Design and Analysis of a Stiffness-Enhanced 3-PPS Parallel Mechanism for Fault-Tolerant Underwater Vectored Thrusters

Abstract

Vectored thrusters can significantly improve the maneuverability of underwater vehicles. However, due to the harsh underwater environment and severe working conditions, the thrust-vectoring device needs to be designed with high stiffness and high reliability. In this paper, a 3-degree-of-freedom (3-DOF) 3-PPS parallel mechanism is employed for the 2-DOF thrust-vectoring device, which has the advantages of high stiffness and a certain level of fault tolerance. The stiffness of the 3-PPS parallel mechanism is enhanced through employing additional passive prismatic joints. Based on the zero-torsion characteristics of the parallel mechanism, closed-form solutions are obtained for displacement analyses, and the orientation workspace of the moving platform under an actuation failure, i.e., one of the active prismatic joints is locked, is particularly investigated through an equi-volumetric partition method. To analyze the orientation workspace distribution under the actuation failure, the fault-tolerant workspace and the maximum inscribed workspace are defined. Furthermore, a new fault-tolerant index is proposed to evaluate the fault tolerance of the parallel mechanism. The proposed design analysis is validated through experiments on an engineering prototype of the parallel mechanism.