas of the part&#39;s surface may have imperfections (also referred to as surface low). A stoning operation is commonly performed to detect such imperfections. Manufacturers of the parts would like to eliminate such imperfections by altering the die. Physically adjusting or modifying a die is also expensive both in terms of time and money. 
     It would, therefore, be desirable to have method and system that can be used for predicting surface imperfections numerically such that expensive die modifications can be minimized. 
     BRIEF SUMMARY OF THE INVENTION 
     This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions in this section as well as in the abstract and the title herein may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the present invention. 
     Methods and systems for numerically predicting surface imperfections in stamped sheet metal parts are disclosed. According to one aspect of the present invention, a FEM mesh is configured for representing a stamped sheet metal part. The FEM mesh includes a plurality of shell elements and a plurality of nodes. At least one surface of the part needs to be examined for imperfections, which can be used for adjusting the die for forming the sheet metal part. Each surface is created by fitting all of nodes of a portion of the FEM mesh in a group-to-group scheme. A group is defined to include a center element and its neighbors. Neighbor elements sharing a side with the center element are always included in the group. Each group includes at least three neighbors in additional to the center element. When there are less than three side-sharing neighbors in the group, node-sharing elements are added into the group such that the criterion of at least three neighbors is met. 
     Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings as follows: 
         FIG. 1A  is a two-dimensional view of an exemplary FEM mesh representing a surface of a stamped sheet metal part in accordance with one embodiment of the present invention; 
         FIG. 1B  is a diagram showing a cross-section of the FEM mesh of  FIG. 1A ; 
         FIG. 1C  is a two-dimensional view of the FEM mesh of  FIG. 1A  along with a stoning direction for checking surface imperfection; 
         FIG. 1D  shows a cross-section of the surface without imperfection in the stoning direction of  FIG. 1C ; 
         FIG. 1E  shows a cross-section of the surface with imperfection in the stoning direction of  FIG. 1C ; 
         FIGS. 2A-2E  are diagrams showing exemplary groups of neighbor elements used for creating a surface in accordance with one embodiment of the present invention; 
         FIG. 3  is a flowchart illustrating an exemplary process of predicting surface imperfection of a stamped sheet metal part in accordance with one embodiment of the present invention; and 
         FIG. 4  is a function diagram showing salient components of a computing device, in which an embodiment of the present invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will become obvious to those skilled in the art that the present invention may be practiced without these specific details. The descriptions and representations herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, and components have not been described in detail to avoid unnecessarily obscuring aspects of the present invention. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the order of blocks in process flowcharts or diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention. 
     Embodiments of the present invention are discussed herein with reference to  FIGS. 1A-4 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments. 
     Referring first to  FIG. 1A , it is shown an exemplary FEM mesh  100  representing a portion of a stamped sheet metal part (e.g., door panel of a car). The FEM mesh  100  comprises a number of shell elements  101 . Since the stamped sheet metal part is generally a non-flat surface, a cross-section  120  of the FEM mesh  100  is a curve. However, each shell element  101  in the FEM mesh  100  has a flat surface, so the cross-section  120  shown in  FIG. 1B  is not a smooth curve. The cross-section  120  contains multiple straight-line segments  111   a - 111   f  between points  110   a - 110   g , which are the intersections at the element&#39;s edge. FEM results are calculated at nodal points of elements (e.g., nodes  101   a - 101   d  for element  101 ). In order to ensure that the cross-section  120  represents a smooth surface of the stamped sheet metal part, a smooth curve  122  is uniquely created from all of the nodal points  110   a - 110   g . All nodal points  110   a - 110   g  are included in the smooth curve  122 . 
     Infinite numbers of cross-sections can be cut through the FEM mesh  100 . A particular direction  130  is shown as a slant line over the FEM mesh  100 . The particular direction  130 , shown in  FIG. 1C , is of interest, because it is the direction to check whether any surface imperfection exists. The direction  130  is the stoning direction in a metal stamping procedure. To use FEM mesh to numerically predict any surface imperfection, element edge intersections  132   a - 132   n  along direction  130  are used for creating a unique smooth curve (not shown but similar to curve  122 ). The unique smooth curve is then used to determine whether there is any surface imperfection. 
     To uniquely create a smooth curve from a plurality of element edge intersection points  132   a - 132   n , a systemic approach based on at least three neighboring elements is used. For example, points  132   a  and  132   b  intersect edges of element  141 . To create a smooth surface, three neighboring elements  142 ,  152  and  153  are included. Selections of neighboring elements are graphically shown in  FIGS. 2A-2E . In other words, nodal points of elements  141 ,  142 ,  152  and  153  are used to create a surface (shown in  FIG. 2C ). Many well known procedures can be used to accomplish the curve fitting procedure, for example, B-spline, smooth patch, etc. 
     For element  142 , the neighboring elements are  141 ,  143  and  153  as graphically shown in  FIG. 2B . For element  153 ,  FIG. 2A  shows a scheme depicting how neighbors  142 ,  163 ,  152  and  154  are selected. 
     An exemplary smooth surface  172  without any surface imperfection is shown in  FIG. 1D . In other words, a curved or rounded outward surface  172  is referred to as a convexed surface.  FIG. 1E  shows an exemplary concaved surface  182  with surface imperfection. Surface imperfection  184  is exaggerated for illustration. 
     For illustration simplicity, the FEM mesh  100  is shown with same sized quadrilateral elements neatly arranged in rows and columns. In reality, elements can be irregular in terms of their sizes and/or orientations and/or types (e.g., triangular elements). The present invention sets no limit as to what type, shape and/or orientation of elements in the FEM mesh. 
     Whereas the FEM mesh  100  contains only quadrilateral elements, the present invention allows other types of shell elements (e.g., triangular elements). The neighbor selection schemes for triangular elements are shown in  FIGS. 2D-2E . 
       FIG. 3  is a flowchart illustrating an exemplary process  300  of numerically predicting surface imperfection of a stamped sheet metal part using a FEM mesh in accordance with one embodiment of the present invention. Process  300  is preferably implemented in software. 
     Process  300  starts by receiving a FEM mesh model in a computer system at step  302 . The FEM model represents a stamped sheet metal part. The model includes a plurality of nodes and a plurality of shell elements (e.g., quadrilateral and/or triangular elements). Next, at step  304 , a group of neighboring elements is defined for each element (referred to as master or center element) in the FEM mesh model. In the initial definition, the group includes all side-sharing or edgy-sharing elements. In other words, each of the neighbors in the group shares one side or edge with the master element at this step. Then process  300  moves to decision  306  to check whether there are at least three neighbors in the group. If not, process  300  moves to step  308  to add one or more node-sharing neighboring elements into the groups with less than three neighbors. Otherwise, process  300  follows the “yes” branch of decision  306  to step  310  to create a smooth surface that encompasses all nodes of the elements in each group. The smooth surface is created in a group-to-group scheme. Adjustments are made between groups to ensure there is continuous curvature between groups. Once the smooth surface has been constructed or created, process  300  uses a cross-section in one or more directions of stoning to determine whether there is any surface imperfection at step  312 . A surface imperfection or surface low can be defined as a concaved cross-section (e.g., a surface low  184  shown in  FIG. 1E ). 
     According to one aspect, the present invention is directed towards one or more computer systems capable of carrying out the functionality described herein. An example of a computer system  400  is shown in  FIG. 4 . The computer system  400  includes one or more processors, such as processor  404 . The processor  404  is connected to a computer system internal communication bus  402 . Various software embodiments are described in terms of this exemplary computer system. After reading this description, it will become apparent to a person skilled in the relevant art(s) how to implement the invention using other computer systems and/or computer architectures. 
     Computer system  400  also includes a main memory  408 , preferably random access memory (RAM), and may also include a secondary memory  410 . The secondary memory  410  may include, for example, one or more hard disk drives  412  and/or one or more removable storage drives  414 , representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive  414  reads from and/or writes to a removable storage unit  418  in a well-known manner. Removable storage unit  418 , represents a floppy disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive  414 . As will be appreciated, the removable storage unit  418  includes a computer readable medium having stored therein computer software and/or data. 
     In alternative embodiments, secondary memory  410  may include other similar means for allowing computer programs or other instructions to be loaded into computer system  400 . Such means may include, for example, a removable storage unit  422  and an interface  420 . Examples of such may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an Erasable Programmable Read-Only Memory (EPROM), Universal Serial Bus (USB) flash memory, or PROM) and associated socket, and other removable storage units  422  and interfaces  420  which allow software and data to be transferred from the removable storage unit  422  to computer system  400 . In general, Computer system  400  is controlled and coordinated by operating system (OS) software, which performs tasks such as process scheduling, memory management, networking and I/O services. 
     There may also be a communications interface  424  connecting to the bus  402 . Communications interface  424  allows software and data to be transferred between computer system  400  and external devices. Examples of communications interface  424  may include a modem, a network interface (such as an Ethernet card), a communications port, a Personal Computer Memory Card International Association (PCMCIA) slot and card, etc. 
     The computer  400  communicates with other computing devices over a data network based on a special set of rules (i.e., a protocol). One of the common protocols is TCP/IP (Transmission Control Protocol/Internet Protocol) commonly used in the Internet. In general, the communication interface  424  manages the assembling of a data file into smaller packets that are transmitted over the data network or reassembles received packets into the original data file. In addition, the communication interface  424  handles the address part of each packet so that it gets to the right destination or intercepts packets destined for the computer  400 . 
     In this document, the terms “computer recordable storage medium”, “computer recordable medium” and “computer readable medium” are used to generally refer to media such as removable storage drive  414 , and/or a hard disk installed in hard disk drive  412 . These computer program products are means for providing software to computer system  400 . The invention is directed to such computer program products. 
     The computer system  400  may also include an input/output (I/O) interface  430 , which provides the computer system  400  to access monitor, keyboard, mouse, printer, scanner, plotter, and alike. 
     Computer programs (also called computer control logic) are stored as application modules  406  in main memory  408  and/or secondary memory  410 . Computer programs may also be received via communications interface  424 . Such computer programs, when executed, enable the computer system  400  to perform the features of the present invention as discussed herein. In particular, the computer programs, when executed, enable the processor  404  to perform features of the present invention. Accordingly, such computer programs represent controllers of the computer system  400 . 
     In an embodiment where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system  400  using removable storage drive  414 , hard drive  412 , or communications interface  424 . The application module  406 , when executed by the processor  404 , causes the processor  404  to perform the functions of the invention as described herein. 
     The main memory  408  may be loaded with one or more application modules  406  that can be executed by one or more processors  404  with or without a user input through the I/O interface  430  to achieve desired tasks. In operation, when at least one processor  404  executes one of the application modules  406 , the results are computed and stored in the secondary memory  410  (i.e., hard disk drive  412 ). The status of the time-marching engineering simulation (e.g., deformed beam element, deformed surface, and their relative position, etc.) is reported to the user via the I/O interface  430  either in a text or in a graphical representation. 
     Although the present invention has been described with reference to specific embodiments thereof, these embodiments are merely illustrative, and not restrictive of, the present invention. Various modifications or changes to the specifically disclosed exemplary embodiments will be suggested to persons skilled in the art. For example, whereas the global searching schemes are described and shown using bucket sorting algorithm, other equivalent techniques can be used to accomplish the same. Additionally, whereas the surface mesh has been shown to be a finite element mesh with a circular shape, other types of surface mesh can be used. Furthermore, the surface mesh has been shown in a two-dimensional for illustration simplicity, the surface mesh can be a surface of an arbitrary shaped three-dimensional object. In summary, the scope of the invention should not be restricted to the specific exemplary embodiments disclosed herein, and all modifications that are readily suggested to those of ordinary skill in the art should be included within the spirit and purview of this application and scope of the appended claims.