Research

"There are no shortcuts to any place worth going."  - Beverly Sills

 

The research interests of our research group are centered around Direct Digital Manufacturing (DDM) in micro- and meso-scales.  DDM is a fabrication approach that can directly convert a computer-aided design model into a physical object.  Hence it has been widely recognized as a disruptive manufacturing technology for a wide variety of applications. To achieve the full potentials of DDM on cost and time savings, the development of novel DDM process, new process planning methodologies and a wide selection of materials are critical.  In addition, DDM enables revolutionary new design by using complex three-dimensional shapes, heterogeneous material properties and multi-functionality.  Systematic knowledge regarding modeling, analyzing, synthesizing, and optimizing such designs are required in order to achieve desired performance.

At USC’s ISE department, we aim to develop DDM technologies that enable people to revolutionize future product design and manufacturing.  Our multi-disciplinary research and training require bridging theoretical, computational and practical knowledge with systematic thinking. By integrating both physics-based manufacturing and computation-based design and control, some of our research results are listed as follows.

 

Novel process and machine development for direct digital manufacturing and 3D printing

 

Rapid Manufacturing in Mintues: The Development of a Mask Projection Stereolithography Process for High-speed Fabricating

Yayue Pan, Chi Zhou, Yong Chen

Proceedings of ASME-Intl. Manufacturing Sci. and Engineering Conference, 2012 (paper)

 

The purpose of this paper is to present a direct digital manufacturing (DDM) process that is an order of magnitude faster than other DDM processes currently available.  The developed process is based on a mask-image-projection-based Stereolithography process (MIP-SL), during which a Digital Micromirror Device (DMD) controlled projection light cures and cross-links liquid photopolymer resin.  In order to achieve high-speed fabrication, we investigated the bottom-up projection system in the MIP-SL process. A set of techniques including film coating and the combination of two-way linear motions have been developed for the quick spreading of liquid resin into uniform thin layers.  The process parameters and related settings to achieve the fabrication speed of a few seconds per layer are presented. Additionally, the hardware, software, and material setups developed for fabricating given three-dimensional (3D) digital models are presented. Experimental studies using the developed testbed have been performed to verify the effectiveness and efficiency of the presented fast MIP-SL process.  The test results illustrate that the newly developed process can build a moderately sized part within minutes instead of hours that are typically required.

 

Direct Geometry Processing for Tele-fabrication

Yong Chen, Kang Li, Xiaoping Qian

Proceedings of ASME Computers and Information in Engineering Conference, 2012 (paper)

 

This paper presents a new approach for tele-fabrication where a physical object is scanned in one location and fabricated in another location. This approach integrates three-dimensional (3D) scanning, geometric processing of scanned data, and additive manufacturing technologies. In this paper, we focus on a set of direct geometric processing techniques that enable the tele-fabrication. In this approach, 3D scan data is directly sliced into layer-wise contours. Sacrificial supports are generated directly from the contours and digital mask images of the objects and the supports for Stereolithography Apparatus (SLA) processes are then automatically generated. The salient feature of this approach is that it does not involve any intermediate geometric models such as STL, polygons or non-uniform rational B-splines that are otherwise commonly used in prevalent approaches. The experimental results on a set of objects fabricated on several SLA machines confirm the effectiveness of the approach in faithfully tele-fabricating physical objects.

 

Ultrasound Transducer Array Fabrication based on Additive Manufacturing of Piezocomposite

Hamid Chabok, Chi Zhou, Yong Chen, Arash Eskandarinazhad, Qifa Zhou, Kirk Shung

Proceedings of the ASME International Sysmposium on Flexible Automation, 2012 (paper)

 

Conventional methods for fabricating ultrasound imaging transducer arrays, especially for high frequency range (>20 MHz), are expensive, time consuming and limited to relatively simple geometries. In this paper, the development of an additive manufacturing (AM) process based on digital micromirror devices (DMDs) is presented for the fabrication of piezoelectric devices such as ultrasound transducer arrays. Both green-part fabrication and the sintering of fabricated green-parts have been studied. A novel two-channel design in the bottom-up projection system is presented to address the piezo-composite fabrication challenges including a small curing depth and viscous ceramic slurry recoating.  A prototype system has been developed for the fabrication of green-parts with complex shapes and small features. Based on the fabricated green-parts, the challenges in the sintering process for achieving desired functionality are discussed. Various approaches for increasing the density of sintered components are presented. Dielectric and piezoelectric properties of the fabricated samples are measured and compared with those of bulk PZT samples. Based on the identified challenges in the DMD-based AM process, future work for achieving fully functional piezoelectric ceramic components is discussed.

 

Smooth Surface Fabrication in the Mask Projection based Stereolithography

Yayue Pan, Xuejin Zhao, Chi Zhou, Yong Chen

Proceedings of NAMRI/SME, 2012 (paper)

 

The surface finish is critical for applications such as optics, micro-fluid mixing and mechanical assembly, in which optical lens, fluidic flow channels and rotating components are all required to be smooth. However, the stair-stepping effect is well known in the layer-based additive manufacturing processes, in which a three-dimensional model is approximated by a set of two-dimensional layers.  Consequently the fabricated surfaces have poor surface finish especially for the ones that are close to the horizontal plane.  In this paper, a novel approach for achieving improved surface finish is presented for the mask-image-projection-based Stereolithography (MIP-SL) process.  Theoretical models and parameter characterization are presented for the developed methods. Accordingly, the process planning and calibration approaches for fabricating smooth surfaces in the MIP-SL process have been developed.  Multiple test cases based on various types of curved surfaces have been performed.  A comparison of the built results based on the traditional and the newly developed approaches illustrates the effectiveness of our method.

 

Digital Material Fabrication Using Mask-Image-Projection-based Stereolithography

Chi Zhou, Yong Chen, Zhigang Yang, Behrokh Khoshnevis

Rapid Prototyping Journal, 2012 (paper)

 

Digital materials such as the ones shown by Objet’s Connex family demonstrate that a new material with desired characteristics can be achieved by combining two different base materials with various concentrations and structures.  We investigate the feasibility of using additive manufacturing processes based on digital mask projection in the fabrication of such digital materials.  A multi-material mask-image-projection-based Stereolithography process is presented.  The related challenges and approaches of addressing them on the development of such a process are identified. A testbed has been developed to fabricate objects with designed digital materials.  Experimental results illustrate that desired material properties can be achieved through the developed process. Several multi-material designs have been produced to highlight the capability of this promising technology for fabricating three-dimensional, multi-material objects with spatial control over placement of both material and structure. The limitations and challenges for future development have also been identified.

 

Fabrication of Conformal Ultrasound Transducer Arrays and Horns based on Multi-axis CNC Accumulation

Yayue Pan, Chi Zhou, Yong Chen

Proceedings of ASME-Intl. Manufacturing Sci. and Engineering Conference, 2011 (paper)

 

Ultrasonic imaging is an important medical imaging technique. It uses ultrasound over 20K Hz to detect and visualize muscles, tendons, and many internal organs. Previous studies have shown that an improved acoustic performance can be achieved by conformal ultrasound transducer arrays and horns that can wrap conformably around curved surfaces. To address challenges in fabricating such curved ultrasound transducer arrays and horns, we investigate the possibility of using a newly developed additive manufacturing (AM) process named CNC accumulation. In such an AM process, an accumulation tool can have multi-axis motion, which is beneficial for building conformal ultrasound transducer arrays and horns on a curved surface. To address different resolution requirements, we illustrate the use of multiple accumulation tools that can have different curing sizes and power in the fabrication of a single component. The tool path planning methods for any given cylindrical and spherical surfaces have been discussed. Based on the developed prototype system, various test cases have been performed. The experimental results have illustrated the capability of the process and its potential use in the fabrication of conformal ultrasound transducer arrays and horns. The current limitations and future development have also been discussed.

Metallic Part Fabrication Using Selective Inhibition Sintering

Behrokh Khoshnevis, Mahdi Yoozbashizadeh, Yong Chen

Rapid Prototyping Journal, 2011 (paper)

 

The purpose of this research is to investigate the fundamentals of the Selective Inhibition Sintering (SIS) process for the fabrication of metallic parts. A SIS-Metal process has been developed based on the microscopic mechanical inhibition principle. In this process metal salt solution is printed in the selected areas of each metal powder layer; the salt re-crystallizes when water evaporates; salt crystals decompose and grow rapidly prior to sintering; during the sintering process the constituents of decomposed salt particles spread between metal powder particles and prevent the fusing of these particles together; consequently, the sintering process in the affected regions is inhibited. This paper presents the research result on the inhibition mechanism and process control of the SIS process. Experimental results are also presented to demonstrate the capability of the process in fabricating metal parts with various geometries.  The SIS-Metal process has numerous advantages including low cost, minimal shrinkage and deformation effects, and independence from polymeric binders.

A Layerless Additive Manufacturing Process based on CNC Accumulation

Yong Chen, Chi Zhou, Jingyuan Lao

Rapid Prototyping Journal, 2011 (paper)

 

Most current additive manufacturing processes are layer-based, that is building a physical model layer-by-layer.  By converting 3-dimensional geometry into 2-dimensional contours, the layer-based approach can dramatically simplify the process planning steps.  However, there are also drawbacks associated with the layer-based approach such as inconsistent material properties between various directions.  In a recent NSF workshop on additive manufacturing, it is suggested to investigate alternative non-layer based approaches.  In this paper, we present an additive manufacturing process without planar layers. In the developed testbed, an additive tool based on a fiber optics cable and a UV-LED has been developed.  By merging such tools inside a liquid resin tank, we demonstrate its capability of building various 2D and 3D structures.  The technical challenges related to the development of such a process are discussed.  Some potential applications including part repairing and building around inserts have also been demonstrated.

 

Modeling and control techniques for accurate and reliable direct digital manufacturing processes

 

An Integrated CNC Accumulation System for Automatic Building-around-inserts

Xuejin Zhao, Yayue Pan, Chi Zhou, Yong Chen, Charlie C. L. Wang

Proceedings of NAMRI/SME, 2011 (paper)

A non-layer-based additive manufacturing (AM) process named computer numerically controlled (CNC) accumulation process is presented for applications such as plastic part repairing and modification. To facilitate the CNC accumulation process, a novel three-dimensional (3D) laser scanning system based on a micro-electo-mechanical system (MEMS) device is developed for in situ scanning of inserted components. The integration of the scanning system in the CNC accumulation process enables the building-around-inserts with little human efforts. A point processing method based on the Algebraic Point Set Surface (APSS) fitting and Layered Depth-normal Image (LDNI) representation is developed for converting the scanning points into triangular meshes. The newly developed 3D scanning system is compact and has sufficient accuracy for the CNC accumulation process. Based on the constructed surface model, data processing operations including multi-axis tool path planning and motion control are also investigated. Multiple test cases are performed to illustrate the capability of the integrated CNC accumulation process on addressing the requirements of building-aroundinserts.

Additive Manufacturing based on Optimized Mask Video Projection for Improved Accuracy and Resolution

Chi Zhou, Yong Chen

Proceedings of NAMRI/SME, 2011 (paper)

 

Additive manufacturing (AM) processes based on mask image projection such as digital micro-mirror devices (DMD) have the potential to be fast and inexpensive. More and more research and commercial systems have been developed based on such digital devices. However, the accuracy and resolution of the related AM processes are constrained by the limited number of mirrors in a DMD. In this paper, a novel AM process based on the mask video projection has been presented. For each layer, a set of mask images instead of a single image are planned based on the principle of optimized pixel blending. The planned images are then projected in synchronization with the small movement of the building platform. A mask image planning method has been presented for the formulated optimization problem. Experimental results have verified that the mask video projection process can significantly improve the accuracy and resolution of built components.

Additive Manufacturing based on Multiple Calibrated Projectors and its Mask Image Planning

Chi Zhou, Yong Chen

Proceedings of ASME International Design Engineering Technical Conferences, 2010 (paper)

 

Additive manufacturing (AM) processes based on mask image projection such as digital micro-mirror devices (DMD) have the potential to be fast and inexpensive. More and more research and commercial systems have been developed based on such digital devices.  However, a digital micro-mirror device such as a digital light processing (DLP) projector has limited accuracy and resolution.  Based on the principle of pixel blending, we present a novel AM process by using multiple DMDs to significantly improve the accuracy and resolution of built components. In order to achieve the desired pixel blending result for a given layer, it is critical to plan the mask images that will be used by the multiple projectors.  In addition, the mask image planning needs to compensate the calibrated light intensity in a projection image that is usually non-uniform and non-linear. We present a general optimized pixel blending method based on direct discrete search (DDS).  Its mathematic model and computing method for the mask image planning are presented. Various test cases have been performed to verify its effectiveness and efficiency

Calibrating Large-area Mask Projection Stereolithography for Its Accuracy and Resolution Improvements

Chi Zhou, Yong Chen

Proceedings of Solid Freeform Fabrication Symposium, 2009 (paper)

 

Solid freeform fabrication (SFF) processes based on mask image projection such as digital micro-mirror devices (DMD) have the potential to be fast and inexpensive. More and more research and commercial systems have been developed based on such digital devices.  However, a digital light processing (DLP) projector based on DMD has limited resolution and certain image blurring.  In order to use a DLP projector in the large-area mask projection stereolithography, it is critical to plan mask images in order to achieve high accuracy and resolution.  Based on our previous work on optimized pixel blending, we present a calibration method for capturing the non-uniformity of a projection image by a low cost off-the-shelf DLP projector. Our method is based on two calibration systems, a geometric calibration system that can calibrate the position, shape, size, and orientation of a pixel and an energy calibration system that can calibrate the light intensity of a pixel. Based on both results, the light intensity at various grayscale levels can be approximated for each pixel. Developing a library of such approximation functions is critical for the optimized pixel blending to generate a better mask image plan. Experimental results verify our calibration results.

Self-intersection Free and Topologically Faithful Slicing of Implicit Solid

Pu Huang, Charlie Wang, Yong Chen

Proceedings of International Design Engineering Technical Conferences, 2011 (paper)

We present a robust and efficient approach to directly slicing implicit solids. Different from prior slicing techniques that reconstruct contours on the slicing plane by tracing the topology of intersected line segments, which is actually not robust, we generate contours through a topology guaranteed contour extraction on binary images sampled from given solids and a subsequent contour simplification algorithm which has the topology preserved and the geometric error controlled. The resultant contours are free of self-intersection, topologically faithful to the given r-regular solids and with shape error bounded; therefore, correct objects can be fabricated from them by rapid prototyping. Moreover, since we do not need to generate the tessellated B-rep of given solids, our approach is memory efficient – only the binary image and the finest contours on one particular slicing plane need to be stored in-core. Our method is general and can be applied to any implicit representations of solids.

Manufactruability Analysis of Infeasible Features in Polygonal Models for Web-based Rapid Prototyping

Yong Chen, Xiaoshu Xu

Proceedings of International Conference on Manufacturing Automation, 2010 (paper)

Web-based online submission of computer-aided design (CAD) models for prototype parts is getting popular among service bureaus and gaining wider acceptance among rapid prototyping users. In the web-based rapid prototyping (RP), an instant quote needs to be generated for an arbitrary polygonal model (usually in STL format).  Hence the manufacturability analysis of polygonal models is critical in automating the instant quoting process.  In this paper two common problems are addressed including: (1) small features that are infeasible to be made by a RP process; and (2) cost analysis of a given polygonal model. We present two methods based on the analysis of offsetting results: (i) a fast approach of identifying infeasible features in order to provide an instant feedback to users; and (ii) an accurate approach of calculating the information of infeasible features for a given tool size in order to facilitate cost analysis.  The related data structures and algorithms are presented.  Both 2-dimensional and 3-dimensional examples are provided to illustrate the effectiveness of our approach.

Three-Dimensional Digital Halftoning for Layered Manufacturing based on Droplets

Chi Zhou, Yong Chen

Transactions of North American Manufacturing Research Institute of SME, Vol. 37, pp. 175-182, 2009 (paper)

 

Layered manufacturing based on droplets, such as multi-jet modeling and polyjet processes, shows great promises in fabricating accurate, smooth and highly detailed 3-dimensional models.  It has been widely used in fabricating prototypes and investment casting patterns.  In this paper, we present a 3-dimensional digital halftoning method which can significantly reduce the building time of these layered manufacturing processes.  The key idea of the halftoning method is to intelligently control the printed droplet layout to form a slanted layer, which can closely match the surface of an input geometry.  We present a mathematical model for finding the optimized droplet layout, and discuss various solution strategies.  The experimental results showed that a revised DBS method can solve the formulated problem effectively and efficiently.

Optimized Mask Image Projection for Solid Freeform Fabrication

Chi Zhou, Yong Chen, Richard A. Waltz

Proceedings of ASME Internal Design Engineering Technical Conferences, 2009 (paper)

 

Solid freeform fabrication (SFF) processes based on mask image projection have the potential to be fast and inexpensive.  More and more research and commercial systems have been developed based on these processes.  For the SFF processes, the mask image planning is an important process planning step.  In this paper, we present an optimization based method for mask image planning.  It is based on a light intensity blending technique called pixel blending.  By intelligently controlling pixels’ gray scale values, the SFF processes can achieve a much higher XY resolution and accordingly better part quality.  We mathematically define the pixel blending problem and discuss its properties.  Based on the formulation, we present several optimization models for solving the problem including a mixed integer programming model, a linear programming model, and a two-stage optimization model.  Both simulated and physical experiments for various CAD models are presented to demonstrate the effectiveness and efficiency of our method.

Non-uniform Offsetting and its Applications in Laser Path Planning of Stereolithography Machine

Yong Chen

Proceedings of Solid Freeform Fabrication Symposium, 2007 (paper)

 

Laser path planning is an important step in solid freeform fabrication processes such as Stereolithography (SLA).  An important consideration in the laser path planning is to compensate the shape of laser beam.  Currently the compensation is divided into two steps, Z-compensation and X-Y compensation, and the shape of laser beam is assumed to be uniform for the whole platform.  In this research, we present a sampling based non-uniform offsetting method which accounts for the different shapes of laser beam at various locations.  We discuss the related steps and algorithms.  We demonstrate its effectiveness by using various test cases.  Besides improving the accuracy of SLA machine, non-uniform offsetting can also be applied to address other accuracy issues caused by thermal and structural variations.

 

 

Design methods and tools for DDM-enabled products and applications

 

Mesh Thickening for Freeform Surface Patches

Charlie Wang, Yong Chen

Rapid Prototyping Journal, 2012 (paper)

Given an intersection-free mesh surface patch S, we introduce a method to thicken S into a solid H located at one side of S, where the thickness r of H is specified by users. With such a surface-to-solid conversion operation, industrial users are able to fabricate a designed (or reconstructed) surface by rapid prototyping. To develop this thickening operator for freeform mesh surfaces, we first investigate an implicit representation of the thickened solid H according to an extension of signed distance function. After that, a partial surface reconstruction algorithm is proposed to generate the boundary surface of H, which remains the given surface S exactly on the resultant surface. Experimental tests show that the thickening results generated by our method give nearly uniform thickness and meanwhile do not present shape-approximation error at the region of input surface S. These two good properties are very important to the industrial applications that need to use this thickening operation.

Joint Design for 3-D Printing Non-assembly Mechanisms

Xuan Song, Yong Chen

Proceedings of ASME IDETC, 2012 (paper)

The layer-based additive manufacturing (AM) processes can directly fabricate sub-systems with multiple components during the building process. Novel applications in robotics and many others have been demonstrated by removing the need of component assembly. However, the AM processes also have inferior accuracy compared to the Computer Numerical Control (CNC) machining process. Hence the joint clearance that can be achieved in a 3D-printed mechanism is large. This would significantly limit the use of AM in directly building movable sub-systems without further assembly operations after the building process. To reduce the joint clearance, we present a novel joint design by considering the fabrication limitation of AM processes. A novel marker structure is developed for various types of joints including cylindrical pin joints. The relation of the marker design and the rotation performance of the 3D-printed joint is modeled. Test cases based on the Stereolithography Apparatus (SLA) process have been performed to verify the effectiveness of the developed joint design. Compared to the traditional pin joint design, the new design can achieve a smaller clearance during rotation while still be able to be fabricated by the SLA process. Consequently its rotation performance can be improved.

Design of Origami Sheets for Foldable Object Fabrication

Dongping Deng, Yong Chen

Proceedings of ASME IDETC, 2012 (paper)

Reconfigurable structures that are enabled through the integration of multiple materials are important for future design and manufacturing practice. We investigate one of such reconfigurable structures - an origami sheet, which can be designed based on a 3D object and unfolded into a 2D sheet with complex creases. A fabrication approach based on a hybrid manufacturing process by integrating layer-based additive manufacturing and silicon molding techniques is developed. Related challenges on designing creases for given folding requirements and the related material properties are discussed. A novel structure design is presented to ensure the fabricated creases that are in soft materials can be folded and unfolded without failures. The design method can be applied to different scale levels. The origami sheets for test cases in different complexity have been tested. The experimental results illustrate that the designed and fabricated origami sheets can be folded and used for product components with reconfigurable shapes.

A Rapid Shape Acquisition Method by Integrating User Touching Input

Yong Chen, Jinho Jung, Yongqiang Li

Virtual and Physical Prototyping, 2011 (paper)

The easiness of creating three-dimensional (3D) models from physical objects is one of the core challenges that remain to be addressed in reverse engineering. In this paper, a touch-based 3D shape acquisition method is presented that is easy and intuitive to use. Based on the method, a user can easily interact with both real and virtual objects and directly generate feature-based CAD models. The key technical challenges on developing the related hardware and software systems are discussed. By using widely available consumer electronic devices, a low-cost prototype system is designed and built. Based on the designed system, a novel 3D coordinate computation method is developed to obtain the touching point positions. Related challenges on using such a system in generating 3D models are also discussed. Multiple examples are presented to illustrate the effectiveness and efficiency of the developed method.

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Multi-Piece Mold Design Based on Linear Mixed-Integer Program Toward Guaranteed Optimality

Stephen Stoyan, Yong Chen

Proceedings of International Conference on Manufacturing Automation, 2010 (paper)

Multi-piece molds are a type of molding technology, which consist of more than two mold pieces and are assembled/dissembled like a space puzzle.  Based on such molds, complex parts can be made for limited run production.  Compared to traditional two-piece molds, parts with much more complex geometries can be made; however, this also brings challenge in designing such multi-piece molds.  Previous works to address the problem are all based on heuristics.  In this paper, we present a multi-piece mold design framework based on linear mixed-integer program.  In our method, multi-piece mold design with guaranteed optimality on the number of mold pieces can be generated for any given CAD model of a molded part.  The formulation of multi-piece mold design as a linear mixed-integer program is presented.  The related multi-piece mold design framework is discussed.  Some examples are provided which illustrate the effectiveness and efficiency of our approach.

Design of Flexible Skin for Target Displacements based on Meso-Structures

Yongqiang Li, Yong Chen, Chi Zhou

Proceedings of ASME Computers and Information in Engineering Conference, 2009 (paper)

Recent advances in sold freeform fabrication (SFF) present tremendous design freedom for a product design with complex geometries.  In this paper, we consider the problem of using rigid materials to design flexible skin of a product component for SFF.  A design strategy based on the combination of well-defined meso-structures is presented to achieve desired heterogeneous material properties, and consequently desired flexibility in target directions and positions.  We present our computational framework to automate the design optimization process.  Due to the dramatically increased design space, a brute force or traditional design optimization method such as the genetic algorithm (GA) and particle swam optimization (PSO) is not efficient.  We present a design method based on the idea of analyzing the flexibility of each link for given meso-structures.  Two experimental examples are presented to demonstrate its usage in generating the maximum/minimum and target displacements.  We also present its comparison with the GA and PSO methods.

Computer-aided Product Design with Performance-Tailored Mesostructures

Yong Chen, Shanglong Wang

Computer-aided Design and Application, 2008 (paper)

Motivated by the design and manufacturing of customized cushions for debilitated patients, we propose the idea of performance-tailored mesostructures which can change or adapt their design performance based on given requirements.  Our goal is to develop a CAD tool with the capability of specifying heterogeneous material properties in a product component such that a designer can design the component with better performance. Based on the object oriented programming paradigm, we first define a performance-tailored mesostructure and its properties.  We then demonstrate an approach of building discipline-specific performance models for such mesostructures.  For a product component design, we present a performance-tailored mesostructure design method based on a hierarchical design framework.  A CAD system based on the proposed design framework is being developed which can generate CAD models with performance-tailored mesostructures.  The generated CAD models can then be fabricated by a layer-based rapid manufacturing system.  Several experimental examples are given to demonstrate the capability of our method.

3D Texture Mapping: A Microstructure Design Method for Rapid Manufacturing

Yong Chen

Computer-aided Design and Application, Vol. (4), No. 6, 2007, pp.761-771 (paper)

Inspired by the developments of biomimetic design and layer manufacturing, we present a microstructure design method which uses complex internal structures to achieve an optimal design. Similar to 2D texture mapping, our approach is based on mapping a 3D microstructure into a design space to generate internal structures. We develop a texture mapping design system which enables a designer to select a microstructure from a library. It then automatically generates a CAD model with internal structures based on given design requirements. The generated CAD model can be fabricated using a layer manufacturing system. The ability to design microstructures within a part creates tremendous potential for lightweight and high performance components and devices.

A Mesh-based Geometric Modeling Method for General Structures

Yong Chen

Proceedings of ASME Design Engineering Technical Conferences, 2006 (paper)

This paper presents a mesh-based geometric modeling method to create tessellated models of general structures, which can be the design results of conformal truss structure design, thermal design, topology design, material design, compliant mechanism design and others. A general structure configuration design format is proposed to describe the structures. A universal structure generating system is developed based on the mesh-based geometric modeling method. Besides generating a watertight surface model for an input structure configuration, the modeling approach can automatically add an important design feature, fillets, in the generated model. Therefore the structures’ mechanical performance is improved comparing to results generated by other methods. Examples from different structure design areas are presented.

Hybrid Geometric Modeling Method for Large Scale Conformal Cellular Structures

Hongqing Wang, Yong Chen, David W. Rosen,

ASME Journal of Computing and Information Science in Engineering, 2006, accepted (paper)

This paper presents a hybrid geometric modeling method to create CAD models of large-scale conformal cellular structures effectively and efficiently. Cellular material structures can be engineered at the mesoscopic scale for high performance and multifunctional capabilities. One type of cellular structure is conformal lightweight truss. A simple method of constructing models of uniform trusses is to pattern unit cells linearly within a CAD system. However, by orienting strut directions and adjusting strut sizes, such trusses can be optimized to achieve superior strength, stiffness, and weight characteristics. For large truss structures, computational and storage complexities cause difficulties in CAD system modeling. In this paper, a new hybrid geometric modeling method by using both solid modeling and surface modeling techniques is developed to directly create tessellated models and automate the geometric modeling process of conformal truss structures efficiently. This hybrid modeling method is intended to support the design, analysis, optimization, and manufacture of conformal truss structures. Examples are presented and the computational efficiency of the hybrid method is compared with the approach of creating the complete solid model of cellular structures. The hybrid geometric modeling method can be generalized to various types of cellular structures as well as other periodic structures.

Geometric Tailoring: A Design for Manufacturing Method for Rapid Prototyping and Rapid Tooling

Shiva Sambu, Yong Chen, David W. Rosen

ASME Journal of Mechanical Design, 2004, Vol(126), 571-580 (paper)

The goal of fabricating functional prototypes quickly is hindered by a mismatch of material properties between production materials and those used in rapid prototyping (RP) machines, such as stereolithography. Even when rapid tooling (RT) technologies are utilized for injection molded parts, differences in mold materials cause differences in molded part properties. To compensate for these material and process differences, a design for manufacturing (DFM) method is introduced, called geometric tailoring. The idea is to modify dimensions of prototype parts to match key characteristics of production parts, such as stress and deflection behaviors. For RP parts, the geometric tailoring DFM method integrates two sub-problems, one for achieving functional requirements by matching part behaviors, and one for RP process planning to incorporate manufacturing capabilities and limitations. For parts fabricated by RT, an additional sub-problem is integrated, namely injection molding process planning. Problem decomposition is critical due to the coupled nature of the sub-problems. A problem decomposition and solution procedure is presented. The geometric tailoring method is shown to enable the matching of prototype to production part behaviors, while improving manufacturability.

The Rapid Tooling Testbed: A Distributed Design-for-manufacturing System

David W. Rosen, Yong Chen, Shiva Sambu, Janet K. Allen, Farrokh Mistree

Journal of Rapid Prototyping, 2003, Vol(9), 122-132 (paper)

A new design-for-manufacturing method, called the geometric tailoring (GT), and the associated digital interface concept have been developed that enable the design activities to be separated from the manufacturing activities. Conditions for the successful application of this method are investigated. The GT method is demonstrated for rapid prototyping and rapid tooling technologies, where prototype parts are required to match the production properties as closely as possible. This method is embodied in a system called the rapid tooling testbed (RTTB). Research work is presented on GT and the distributed computing environment underlying the RTTB. Examples are summarized from the usage of this method and testbed.

Computer-Aided Design for Rapid Tooling: Methods for Mold Design and Design-for-Manufacture

Yong Chen

Ph.D. Dissertation, Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 2001. (dissertation)

 

 

New modeling and computing methods for DDM-enabled complex geometries

 

Parallel and Efficient Boolean on Polygonal Solids

Hanli Zhao, Charlie C. L. Wang, Yong Chen, Xiaogang Jin

Visual Computer, Vol. 27, pp. 507-517, 2011 (paper)

We present a novel framework which can efficiently evaluate approximate Boolean set operations for Brep models by highly parallel algorithms. This is achieved by taking axis-aligned surfels of Layered Depth Images (LDI) as a bridge and performing Boolean operations on the structured points. As compared with prior surfel-based approaches, this paper has much improvement. Firstly, we adopt key-data pairs to store LDI more compactly. Secondly, robust depth peeling is investigated to overcome the bottleneck of layer-complexity. Thirdly, an out-of-core tiling technique is presented to overcome the limitation of memory. Real-time feedback is provided by streaming the proposed pipeline on the many-core graphics hardware.

Uniform Offsetting of Polygonal Model based on Layered Depth-Normal Images

Yong Chen, Charlie C. L. Wang

Computer-Aided Design, Vol. 42, 2010 (paper)

Uniform offsetting is an important geometric operation for computer-aided design and manufacturing (CAD/CAM) applications such as rapid prototyping, NC machining, coordinate measuring machines, robot collision avoidance, and Hausdorff error calculation.  We present a novel method for offsetting (grown and shrunk) a solid model by an arbitrary distance r.  First, offset polygons are directly computed for each face, edge, and vertex of an input solid model. The computed polygonal meshes form a continuous boundary; however, such a boundary is invalid since there exist meshes that are closer to the original model than the given distance r as well as self-intersections.  Based on the problematic polygonal meshes, we construct a well-structured point-based model, Layered Depth-Normal Image (LDNI), in three orthogonal directions.  The accuracy of the generated point-based model can be controlled by setting the tessellation and sampling rates during the construction process. We then process all the sampling points in the model by using a set of point filters to delete all the invalid points .  Based on the remaining points, we construct a two-manifold polygonal contour as the resulting offset boundary.  Our method is general, simple and efficient.  We report experimental results on a variety of CAD models and discuss various applications of the developed uniform offsetting method.

Layer Depth-Normal Images for Complex Geometries - Part I: Accurate Modeling and Adaptive Sampling

Yong Chen, Charlie C. L. Wang

Proceedings of ASME Design Engineering Technical Conferences, 2008 (paper)

The layered depth-normal images (LDNIs) is an implicit representation of solid models that sparsely encodes the shape boundary in three orthogonal directions.  We present a LDNI-based geometric modeling method for applications with high accuracy requirements.  In our method, we first construct LDNIs models from input polygonal models.  The accuracy of the generated LDNIs models can be controlled by setting pixel width during the construction process. Even for very complex geometries and high accuracy requirements, the construction process is fast with the aid of graphics hardware. Based on the LDNIs models, we then perform geometric modeling operations.  Two types of operations are presented including regularizing and Boolean operations.  The geometric modeling operations are straightforward and easy to be implemented robustly.  From the processed LDNIs model, an adaptive sampling method is presented to construct a cell representation that includes both uniform and octree cells.  Finally 2-manifold polygonal mesh surfaces are constructed from the cell representation.  For high accuracy requirements that are typical in CAD/CAM applications, we present a volume tiling technique and a parallel implementation to accelerate the computation.  Our method achieves a good balance between the accuracy and computational resources. We report experimental results on a variety of CAD models.  The results demonstrate the effectiveness and efficiency of our approach especially for modeling complex geometries.

Layer Depth-Normal Images for Complex Geometries - Part II: Manifold-Preserved Adaptive Contouring

Charlie C. L. Wang, Yong Chen

Proceedings of ASME Design Engineering Technical Conferences, 2008

We present an adaptive contouring approach to generate contour surface from solid models represented by Layered Depth-Normal Images (LDNI) sampled in three orthogonal directions. Our contouring algorithm builds an octree structure for mesh generation in a top-down manner: starting from the bounding box of a LDNI solid model, the cells are recursively subdivided into smaller sub-cells based on the topology and geometry criteria of refinement until both of the requirements, the topology in cell is simple and the geometry approximation error is less than a user defined tolerance, are satisfied. The subdivision also stops when the processed cells reach the finest resolution of LDNI models. In order to overcome the topology ambiguity inside a cell that leads to the occurrence of nonmanifold entities, we analyze the possible inside/outside configurations of cell-nodes and exploit two strategies to generate manifold-preserved mesh surfaces. Moreover, the most time-consuming step of our contouring algorithm – the construction of octree structure can be easily parallelized to run under a computer framework with multiple-processors and shared memory. Several examples have been tested in the paper to demonstrate the success of our method.

An Accurate Sampling-based Method for Approximating Geometry

Yong Chen

Journal of Computer-Aided Design 2007, Vol. 39, No. 11, pp. 975-986 (paper)

We present a sampling-based method for approximating the boundary of a geometry defined by various geometric operations. Based on a novel adaptive sampling condition, we first construct a volumetric grid such that in each cell an error-minimizing point can be found to captures all the geometric objects inside the cell. We then construct a polygonal model from the grid. We guarantee the boundary approximation has the same topology as the exact surfaces, and the maximum approximation error from the exact surfaces is bounded by a user specified tolerance. Our method is robust and easy to implement. We have applied it in applications such as remeshing of polygonal models, Boolean operations and offsetting operations. We report experimental results on a variety of CAD models.

    

Robust and Accurate Boolean Operations on Polygonal Models

Yong Chen

Proceedings of ASME Design Engineering Technical Conferences, 2007 (paper)

We present a new sampling-based method for the efficient and reliable calculation of boundary surface defined by a Boolean operation of given polygonal models.  We first construct uniform volumetric cells with sampling points for each geometric element of polygonal models.  We then calculate an error-minimizing point in each cell based on a quadratic error function (QEF).  Based on a novel adaptive sampling condition, we construct adaptive octree cells such that a QEF point in each cell can capture the shape of all the geometric elements inside the cell.  Finally we reconstruct a polygonal model from the volumetric grids and QEF points for approximating the boundary of a solid defined by the Boolean operation.  Our method is robust since we can handle different types of topological inconsistency including non-manifold configurations. It is also accurate since we guarantee the boundary approximation has the same topology as the exact surface, and the maximum approximation error from the exact surface is bounded by a user specified tolerance.  The efficient hierarchical scheme based on octree enables using sufficient grid resolutions on a commodity PC.  We demonstrate our algorithm for a number of test cases and report experimental results.

A Point-Based Offsetting Method of Polygonal Meshes

Yong Chen, Hongqing Wang, David W. Rosen, Jarek Rossignac

ASME Journal of Computing and Information Science in Engineering 2006, in review (paper)

We address the delicate problem of offsetting polygonal meshes. Offsetting is important for stereolithography, NC machining, coordinate measuring machines, robot collision avoidance, and Hausdorff error calculation. We introduce a new fast, and very simple method for offsetting (growing and shrinking) a solid model by an arbitrary distance r. Our approach is based on a hybrid data structure combining point samples, voxels, and continuous surfaces. Each face, edge, and vertex of the original solid generates a set of offset points spaced along the (pencil of) normals associated with it. The offset points and normals are sufficiently dense to ensure that all voxels between the original and the offset surfaces are properly labeled as either too close to the original solid or possibly containing the offset surface. Then the offset boundary is generated as the isosurface using these voxels and the associated offset points. We provide a tight error bound on the resulting surface and report experimental results on a variety of CAD models.

Filleting and Rounding Using a Point-based Method

Yong Chen, Hongqing Wang, David W. Rosen, Jarek Rossignac

Proceedings of ASME Design Engineering Technical Conferences, 2005 (paper)

Rounds and fillets are important design features. We introduce a new point-based method for constant radius rounding and filleting. Based on the mathematical definitions of offsetting operations, discrete offsetting operations are introduced. Steps of our approach are discussed and analyzed. The methodology has been implemented and tested. We present the experimental results on accuracy, memory and running time for various input geometries and radius. Based on the test results, the method is very robust for all kinds of geometries.

  

A Region Based Method to Automated Design of Multi-Piece Molds with Application to Rapid Tooling

Yong Chen, David W. Rosen

ASME Journal of Computing and Information Science in Engineering, 2002, Vol(2), 86-97 (paper).

Particularly for rapid tooling applications, delivering prototype parts with turn-around times of less than two weeks requires fast, proven mold design methods. We present a region-based approach to automated mold design that is suitable for simple two-piece molds (consisting of core and cavity), as well as molds with many additional moving sections. In our region-based approach, part faces are partitioned into regions, each of which can be formed by a single mold piece. The basic elements of our approach are concave regions (generalized pockets) and convex faces since these elements are central to the identification of regions. This paper focuses on the initial steps of automated mold design, including a problem formulation, methods for identifying the basic elements from part faces, and combining them into regions. By seeking to minimize the number of mold pieces, different partitions of faces into regions are explored until the smallest number of regions is found. During this process, a linear programming problem is adopted for finding a satisfactory parting direction of a region. Algorithms are presented for the region generating and combining process. Our approach is illustrated with several examples of industrial injection molded parts.

A Reverse Glue Approach to Automated Construction of Multi-Piece Molds

Yong Chen, David W. Rosen

ASME Journal of Computing and Information Science in Engineering, 2003, Vol(3) (paper).

Mold design can be a difficult, time-consuming process. Determining how to split a mold cavity into multiple mold pieces (e.g., core, cavity) manually can be a tedious process. This paper focuses on the mold construction step of the automated mold design process. By investigating glue operations and its relations with parting faces, an approach based on a new reverse glue operation is presented. The key to the reverse glue operation is to generate parting faces. A problem definition of parting face generation for a region is provided. Correspondingly, three face generating criteria are identified. Based on the parting lines of a region, our algorithms to generate the parting faces are presented. Our mold construction algorithms for two-piece molds and multi-piece molds are also presented with brief discussions. Some industrial examples are provided which illustrate the efficiency and effectiveness of our approach. We tested our mold designs by fabricating stereolithography mold inserts (a rapid tooling method) and molding parts.