Uwe R. Zimmer

Distributed shape control of homogeneous swarms of autonomous underwater vehicles

In this paper, we study large formations of underwater autonomous vehicles for the purposes of exploration and sampling the ocean surface. The formations or aggregates we consider are composed of up to hundreds of robots with the capability of forming various complex shapes dictated by the shape of the region to be explored, as well as special shapes suitable for migration. The shapes are determined through bathymetric maps and described with reduced-dimensional representation techniques. The approach we propose is that of breaking up the control and coordination strategy into two decoupled problems, i.e., partitioning the aggregate into two non-overlapping sets: its boundary and its interior. The boundary uses general theory of curve evolution to form shapes while the interior passively complies, using attraction-repulsion forces, to form a uniform distribution inside the boundary. This makes the problem much more tractable than previous methods. Decision making by individual robots is entirely based on local information, autonomous underwater vehicles, formation control, swarm control.

Control of Open Contour Formations of Autonomous Underwater Vehicles

In this paper, we propose a distributed elastic behaviour for a deformable chain-like formation of small autonomous underwater vehicles with the task of forming special shapes which have been explicitly defined or are defined by some iso-contour of an environmental concentration field. In the latter case, the formation has to move in such a way as to meet certain formation parameters as well as adapt to the iso-line. We base our controller on our previous models (for manually controlled end points) using general curve evolution theory but will also propose appropriate motions for the end robots of an open chain.

Contour shaped formation control for autonomous underwater vehicles using canonical shape descriptors and deformable models

Cooperative tasks such as swarms following gradients, detecting environmental boundaries (defining natural features) and exploration by aggregates of mobile autonomous robots have gained much attention in the past few years. Aggregates come in various forms including rigid formations, involving a few robots, with clearly-defined simple shapes or swarms, with up to hundreds of robots, but with no particular shape characteristics. In the afore-mentioned tasks, a large aggregate is required to form complex shapes. When the number of robots increases, it is very difficult or even senseless to manually determine the position of each and every agent within the formation. In this paper, we concentrate on a special kind of geometric formation (open and closed contours). We use Fourier descriptors, as one of the most natural ways of representing curves, and active contour models, to make the aggregate exhibit desirable physical behaviours. We apply these kinds of formations to the case of autonomous underwater vehicles collectively exploring, mapping, and adapting to environmental features. Many of the features found in underwater landscapes are shaped as open or closed contours so that the proposed combination makes sense. In our approach, a group of mobile agents can synthesize a curve given a small set of canonical invariant descriptors.

Underwater acoustic localization for small submersibles

This article discusses the complete processing chain of a high-precision as well as robust underwater localization method. Range and azimuth as well as relative heading are determined for ranges up to 90 m. For ranges up to 10 m the measured precision is 0.05 m for range, 2 deg for direction, and 5 deg for heading of neighboring vehicles. The system is also designed to be deployable in swarms of vehicles as acoustic localization signals are separated by time slots and maximum length sequence (MLS) signatures. The proposed method is based on the consequent exploitation of MLS broadband signal characteristics and the exploration of several methods for each vehicle to estimate ranges, azimuths, and headings of neighboring vehicles. An existing radio-based swarm communication system provides additional support and leads to enhanced measurement precision and interference robustness. The capabilities and limitations of the proposed systems are experimentally determined and discussed.

Relative localisation for AUV swarms

In most robotic swarming applications each individual needs to know the positions and orientations of at least its near neighbours. One way of achieving this local estimate underwater is by means of wide-bandwidth acoustic signals of specific statistical characteristics (maximum length sequence -MLS). The principal capability of local posture estimation has been demonstrated. While previous results have been achieved offline, e.g. have not been available to the vehicles while being in the swarm, this article reports about the first on-line evaluation test-runs.

Acoustical Methods for Azimuth, Range and Heading Estimation in Underwater Swarms

Acoustical Methods for Azimuth, Range and Heading Estimation in Underwater Swarms Enhanced, embodied autonomy in small submersibles enables the design and deployment of practical swarms of autonomous underwater vehicles (AUVs). The swarming paradigm requires for each vehicle location awareness of at least its near neighbors. The Serafina AUV swarming project additionally requires a localisation system which could cope with the dynamic and fast changing vehicle configurations while being small, reliable, robust, and energy efficient and not dependent on pre‐deployed acoustic beacons. The short‐range acoustical relative localisation system proposed here, uses hyperbolic and spherical localization concepts and provides each vehicles with the azimuth, range and heading of its near neighbours. The implementation utilises an acoustically transmitted MLS signal which provides extremely high robustness against interference by stochastic and systematic disturbances which are typical for underwater environments. Th...

Effective Communication in Schools of Submersibles

Effective communication mechanisms in schools of submersibles are a key requirement for their meaningful deployment. Furthermore a fully distributed communication schema is preferable for reasons of reliability. The required communication form is usually many-to-many, especially omnicast [4] (or gossiping ). All these constraints are hard to achieve at the same time in a low-bandwidth, short range communication setup. The theoretical findings in [4] and [5] are expanded and employed for the actual underwater schools communication in the Serafina project (e.g. [2], [3]). The results are reported here.

Motion Control of Mobile Robots—From Static Targets to Fast Drives in Moving Crowds

Motion control of vehicles under uncertain, noisy, and discontinuous positioning is essential in autonomous navigation in unknown environments. This article suggests two methods for motion control, where the initial parameters of the on-line control are physically explainable, the resulting trajectory as well as the control parameters are asymptotically converging and glitches in the localization are handled robustly. The differences to a known method based on Lyapunov functions are discussed theoretically as well as in terms of actual motion measurements. Physical experiments with landbound vehicles show the reliability and limitations of these different methods in setups employing static attractors, systematically moving targets and fast, unpredictable moving targets in highly dynamical environments. Mainly due to the physical meaning of the control parameters the adaptation to actual kinematics and dynamics is significantly simplified.

Motion Planning for Small Formations of Autonomous Vehicles Navigating on Gradient Fields

In this paper, we present a motion planning scheme for navigation of a contour-like formation of autonomous underwater vehicles on gradient fields and subsequent convergence to desired isoclines, inspired by evolution of closed planar curves. The basic evolution behaviour is modified to include moving boundary points and incorporate safety constraints on formation parameters. Also, the whole process is decomposed into a sequence of well-behaving states. As opposed to the basic model, the regularized solution is characterized by the maximum allowable curvature rather than balance of forces determined by fixed coefficients. Nevertheless, the proposed framework subsumes the original model. Blocking states and fairness are briefly discussed.

MLS Based Distributed, Bearing, Range and Posture Estimation for Schools of Submersibles

Continuous relative localization among all local members of a school of submarines is an essential pre-condition for distributed motion control. Exploiting the short-range, underwater electromagnetic as well as the acoustical channel, the proposed approach delivers sufficient bearing, range and posture estimations based on local sensing in small, autonomous submersibles with limited energy capacities and computational resources. This research is part of the Serafina project at the Australian National University, aiming for large schools of autonomous underwater vehicles.