James D. English's PhD Thesis Abstract

Free-Swinging Failure Tolerance for Robotic Manipulators

Ph.D., Purdue University, Dec. 1996

Major Professor: Anthony A. Maciejewski

This thesis addresses the issues of tolerating and reducing the likelihood of the free-swinging type of robotic joint failure. A free-swinging failure is one that results in the lack of actuator torque. The failure-tolerance goals addressed are as follows: 1) before a failure, to prevent a failure or reduce negative effects should a failure occur and 2) after a failure, to continue to perform the desired task. The first of these, failure tolerance and prevention before a failure, is achieved by defining several measures of failure susceptibility and minimizing them under the constraint of prescribed hand motion. Each measure addresses a different failure tolerance or prevention aspect, using torque, acceleration, or displacement. The torque-based measure is appropriate for failure prevention and for reducing the force-domain aspects of a failure. The acceleration-based measure is meant for use with a failure-tolerant controller, as reduced joint acceleration after a failure gives more time to compensate. The displacement-based measures gauge the motion of a manipulator after a failure, i.e., the change resulting from the manipulator settling into a zero-gravitational-torque configuration. They are appropriate for preventing secondary damage and for reducing errors in the task. The second primary goal, use after a failure, is addressed by presenting ways to find which hand poses (position/orientation) are reachable after a failure. A method using the roots of a sixth-order polynomial is given for finding the zero-torque inverse kinematics of any planar three-link arm with any mass distribution. This is used to find postfailure workspaces by exhaustive search. Also, a differential method is given for tracing postfailure-workspace boundaries. It is valid for any n-link manipulator with a general static torque definition.