Reverse Engineering
How a team from UConn used 3D scanning in a competition
The Formula SAE competitions challenge teams of university undergraduate and graduate students to design and fabricate a small, formula-style vehicle. There are very few restrictions to the overall car design. Teams typically spend eight to twelve months building their vehicles before competition.
The space constraints within the cars are such that accurate CAD models of the engines’ external geometries are needed. Furthermore, the FSAE rule book imposes dimensional limits for certain aspects of the racecars as well. A mere rough-sketched envelope of an engine and its accessories in 3D CAD will no longer suffice to find the most optimal packaging solution. The engine is an integral part of the vehicle, and a need exists for 3D CAD models. As engine manufacturers will not provide design data, 3D scanners are used to create the CAD files.
The use of 3D scanning equipment to make CAD files is not new within the FSAE student community and several engine makes and models have been brought into 3D CAD over the years. This article demonstrates the improvements which continue to make 3D scanning even more valuable.
University of Connecticut FSAE Team—Bolton Works
The University of Connecticut (UConn) has fielded a vehicle in Formula SAE Michigan since 2007. With over 120 colleges and universities registered, Formula SAE Michigan is the largest of its kind. Over the course of four days, the cars are judged in a series of static and dynamic events including technical inspection, cost, presentation, engineering design, solo performance trials, and high-performance track endurance. For the class in which the UConn team competes, the engine must be a four-stroke, Otto-cycle piston engine with a displacement no greater than 610 cc. An air restrictor must be fitted downstream of the throttle and upstream of any compressor. Production four-cylinder 600 cc sport bike engines are typically used due to their availability and displacement.
As sponsor of the FSAE teams, Bolton Works offers reverse engineering services. In July of 2014, Bolton Works was approached by the UConn team to assist in the creation of 3D CAD models (SolidWorks) of the Suzuki GSX-R600 engine for the 2014–2015 competition. The CAD models are used to check for interference and to facilitate design modification of the engine. An additional request was made to provide the geometry of the flow path of the intake port, exhaust port, combustion chamber, valves and piston. The profile of the cam shafts was to be measured as well. The data is used to assist in CFD modeling of the engine’s intake system.
To provide UConn with a comprehensive 3D model, the following was determined:
–Industrial grade, high-resolution Zeiss Comet scanners, with high-end optics and sensors, must be used to ensure detail and accuracy.
–The accuracy of critical dimensions of the CAD model should be verified with a Zeiss Contura CMM.
–The engine model should include all factory bolted-on accessories of the engine.
–The model should be subdivided into its 25 individual components so they can be replaced by a UConn-designed one in SolidWorks, as needed.
–Internal geometry of the complete cylinder head’s flow path should be provided.
Engine
The team uses a Suzuki GSX-R600 engine, model year 2001–2003. It is a water-cooled, 599 cc, DOHC, 16-valve engine capable of delivering 115 hp (85 kW) at 13,000 rpm and 51 foot pounds (69 Nm) at 10,500 rpm. However, the FSAE-enforced restrictor does bring the performance below 100 hp.
The engine comprises four load bearing components:
–Die-cast aluminum lower crank case, which includes the upper transmission case.
–Cast aluminum cylinder block with integrated upper crankcase.
–Cast aluminum cylinder head.
–Die-cast lower transmission case.
Bolted to these four components are covers for the clutch, stator, camshafts, vents, crankcase and sprocket. Furthermore, an external water pump assembly, the starter motor, oil filter, oil cooler, water inlet housing and several sensors are attached as well. The original oil sump has been replaced by a UConn designed one, to accommodate the different driving dynamics of the race (car track use versus motorcycle use). Twenty-five components have surfaces external to the engine and all were scanned and modeled to become part of the SolidWorks assembly.
3D Scanner
There are various techniques for the measurement of a three-dimensional shape. One method is “structured light projection.” As white light orblue light can be used as a lighting source for the projection of the fringe patterns, these scanners are also known as “white light” or “blue light” scanners (as opposed to a laser scanner).
The operating principle is the identification of object points by a pattern projected onto the surface and observed by a camera under a different view. The camera to surface measurement is based on triangulation. Because the angles and distances between the light source and camera are fixed, and the direction of the light ray is known, the depth of the surface where the light strikes can be calculated.
German Zeiss Comet scanner used for this project is a Structured Light scanning system for applications ranging from small, precision components to large tools, dies and vehicles. The Zeiss Comet produces dense, point cloud data, which permits inspection (metrology) and reverse engineering. The Comet scanner has an adjustable measuring volume of 75 × 75 × 50 mm to 900 × 600 × 600 mm, and accuracy of up to ±0.005 mm (0.0002″). Each view or “scan” can measure up to about 11 million X-Y-Z points. With large objects or objects with complex surface geometry, it is necessary to take several measurement positions to ensure that all surfaces are recorded. There is no limit to the number of views, or “patches,” that can be recorded per object other than computer memory. After the scan, the patches are globally aligned to form one 3D point cloud.
3D Scanning
Before scanning, the engine was drained and thoroughly cleaned, as the resolution of the scanner is such that dirt particles, scratches, and other undesirable residue will show up in the scan data. After removal of most of the internal components (crankshaft, pistons, transmission, etc.), the external visible components were re-assembled. For the scanning, the engine was placed on a high accuracy rotary table, of which the position is tracked to enable automatic alignment of the different scan patches. For the complete assembly, 153 patches were created, resulting in a scan data set of 4 GB. This scan data set was than reduced in size by removing overlapping points and connecting the remaining points with triangles.
This triangulated file, called an STL file, is a facetted representation of the scan geometry 1.5 GB in size (30 million triangles). As parts of the bolted-on accessories cast a shadow on the engine during scanning, large areas are not captured during the scanning process. For example, the backside of the water pump is not visible to the scanner and therefore will leave a gap in the data. To ensure completeness, the engine was disassembled and each component was scanned separately. STL files were created and then matched (best fitted) to the assembly file to ensure correct positioning in 3D space. The individual STL files were then brought into Geomagic Software (3D Systems Inc., Rock Hill, SC) to fill in gaps of areas still missed during the scanning process. The gap filling process is essential, as we want to end up with completely closed geometry in order to create a solid for use in SolidWorks.
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Conversion of Components into SolidWorks
Bringing large scan data sets in STL format into SolidWorks is a challenge. SolidWorks offers two options to import STL files: as a graphics file or as geometry. In the case of the graphics file, large STL files can be loaded and visualized. However, they do not behave like solids, surfaces or any other geometry which can be referenced. Therefore, they cannot be used to build upon.
The second option in SolidWorks is to translate each triangle in the STL file as a surface. The practical limit of SolidWorks 2015, used at the time of writing, is around 20,000 triangles. The accumulated total amount of triangles of each component in the assembly exceeds 100 million triangles, therefore prohibiting the use of the STL files as reference geometry in SolidWorks.
As bringing the STL files into SolidWorks is not a viable option, Nurbs surfaces are created outside SolidWorks and then imported as parasolids. To create the Nurbs surfaces, Geomagic was used. Geomagic automatically generates a network of splines on top of the STL files, which approximate the geometry. How closely they will follow the STL file will depend on the amount of node points in the spline, which can be adjusted. From this network of splines, Nurbs surfaces are created. This method is efficient and reduces the file sizes to more manageable levels. As a final step, the surface patches are “sewn” together to form a closed, solid, CAD model.
This solid, constructed in Nurbs Surfaces, represents the engine as is, including any flaws there might be in the engine’s surfaces when scanned. Consequently, machining inaccuracies, worn areas, dents and scratches, etc., will be part of the model unless further steps are taken. As the goal is to be able to use the CAD models as a reference, the attachment points for frame, intake manifold and exhaust manifold need to be free of defects. They are therefore modeled in separately. Where, for example, a bolt hole would exist, a cylinder would be fitted to the scans. SolidWorks can recognize the cylinder and use this as reference to which other geometry can be easily built upon.
All models are exported from the Geomagic software into the neutral parasolid format, which SolidWorks can read. As all models are already in the correct position, they can be loaded as an assembly in SolidWorks. The total size of the complete GSX-R600 engine SolidWorks assembly file is about 500 MB, requiring about 1.5 GB memory when loaded in SolidWorks. Although the size of the file was not an issue for any of the computer workstations in use, it is possible to suppress any of the components in the subassembly to focus on, for example, the cylinder head.
Verification of Dimensions
To verify that the dimensions of the attachment points are correct, the solid model is also used to program the CMM and re-measure these points on a second, still-assembled engine. The solid model was then updated in SolidWorks to reflect these CMM values as needed.
Reverse Engineering Cylinder Head—Inside Geometry
The mandatory restrictor changes the characteristics of the engine entirely and the intake system has to be redesigned to minimize performance loss. A 3D CAD model of the internal and external geometries of the cylinder head was made. This would make CFD analysis during the design and optimization of a new intake more complete.
To scan the flow path, a second cylinder head was sacrificed. By cutting it up in sections, the internal geometry was exposed and could be scanned. The different scanned patches of the intake and exhaust port were than realigned to a reference scan of the cylinder head.
In the last part of the process, the valves and pistons were scanned and modeled and a 3D model emerged representing the complete flow path, from intake port to combustion chamber and exhaust port.
Total Effort
The reverse engineering process consists of two steps: the data acquisition (3D scanning) and the processing of the files into solid models.
The scanning of the assembled engine was accomplished in one day, including the generation of the STL file. The scanning of the individual 25 components took another seven workdays. When the STL file of the first separate component became available, the creation of a solid was started; therefore, scanning and modeling was essentially done in tandem. The process of creating solid models was completed within nine workdays for all components.
An additional four hours were spent for the CMM measurements and adjustment of the SolidWorks model features was needed. The complete reverse engineering of the 25-part engine assembly was accomplished within 15 workdays.
Since completion of the Suzuki GSX-R600 CAD model, 10 different engine models have been made by Bolton Works and are in use by more than 50 teams worldwide.