Underwater Robots - Gianluca Antonelli.pdf

4. Fault Detection/Tolerance Strategies for
AUVs and ROVs
4.1 Introduction
Autonomous Underwater Vehicles (AUVs) and Remotely Operated Vehicles
(ROVs) received increasing attention in the last years due to their significant
impact in several underwater operations. Examples are the monitoring and
maintenance of off-shore structures or pipelines, or the exploration of the sea
bottom; see, e.g., reference [321] for acomplete overview of existing AUVs
with description of their possible applications and the main subsystems. The
benefit in the use of unmanned vehicles isinterms of safety, due to the
possibility toavoid the risk of manned missions, and economic. Generally,
AUVs are required to operate over long periods of time in unstructured environments
in which anundetected failure usually implies loss of the vehicle.
It is clear that, even in case of failure detection, in order to terminate the
mission, or simply to recover the vehicle, afault tolerant strategy, inawide
sense, must be implemented. In fact, simple system failure can cause mission
abort [154] while the adoption of afault tolerant strategy allows to safely
terminate the task as in the case of the arctic mission of Theseus [118]. In
case ofthe use of ROVs, askilled human operator isincharge ofcommand
the vehicle; afailure detection strategy is then of help inthe human decision
making process. Based on the information detected, the operator can
decide in the vehicle rescue or to terminate the mission by, e.g., turning off
athruster.
Fault detection is the process of monitoring asystem in order to recognize
the presence ofafailure; fault isolation or diagnosis isthe capability to
determine which specific subsystem is subject to failure. Often in literature
there isacertain overlapping in the use of these terms. Fault tolerance is the
capability tocomplete the mission also in case of failure ofone or more subsystems,
it is referred also asfault control, fault accommodation or control
reconfiguration. Inthe following the terms fault detection/tolerance will be
used.
The characteristics of afault detection scheme are the capability ofisolate
the detected failure; the sensitivity, in terms of magnitude of the failure that
can be detected and the robustness inthe sense of the capability ofworking
properly also innon-nominal conditions. The requirements ofafault tolerant
scheme are the reliability, the maintainability and survivability [240]. The
G. Antonelli: Underwater Robots, 2nd Edition, STAR 2, pp. 79–91, 2006.
© Springer-Verlag Berlin Heidelberg 2006