A versatile virtual prototyping system for rapid product ......A versatile virtual prototyping...

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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 CAVE- based 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 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 multi- material 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 develop- ment, and it has been successfully used in ship-building and car www.elsevier.com/locate/compind Available online at www.sciencedirect.com Computers in Industry 59 (2008) 477–488 * 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

Transcript of A versatile virtual prototyping system for rapid product ......A versatile virtual prototyping...

Page 1: A versatile virtual prototyping system for rapid product ......A versatile virtual prototyping system for rapid product development S.H. Choi*, H.H. Cheung Department of Industrial

www.elsevier.com/locate/compind

Available online at www.sciencedirect.com

(2008) 477–488

Computers in Industry 59

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 CAVE-

based 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) L

ayer manufacturing (LM) technology for physical

fabrication of product prototypes, rapid tooling, and direct

manufacture of components;

(ii) H

eterogeneous object modelling schemes and multi-

material toolpath generation algorithms for design and

subsequent fabrication of composite and functionally

graded objects, such as bio-degradable scaffolds;

(iii) V

irtual 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 develop-

ment, 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) 477–488478

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 environ-

ment 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) tech-

nologies. It comprises mainly a suite of software packages for

simulation of MMLM processes, including multi-toolpath

planning, build-time estimation, accuracy analysis, a desktop-

based VR system, and a CAVE-based VR system. The desktop-

based 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 simula-

tions, 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 manu-

facturing 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 cell-

containing 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 differenti-

ate 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, surgi-

cal 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.

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S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488 479

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 semi-

immersive 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 stereo-

scopic 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 CAVE-

like 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 desktop-

and the CAVE-based VR systems have different worthiness and

limitations. It would therefore be beneficial to integrate and

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.

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488480

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 algo-

rithms 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 multi-

toolpaths, 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 desktop-

and 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.

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488 481

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 multi-

material 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 multi-

toolpaths 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.

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488482

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.

Fig. 4. Two perspectives of

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,

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.

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488 483

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 high-

resolution stereoscopic images. A VR engine, consisting of a

cluster of network PCs, coordinates the projectors to project

Fig. 6. (a) The architecture of the imseCAVE. (

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.

b) Physical construction of the imseCAVE.

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

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488484

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 object-

oriented programming tool, called the Virtools Dev, and its add-

on 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 experi-

ences 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 proto-

type. 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.

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488 485

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

Fig. 9. Digital fabrication of

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

the shoe sole prototype.

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

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

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488486

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 desktop-

based 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 multi-

material shoe sole prototype is fabricated, either in a semi- or

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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.

S.H. Choi, H.H. Cheung / Computers in Industry 59 (2008) 477–488 487

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 multi-

toolpath 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 semi-

immersive 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 cluster-

based 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 ever-

increasing 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.

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S.H. Choi is associate professor in the IMSE Depart-

ment at the University of Hong Kong. He obtained

both his BSc and PhD degrees at the University of

Birmingham. He worked in computer industry as

CADCAM consultant before joining the University

of Hong Kong. His current research interests include

CADCAM, advanced manufacturing systems and

virtual prototyping technology.

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.