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Selected Readings in Vision and Graphics
edited by Luc Van Gool, Gábor Székely, Markus Gross, Bernt Schiele
Haptic rendering of
frictional tool-tissue contact
First edition 2008. 198 pages (10 of them in color), 64,00.
ISBN-10: 3-86628-207-9. ISBN-13: 978-3-86628-207-0
The sensation of interaction forces in virtual reality applications enhances the immersion of users in computer generated environments. It helps them to move objects embedded in the virtual environments in a natural way by using all the input stimuli they would get from the real world. The display of contact forces has a big potential in virtual reality based training systems where high dexterity of tool manipulation is required. These are for example surgical simulators.
Providing haptic feedback in surgical simulators is a challenging task, for which the main bottleneck is the computational power of current hardware. In general, a refresh rate of 1 kHz is required for a stable force feedback. This is hard to achieve in virtual environments which model the interaction with deformable bodies. On one hand it is due to the high number of degrees of freedom needed for the representation of the soft body, on the other hand it is due to a possibly large number of contact points acting at a single moment. In the existing simulators, the force output has been therefore often simplified, modeling contacts with single points and with the assumption of frictionless contact.
In this thesis we present two methods for a realistic 6-Degree-Of-Freedom force feedback, considering frictional contact between rigid tools and deformable bodies. The first one is an extension of the popular virtual proxy point method, applied to multiple contacts. The contact points are treated independently, which promises low complexity of the algorithm. Nevertheless, it is based on penalty forces, which can lead to false behaviour under high loads. The second method follows a constraint based contact formulation, which provides physically more precise contact forces than the penalty based approach. Nevertheless, it is computationally more expensive than the first approach, for what it is hard to compensate in real-time. Therefore, in this method we focus on the development of an efficient multirate algorithm, which decouples the full update of the contact forces from their display on the haptic device. A computationally effective haptic rendering algorithm is achieved by linearising the inverse of the contact Jacobians in the active space.
To analyse the fidelity of the provided haptic feedback, we propose a Turing like test, where the participants have to discern blindly between the groups of real objects and virtually generated ones. Our pilot studies have been simple in the sense that it allowed the user to push on the sample only at one point. Nevertheless, they proved that in our setup we achieved high realism when haptically presenting soft virtual objects, with the stiffness range of soft tissues, to the users. As a result, the relative stiffnesses of the virtual samples have been truthfully recovered by all the participants. Moreover, we observed, that if little noise is present in the haptic output, even if being hardly detectable by the users, it unconsciously amplifies the perceptual difference between the real and the virtual objects.
Finally, the results of these experimets identify the limited stiffness of our haptic hardware and the not tuned dynamic properties of the virtually generated objects as the main indicators upon which it is possible to discern between the real and virtual objects.
About the author:
Peter Lekovskı studied Computer Science at the Comenius University in Bratislava, Slovakia, where he obtained his M.Sc. in Computer Science in 2002. His focus was on Computer Graphics, Combinatorics and Computation. In 2002 he attended a CGC Pre-Doc program at the ETH Zürich, Switzerland, as a preparation for his future doctoral studies. In May 2003 he became a member of the Computer Vision Lab at ETH Zürich. Here he led his research on haptic interaction with soft tissues, with the focus on generating a realistic real-time 6DOF haptic feedback for a virtual surgery simulator. The work was performed under the scope of the European TOUCH-HapSys project. In 2007, he finished his doctoral thesis and was awarded a Ph.D. degree (Doctor of Sciences) from the ETH Zürich.
Keywords / Schlagwörter:
PhD, haptics, 6DOF, contact problem, friction, physically based simulation, virtual reality, deformable objects, Turing test
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