Date: Tue, 14 Jan 1997 19:35:44 GMT Server: Apache/1.1.1+ Content-type: text/html Content-length: 7743 Last-modified: Tue, 14 Jan 1997 18:34:21 GMT Virtual Prototyping of Mechanical Assemblies

Virtual Prototyping of Mechanical Assemblies

Current generation CAD systems provide little support for prototyping mechanical assemblies. We are creating an integrated design and simulation system to address this problem. Fundamental to the effort is the investigation of effective mechanisms for allowing a designer to interact with an assembly of parts in a ``virtual'' manner, prior to creation of any physical prototypes. The designer will be able to touch, hold, and move models of parts and assemblies as well as see them in rendered views. The result will be an environment in which part interactions can be considered in a more natural manner than is possible with current technology. Part interactions and articulations of high DOF assemblies can be considered early in the design process. Assembly procedures can be evaluated at the time when parts are designed, without the need to explicitly specify assembly sequences or constraints. Even greater advantages will accrue in the design of devices intended for human use, since manipulability can be examined without the need to fabricate a physical prototype. Finally, the virtual display tools that are integral to this project will give designers a better appreciation for complex part geometry than can be gained from traditional CAD displays, which have only limited 3-D information.

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CAD systems for all but the simplest of mechanical parts must be able to provide the designer with a clear sense of three-dimensional object geometry. Usually this is done with standard graphical rendering techniques involving perspective, shading, and simple animation. When such tools are used for assemblies of parts, we often hear designers complain: ``What I really want to do is get my hands on the parts and see how they fit together.'' In fact what is happening is that the conventional display technologies are not providing the user with sufficient information about the geometry and manipulability of the objects in question.

A key aspect of addressing this limitation is to augment visual rendering of geometric models with haptic interfaces which allow direct manipulation of modeled objects. Such interfaces must include accurate anthropomorphic geometry and provide realistic force feedback if they are to be of real value. The Sarcos Dextrous Arm Master represents arguably the most advanced force-reflecting exoskeleton available today. The upper seven degree-of-freedom arm component matches the redundancy of the human arm, while three degrees of freedom are provided to the hand; in total, there are 10 degrees of freedom and of force reflection per master arm. In the hand, there is a two degree-of-freedom thumb, a fixed forefinger, and a one degree-of-freedom flexing index/middle finger. The Sarcos Dextrous Arm is based on hydraulic actuation, which is unmatched for torque density and bandwidth for this application. The result is a master that is much more compact and of higher performance than would be possible with electric motor drives.

The pictures above shows a one third scale model for a novel diesel engine with one cylinder, two pistons, and four crankshafts and represents of the kinds of assemblies that we are working towards accommodating in a virtual prototyping system. The engine involves complex and interrelated part motions that should be well understood by designers prior to committing to expensive fabrication operations. A full-function virtual prototyping system will let the designer manipulate individual parts to see how the entire assembly moves. Virtual prototyping furthermore would allow a designer to consider possible assembly sequences, modifying parts as necessary to ease the assembly process. Finally, a virtual prototyping system with a sufficiently complete model of the human body could be used to validate assembly operations for the engine.

Creating a user interface for virtual prototyping systems involves solving three largely independent problems:

Geometric modeling systems able to support virtual prototyping of mechanical assemblies need capabilities beyond those of the typical CAD/CAM system. Not only must it be possible to design and render individual parts, but part interactions involving contact, forces, and torques must be computable in real-time. When a designer reaches out to ``grab'' a (virtual) part, the system needs to know when contact with the part has been made so that appropriate feedback can be generated. Once a solid grip has been established, user feedback must reflect object inertial and gravity forces and assembly forces due to contact. For those assemblies capable of articulation, a manipulation of one part of the assembly must both cause the appropriate motions in other parts of the assembly and reflect appropriate sensations to the user.

Standard CAD systems are unable to supply much of the information needed to support either assembly operations themselves or the haptic rendering required in a virtual prototyping system.