Karl D. von Ellenrieder

Unmanned Autonomous Sailing: Current Status and Future Role in Sustained Ocean Observations

Sea surface measurements with higher temporal and spatial resolution than satellite observations, manned expeditions, moored buoys, Lagrangian floats, and other contemporary methods could result from using unmanned autonomous sailing craft for sustained oceanographic observations. Energetically sustainable mission-specific systems for environmental event forecasting and meteorological and ocean condition distribution tracing over a long-term period could result from mission-specific autonomous sailing platforms. The motivation for wind-powered unmanned autonomous vehicle use for oceanographic/meteorological measurements is briefly discussed in this paper, which also introduces aspects of a Florida Atlantic University-developed vehicle, outlines some current and past work, comments on significant historical considerations, and concludes by pressing policy issues and future work recommendations.

Dynamics-aware target following for an autonomous surface vehicle operating under COLREGs in civilian traffic

We present a model-predictive trajectory planning algorithm for following a target boat by an autonomous unmanned surface vehicle (USV) in an environment with static obstacle regions and civilian boats. The planner developed in this work is capable of making a balanced trade-off among the following, possibly conflicting criteria: the risk of losing the target boat, trajectory length, risk of collision with obstacles, violation of the Coast Guard Collision Regulations (COLREGs), also known as “rules of the road”, and execution of avoidance maneuvers against vessels that do not follow the rules. The planner addresses these criteria by combining a search for a dynamically feasible trajectory to a suitable pose behind the target boat in 4D state space, forming a time-extended lattice, and reactive planning that tracks this trajectory using control actions that respect the USV dynamics and are compliant with COLREGs. The reactive part of the planner represents a generalization of the velocity obstacles paradigm by computing obstacles in the control space using a system-identified, dynamic model of the USV as well as worst-case and probabilistic predictive motion models of other vessels. We present simulation and experimental results using an autonomous unmanned surface vehicle platform and a human-driven vessel to demonstrate that the planner is capable of fulfilling the above mentioned criteria.

Thrust measurements from a finite-span flapping wing

The thrust per unit length behind a flapping NACA0030 airfoil with an aspect ratio of three is measured and presented. Aspects of the evolution of vorticity behind the thrust-producing wing are discussed based on quantitative experiments. Multiple planes of stereoscopic particle image velocimetry measurements are conducted at several locations along the span of the wing at a Strouhal number of 0.35. Of particular interest is the effect of wingtip vortices on the structure of the flow behind the oscillating wing. Wing kinematics is responsible for the flow structure in the 2-D airfoil case. Here, the spanwise distribution of vorticity is found to be dominated, in the large scale, by a single pair of intense counter-rotating vortices. Each member of the large-scale vortex pair is constituted by two smaller corotating vortices that constructively merge in the initial stages of flow separation. Toward the wingtips, three-dimensional effects are more significant. The spatiotemporal variations of transverse and spanwise vorticity in these regions suggest severe local flow deformation. Measurements reveal that flow morphology is highly complex and three-dimensional, unlike any previously observed 2-D wing-based vortex sheets. Furthermore, using 2-D particle image velocimetry data, a sinusoidal variation in thrust force, 90 deg out of phase with the airfoil motion, is measured in the midspan region of the airfoil. The largest measured thrust occurs at the maximum angles of attack, corresponding to the creation of strong leading-edge vortices.

Measurements of a wall-bounded, turbulent, separated flow using HPIV

In this work, a lens-less, off-axis, holographic particle image velocimetry (HPIV) system is employed to investigate the separated flow over the blunt leading-edge of a flat plate. Multiple two-dimensional velocity maps within a three-dimensional region of interest (at a single instant in time) are obtained using a multigrid cross-correlation digital PIV technique. The out of plane vorticity is extracted from each of the velocity fields. An attempt to determine the out of plane velocity for each image was also made using a stereoscopic scanner. However, owing to the small effective aperture of the holographic image it was not possible to stereoscopically measure the third component of velocity. The limitations of the stereoscopic scanning method for HPIV and some potential solutions are discussed. This article was chosen from selected Proceedings of the Eighth European Turbulence Conference (Advances in Turbulence VIII (Barcelona, 27–30 June 2000) (Barcelona: CIMNE) ed C Dopazo. ISBN: 84-89925-65-8).

Experimental Evaluation of Approach Behavior for Autonomous Surface Vehicles

This article presents an experimental assessment of an Unmanned Surface Vehicle (USV) executing an approach behavior to several stationary targets in an obstacle field. A lattice-based trajectory planner is implemented with a priori knowledge of the vehicle characteristics. In parallel, a low-level controller is developed for the vehicle using a proportional control law. These systems are integrated on the USV control system using the Lightweight Communications and Marshalling (LCM) message passing system. Filtered vehicle-state information from onboard sensors is passed to the planner, which returns a least-cost, dynamically feasible trajectory for achieving the ascertained goal. The system was tested in a 750 m by 150 m area of the US Intracoastal Waterway in South Florida in the presence of wind and wave disturbances to characterize its effectiveness in a real-world scenario. The vehicle was able to replicate behavior predicted in simulations when navigating around obstacles. The approach distance to each target was favorably lower than the user-defined limit. Owing to the fact that the USV uses differential thrust for steering, the vehicle tracked the planned trajectories better at lower speeds.Copyright

Development and Preliminary Experimental Validation of a Wind- and Solar-Powered Autonomous Surface Vehicle

The design and initial testing of a wind- and solar-powered (WASP) autonomous surface vehicle (ASV) are presented. The concept vehicle is a 4.2-m length overall monohull keelboat powered by a rigid wing sail with a 5-m span. It is designed to operate in 7-10 km of wind and a maximum sea state of 2. Specific attention is placed on the aerodynamic, hydrodynamic, and systems integration aspects of the design. Rigid wing sails are shown to have superior performance to conventional cloth sails for use on ASVs owing to their higher aerodynamic efficiency and robustness. The manual design process involved in selection of the airfoil cross section for the wing sail and the resulting aerodynamic performance predictions are presented. Through mathematical derivation, the optimal angle for switching the sail configuration from an upwind/crosswind lift-generating mode to a downwind drag-generating mode is found to be 135 . To simplify implementation of the control system when operating in the lift-generating sailing mode, a control scheme that decouples heading and speed control by simply setting the wing sail to an angle of attack of 10 and independently controlling heading was used. The simulation results from a velocity prediction program (VPP) used to verify the feasibility of this control approach are presented. Initial field trials of the vehicle using the control approach with autonomous wing control and manual rudder control are presented. It is shown that the measured boat speeds and wind speed/directions are within the range of values expected from the VPP. Temperatures recorded in a thermal plume during the field trials of the ASV are shown as a function of global positioning system (GPS) location and time.

Characterization and System Identification of an Unmanned Amphibious Tracked Vehicle

Experimental testing of an unmanned amphibious tracked vehicle on dry sand, in the surfzone and in water, has been performed to explore its maneuvering and performance characteristics for the planned future development of an automatic control system. The 2.69-m-long, 295-kg concept vehicle utilizes a small waterplane area twin hull (SWATH) configuration with integrated crawler tracks and twin propellers for propulsion. Maximum traversable incline tests on dry sand reveal that the vehicle has a drawbar pull (DP) of about 1000 N (about one third of its weight) and can operate on slopes of 15 ° briefly and 10 ° for extended periods of time. Maneuvering tests were performed on flat, dry sand, as well as into and out of a surfzone at a beach site with inclines ranging from about 3 ° to 6 °. In each of the tests, the track forces, speed over ground, and both linear and rotational accelerations were measured. Also recorded were environmental conditions, such as wind speed, significant waveheight/period, and ground slope. The tests reveal that the vehicle has a maximum straight-line flat ground speed of about 2 m/s, a minimum flat ground turning radius of 2.4 m, an in-water minimum turning radius of 1.9 m, and a maximum straight-line waterborne speed of 1.2 m/s. It is also shown that system identification applied to the data recorded from the flat ground and waterborne maneuvering tests can be used to find a linear, parametric state-space model for the vehicle that adequately reproduces its motion. Land-to-sea transition tests in two different seas of one-third significant waveheights of 8 and 28 cm (both measured at a distance of 6 m from the mean waterline on the beach) show that the vehicle exhibits substantial track slip as it traverses into the surfzone and its weight becomes increasingly supported by the displacement of its hulls. The tractive force model developed by Wong (Theory of Ground Vehicles, New York, NY, USA: Wiley, 2001) is modified to account for the reduction in track-supported vehicle weight as the vehicle becomes waterborne. It is shown that this adapted model captures the main physical features of the measured track forces. The waveheights and periods recorded during surfzone transition tests are used to examine the seakeeping properties of the vehicle. It is found that the vehicles natural frequency of roll is near the dominant wave frequencies measured. A Froude-Krylov strip theory simulation shows that the wave forces acting on the vehicle in 28-cm waves may be slightly larger than the force measured on each track when the DUKW-Ling is crossing the transition zone between the beach and surfzone and should not be ignored in modeling and simulation studies.

Trajectory planning with adaptive control primitives for autonomous surface vehicles operating in congested civilian traffic

We introduce a model-predictive trajectory planning algorithm for unmanned surface vehicles (USVs) operating in congested civilian traffic. The planner reasons about the availability of contingency maneuvers needed in case of any of the civilian vessels breaches the International Regulations for the Prevention of Collisions at Sea (COLREGs). Our exploratory study indicated that implementing the envisioned planner requires significant speed up of trajectory planning to cope with the dynamics of the scene, and evaluation of collision risk. We describe a new method for efficiently searching 5D state space for a dynamically feasible trajectory using adaptive control action primitives. The algorithm estimates the congestion of the state space regions to evaluate collision risk, and then dynamically scales action primitives used during the search while preserving their dynamical feasibility. Our simulation experiments demonstrate that this leads to a substantial increase in the search efficiency and a decrease in the number of collisions, especially in complex scenarios with a higher number of civilian vessels.

High-level fuzzy logic guidance system for an unmanned surface vehicle (USV) tasked to perform autonomous launch and recovery (ALR) of an autonomous underwater vehicle (AUV)

There have been much technological advances and research in Unmanned Surface Vehicles (USV) as a support and delivery platform for Autonomous/Unmanned Underwater Vehicles (AUV/UUV) or Remotely Operated Vehicles (ROV). Advantages include extending underwater search and survey operations time and reach, improving underwater positioning and mission awareness, in addition to minimizing the costs and risks associated with similar manned vessel operations. The objective of this paper is to present the design and development a high-level fuzzy logic guidance controller for a WAM-V 14 unmanned surface vehicle (USV) in order to autonomously launch and recover a REMUS 100 autonomous underwater vehicle (AUV). The approach to meeting this objective is to develop ability for the USV to intercept and rendezvous with an AUV that is in transit in order to maximize the probability of a final mobile docking maneuver. Specifically, a fuzzy logic Rendezvous-Docking controller has been developed that generates Waypoint-Heading goals for the USV to minimize the cross-track errors between the USV and AUV. A subsequent fuzzy logic Waypoint-Heading controller has been developed to provide the desired heading and speed commands to the low-level controller given the Waypoint-Heading goals. High-level mission control has been extensively simulated using Matlab and partially characterized in real-time during testing. Detailed simulation, experimental results and findings will be reported in this paper.

Control of an Unmanned Surface Vehicle With Uncertain Displacement and Drag

Experimental testing of an unmanned surface vehicle (USV) has been performed to evaluate the performance of two low-level controllers when displacement and drag properties are time varying and uncertain. The USV is a 4.3-m-long, 150-kg wave adaptive modular vessel (WAM-V) with an inflatable twin-hull configuration and waterjet propulsion. Open-loop maneuvering tests were conducted to characterize the dynamics of the vehicle. The hydrodynamic coefficients of the vehicle were determined through system identification of the maneuvering data and were used for simulations during control system development. The resulting controllers were experimentally field tested on-water. Variable mass and drag tests show that the vehicle is best controlled by a model reference adaptive backstepping speed and heading controller. The backstepping controller developed by Liao et al. (2010) is modified to account for an overprediction of necessary control action and motor saturation. It is shown that when an adaptive algorithm is implemented for the surge control subsystem of the modified backstepping controller, the effects of variable mass and drag are mitigated.