Underwater Robots - Gianluca Antonelli.pdf

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FTC UUV
4.2 Experienced Failures 81
FD
high level
Fig. 4.1. General fault detection/tolerant control scheme for an Unmanned Underwater
Vehicle (UUV). The Fault Detection (FD) block is in charge of detecting
the failure, send amessage to the higher level supervisor and, eventually, modifying
the Fault Tolerant Controller (FTC)
sensor failure The underwater vehicles are currently equipped with several
sensors in order to provide information about their localization and velocity.
The problem is not easy, itdoes not exist asingle, reliable sensor that gives
the required position/velocity measurement orinformation about the environment,
e.g., about the presence of obstacles. For this reason the use of
sensor fusion by, e.g., aKalman filtering approach, is acommon technique to
provide to the controller the required variables. This structural redundancy
can be used toprovide fault detection capabilities tothe system. Indetail, a
failure can occur in one of the following sensors:
• IMU (Inertial Measurement Unit): it provides information about the vehicle’s
linear acceleration and angular velocity;
• Depth Sensor: by measuring the water pressure gives the vehicle’s depth;
• Altitude and frontal sonars: they are used to detect the presence of
obstacles and the distance from the sea bottom;
• Ground Speed Sonar: itmeasures the linear velocity ofthe vehicle with
respect to the ground;
• Currentmeter: it measures the relative velocity between vehicle and water;
• GPS (Global Positioning System): it is used to reset the drift error of the
IMU and localize exactly the vehicle; itworks only at the surface;
• Compass: it gives the vehicle yaw;
• Baseline Acoustic: with the help of one or more transmitters itallows exact
localization of the vehicle in aspecific range of underwater environment;
• Vision system: it can be used totrack structures such aspipelines.
Foreachofthe above sensors the failurecan consist in an output zeroing if,
e.g., there isanelectrical trouble or in aloss of meaning. It can be considered
as sensor failure also an external disturbance such asamulti-path reading
of the sonar that can be interpreted as asensor fault and correspondingly
detected.