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Computer-Aided Design 37 (2005) 45–54 www.elsevier.com/locate/cad

A reverse engineering method based on haptic volume removing
Zhengyi Yang, Yonghua Chen*
Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China Received 12 December 2003; received in revised form 19 March 2004; accepted 23 March 2004

Abstract This paper presents a new reverseengineering methodology that is based on haptic volume removing. When a physical object is to be digitized, it is first buried in a piece of virtual clay that is generated with the help of a fixture. Now digitizing the physical object is by simply chipping away the virtual clay with a position tracker that is attached to a haptic device PHANToMw. While chipping away the clay, the user can see on thecomputer monitor what is emerging and at the same time feel the chipping force from the haptic device. By so doing, reverse engineering is seamlessly integrated into haptic volume sculpting that is now widely used for conceptual design. Furthermore, the proposed method has eliminated the need to merge point clouds that are digitized from different views using current digitizers. The virtual clayvolume is represented by a spatial run-length encoding scheme. A prototype system has been developed to demonstrate the feasibility of the proposed new method through a case study. The strengths and weaknesses of the presented method are analyzed and the applicability is discussed. q 2004 Elsevier Ltd. All rights reserved.
Keywords: Reverse engineering; Conceptual design; Volume sculpting; Hapticshape modeling

1. Introduction Reverse engineering (RE) starts from an existing physical object. By various digitizing methods, point clouds of the object’s surfaces are collected automatically or manually. The point clouds are then segmented and fitted to different surface features so that a Computer-Aided Design (CAD) model of the object can be generated. In this paper, a novel method isintroduced to reverse engineering practice. It is a freehand haptic volume sculpting method. The basic principle of the proposed method is best explained by a graphical illustration. Fig. 1(a) shows a physical hand model to be digitized. In Fig. 1(b), two bounding boxes of the same size for the hand model are created with the help of a set of fixtures. One of the bounding boxes can be seen on the screenand the other is invisible but can be felt with the haptic device. Both of the two bounding boxes are treated as virtual clay. Object digitizing is by chipping away the delta volume (delta volume is defined as the difference between the virtual clay and the physical object) of the virtual clay using the position
* Corresponding author. Tel.: þ 86-852-2859-7910; fax: þ 86-852-28585415. E-mailaddresses: yhchen@hkucc.hku.hk (Y. Chen), yangzy@ graduate.hku.hk (Z. Yang). 0010-4485//$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cad.2004.03.003

tracker. The physical object to be digitized should be solid and impenetrable because there is physical contact between the position tracker and the object. This requirement indicates that the proposed method cannot workwell on soft objects. For the delta volume, it is deformable and assigned a spring constant k for haptic feedback. As shown in Fig. 1(c), while the delta volume is being removed, the model is seen emerging. When the entire delta volume of the virtual clay is removed, the volume left is a volume representation of the physical object. The haptic force is important while sculpting as the virtual claysurrounding the physical object is invisible, a sense of touch gives the user as to which part is yet to be removed. During sculpting, triangular meshes can also be displayed by ‘marching cube’ family algorithms. In Fig. 1(c), the volume representation is shown in the left viewport on the screen and the surface model obtained with marching cube algorithms in the right viewport. The tracker used...
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