Characterized by their elongate bodies and relatively simple legs, multi-legged robots have the potential to locomote through complex terrains for applications such as search-and-rescue and terrain inspection. Prior work has developed effective and reliable locomotion strategies for multilegged robots by propagating the two waves of lateral body undulation and leg stepping, which we will refer to as the twowave template. However, these robots have limited capability to climb over obstacles with sizes comparable to their heights. We hypothesize that such limitations stem from the twowave template that we used to prescribe the multi-legged locomotion. Seeking effective alternative waves for obstacleclimbing, we designed a five-segment robot with static (nonactuated) legs, where each cable-driven joint has a rotational degree-of-freedom (DoF) in the sagittal plane (vertical wave) and a linear DoF (peristaltic wave). We tested robot locomotion performance on a flat terrain and a rugose terrain. While the benefit of peristalsis on flat-ground locomotion is marginal, the inclusion of a peristaltic wave substantially improves the locomotion performance in rugose terrains: it not only enables obstacle-climbing capabilities with obstacles having a similar height as the robot, but it also significantly improves the traversing capabilities of the robot in such terrains. Our results demonstrate an alternative actuation mechanism for multilegged robots, paving the way towards all-terrain multi-legged robots.

Drag-based swimming using rowing appendages, fins, and webbed feet is a widely adopted mode of locomotion in aquatic animals. To develop efficient underwater and swimming vehicles, various bioinspired drag-based paddle designs have been proposed, often facing a trade-off between propulsive efficiency and versatility. Webbed feet generate effective propulsive force during the power phase, while being lightweight, robust, and partially foldable during the recovery phase. However, the time-consuming process of mechanically folding and unfolding webbed feet extends the transition periods between the recovery and power phases, which in turn increased drag, and reduces overall paddling efficiency. In this study, we draw inspiration from the coupling tendons of aquatic birds. We implement tendon coupling mechanisms to minimize the transition time between the recovery and power phases. Hardware experiments demonstrate that our proposed mechanism improves propulsive efficiency by factors of$\mathbf{2. 0}$and$\mathbf{2. 4}$compared to designs without extensor tendons and based on passive paddles, respectively. Additionally, we find that distal leg joint clutching-previously shown to enhance efficiency in terrestrial walking-plays a negligible role in swimming locomotion. In sum, we present a novel principle for efficient drag-based leg and paddle design, with implications for understanding the swimming mechanics of aquatic birds and advancing bioinspired aquatic propulsion systems.

Achieving robust legged locomotion on complex terrains poses challenges due to the high uncertainty in robot-environment interactions. Recent advances in bipedal and quadrupedal robots demonstrate good mobility on rugged terrains but rely heavily on sensors for stability due to low static stability from a high center of mass and a narrow base of support [1]. We hypothesize that a multi-legged robotic system can leverage morphological redundancy from additional legs to minimize sensing requirements when traversing challenging terrains. Studies suggest [2], [3] that a multi-legged system with sufficient legs can reliably navigate noisy landscapes without sensing and control, albeit at a low speed of up to 0.1 body lengths per cycle (BLC). However, the feedback control framework to enhance speed of multi-legged robots on challenging terrains remains underexplored due to diverse environmental interactions. Such complexity makes it difficult to identify the key parameters to control in these high-degree-of-freedom systems. Here, using laboratory and field experiments, we demonstrate that a vertical body undulation wave helps mitigate environmental disturbances that affect robot speed. These findings are supported by probabilistic models. Using such insights, we introduce a control framework which monitors foot-ground contact patterns on rugose landscapes using binary foot-ground contact sensors to estimate terrain rugosity. The controller adjusts the vertical body wave based on the deviation of the limb's averaged actual-to-ideal foot-ground contact ratio, achieving a significant enhancement of up to 0.235 BLC on rugose laboratory terrain. We observed a 50% to 60% increase in speed and a 30% to 50% reduction in speed variance compared to the open-loop controller. Additionally, the controller operates in complex terrains outside the lab, including pine straw, robot-sized rocks, mud, and leaves. Project website:https://juntaohe.github.io/TRO_2025/

Modulating the stiffness of soft actuators is crucial for improving the efficiency of interaction with the environment. However, current stiffness modulation mechanisms are hard to achieve high lateral stiffness and a wide range of bending stiffness simultaneously. Here, we draw inspiration from the anatomical structure of the finger and propose a bi-directional tunable stiffness actuator (BTSA). BTSA is a soft-rigid hybrid structure that combines air-tendon hybrid actuation (ATA) and bone-like structures (BLS). We develop a corresponding fabrication method and a stiffness analysis model to support the design of BLS. The results show that the influence of the BLS on bending deformation is negligible, with a distal point distance error of less than 1.5 mm. Moreover, the bi-directional tunable stiffness is proved to be functional. The bending stiffness can be tuned by ATA from 0.23 N/mm to 0.70 N/mm, with a magnification of 3 times. The addition of BLS improves lateral stiffness up to 4.2 times compared with the one without BLS, and the lateral stiffness can be tuned decoupling within 1.2 to 2.1 times (e.g. from 0.35 N/mm to 0.46 N/mm when the bending angle is 45 deg). Finally, a four-BTSA gripper is developed to conduct horizontal lifting and grasping tasks to demonstrate the advantages of BTSA.