Report Number: CS-TR-95-1537
Institution: Stanford University, Department of Computer Science
Title: Real-Time Modification of Collision-Free Paths
Author: Quinlan, Sean
Date: January 1995
Abstract: The modification of collision-free paths is proposed as the basis for a new framework to close the gap between global path planning and real-time sensor-based robot control. A physically-based model of a flexible string-like object, called an elastic band, is used to determine the modification of a path. The initial shape of the elastic is the free path generated by a planner. Subjected to artificial forces, the elastic band deforms in real time to a short and smooth path that maintains clearance from the obstacles. The elastic continues to deform as changes in the environment are detected by sensors, enabling the robot to accommodate uncertainties and react to unexpected and moving obstacles. While providing a tight connection between the robot and its environment, the elastic band preserves the global nature of the planned path. The greater part of this thesis deals with the design and implementation of elastic bands, with emphasis on achieving real-time performance even for robots with many degrees of freedom. To achieve these goals, we propose the concept of bubbles of free-space---a region of free-space around a given configuration of the robot generated from distance information. We also develop a novel algorithm for efficiently computing the distance between non-convex objects and a real-time algorithm for calculating a discrete approximation to the time-optimal parameterization of a path. These various developments are combined in a system that demonstrates the elastic band framework for a Puma 560 manipulator.