Patent Publication Number: US-10325413-B2

Title: Generating smooth optimized parts

Description:
BACKGROUND 
     Additive manufacturing (AM) is increasingly used to manufacture workpieces for many applications. AM is particularly useful in aerospace production, as it enables designs with complex geometries but without additional cost. Workpieces designed to be additively manufactured can be optimized to enhance performance by departing from the simple geometric designs that can economically be produced through conventional manufacturing techniques (e.g., casting and subtractive machining). One class of optimization methods is topology optimization (TO). TO allows a workpiece shape to be iteratively optimized via finite element analysis using specified (i.e. desired) optimization objectives and boundary conditions. TO can, for example, be used to generate workpiece design shapes for optimal fluid flow, structural stiffness, load capacity, and other design goals. 
     TO processes typically produce output surface designs in the form of stereolithography (STL) files that commonly exhibit undesirable artifacts. In particular, STL outputs from TO processes are rarely smooth, and sometimes include unwanted voids or tabs of additional material. Before the STL output of a TO process is usable, it must ordinarily be edited or entirely recreated to remove these artifacts. This clean-up process is conventionally a manual and lengthy one. 
     SUMMARY 
     A method of generating a smooth optimized part design for a workpiece is presented. Topology optimization is performed based on design objectives, to generate surface data describing an optimized but unfinished surface of the workpiece. The surface data is used to generate volumetric data describing the workpiece structure. A three dimensional smoothing filter is applied to the volumetric data. A manufacturing design is generated from the resulting smoothed volumetric data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating a method of generating smooth optimized part designs. 
         FIG. 2  is a flowchart illustrating an expansion of the method of  FIG. 1 , for use with flow optimized surfaces. 
         FIG. 3  is a flowchart illustrating an alternative to the method of  FIG. 2 . 
     
    
    
     While the above-identified figures set forth one or more embodiments of the present disclosure, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features and components not specifically shown in the drawings. 
     DETAILED DESCRIPTION 
     The present disclosure concerns methods of artifact removal from workpiece designs generated via topology optimization (TO). In particular, the following methods remove artifacts from TO output surfaces by converting those surfaces into voxel data describing the surfaced workpiece volumetrically, applying smoothing filters to that voxel data, and fitting a surface to the resulting volumetric design. This process smoothes the TO output design, and allows other artifacts to be removed (manually or automatically) with greater ease. 
       FIG. 1  is a flowchart of method  100 , a design method for generating smooth optimized part designs for a workpiece. First, an output surface is generated via TO. (Step  102 ). Generally, any TO-optimized workpiece surface can be used. TO can be used to optimize for any applicable design objective, including objectives of the workpiece itself (e.g., structural strength, rigidity, mass) and objectives determined or affected by the workpiece, but external to it (e.g., pressure loss or temperature change through a flow passage). This TO-generated output surface is described via a data set such as a stereolithographic (STL) surface contour. This surface data describes an optimized but unfinished surface. 
     Surface data S TO  generated via TO is used to generate volumetric data V U  describing the (still unfinished) workpiece design. (Step  104 ). This data can, for example, consist of a voxel-based volumetric data set wherein each voxel is assigned a density from 0 to 1, with 0 corresponding to empty space and 1 to solid workpiece. More generally, and more usefully in complex cases, the voxel-based volumetric data set can assign each voxel to a particular phase, e.g., to a particular part, or to the environment. In at least some embodiments the generation of volumetric data begins with distinguishing interior from exterior volume of the workpiece using the unfinished surface as a delineating tool. For a monolithic workpiece, voxels within the interior of the workpiece are assigned to the workpiece, while voxels exterior to the workpiece are assigned to the environment. In some embodiments of the present invention, for example, voxels along a boundary defined by the unfinished surface can, for example, be given fractional assignments (e.g., 60% environment, 40% workpiece) although this is not required. More generally, the above process can be used not only to distinguish between workpiece and non-workpiece material, but to distinguish between multiple workpiece parts designed together to operate as a part of the same structure. The resulting unfinished volumetric data set V U  can, for example, include a voxelized three-dimensional (3D) map of multiple interrelated parts. In an illustrative example having three distinct parts, each voxel has four assignment weightings: one environmental, and one for each of the three parts. These four assignments sum to 1. 
     The volumetric extrapolation of the TO output surface generated in step  104  can exhibit undesirable artifacts, including unwanted roughness or irregularity, voids, and extraneous material. At least some of these artifacts can, in some embodiments, be removed by editing unfinished volumetric data V U  directly. (Step  106 ). This editing can include manual addition or subtraction of material (i.e. changing voxel assignment to part or environment, respectively) to remove voids or cavities (whether fully enclosed or exposed to the environment), or extraneous material, or to thicken or thin portions for structural purposes. In other embodiments this process can be automated or partly automated (e.g., with operator supervision) using computer learning subroutines and/or algorithms. Computer learning algorithms can also be used to recognize and flag structures with simple analytic geometries, such as flat planes, regular polygons, and cylindrical elements. Such analytic geometries are often necessary at boundary locations such as bolt holes and mating surfaces between adjacent parts. Accordingly, voxel manipulation in step  106  can include flagging voxels encompassed by such analytic structures to prevent their adjustment by loss through subsequent smoothing processes. A structure intended to include a bolt collar, for example, could be analyzed to recognize and preserve structures resembling analytic cylinders, through subsequent smoothing. Even if some or all corrections made in step  106  are performed by a human operator, method  100  enables some labor savings by allowing these manual edits to be relatively rough, due to subsequent smoothing (Step  108 ; see below). In some embodiments, voxel manipulation at step  106  can include the addition of sacrificial voxels (by assigning extra voxels to the workpiece) that are removed (i.e. reassigned to the environment) after smoothing, so as to form a sharp-edged final structure. 
     Voxel-based smoothing is accomplished by applying three-dimensional smoothing filters to each voxel within or surrounding the workpiece (Step  108 ). Smoothing filters adjust assignment of each voxel based on assignments of adjacent or nearby voxels. Filter operations can include Gaussian filters, mean or median value filters, max or min filters, Lanczos filters, Bilateral filters, anisotropic diffusion filters, curvature anisotropic diffusion filters, gradient anisotropic diffusion, curvature flow filters, min/max curvature flow, non-local means filters, box filters, majority filters, recursive exponential filters, SNN filters, sigma filters, Nagao filters, despeckle filters, and other arithmetic smoothing filters or combinations of filters. 
     Smoothing can require several successive iterations of filtering. In many embodiments, different three-dimensional smoothing filters can be applied in successive filtering steps. This filtering tends to reduce surface irregularity on the volumetrically-described workpiece, as defined by a resulting finished volumetric data set V F . As noted above, some analytic structures to be preserved or inserted into the volumetric data set (e.g., bolt holes, mating surfaces) can be exempted from smoothing. 
     Some filters do not preserve volume, allowing total workpiece volume to be lost during the smoothing process. To accommodate such cases, the present method can include a further step of dilating a part to preserve volume. (Step  109 ). Dilation is accomplished by adding a small number of voxels (e.g., 1 to 3) surrounding the voxelized (volumetric) part model, i.e. by flagging those immediately surrounding voxels as belonging to the workpiece. This addition of volume to the workpiece offsets volume lost to smoothing. Although step  109  is illustrated in  FIG. 1  as following smoothing step  108  and preceding step  110 , step  109  can in some cases be performed prior to filtering step  108 . 
     After voxel-based smoothing and any necessary dilation to preserve volume are complete, a smooth surface S S  is fitted to finished volumetric data set V F . (Step  110 ). Smooth surface S S  describes a boundary between the workpiece and its environment, and/or between separate regions of the workpiece. Smooth surface S S  is then exported in a manufacturing design format acceptable as input to additive manufacturing apparatus or other downstream design process. (Step  112 ). This format can, for example, be a surface computer aided design (CAD) format accepted by manufacturing apparatus or CAD software. In some embodiments, however, this format can be volumetric, and step  110  can accordingly be omitted. Post-processing can be performed on the resulting manufacturing-format design, as needed. (Step  114 ). In some embodiments, method  100  can enforce symmetry in a designed workpiece by a defining a plane of symmetry, deleting all design located on a first side of the plane, and mirroring the design on the opposite side of the plane across to the first side. (Step  116 ). Alternatively, symmetry can be enforced via a Boolean union of each side with a reflection of the opposite side. In general, any seams in surface contour resulting from symmetry enforcement can be removed by manual editing, through ordinary filtering, or through the combination of normal filtering (as described above) with additional localized filtering in the vicinity of the plane of symmetry. Although step  116  is shown following design post-processing step  114 , some embodiments of the present method can enforce symmetry earlier in the design process, e.g., together with step  106 , between steps  104  and  106 , or after steps  108  or  110 . 
       FIG. 2  is a flowchart of method  200 , a design method similar to method  100 . Method  200  is used to smooth flow-interacting structures such as ducts or channels. Like method  100 , method  200  begins with the generation of an output surface via TO. (Step  202 ). In method  200 , this TO specifically optimizes for flow objectives (e.g., flow speed, pressure drop), and generates a flow surface describing a fluid flow boundary. Steps  104 ,  106 ,  108 ,  112 , and  114  are substantially as described above with respect to method  100 . Steps  104 ,  106 , and  108  together generate a volumetric flowfield describing the flow volume of the flow-interacting structure. In some embodiments, symmetry can be enforced as discussed above with respect to  FIG. 1 . After the three-dimensional smoothing filters are applied (step  108 ), this smoothed flow field is dilated by the addition of extra voxels to the workpiece (i.e. by assignment of extra voxels to the workpiece) in a region immediately surrounding the flow field. (Step  204 ). This dilation produces a dilated or “thickened” flow field. Dilation can, for example, be accomplished by reassigning environmental voxels to the workpiece if they are sufficiently close to workpiece voxels. This dilated flow field can optionally be dilated differently in different regions of workpiece, or edited after dilation, such that the thickness of the dilation is varied for structural strength or weight savings. (Step  206 ). Surfaces are then fit to both the dilated and undilated (primary) flow fields, substantially as set forth above with respect to step  110  of method  100 . (Step  208 ). The surface fitted to the dilated flow field corresponds to an outer surface of the resulting duct or passage structure, while the surface fitted to the undilated flow field corresponds to an internal flow-facing surface of the structure. These surfaces are exported to a design format as described above. (Step  112 ). The resulting workpiece can, for the example of solid modeling, be defined by Boolean subtraction of the solid defined by the inner surface from the solid defined by the outer surface. (Step  210 ). Alternatively, in the case of surface-based CAD modeling, a CAD model can be constructed in which the inner and outer surface both form exterior walls of the duct or passage structure. Post-processing can be done as in method  100 , e.g., to remove sacrificial design material or add preset features such as mating flanges and fastener receivers. (Step  114 ) 
       FIG. 3  is a flowchart of method  300 , an alternative method to method  200 . Like method  200 , method  300  is used to smooth flow-interacting structures such as ducts or channels. Method  300  differs from method  200  primarily in that Boolean subtraction of the undilated flowfield from the dilated flowfield is performed in volumetric data, and immediately follows the structural editing of step  206 . (Step  302 ). Rather than generating inner and outer smoothed surfaces from separate sets of finished volumetric data V F  corresponding to the dilated and undilated flowfields as in method  200 , subtraction step  302  generates a single volumetric data set V F  of the workpiece as a hollow, smoothed structure from which all smoothed surfaces S S  are generated (Step  304 ). 
     In at least some embodiments, subsequent TO of the non-flow-facing portions for structural parameters (e.g., rigidity, strength) can be performed on the resulting hollow structure, while holding the flow-facing surface fixed to preserve desired flow characteristics of the structure as a whole. (Step  306 ). This optimization process can include a second iteration of smoothing of the non-flow-facing portion, substantially as described above with respect to method  100 . Once both (or all) portions of the workpiece have been optimized for appropriate flow and structural parameters and smoothed, the resulting geometry is exported in a CAD or otherwise manufacturing-friendly format (Step  112 ), and post-processed as necessary (Step  114 ). 
     Although the aforementioned method contemplates optimizing portions for flow characteristics and structural characteristics sequentially, any of the above methods can be modified to integrate surfaces generated in parallel via different TO processes to form a single unfinished volumetric data set V U  that is smoothed via three-dimensional filtering, as described above. In general, the present invention encompasses the use of three-dimensional volumetric smoothing filters with a volumetric data set generated from any number of topology-optimized surfaces. These methods reduce or eliminate manual artifact correction necessary to transform surfaces created via TO processes into workable part designs. 
     DISCUSSION OF POSSIBLE EMBODIMENTS 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A method of generating a smooth optimized part design for a workpiece, the method comprising: performing topology optimization based on design objectives and constraints, to generate surface data describing an optimized but unfinished surface; generating volumetric data describing the workpiece structure based on the surface data; applying a three dimensional smoothing filter to the volumetric data; and generating a manufacturing design from the smoothed volumetric data. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A further embodiment of the foregoing method, wherein the surface data is stereolithographic data. 
     A further embodiment of the foregoing method, wherein the volumetric data comprises a voxel representation of a workpiece structure delineated by the unfinished surface. 
     A further embodiment of the foregoing method, wherein generating the volumetric data comprises: differentiating interior from exterior volume of the workpiece, as delineated by the unfinished surface; and assigning voxels of the interior volume to the workpiece. 
     A further embodiment of the foregoing method, wherein the workpiece comprises multiple separate regions, the wherein the method further comprises: differentiating volumes of the separate regions and assigning voxels to those volumes, separately. 
     A further embodiment of the foregoing method, wherein voxel assignment to each volume can be fractional. 
     A further embodiment of the foregoing method, wherein the smoothing filter includes at least one filter step from a set consisting of: Gaussian filtering, mean filtering, median filtering, max or min filtering, Lanczos filtering, Bilateral filtering, anisotropic diffusion filtering, curvature anisotropic diffusion filtering, gradient anisotropic diffusion, curvature flow filtering, min/max curvature flow, non-local means filtering, box filtering, majority filtering, recursive exponential filtering, SNN filtering, sigma filtering, Nagao filtering, and despeckle filtering. 
     A further embodiment of the foregoing method, wherein the smoothing filter includes a plurality of non-identical iterative filtering steps. 
     A further embodiment of the foregoing method, further comprising editing the volumetric data to add, edit, or remove material by assigning voxels to the workpiece, to a region, or to environment, prior to smoothing. 
     A further embodiment of the foregoing method, wherein editing the volumetric data comprises assigning voxels to the workpiece so as to fill unwanted voids or cavities. 
     A further embodiment of the foregoing method, wherein editing the volumetric data comprises assigning voxels to the workpiece so as to add sacrificial voxels, and wherein the method further comprises: assigning the sacrificial voxels to environment after applying the three-dimensional smoothing filter. 
     A further embodiment of the foregoing method, further comprising defining analytic structures in the volumetric data, wherein applying the smoothing filter does not alter the analytic structures. 
     A further embodiment of the foregoing method, wherein defining the analytic structures comprises recognizing and flagging structures with simple analytic geometries. 
     A further embodiment of the foregoing method, wherein generating a manufacturing design from the smoothed volumetric data comprises: fitting a part surface to the smoothed volumetric data; and exporting the part surface in a manufacturing or computer aided design format. 
     A further embodiment of the foregoing method, wherein the design objectives include flow objectives, and the unfinished surface is a flow surface. 
     A further embodiment of the foregoing method, further comprising: dilating a flowfield described by the volumetric data; and subtracting the undilated flowfield from the dilated flowfield to form a hollow shell. 
     A further embodiment of the foregoing method, further comprising adjusting a surface defined by the dilated flowfield through topology optimization based on structural design objectives and constraints. 
     A further embodiment of the foregoing method, further comprising enforcing symmetry of the volumetric data, before applying the smoothing filter. 
     A further embodiment of the foregoing method, wherein the surface data describes at least a second optimized but unfinished surface, and wherein each optimized but unfinished surface is generated using different design objectives and constraints. 
     A further embodiment of the foregoing method, wherein the different design objectives and constraints include both structural and flow objectives and constraints. 
     SUMMATION 
     Any relative terms or terms of degree used herein, such as “substantially”, “essentially”, “generally”, “approximately” and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, alignment or shape variations induced by thermal, rotational or vibrational operational conditions, and the like. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.