Designing Fault-Tolerant Manipulators: How Many Degrees-of-Freedom?

Abstract

One of the most important parameters to consider when de signing a manipulator is the number of degrees of freedom (DOFs). This article focuses on the question: How many DOFs are necessary and sufficient for fault tolerance, and how should these DOFs be distributed along the length of the manipula tor ? A manipulator is fault tolerant if it can complete its task even when one of its joints fails and is immobilized. The num ber of DOFs needed for fault tolerance strongly depends on the knowledge available about the task. In this article, two approaches are explored. First, for the design of a general purpose fault-tolerant manipulator, it is assumed that neither the exact task trajectory nor the redundancy resolution algo rithm are known a priori and that the manipulator has no joint limits. In this case, two redundant DOFs are necessary and sufficient to sustain one joint failure, as is demonstrated in two design templates for spatial fault-tolerant manipulators. In a second approach, both the Cartesian task path and the redundancy resolution algorithm are assumed to be known. The design of such a task-specific fault-tolerant manipulator requires only one degree of redundancy.