Underwater Robots - Gianluca Antonelli.pdf

80 4. Fault Detection/Tolerance Strategies for AUVs and ROVs
common concept is that, to overcome the loss of capability due to afailure, a
kind of redundancy is required in the system. Ageneral scheme is presented
in Figure 4.1.
In this chapter, asurvey over existing fault detection and fault tolerant
schemes for underwater vehicles ispresented. Forthese specific systems, adopting
proper strategies, an hardware/software (HW/SW) sensor failure or
an HW/SW thruster failure can besuccessfully handled indifferent operating
conditions as it will be shown in next Sections. In some conditions, it is required
that the fault detection scheme is also able to diagnostic some external
not-nominal working conditions such asamulti-path phenomena affecting
the echo-sounder system [61]. It is worth noticing that, for autonomous systems
such asAUVs, space systems or aircraft, afault tolerant strategy is
necessary to safely recover the damaged vehicle and, obviously, there isno
panic button in the sense that the choice ofturning off the power oractivate
some kind of brakes is not available.
Most of thefault detection schemes aremodel-based [1, 3, 4, 5, 52,61, 140,
143, 238, 306, 307] and concern the dynamic relationship between actuators
and vehicle behavior or thespecific input-output thruster dynamics. Amodelfree
method ispresented in[43, 173]. Higher level fault detection schemes are
presented in[117, 118, 146, 324]. References [42, 177, 212, 222, 279, 308]
deal with hardware/software aspects of a fault detection implementation
for AUVs. Neural Network and Learning techniques have also been presented
[102, 113, 142, 144, 284].
Concerning fault tolerant schemes, most ofthem consider athruster redundant
vehicle that, after afault occurred in one of the thrusters, still is
actuated in 6degrees of freedom (DOFs). Based on this assumption areallocation
ofthe desired forces onthe vehicle over the working thrusters is
performed [3, 4, 5, 52, 61, 232, 233, 234, 236, 251, 306, 307]. Of interest
is also the study ofreconfiguration strategies if the vehicle becomes underactuated
[228].
Only few papers concern the experimental results of fault detection and
fault tolerant schemes [3, 4, 5, 42, 52, 61, 117, 118, 212, 232, 233, 251, 306,
307]; for all the above references it is worth noticing that the successfully
results has been achieved with the implementation ofsimple algorithms.
In Section 4.2 asmall list of failures occurred during wet operations is
reported; Section 4.3 and 4.4 report the description of fault detection and
tolerant strategies for underwater vehicles. Since the implementation of such
strategies in areal environment isnot trivial Section 4.5 describes inmore
detail some successfully experiments. Finally, the conclusions are drawn in
Section 4.6.
4.2 Experienced Failures
In this section, asmall list of possible ROVs/AUVs’ failures is reported.