This paper proposes a manipulator-effort-aware model predictive control (MPC) framework for coordinating body motion of a multi-legged underwater walking robot during moored mine clearance under ocean-current disturbances. The method treats the norm of manipulator joint torques as an effort-related input and uses it in an upper-layer MPC to adapt the robot’s body approach motion and posture, while lower-level whole-body control and impedance control handle body stabilization and rope grasping. Simulation studies in a ROS1 Noetic–Gazebo environment with a UUV-Simulator-based model show that, under increasing unidirectional current, conventional decoupled controllers cause manipulator torques to grow and approach saturation, whereas the proposed framework keeps torques within safe limits by generating adaptive body-motion compensation. These results indicate improved mechanical stability and reduced manipulator burden during rope-grasping interactions, though validation is currently limited to a simplified unidirectional flow without full locomotion and is sensitive to model accuracy, motivating future experiments with more complex currents, dynamic walking scenarios, and refined hydrodynamic modeling.

This study presents an experimentally validated multibody-particle coupling framework for predicting time-history hydrodynamic loads of dynamically moving underwater mechanisms. The motivation is the limited availability of validated datasets and workflows for multi-jointed robotic motions, where motion-dependent hydrodynamic loads can cause prediction errors and degraded control performance if not properly accounted for. A commercial-solver co-simulation is established by integrating RecurDyn as the multi-body dynamics solver with Particleworks as the MPS-based particle fluid solver under identical prescribed rotational kinematics. To ensure practical numerical settings under computational constraints, a parameter study is first conducted to select robust pressure-related options and time-integration settings. Single-and double-pendulum underwater drop tests are then performed to obtain angular displacement, tangential speed, and reconstructed resultant force and impulse time histories, together with an energy-loss metric across varying initial angles. The coupled simulations show strong waveform agreement with experiments, achieving a cross-correlation function of 94 percent for the single-pendulum force and impulse signal and 72 percent and 93 percent for the first and second links of the double pendulum. Trend-level validation based on fluid-induced energy loss yields a maximum relative error of 15 percent over the tested initial angles. This study advances previous SPH/MPS-MBD coupling studies by quantitatively validating hydrodynamic loads predicted by the coupled simulation through direct comparison with measured force and impulse time histories. The proposed benchmark and validated workflow provide a practical basis for constructing state-dependent hydrodynamic load maps and embedding reduced-order load models for real-time simulation and control of underwater robotic appendages.

The escalating demand for precision in maritime missions has led to the development of collaborative heterogeneous multi-robot systems, specifically pairing Autonomous Surface Vehicles (USVs) with Autonomous Underwater Vehicles (AUVs). Autonomous docking is essential for mission persistence, allowing AUVs to use USVs for recharging and data offloading, yet achieving reliable docking is difficult because these underactuated platforms are highly susceptible to wind and current disturbances. This paper introduces a specialized simulation framework utilizing a MATLAB-based Graphical User Interface (GUI) and 6-DOF equations of motion to evaluate docking success rates in real-time by analyzing measured environmental vectors. Through a scoring framework incorporating the Continuous Ranked Probability Score (CRPS), the system identifies optimal docking headings where environmental forces are minimized or exhibit a force-offsetting effect. To ensure kinematic feasibility, the trajectory planning logic integrates minimum turning radii of USV and AUV, while temporal synchronization is maintained via Estimated Time of Arrival (ETA) calculations at each waypoint. The proposed algorithm was implemented in C++ within the ROS2 framework and validated through stationary and collaborative docking scenarios under stochastic loads. Experimental results confirm that aligning the docking axis with optimized directions allows for stable docking performance.

This paper presents the mechanical design and hydrodynamic analysis of a six-legged underwater walking robot developed for efficient mine hunting and retrieval operations in challenging coastal environments. Inspired by the locomotion of ghost crabs, the robot integrates a bio-inspired morphology with both walking and swimming capabilities to ensure robust mobility and stability under strong currents, low visibility, and uneven seabed. The mechanical design emphasizes ground contact through six articulated legs for enhanced posture control, while a hybrid propulsion system enables seamless transition between walking, paddling, and thruster-based swimming modes. Simulation studies validate the effectiveness of various walking gaits and posture control strategies, while Computational Fluid Dynamics (CFD) analysis confirms the robot's streamlined form reduces hydrodynamic drag. The results demonstrate that the robot is well-suited for close-range inspection, object recovery, and navigation in turbulent underwater conditions. This research contributes to the advancement of legged marine robotics by offering a versatile platform for precise seabed interaction and mission adaptability.

The Crabster Robot for Mine disposal (CRM) is a hexapod underwater walking platform developed to detect and neutralize seabed mines by navigating across the ocean floor using six articulated legs and four integrated thrusters. A buoyancy control mechanism based on a compressed air system enables seamless transition between walking and propulsion modes. One of the principal challenges in subsea operation is maintaining dynamic stability under hydrodynamic disturbances, particularly the risk of tumbling. To address this, we propose an extended dynamic tumble stability margin tailored for underwater locomotion, adapted from methods originally designed for terrestrial quadrupeds. Under the assumption of a stationary robot on a flat seabed, we evaluate the influence of buoyancy and fluid dynamics on overall stability. Our findings demonstrate posture-dependent stability, with lateral flows posing less risk of tumbling than frontal currents. These results provide insights into robust operation strategies for underwater walking robots in hazardous environments.

There is a critical requirement for a LARS (Launch and Recovery System) to operate UUVs (Unmanned Underwater Vehicles) in ocean environments. Especially when UUVs are launched from and recovered by USVs (Unmanned Surface Vehicles), an automatic LARS is a very important device. This paper comprehensively presents the development of a novel launch and recovery system specifically designed for UUV operations at the bottom of USVs. The mechanical aspect of the LARS can be developed using various mechanisms; in this study, the Sarrus mechanism was applied to achieve the vertical movement required by the system. The structural analysis of the proposed the LARS has confirmed its reliability, structural integrity and operational effectiveness in ocean environments. And, a hydraulic system was applied to operate the LARS, and the overall hydraulic system was designed by calculating the capacity of each hydraulic actuator. In parallel, a complementary electrical and electronic control system was developed to operate the hydraulic systems. A testbed was built for performance testing, and LARS was installed on the testbed to perform LAB Test and Ocean Basin Test. After the performance test, the LARS was disassembled and anodised to prevent corrosion. The completed LARS was mounted on the USV. This research contributes to technologically advanced launch and recovery solution that addresses significant issues in the operation of unmanned underwater vehicles.

With the growing importance of marine exploration and military operations, the demand for long-duration missions of autonomous underwater vehicles (AUVs) has increased. However, limitations in charging and data transmission restrict their independent operation. To overcome these challenges, cooperative operation between autonomous surface vehicles and AUVs, along with reliable docking technology, is essential. Nevertheless, docking success rates vary depending on the nonholonomic constraints of the platforms and the influence of marine environmental conditions. In this study, a MATLABTM-based simulator was developed that incorporated current, wind, and wave effects to analyze the dynamic characteristics of both platforms under diverse conditions and evaluate docking success rates. Simulation results showed that docking success was highest when disturbances were aligned with or opposite to the direction of motion, whereas lateral disturbances reduced performance. In combined environments, compensating effects improved outcomes, leading to the derivation of optimal docking approach directions under specific environmental conditions.

The needs for underwater robots increased to explore and develop ocean resources safely and efficiently. Traditionally propeller-based ROVs(Remotely Operated Vehicles) and AUVs(Autonomous Unmanned Vehicles) have been used for ocean exploration and development. However, over the past decade, a new type of underwater robot that uses legs has emerged. These robots offer advantages such as stable movement over uneven or complex sea floor terrains and minimizing ecosystem disruption. However, due to legged locomotion, they have slower movement speeds compared to traditional underwater robots. To overcome this limitation, a swimming function using legs has been introduced. This paper aims to discuss the design of the swimming motion for underwater legged robots. Additionally, we define the hydrodynamic resistance that occurs due to swimming with legs underwater and calculate it to determine the thrust generated during swimming.

The Crabster CR200 is an underwater walking robot inspired by crabs and lobsters, designed for precise seabed inspection and manipulation. It maintains stability and position on the seafloor, even in strong currents, by adjusting its posture through six legs, each with four degrees of freedom. The key advantage of the CR200 lies in its ability to resist drifting in strong currents by adapting its posture to maintain its position on the seafloor. However, information is still lacking on which specific posture generates the maximum downforce to ensure optimal stability in the presence of currents and the seabed. This study aims to determine the fluid forces acting on the CR200 in various postures using Computational Fluid Dynamics (CFD) and identify the posture that generates the maximum downforce. The posture is defined by two parameters: angle of attack and seafloor clearance, represented by the combination of the robot’s pitch angle and distance to the seabed. By varying these parameters, we identified the posture that produces the greatest downforce. Through a series of analyses, we identified two main fluid dynamic principles affecting the downforce on a robot close to the seabed. First, an optimal pitch angle exists that generates the maximum downward lift on the robot’s body. Secondly, there is an ideal distance from the seabed that produces maximum suction on the bottom surface, thereby creating a strong Venturi effect. Based on these principles, we determined the optimal robot posture to achieve maximum downforce in strong current conditions. The optimal underwater robot posture identified in this study could be applied to similar robots operating on the seafloor. Furthermore, the methodology adopted in this study for determining the optimal posture can serve as a reference for establishing operational postures for similar underwater robots.

Underwater vehicles are becoming the alternatives of the traditional way of mine searching based on Mine Hunter Coastal (MHC) or Mine Sweeper Hunter (MSH). The floating vehicles, however, fundamentally have difficulties to operate in strong current and turbid water area. A hexapod underwater walking robot CRM is proposed for mine hunting in sea-current, turbid, rocky, and burying area. We designed the CRM by extending and adapting the Crabster technologies to the operational concept of mine hunting. The operational concept, functional requirements and conceptual design of CRM are presented in this paper. The focus is on the designing the operating concept and robot platform.

Biofouling on ship hull should be removed for energy efficiency of ships in service, also for marine environmental protection against invasive aquatic species. Underwater navigation is one of the core technologies of hull cleaning robots (HCR), and it is essential for the intelligence and autonomy of the HCR. In this paper, we proposes a designed of an underwater navigation system for multiple HCRs using USBL acoustic positioning as an auxiliary sensor. Two USBL transceivers will be placed in the bow and stern respectively to measure four HCRs’ position. The position measurement distortion of USBL caused by diffraction waves and the effect of reflection waves between the dock wall (or seabed) and docked ship will be discussed. This paper also proposes a method for correcting diffraction errors, judging and rejecting outliers.

In underwater with high levels of turbidity, a multi-beam acoustic camera proves to be advantageous for obtaining visibility. Unfortunately, the loss of elevation information during the making process of 2D acoustic images causes it challenging to accurately determine the three-dimensional information of objects. This paper proposes a method that applies the Radon transform and back-projection method to restore elevation and reconstruct objects in 3D from 2D acoustic images. For this, 2D acoustic images need to be taken at different perspective views as the camera undergoes a rolling motion from 0 to 90 degrees. Experiment results are included as well.

AUVs (Autonomous Underwater Vehicles) have been widely used for various underwater survey missions. In general, cruising type AUVs have torpedo-shaped hull forms for minimizing the hull resistances. It is necessary to ensure there are sufficient straight-line stabilities for a cruising type AUV in order to maintain its desired courses and depths with minimum controls during operations. In this study, vertical and horizontal plane straight-line stabilities of a designed AUV are analyzed based on the captive model tests. VPMM (Vertical Planar Motion Mechanism) tests are carried out to obtain hydrodynamic coefficients of the designed AUV. The AUV shows both vertical and horizontal plane instabilities, its stability margin indices are unsatisfying when compared with the recommended values. But the recommended vertical plane stability margins are subject to very high-speed conditions, so the characteristic equations of the AUV are reconsidered to investigate low-speed vertical plane stabilities in more detail. Next, a pair of horizontal and vertical fins are respectively designed to improve the vertical and horizontal plane stabilities of the AUV. The AUV with fixed fins shows improved stability margin indices within the recommended levels. In addition, the shape of the horizontal fins is properly modified for safe operations. The AUV with the designed vertical fins shows good horizontal plane stability in straight run simulations with step disturbances. And the effects of the horizontal fins are investigated through depth control simulations with biased random disturbances.

The effect of electron beam (EB) irradiation on superconducting properties and microstructures of MgB2 bulk superconductors were investigated. At E-beam doses of 1x10(16) e/cm(2) and 1x10(17) e/cm(2), the effect of irradiation on a superconducting transition temperature (T-c) of MgB2 was weak. As a dose increases to 5x10(17) e/cm(2), T-c decreases by 0.5 K. The critical current density (J(c)) measured at 4.2 K and 20 K, and 0 T - 5 T increases slightly as exposure time increases. X-ray diffraction for the irradiation surface of MgB2 shows that the diffraction intensity of (hkl) peaks decreases proportionally as the exposure time increases. This indicates that the crystallinity of MgB2 was degraded by irradiation. TEM investigation for the irradiated sample showed distorted lattice structure, which is consistent with the XRD results. The J(c) increase and T-c reduction of MgB2 by irradiation are believed to be caused by the lattice distortion.

It is necessary to estimate manoeuvring characteristics of submerged bodies at the design stage in order to ensure the safe operations. In this study, added mass coefficients in the mathematical model of submerged bodies are estimated by captive model tests and numerical calculations. Two kinds of models, MARIN ‘BB2’submarine model and AUV (Autonomous unmanned vehicle) model are utilized in the forced oscillation tests. Compared to BB2 submarine, AUV with cylindrical type hull form shows relatively small added masses in roll, pitch, and yaw directions. Next, numerical calculations based on potential theory are performed under the assumption that viscous effects on inertia forces are negligible. Added masses obtained by numerical calculations are in good agreements with forced oscillation test results. And if slow manoeuvres of submerged bodies are presumed, some of velocity coupled terms can be approximated by combinations of added mass coefficients.

This article presents a modeling method for the uncorrelated measurement error of the ultra-short baseline (USBL) acoustic positioning system for aiding navigation of underwater vehicles. The Mahalanobis distance (MD) and principal component analysis are applied to decorrelate the errors of USBL measurements, which are correlated in the x- and y-directions and vary according to the relative direction and distance between a reference station and the underwater vehicles. The proposed method can decouple the radial-direction error and angular direction error from each USBL measurement, where the former and latter are independent and dependent, respectively, of the distance between the reference station and the vehicle. With the decorrelation of the USBL errors along the trajectory of the vehicles in every time step, the proposed method can reduce the threshold of the outlier decision level. To demonstrate the effectiveness of the proposed method, simulation studies were performed with motion data obtained from a field experiment involving an autonomous underwater vehicle and USBL signals generated numerically by matching the specifications of a specific USBL with the data of a global positioning system. The simulations indicated that the navigation system is more robust in rejecting outliers of the USBL measurements than conventional ones. In addition, it was shown that the erroneous estimation of the navigation system after a long USBL blackout can converge to the true states using the MD of the USBL measurements. The navigation systems using the uncorrelated error model of the USBL, therefore, can effectively eliminate USBL outliers without loss of uncontaminated signals.

Effects of electron beam (EB) irradiation on the mechanical strength of Cu (conducting sheath) and Nb (diffusion barrier) of Cu/Nb/MgB2 superconducting was investigated. Wire- and tape-type Cu/Nb/MgB2 samples were irradiated at E-beam energy of 2.5 MeV and 5 mA and a maximum E-beam dose was 5x1017 e/m(2). The hardness value of Cu and Nb region was measured by the Vickers micro-hardness method. In the case of the wire sample, the hardness of Cu and Nb increased proportionally as the dose was increased up to 5x1017 e/m(2), whereas in the case of the tape sample, the hardness increased up to a dose of 0.5x1017 e/m(2), and decreased slightly 5x1017 e/m(2). The hardness increase of Cu and Nb is believed to be due to the decrease of the deformability of Cu and Nb due to the defects formed inside the materials by E-beam irradiation.