Fumihiko Asano

Impulse-driven limbless robots offer a promising platform for locomotion in environments where body deformation or complex actuation is undesirable. Prior work has shown that symmetric designs can achieve net displacement through temporally asymmetric internal actuation, yet such systems often exhibit limited adaptability due to weak coupling with environmental parameters. By introducing geometric curvature into the robot's chassis, we show that a minimal morphological modification can enrich the locomotion dynamics of impulse-driven robots with a symmetric body. Through dynamic modeling and simulation, we demonstrate that curvature strengthens the interaction between internal impulses and ground contact, allowing the robot's speed to respond to environmental friction. Experimental validation supports that actuation-dependent trends align closely with simulation predictions, and an omnidirectional extension further highlights the design's scalability. Our findings suggest that even in simple robotic systems, small morphological changes can significantly enhance performance by shaping how internal actuation couples with environmental interaction. This work advances the understanding of embodied symmetry-breaking and offers a low-complexity pathway toward adaptable and agile robotic locomotion.

This paper investigated the dynamics and control of an underactuated rimless wheel (RW) with semicircular feet driven by two pendulum-like arms. We focused on the entrainment property of the RW walking to the periodic oscillations of the arms and analyzed how different arm coordination patterns, i.e., in-phase and anti-phase swingings, affect the walking stability and performance of the system. A mathematical model was developed that captured the continuous dynamics, collision events, and control method for this hybrid mechanical system. Numerical simulations demonstrated that the anti-phase arm swinging, similar to human walking, provided superior performance compared to the in-phase swinging. Specifically, the anti-phase coordination yielded a wider range of entrainment, as well as a wider range of stable walking, across various control parameters including the forcing frequency and the arm weight. Through rigorous Poincaré map stability analysis, we further quantified the system’s robustness to perturbations, revealing that anti-phase arm swinging maintains stability even with increased arm mass, while in-phase swinging becomes unstable. The stable walking of the anti-phase coordination was supported by the opposing movements of the left and right arms, which effectively counterbalance the forward and backward momentums to suppress the ground reaction forces. These results may provide design principles for controlling underactuated walking robots with arms.

Legged locomotion animals produce various gaits, e.g., skipping, walking, running, and crawling, depending upon the environmental situation. Such an autonomous selection of distinct gait patterns is highly advantageous for energy efficiency, stress minimization, and postural stability. It should be of interest to introduce such a remarkably flexible mechanism to robotics research. This study addresses the mechanism of generating various gait patterns in a passive legged locomotion system by introducing a tensegrity structure. Building upon the classical model of the rimless wheel, we propose a novel model, called rimless wheel-like tensegrity walker (RTW). Numerical simulations show that the RTW system can generate skipping, walking, and crawling gaits depending upon the strength of the body-leg coupling, which controls independence level of the leg movements. Smooth gait transition can also be realized by a change in the body-leg coupling or the environmental parameter. An experimental study using physical models of the RTW confirmed the validity of the numerical results. The RTW may provide a minimal locomotion model to generate various gaits and to induce their transitions.

The authors have studied models and control methods for legged robots without having active ankle joints that can not only walk efficiently but also stop and developed a method for generating a gait that starts from an upright stationary state and returns to the same state in one step for a simple walker with one control input. It was clarified, however, that achieving a perfect upright stationary state including zero dynamics is impossible. Based on the observation, in this paper we propose a novel robotic walker with parallel linkage legs that can return to a perfect stationary standing posture in one step while simultaneously controlling the stance-leg motion and zero-moment point (ZMP) using two control inputs. First, we introduce a model of a planar walker that consists of two eight-legged rimless wheels, a body frame, a reaction wheel, and massless rods and describe the system dynamics. Second, we consider two target control conditions; one is control of the stance-leg motion, and the other is control of the ZMP to stabilize zero dynamics. We then determine the control input based on the two conditions with the target control period derived from the linearized model and consider adding a sinusoidal control input with an offset to correct the resultant terminal state of the reaction wheel. The validity of the proposed method is investigated through numerical simulations.