PuffyBot: An Untethered Shape Morphing Robot for Multi-environment Locomotion

TL;DR

PuffyBot tackles the challenge of autonomous, multi-environment locomotion by integrating a shape-m morphing scissor-lift body with a bell-crank-driven fin orientation, enabling transitions between crawling and swimming using a single low-power actuator. Buoyancy is actively modulated through volume change, with a design-space model confirming how geometry and mass govern net buoyant force and enabling reliable negative-to-positive buoyancy shifts. The approach achieves untethered operation with energy per morph well below thermal-actuated counterparts and demonstrates locomotion on land, underwater floors, and the water surface, highlighting the potential for compact, energy-efficient amphibious robots. Limitations include open-loop buoyancy control and tank-based validation; future work targets closed-loop buoyancy regulation, onboard sensing, active pressure modulation, and deployment in larger or outdoor environments.

Abstract

Amphibians adapt their morphologies and motions to accommodate movement in both terrestrial and aquatic environments. Inspired by these biological features, we present PuffyBot, an untethered shape morphing robot capable of changing its body morphology to navigate multiple environments. Our robot design leverages a scissor-lift mechanism driven by a linear actuator as its primary structure to achieve shape morphing. The transformation enables a volume change from 255.00 cm3 to 423.75 cm3, modulating the buoyant force to counteract a downward force of 3.237 N due to 330 g mass of the robot. A bell-crank linkage is integrated with the scissor-lift mechanism, which adjusts the servo-actuated limbs by 90 degrees, allowing a seamless transition between crawling and swimming modes. The robot is fully waterproof, using thermoplastic polyurethane (TPU) fabric to ensure functionality in aquatic environments. The robot can operate untethered for two hours with an onboard battery of 1000 mA h. Our experimental results demonstrate multi-environment locomotion, including crawling on the land, crawling on the underwater floor, swimming on the water surface, and bimodal buoyancy adjustment to submerge underwater or resurface. These findings show the potential of shape morphing to create versatile and energy efficient robotic platforms suitable for diverse environments.

Figures (10)

• Figure 1: (A) PuffyBot photographed in natural terrain, equipped with onboard actuation, control electronics, and power supply. (B) Crawling mode: the scissor-lift body is fully compressed and the fins lie flat, providing stability and traction for crawling on land or along the underwater floor. (C) Swimming mode: the scissor-lift body expands and the fins rotate downward to align with the body, forming a streamlined configuration for surface swimming.
• Figure 2: An exploded view of the CAD model shows the robot design including links, mechanisms and actuators.
• Figure 3: Bell-crank linkage coupling the scissor-lift link and servo mount. The mechanism converts vertical motion into a 90° servo reorientation, enabling transitions between horizontal (crawling) and vertical (swimming) mode.
• Figure 4: (A) 3D printing of a compliant fin directly onto TPU fabric using TPU filament. (B) The soft compliant fin acts as a flexible joint: it self-locks during crawling and backward stroke to generate thrust, and bends upward during the forward recovery stroke to minimize drag in water.
• Figure 5: Free-body diagram showing three buoyancy states of the robot with change in volume: $F_\text{w} > F_\text{b}$: negative, $F_\text{w} = F_\text{b}$: neutral, and $F_\text{b} > F_\text{w}$: positive buoyancy.