ReefGlider: A highly maneuverable vectored buoyancy engine based underwater robot

TL;DR

ReefGlider addresses the gap between low-noise, energy-efficient buoyancy gliders and high-mobility thruster AUVs by introducing vectored buoyancy control (VBC) that enables $6$-DOF motion with sub-meter precision. The paper details the hardware design (two buoyancy engines with four linear actuators, passive plate, and compact electronics), a simplified linearized modeling framework, and a PID-based feedback control architecture, augmented by open-loop compensation. Tank experiments demonstrate depth-hold, three-dimensional sawtooth trajectories, and yaw maneuvers, validating non-holonomic yet high-precision capabilities with ground-truth via AprilTag tracking. The work highlights ReefGlider's potential for long-term coral reef monitoring, offering a low-noise, energy-efficient platform and a modular design framework that can accommodate additional payloads and sensing while maintaining maneuverability.

Abstract

There exists a capability gap in the design of currently available autonomous underwater vehicles (AUV). Most AUVs use a set of thrusters, and optionally control surfaces, to control their depth and pose. AUVs utilizing thrusters can be highly maneuverable, making them well-suited to operate in complex environments such as in close-proximity to coral reefs. However, they are inherently power-inefficient and produce significant noise and disturbance. Underwater gliders, on the other hand, use changes in buoyancy and center of mass, in combination with a control surface to move around. They are extremely power efficient but not very maneuverable. Gliders are designed for long-range missions that do not require precision maneuvering. Furthermore, since gliders only activate the buoyancy engine for small time intervals, they do not disturb the environment and can also be used for passive acoustic observations. In this paper we present ReefGlider, a novel AUV that uses only buoyancy for control but is still highly maneuverable from additional buoyancy control devices. ReefGlider bridges the gap between the capabilities of thruster-driven AUVs and gliders. These combined characteristics make ReefGlider ideal for tasks such as long-term visual and acoustic monitoring of coral reefs. We present the overall design and implementation of the system, as well as provide analysis of some of its capabilities.

Figures (8)

• Figure 1: ReefGlider show here in a tank is a highly maneuverable underwater robot that uses buoyancy for propulsion, but unlike other gliders is highly maneuverable.
• Figure 2: Isometric view of the ReefGlider and relevant frames of reference.
• Figure 3: Section-view of a single Buoyancy Engine.
• Figure 4: Feedback control diagram for the VBC. The mixer sums the pressure, roll, pitch and Open Loop (OL) control signals and sends the corresponding actuation to each of the linear actuators.
• Figure 5: (a) Depth-hold experiment: ground-truth pose data from AprilTag tracking by a GoPro submerged in test tank. (b) Depth-hold experiment: roll, pitch, and depth readings (in blue) with their corresponding set points plotted (in red) for changing depth set points.