In nature, certain aquatic animals and seabirds are capable of leaping from water surface and overcoming aquatic obstacles with ease. Inspired by that, researchers have developed various underwater robots, which can perform the aquatic jumping motion. Although there are several ways to achieve it, the water jet propulsion is the most appropriate approach for the amphibious jumping robot, which is under development. In this paper, a high-performance water jet thruster powered by liquid nitrogen is proposed to be the potential actuator for the amphibious jumping robot. The theoretical jumping model is built to optimize the initial volume fraction of water inside thruster and analyze its parameters' variation during water jet. Then the computational fluid dynamics simulations by ANSYS FLUENT software are carried out to analyze the self-pressurization process of liquid nitrogen as well as the water jet process. Finally, the proof-of-concept outdoor experiments present that the 3.7 kg thruster's maximum aquatic and terrestrial jumping heights are 25.1 m and 24.4 m, respectively. A simple heat transfer analysis between water and liquid nitrogen is also conducted, and the order of magnitude estimation of heat transfer coefficient is given to be 265W/ (m2·K) based on the experimental reaction time.
While exploring complex environments, conventional mobile robots can overcome obstacles of limited height and jumping robots can overcome obstacles several times their own size. When jumping motion and wheeled or tracked motion are combined to form a composite motion, robots have even better ability to traverse rugged terrain. Therefore, we designed a wheeled hopping robot with a novel combustion-powered actuator and carried out structural design and experimental studies. The combustion-powered actuator has a unique inlet scheme and good sealing performance and is easy to install. The jump height equation was obtained by analyzing the combustion process. With the help of orthogonal array design methods, the factors that affect the wheeled hopping robot's jumping height were analyzed to improve its key design parameters and optimize its jumping performance. Finally, experiments were performed with the actuator and the wheeled hopping robot. When the stoichiometric ratio of the mixed fuels nears complete combustion, the combustionpowered actuator can jump 7.5 m high with a payload of 3.75 kg and its own weight of 1.25 kg, and the wheeled hopping robot can jump 4.5 m high with a payload of 6 kg. Evaluating the robot's performance in terms of energy efficiency reveals that it has a significant jumping advantage.
Frogs can keep an excellent aerial balance for landing and achieve consecutive jumps reliably. A safe landing requires an accurate body righting in the air. However, there is no systematic study on how the frogs adjust the aerial postures and body attitudes after jumping. The stretched long hindlegs swung quickly in the aerial phase, which revealed a clear relationship with the body attitudes. This study aimed to verify the function of frogs' hindlegs on aero body righting in the air. We captured the motions of both hindlegs and found the hindlegs adopted two movement modes, the bilateral parallel, and separated swings. The hindleg-induced torques by the two movements were negatively correlated with the body's angular accelerations on pitch and roll, respectively. Moreover, an analytical model was derived based on the conservation of angular momentum and verified by the dynamic simulations. Thus, we confirmed that the hindlegs are the dominant mechanism in aerial pitch and roll controls.We anticipate our achievements to inspire the design of air-righting tools.
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