Underwater Robots - Gianluca Antonelli.pdf

5.4 Experiments of Fault Tolerance to Thrusters’ Fault 101
5.4 Experiments of Fault Tolerance to Thrusters’ Fault
The desired trajectory is the same shown in the previous Section. The experiments
were run several times simulating different thruster’s fault. The
fault was simulated via software just imposing zero voltage to the relevant
thrusters. It is worth noticing that, while the desired trajectory is always
the same, some minor differences arise among the different experiments due
to different factors: the presence ofstrong noise on the sonar that affected
the xy movement (see [215] for details), the presence of apool current. Meanwhile
the controller guarantees agood tracking performance tolerant to the
occurrence of thruster’s fault and it is robust with respect to the described
disturbances.
In next plots the movement ofthe vehicle without fault and with 2faults
are reported. The two faults arise asfollow: at t = 260 s, one horizontal
thruster is off, at t =300 salso the corresponding vertical thruster is off. We
chose to test afault of the same side thrusters because this appears to be the
worst situation. Notice that the control law isdifferent from the control law
tested in the previous Section. Fordetails, see [232, 233, 234, 251].
In Figure 5.7 the behavior of the vehicle along xy without and with fault
is reported. It can be noted that, as for the previous experiments, the vehicle
is controlled only after 100 sinorder for the sonar towork properly and wait
for the transient ofthe position filter [216]. From the right plot in Figure 5.7
we can see that the first fault, at t =260 s, it does not affect the tracking
error while for the second, at t =300 s, it causes only asmall perturbation
that is fully recovered by the controller after atransient. Some comments are
required for the sonar based error: during the experiment with fault it has
been possible to see asmall perturbation in the xy plane but, according to
the data, this movement has been of 60 cm in 0 . 2s,this is by far awrong
data caused by the sonar filter.
In Figure 5.8 the behavior of the vehicle along z without and with fault
is reported. It can be noted that the vehicle tracks the trajectory with the
same error with and without thrusters’ faults. Since the control law was
designed in 6DOFs, the difference in the behavior between the horizontal
plane, where asmall perturbation has been observed, and the vertical plane
is caused mainly by the different characteristics ofthe sensors rather than by
the controller itself. In other words, we can expect this nice behavior also in
the horizontal plane if amore effective sensorial system would be available.
Another comment need to be done about the depth data. In Figure 5.3
the plot of the depth for adifferent control law isshown. It could appear that
the tracking performance is better in the last experiment shown in Figure 5.8.
However ithas to be underlined that the experiments are expensive interms
of time and of people involved, the latter are simply successive tothe first,
the gains, thus, are better tuned.
In Figure 5.9 the attitude without and with fault is reported. It can be noted
that, in case of fault, only after when the two faults arise simultaneously