Bai Chen

Flapping-wing propulsion offers high hydrodynamic efficiency but causes unstable pitch oscillations due to asymmetric forces. Conventional stabilization methods use external control surfaces, which increase drag and compromise efficiency in aquatic environments. This paper proposes a new stabilization strategy using the tilt degree of freedom (DOF) of the flapping wings. By coordinating tilt, flap, and feather motions, the hydrodynamic force vector is reconfigured to counteract pitch moments without extra appendages. A Pitch-Range-Suppression Model (PRSM) is developed to generate optimal kinematic setpoints. Constrained by the Strouhal number ($St \in [0.2, 0.4]$) and stall limits, the model maximizes thrust while minimizing lateral forces. The optimization is reduced to a closed-form Tilt Law Modulator (TLM), enabling microsecond-scale execution on embedded hardware. Experiments on a penguin-mimetic robot show a 52.2\% reduction in pitch oscillation amplitude compared to the no-tilt case. The results confirm that the optimal tilt amplitude is invariant to flow speed, validating the robustness of the kinematic mapping for stable, low-drag underwater locomotion.

Conventional hip exoskeletons typically employ multiple actuators to provide effective assistance to the corresponding joint, leading to an increase in the weight of the exoskeleton. Underactuated designs reduce the number of actuators, thereby lowering system weight and cost. However, existing single-motor underactuated hip exoskeletons still face challenges in achieving precise assistance and accommodating non-walking movements such as free sitting. In this work, we propose an underactuated hip exoskeleton with a series elastic actuator (SEA) and two independent cables for walking assistance. The incorporation of the SEA contributes to system safety and precise assistive force control. Furthermore, the proposed differential cable structure enables free sitting movement and allows for non-strictly symmetric hip motion. In experiments, with a target assistive force of 300 N, the proposed actuator achieves a peak force-tracking accuracy of 98.01% in walking tests, and the hip exoskeleton reduces peak muscle activation by up to 23.62% during walking.

Structural static testing is paramount for validating the structural integrity of critical aerospace components. However, conventional test rigs are often constrained to fixed loading axes and frequently induce parasitic torques. Accurate reproduction of aero-engine pylon flight loads therefore requires a mechanism that combines omnidirectional vector loading, high stiffness, and efficient force transmission. Achieving these coupled requirements is primarily a geometric synthesis problem, yet the associated workspace, stiffness, and load–capacity indices are nonlinear, mutually coupled, and expensive to evaluate over dense pose samples. To address this optimization bottleneck, this work develops a task-specific 6-UPS loading mechanism and a bio-inspired sensitivity-weighted NSGA-II algorithm for its geometric synthesis. Inspired by gene/locus-specific heterogeneity in biological evolution, the algorithm assigns variable-wise search intensities according to design-variable sensitivities, which are estimated using Multivariate Adaptive Regression Splines (MARS). In this way, influential design genes receive stronger local exploitation, whereas less sensitive ones retain broader exploration. Numerical simulations demonstrate that the proposed approach reduces computation time from about 30 h to 3 h relative to direct optimization with the baseline NSGA-II, while simultaneously improving workspace, stiffness, and load-carrying capacity. A hybrid physical prototype was further tested under 240 loaded pose conditions; the system maintained force magnitude errors below 0.64% (63.42 N) and directional deviations below 1.15°. These results support the efficacy of the proposed bio-inspired optimization-based design methodology for high-fidelity static testing of aero-engine pylons under the adopted hybrid setup.

Performance indices are crucial for performance analysis and evaluation of parallel mechanisms. However, conventional performance indices are not dimensionally homogeneous for the parallel mechanisms with both translational and rotational degrees of freedom. In this article, we propose a new method for formulating the Jacobian matrix. Homogeneous dynamic accuracy, dexterity, and manipulability are derived by evaluating the minimum condition number and geometric mean of singular values of the natural Jacobian matrix. The design optimization is performed to obtain the optimal architectures based on the proposed indices. The dynamic dexterity index is then applied to the motion planning. An experimental system is established to conduct comparative tests between trajectories planned using the proposed performance index in task space and the conventional trajectories planned in joint space. Conventional methods exhibit tracking errors up to 2.78 mm, while the proposed method achieves 85.23% and 31.77% improvements in MAE and RMSE, respectively. The proposed natural Jacobian formulation provides homogeneous indices that improve dynamic performance through design optimization and motion planning, and it is also applicable to serial and hybrid mechanisms, as well as any situations requiring the use of the Jacobian matrix.

Exoskeletons play a huge role in human body enhancement and physical rehabilitation. In this paper, a new modular exoskeleton driven by double-tendon-sheath variable stiffness actuator (DTS-VSA) is designed to achieve effective human power assistance. The modular and variable stiffness structure of exoskeleton enable the adaptation to different human joint, improving the characteristics of physical human-robot interaction. The DTS-VSA is designed based on the pulley-cable-spring preloading principle and tendon sheath transmission, and its stiffness model is developed through quasi-static force balance analysis. To realize coordinated and active power argumentation, a fuzzy adaptive assistive controller integrated with human joint torque and stiffness estimation is proposed based on surface electromyography. Feasibility is experimentally verified via three typical load-carrying experiments and ten volunteers. The experimental results show that the average assistance efficiencies of elbow motion and knee motion in different experiment conditions are higher than 44.72% and 38.41%.

Exoskeletons have great potential to enhance the daily activities of individuals needing robotic assistance. This article presents a design for an underactuated hip exoskeleton providing both active and passive walking assistance. The system integrates a driving reversal device (DRD) and a hip energy storage walking device (HESWD) to improve performance. The DRD allows a single motor to drive both hips during the movement phase, while the HESWD stores negative work generated by the human hip joint and uses it as positive work during the “blank” phase of underactuated walking. Specialized motor trajectory planning and optimization are used to meet the active assistance phase requirements. A radial basis function (RBF) neural network-based trajectory tracking control method adapts to changes in human gait. Treadmill experiments demonstrate that the tracking error is below 0.4146°. The results show the effectiveness of the proposed control method and the exoskeleton’s ability to adapt to slight gait variations.

This review synthesizes current research on hydrofoil-propelled robots inspired by the swimming mechanisms of sea turtles and penguins. It begins by summarizing the swimming kinematics of these organisms, highlighting their superior aquatic performance as the primary motivation for biomimetic design. Next, established analytical methods for characterizing hydrofoil locomotion patterns are presented, along with a clear delineation of the decoupled motion components exhibited by sea turtle flippers and penguin wings. Such decoupling provides a systematic framework for guiding the design of driving mechanisms. Building on this biomechanical foundation, the review critically examines recent advances in biomimetic flexible hydrofoils that enhance propulsion efficiency through three synergistic mechanisms to enhance thrust generation, while identifying key challenges in material durability and non-linear fluid–structure interactions. The review then surveys existing hydrofoil actuation systems, which commonly reproduce coupled motions with multiple degrees of freedom (DOFs). Finally, representative biomimetic robots are examined: sea turtle-inspired forelimbs typically incorporate three DOFs, whereas penguin-inspired wings usually offer two DOFs. By aligning robotic designs with the decoupled motion patterns of the source organisms, this review offers critical insights to advance the development of hydrofoil propulsion systems for enhanced aquatic performance.

Multi-degree-of-freedom (DOF) precision positioning stages (PPS) based on displacement amplifiers are commonly designed and used in precision engineering applications. However, the equivalent load and cross-coupling effects in these PPS can adversely impact the kinematic performance of the displacement amplifiers, resulting in a significant reduction in positioning accuracy. To this end, a novel mathematical model has been developed based on the chain-based compliance matrix (CMM) and the beam constraint model, enabling precise calculation of the amplification ratio and input stiffness of the compound bridge-type amplifier (CBTA). Firstly, an analytical model of the CBTA with asymmetric input force characteristics has been developed based on the CMM. A comparison with existing analytical models and finite element analysis (FEA) results demonstrates that the proposed model offers higher computational accuracy than the current models. Then, the structural stiffness and cross-coupling model of the PPS were derived using the beam constraint model. To further validate the effectiveness and scalability of the proposed method, the kinematic model of the XY PPS, based on the CBTA, was investigated. Finally, the correctness of the proposed theoretical model was verified through FEA and experimental validation. Taking into account the effects of load and cross-coupling, the amplification ratio and input stiffness errors of the XY PPS were both found to be less than 9.44% and 9.52%, respectively. Therefore, the proposed analysis model provides a new perspective for the design and application of multi-DOF PPS based on displacement amplifiers.