A non-electrical pneumatic hybrid oscillator for high-frequency multimodal robotic locomotion

Abstract

Pneumatic oscillators, incorporating soft non-electrical logic gates, offer an efficient means of actuating robots to perform tasks in extreme environments. However, the current design paradigms for these devices typically feature uniform structures with low rigidity, which restricts their oscillation frequency and limits their functions. Here, we present a pneumatic hybrid oscillator that integrates a snap-through buckling beam, fabric chambers, and a switch valve into its hybrid architecture. This design creates a stiffness gradient through a soft-elastic-rigid coupling mechanism, which substantially boosts the oscillator's frequency and broadens its versatility in robotic applications. Leveraging the characteristic capabilities of the oscillator, three distinct robots are developed, including a bionic jumping robot with high motion speed, a crawling robot with a pre-programmed logic gait, and a swimming robot with adjustable motion patterns. This work provides an effective design paradigm in robotics, enabling autonomous and efficient execution of complex, high-performance tasks, without relying on electronic control systems.

Fig. 1. Overview of the pneumatic hybrid oscillator (PHO).

a Overview of the high frequency, cascade ability, and phase tunable pneumatic hybrid oscillator (PHO) for various robot actuations. b Comparison of the motion speed and frequency of the bionic kangaroo robot designed based on the proposed PHO with the same type of pneumatic robot in the literatures^,,,–,,,–. c The design of PHO.

Fig. 2. Concept and working principle of the pneumatic hybrid oscillator (PHO).

a Design principle of PHO. b Demonstration of the oscillation process of PHO. c The relation between the stored potential energy of the elastic beam and the active joint. d The nonlinear relationship between the rotation angle of the active joint and the passive joint, where ${\theta }_c$ and ${\theta }_p$ are the critical angle of the active joint and passive joint, respectively. e During the oscillation, the active joint rotates continuously, and the passive joint steps. There is a difference in the phase of their rotation. f Construction of the PHO: the actuation chambers, the rotating-based bistable beam, and the switching valve. g High-speed camera images of one oscillation period with a rotation period of 19 ms.

Fig. 3. Analytical model for pneumatic hybrid oscillator (PHO) to predict oscillation frequency with varying design parameters.

a PHO can be modeled using RC analogue electrical circuit. One oscillation period contains two deformation processes with one chamber inflating and the other deflating, shown by flow-indicating diagrams and two snap-through processes. b–d Characteristic values of electronic components including capacitance of chamber, capacitance for elastic beam, and valve blocking factor over design parameters, guiding the optimized design for higher frequency. R_f represents the equivalent resistance of the valve, while the R_all is the maximum value of R_f. e–g Predictions from the analytical RC circuit model (dashed lines) and experiment data points of oscillation frequency for PHOs with distinct normalized length and bending stiffness. Error bars represent the standard deviation of the measured oscillation frequency from 10 measurements.

Fig. 4. Kangaroo-inspired high-speed and high-frequency robot.

a Mechanical design of the robotic kangaroo. b i: the motion of a kangaroo, which jumps with its rear legs, and uses its tail to maintain balance; ii: the robotic counterpart mimicking this posture; iii: schematic diagram of the kangaroo-inspired robot, where β is the initial angle of the assembled rear leg and ф, half of the rotation range; iv: anisotropy friction of the rear leg. c Similar running gaits between a real kangaroo and the robotic one. d High-speed running with a tunable actuation frequency. e Characterization of the jumping speed under different initial angles. Shaded regions represent 1 standard deviation. f Characterization of the jumping step length under different initial angles. Shaded regions represent 1 standard deviation. g Characterization of the ground duty cycle ratio.

Fig. 5. Cascaded-loop crawling robot.

a i: Design principle of the cascaded-loop of two pneumatic hybrid oscillators (PHOs). ii: The inner pressures of the four chambers change periodically over time in the cascade configuration. b Waveforms of chamber pressure, active joint angles, and passive joint angles at a constant supply pressure. c The design of the crawling robot. d The crawling gait of the robot. e The robot crawls forward under constant input pressure. f The speed of the crawling robot varies with frequency. Shaded regions represent 1 standard deviation. g Velocity comparison of the pneumatic crawling robots.^,,–.

Fig. 6. Phase modulation of pneumatic hybrid oscillator (PHO) enables a multi-directional untethered swimming robot.

a i: the modulation principle of PHO. ii: the effect of tuning the fixed angle of the auxiliary elastic beam. b Design of the multi-directional swimming robot. A CO₂ gas cylinder is integrated to make it untethered. c Direction changing of the swimming robot. d Various motion trajectories under different phase settings.