Ralf Bachmayer

Multi-AUV Control and Adaptive Sampling in Monterey Bay

Operations with multiple autonomous underwater vehicles (AUVs) have a variety of underwater applications. For example, a coordinated group of vehicles with environmental sensors can perform adaptive ocean sampling at the appropriate spatial and temporal scales. We describe a methodology for cooperative control of multiple vehicles based on virtual bodies and artificial potentials (VBAP). This methodology allows for adaptable formation control and can be used for missions such as gradient climbing and feature tracking in an uncertain environment. We discuss our implementation on a fleet of autonomous underwater gliders and present results from sea trials in Monterey Bay in August, 2003. These at-sea demonstrations were performed as part of the Autonomous Ocean Sampling Network (AOSN) II project

Vehicle networks for gradient descent in a sampled environment

Fish in a school efficiently find the densest source of food by individually responding not only to local environmental stimuli but also to the behavior of nearest neighbors. It is of great interest to enable a network of autonomous vehicles to function similarly as an intelligent sensor array capable of climbing or descending gradients of some spatially distributed signal. We formulate and study a coordinated control strategy for a group of autonomous vehicles to descend or climb an environmental gradient using measurements of the environment together with relative position measurements of nearest neighbors. Each vehicle is driven by an estimate of the local environmental gradient together with control forces, derived from artificial potentials, that maintain uniformity in group geometry.

Multi-AUV control and adaptive sampling in Monterey Bay

Multi-AUV operations have much to offer a variety of underwater applications. With sensors to measure the environment and coordination that is appropriate to critical spatial and temporal scales, the group can perform important tasks such as adaptive ocean sampling. We describe a methodology for cooperative control of multiple vehicles based on virtual bodies and artificial potentials (VBAP). This methodology allows for adaptable formation control and can be used for missions such as gradient climbing and feature tracking in an uncertain environment. We discuss our implementation on a fleet of autonomous underwater gliders and present results from sea trials in Monterey Bay in August 2003. These at-sea demonstrations were performed as part of the Autonomous Ocean Sampling Network (AOSN) II project.

An accurate four-quadrant nonlinear dynamical model for marine thrusters: theory and experimental validation

This paper reports two specific improvements in the finite-dimensional nonlinear dynamical modeling of marine thrusters. Previously reported four-quadrant models have employed thin airfoil theory considering only axial fluid flow and using sinusoidal lift/drag curves. First, we present a thruster model incorporating the effects of rotational fluid velocity and inertia on thruster response. Second, we report a novel method for experimentally determining nonsinusoidal lift/drag curves. The model parameters are identified using experimental thruster data (force, torque, and fluid velocity). The models are evaluated by comparing experimental performance data with numerical model simulations. The data indicates that thruster models incorporating both reported enhancements provide superior accuracy in both transient and steady-state responses.

Underwater gliders: recent developments and future applications

Autonomous underwater vehicles, and in particular autonomous underwater gliders, represent a rapidly maturing technology with a large cost-saving potential over current ocean sampling technologies for sustained (month at a time) real-time measurements. We give an overview of the main building blocks of an underwater glider system for propulsion, control, communication and sensing. A typical glider operation, consisting of deployment, planning, monitoring and recovery are described using the 2003 AOSN-II field experiment in Monterey Bay, California. We briefly describe the recent developments at NRC-IOT, in particular, the development of a laboratory-scale glider for dynamics and control research and the concept of a regional ocean observation system using underwater gliders.

Surveying a subsea lava flow using the Autonomous Benthic Explorer (ABE)

This paper summarizes results from the first science deployment of the Autonomous Benthic Explorer (ABE), conducted on the Juan de Fuca Ridge (46°N, 129°W) at depths between 2200 and 2400 m. Using long baseline acoustic transponders, the ABE descended with precision to a preassigned starting point, then executed dead-reckoned tracklines. It followed the bottom at distances between 7 and 20 m using an acoustic fathometer as a reference sensor. The ABE mapped a new subsea lava flow with a magnetometer, imaged the seafloor with a stereo snapshot video system, and mapped a hydro thermal plume with conductivity and temperature sensors. The ABE completed 7 successful dives and covered over 35 km of tracklines. Detailed power records were logged, which permits extrapolation of the ABEs performance to other missions and higher capacity batteries.

Development of an Auxiliary Propulsion Module for an Autonomous Underwater Glider

A low-power propeller-based propulsion module has been developed to augment the buoyancy engine of a 200 m Slocum electric glider. This device is introduced to allow new behaviours such as horizontal flight and faster overall speeds to expand the existing operational envelope of underwater gliders. The design of the system is optimized for use at the typical horizontal glider speed of 0.3 m/s. Before integration into the glider the stand-alone propulsion module has been tested in a small flume tank to verify the systems performance. The validity of a previously published hydrodynamic model of the glider at zero angle of attack was verified by conducting drag measurements at various flow velocities at full scale in a larger flume tank. Self-propulsion tests were also performed to establish the performance of the glider with the new propulsion module in the larger flume tank and in the university tow tank. The results from these tests show that the new propulsion module is able to match the performance of the conventional glider for full depth profiles and to exceed it for limited depth profiles.

Glider control: a close look into the current glider controller structure and future developments

Simplicity of operation, extreme endurance and stealth are some of the principal advantages of autonomous underwater gliders. These advantages come at the expense of a minimalist approach in the sense of actuation and permissible power consumption. Also by nature the gliders are highly dependent on their operational environment, notably currents and density gradients. A control system that is able to cope with this challenging dynamic environment and the actuation and sensing constraints has to be robust with respect to environmental uncertainties and at the same time it has to be optimal in the sense of power consumption and accuracy. In this paper we look at the existing controller structure and show possibilities for improvements within the framework of the available sensor information. We summarize the existing controller structure and show results from recent field-tests of Slocum gliders in the Gulf of Mexico that demonstrate the potential for improvements in the sense of reduced power consumption associated with actuation and decreased response time enabling shallow water operations.

Coordinated gradient descent: A case study of Lagrangian dynamics with projected gradient information

Abstract The paper studies gradient descent algorithms for vehicle networks. Each vehicle within the network is modeled as a double integrator in the plane. For each individual vehicle, the control input enabling coordinated gradient descent consists of a gradient descent control term and additional inter-vehicle forcing terms. When each vehicle has enough sensors to measure the full gradient at its current position, then the closed-loop system becomes Lagrangian. We focus in the present paper upon the more practical situation where each vehicle has only one sensor with which to sample the environment. We take this into account by replacing the full gradient in the closed-loop equations by its projection on the direction of motion for each individual vehicle. This gives rise to a differential equation with discontinuous right-hand side. In order to avoid the (practical and theoretical) complications that arise as a consequence of these discontinuities, we modify the inter-vehicle forcing terms and represent the velocity of each vehicle by a magnitude and an angle, resulting in a set of smooth differential equations. We demonstrate our approach with simulations

Analysis and development of a buoyancy-pitch based depth control algorithm for a hybrid underwater glider

The hybrid glider augments a Slocum electric glider with a propeller based propulsion device enabling new modes of operation. One of the new modes available is constant depth flight. The glider has two mechanisms which lend themselves to a control scheme for depth control, a ballast system and an internal mass shifting mechanism for pitch control. This paper examines the use of a ballast depth controller and a pitch based depth controller. The detailed implementation of both controllers is described and experimental results are presented.