Underwater Robots - Gianluca Antonelli.pdf

82 4. Fault Detection/Tolerance Strategies for AUVs and ROVs
thruster blocking It occurs when asolid body isbetween the propeller
blades. It can be checked bymonitoring the current required by the thruster.
It has been observed, e.g., during the Antarctic mission ofRomeo [61]: in
that occurrence it was caused byablock of ice.
flooded thruster Athruster flooded with water has been observed during
aRomeo’s mission [61]. The consequence has been an electrical dispersion
causing an increasing blade rotation velocity and thus athruster force higher
then the desired one.
fin stuck or lost This failure can causes aloss of steering capability as
discussed bymeans of simple numerical simulations in [143].
rotor failure Apossible consequence of different failures of the thrusters
is the zeroing of the blade rotation. The thruster in question, thus, simply
stops working. This has been intentionally experienced during experiments
with ODIN [232, 233, 306, 307], RAUVER [140] and Roby 2[4] and during
another Romeo’s mission [61].
hardware-software failure Acrash in the hardware orsoftware implemented
onthe vehicle can beexperienced. In this case, redundancy techniques
can be implemented to handle such situations [42].
4.3 Fault Detection Schemes
In [3,4]amodel-based fault detection scheme is presented to isolate actuators’
failures in the horizontal motion. Each thruster is modeled as in [127]. The
algorithm is based onabank of Extended Kalman Filters (EKFs) the outputs
of which are checked in order to detect behaviors not coherent with the
dynamic model. In case of two horizontal thrusters and horizontal motion
3EKFs are designed tosimulate the 3behaviors: nominal behavior, left
thruster fault, right thruster fault. The cross-checking of the output allows
efficient detection as it has been extensively validated experimentally (details
are given in Section 4.5). Asketchofthis schemeisgiven in Figure 4.3 where u
is the vector of thruster inputs and the vehicle yaw ψ is measured by means
of acompass. In[5], the same approach isinvestigated with the use of a
sliding-mode observer instead of the EKF. The effectiveness of this approach
is also discussed by means of experiments.
The work in [52, 61] focuses on thethruster failure detection by monitoring
the motor current and the propeller’s revolution rate. The non-linear nominal
characteristichas been experimentally identified,thus, if themeasured couple
current-propeller’s rate is out of aspecific bound, then afault is experienced.
Based onamapping ofthe i-o axis the possible cause is also specified with
amessage tothe remote human operator. The two failures corresponding to
athruster flooding or to arotor failure, in fact, fall in different axis regions
and can be isolated. Interesting experiments are given in Section 4.5.