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Abstract: Impulse-based Dynamic Simulation of Rigid Body Systems

Impulse-based Dynamic Simulation of Rigid Body Systems

Brian Vincent Mirtich

Doctor of Philosophy in Computer Science, University of California at Berkeley

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Abstract:

Dynamic simulation is a powerful application of today's computers, with uses in fields ranging from engineering to animation to virtual reality. This thesis introduces a new paradigm for dynamic simulation, called impulse-based simulation. The paradigm is designed to meet the twin goals of physical accuracy and computational efficiency. Obtaining physically accurate results is often the whole reason for performing a simulation, however, in many applications, computational efficiency is equally important. Impulse-based simulation is designed to simulate moderately complex systems at interactive speeds. To achieve this performance, certain restrictions are made on the systems to be simulated. The strongest restriction is that they comprise only rigid bodies.

The hardest part of rigid body simulation is modeling the interactions that occur between bodies in contact. The most commonly used approaches are penalty methods, followed by analytic methods. Both of these approaches are constraint-based, meaning that constraint forces at the contact points are continually computed and applied to determine the accelerations of the bodies. Impulse-based simulation is a departure from these approaches, in that there are no explicit constraints to be maintained at contact points. Rather, all contact interactions between bodies are affected through collisions; rolling, sliding, resting, and colliding contact are all modeled in this way. The approach has several advantages, including simplicity, robustness, parallelizability, and an ability to efficiently simulate classes of systems that are difficult to simulate using constraint-based methods. The accuracy of impulse-based simulation has been experimentally tested and is sufficient for many applications.

The processing of collisions is a critical aspect of the impulse-based approach. Efficient algorithms are needed for detecting the large number of collisions that occur, without missing any. Furthermore, the physical accuracy of the simulator rests upon the accuracy of the collision response algorithms. This thesis describes these essential algorithms, and their underlying theory. It describes how the algorithms for simple rigid body simulation may be extended to systems of articulated rigid bodies. To prove the method is truly practical, the algorithms have been implemented in the prototype simulator, Impulse. Many experiments performed with Impulse are described.

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