cs.ROSep 23, 2026

Omnidirectional Amphibious Locomotion via Internal Mass Actuation

Authors: Niko Weaver, Boxi Xia, Li-Yu Lo, Yuhao Huang, Boyuan Chen

Organizations: Duke University

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Abstract

Field robots must traverse varied terrain and obstacles while remaining robust to water, debris, vegetation, and physical contact. We present MARBLE, a fully enclosed omnidirectional amphibious rolling robot driven entirely by internal mass redistribution. Three mutually orthogonal linear sliders shift internal masses to generate body rotation, while an orientation-aware controller maps planar velocity commands into slider positions. A rigid spherical shell encloses all active mechanisms and simultaneously serves as the terrestrial contact surface, buoyant enclosure, and mounting structure for passive fins that enable water-surface propulsion. Rotation of the same shell architecture hence produces rolling on land and surface propulsion in water without mechanical reconfiguration or separate locomotion actuators. The spherical morphology further allows the robot to accommodate changes in body orientation and contact location during direct interactions with terrain and obstacles. We evaluate MARBLE through omnidirectional locomotion characterization, traversal across heterogeneous terrestrial environments, aquatic surface locomotion, land-water transitions, and deliberate obstacle interactions. These experiments demonstrate how a single enclosed mechanical architecture can combine omnidirectional mobility, cross-medium locomotion, and tolerance to environmental contact. MARBLE provides a compact design for field mobility across heterogeneous terrain, obstacles, and land-water transitions. We will open-source all software and hardware design. Our website is https://generalroboticslab.com/MARBLE

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Fig. 1: Time-lapse images of MARBLE showing (a) land-to-water entry, (b) terrestrial rolling, and (c) water-surface propulsion. The same internal mass-actuation mechanism produces motion on land and water without mechanical reconfiguration.
Fig. 2: Mechanical design of MARBLE. (a) Linear mass-slider module. (b) Internal configuration with three orthogonal sliders and the electronics capsule. (c) Internal frame without the outer shell. (d) Complete robot with the sealed shell and passive fins.
Fig. 3: Locomotion principle of MARBLE. Top: displacing the internal slider masses offsets the center of gravity from the shell center, and the resulting moment about the ground contact rolls the shell. Below: simulation of ground and water-surface locomotion at 0.5 m s−10.5\text{\,}\mathrm{m}\text{\,}{\mathrm{s}}^{-1} using geometric and learned controllers, sampled at the same instants. The transparent shell shows the three sliding masses, colored one per rail. The quartered circle denotes their center of gravity, the red dot the shell center, and the blue arrow the heading direction.
(a) Reward terms
TermExpression rir_{i}wiw_{i}
Velocity trackingexp⁡(−(∥vxyc−vxy∥2+λzvz2)/β2)\exp\bigl(-(\lVert\mathbf{v}^{c}_{xy}-\mathbf{v}_{xy}\rVert^{2}+\lambda_{z}v_{z}^{2})/\beta^{2}\bigr)+2.0+2.0
Slider velocity∑jq˙j2\sum_{j}\dot{q}_{j}^{2}−0.002-0.002
Travel limit∑j[(qj−qjmax⁡)+2+(qjmin⁡−qj)+2]\sum_{j}\bigl[(q_{j}-q_{j}^{\max})_{+}^{2}+(q_{j}^{\min}-q_{j})_{+}^{2}\bigr]−1.0-1.0
Action rate∥at−at−1∥2\lVert\mathbf{a}_{t}-\mathbf{a}_{t-1}\rVert^{2}−0.05 ⁣→ ⁣−0.2-0.05\!\rightarrow\!-0.2
Action acceleration∥at−2at−1+at−2∥2\lVert\mathbf{a}_{t}-2\mathbf{a}_{t-1}+\mathbf{a}_{t-2}\rVert^{2}−0.05-0.05
Table 4
Fig. 4: Terrestrial, aquatic, and transitional locomotion of MARBLE. Top row: representative time-lapse sequences. Bottom row: corresponding planar position trajectories for (a) terrestrial, (b) aquatic, and (c) transitional locomotion. Trajectory color indicates temporal progression, while the circle and cross mark the start and end positions, respectively.
Fig. 5: Measured velocity vectors under joystick commands for (a) the learned controller and (b) the geometric controller. Velocities are estimated by numerical differentiation of camera-relative trajectories and plotted from a common origin. Each panel contains 7,568 samples with identical axis and color scales.
Fig. 6: Representative time-lapse sequence of obstacle interaction showing approach, direct shell contact, and continuous interaction with the buoy (red).
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