Feature based haptic rendering

Abstract

3D I/O devices, long awaited in Computer Aided Design (CAD) systems, are finally available as force feedback devices such as PHANToM (SensAble Technologies, Inc.). This paper presents Feature Based Haptic Rendering (FBR), which provides haptic feedback of a more accurate shape required by the haptic man-machine interface for surface CAD than intermediate representation (IR) [1][2]. Fig. 1 shows the FBR architecture. Virtual surface modelling process is connected to a haptic rendering loop using a single asynchronous TCP/IP socket stream. As [2] states, decoupling haptic rendering process is indispensable to obtaining high haptic fidelity that requires device control latency of ~kHz, since shape modelling, e.g. NURBS knot insertion, may take time. IR is data exchanged between decoupled processes, which actually is a plane in [1] and to which [2] added a few more primitives. Because only received IR is used in haptic rendering, the haptic process does not need a geometry database. The database coherency problem that arises with such architecture, in that each process has its own geometry database copy, can thus be avoided. FBR generalizes the idea of IR to a feature, which is the unit of communication and can be categorized into two groups -data and commands. The data feature includes local haptic properties of virtual objects, such as shape (Fig. 2), texture, stiffness, and position. Command feature is used to control other processes, e. g., switching interpolation strategies of received shape features in the haptic rendering loop. FBR provides a communication protocol to exchange features. A feature is split into one or more fixed-length packets from which the receiver reconstructs the original feature. The haptic renderer has feedback force calculation algorithms and interpolation strategies for features. FBR can be used by any point-contact type force feedback device as IR and can be easily extended to multiple-contact type devices by introducing an object momentum feature. Haptic volume rendering is also easy: shape features can be constructed as local isosurfaces by the Marching Cube algorithm [3], for example. Physically correct haptic rendering of a thin, elastic object is then possible, which is important for surgical simulation but is difficult with the voxel density field algorithm [4]. A virtual surface modeler with force feedback has been implemented with FBR, whose modelling and graphic rendering run on a SGI Indigo2 Elan (200 MHz R4400) and the haptic display is PHANToM (6 DOF input and 3 DOF force output) on a PC (200 MHz Pentium Pro). The user can touch, trace, and deform a B-Spline and a triangular surface directly [5] with virtual tools catching haptic feedback (Fig. 3). Rich shape features display the surface shape accurately. A 5-10 kHz device control latency has been achieved with 20-30 Hz communication and graphics refresh rate. Quantitative and qualitative evaluation of FBR using psychophysical methods are in work.