Catapult

Abstract: Coaxial launchers have received very much less attention than railguns because of their greater complexity, but they offer several significant advantages. They require no physical contact with the projectile, scale readily to very large diameter, can distribute thrust over the length of the projectile, have more adaptable energy supply and impedance requirements (being a multi-turn device), offer higher efficiency, positive control over the launch cycle, and permit component redundancy to achieve any desired degree of reliability. Thrust for a given current can be a hundred times higher than in a railgun, but the current must be synchronized with projectile motion. The voltage required to do so increases with velocity, and high voltage commutation capability represents the technological limit to launch velocity. Present research activity is concerned with commutation in the low and high velocity domains, position sensing techniques, dynamic stress containment in drive coils, and design of the first practical EM launcher: a Nimitz class aircraft catapult.

Abstract: Fleas have a unique catapult mechanism with a special muscle configuration. Energy is stored in an elastic material, resilin, and the extensor muscle. Force is applied by the extensor muscle to generate a torque. Energy is released as a small triggering muscle reverses the direction of the aforementioned torque. A flea can jump 150 times its body length using this elastic catapult mechanism. In this paper, a flea-inspired catapult mechanism is presented. This mechanism can be categorized as an active storage and active release elastic catapult. Owing to its unique stiffness change characteristic, a shape-memory-alloy coil spring actuator enables the mimicking of the flea's catapult mechanism. Two types of flea-inspired jumping mechanisms were developed for verifying the feasibility of applying the concept to an efficient jumping robot. The first prototype has a flea-like appearance and the second is simplified to contain just the essential components of the flea-inspired catapult mechanism. The two prototypes are 20-mm- and 30-mm-long and can jump 64 cm and 120 cm, respectively. This unique catapult mechanism can be used not only for jumping robots but also for other small-sized robots to generate fast-releasing motion.

Abstract: In this paper, we propose a novel robotic catapult for generating repeated impulsive motions which are considered as a key to achieve creature-like motions by a compact autonomous robot. The proposed robotic catapult is just a bended elastic strip whose two ends are fixed to two rotational joints, i.e., a mechanical closed-loop of an elastic material named the 'closed elastica'. By only driving one joint back and forth gradually, we can obtain repeated impulsive motions of the elastic strip. It is shown that this robotic catapult can be applied to some robotic tasks such as impulsive robotic swimming and fly casting manipulation.