Underwater Robots - Gianluca Antonelli.pdf

88 4. Fault Detection/Tolerance Strategies for AUVs and ROVs
4.5 Experiments
Roby 2isaROV developed at the National Research Council-ISSIA, Italy.
It has been object of several wet tests aiming at validating fault detection
approaches. The horizontal motion isobtained bythe use of two fore thrusters
that control the surge velocity aswell as the vehicle heading; the depth
is regulated by means of two vertical thrusters. In[3, 4, 5] experiments of
different fault detection schemes have been carried out by causing, on purpose,
an actuator failure: one ofthe thrusters has been simply turned off. The
experiments in [3, 4] have been carried out in apool, in [5] the experiments
also concern acomparison between EKFs and sliding-mode observers. The
latter is aresult of abilateral project with the Naval Postgraduate School,
Monterey, CA.
The Italian National Research Council (CNR-ISSIA) also developed the
ROV ROMEO and tested, in an antarctic mission, both fault detection and
tolerant schemes [52, 61]. In particular the case of flooded and blocked thrusters
occurred. Inboth cases the fault has been detected and the information
could be reported tothe human operator in order to activate the reconfiguration
procedure. Figure 4.7 shows the expected and measured motor currents
in case of flooded thruster: itcan be observed apersistent mismatching between
the output ofthe model and the measured values.
The vehicle Theseus manufactured by ISE Research Ltd with the Canadian
Department ofNational Defence successfully handled afailure during
an Arctic mission of cable laying [117, 118]. In details, the vehicle did not
terminate ahoming step, probably due to poor acoustic conditions and the
Fault Manager activated asafe behavior: stop under the ice and wait for further
instructions. This allowed tore-establish acoustic telemetry and surface
tracking and safely recover the vehicle. Notice that his fault wasn’t intentionally
caused [118].
The vehicle ODIN, anAUV developed at the Autonomous Systems Laboratory
(ASL) of the University ofHawaii, HI, USA, has been used for several
experiments. In [306, 307] the fault detection and tolerant schemes are experimentally
validated. The thruster fault hasbeen tested by zeroing the output
voltagebymeans of software, the fault detection schemeidentified the trouble
and correctly reconfigured the force allocation byproperly modifying of the
TCM. The fault tolerant scheme with respect to depth sensor fault has also
been tested by zeroing the sensor reading and verifying that the algorithm,
after aprogrammed time of 1s,correctly switched on the other sensor. While
the theory has been developed for a6-DOFs vehicle, the experiments results
only present the vehicle depth.
The same vehicle has been used to validate the fault tolerant approach
developed in [232, 233, 251] in 6-DOFs experiments. Different experiments
have been carried out by zeroing the voltage onone or two thrusters simultaneously
that, however, did not cause the vehicle becoming under-actuated.
The implemented control law isbased onanidentified reduced ODIN model