Autonomous Underwater Gliders Research Articles

Heuristic Surface Path Planning Method for AMV-Assisted Internet of Underwater Things

Ocean exploration is one of the fundamental issues for the sustainable development of human society, which is also the basis for realizing the concept of the Internet of Underwater Things (IoUT) applications, such as the smart ocean city. The collaboration of heterogeneous autonomous marine vehicles (AMVs) based on underwater wireless communication is known as a practical approach to ocean exploration, typically with the autonomous surface vehicle (ASV) and the autonomous underwater glider (AUG). However, the difference in their specifications and movements makes the following problems for collaborative work. First, when an AUG floats to a certain depth, and an ASV interacts via underwater wireless communication, the interaction has a certain time limit and their movements to an interaction position have to be synchronized; secondly, in the case where multiple AUGs are exploring underwater, the ASV needs to plan the sequence of surface interactions to ensure timely and efficient data collection. Accordingly, this paper proposes a heuristic surface path planning method for data collection with heterogeneous AMVs (HSPP-HA). The HSPP-HA optimizes the interaction schedule between ASV and multiple AUGs through a modified shuffled frog-leaping algorithm (SFLA). It applies a spatial-temporal k-means clustering in initializing the memeplex group of SFLA to adapt time-sensitive interactions by weighting their spatial and temporal proximities and adopts an adaptive convergence factor which varies by algorithm iterations to balance the local and global searches and to minimize the potential local optimum problem in each local search. Through simulations, the proposed HSPP-HA shows advantages in terms of access rate, path length and data collection rate compared to recent and classic path planning methods.

Detection efficiency of an autonomous underwater glider carrying an integrated acoustic receiver for acoustically tagged Pacific herring

AbstractAutonomous underwater vehicles (AUVs) or gliders are increasingly being used with acoustic telemetry to elucidate fish movements while collecting simultaneous environmental data. We assessed the utility of an AUV equipped with an integrated acoustic receiver to detect Pacific herring (Clupea pallasii) in Prince William Sound, AK, USA. A range test evaluated the effect of glider flight characteristics and environmental conditions on the detection efficiency of transmitters at varying depths. While distance from transmitters was the strongest predictor of detections, glider depth had a variable effect on detection efficiency which depended on transmitter depth and dive orientation. The detection efficiency of the glider-mounted acoustic receiver was less affected by wind speed and water level than that of stationary acoustic receivers deployed within the study area. The AUV also performed repeated, adaptive transects in an area of ∼630 km2 area and detected 30 Pacific herring transmitters without a priori knowledge of their locations. Of these herring transmitters, 14 were presumed shed after repeated detections within the same area, and 2 were detected at multiple locations. This study is the first to demonstrate that glider-mounted acoustic receivers have high detection efficiency for transmitters at varying depths and can detect movements of migratory forage fish in large search areas.

Design and Construction of Hybrid Autonomous Underwater Glider for Underwater Research

The main goal of this paper was to design and construct a hybrid autonomous underwater glider (HAUG) with a torpedo shape, a size of 230 cm in length and 24 cm in diameter. The control, navigation, and guidance system were executed simultaneously using a Udoo X86 minicomputer as the main server and three BeagleBone Black single-board computers as the clients. The simulations showed a controlled horizontal speed of 0.5 m/s in AUV mode and 0.39 to 0.51 m/s in glide mode with a pitch angle between 14.13∘ and 26.89∘. In addition, the field experiments under limited space showed the proposed HAUG had comparable results with the simulation, with a horizontal speed in AUV mode of 1 m/s and in glide mode of around 0.2 m/s. Moreover, the energy consumption with an assumption of three cycles of gliding motion per hour was 51.63 watts/h, which enabled the HAUG to perform a mission for 44.74 h. The proposed HAUG was designed to hold pressure up to 200 m under water and to perform underwater applications such as search and rescue, mapping, surveillance, monitoring, and maintenance.

Acoustic sampling of Antarctic krill with simulated underwater buoyancy gliders: Does the sawtooth dive pattern work?

Autonomous underwater gliders may be viable adjuncts to or in some cases replacements for ship-based oceanographic sampling. Gliders and ships acoustically sample the water column differently, with ships sampling all depths simultaneously in a single vertical pulse and gliders sampling shorter vertical segments of the water column in an up and down, sawtooth pattern. We simulated gliders following this flight pattern to sample the densities at depth of Antarctic krill (Euphausia superba), a patchily-distributed crustacean that is targeted by an international fishery. Krill densities from ship-based surveys conducted between 2001 to 2011 were treated as the “true” population densities sampled by the simulated gliders. Depth-integrated densities estimated from the glider sampling were compared to the population densities for each year. Coverage probabilities (the proportion of population means within a standard deviation of the glider sample means) for gliders diving to 150 m were near 100% in most years, better than the nominal 68%. Gliders diving to a maximum depth of 150 m estimated the annual population means better than gliders diving deeper because shallow dives provided more samples for a given length of trackline. Modeling the zero and non-zero data as separate distributions (the delta approach), an alternative to the lognormal CV approach used in this study, resulted in less accurate estimates of krill population densities. These results suggest that the sawtooth flight pattern of gliders can produce density estimates of krill comparable to the annual time series of density estimates from ship-based surveys. Gliders may also be useful to survey other patchily-distributed pelagic organisms.

Canyon Downwelling and Water Mass Influences on Winter Zooplankton Distributions in the Coastal Northeast Pacific

AbstractSubmarine canyons are considered biological hotspots due to enhanced mixing, upwelling, and other dynamic processes. There have been few observational surveys of canyon processes or zooplankton‐watermass relations during stormy downwelling seasons. In winter 2017, three autonomous underwater gliders provided highly‐resolved hydrographic and zooplankton backscatter observations in and around a canyon in the Northeast Pacific. While a glider's slow speed typically confounds spatial and temporal signals, the concurrent use of multiple gliders, sampling a canyon transect repeatedly, allowed us to explore the canyon system both spatially (i.e., canyon influence) and temporally (i.e., changing water type fractions). Canyon downwelling displaced the pycnocline by about 30 m depth over the scale of the canyon, which is ∼30% of the total pycnocline depth deviation observed over the course of the study. Diel vertically migrating (DVM) zooplankton generally stayed above the pycnocline and mean zooplankton volume scattering strength (Sv) in the upper water column (above the pycnocline) was higher when the pycnocline was deeper. At mid‐depths (150–200 m depth), zooplankton backscatter and DVM behavior were correlated with water type fractions, with higher Sv being associated with higher proportions of Pacific Equatorial Water and stronger DVM being associated with higher proportions of Pacific Subarctic Upper Water. Winter zooplankton distributions appear to be influenced by multiple, interacting drivers including canyon dynamics and water mass composition. In our study, the latter was found to have the stronger influence on zooplankton distribution.

Fast fixed-time vertical plane motion control of autonomous underwater gliders in shallow water

In this paper, a fast fixed-time vertical plane motion controller is proposed for autonomous underwater gliders (AUGs) gliding in shallow water. The influence of speed-sensorless conditions, model uncertainties, unknown time-varying external disturbances, input saturations, and state delay are taken into account. To improve control performance, a fast fixed-time stable system is first presented. Based on the system, an adaptive extended state observer (ESO) is developed for estimating speed, model uncertainties, and external disturbances. A fast fixed-time controller is designed for improving the gliding efficiency and reducing the risk of hitting the ocean floor. Moreover, an input saturation auxiliary system and an advance compensation method are presented to cope with input saturations and state delay. According to Lyapunov theory, it is proved that the AUG states can converge into a small neighborhood within a fixed time. Finally, simulation results demonstrate the rapidity and effectiveness of the designed control method.

Hydrodynamic design and analysis of a hybrid-driven underwater vehicle with ultra-wide speed range

This paper presents a hybrid-driven underwater vehicle with ultra-wide speed range, which combines the conventional autonomous underwater glider with a shaftless propeller. By designs of low-resistance structures in wide speed range, the vehicle has a low-speed gliding mode within 3 kn driven by buoyancy, and a low-medium-high speed navigation mode up to 30 kn driven by the propeller. In order to analyze the effectiveness of the structure designs, an analysis method of the flow energy dissipation (FED) is proposed. By solving the cumulative function of the FED, this method can quantitatively characterize the influence of the vehicle's local structures on navigation power consumption. On this basis, the hydrodynamic numerical models for the two modes are established by the computational fluid dynamics (CFD) methods. The numerical analysis indicates that this vehicle has a good lift-drag ratio in glide mode and a superior fluidity in propelled mode. It can maintain low resistance and low power consumption in both modes. Finally, the numerical models are verified to be accurate by prototype tests. This vehicle can provide reference for the designs of the integrated underwater platform with wide speed range and multi missions.

Potential Vorticity and Instability in the Pacific Equatorial Undercurrent West of the Galápagos Archipelago

Abstract The Galápagos Archipelago lies on the equator in the path of the eastward flowing Pacific Equatorial Undercurrent (EUC). When the EUC reaches the archipelago, it upwells and bifurcates into a north and south branch around the archipelago at a latitude determined by topography. Since the Coriolis parameter (f) equals zero at the equator, strong velocity gradients associated with the EUC can result in Ertel potential vorticity (Q) having sign opposite that of planetary vorticity near the equator. Observations collected by underwater gliders deployed just west of the Galápagos Archipelago during 2013–16 are used to estimate Q and to diagnose associated instabilities that may impact the Galápagos Cold Pool. Estimates of Q are qualitatively conserved along streamlines, consistent with the 2.5-layer, inertial model of the EUC by Pedlosky. The Q with sign opposite of f is advected south of the Galápagos Archipelago when the EUC core is located south of the bifurcation latitude. The horizontal gradient of Q suggests that the region between 2°S and 2°N above 100 m is barotropically unstable, while limited regions are baroclinically unstable. Conditions conducive to symmetric instability are observed between the EUC core and the equator and within the southern branch of the undercurrent. Using 2-month and 3-yr averages, e-folding time scales are 2–11 days, suggesting that symmetric instability can persist on those time scales. Significance Statement The Pacific Ocean contains fast-moving currents near the equator and below the surface that result in instabilities and mixing. The Galápagos Archipelago lies directly in the path of the eastward-flowing Pacific Equatorial Undercurrent. There are few observations of what happens to the current when it reaches the Galápagos Archipelago, so theories and models of the instabilities and mixing resulting from these strong currents have not been well verified. The Repeat Observations by Gliders in the Equatorial Region (ROGER) project deployed autonomous underwater gliders to observe the current system in this region. The results show that a range of instabilities may be responsible for the cold sea surface temperature of the Galápagos Cold Pool and the generation of tropical instability waves.

Vibration Analysis and Isolator Component Design of the Power System in an Autonomous Underwater Glider

Vibration isolation technology is one of the main methods for controlling the vibration and noise of underwater vehicles. This paper presents a vibration analysis and vibration isolator component design method of the power system in autonomous underwater gliders (AUGs). The dynamic models of the single-layer and double-layers vibration isolation systems of the power system of the AUG are established to analyze the force transfer rate in the system. A reasonable stiffness range of the vibration isolation system is obtained according to the results of the dynamic model. To validate the vibration isolation performances of the presented vibration isolation systems, the finite element (FE) models of single-layer and double-layers vibration isolation systems are established. The vibration responses and vibration isolation performances of the system with and without the rubber isolation systems are compared. Moreover, the effect of stiffness on the vibration isolation performances of the system is discussed. The presented results can provide some guidance for the design method of the vibration isolation system in the power system of AUGs.

A digital twins enabled underwater intelligent internet vehicle path planning system via reinforcement learning and edge computing

The Autonomous Underwater Glider (AUG) is a kind of prevailing underwater intelligent internet vehicle and occupies a dominant position in industrial applications, in which path planning is an essential problem. Due to the complexity and variability of the ocean, accurate environment modeling and flexible path planning algorithms are pivotal challenges. The traditional models mainly utilize mathematical functions, which are not complete and reliable. Most existing path planning algorithms depend on the environment and lack flexibility. To overcome these challenges, we propose a path planning system for underwater intelligent internet vehicles. It applies digital twins and sensor data to map the real ocean environment to a virtual digital space, which provides a comprehensive and reliable environment for path simulation. We design a value-based reinforcement learning path planning algorithm and explore the optimal network structure parameters. The path simulation is controlled by a closed-loop model integrated into the terminal vehicle through edge computing. The integration of state input enriches the learning of neural networks and helps to improve generalization and flexibility. The task-related reward function promotes the rapid convergence of the training. The experimental results prove that our reinforcement learning based path planning algorithm has great flexibility and can effectively adapt to a variety of different ocean conditions.

Autotune control algorithm based on relay feedback and adaptive neural network for attitude tracking of nonlinear AUG system

Due to the complexity and uncertainty of the nonlinear autonomous underwater glider (AUG) system, the control algorithms for attitude tracking of the AUG system are very difficult to directly design. In this paper, a novel autotuning control algorithm (ATCA) based on relay feedback and adaptive neural network is proposed to effectively implement the attitude tracking of the AUG system. The proposed algorithm only utilizes the online input/output (I/O) data to achieve the AUG system attitude control, ignoring the mathematical system model. The ATCA control parameters are initialized by relay feedback and adjusted online based on gradient descent algorithm with the partial derivative of the AUG system provided by adaptive neural network. Besides, in the ATCA, the fast adaptive learning factor is employed to make the AUG system respond quickly to the evolving reference trajectory. Furthermore, the complete stability of the closed-loop AUG system with the ATCA has been proven via the Lyapunov stability theory. The simulation studies illustrate the correctness the proposed algorithm. Compared with three popular data driven control algorithms, the proposed algorithm has superiority in terms of system response time, integral squared error (ISE) and integral absolute error (IAE). © 2014 xxxxxxxx. Hosting by Elsevier B.V. All rights reserved.

Dynamic Analysis of an Autonomous Underwater Glider with Single- and Two-Stage Vibration Isolators

Vibrations from the power system can significantly affect the working performances (ocean observation) of autonomous underwater gliders (AUGs). In order to reduce the vibration transmission from vibration sources to the precision instruments in AUGs, single- and two-stage vibration isolator rings are designed in this paper. The dynamic models of the single- and two-stage vibration isolation of the AUG are presented. The force transmission ratio of the AUG is calculated in MATLAB code. The influences of the isolator and the structure stiffness are analyzed. The dynamic stiffness of the designed isolators, as an important design parameter, is calculated using the finite element method. The influence of the designed parameter on the dynamic stiffness of the rubber ring isolator is discussed. The coupled vibro-acoustic finite element method is used to analyze the vibration and acoustic response of an AUG with the single- and two-stage vibration isolators. The insertion loss is calculated in order to assess the vibration isolation performance of the single- and two-stage vibration isolators. The results from the dynamic models and the finite element models both show that the vibration isolation performance of the two-stage vibration isolator ring performs better than that of the single-stage vibration isolator ring.

Autonomous underwater glider observations in southern Lake Ontario and Niagara River plume.

The nearshore areas of the Laurentian Great Lakes provide valuable ecosystem services including habitat for a variety of species and water for surrounding communities. Recent declines in nearshore water quality have increased the need for understanding the connectivity between nearshore and offshore areas; however, observing water quality variability across the dynamic nearshore to offshore transition zone poses logistical challenges for traditional observing systems. Here we evaluate cross-shore and along-shore water quality gradients using observations from two three-week deployments of a Slocum autonomous glider in southern Lake Ontario. The glider was deployed between the Niagara River mouth and Rochester, NY during early and late summer 2018, and each deployment resulted in over 3000 vertical profiles of the water column, and several transects between 2 km and 20 km from shore. In early summer, the cross-shore chlorophyll gradient was characterized by highest values just below the surface, at the frontal zone between weakly stratified conditions closer to shore and unstratified conditions offshore. In late summer, stratified conditions extended across the entire survey area. The depth of the thermocline was deeper and chlorophyll values were lower within 10 km of shore than offshore, where the highest chlorophyll values were observed in a distinct layer below the thermocline. In both early and late summer, the frontal boundary indicated by the cross-shore chlorophyll gradient was located below the surface and well offshore of what is typically considered the nearshore zone but was within the width of the coastal boundary layer. The high-resolution glider observations provide a detailed view of patterns of variability across a dynamic coastal zone and pinpoint the cross-shore frontal boundary that may be important for biologists to differentiate biological communities.

Simultaneous assessment of oxygen- and nitrate-based net community production in a temperate shelf sea from a single ocean glider

Abstract. The continental shelf seas are important at a global scale for ecosystem services. These highly dynamic regions are under a wide range of stresses, and as such future management requires appropriate monitoring measures. A key metric to understanding and predicting future change are the rates of biological production. We present here the use of an autonomous underwater glider with an oxygen (O2) and a wet-chemical microfluidic total oxidised nitrogen (NOx-=NO3-+NO2-) sensor during a spring bloom as part of a 2019 pilot autonomous shelf sea monitoring study. We find exceptionally high rates of net community production using both O2 and NOx- water column inventory changes, corrected for air–sea gas exchange in case of O2. We compare these rates with 2007 and 2008 mooring observations finding similar rates of NOx- consumption. With these complementary methods we determine the O2:N amount ratio of the newly produced organic matter (7.8 ± 0.4) and the overall O2:N ratio for the total water column (5.7 ± 0.4). The former is close to the canonical Redfield O2:N ratio of 8.6 ± 1.0, whereas the latter may be explained by a combination of new organic matter production and preferential remineralisation of more reduced organic matter at a higher O2:N ratio below the euphotic zone.

Self-triggered three-dimensional coordinated path following of disk-type autonomous underwater gliders based on low-frequency learning fuzzy predictors

This paper is concerned with a self-triggered three-dimensional coordinated path following problem for a fleet of under-actuated disk-type autonomous underwater gliders (AUGs) subject to input constraints as well as internal model uncertainties and external ocean disturbances. A modular backstepping design approach is employed to devise three-dimensional fuzzy coordinated path following controllers for networked AUGs. Especially, a three-dimensional kinematic control law is firstly constructed by using a line-of-sight guidance scheme. Secondly, a predictor-based self-triggered communication mechanism is proposed to guarantee that each AUG broadcasts information at its own triggering instants, and to listen to and receive the incoming information from the neighboring AUGs at their triggering instants. Finally, an adaptive kinetic control law is designed based on low-frequency learning fuzzy predictors, which are able to extract the high-frequency components of uncertainties and disturbances. The closed-loop system is proven to be input-to-state stable via a cascade stability analysis. Simulation results validate the feasibility and efficacy of the proposed self-triggered three-dimensional coordinated path following controllers for the under-actuated disk-type AUGs subject to uncertainties and disturbances.

Variable Gain Super Twisting Sliding Mode Control (VGSTW) application in longitudinal plane of Autonomous Underwater Glider (AUG)

This paper presents a robust control design using variable gain super twisting sliding mode control application in Autonomous Underwater Glider (AUG). AUGs are known as underactuated systems and very nonlinear in nature make difficult to control. The controller is designed for longitudinal plane of an AUG that tracks the pitching angle and net buoyancy of a ballast pump for nominal system, system in existence of external disturbance and parameter variations in hydrodynamic coefficients. The Lyapunov stability theorem has proved that the proposed control law is satisfied the stability sufficient condition. The simulation results have shown that the proposed controller has improved the transient response, reduced steady error and chattering effects in control input and sliding surface in all cases.

Typhoon-induced Full Vertical Mixing and Subsequent Intrusion of Yangtze Fresh Waters in the Southern Yellow Sea: Observation with an Underwater Glider and GOCI Ocean Color Imagery

Lim, H.S.; Kim, D.; Lee, H.J.; Kim, M.; Jin, S.H.; Miles, T.N., and Glenn, S., 2021. Typhoon-induced full vertical mixing and subsequent intrusion of Yangtze fresh waters in the Southern Yellow Sea: Observation with an underwater glider and GOCI ocean color imagery. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 171–175. Coconut Creek (Florida), ISSN 0749-0208. Typhoons have been regarded as an important forcing to control oceanographic phenomena, particularly in the Yellow and East China Seas. The influences of typhoons have become increasingly severe due to global warming. An autonomous underwater glider was deployed west of Jeju Island for 10 days from 15th to 25th August, 2018 to observe changes in physical environments induced by Typhoon Soulik. The glider data show that the stratified water masses were destroyed by the typhoon into a fully mixed stage of the entire water column. This destratification is manifested by many environmental parameters including temperature, salinity, chlorophyll-a, and suspended sediment concentrations. Accordingly, calculated parameters, density, and Richardson number, indicate de-stratification. The water column displayed, however, a rapid return to the stratification stage immediately after the typhoon passage. In addition, the GOCI geostationary ocean color imagery was analyzed that were obtained during and after the passage of Soulik between 15-25 August, 2018. These satellite images suggest that the discharge of the Yangtze River fresh water so increased during the typhoon that the intensified freshwater plume could move toward Jeju Island. As a result, observations with an autonomous glider may provide a promising means in analyzing oceanographic processes occurring during the peak of typhoons.

Sensitivity Analysis of the Turning Motion of an Underwater Glider on the Viscous Hydrodynamic Coefficients

Autonomous underwater gliders (AUG) are a class of underwater vehicles that move using a buoyancy engine and forces from wings. Gliders execute turning motion with the help of a rudder or an internal roll control mechanism and the trajectory of the turn is a spiral. This paper analyses the sensitivity of the characteristics of spiral manoeuvre on the hydrodynamic coefficients of the glider. Based on the dynamics model of a gliding fish whose turn is enabled by a rudder, the effect of hydrodynamic coefficients of the hull and the rudder on the spiral motion are quantified. Local sensitivity analysis is undertaken using the indirect method. The order of importance of hydrodynamic coefficients is evaluated. It is observed that the spiral path parameters are most sensitive to the side force created by the rudder and the effect of the drag coefficient is predominant to that of the lift coefficients. This study will aid in quantifying the effect of change of geometry on the manoeuvrability of AUGs.

Research on Anti-Interference Nonlinear Adaptive Control Method for Autonomous Underwater Gliders

In this paper, the characteristics of underactuation are analyzed according to the working principle of underwater glider, a complete 6DOF model of underwater glider is established, and the error equation of 3D path tracking is given. Based on the line-of-sight navigation method in three-dimensional space, the nonlinear adaptive control strategy is designed, and the roll angle control law, pitch angle control law, and depth control law of underwater glider are given. The tracking error of underwater glider is proved to be convergent by Lyapunov stability theorem. Finally, the results of marine experiments verify that the nonlinear controller designed in this paper can overcome the influence of constant current disturbance under certain conditions and has good robustness and good tracking effect under different ocean current sizes and course angles. Of course, one control method cannot be applicable to all sea conditions. When the angle between the size and direction of the ocean current exceeds a certain value, the tracking control of the underwater glider will show obvious divergence. Therefore, the stability range against the interference of the sea current is given in this paper.

DESIGN OF DELTA WING AUTONOMOUS UNDERWATER GLIDERS FOR LONG TERM SUSTAINABLE AND STEALTH RECONNAISSANCE USING NUMERICAL STUDIES

Autonomous Underwater Gliders (AUGs) are a unique source of collecting oceanographic data for varying depths with longer endurance than other types of unmanned underwater vehicles. The buoyancy-driven propulsion mechanism assists in diving deep into the ocean, collect data, resurface in saw-tooth and spiral motions. There have been various hull-forms introduced for this purpose starting from - Torpedo hull, XRay Flying-Wing Glider, etc., all of which have seen an improved understanding of the hydrodynamics associated with the hull-form of the gliders. The saw-tooth motion represents the glider's capacity to cover larger distances with a smaller angle of attack, thereby achieving an increased range for the same power included within the hull batteries. In this paper, a design study of delta wing hull-forms, which are proven to have better longitudinal motion characteristics, is proposed to arrive at a preliminary hull form that can be used for long term reconnaissance missions, remaining in stealth to assist submarines with valuable data. Delta wing hull forms are varied using different NACA sections, and numerical analysis of the obtained hull forms is performed to arrive at the hydrodynamics; namely, the drag, lift forces and coefficients for varying angles of attack, using commercial CFD software – StarCCM+. These results are then compared against each other to derive the preliminary hull form for further study on better longitudinal motion characteristics, thereby using less power for its motion underwater.