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Underwater Robots - Gianluca Antonelli.pdf

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4.4 Fault Tolerant Schemes 87<br />

basedand it handlesthe thrusterredundancy by apseudo-inverse approachof<br />

the TCM that guarantees the minimization of the actuator quadratic norm.<br />

The thruster dynamics, with the model described in [147], is also taken into<br />

account.<br />

The work [306, 307] presents afault detection-tolerant scheme for sensor<br />

faults. In detail, the depth of the vehicle is measured by using apressure<br />

sensor and abottom sonar sensor. Athird, virtual, sensor isadded: this is<br />

basically anARX (AutoRegressive eXogeneous) model of the vehicle depth<br />

dynamics. Bycomparing the measured values with the predicted ones, the<br />

residual is calculated and the failed sensor eventually disconnected for the remaining<br />

portion of the mission. Asketch ofthe scheme isshown in Figure 4.6,<br />

in nominal working conditions, the 3residual R i are close to the null value.<br />

It is worth noticing that this approach requires exact knowledge of the ocean<br />

depth.<br />

bottom<br />

knowledge<br />

u<br />

vertical<br />

sonar<br />

pressure<br />

sensor<br />

analytical<br />

model<br />

− +<br />

Fig. 4.6. Fault tolerance strategy for sensor fault proposed by K.C. Yang, Y. Yuh<br />

and S.K. Choi<br />

In [228], the case of an underactuated AUV controlled by control surfaces<br />

is considered. The vehicle tries to move in the unactuated DOFs by using<br />

elementary motions inthe actuated DOFs. The method has been tested on<br />

the vehicle ARCS and showed that, in this form, it is not applicable. This<br />

study provides information on structural changes to be adopted inthe vehicle<br />

in order to develop avehicle suitable for implementing this method.<br />

One of the first works of reconfiguration control for AUVs is given in [283],<br />

where, however, only asuperficial description of apossible fault tolerant<br />

strategy is provided. Recently, [288] proposed areconfiguration strategy to<br />

accommodate actuator faults: this is based onamixed H 2 /H∞ problem.<br />

Simulation results are provided.<br />

+<br />

−<br />

−<br />

+<br />

R 1<br />

R 3<br />

R 2<br />

FD

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