The U.S. Air Force and DRS Technologies, a supplier of defense electronic systems, commissioned the University of Oklahoma to study developments in scanners in reverse engineering projects. The authors examined several 3D scanning software packages. Here are excerpts from their report.
By Kuang-Hua Chang and Chienchih Chen
Univeristy of Oklahoma
The U.S. Air Force and DRS Technologies, a supplier of defense electronic systems, commissioned the University of Oklahoma to study developments in scanners in reverse engineering projects. The authors examined several 3D scanning software packages. Here are excerpts from their report.
Today’s 3D scanners are more portable, affordable; and capture points faster and more accurately. Many hand-held laser scanners can capture tens of thousands points per second with a level of accuracy around 40 μm. You can find such scanners for about $50,000. Only a few geometric modeling software tools, though, can convert all of those points into useful models.
Geomagic, Rapidform, PolyWorks, and SolidWorks/Scan to 3D, are among the programs with auto-surfacing technology that automatically converts point clouds into NURBS surface models. But NURBS surface models contain only surface patches without the additional semantics and topology inherent in feature-based parametric representation. Therefore, they are not suitable for design changes, feature-based NC tool-path generation, and technical data package preparation. If an application will experience design changes, (and which application does not?), then software must handle parametric solid modeling. Unfortunately, parametric solid modeling may never be fully automated because no one, yet, has discovered how to automatically or interactively extract the original design intent from the data points.
Noted the authors, the ideal scenario is having software tools that automatically take care of labor-intensive tasks—such as managing point cloud and triangulation—and offer capabilities that allow designers to recover design intents interactively.
Once a design has been parameterized, the next steps are to undergo shape engineering and parametric solid modeling. Shape engineering involves four main phases:
(1) triangulation that converts data points to polygon mesh,
(2) mesh segmentation that separates polygon mesh into regions based on the characteristics of the surface geometry they respectively represent,
(3) solid modeling that converts segmented regions into parametric solid models,
(4) model translation that exports solid models constructed to mainstream CAD systems.
Ideally, the entire process is fully automated; except for Phase 3, which requires designer’s interaction mainly to recover original design intents.
Mesh segmentation is one of the most important steps. It groups the original data points or mesh into subsets, each of which logically belongs to a single primitive surface.
Solid modeling is probably the least developed in the overall shape engineering process. Boundary representation (B-rep) and feature-based are the two basic representations for solid models. Some proposed methods automatically construct B-rep models from point clouds or triangular mesh. Some focus on manufacturing feature recognition for process planning purpose. One promising development in recent years was the geometric feature recognition (GFR), which automatically recognizes solid features embedded in B-rep models. However, none of the methods fully automates the construction process and generates fully parametric solid models.
One of the most successful algorithms for geometric feature recognition has been proposed by Venkataraman. The algorithm uses a simple four step process: (1) simplify imported faces, (2) analyze faces for specific feature geometry, (3) remove recognized feature and update model, and (4) return to Step 2 until all features are recognized. Once all possible features are recognized, they are mapped to a new solid model of the part, which is parametric with a feature tree that defines the feature regeneration (or model rebuild) sequence.
Venkataraman’s method was recently commercialized by Geometric Software Solutions, Ltd., and implemented in a number of CAD packages, including SolidWorks and CATIA. It recognizes basic features, such as extrude, revolve, and more recently, sweep. This capability primarily supports solid model translations between CAD packages in which not only geometric entities (as has been done by IGES Initial Graphics Exchange Standards) but also parametric features are translated.
One of the major issues in commercial GFR software is design intent recovery. Current GFR implementations are not flexible. Without adequate user interaction, the single sketch of a flange of an airline tube may be recognized as four or more separate features. While the final solid parts are physically the same, their defining parameters are not. Such a batch mode implementation may not be desired in recovering meaningful design intents.
A feature-based parametric solid model consists of two key elements: a feature tree, and fully parameterized sketches used for protruding solid features. A fully parameterized sketch implies that the sketch profile is fully constrained and dimensioned, so that a change in dimension value yields a rebuilt as anticipated with design intents. To the authors’ knowledge, there is no such method proposed or offered that fully automates the process. Some capabilities are offered by commercial tools, such as Rapidform, that support designers to interactively create fully parameterized sketches, which accurately conform to the data points and greatly facilitate the solid modeling effort.
Since most of the promising shape-engineering capabilities are not offered in CAD packages, the solid models constructed in reverse engineering software will have to be exported to mainstream CAD packages. The conventional solid model translation through standards, such as IGES or STEP AP, are inadequate since parametric information, including solid features, feature tree, sketch constraints and dimensions, are completely missing in the translation.
Although feature recognition capability offers some relief in recognizing geometric features embedded in B-rep models, it is still an additional step that is often labor intensive. Direct solid model translations have been offered in some software, such as liveTransfer™ module of Rapidform XOR3 and third party software, such as TransMagic.
The most useful and advanced shape engineering capabilities are offered in specialized, non-CAD software, such as Geomagic, and Rapidform, that are intended to support reverse engineering. Some CAD packages, such as SolidWorks, Pro/ENGINEER Wildfire, and CATIA, offer limited capabilities for shape engineering. In general, capabilities offered in CAD are labor intensive and inferior to specialized codes while dealing with shape engineering.
After intensive review and survey, to the authors’ knowledge, the best software on the market for reverse engineering is Geomagic Studio v.11 and Rapidform XOR3. This was determined after an intensive study, following a set of prescribed criteria including auto-surfacing, parametric solid modeling, and software usability. Between the two, Geomagic has a slight edge in geometric entity editing, which is critical for auto-surfacing (construction of NURBS surface models). In terms of solid modeling, Geomagic stops short at only offering primitive surfaces, such as plane, cylinder, and sphere from segmented regions.
Rapidform is excellent in auto-surfacing and superior in support of solid modeling that goes beyond primitive surface fitting. Rapidform offers convenient sketching capabilities that support feature-based modeling. As a result, it often requires less effort yet yields a better solid model by interactively recovering solid features embedded in the segmented regions.
The interactive approach mainly involves creating or extracting section profiles or guide curves from polygon mesh, and following CAD-like steps to create solid features; for example, sweep a section profile along guide curves for a sweep solid feature.
Test Examples
Geomagic automatically recognizes primitive surfaces from segmented regions. If a primitive surface is misrecognized or unrecognizable, you can interactively choose the segmented region and assign a correct primitive type. Often, this interactive approach leads to a solid model with all bounding surfaces recognized. Unfortunately, there is no feature tree and no CAD-like capabilities in Geomagic. You will not be able to see any sketch or dimensions in Geomagic Studio v.11. Therefore, you will not be able to edit or add any dimensions or constraints to parameterize the sketch profiles.
Section sketches only become available after exporting the solid model to a selected CAD package supported by Geomagic. Primitive surfaces in most regions are recognized correctly. However, there are some regions incorrectly recognized; for example, the hole in the middle of the block was recognized as a free-form primitive, instead of a cylinder. There are also regions that remained unrecognized, such as the middle slot surface.
Although most primitives are recognized in Geomagic, there are still issues to address. One of them is misrepresented profile. The sketch profile will have to be carefully inspected to make necessary corrections, as well as adding dimensions and constraints to parameterize the profile.
Unfortunately, such inspections cannot be carried out unless the solid model is exported to supported CAD systems. Lack of CAD-like capability severely restricts the usability of the solid models in Geomagic, let alone the insufficient ability for primitive surface recognition.
For parametric solid modeling, Rapidform offers excellent CAD-like capabilities, including feature tree. These capabilities let you create solid models and make design changes directly in Rapidform. For example, you will be able to create a sketch profile by intersecting a plane with polygon mesh and extrude the sketch profile to match the bounding polygon mesh for a solid feature. With the feature tree, you can always roll back to previous entities and edit dimensions or redefine section profiles. These capabilities make Rapidform particularly suitable for parametric solid modeling.
Rapidform offers two methods for solid modeling, Sketch and Wizard, which offer fast and easy primitive recognition from segmented mesh. The major drawback of the Wizard is that some guide curves and profile sketches generated are non-planar spline curves that cannot be parameterized. You can use either or both methods to generate solid features in a single part.
Method 1: Sketch
In general, there are six steps employed in using the sketch method, (1) creating reference sketch plane, (2) extracting sketch profile by intersecting the sketch plane with the polygon mesh, (3) converting extracted geometric entities (usually as planar spline curves) into standard line entities, such as arcs and straight lines, (4) parameterizing the sketch by adding dimensions and constraints, (5) extruding, revolving, or lofting the sketches to create solid features; and (6) employing Boolean operations to union, subtract, or intersect features if necessary.
Rapidform has an Auto Sketch capability that automatically converts extracted spline curves into lines, circles, arcs, and rectangles, with some constraints added. Most constraints and dimensions will have to be added interactively to fully parameterize the sketch profile. Steps 4 to Step 6 are similar to conventional CAD operations. With these capabilities, you can efficiently create fully constrained parametric solid models.
For the block example, a plane that is parallel to the top (or bottom) face of the base block was created first by clicking more than three points on the surface. The plane is offset vertically to ensure a proper intersection between the sketch plane and the polygon mesh. The geometric entities obtained from the intersection are planar spline curves. The Auto Sketch capability can extract a set of
standard CAD-like line entities to best fit the spline curves. These standard line entities can be joined and parameterized by manually adding dimensions and constraints for a fully parameterized section profile. Once the sketch profile is parameterized, it can be extruded to generate an extrusion feature for the base block. Boolean operations can be used to union, subtract, or intersect solid features for a fully parameterized solid model. The final solid model is analyzed by using Accuracy Analyzer. The solid model generated is extremely accurate, where geometric error measured in average and standard deviation is 0.0002 and 0.0017 in., respectively between the solid model and point cloud.
Method 2: Wizard
Wizard, or Modeling Wizard, automatically extracts Wizard features such as extrude, revolve, pipe, and loft, and so on, to create solid models from segmented regions. There are five Wizard features: extrusion, revolution for extracting solid features; and sweep, loft, and pipe for surface features. There are three general steps to extract features using Wizard, (1) select mesh segments to generate individual features using Wizard, (2) modify the dimensions or add constraints to the sketches extracted to parameterize the sketches, and (3) use Boolean operations to union, subtract, or intersect individual features for a final model if needed.
Although Wizard offers a fast and convenient approach for solid modeling, the solid models generated should be closely examined for validation.
In summary, Rapidform is the only reverse engineering software that supports creating parametric solid models from scanned data. It offers CAD-like capabilities that let you add dimensions and constraints to sketches and solid features for a fully parametric solid model. Design intent and model accuracy can be achieved using the Sketch method, which is in general a much better option for creating parametric solid models.
The solid models created in specialized software, such as Rapidform and Geomagic, have to be translated to mainstream CAD systems to support engineering applications. Both Rapidform and Geomagic offer capabilities that export solid models to numerous CAD systems.
The solid model of the block example created in Geomagic was exported to SolidWorks and Wildfire using Parametric Exchange of Geomagic. For SolidWorks, all seventeen features recognized in Geomagic were translated as individual features. Note that since there are no Boolean operations offered in Geomagic Studio v.11, these features are not associated. There is no relation established between them. As a result, they are just “piled up” in the solid model. Subtraction features, such as holes and slots, simply overlap with the base block. Similar results appear in Wildfire, except that one extrusion feature was not exported properly.
The liveTransfer™ module of Rapidform XOR3 exports parametric models directly into major CAD systems, including SolidWorks 2006+, Siemens NX 4+, Pro/ENGINEER Wildfire 3.0+, CATIA V4 and V5 and AutoCAD.
The block example that was fully parameterized in Rapidform was first exported to SolidWorks. All the solid features were seamlessly exported, except for some datum points. Since entities such as polygon mesh and segmented regions are not included in SolidWorks database, they cannot be exported. As a result, geometric datum features associated with these entities are not exported properly. The dimensions and constraints added to the sketches and solid features in Rapidform exported well, except again for those referenced to entities that are not available in SolidWorks. Fortunately, it only requires you to make a few minor changes (such as adding or modifying dimensions or constraints) to bring back a fully parametric solid model in SolidWorks. Similar translation results were observed in NX. However, model translation to Wildfire 4.0 is problematic; numerous issues, such as missing and misinterpretation portion of the section profile, are encountered. In general, parametric solid models created in Rapidform can be exported well to SolidWorks and NX. The translation is almost seamless.
University of Oklahoma
University of Oklahoma, khchang@ou.edu
University of Oklahoma, chienchih.chen-1@ou.edu
Some of this material was excerpted from the full paper with permission by Computer-Aided Design and Applications.
