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19 pages, 3735 KB

Open AccessArticle

Trajectory Tracking of Underwater Hexapod Robot Based on Model Predictive Control

by Ruiwei Liu, Jieyu Zhu, Manjia Su, Xianyan Gu, Shuohao Fang, Dehui Zheng and Haoyu Yang

Machines 2026, 14(2), 171; https://doi.org/10.3390/machines14020171 - 2 Feb 2026

Viewed by 785

To achieve high-precision trajectory tracking control for an underwater hexapod robot, this paper proposes a hierarchical control architecture. Firstly, a multi-rigid-body dynamic model for the robot is established based on the Newton-Euler method and reasonably simplified. Secondly, a Central Pattern Generator (CPG) network with the Hopf oscillator as its core is designed to generate stable and coordinated crawling gaits. By introducing a steering parameter, a kinematic model connecting the CPG output is constructed. Furthermore, based on this dynamic and kinematic model, an upper-layer Model Predictive Controller (MPC) is designed. The optimized control quantities output by the MPC are mapped into the rhythmic parameters of the CPG network via a transfer function established by fitting experimental data, thus forming the complete MPC-CPG controller. Finally, the proposed method is validated through simulations of circular trajectory tracking. The results show that even in the presence of initial errors, the controller can converge rapidly, with trajectory position error consistently maintained within −0.1 m~0.1 m, and heading angle error confined to the range of −15~15°. The experiments fully demonstrate the effectiveness of the proposed MPC-CPG controller in ensuring trajectory tracking accuracy, motion smoothness, and system stability. Full article

(This article belongs to the Special Issue Design, Control and Application of Precision Robots)

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20 pages, 7180 KB

Open AccessArticle

An Indirect Foot-End Touchdown Detection Method for the Underwater Hexapod Robot

by Zonglin Liu, Meng Wang, Tong Ge, Rui Miao and Gangtai Lu

J. Mar. Sci. Eng. 2026, 14(1), 9; https://doi.org/10.3390/jmse14010009 - 19 Dec 2025

Cited by 1 | Viewed by 979

Abstract

The underwater hexapod robot has advantages such as lower energy consumption and reduced environmental interference compared to ROVs and AUVs. The foot-end contact detection with the seabed is the key technology for adapting to complex terrains. This paper focuses on the ‘Dragon Crab’ underwater hexapod robot developed by Shanghai Jiao Tong University and proposes an indirect detection method that does not require foot-end contact sensors. By establishing the kinematic and dynamic models of the robot’s legs, combined with multi-order polynomial trajectory planning to reduce non-contact force interference, the foot-contact determination condition is defined. Through simulation experiments and force analysis of the legs, the contact detection parameters are estimated. Then, single-leg contact tests are conducted to obtain joint motor torque variation curves and foot-end height variation curves through the kinematic model, verifying the proposed contact detection conditions and parameters. Finally, the method is applied to underwater obstacle-crossing experiments of the underwater hexapod robot using triangular and wave gait patterns. Experimental results show that the method can accurately identify the foot-end contact state and has high applicability in complex underwater terrains. Full article

(This article belongs to the Special Issue Underwater Robots)

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12 pages, 2791 KB

Open AccessArticle

Low-Complexity 2D DOA Estimation via L-Shaped Array for Underwater Hexapod Robot

by Yingzhe Sun, Qiyan Tian, Di Yao and Qifeng Zhang

J. Mar. Sci. Eng. 2025, 13(2), 229; https://doi.org/10.3390/jmse13020229 - 25 Jan 2025

Cited by 1 | Viewed by 1341

Abstract

This paper takes underwater hexapod robot target grasping in an extremely shallow water environment as the research goal and carries out the research on a high-precision and low-complexity method of target positioning. We address the above problem of estimating the two-dimensional (2D) directions of arrival (DOAs) of targets, using an L-shaped ultrasonic array. Based on the above considerations, low-complexity 2D multiple signal classification (MUSIC) based on sparse signal recovery (SSR) is proposed to enhance the super-resolution capability and DOA estimation accuracy. In the first step, subspace dimension is determined based on space distance. Then, a mixed-norm method is exploited to construct the projection subspace and new noise subspace. Finally, the orthogonality between the noise and signal subspaces is used to estimate DOAs. Via a numerical simulations analysis, we illustrate that the proposed technique can enhance the accuracy of DOA estimation while also being robust against coherent sources and limited snapshots. Full article

(This article belongs to the Special Issue Recent Advances on Intelligent Maintenance and Health Management in Ocean Engineering)

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20 pages, 1573 KB

Open AccessArticle

Simultaneous Localization and Guidance of Two Underwater Hexapod Robots under Underwater Currents

by Jonghoek Kim

Sensors 2023, 23(6), 3186; https://doi.org/10.3390/s23063186 - 16 Mar 2023

Viewed by 2867

Abstract

This paper addresses the simultaneous localization and guidance of two underwater hexapod robots under sea currents. This paper considers an underwater environment where there are no landmarks or features which can assist a robot’s localization. This article uses two underwater hexapod robots that move together while using each other as landmarks in the environment. While one robot moves, another robot extends its legs into the seabed and acts as a static landmark. A moving robot measures the relative position of another static robot, in order to estimate its position while it moves. Due to underwater currents, a robot cannot maintain its desired course. Moreover, there may be obstacles, such as underwater nets, that a robot needs to avoid. We thus develop a guidance strategy for avoiding obstacles, while estimating the perturbation due to the sea currents. As far as we know, this paper is novel in tackling simultaneous localization and guidance of underwater hexapod robots in environments with various obstacles. MATLAB simulations demonstrate that the proposed methods are effective in harsh environments where the sea current magnitude can change irregularly. Full article

(This article belongs to the Section Physical Sensors)

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