A novel 3D bit-map-based CAD has been developed. This program, named CellCAD,
relies on techniques which provides easy manipulation of huge sized 3D bit-map, poly-triangle
and 2D bit-map data. It can be attached to various kinds of additional plug-in processors to
extend its functions and to customize it highly for broad applications. CellCAD can be applied
in fields using computed tomography digitizers and/or layering fabricators. The authors applied
CellCAD to hydraulic pump model fabrication. This paper reports the basic design concept and
implementation techniques of CellCAD from the viewpoint of design engineering, and also
introduces the methods and results of an actual CellCAD application.Mechanical Engineerin

Nagumo, Yasushi

Numata, Shouhei

Sadaoka, Noriyuki

Takagi, Tarou

Yashiki, Tatsuro

English

Texas ScholarWorks

DEVELOPMENT OF 3D BIT-MAP-BASED CADAND ITS APPLICATION TO HYDRAULIC PUMP MODEL FABRICATIONTarou Takagi*, Tatsuro Yashiki*, Yasushi Nagumo*, Shouhei Numata* and Noriyuki Sadaoka**Power and Industrial Systems R&D Laboratory, Hitachi, Ltd.AbstractA novel 3D bit-map-based CAD has been developed.  This program, named CellCAD,relies on techniques which provides easy manipulation of huge sized 3D bit-map, poly-triangleand 2D bit-map data.  It can be attached to various kinds of additional plug-in processors toextend its functions and to customize it highly for broad applications.  CellCAD can be appliedin fields using computed tomography digitizers and/or layering fabricators.  The authors appliedCellCAD to hydraulic pump model fabrication.  This paper reports the basic design concept andimplementation techniques of CellCAD from the viewpoint of design engineering, and alsointroduces the methods and results of an actual CellCAD application.1. IntroductionRecently, many tools and methods for digital engineering are being developed and utilizedin manufacturing.  To obtain digital shape data from real objects, high energy X-ray computedtomography (CT) is very useful, because it can digitize entire shapes including interior regionsaccurately and automatically in very short time [1].X-ray CT apparatuses obtain sectional images as two-dimensional (2D) bit-map data, whileconventional CAD systems handle boundary representation (B-rep) data [2].  B-rep data containgeometrical and topological definitions, called "features", which are not contained in bit-mapdata.  Therefore, there is poor compatibility between bit-map data and B-rep data, and it is verydifficult to convert bit-map data into B-rep data.The purpose of this study is to utilize piled-up X-ray CT sectional images, that is three-dimensional (3D) bit-map data, for various engineering activities.  The authors developed anovel CAD program, named CellCAD, which can hold and manipulate 3D bit-map data asengineering shape data.  This paper reports the design concept and implementation techniquesfor CellCAD.4242. B-rep data and 3D bit-map dataThe difference in the two data formats, B-rep and bit-map, can be explained using Figure 1.A 3D B-rep datum is represented as a set of geometrical and topological definitions of shapeelements like vertices, edges, faces and bodies.  On the other hand, a 3D bit-map datum isrepresented as a three-dimensional array of tiny cells known as voxels (volume picture cells).Each voxel contains inside/outside discriminant or relative density at its position.The greatest advantage of a 3D bit-map should be X-ray CT data acceptability.  Sectionalimages obtained from X-ray CT apparatuses can be converted into 3D bit-map data by a piling-up method very easily.  Additionally, the 3D bit-map has other two significant advantages,"uniqueness" and "compliance".  Uniqueness is the characteristic that the same shapes must berepresented with the same data.  This is important for comparing two or more shapes to evaluatetheir sameness or to extract the difference between them.  Compliance is the characteristic thatthe datum structure changes very little when a small deformation is applied to the shape.  This isimportant for creating a shape by iterative computation like topology optimization.Even with these advantages, there are severe issues to deal with the large data size of 3Dbit-map.  When representing even quite simple shapes, sometimes the 3D bit-map requiresmemory and storage capacity of 1 GB or more, while a B-rep requires less than 10 KB.  Such ahuge amount of data is very difficult to hold, manipulate and display.  This is the main reasonwhy all conventional CAD systems handle B-rep data.However, the situation has been changing recently.1. Computer performance has been drastically increased since the 1970s, when thefirst CAD system was developed.  The latest PC models are able to handle hugeamounts of 3D bit-map data in reasonable times.V1:(1.0,1.0,1.0)V2:(1.0,0.0,1.0)V3:(0.5,0.0,1.0)V4:(0.5,0.5,1.0)E1:V1,V2E2:V2,V3E3:V3,V4E4:V4,V5F1:E1,E2,E3,   E4,E5,E6B :F1,F2,F3,F4,   F5,F6,F7,F8V1V2V3V5V6V8V9V11V12E1E2E3E4E5E6E8E12E14E17F1F3F4F5F6Top layerBottom layerVoxelsFigure 1. B-rep and bit-map formats4252. Many "layered manufacturing" tools have been developed.  Rapid fabricators and"voxel engineering" programs give 3D bit-map great utility in engineering fields.3. The quality and performance of X-ray CT has been greatly improved.  3D bit-mapdata containing even 2000´2000´2000 voxels can be obtained in a few days,from target objects with a 1000 mm diameter.Such significant changes have mitigated the weak points of the 3D bit-map, allowing itsadvantages to become the focus so there is a strong motivation for developing the 3D bit-mapCAD.3. Conceptual design of 3D bit-map CADAs the first step to develop a 3D bit-map CAD program, the authors started with theconceptual design.  In order to determine its functional requirements, the authors made a list ofissues to be considered.  Comparison of 3D bit-map CAD with conventional B-rep CAD led toidentification of the following issues.1. Large data size.  A 3D bit-map datum containing 3000´3000´3000 voxelsrequires memory and storage capacity of more than 2.5 GB.  This issue in turnleads to the following derivative issues.1-1. Network capacity.  In the case of cooperative work by many operators, adistributed CAD system may cause network congestion due to largedata exchanges.1-2. Memory addressing limitation.  Current Pentium-compatible 32-bitCPUs cannot address from a 4 GB memory space.  Also the WindowsOS cannot address from a 2 GB memory space.1-3. Long process time.  3D bit-map CAD operators have to wait for a longtime until operations are completed.  If the operators make a wrongoperation, they experience a terrific loss of time.2. Immature concept.  The basic idea and methodology of engineering using 3D bit-map data have not been applied to many cases of practical production.  Therefore,requirements for 3D bit-map engineering tools have not yet been clearly defined.Functional changes or additions may be required in the 3D bit-map CADprogram.3. Analytical usage.  Conventional CAD operators know what they are handling, butthis is different for 3D bit-map CAD operators handling X-ray CT data, becausethe target objects may not be their own products.  The operators may want to usethe 3D bit-map CAD for X-ray CT data analysis.426Many of these issues change the 3D bit-map CAD specifications.  The authors found thefollowing solutions for the issues.1. PC cluster architecture (for issue 1).  A PC cluster is a set of PCs connected by ahigh speed network.  It is very useful for realizing scalable parallel processing.  Abit-map datum is a set of local information, therefore parallel processing is veryeffective to reduce process time.2. Client-server system (for issue 1-1).  This consists of one server PC and one ormore client PCs.  The server holds 3D bit-map data and processes them uponreceiving commands from the clients.  In this system, large amounts of data arenot delivered to all clients to avoid network congestion.3. Partial loading and saving (for issue 1-2).  Functions extracting partial data from3D bit-map files and functions inserting partial data into 3D bit-map files arerequired for handling large amounts of data over 2 GB.4. Pre-process and pre-load viewing (for issue 1-3).  To avoid wrong operations, theresults are shown to the operators before they start the operations.  Additionally, itis also better to show the results of threads, that is sequences including two ormore operations, before the operators start the threads.5. Operation describing language (for issue 1-3).  To reduce the operating time,sequential execution of many operations is very effective. A special languageallows recording of operations and threads in literal scripts, modifying them usingtext editing programs, and executing them manually and automatically.6. Plugged-in functional modules (for issue 2).  It is very effective to define theinterface between the kernel part and the plug-in modules to ease modification ofthe function set.  Functions are attachable and removable when needed.7. Multi-viewer system (for issue 3).  Many other CAD programs assign one viewerfor one datum, nevertheless, it is better to provide two or more viewers to easethree-dimensional shape recognition.8. Analytical functions (for issue 3).  Extracting information from 3D bit-map data isvery important to handle X-ray CT data from unknown target objects.  Analysis,measurement and recognition functions are very useful for this purpose.4. Implementation of CellCADConsidering the results of the conceptual design, the authors have built a multi-purpose 3Dbit-map CAD program named CellCAD.  It works on Windows NT 4.0 or Windows 2000operating systems over IBM PC/AT compatible PCs.427The "geometry handling layer" containsthe original 3D bit-map handling library"CellGeom".  This library provides hundredsof functions handling 3D bit-map, poly-triangle and 2D bit-map data.  CellGeomworks on both stand alone and client-serversystems, on both single PCs and PC clusters.The "CAD kernel layer" is implementedas the CellCAD kernel module.  This moduleis an executable program provided with agraphic user interface (GUI).  Figure 3 showsan example screen display.  The kernelmodule provides essential functions ofCellCAD including the following items.Hardware layer PC cluster architecture- PC cluster managing middleware ClusterWareClient-server systemGeometry handling layer3D bit-map handling library CellGeomCAD kernel layer CellCAD kernel moduleCAD plug-in layer CellCAD plug-in modules- Rendering (volume rendering, cut rendering, slicing, etc.)- Shape operation (primitive creation, transformation, cell operation, optimization, etc.)- Measurement (dimension measurement, shape comparison, density analysis, etc.)- Data conversion (rasterization, RP data export, simulation data export, etc.)- Feature extraction (geometric fitting, coordinate definition, alignment, etc.)- Poly-triangle manipulation (remeshing, inconsistency fixing, etc.)- Other functionsFigure 2. 4-layer structure of CellCAD1. Basic data management (data holding, memory management, display, etc.)2. Basic user interface (command menu, information, help system, etc.)3. Plug-in module interface4. License controlFigure 3. Example CellCAD screen displayCellCAD is designed as a 4-layer structure (Figure 2).  The "hardware layer" realizes acomputing environment for CellCAD.  This layer contains the original middleware "Cluster-Ware", designed for Windows-based PC cluster management.428The "CAD plug-in layer" consists of many CellCAD plug-in modules.  They providevarious useful functions, some of which are listed in Figure 2.5. CellCAD applied to pump model fabricationCellCAD has been applied to hydraulic pump model fabrication.  The target object was anexperimental model of a mixed flow pump impeller, designed with a 3D CAD program (Figure4).  The diameter and the height were approximately 100 mm and 74 mm.  The object wasfabricated by the selective laser sintering (SLS) method using metal material [3].  The SLSmethod, which is a rapid prototyping (RP) technique, included a furnace cycle to sinter thelaminated objects to give them high strength.  The final material density was 7.9 g/cm3.This furnace cycle sometimes causes deformation of the objects, therefore a free-form shapevalidation method is required.  For this purpose, the pump developer chose an X-ray CTapparatus (Hitachi HiXCT-6M).  It worked very effectively to digitize the curved shape of thepump impeller into a set of 210 planes of sectional images, every one containing 900´900 cells.The total amount of the data was 162 MB.The shape validation was achieved with the following sequence.  Views of the data duringthis sequence are shown in Figure 5.1. Input the sectional images.  The sectional images were input and piled up to builda 3D bit-map datum (a).2. Input the designed shape.  A poly-triangle datum, with the designed shape, wasinput from the 3D CAD program (b).3. Crop the 3D bit-map datum.  The unused region around the object in the 3D bit-50mmmap datum was cropped to reduce thedatum size (c).  The cropped datumcontained 360´360´210 voxels.4. Adjust the scale of the poly-triangledatum.  The 3D bit-map datum consistedof 0.35 mm-sized cubic voxels.  Thepoly-triangle datum was resized tochange its unit from mm to voxel.5. Obtain the geometric relationshipbetween the poly-triangle and 3D bit-mapdata.  The poly-triangle datum wasoverlaid on the 3D bit-map datum .  Thealignment could be done geometrically Figure 4. Target pump impeller429and visually.6. Transform the 3D bit-map datum.  The 3D bit-map datum was moved to thedesigned position, and rotated into the designed orientation (d).7. Obtain the density distribution.  To know the relative density threshold whichseparates the internal and external regions, the relative density for the air part andthe metal part were measured, and their mean was taken as the threshold.8. Remesh the polygon facets.  The facets, which are elements of the poly-triangledatum, were divided into smaller facets (e).  This process is important todistribute the measured points closed enough together. 9. Measure the surface offset.  The original algorithm "virtual probe measurement(VPM)", which is a simulation of coordinate measuring machines, was applied toquantify the surface offset distribution between the 3D bit-map datum and thepoly-triangle datum.  The accuracy of the VPM was higher than the voxel size.  Inmany cases, it was within 0.1 voxels.  Measured offset was converted into a colorand mapped on the facets.acdb ef g0.2 mm-0.2 mmFailedFigure 5. Views of data during the shape validation sequence43010. Render the colored poly-triangle datum.  The color-mapped poly-triangle datumwas rendered and the rendered images were pasted into a report which indicatedthe distribution of the deformation of the pump impeller (f, g).Significantly, all of these 3D bit-map operations are carried out within a day usingCellCAD.  The report is very useful to analyze what happens during the furnace cycle and toimprove the furnace conditions.6. ConclusionsThe conclusions of this paper are as follows.1. A novel 3D bit-map CAD was developed, in order to utilize X-ray CT data and toenjoy the exclusive advantages of bit-map data.2. Issues for realizing the 3D bit-map CAD were listed, and ways to deal with themwere found.3. The 3D bit-map CAD, CellCAD, was implemented as a 4-layer structure.  It ispowered on client-server systems and/or PC cluster architecture.4. CellCAD was applied to hydraulic pump model fabrication.  It is useful to analyzephenomena during the fabrication process of experimental models and to improvethe conditions for their fabrication.AcknowledgementsThe authors are very grateful to Prof. Noboru Kikuchi, the University of Michigan, for hiskind advice regarding this study.  The authors are also very grateful to Mr. Ichiro Harada, HitachiIndustries Co., Ltd., for giving them the chance to apply CellCAD to a practical problem.The authors are also very grateful to Mr. Ken Yamauchi and Mr. Kazuo Ohtomo, IbarakiHitachi Information Service Co., Ltd., for their unfailing cooperation.References[1] J. H Stanley, R. N. Yancey, CT-assisted Agile Manufacturing, Proceedings of SPIE,Nondestructive Evaluation for Process Control in Manufacturing (1996).[2] J. D. Foley, A. Dam, S. K. Feiner, J. F. Hughes, Computer Graphics, Addison-Wesley(1990).[3] N. Sadaoka et al., Metal Selective Laser Sintering Technology Applied on HydraulicTurbine Design, The 8th International Conference on Rapid Prototyping (2000).431

Development of 3D Bit-Map-Based CAD and Its Application to Hydraulic Pump Model Fabrication

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Development of 3D Bit-Map-Based CAD and Its Application to Hydraulic Pump Model Fabrication

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