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 displace the center of mass and generate body rotation, while an orientation-aware controller maps planar velocity commands into slider positions as the body continuously reorients. 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 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.

Video

Overview

MARBLE rolling from land into water

Cross-Medium Amphibious Locomotion

MARBLE navigates amphibious environments without hardware reconfiguration, using one locomotion system.

Terrestrial Rolling (1.5×)

MARBLE terrestrial rolling

Water-Surface Propulsion (2×)

MARBLE water-surface propulsion

Land-Water Transition (3×)

MARBLE land-water transition
TerrestrialAquaticTransition
Distance10.60 m5.24 m5.21 m
Mean speed0.745 ± 0.163 m/s0.375 ± 0.059 m/s0.215 ± 0.146 m/s
Max speed1.025 m/s0.488 m/s0.840 m/s

Design

Three orthogonal mass sliders shift MARBLE’s center of mass to induce rolling. All active mechanisms are sealed inside the spherical shell.

Mechanical design of MARBLE: linear mass-slider module, internal orthogonal configuration, frame, and sealed shell
SpecificationValue
Outer diameter387 mm
Shell mass1.2 kg
Fin height8 mm
Mass sliders3 × 700 g
Slider stroke220 mm
MotorsCubeMars GL40 II
Position control100 Hz
Battery4S LiPo, 16 V
Locomotion principle of MARBLE

Omnidirectional Velocity Characterization

Geometric and learned controllers in MuJoCo, on ground and on water.

Geometric and learned controllers in simulation, on ground and on water

Measured velocity vectors on hardware under joystick commands (7,568 samples each). The learned controller reaches 0.348 m/s mean speed versus 0.252 m/s for the geometric controller.

Learned Controller

Measured velocity vectors, learned controller

Geometric Controller

Measured velocity vectors, geometric controller

Obstacle Interaction

The enclosed actuation makes MARBLE contact-tolerant: it can push against obstacles with its shell without exposing any mechanism.

MARBLE pushing a floating buoy with the shell Obstacle interaction time-lapse: approach, contact, push

Photos

MARBLE at the land-water boundary MARBLE interacting with a buoy MARBLE shell and fin close-up Pond test site: boardwalk Pond test site: viewing platform

Citation

1
2
3
4
5
6
7
8
9

@misc{weaver2026omnidirectionalamphibiouslocomotion,
      title={Omnidirectional Amphibious Locomotion via Internal Mass Actuation},
      author={Niko Weaver and Boxi Xia and Li-Yu Lo and Yuhao Huang and Boyuan Chen},
      year={2026},
      eprint={2609.27358},
      archivePrefix={arXiv},
      primaryClass={cs.RO},
      url={https://arxiv.org/abs/2609.27358},
}