Design and Attitude Control of an Underwater Quadruped Robot

Davide Molinaroli Mohit Singh Kostas Alexis
Autonomous Robots Lab · Norwegian University of Science and Technology
Paper arXiv Open-source CAD Video
The underwater quadruped robot submerged in a water tank while performing roll and pitch maneuvers

The underwater quadruped robot tracking roll and pitch attitude setpoints in a water tank.

Abstract

Legged robots are versatile on land, but their use in underwater environments remains limited. Extending quadruped locomotion to water enables amphibious mobility with applications in inspection, environmental monitoring and disaster response. This paper presents the design, modeling, and experimental validation of a reproducible underwater quadruped robot. The robot is built around custom waterproof motor housings machined from polyoxymethylene plastic, which use off-the-shelf O-rings and dynamic shaft seals. A simplified model is derived to describe the dynamics of this underwater legged system, capturing how drag forces on spherical end effectors transmit torque to the floating base. Building on this model, a closed-loop attitude controller is developed using an error formulation defined on the special orthogonal group SO(3). The controller is evaluated both in simulation and experimentally in a water tank, where the robot tracks desired orientation setpoints in roll, pitch and yaw.

Robot Design

3D Model

Drag to orbit · scroll to zoom
  • Body: 58.99 × 34.44 × 15.33 cm, 4.9 kg
  • Actuators: 12 × AGF-RC IB53BHU, 3.9 Nm
  • OBC: NVIDIA Jetson Orin NX + A603 Carrier Board
  • Power: 2S 3500 mAh + 4S 550 mAh LiPo

Waterproofing

Servo enclosure

The assembled two-servo waterproof casing
Loading assembly sequence
Top cover
Drag the slider or press Play to build the housing up part by part.
Exploded view of the waterproof casing containing two servos, beside the machined part
Exploded view and real-world assembly of the servo enclosure.

Main electronics enclosure

Bottom part of the main electronics enclosure: Jetson Orin NX on its carrier board, I2C multiplexer, PWM driver and ADC stack
Bottom half of the main electronics enclosure.
Top part of the main electronics enclosure: power lines, VN-100 IMU, switch, tether interface, pressure vent and depth sensor
Top half of the main electronics enclosure.

Model and Controller

Simplified model

Diagram of the simplified dynamic model: body and world frames, leg kinematics, and drag forces acting on the spherical end effectors

Attitude Controller

Block diagram of the attitude control loop: setpoint, SO(3) error, control torque, leg allocation, robot, IMU feedback

Experiments

Maneuver

Setpoint

Cite

BibTeX

Reference

@misc{molinaroli2026designattitudecontrolunderwater,
      title={Design and Attitude Control of an Underwater Quadruped Robot}, 
      author={Davide Molinaroli and Mohit Singh and Kostas Alexis},
      year={2026},
      eprint={2609.09217},
      archivePrefix={arXiv},
      primaryClass={cs.RO},
      url={https://arxiv.org/abs/2609.09217}, 
}