Underwater vehicles (UVs) are subjected to various environmental disturbances due to ocean currents, propulsion systems, and un-modeled disturbances. In practice, it is very challenging to design a control system to maintain UVs stayed at the desired static position permanently under these conditions. Therefore, in this study, a nonlinear dynamics and robust positioning control of the over-actuated autonomous underwater vehicle (AUV) under the effects of ocean current and model uncertainties are presented. First, a motion equation of the over-actuated AUV under the effects of ocean current disturbances is established, and a trajectory generation of the over-actuated AUV heading angle is constructed based on the line of sight (LOS) algorithm. Second, a dynamic positioning (DP) control system based on motion control and an allocation control is proposed. For this, motion control of the over-actuated AUV based on the dynamic sliding mode control (DSMC) theory is adopted to improve the system robustness under the effects of the ocean current and model uncertainties. In addition, the stability of the system is proved based on Lyapunov criteria. Then, using the generalized forces generated from the motion control module, two different methods for optimal allocation control module: the least square (LS) method and quadratic programming (QP) method are developed to distribute a proper thrust to each thruster of the over-actuated AUV. Simulation studies are conducted to examine the effectiveness and robustness of the proposed DP controller. The results show that the proposed DP controller using the QP algorithm provides higher stability with smaller steady-state error and stronger robustness.

/pdf/robust-position-control-of-an-over-actuated-underwater-1isvx1c8eo.pdf

Robust Position Control of an Over-actuated Underwater Vehicle under Model Uncertainties and Ocean Current Effects Using Dynamic Sliding Mode Surface and Optimal Allocation Control.

Nowadays, underwater vehicles (UVs) are applied to various tasks such as carrying objects or maintenance of underwater structures. To carry out well these tasks, UVs should keep the position and orientation in the water to perform the specified tasks. However, the systems used in underwater operations are always under the influence of disturbance such as ocean currents and model uncertainties. In this paper, the robust station-keeping (SK) control algorithm based on a sliding mode control (SMC) theory is designed to guarantee stability and better performance of a hovering over-actuated autonomous underwater vehicle (HAUV) despite the existence of model uncertainties and ocean current disturbance in the horizontal plane (HP). Using the Lyapunov theorem, the stability of the proposed controller is demonstrated. Besides, an optimal allocation control (AC) algorithm is also designed to keep the linear position and Euler angles of the HAUV in the presence of model uncertainties as well as ocean currents and to minimize the energy consumption of the system. Finally, a series of simulations and experiments for the HAUV system is conducted to demonstrate the superior performance of the proposed method.

/pdf/station-keeping-control-of-a-hovering-over-actuated-k5bxmw9jkr.pdf

Station-Keeping Control of a Hovering Over-Actuated Autonomous Underwater Vehicle Under Ocean Current Effects and Model Uncertainties in Horizontal Plane

Shape optimization of underwater glider based on approximate model technology

System design optimization of an underwater glider involves various disciplines, such as hydrodynamics, structure, sizing, control, power, payload, and trajectory, which overlap with each other and influence the endurance of the system. Compared with traditional optimization methods, multidisciplinary design optimization provides a more efficient method for complex engineering problems involving overlapping disciplines. This article establishes a universal disciplinary framework of an underwater glider after the system analysis, which involves five disciplines, including hydrodynamic shape, pressure hull, buoyancy, attitude, and energy. To decouple the relationship among these disciplines and improve the efficiency of optimization, approximate models in the disciplines of hydrodynamic shape and pressure hull are established. After the coupling analysis, a novel approach is proposed for the system design optimization of an underwater glider, in which the concurrent subspace optimization, penalty function method, and the multipopulation genetic algorithm are combined together. The approach proposed in this article can meet the diversified design requirements of underwater gliders in different observation missions, which is verified by a sea trial of an improved Petrel-L glider. 

A New Approach to System Design Optimization of Underwater Gliders

System design optimization of an underwater glider involves various disciplines, such as hydrodynamics, structure, sizing, control, power, payload, and trajectory, which overlap with each other and influence the endurance of the system. Compared with traditional optimization methods, multidisciplinary design optimization provides a more efficient method for complex engineering problems involving overlapping disciplines. This article establishes a universal disciplinary framework of an underwater glider after the system analysis, which involves five disciplines, including hydrodynamic shape, pressure hull, buoyancy, attitude, and energy. To decouple the relationship among these disciplines and improve the efficiency of optimization, approximate models in the disciplines of hydrodynamic shape and pressure hull are established. After the coupling analysis, a novel approach is proposed for the system design optimization of an underwater glider, in which the concurrent subspace optimization, penalty function method, and the multipopulation genetic algorithm are combined together. The approach proposed in this article can meet the diversified design requirements of underwater gliders in different observation missions, which is verified by a sea trial of an improved Petrel-L glider.

Glider performance analysis and intermediate-fidelity modelling of underwater vehicles

Maribot LoLo is an autonomous underwater vehicle (AUV) developed at the KTH Centre for Naval Architecture as part of the Swedish Maritime Robotics Centre (SMaRC). The center’s cross-disciplinary activities require an AUV research platform that can be used for data collection and to test and demonstrate novel technologies. The challenge herein is to create a well-performing and yet versatile vehicle. This paper introduces Maribot LoLo and presents the underlying design philosophy which focuses on versatility and endurance. A free-flooded hull offers modularity and modifiability while reliability and robustness are achieved through hardware redundancy and a hierarchical captain-scientist relationship in the embedded system. The vehicle is designed to be operated at moderate water depths and on long-range missions. This leads to challenges in the design of the variable buoyancy system (VBS) which also is presented. The achievable range of the AUV is evaluated with a simple hydrodynamics model based on frictional drag.

Design of an AUV Research Platform for Demonstration of Novel Technologies

This study combines high-fidelity simulation models with experimental power consumption data to evaluate the performance of Energy Management Strategies (EMS) for fuel cell/battery hybrid Autonomous Underwater Vehicles (AUV). The performance criteria are energy efficiency, power reliability and system degradation. The lack of standardized drive cycles is met by the cost-efficient solution of synthesizing power profiles from sampled AUV field trial data. Three power profiles are used to evaluate finite-state machine, fuzzy logic and two optimization-based EMS. The results reveal that there is a trade-off between the objectives. The rigidity of the EMS determines its load-following behavior and consequently the performance regarding the objectives. Rule-based methods are particularly suitable to design energy-efficient operations, whereas optimization-based methods can easily be tuned to provide power reliability through load-following behavior. Both classes of EMS can be best-choice methods for different types of missions. 

Evaluation of Energy Management Strategies for Fuel Cell/Battery-Powered Underwater Vehicles against Field Trial Data

This paper presents a comparison of different energy management strategies (EMS) for autonomous underwater vehicles (AUV) with fuel cell-battery hybrid power systems. Sophisticated EMS can decrease energy consumption, limit fuel cell degradation or increase reliability. EMS for hybrid vehicles have been studied extensively in the automotive industry where standardised drive cycles are applied. As for AUVs, there are no standard drive cycles and power profiles can vary significantly depending on the type of mission. In this study, rule-based and optimization-based EMS are compared. The rule-based strategies rely on deterministic rules and fuzzy logic, the optimization-based strategies minimize a constrained cost function to efficiently split the power demand. The EMS are evaluated against a previously sampled power profile of a Hugin 3000 AUV. The evaluation against real power profiles adds significant value to this study.

Energy Management Strategies for Fuel Cell-Battery Hybrid AUVs

This study focuses on the feasibility and benefits of implementing of a fuel cell/battery hybrid power system in the autonomous underwater vehicle LoLo. We highlight the practical implications and challenges related to such a power system conversion and compare the benefits of using a fuel cell system rather than a pure battery system. Storage of hydrogen and oxygen as pressurized gases is considered most suitable for a conversion of this kind. In order to outperform Li-ion battery packs, high-pressure gas storage is required, preferably at pressures of 500 MPa or higher. Through a market analysis, we analyze the performance of commercial gas cylinders in terms of volumetric and gravimetric densities. This information can subsequently be used to compare energy density and effective energy density for the respective power systems. The study exemplifies how fuel cell/battery hybrid systems can provide up to 75% higher effective energy density compared to conventional battery packs. Ongoing developments in the fuel cell and auxiliary system markets are likely to allow for further optimization of the fuel cell system.

Clemens Deutsch

Papers

Glider performance analysis and intermediate-fidelity modelling of underwater vehicles

Design of an AUV Research Platform for Demonstration of Novel Technologies

Evaluation of Energy Management Strategies for Fuel Cell/Battery-Powered Underwater Vehicles against Field Trial Data

Energy Management Strategies for Fuel Cell-Battery Hybrid AUVs

Fuel Cells in Practice: Challenges and Benefits for AUVs