Fault Tolerant Control of Nonholonomic Mobile Robot Formations

Fault Tolerant Control of Nonholonomic Mobile Robot Formations

T. Dierks, B. T. Thumati, S. Jagannathan
Copyright: © 2010 |Pages: 34
DOI: 10.4018/978-1-61520-849-4.ch003
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Abstract

In this chapter, a fault tolerant kinematic/torque control law is developed using backstepping for leader-follower based formation control in order to accommodate the dynamics of the robots and the formation in contrast with kinematic-based formation controllers. First, nominal control laws are derived for the leader and follower robots under the assumption of normal operation (no faults), and the stability of the individual robots and the formation is verified using Lyapunov methods. Subsequently, in the presence of state faults such as actuator fault, flat-tire etc., which could be incipient or abrupt in nature, an online fault detection and accommodation (FDA) scheme is derived to mitigate the effects of a fault by modifying the nominal controller. In other words, an additional term is introduced to the existing control law to minimize the effects of the fault, and this additional term is a function of the unknown fault dynamics which are recovered using the online learning capabilities of a neural network. Further, mathematical stability results are derived using Lyapunov theory, and both the FDA scheme and the formation errors are guaranteed to render asymptotic stability in the presence of faults. Numerical results are provided to verify the theoretical conjectures.
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Introduction

Over the past decade, the attention has shifted from the control of a single nonholonomic mobile robot to the formation control of mobile robots because of the advantages a team of robots offers in terms of increased efficiency for search and rescue operations, mapping unknown or hazardous environments, perimeter security and bomb sniffing (Chen & Wang, 2005). Leader-follower formation control is a popular methodology where followers stay at a specified separation distance and bearing angle from a designated leader. In such tasks, it is essential to consider the dynamics of each robot as well as the dynamics of the formation in order to ensure that the overall group objectives are achieved (Dierks & Jagannathan, 2009a). Additionally, due to the complexities of the mobile robot system and its interactions with the formation, the risk of failure is very high and jeopardizes the performance or success of the entire formation. Hence a robust, fault tolerant mobile robot formation control is needed. The purpose of the scheme is to maintain a reliable mobile robot formation even in the presence of system uncertainty and faults. Thus, in this work, a novel control design is proposed for leader-follower formation control under normal system operation (i.e., formation control without any faults). Then, a fault tolerant formation control scheme using neural networks (NN’s) is proposed for robots with incipient faults.

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