Inertia-driven amphibious robot with asymmetric microundulatory fin arrays

Centimeter-scale amphibious robots are promising for versatile tasks. Existing solutions use active and multiple mechanisms for environmental interaction; however, such designs face sealing challenges at small scales and are often complex and unreliable. Here, we present an inertia-driven actuation strategy combining a variable-output voice coil motor (VCM) with a fully sealed rigid shell. By modulating the VCM output, the robot achieves jumping, full-stroke vibration for terrestrial locomotion and small-stroke vibration for aquatic propulsion. Terrestrial tests demonstrate rapid motion on granular media, continuous jumping, and load carrying. The robot also uses passive tilted fins that convert reciprocating motion into steerable aquatic thrust, realizing an inertia-driven multidirectional propulsion mechanism. Thrust generation and frequency-dependent propulsion were analyzed through aquatic experiments, high-speed particle image velocimetry, and simulations. Last, a 24-gram legless prototype (Leglessbot) demonstrated effective locomotion across diverse terrain, offering a compact solution for underactuated amphibious mobility.