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Under construction. Virtual Prototyping for Human-Centric Design The goal of this project is to add a sense of contact and manipulation in the CAD design of mechanical assemblies. Part interaction, assembly, and manipulability can then be evaluated without fabrication of physical prototypes. A haptic device, such as the Sarcos Dextrous Arm Master, is being employed as a real-time interface to the Geometric Design and Computation (GDC) research group's Alpha_1 CAD/CAM system. This haptic interfacing allows forces of contact to be simulated, such as surface tracing, assembly forces, and grasping.
The particular challenges in this project are that mechanical CAD sytems are not designed for real-time interaction with haptic interfaces and for incorporating physics, while haptic interfaces have not been applied to operate on complex curved surfaces such as those typical of many manufactured parts [JMH96]. A key issue is how to partition the computations between a workstation, on which the CAD model resides, and a microprocessor controller for the haptic interface [JMH97]. Global Minimum Distance Calculation 
As a haptic interface moves around in the virtual environment, its
position has to be monitored for possible interaction with objects.
The haptic interface may be holding a virtual part, and the modeled
gripper
itself has a certain geometry, so arbitrary surface-to-surface minimum
distance calculation is required. Algorithms have been developed that
allow the minimum distance to be calculated at interactive rates,
based upon a hierarchical bounding boxes, control polygons of NURBS
surfaces, and subdivision [DEJ98a]. These algorithms can deal with
different types of surface description, including NURBS and polygons.
Recent work has employed coherence between time steps to make the
computation more efficient [DEJ99a].
The regions of minimum distance are the points of interest for haptic interaction. The corresponding surface portions are sent to the haptic controller for the real-time interaction using TCP/IP. The workstation receives regular updates from the micros using UDP about the haptic interface position, and caches sufficient surface patches on the micros around the haptic working point so that the haptic device will not run off the local surface. Local Collision Detection and Surface Tracing
The work to date has considered the end effector finger and thumb as a point. Work is currently progressing on surface-to-surface geometric computations, to model the 3D geometry of the arm and of part-to-part interactions [HJ99]. This project was supported by NSF Grant DMI-9978603, Dr. George Hazelrigg, Program Manager. Publications
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The actual collision detection and generation of contact forces must
be computed on the micros because of low-latency and high servo rate
requirements. The micros operates only on a local portion of the CAD
model. Methods have been developed whereby the geometric computations
occur directly on the NURBS model [TVTII97a], rather than on some form
of intermediate representation such as meshes or intermediate planes.
These methods take advantage of mathematical properties of NURBS and
run at interactive rates. Since NURBS are the surface representation
of choice in CAD/CAM, these methods make it possible to use a wide
collection of models. Also the compact representation of NURBS permits
both faster transmission and smaller storage requirements within the
micros. These methods have been extended to handle relative moving
surfaces, such as simulating model manipulation, a push button or
pendulum [TVTII97b]. Numerical aspects of the tracing have been
recently improved 
Trimming curves are normally added to create sharp edges, for holes
and other patterns; they stamp out shapes from the underlying surface
like a cookie cutter. The geometric computations are substantially
complicated by the addition of trimming curves to NURBS. However, we
have been able to modify the computations to still run at haptic rates
with trimmed NURBS [TVTII99].