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Hull design
When designing an AUV, one of the most important components is its hull (sometimes called shell or external structure). This component not only determines how the robot will look like, but also will have a deep impact in the overall performances of the resulting system. As such, the design of the hull can not be taken lightly.
The characteristics of the hull depend on the application of the AUV, however, when designing a hull some general aspects need to be considered: External conditions: working pressure and temperature, impacts etc.
As pointed in the list the hull must be able to resist the hydrostatic pressure of the water at working conditions. Additionally, the hull needs to minimize the drag forces of the water. These two characteristics depend on the geometry of the hull, which in turn will also a↵ect the AUV maneu-verability and power consumption. Given that pressure resistance and drag reduction play such an important role in AUVs performance, the definition of the shape of the hull is almost an unavoidable step in the design process of these vehicles [30,31].
Several shapes are used in order to create AUVs [32–34]. From a pressure point of view, an spherically shaped AUV would be suitable, however, this shape at high speeds can lead to instability due to hydrodynamic e↵ects [35]. In the other hand, cylindrical hulls have a series of interesting characteristics such as good pressure resistance and low longitudial drag [36]. Addition-ally, due to its longitudinal shape, the cylinder is more compatible with the shape of the inner components of an AUV (instruments, batteries, etc). A drawbacks of this shape are the cavitation e↵ect [35] and instability of the robot [37].
Evidently, simple geometrical shapes are only a first approach, since usu-ally these are based on previous experiences and studies. More sophisticated methods exist, in which the determination of the better suited hull for a given application is treated as an optimization problem. Indeed, optimiza-tion techniques are also used in order to find shapes to minimize the drag and improve pressure resistance [38]. Some studies use multi-objective optimiza-tion approaches in order to find suitable shapes from a hydrodynamic point of view and taking into account design considerations such as component placement [39] and cost [40].
Given that the determination of the drag forces relies heavily on hy-drodynamic considerations, many studies are also carried out in order to characterize the hydrodynamic properties of a robot [41, 42]. These studies usually involve the use of finite element methods using specialized software such as Fluent [43,44]. Hydrodynamic studies prove to be more useful in the design stage of the robot. However, even when used as an evaluation tool, they provide important information for the AUV user.
Propulsive topologies
The degree of actuation, the amount of degrees of freedom actuated in the AUV, defines what kind of movements can be generated by the robot propul-sive system (propulsive topology). Evidently, the more an underwater robot is actuated, the more complex tasks it will be able of perform. At the design stage, the designer needs to imagine the type of tasks that the AUV will perform in order to determine the needed degree of actuation of the vehicle. This task, simple at first sight, is decisive not only for the final capabilities of the robot but also for its overall performance. Indeed, a poorly chosen propulsive configuration can lead to an AUV incapable of performing certain tasks or, at the least, to an under-performant vehicle.
To make sure an AUV is maneuverable enough to perform a great amount of tasks, an immediate approach would be actuate all six degrees of freedom of the Cartesian space, making it fully actuated. However, as stated in the introduction of this thesis, another desired characteristic of an AUV is a high degree of autonomy. An AUV actuated in all of its degrees of freedom will certainly need many actuators, which would make the vehicle power-hungry. This could cause a rapid consumption of the, necessarily limited, energetic resources of the robot. In the process of creating an AUV, at some point the designer must face this dichotomy between actuation capabilities and autonomy (power consumption). This is a key point in the development of the robot, since the solution to this dilemma will be fundamental for the rest of the process.
Several kind of topologies can be found in the literature. This shows the importance of this design domain, as well as the lack of a general solution capable of satisfying most application cases. We can divide these robots in two groups: fully actuated and underactuated robots.
Fully actuated robots
This type of underwater robots have six or more actuators actuating the six degrees of freedom of the robot. This property allows these robots to perform a great variety of tasks, since with their propulsive resources they can move in the space with complete freedom (or holonomy). This freedom comes, however, at the cost of consuming great amounts of energy. Despite their outstanding maneuverability, there are relatively few fully actuated AUV. This could be because of their power consumption or due to the fact that their potential agility is not yet matched with reliable control and navigation systems. Another reason for the reduced number of fully actuated AUVs can be the lack of appropriate missions. Indeed, for a great number of tasks nowadays there is not a valid reason to actively control all of the degrees of freedom of an AUV. This is due to the fact that AUV operations are mostly meant to be planar and at a certain depth, without many changes in orientation.
The importance of this type of vehicles; however, can not be neglected. A testimony of this, are the research e↵orts made in order to correctly control this AUV category. Some of these works mostly take a general approach at the control of these vehicles, aiming their developments to a conceptual level and not focusing on the robot particularities nor on a direct application [45–47], while others tackle the control of an specific vehicle [3].
Industrial AUV tend to focus on survey-style tasks [48–50], in which ma-neuverability is not as important as autonomy and velocity. In such an sce-nario, fully actuated AUVs are not yet sufficiently attractive. However, for academic proposes, fully actuated AUVs are very appealing. Indeed, due to their maneuverability, these vehicles can be used as a test bench in order to develop new control and localization methods [51–53]. Evidently, given the maneuverability of these robots and thanks to new advances in control, these underwater vehicles are receiving more attention from companies willing to invest in this kind of technology [54].
Under actuated robots
The second type of robots reviewed in this section are called underactuated. The main characteristic of thee AUV is that they are not able to fully control all of their six degrees of freedom.
The amount of underactuated AUVs found in the literature is greater than the amount of their fully actuated counterparts. This could be due to the reduced power consumption (less actuators means less consumed energy) or to the fact that most of the tasks for these robots only need the control over a few degrees of freedom.
Many examples of underactuated AUVs are available in the literature. Historically, the most common type were the torpedo-shaped underwater robots [55]. These vehicles, designed to travel great distances in order to perform survey-style missions, were the first AUVs to find their way out of research facilities and get adopted to commercial use [56, 57]. The range of applications of torpedo-shaped AUVs is quite large, they are used to collect data [58], to map the sea floor [59] and to inspect pipelines [60]. Motivated by the need of autonomy and to travel great distances, a type of torpedo-shaped AUV called glider has been developed in the last decades [61,62]. The main advantages of these vehicles are the long range mission capabilities with low energy consumption. After the first wave of torpedo-shaped AUVs and thanks to advancements in control, localization and path planing, AUVs applications have mutated to other areas of interest. Along with the change of mission, the structure of these AUVs have mutated to adapt to new missions. Nowadays, we can find (mostly underactuated) AUVs acting as diving companions [63], cleaning ship hulls [64] and reaching their application scope to intervention tasks [65]. Even if they seem complex, these tasks do not demand the control of all degrees of freedom of the robot.
Control over this type of vehicle is more challenging than over fully ac-tuated robots. Indeed, for underactuated AUVs, not every trajectory is reachable, due to their propulsive limitations. Great e↵orts have been made in order to overcome this drawback, in the literature we can find advanced and creative control techniques [66–69] trying to solve this problem.
Propulsion means
Equally important as the propulsive technology, is the way in which the propulsive forces are created. In this section, di↵erent propulsive means will be discussed, ranging from classic technologies, such as control and control surface, to the innovative biomimetic approach.
Fixed propulsion
The first type of propulsion technology analyzed in this section is the fixed one. These actuators are able to generate the propulsive force along a fixed direction with regard to the robot body. Usually, in order to properly con-trol the degrees of freedom required by a mission, many fixed actuators are needed.
The number, orientation and position of these actuators on the hull of the AUV depend on the type of mission performed by the robot. In virtue of this, no general rules exist to define said design parameters. This step of the design process relies heavily on the previous experience of the designer.
Table of contents :
List of Figures
List of Tables
1 R´esum´e
1.1 Introduction
1.1.1 V´ehicules sous-marins non habit´es
1.1.2 M´ethodologie de recherche
1.2 Modelisation de l’AUV
1.2.1 Mod´elisation des robots sous-marins autonomes
1.2.2 Mod´elisation de la propulsion
1.3 Commande bas´ee mod`ele
1.3.1 Bouclage dynamique lin´earisant
1.4 Optimisation g´en´etique de la propulsion
1.4.1 Algorithmes g´en´etiques
1.4.2 Optimisation globale des robot sous-marins autonomes
1.4.3 Reconfiguration dynamique de la propulsion
1.4.4 Discussion
1.5 Conclusion
2 Introduction
2.1 Unmanned underwater vehicles
2.1.1 Underwater robots classification
2.1.2 Motivation
2.2 Research methodology
2.3 Organization of this manuscript
3 Underwater robot design: State of the art
3.1 Sub-systems design
3.1.1 Hull design
3.1.2 Propulsive topologies
3.1.2.1 Fully actuated robots
3.1.2.2 Under actuated robots
3.1.3 Propulsion means
3.1.3.1 Fixed propulsion
3.1.3.2 Vectorial propulsion
3.1.3.3 Control surfaces
3.1.3.4 Biomimetics
3.1.4 Control techniques
3.1.4.1 Linear
3.1.4.2 Nonlinear
3.2 Design methods
3.3 Conclusion
4 Modeling and simulation
4.1 Autonomous underwater vehicles modeling
4.1.1 Kinematics
4.1.2 Dynamics
4.1.3 Hydrodynamics
4.1.3.1 Added mass
4.1.3.2 Damping forces
4.1.3.3 Gravitational forces
4.2 Propulsion modeling
4.2.1 Thrust configuration
4.2.2 Fixed propulsion modeling
4.2.2.1 Steady-state model
4.2.2.2 Dynamic model
4.2.2.3 Parameter identification
4.2.3 Vectorial propulsion modeling
4.2.3.1 Mechanical model
4.2.3.2 Magnetic model
4.2.3.3 Electro-mechanical model
4.2.3.4 Experimental validation
4.3 Simulation
4.3.1 The RSM robot model
4.3.1.1 Hydrodynamic model
4.3.1.2 Propulsion system model
4.3.2 EAUVIVE simulator
4.3.2.1 Numeric resolution
4.3.2.2 Tasks and trajectories
4.3.2.3 Results
4.4 Conclusion
5 Control and estimation
5.1 Torque computation
5.1.1 Principle
5.1.2 Kinematic level
5.1.3 Dynamic level
5.1.4 Thrust allocation
5.1.5 Space reduction
5.1.5.1 Evaluation trajectory
5.1.5.2 Kinematic control law reduction
5.1.5.3 Dynamic control law reduction
5.1.5.4 Thrust allocation reduction
5.1.6 Numerical validation
5.1.6.1 Mission simulation
5.1.6.2 Results
5.2 Controllability
5.2.1 T matrix
5.2.2 B matrix
5.3 Kalman filter
5.3.1 Principle
5.3.2 Extended Kalman filter
5.3.3 Application
5.4 Validation
5.4.1 ROS programming
5.4.2 Control architecture
5.4.3 Software-in-the-loop
5.4.4 Processor-in-the-loop
5.5 Conclusion
6 Genetic optimization
6.1 Introduction
6.1.1 Global optimization problem
6.1.1.1 Di↵erent types of methods
6.1.1.2 Evolutionary algorithms
6.2 Genetic algorithms
6.2.1 Principle
6.2.2 Components
6.2.2.1 Population initialization
6.2.2.2 Evaluation
6.2.2.3 Selection
6.2.2.4 Crossover
6.2.2.5 Mutation
6.2.2.6 Replacement
6.2.3 Application example
6.2.3.1 Rastringin function
6.2.3.2 Parameters
6.2.3.3 Results
6.3 Adaptation to AUV propulsion design
6.3.1 Design parameters
6.3.1.1 Propulsion parameters
6.3.1.2 Controller parameters
6.3.2 Representation of solutions
6.3.2.1 Number of thrusters
6.3.2.2 Thruster position
6.3.2.3 Thruster orientation
6.3.2.4 Control gains
6.3.2.5 Tracking point position
6.3.3 Optimization problem
6.3.4 Evaluation
6.3.4.1 Simulation based
6.3.4.2 Reality based
6.3.4.3 Robot-in-the-loop based
6.4 Global optimization of underwater robots
6.4.1 Separate optimization of sub-problems
6.4.1.1 Optimization of control gains
6.4.1.2 Optimization of point e position
6.4.1.3 Optimization of thrusters position
6.4.1.4 Fixed thrusters orientation
6.4.2 Global vs. sequential optimization
6.4.2.1 Sequential optimization
6.4.2.2 Global optimization
6.4.3 Topology optimization
6.4.3.1 Optimization for scanning of the seabed .
6.4.3.2 Optimization for diving toward water turbine
6.4.3.3 Optimization for tomography of water turbine
6.5 Dynamic configuration
6.5.1 Seabed inspection
6.5.2 Diving toward water turbine
6.5.3 Tomography
6.6 Conclusion
7 Conclusions and perspectives
References
