Sunquan Yu

This work reveals a tension-induced out-of-plane buckling mechanism in square-wave thin sheets, which exhibit distinctly different pre-buckling and post-buckling stiffness. Simplified theoretical analysis identifies that bending of the transverse beams about their weak axis is the dominant mode, while the torsional critical load is substantially higher. Based on this bending-dominated mechanism, analytical solutions for the critical buckling load and dual stiffness are derived and validated through finite element simulations. In the post-buckling regime, the structure maintains a stable load-carrying capacity despite the reduced stiffness. For series-connected arrays, the critical buckling load is found to be independent of the number of periods, whereas both pre- and post-buckling tensile stiffness scale inversely with the period count. When extended to orthogonal mesh structures, the supporting link length exhibits a non-monotonic effect on the critical load: increasing the link length initially raises the critical load, but beyond a threshold, bending of the links themselves weakens the constraint and reduces the load. This mechanism is further confirmed by comparative simulations with rigid links, where the non-monotonicity disappears and the critical load monotonically increases with length. These results collectively demonstrate that the stiffness transition from high to low with displacement provides a natural threshold-triggered switching mechanism. Quantitatively, the ratio of pre- to post-buckling stiffness exceeds an order of magnitude, demonstrating a pronounced switching capability. Overall, this work not only deepens the fundamental understanding of tensile buckling in periodic thin-walled structures but also provides explicit theoretical foundations and design guidelines for threshold-triggered sensors, impact recorders, soft robotics, and mechanical metamaterials.

Soft robotic arms offer a novel and secure technical approach for assisting astronauts in space stations, owing to their unique advantages such as flexibility and safety in human–machine interaction. During astronaut operational assistance, the forward and inverse kinematics models serve as the theoretical foundation for achieving interactive control. Typically, forward kinematics calculates the end position based on control inputs, while inverse kinematics determines the necessary inputs to achieve a desired position. Existing methods encounter challenges due to the large deformations and redundancies of the arms, which can lead to numerical instability or high data requirements. This study presents an innovative kinematic approach specifically designed for pneumatic soft arms. The forward model employs a fitting function, while the inverse kinematics is formulated as a constrained optimization problem that minimizes the deviation of the end position from the target. Experiments demonstrate that the forward fitting method is effective for both single and multichamber actuators. The inverse optimization substantially enhances control robustness and ensures motion convergence. This algorithm serves as a foundation for closed-loop feedback control, facilitating precise positioning. Demonstrations indicate that the proposed method enables rapid positioning through a single optimization calculation and achieves precise positioning with closed-loop control based on the optimization, offering promising extensions to other types of pneumatic robotic arms. The forward and inverse kinematics algorithm proposed in this paper provides a theoretical basis for the application of soft robotic arms in space operation assistive missions.

Soft robots are characterized by ultraflexible electromechanical structures that are capable of large deformations and movements. However, the challenge of generating the high output forces necessary for manipulation persists because of the low Young’s moduli of soft materials, which limit their effectiveness in tasks such as carrying, grasping, and providing support. Inspired by shrimp exoskeletons, we designed an origami exoskeleton that features flexible-rigid arrangements and mechanical multistability. This design offers high stiffness in stable states while allowing for long-stroke transitions, making it suitable for integration with soft actuators to balance deformability and load-bearing capacity. With advantages such as high compressibility and rapid integration, these exoskeleton-enhanced arms have promising applications when mounted on unmanned ground and aerial vehicles, facilitating large-scale dexterous manipulation with the necessary load capacities. For existing soft arms, the proposed origami exoskeleton enhances the potential for manipulation within their original design framework.

The Satellite-based Internet of Things (S-IoT) system offers a promising solution for accessing IoT devices in areas where terrestrial networks are inaccessible. These devices have significant roles in various scenarios, including aeronautic and maritime surveillance, data collection from sensors, and distress signals from sinking ships. However, covering numerous IoT targets with a minimal number of satellites poses a challenge. This paper proposes a solution known as the 5S payload designed for S-IoT. The 5S payload integrates five subsystems into a single unit, comprising the following components: (1) Automatic Identification System (AIS) for ship surveillance, (2) Very High-Frequency Data Exchange System (VDES) enabling two-way communication for ships, (3) Automatic Dependent Surveillance-Broadcast (ADS-B) system for aircraft surveillance, (4) Data Collection System (DCS) for user-defined sensors, and (5) Emergency Search and Rescue (ESR) system. The design of the 5S payload adheres to the CubeSat standard and occupies a compact 1.5U size, enabling rapid manufacturing and deployment. The feasibility of the 5S payload was demonstrated on the TianTuo-5 satellite launched on August 23, 2020. Additionally, this paper presents in-orbit experimental results spanning three years to further illustrate the effectiveness of the 5S payload.

This study proposes an improved two-dimensional finite-difference time-domain method (2D-FDTD) to simulate the propagation of guided waves in three-dimensional (3D) stiffened plates, which can greatly reduce the calculation amount and accelerate the simulation process. This method can be used for damage-detection imaging based on time-reversal with wavefield reconstruction (TR-RW) as a quick numerical simulation of wavefield is available. First, the piezoelectric wafer active sensors excite and collect Lamb wave signals in the damaged stiffening plate, after which these signals are reversed in the time domain. Second, the TR signals are again triggered by each sensor in turn on the reconstructed numerical model of the tested structure with 2D-FDTD. Third, the wavefield animation is obtained through simulation. Finally, the energy of the wavefield can be focused in certain areas, indicating the location of the damage. As an example, TR-RW, combined with 2D-FDTD method, is adopted to detect damage in an integral grid-stiffened plate, showing that the proposed 2D method can decrease the calculation time from dozens of minutes in the 3D model to dozens of seconds, which is of great significance for damage-detection applications. Furthermore, compared with the virtual TR method, the combined TR-RW and 2D-FDTD method can overcome the multipath effect and improve the damage location accuracy of stiffened structures.

The integral grid-stiffened plate is often used as the outer wall structure of spacecraft, which can reduce weight and increase strength. The Lamb wave is often used to detect damage of plate structure due to its advantages of strong penetration and minimum attenuation over long distances. When the Lamb wave crosses the stiffeners, the dispersion, reflection, transmission and mode conversion waves will be superimposed, resulting in difficulties in damage detection and structural health monitoring (SHM). This paper proposed an improved total focusing method (TFM) to locate damage in integral grid-stiffened plates based on distributed piezoelectric wafer active sensors (PWAS) array. Firstly, the appropriate working frequency was selected to generate the A0 mode Lamb wave depending on the structure and sensors, and the full matrix capture (FMC) method was introduced to collect data. Secondly, TFM was used to obtain the initial imaging results and the possible area of damage can be assessed and identified. Thirdly, the double window functions were proposed to extract the scattering signal related to the damage for the specific mode (A0). Lastly, the TFM was used again to obtain the final imaging results and the damage can be correctly identified. The proposed method has been verified by three-dimensional finite element (FE) simulation as well as experiments. The experimental results show that the damage in an aluminum alloy integral grid-stiffened plate can be located with the error of 13mm.