A Fault-Tolerant Fuzzy-Logic Based Redundancy Resolution Method for Underwater Mobile Manipulators

Abstract

In this work, a fault-tolerant redundancy resolution scheme is presented that allows a single 6-DOF command to be distributed over a small URVM system composed of an otherwise underactuated URV and serial manipulator. The URVM system admits an infinite number of joint-space solutions for each commanded end-effector state due to its inherent redundancy. The primary objective is realized using the right Moore-Penrose pseudoinverse solution. The secondary objectives are: avoiding manipulator joint limits, avoiding singularity and high joint velocity; keeping the end-effector in sight of the on-board camera minimizing the URV motion; and minimizing the drag-force resistance, or weathervaning. Each criterion is defined within the framework of the Gradient Projection Method. The hierarchy for the secondary tasks is established by a low-level artificial pilot that determines a weighting factor for each criterion based on if- then type fuzzy rules that reflect an expert human pilot's knowledge. A Mamdani fuzzy inference system is used to interpret the fuzzy rules based on the sensory knowledge. The resulting weight schedule yields a self-motion (null-space motion) that emulates how a skilled operator would utilize the full capabilities of the URVM to achieve the secondary objectives. The proposed redundancy resolution scheme has a fault-tolerant property. When a joint failure occurs, the scheme automatically redistributes the end-effector velocity command taking into account the faulty joints. To demonstrate the efficacy of the proposed scheme, several numerical simulations are performed The results illustrate the validity of the proposed redundancy scheme.