Underwater Glider Research Articles

On the Spatio‐Temporal Scales of the Subtropical Mode Water Formation in the South Atlantic

AbstractWe examined the changes in the ocean’s upper layer structure involved in the formation of the subtropical mode water in the South Atlantic. Here we present the results from a survey done in the region of formation between 37°W and 32°W and 34°S and 37°S from July–October 2018, using high‐resolution measurements obtained from an underwater glider. From its records, the mode water development was observed for the first time in the South Atlantic as localized chimney‐like patterns in the temperature, salinity, and potential vorticity (PV) measurements with a horizontal scale of (7 ± 2) km, associated with wintertime oceanic convective processes. Over time, these submesoscale structures tend to expand and dominate the region as a large volume of mode water. These vertical structures present highly homogeneous temperature and salinity values. The observed PV was on the order of 5.0 × 10−11 m−1 s−1, which is one order of magnitude smaller than the pre‐existent mode water at the lower layers; negative values were observed, indicating active convection. A 1‐D vertical model was used to predict the depth of the mixed layer conditioned by the surface fluxes and triggered by its changes in winter to late spring. Comparisons of the sea surface temperature measured from satellite collocated with the glider’s measurements showed a very good agreement. From this analysis, we were able to establish the decorrelation scales that characterize mode water formation. Further inspection of the glider data yielded temporal decorrelation scales indicative of convection modified by planetary rotation.

Based on Deep Reinforcement Learning to path planning in uncertain ocean currents for Underwater Gliders

This paper studies the application of Deep Reinforcement Learning (DRL) in the field of underwater glider (UG) formation path planning and solves the path planning of glider formation in uncertain ocean currents. Facing the uncertain ocean current environment, based on the observation and collection of ocean current data by the overall ocean observation system, the glider formation needs to have the ability of path planning. The original DRL algorithm network architecture is simple, and the feature extraction ability is limited. According to the characteristics of ocean current and path planning, a Deep Convolutional Neural Network for uncertain ocean current environment (Duo-CNN) network architecture with a dual-branch structure is proposed. Based on the DRL, the glider model and the interaction between the glider and the environment are used to guide the path planning through the implicit prediction of the known ocean current data. A solution for formation construction, obstacle avoidance, and path planning of glider formation in a dynamic uncertain ocean current environment is provided. The simulation verifies the effectiveness of the path planning of the glider formation using the DRL of the dual-branch Duo-CNN architecture. Compared with other architectures with feature extraction ability, Duo-CNN has advantages.

Sensitivity analysis of profile navigation command of underwater gliders to the initial heading error for improving trajectory accuracy

With the widespread application of underwater gliders (UGs) in marine research, there is an increasing demand for improved trajectory control accuracy. However, the initial heading error is unavoidable in internally actuated UGs, making it challenging to rectify the trajectory error generated in the heading error correction process. To effectively mitigate the impact of this error on trajectory accuracy, this study analyzes the sensitivity of profile navigation command parameters to the initial heading error through dynamic modeling of Petrel-L UG. Firstly, a simulator is developed to model the behaviors of Petrel-L, mainly involving dynamic modeling, underwater navigation, low-level control, and environment model. Subsequently, the sensitivity of profile navigation parameters in various commands to the initial heading error is analyzed. Surrogate model and Monte Carlo simulation are integrated to improve the computational speed of the analysis while preserving calculation precision. The average maximum single-profile horizontal error is 661 m, and the average maximum single-profile course error is 6.47°. Among profile navigation parameters, the target pitch during diving is proved to be the primary factor affecting the maximum trajectory error, and the increase of this parameter can enhance trajectory accuracy. This research also provides valuable reference for path following and station-keeping control of other UGs.

A self-powered underwater glider using bidirectional swing-rotation hybrid nanogenerator

With the development of marine exploitation, ocean robots pose a great challenge to traditional battery power supply, scavenging green and sustainable energy from the ocean environment to improve duration time and operation range has become a feasible solution. Herein, a self-powered underwater glider using a bidirectional swing-rotation triboelectric-electromagnetic hybrid nanogenerator (BSR-HNG) has been fabricated to harvest energy from water waves. A mechanical rectification method using a novel gear escapement mechanism is introduced to rectify irregular low-frequency wave excitation into the stable high-frequency rotation, enabling the BSR-HNG to operate stably even at an excitation frequency of 0.1 Hz. Compared to traditional swing-rotation nanogenerators using one-way bearing, the BSR-HNG utilizes symmetrically arranged gear escapement mechanisms to convert bidirectional swing into unidirectional rotation. With the optimized design, the average output power generated in one cycle increases by 48%, and the peak power of the triboelectric nanogenerator (TENG) and electromagnetic nanogenerator (EMG) at 0.8 Hz reaches about 0.4 mW and 0.12 W, respectively. Furthermore, a commercial thermometer and a calculator are utilized to demonstrate the high performance of the BSR-HNG in a simulated wave environment. The BSR-HNG is also integrated into the underwater glider as a power source, and a depth-sensor in the underwater glider is successfully powered by BSR-HNG, which provides a new solution to improve the endurance of marine robots.

Fault-tolerant heading control for a bionic gliding robotic dolphin with flipper faults

This paper presents a fault-tolerant heading-control method for a bionic gliding robotic dolphin operating in the presence of flipper faults. To achieve both high manoeuvrability and long endurance, a bionic gliding robotic dolphin that integrates the propulsion modes of dolphins and underwater gliders is introduced. The operational conditions of the flippers are considered meticulously, and common fault types that can arise during the gliding process, such as stuck, failure, and efficiency-loss faults, are analysed. These faults can affect the heading capabilities of the gliding robotic dolphin. To ensure normal navigation in the presence of flipper faults, a fault-tolerant heading control method is devised for the gliding robotic dolphin. This method applies backstepping sliding control to establish control laws based on a dynamic model. Subsequently, a flipper angle solver is designed to convert the controller outputs into deflection angles for the flippers. Simulation results obtained under three typical flipper faults demonstrate the effectiveness of the proposed fault-tolerant control method. This fault-tolerant control method ensures the safety of gliding robotic dolphins under complex deep-sea conditions.

Numerical investigation for the blockade capability of the multi-BWBUG cooperative system

ABSTRACT The multiple blended-wing-body underwater glider (multi-BWBUG) cooperative system features high endurance and low noise. This system is highly suitable for performing the blockade mission offshore. However, the research on this mission has not yet been reported. Thus, this study first proposes a blockade scenario for a multi-BWBUG cooperative system. The corresponding blockade mechanism is also described. Then, several typical topologies, the occurrence probability of network state, inherent capability, combat capability, and blockade capability, are defined. Finally, a novel method is proposed to calculate the blockade capability of the multi-BWBUG cooperative system. The simulation results indicate that the transmitting source level and communication frequency of communication equipment influence the occurrence probability of the network state. In addition, the failure ratio of communication links and the number of primary communication links affect the combat and blockade capabilities of the multi-BWBUG cooperative system. The blockade capability generally increases with the increase in the detection range of passive sonar.

A Soft Robotic Morphing Wing for Unmanned Underwater Vehicles

Actuators based on soft elastomers offer significant advantages to the field of robotics, providing greater adaptability, improving collision resilience, and enabling shape‐morphing. Thus, soft fluidic actuators have seen an expansion in their fields of application. Closed‐cycle hydraulic systems are pressure agnostic, enabling their deployment in extremely high‐pressure conditions, such as deep‐sea environments. However, soft actuators have not been widely adopted on unmanned underwater vehicle control surfaces for deep‐sea exploration due to their unpredictable hydrodynamic behavior when camber‐morphing is applied. This study presents the design and characterization of a soft wing and investigates its feasibility for integration into an underwater glider. It is found that the morphing wing enables the glider to adjust the lift‐to‐drag ratio to adapt to different flow conditions. At the operational angle of attack of 12.5°, the lift‐to‐drag ratio ranges from −70% to +10% compared to a rigid version. Furthermore, it reduces the need for internal moving parts and increases maneuverability. The findings lay the groundwork for the real‐world deployment of soft robotic principles capable of outperforming existing rigid systems. With the herein‐described methods, soft morphing capabilities can be enabled on other vehicles.

A data-driven approach for predicting depth-averaged velocities in the early stages of underwater glider navigation

Predicting the depth-averaged velocities of underwater gliders in the early stages of navigation is crucial for task performance optimization. However, due to the insufficient number of data samples for the depth-averaged velocities of underwater gliders in the early stages of navigation and the complex mechanisms of depth-averaged flow, task performance optimization remains challenging. This study presents a data-driven approach, the CutMix-augmented Extreme Gradient Boosting (CM_XGB) method, to effectively address this issue. Initially, the CutMix method is employed to augment the depth-averaged velocity samples. Then, the XGB method is applied for prediction. The proposed CM_XGB method is compared to five different prediction methods using three real-world depth-averaged velocity datasets and three error evaluation metrics, and its effectiveness in accurately predicting depth-averaged velocities under the same parameter conditions during the early stages of observation missions is demonstrated. This study shows that the CM_XGB method accurately predicts depth-averaged velocities even with limited data. This method is especially useful for underwater glider missions with data challenges in the initial stages. The success of the CM_XGB method highlights its potential for broader applications in oceanographic research and related fields, providing a valuable tool for scientists and researchers working with limited datasets.

Underwater glider 3D path planning with adaptive segments and optimal motion parameters based on improved JADE algorithm

This paper presents a novel 3D path planning method for the underwater glider (UG) that incorporates adaptive segment strategy, motion parameters optimization, and an improved JADE algorithm. The method aims to generate an energy-efficient path by adapting to ocean currents and seabed topography and selecting favorable motion parameters. We establish an energy consumption model for a blended-wing-body UG, examining the influence of motion parameters and ocean currents on its performance. The proposed method encodes an energy-optimal gliding path through multiple path segments, each defined by a set of path points, pitch angles, and diving depths. The fitness function, based on energy consumption, guides the optimization process. To enhance the optimization, we present an improved JADE algorithm with multi-mutation strategies, which adaptively updates the mutation operation and mean crossover probability. Our method was assessed and compared with a classical UG path planning method on 6 test scenarios. Simulation results confirm that adaptive segments and motion parameter optimization contribute to better adaptation to ocean environments and reduced energy consumption.

A multi-platform Guidance, Navigation and Control system for the autosub family of Autonomous Underwater Vehicles

Over the last decade, there has been a proliferation of marine robots such as Unmanned Surface Vehicles (USVs), Autonomous Underwater Vehicles (AUVs) and underwater gliders. The variety of vehicles’ capabilities and styles has led to numerous strategies to address the Guidance, Navigation and Control (GNC) tasks. In particular, a recent trend in the advancements of GNC systems is to develop modular, multi-platform solutions. This paper illustrates the approach followed in the development of the GNC system for the different types of AUV of the Autosub family. The presented system is decomposed into several interconnected components, each implemented as a standalone software module. Each component is highly-reconfigurable and easily extensible, and the interactions between the modules rely on generic-purpose interfaces. Such a design enables the execution of a broad range of autonomous missions, as well as the integration and deployment of the system on marine platforms with very diverse characteristics, not limited to the vehicles of the Autosub class. Experimental results obtained in real-time during in water campaigns are provided to demonstrate the effectiveness of the proposed system and its versatility on a wide range of platforms and missions.

Nonsingular fast terminal sliding mode tracking control for underwater glider with actuator physical constraints

An adaptive finite-time composite control (AFTCC) scheme is designed for the pitch angle trajectory tracking control of Underwater Glider (UG) with actuator physical constraints, uncertain dynamics and external disturbances. Firstly, a novel nonsingular fast terminal sliding mode control (NFTSMC) law is designed that can avoid singular problems, significantly reduce chattering, and achieve finite-time convergence of the system states. Secondly, a novel fixed-time extended state observer (FxTESO) is established to estimate the pitch angular velocity and lumped disturbances within a fixed time. Furthermore, a novel adaptive fixed-time saturation compensation system (AFxTSCS) is proposed to mitigate the effect caused by actuator saturation, and it can adjust the parameter adaptively when the actuator is in saturation or out of saturation. Finally, the AFTCC scheme, which is based on the NFTSMC framework and combines FxTESO and AFxTSCS, is designed to achieve the pitch angle trajectory tracking of UG, and the finite-time convergence of the whole closed-loop system is proved by the Lyapunov stability theory, and the simulations verify the availability and superiority of the proposed AFTCC scheme.

An analytical vibration model of a one-dimensional two-stage periodic isolation system for the broadband vibration suppression of an underwater glider

Periodic isolation system is effectively applied in broadband vibration control. To further enhance the broadband vibration attenuation effect, the paper proposes a two-stage periodic isolation system for an underwater glider. The analytical model of the one-dimensional two-stage periodic isolation system is developed through the multi-degree of freedom spring mass model method. For illustrating the superiority of the proposed two-stage periodic isolator, the force transmission ratio and the wave propagation constant of the SDOF isolator, the single-stage periodic isolator, and the two-stage periodic isolator are calculated and compared. In order to obtain the dynamic parameter influences on the vibration isolation performances as the design guidelines of the two-stage periodic isolator, the parametrical study is carried out based on the analytical model. Furthermore, a two-stage periodic isolator is designed for an underwater glider. The application effect of the two-stage periodic isolator is investigated through analytical modeling and finite element method, comparing to the single-stage periodic isolator. The research results from the analytical models show the proposed two-stage periodic isolator could strength the broadband vibration suppression. The parametrical study results demonstrate the vibration attenuation bandgap and attenuation amount are greatly influenced by the designed dynamic parameters, such as the mass unit and the spring unit of the periodic isolator, the intermediate mass of the two-stage isolator, and the number of the periodic cells. In application study of an underwater glider, the finite element results verify that the two-stage periodic isolator has more vibration attenuation effect than the single-stage periodic isolator. The vibration isolation assessment according to the proposed analytical model gives good predictive performance before the finite element model verification.

Control parameter optimization of underwater gliders with uncertainty using hierarchical optimization method and interval analysis theory

This article attempts to solve the engineering optimization problem for control parameter configuration of underwater gliders with parameter uncertainty. The optimization objectives are maximizing the glider energy utilization rate and average voyage velocity; design variables include the net buoyancy adjustment amount and movable mass block translation amount; and uncertain parameters include the glider control parameter errors, manufacturing errors and sensor measurement errors. First, a dynamic simulation-based performance evaluation model of the glider is established, and interval number order theory is used to quantify the uncertainty. Then, surrogate models of the performance evaluation model are established, and the Sobol’ method-based sensitivity analysis is executed to identify the key uncertain parameters. Thus, a hierarchical optimization framework is introduced, and its system level and sub-level are used for determining optimal control parameter values and performance evaluation parameter intervals, respectively. Finally, optimization calculation is executed under different parameter configurations, and some rules are summarized.

Gliding strategy analysis and optimisation of underwater gliders balancing energy consumption, motion accuracy and voyage velocity

ABSTRACT This paper presents a novel gliding strategy for underwater gliders, which can better balance the glider energy consumption, motion accuracy and voyage velocity. This strategy is designed according to the performance analysis results of the glider under the conventional gliding strategy. In the shallow water areas with the higher current intensity, the glider adjusts the net buoyancy to a larger value, which can ensure the higher voyage velocity and motion accuracy. In the deep water areas with the smaller current intensity, the glider adjusts the net buoyancy to a smaller value, which can ensure the higher energy utilisation rate. In the different water areas, the movable mass block translation amount will also be adjusted. To improve the performance analysis efficiency, the radial basis function (RBF) neural network is employed to establish surrogate models of the glider dynamic model. Then, we carry out the glider control parameter optimisation using the surrogate models and non-dominated sorting genetic algorithm II. The comparison research between the conventional gliding strategy and the novel gliding strategy shows that the novel gliding strategy can make the glider have the higher motion accuracy under the specific energy utilisation rate. Besides, the novel gliding strategy will not affect the optimal solution of the glider voyage velocity and energy utilisation rate, and it improves the glider motion accuracy by mainly reducing the voyage velocity. The advantage of the novel strategy is mainly to improve the glider motion accuracy, which may provide certain guidance for the actual application of underwater gliders.

Motion states identification of underwater glider based on complex networks and graph convolutional networks

Underwater glider (UG) plays an important role in ocean observation and exploration for a more efficient and deeper understanding of complex ocean environment. Timely identifying the motion states of UG is conducive for timely attitude adjustment and detection of potential anomalies, thereby improving the working reliability of UG. Combining limited penetrable visibility graph (LPVG) and graph convolutional networks (GCN) with self-attention mechanisms, we propose a novel method for motion states identification of UG, which is called as visibility graph and self-attention mechanism-based graph convolutional network (VGSA-GCN). Based on the actual sea trial data of UG, we chose the attitude angle signals of motion states related sensors collected by the control system of UG as the research object and constructed complex networks based on the LPVG method from pitch angle, roll angle, and heading angle data in diving and climbing states. Then, we build a self-attention mechanism-based GCN framework and classify the graphs under different motion states constructed by a complex network. Compared with support vector machines, convolutional neural network, and GCN without self-attention pooling layer, the proposed VGSA-GCN method can more accurately distinguish the diving and climbing states of UG. Subsequently, we analyze the variation of the transitivity coefficient corresponding to these two motion states. The results suggest that the coordination of the various sensors in the attitude adjustment unit during diving becomes closer and more efficient, which corresponds to the higher network measure of the diving state compared to the climbing state.

Estimating the Energy Flux of Internal Tides in the Northern South China Sea Using Underwater Gliders

AbstractThe characteristics of the energy flux of internal tides in the northern South China Sea (SCS) were explored by analyzing a fleet of underwater gliders. It was found that the low‐mode diurnal internal tides with ∼300 km wavelength in the middle basin are generated predominantly in the Luzon Strait (LS) and propagate over 1,000 km to the western SCS. However, the semidiurnal internal tides are generated in multiple regions, including the LS in the east part, the continental shelf, and the islands in the west. The energy flux of the mode‐1 diurnal internal tides attenuated rapidly within 450 km of the LS and was less pronounced after that. The estimated dissipation rate based on the mode‐1 energy flux is about 10−8 W/kg, underling the significant role of mode‐1 diurnal internal tides in bolstering far‐field mixing. This study provides a unique view of the spatial pattern, energy flux, and energy sink of the internal tides in the northern SCS, which could supplement the altimetry‐based results and improve the parameterization in ocean models.

Submesoscale variability on the edge of Kuroshio-shed eddy in the northern South China Sea observed by underwater gliders

Based on a submesoscale-resolving glider observation from April 25 to May 4, 2018, characteristics and underlying dynamics of submesoscale variability at the edge of an anticyclonic eddy shed from Kuroshio in the Northern South China Sea are explored in this study. Three underwater gliders traveled across the frontal zone and implemented ~ 300 dives, covering a horizontal distance of ~ 160 km and a vertical depth of ~ 500 m in 9 days. The character of k−2 slope for spectral potential energy and the strong lateral buoyancy gradient indicate frontogenesis-induced submesoscale motions on the eddy edge. Further analysis focusing on the potential vorticity and balanced Richardson number reveals the development of symmetric instability (SI), which is associated with the strong lateral gradient of buoyancy at the edge of the anticyclonic eddy in the late spring.

Examining Modulations of Internal Tides within An Anticyclonic Eddy Using a Wavelet-Coherence Network Approach

Interactions between internal tides and mesoscale eddies are an important topic. However, examining modulations of internal tides inside a mesoscale eddy based on observations is difficult due to limited observation duration and inaccurate positioning within the eddy. In order to overcome these two practical limitations, we use the active navigation capability of underwater gliders to conduct measurements inside the targeted eddy and utilize the wavelet approach to investigate modulations of internal tides with diurnal and semidiurnal periods inside the eddy. Based on the wavelet’s frequency–time locality, we construct scale-specific networks via wavelet coherence (WC) from multivariate timeseries with a small sample size. The modulation of internal tides is then examined in terms of temporal evolutionary characteristics of the WC network’s topological structure. Our findings are as follows: (1) the studied eddy is vertically separated into two layers, the upper (<400 m) and lower (>400 m) layers, indicating that the eddy is surface intensified; (2) the eddy is also horizontally divided into two domains, the inner and outer centers, where the modulation of internal tides seems to actively occur in the inner center; and (3) diurnal internal tides are more strongly modulated compared to semidiurnal ones, indicating the influence of spatial scales on the strength of interactions between internal tides and eddies.

Intrathermocline eddies observed in the northwestern subtropical Pacific Ocean

Two anticyclonic intrathermocline eddies (ITEs) were detected by an underwater glider in the northwestern subtropical Pacific Ocean during August-October 2019. They both exhibited a lens-shaped vertical structure within the thermocline with their cores located at ~170 m. The North Pacific Subtropical Mode Water (STMW) was found within the cores of these two ITEs. The lens-shaped structure of ITE1 observed by the glider was very clear since the glider seemed to have moved into its core during the observation. Further analysis reveals that ITE1 displayed no signals at the sea surface and lasted for about 20 days (26 August-14 September 2019). ITE1 was locally formed and the water inside it was a mixture of local water and the water in the northern adjacent area. The low-salinity water at 0-50 m from the northern adjacent area extended southwestward and mixed with the local water. As a result, the local salinity-forced restratification caused a potential vorticity (PV) decrease in the subsurface and finally resulted in the generation of ITE1. The baroclinic instability at 50-170 m may be the main energy source for ITE1 generation. On the other hand, the lens-shaped structure of ITE2 observed by the glider was less prominent since the glider did not move into its core. Further analysis reveals that the lens-shaped structure of ITE2 was also very clear near its core and ITE2 displayed clear signals at the surface as an anticyclonic eddy (AE2). AE2/ITE2 was remotely generated within the main formation region of STMW and then moved southwestward. The low PV STMW was trapped in AE2 and a lens-shaped structure developed in the subsurface. Subduction of the STMW caused the generation of ITE2.

Robust adaptive control of underwater glider for bottom sitting-oriented soft landing

This paper deals with the formalization, design, theorization, and implementation of a robust adaptive nonlinear control scheme to solve the bottom sitting-oriented soft landing control problem for an underwater glider under model uncertainties, actuator dynamics, and external disturbances. First, uniformized motion equations are derived based on Lie derivatives for describing pitch angle and vertical velocity tracking control. Then, the robust adaptive control framework is enhanced by containing an anti-windup auxiliary system and a disturbance observer. The stability of the closed-loop control system based on Lyapunov theory is proven to guarantee the boundedness of the entire internal signals. Finally, multifarious experimental results are also conducted by a self-developed underwater glider to demonstrate the effectiveness of the proposed controller.