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Computers in Industry 59 (2008) 477–488 www.elsevier.com/locate/compind

A versatile virtual prototyping system for rapid product development S.H. Choi *, H.H. Cheung Department of Industrial and Manufacturing Systems Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Received 25 January 2007; received in revised form 22 June 2007; accepted 17 December 2007 Available online 1 February 2008

Abstract This paper presents a versatile virtual prototyping (VP) system for digital fabrication of multi-material prototypes to facilitate rapid product development. The VP system comprises a suite of software packages for multi-material layered manufacturing (MMLM) processes, including multi-toolpath planning, build-time estimation and accuracy analysis, integrated with semi-immersive desktop-based and full-immersive CAVEbased virtual reality (VR) technology. Such versatility makes the VP system adaptable to suit specific cost and functionality requirements of various applications. The desktop-based VR system creates a semi-immersive environment for stereoscopic visualisation and quality analysis of a product design. It is relatively cost-effective and easy to operate, but its users may be distracted by environmental disturbances that could possibly diminish their efficiency of product design evaluation and improvement. To alleviate disturbance problems, the CAVE-based VR system provides an enclosed room-like environment that blocks out most disturbances, making it possible for a design team to fully concentrate and collaborate on their product design work. The VP system enhances collaboration and communication of a design team working on product development. It provides simulation techniques to analyse and improve the design of a product and its fabrication processes. Through simulations, assessment and modification of a product design can be iterated without much worry about the manufacturing and material costs of prototypes. Hence, key factors such as product shape, manufacturability, and durability that affect the profitability of manufactured products are optimised quickly. Moreover, the resulting product design can be sent via the Internet to customers for comments or marketing purposes. The VP system therefore facilitates advanced product design and helps reduce development time and cost considerably. # 2007 Elsevier B.V. All rights reserved. Keywords: Virtual prototyping; Virtual reality; Immersive visualisation; Product design evaluation; Multi-material layered manufacturing

1. Introduction Mounting pressure of intensifying market globalization and competition has been driving manufacturing industries to compete on incessant reduction in lead-time and cost of product development while assuring high quality and wide varieties. However, conventional manufacturing processes are no longer sufficient to speed up validation of product design and development processes to meet ever-increasing diversities of customer demands, stringent cost control, and complexity of new products. Indeed, the significance of rapid product development or rapid manufacturing has been recognised in recent years. For this, many researchers have worked on developing various * Corresponding author. E-mail address: [email protected] (S.H. Choi). 0166-3615/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.compind.2007.12.003

technologies, which can be roughly categorised into three areas: (i) Layer manufacturing (LM) technology for physical fabrication of product prototypes, rapid tooling, and direct manufacture of components; (ii) Heterogeneous object modelling schemes and multimaterial toolpath generation algorithms for design and subsequent fabrication of composite and functionally graded objects, such as bio-degradable scaffolds; (iii) Virtual prototyping (VP) and virtual manufacturing (VM) simulation techniques for digital fabrication of prototypes, validation and optimisation of product designs, and evaluation of product assemblability and producability. Among these technologies, virtual simulation is regarded as an important technological advancement for product development, and it has been successfully used in ship-building and car

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S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488

industries [1,2]. It is a process of using virtual prototypes and simulation techniques, often in a virtual reality (VR) system with innovative input and stereoscopic output, to evaluate and improve a product design and to validate its planning and manufacturing processes [3–8]. Through simulations, key factors such as the shape and the manufacturability of a product may be optimised without committing much to prototypes and tooling. Indeed, virtual simulation reduces the need for physical prototypes and hence minimizes tooling cost and material waste, and it allows manufacturers to ‘‘get it right the first time’’ and helps them deliver quality products to market on time and within budget. However, the current virtual simulation technique, which often adopts either semi-immersive or full-immersive VR, is not without limitations, particularly with respect to the sense of immersion. Bochenek and Ragusa [6] pointed out that it is important to appropriately select a VR system for product design and development processes. They investigated the use of four commercial VR display systems and found that the sense of immersion plays an important role in improving the design review practices, and that a higher sense of immersion facilitates better improvement. In general, semi-immersive VR systems (single-screen or desktop-based) are relatively easy to use, affordable, and of good resolution, though their users tend to be susceptible to environmental distractions. On the other hand, full-immersive VR systems (multi-screen or CAVE-based) can generate a relatively higher sense of immersion that facilitates user interaction and collaboration, but they are generally more expensive, of less resolution and poor portability, and needs special space requirements [4,9,10]. Hence, it is worthwhile to combine the good features of both semi- and full-immersive VR to enhance the versatility and effectiveness of virtual simulation at affordable cost. This paper therefore proposes a versatile virtual prototyping system for evaluation of product designs and digital fabrication of multi-material prototypes either in a semi-immersive environment or in a full-immersive, disturbance-free environment to facilitate rapid product development. The versatility of choosing between semi- and full-immersive VR environments makes the VP system adaptable to suit the cost and functionality requirements of various applications. The VP system integrates semi- and full-immersive VR with multi-material layered manufacturing (MMLM) technologies. It comprises mainly a suite of software packages for simulation of MMLM processes, including multi-toolpath planning, build-time estimation, accuracy analysis, a desktopbased VR system, and a CAVE-based VR system. The desktopbased system creates a semi-immersive VR environment for stereoscopic visualisation, interaction, and quality analysis of the product design. It is cost-effective and easy to operate, but its users may be distracted by environmental disturbances that could possibly diminish the efficiency of the product design evaluation and improvement process. To alleviate disturbance problems, the CAVE-based VR system provides an enclosed room-like environment that blocks out disturbances, making it

possible for a design team to fully concentrate and collaborate on evaluation and improvements of a product design. Hence, the VP system enhances collaboration and communication of a design team working on product development. It provides effective tools to simulate and optimise MMLM processes that fabricate prototypes for design evaluation and improvement to facilitate subsequent product production. Through simulations, validation of a product design can be readily iterated as required without worrying about the manufacturing and material cost of prototypes. Thus, key factors, such as product shape and manufacturability, can be optimised accordingly. Moreover, the resulting product design can be sent via the Internet to customers for comments or marketing purposes. The VP system therefore facilitates advanced product design and helps reduce the time and cost of product development considerably. 2. Review of related works 2.1. Multi-material layered manufacturing Layered manufacturing (LM), also called rapid prototyping (RP), has been widely used to produce prototypes of complex shapes without tooling, particularly for manufacturing and medical applications [11,12]. Multi-material layered manufacturing is an extension of the existing single-material LM technology [13,14] for fabricating multi-material parts, such as electronic products, advanced communication components, drug delivery devices, and innovative cellular and cellcontaining tissue scaffolds [15–17]. A multi-material prototype may be made of materials that change gradually from one type to another, or of a collection of discrete materials. In contrast to single-material ones, multi-material prototypes can differentiate clearly one part from another, or tissues from blood vessels of a human organ; and they perform better in rigorous environments [18]. 2.2. Virtual reality and virtual prototyping VR systems have been successfully adopted for various applications, especially military training, entertainment, surgical planning, manufacture simulation, marketing, and museum exhibitions. Based on the level of immersion, VR systems are either semi-immersive or full-immersive. A semi-immersive VR system is typified by a desktop monitor or a pair of LCD projectors with a large screen [4,7,9,19–21] on which stereoscopic images are displayed, while a full-immersive VR system is often characterised by a room-like CAVE environment consisting of multiple screens. CAVE was first developed by the Electronic Visualisation Laboratory (EVL) of the University of Illinois-Chicago [22] to create a multi-person, room-sized, 3D video and audio environment. Stereoscopic images are projected onto three walls and the floor and are viewed with active glasses equipped with a location sensor. The following sections review some related works on the main features and limitations of semi- and full-immersive VR systems.


Morar and Macredie [19] pointed out that desktop-based VR systems are relatively cheap and portable, and are popular for 3D interactive computer games, commercial and industrial training, and modelling applications. Wang and Li [21] proposed a desktop VR system for industrial training applications, such as maintenance training for a refinery pump system. They considered the system affordable, portable, and easy-to-use, though it should be enhanced to provide better realistic and interactive performance by upgrading the software and hardware required. Hoffmann et al. [9] reported that full-immersive VR systems, such as the Powerwalls and the traditional CAVEs, are often operated as VR centres or showrooms and are only affordable by large enterprises due to the high cost and the complexity and size of such installations. To overcome such limitations, they attempted to develop a low-cost, compact, and full-immersive VR system by extending the classical desktop workplaces. Fairen et al. [10] also pointed out that CAVE-based systems do not fit in conventional offices due to special space requirements. They proposed a low-cost, portable, and semiimmersive VR system for cooperative inspection of complex computer-aided designs. This portable system was intended for demonstrations and presentations of designs at a client’s office. Indeed, semi-immersive VR systems can be treated as an affordable, easy-to-use, and convenient VR tool for stereoscopic visualisation, inspection, and interactions at locations, such as in a customer’s office and at a trade fair, without the need for large space and complex installation. On the other hand, CAVE-based VR systems are a powerful visualisation tool for collaborative applications, particularly in military, medical, and automobile and aerospace industries. For example, the USA army used a CAVE system to design, test, and review new vehicle models before physical fabrication was committed [23,24]. The development time and cost could be significantly reduced because design errors were reduced as communication between design team members was improved through immersive visualisation of the digital vehicle prototype in lieu of the physical ones. Indeed, CAVE provides a high sense of immersion in real-time for multiple users, and the level of immersion has a significant impact on user performance on collaborative tasks [25]. However, high cost and complex installation hinder the potential applications of CAVE-based VR systems in diverse markets. To overcome such weaknesses, Li et al. [26] developed a PC-based distributed multiple display VR system. As programming of this system was based on the traditional C/ C++ language, it might not be easy to develop relatively complex VR applications. Seron et al. [27] developed a CAVElike environment as a tool for full-sized train design. Although Seron’s system could reduce design errors and streamline the development of train products, the image quality, interactions, and the level of immersion might need further improvement by adding a floor with projected images at a cheaper cost. From the discussions above, it is obvious that the desktopand the CAVE-based VR systems have different worthiness and limitations. It would therefore be beneficial to integrate and

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exploit the good features of the two systems. The following section describes a versatile VP system for evaluation of product designs and digital fabrication of multi-material prototypes either in a semi-immersive environment or in a full-immersive, disturbance-free environment to facilitate rapid product development. This VP system provides flexibility for users to choose either a desktop- or a CAVE-based VR platform according to practical needs and available resources. 3. The proposed versatile virtual prototyping system The proposed versatile VP system consists mainly of a suite of software packages to simulate MMLM processes, including colour STL modelling, slicing, topological hierarchy sorting of slice contours for subsequent process planning, multi-toolpath planning and generation, and build-time estimation [33–36]. In particular, these packages are integrated with a set of control modules and VR graphics kernels that drive both desktop- and CAVE-based VR platforms to create semi- and full-immersive visualisation of the MMLM processes at the user’s choice. With the proposed VP system, designers can fabricate digital multi-material prototypes, in lieu of costly physical ones, to evaluate product designs and visualise the influences of critical process parameters, such as build-direction, layer thickness, and hatch space, on the MMLM process. The resulting digital prototypes can be sent via the Internet to customers to solicit comments, while the process parameters can be used for optimal fabrication of physical prototypes. This approach considerably reduces the number of costly physical prototypes needed for rapid product development. Therefore, the associated manufacturing overheads and product development time can be reduced substantially, because digital prototypes are mostly used and there is no worry about the cost and the quality of physical prototypes. Using the resulting set of optimal process parameters, physical prototypes of desirable quality can be made quickly and economically for detailed design evaluation. The physical prototypes can also be used as master patterns for making tools needed by conventional manufacturing processes, such as injection moulding and CNC machining, for mass production of the final products. Furthermore, the VP system would be particularly useful for small-batch production of customised products, which cannot be produced with conventional processes economically. Recently, LM has been widely explored for direct manufacture of customised products. It is envisaged that when LM becomes viable for direct manufacture of customised products, it will be vital to validate the accuracy and quality of prototypes before committing to physical fabrication. Therefore, the VP system would be a practical simulation tool for rapid product development. Fig. 1 shows the flow of the VP system. Firstly, a product model created by CAD or an MRI/CT digitiser is converted into STL format, which is the industry de-facto standard. As STL is monochrome or single-material, an in-house package is used to paint the STL model, with each colour representing a specific material.

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Fig. 1. The flow of the versatile VP system.

Secondly, a few steps are undertaken to prepare for subsequent simulation of the MMLM process and visualisation of the resulting digital prototypes: (a) slice the colour STL model into a number of layers of a predefined thickness. If the LM machine supports variable layer thickness, the STL model may be sliced with an adaptive slicing algorithm to increase fabrication efficiency. The resultant layer contours and material information are stored in a modified Common Layer Interface (CLI) file; (b) sort the slice contours with a contour sorting algorithm to establish explicit topological hierarchy; (c) based on the hierarchy information, multi-toolpath planning algorithms are used to plan and generate multi-toolpaths by hatching the slice contours with a predefined hatch space. The hatch vectors are stored in the modified CLI file for fabrication of digital prototypes and build-time estimation. Thirdly, a versatile VR simulation system is used for digital fabrication of multi-material prototypes. It allows users to choose either a desktop- or a CAVE-based VR platform to create a semi- or a full-immersive virtual environment,

respectively, for stereoscopic visualisation and quality analysis of the resulting digital prototypes, with which product designs can be reviewed and improved efficiently. A suite of algorithms for LM process planning, such as slicing, choice of build-direction, model orientation and layer thickness, generation of sequential and concurrent multitoolpaths, and build-time estimation, are incorporated in the proposed VP system. The details of these algorithms have been presented in [28–36]. This paper focuses on the development of the VR system. In particular, it addresses the enhancement of versatility and effectiveness of virtual simulation for product design and digital fabrication of multi-material prototypes at affordable cost. The following section describes the desktopand the CAVE-based VR platforms in detail. 3.1. The desktop-based VR platform The desktop-based VR system consists mainly of a software package for stereoscopic visualisation of product designs and


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Fig. 2. LCD projectors with a screen for semi-immersive VR display.

optimisation of MMLM processes. The software interfaces with commercial desktop-based VR hardware to display a model for stereoscopic visualisation. Using a desktop monitor, which is relatively small but highly portable, a user wears a pair of active shutter glasses that generate stereoscopic feelings by synchronising with the display device to switch on and off the images to the left eye and the right eye alternatively. This creates a semi-immersive VR environment in which a designer can stereoscopically visualise product designs and perform quality analysis. If a much larger display is needed, a pair of LCD projectors with a large screen as in Fig. 2 can be used. A user wears a pair of oppositely polarised glasses that filter the polarised images for the left eye and the right eye, respectively. With this wall-sized screen, a group of designers can participate in stereoscopic visualisation and collaborative review of product designs in the semi-immersive VR environment. This indeed improves exchange of ideas among a design team. In addition, the software package consists of a Product Viewer module and a Virtual Prototype Fabrication module, based on the WorldToolkit (WTK) graphic libraries, for simulation of the MMLM process. The Product Viewer module displays a colour product model in a semi-immersive VR environment in which a small group of designers can work together to study and improve the product design; the Virtual Prototype Fabrication module can then fabricate digital multimaterial prototypes of the product. A dexel-based approach is adopted for digital fabrication of prototypes [28]. A dexel is a hatch vector representing the path that a tool has to follow within a contour to build a portion of a layer. By building a volume of a specific height and a width around a dexel, a strip of material may be represented. Hence, rectangular solid strips are laid to form a layer, which is subsequently stacked up to form a prototype during a digital fabrication process. During the fabrication process, a designer can observe how a prototype is fabricated. Once it is finished, the resulting digital multi-material prototype can be studied using the utilities provided to visualise the quality of the prototype that the LM machine will subsequently deliver. The designer can navigate around the internal and opaque structures of the

prototype to investigate the design. Besides, the colour STL model can be superimposed on its digital prototype for comparison, with the maximum and the average cusp highlighted to indicate the dimensional deviations. A tolerance may be set to highlight locations with deviations beyond the limit. The designer may thus identify and focus on the parts that would need modifications. To improve the accuracy and the surface quality of some specific features of the prototype, the process parameters, such as the build-direction, the model orientation, the layer thickness, and the hatch space, may be tuned accordingly. After the visualisation process, the colour STL model of the toy car is sliced, for example, into 120 layers with a thickness of 0.194 mm, because most current LM machines support only uniform layer thickness during a prototype fabrication process. If the LM machine to be simulated supports variable layer thickness, the model may be instead sliced with an adaptive slicing algorithm. The resulting layer contours are then sorted to establish the topological hierarchy for generation of multitoolpaths with a hatch space of 0.400 mm. Subsequently, the Virtual Prototype Fabrication module fabricates a digital prototype by depositing the rectangular solid strips one by one at an appropriate z-height, as shown in Fig. 3. The resulting virtual prototype of the toy car can be manipulated for visual inspection, as in Fig. 4. Furthermore, it can be superimposed on its STL model to highlight the surface texture and the dimensional deviations, as in Fig. 5. The system also calculates the cusp heights to evaluate the overall dimensional deviations. In this example, the average and the maximum cusp heights are 0.098 and 0.164 mm, respectively. Suppose that any deviations more than 0.170 mm are considered unacceptable, the designer may choose to highlight the areas which are out of the design limit for subsequent investigation of these critical features. Excessive deviations are highlighted with red or green pins. The red pins point to the maximum deviations whereas the green ones point to unacceptable deviations. If unsatisfactory deviations are located at important parts of the model, the designer may choose either to change the model orientation to shift the deviations or to reduce the layer thickness and the hatch space to improve the accuracy. When it is necessary to

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Fig. 3. Digital fabrication process of a toy car prototype.

assess detailed assembly fitness of the various parts of the toy car, the parts can be stored as individual STL models for quality analysis and digital fabrication. Digital fabrication of a part prototype can be repeated until a set of acceptable process parameters are obtained. Subsequently, physical prototypes of all parts are produced and assembled to form a complete toy car prototype. Furthermore, the physical prototypes can be processed and used as master patterns to make tools for mass production of the product. Therefore, the proposed desktop-based VR system is an easy-to-use and cost-effective tool for visualisation and digital fabrication of multi-material prototypes to facilitate product design review and improvement. However, the semi-immersive VR environment may be susceptible to environmental disturbances, diminishing the designers’ true feeling and concentration and hence their efficiency in the design process.

To address this problem, the level of immersion is enhanced by integrating the VP system with a CAVE-based VR system with multiple screens to provide a full-immersive virtual environment for vivid stereoscopic visualisation and interaction in a natural way. As such, a design team can fully immerse in exploration, study, and improvement of a product design, including assemblies, sub-assemblies, and components, well before they ever exist physically in reality. Hence, the time and cost of product development can be further reduced. 3.2. The CAVE-based VR platform The CAVE-based VR platform consists of a cluster of PCs with a cubicle of three walls on a floor. An immersive virtual environment is created by projecting stereoscopic images on three 10ft 8ft screens on the walls, namely the front, the right,

Fig. 4. Two perspectives of the toy car prototype.


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Fig. 5. Superimposition of the toy car prototype on its STL model to highlight excessive materials and dimensional deviations.

and the left, respectively, and on a 10ft 10ft screen on the floor. Fig. 6a and b shows, respectively, the architecture and the physical construction of the PC-based CAVE system, called imseCAVE, in the IMSE Department at the University of Hong Kong. Each projection screen has a reflector and two LCD projectors controlled by two related PCs. The LCD projectors are specially designed with polarising lenses to produce highresolution stereoscopic images. A VR engine, consisting of a cluster of network PCs, coordinates the projectors to project

images on the related screens to create an immersive virtual environment. This configuration forms a relatively low-cost, configurable, and flexible CAVE-based VR system, which can be conveniently integrated to form the proposed versatile VP system to facilitate product development. The hardware is controlled by a software package, which can be separated into three layers, as shown in Fig. 7. The bottom layer includes basic system software and hardware, such as Windows XP OS and graphics kernel for control of the PC network, interface devices, and projectors.

Fig. 6. (a) The architecture of the imseCAVE. (b) Physical construction of the imseCAVE.

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Fig. 7. Software architecture of the PC cluster-based CAVE system.

The middle layer is a PC cluster-based library that coordinates all operations of the CAVE system. It synchronises all the devices to create correct perspective for each screen, keeps track of which screens are in use, and provides the applications with the current states of all the CAVE elements. It consists of three sub-systems: (i) I/O device sub-system for controlling I/O devices; (ii) display subsystem for projecting images on the corresponding screens; and (iii) the network subsystem for keeping communication and synchronisation between all clustering PCs. The top layer is a package of tailor-made application programs, developed with the PC cluster-based CAVE library, for immersive visualisation and simulation in the CAVE system. This application package is implemented in an objectoriented programming tool, called the Virtools Dev, and its addon library, called Virtools VR Pack [37]. The Virtools Dev contains a suite of algorithms to help programmers create, visualise, manipulate, and track objects in the virtual environment. Besides, the Virtools VR Pack allows users to tailor applications for producing full-immersive, life experiences using the PC-based distributed computing. In using the Virtools Dev tool, a Script is the visual representation of a behaviour applied to an element. A behaviour is described with Behaviour Building Blocks (BBs) which are a visual representation of a software element known as a function. The software provides a collection of pre-defined BBs that enable users to create an application script conveniently. Scripts are performed by the Behaviour Engine. With this approach, the application package can be conveniently developed for stereoscopic visualisation and simulation of the MMLM process in an immersive CAVE virtual environment. The application package for the PC cluster-based CAVE system contains two modules, namely, Product Viewer and Virtual Fabricator, similar to those for the desk-based system. The Product Viewer displays a virtual product in a CAVE virtual environment in which users can fully immerse to manipulate and study the design, as in Fig. 8. It also facilitates manipulations of a virtual product, including rotation, and scale up/down, toggling visibility/invisibility of a component, using wireless I/O devices, such as a mouse, a keyboard, and a joystick. The designer can hide the external car body to study the gearbox assembly from different perspectives.

Such manipulation functions are developed using the Virtools Dev. A product model created by the CAD software or the digitised equipment is first converted into the VRML file format, and then imported to the Virtools Dev to add functions and behaviour of each product component by linking the BBs accordingly. To develop functions for rotating and scaling up/ down the virtual product via a number of specific keys of a wireless keyboard, three standard BBs, called Switch On Key, Rotate, and Scale, are used. When a user presses down a particular key for rotating the model, the Switch On Key BB is triggered and a output signal is sent to the corresponding Rotate BB to activate the rotation behaviour. Thus, the context of the Product Viewer module can be easily developed. In addition, the Virtual Fabricator, similar to that in the desktop-based system, is created with the Virtools Dev for digital fabrication of multi-material prototypes. When the fabrication completes, designers can fully immerse in the CAVE virtual environment for stereoscopic visualisation and quality analysis of the resulting multi-material virtual prototype. This full-immersive environment blocks out most disturbances and hence enhances the efficiency of the design review and improvement process. For the Virtual Fabricator to simulate a prototype fabrication process, the multi-toolpaths are translated into a dataset supported by the Virtools Dev for the Virtual Fabricator to load and fabricate. The Virtual Fabricator has two scripts created with the Virtools Dev, one for loading the dataset, namely DataLoad, and another for building 3D models for simulation of the prototype fabrication process, namely Build3DModel. The script DataLoad consists of two BBs, called Array Load for loading the tabular data in cells into an array from the formatted file and Activate Script for activating a script, respectively. These two BBs are linked together with behaviour links (bLinks). Each BB has its own parameters. When the DataLoad script finishes the loading of data, the Activate Script BB is triggered to activate the Build3DModel script via a behaviour link (bLink). Then, the Build3DModel starts simulating the prototype fabrication process layer by layer. The Virtual Fabricator also adopts the dexel-based approach for simulation of the prototype fabrication process, as in the desktop-based system. It uses an object Cube, which is a standard 3D entity data resource and is stored in NMO file format, to represent a dexel with its specific position, size, and material property. Hence, three BBS, namely Set Position, Scale, and Set material, are used to represent the position, size, and material of a dexel based on the data stored in a text file. In addition, the Iterator BB is used to control the layer-by-layer loop process. For the scripts of both the Product Viewer and the Virtual Fabricator to function properly in the CAVE-based system, they have to be integrated with a script program called the PC cluster-based CAVE Coordinator. The PC cluster-based CAVE Coordinator is created with the Virtools VR pack, which provides a set of VR libraries containing a package of standard BBs for users to develop applications needed to control a cluster of PCs and projectors and to generate a full-immersive virtual environment. The Coordinator can be treated as a


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Fig. 8. Study of a product design in the CAVE virtual environment.

middle layer between the Product Viewer/the Virtual Fabricator modules and the PC cluster-based CAVE display to coordinate the operations of the whole CAVE-based VR system. It synchronises a cluster of PCs to distribute image signals to the related projectors for projection on the screens, and hence the creation of an immersive virtual environment through the network system. In this cluster of PCs, one is master for receiving user input signals while the remaining ones are slaves for screen display. Firstly, the master receives the peripheral state of the joystick, the keyboard, and the mouse, etc. and then sends this state to each slave. Secondly, the slaves wait for this state to arrive and acknowledge reception to the master. Based on the same shared causes, they will all compute their own frame when every PC holds the shared state. Thus, using the Virtools VR Pack, this distributed computing technique can be easily

developed by logically linking the Virtools VR BBs, such as VR Host Id and VR Distrib, to develop specific applications. It can be seen that the PC cluster-based CAVE system above is relatively convenient and flexible, making full-immersive VR a versatile and affordable tool for small-and-medium sized companies to develop products. 4. A case study Hong Kong produces a wide range of footwear products mainly for export. In recent years, many footwear companies have attempted to develop their own brands by improving capabilities of product design, tooling, and quality control, while some others focus on providing tailor-made services to take care of the special needs of customers. The proposed versatile VP system would therefore be useful for the footwear

Fig. 9. Digital fabrication of the shoe sole prototype.

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Fig. 10. Digital fabrication process of a multi-material shoe sole prototype.

industry to help save product development cost and time. In the following section, a casual shoe sole, which affects the comfort and performance of the footwear, is used to demonstrate a possible application of the proposed versatile VP system. 4.1. A casual shoe sole Using the VP system, a shoe designer can choose a desktopbased VR system to review the shape, colour, and ergonomics of the shoe sole in a semi-immersive virtual environment. But to minimise disturbances and to increase the level of immersion to help a design team focus on exchange of ideas for design improvements, a CAVE VR system can be used instead. This helps reduce product development cost and time substantially since potential errors can be avoided in the early design stage. After visualisation and evaluation of the design, the shoe sole model in STL format is firstly sliced. In this case, it is sliced into 88 layers with a layer thickness of 0.178 mm. Secondly, multi-toolpaths are planned and generated by hatching each layer contours with a hatch space of 0.496 mm. Subsequently, based on the resulting multi-toolpaths containing geometric and material information, a virtual multimaterial shoe sole prototype is fabricated, either in a semi- or

Fig. 11. Superimposition of the shoe sole prototype on its STL model.


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Fig. 12. (a) Measurement of foot plantar pressure using an F-Scan1 system. (b) Foot plantar pressure profiles for the pair of physical shoe soles.

full-immersive environment, as shown in Fig. 9. During the digital fabrication process, the designers can visualise how it is fabricated layer by layer to reveal the detail structure, as shown in Fig. 10. The resulting shoe sole prototype can be superimposed on its colour STL model to highlight dimensional deviations, and the overall dimensional deviations can be analysed accordingly, as in Fig. 11. In this case, the average and the maximum cusp heights are 0.092 and 0.179 mm, respectively, and the regions with deviations exceeding 0.160 mm are highlighted. The red pins point to the maximum deviations whereas the green ones point to unacceptable deviations. With this result clearly visualised, the designer may choose to modify the design or change a new set of process parameters to minimise the dimensional deviations. When the quality is deemed acceptable, the process parameters can be used for subsequent fabrication of physical prototypes. Hence, the MMLM process is optimised and the number of costly physical prototypes reduced accordingly. As such, a pair of physical shoe sole prototypes of elastomeric material with rubber-like properties [38] can be fabricated on a 3D printing machine to test ergonomic fitness by measuring the profiles of the plantar pressure induced by a user’s feet. To do this, an F-Scan1 system with a pair of paper-thick sensors [39], as shown in Fig. 12a, may be put on the shoe soles for the user to step on for testing. By studying the plantar pressure profiles generated as shown in Fig. 12b, the designer can evaluate the ergonomic fitness of the shoe soles quantitatively, and modify the design accordingly, if deemed necessary. The design modification-evaluation-testing process above can be repeated quickly until a pair of shoe soles of satisfactory design is obtained. Therefore, the proposed versatile VP system is useful for reducing the cost and time of product development. 5. Conclusion This paper proposes a versatile VP system which integrates the good features of semi- and full-immersive VR to enhance the versatility and effectiveness of virtual simulation for product design and digital fabrication of multi-material prototypes at affordable cost.

The VP system comprises mainly a suite of software packages for simulation of MMLM processes, including multitoolpath planning, build-time estimation, and accuracy analysis. It can drive a desktop-based VR system with either a monitor or a large non-depolarising screen to generate a semiimmersive VR environment, which is cost-effective, portable, and easy to operate, for review and improvement of product designs. To minimise environment disturbances and to enhance the level of immersion, the VP system can control a PC clusterbased CAVE system to create a full-immersive VR environment that enhances collaboration and communication of a design team working on product development. It is indeed an effective and versatile tool for rapid product development to meet everincreasing diversities of customer demands, stringent cost control, and complexity of new products. Acknowledgements The authors would like to acknowledge the Research Grant Council of the Hong Kong SAR Government and the CRCG of the University of Hong Kong for their financial support for this project. References [1] H.T. Kim, J.K. Lee, J.H. Park, B.J. Park, D.S. Jang, Applying digital manufacturing technology to ship production and the maritime environment, Integrated Manufacturing Systems 13 (5) (2002) 295–305. [2] G. Wo¨hlke, E. Schiller, Digital planning validation in automotive industry, Computers in Industry 56 (4) (2005) 393–405. [3] M. Kerttula, T. Tokkonen, Virtual design of multiengineering electronics systems, IEEE Computer 34 (11) (2001) 71–79. [4] T.S. Mujber, T. Szecsi, M.S.J. Hashmi, Virtual reality applications in manufacturing process simulation, Journal of Materials Processing Technology 155/156 (2004) 1834–1838. [5] A. Gomes de Sa´, G. Zachmann, Virtual reality as a tool for verification of assembly and maintenance processes, Computers & Graphics 23 (3) (1999) 389–403. [6] G.M. Bochenek, J.M. Ragusa, Study results: the use of virtual environments for product design, in: Proceedings of IEEE International Conference on Systems, Man, and Cybernetics, Hyatt Regency La Jolla, San Diego, California, USA, 11–14 October, (1998), pp. 250–1253. [7] M. Weyrich, P. Drews, An interactive environment for virtual manufacturing: the virtual workbench, Computers in Industry 38 (1) (1999) 5–15.

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H.H. Cheung gained his BEng degree from the IMSE Department at the University of Hong Kong. He continued his postgraduate research study in the Department, and his research interest is in virtual prototyping technology.