Station Keeping of Small Outboard-Powered Boats

Three station keeping controllers have been developed which work to minimize displacement of a small outboard-powered vessel from a desired location. Each of these three controllers has a common initial layer that uses fixed-gain feedback control to calculate the desired heading of the vessel. A second control layer uses a common fixed-gain feedback controller to calculate the net forward thrust, one of two algorithms for controlling engine angle (Fixed-Gain Proportional-integral-derivative (PID) or PID with Adaptively Augmented Gains), and one of two algorithms for differential throttle control (Fixed-Gain PID and PID with Adaptive Differential Throttle gains), which work together to eliminate heading error. The three selected controllers are evaluated using a numerical simulation of a 33-foot center console vessel with twin outboards that is subject to wave, wind, and current disturbances. Each controller is tested for its ability to maintain position in the presence of three sets of environmental disturbances. These algorithms were tested with current velocity of 1.5 m/s, significant wave height of 0.5 m, and wind speeds of 2, 5, and 10 m/s. These values were chosen to model conditions a small vessel may experience in the Gulf Stream off of Fort Lauderdale. The Fixed-gain PID controller progressively got worse as wind speeds increased, while the controllers using adaptive methodologies showed consistent performance over all weather conditions and decreased heading error by as much as 20%. Thus, enhanced robustness to environmental changes has been gained by using an adaptive algorithm

Precision stationkeeping with azimuthing thrusters

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references.Precision positioning of an unmanned surface vehicle (USV) in a nautical environment is a difficult task. With a dual azimuthing thruster scheme, the optimization of thruster outputs uses an online method to minimize the amount of error. It simplifies necessary calculations by the assumption that the rotating thrusters are always parallel thus making the system holonomic. The scheme accommodates for limitations in actuator outputs, including rotation limits and time-lagged thrusts and was implemented in a MATLAB simulation that tested its response to step errors and disturbance forces, similar to what it would encounter in actual implementation. It successfully achieved commanded outputs in all three degrees of freedom, typically within 25 seconds. It also rejects constant and sinusoidal disturbance forces. However, specific configurations arise where the USV, at times, is uncontrollable and the system only recovers after being further perturbed into a controllable configuration.by Adam D. Doroski.S.B

Global Estimation Methodology for Wave Adaptation Modular Vessel Dynamics Using a Genetic Algorithm

Determining parameters for a system model for marine vessels becomes more difficult as the model is made more complex. Work has been done to determine the equations of motion, but not to fully define how to estimate all of the system parameters. This work utilizes a global optimization methodology for estimating the system parameters using a genetic algorithm. The optimizer uses training data sets created from a set of ship maneuvering standards to minimize the error in the 3 degree-of-freedom equations of motion. The model has been optimized using a “No Surge-Yaw” model (minimal surge coupling) and a “Full” model (all states have coupling effects to each other) to determine how well each model can be estimated. The “No Surge-Yaw” model had the best results with making a working marine vessel model. The “Full” model was difficult to optimize due to the additional parameters that had unknown, nonlinear constraints. The “No Surge-Yaw” model was compared to linearized, no coupling version of the model that is commonly used. The linearized model vastly overestimated the results in sway and yaw rate motion while the “No Surge-Yaw” captured the expected coupling dynamics that do exist. Overall, the results of this methodology did generate a set of working marine vessel parameters for an unknown, coupled-state dynamic model

A Novel and Inexpensive Solution to Build Autonomous Surface Vehicles Capable of Negotiating Highly Disturbed Environments

This dissertation has four main contributions. The first contribution is the design and build of a fleet of long-range, medium-duration deployable autonomous surface vehicles (ASV). The second is the development, implementation, and testing of inex-pensive sensors to accurately measure wind, current, and depth environmental vari- ables. The third leverages the first two contributions, and is modeling the effects of environmental variables on an ASV, finally leading to the development of a dynamic controller enabling deployment in more uncertain conditions. The motivation for designing and building a new ASV comes from the lack of availability of a flexible and modular platform capable of long-range deployment in current state of the art. We present a design of an autonomous surface vehicle (ASV) with the power to cover large areas, the payload capacity to carry sufficient batteries to power components and sensor equipment, and enough fuel to remain on task for extended periods. An analysis of the design, lessons learned during build and deployments, as well as a comprehensive build tutorial is provided in this thesis. The contributions from developing an inexpensive environmental sensor suite are multi-faceted. The ability to monitor, collect, and build models of depth, wind, and current in environmental applications proves to be valuable and challenging, where we illustrate our capability to provide an efficient, accurate, and inexpensive data collection platform for the community’s use. More selfishly, in order to enable our end- state goal of deploying our ASV in adverse environments, we realize the requirement to measure the same environmental characteristics in real-time and provide them as inputs to our effects model and dynamic controller. We present our methods for calibrating the sensors and the experimental results of measurement maps and prediction maps from a total of 70 field trials. Finally, we seek to inculcate our measured environmental variables along with previously available odometry information to increase the viability of the ASV to maneuver in highly dynamic wind and current environments. We present experimen- tal results in differing conditions, augmenting the trajectory tracking performance of the original way-point navigation controller with our external forces feed-forward algorithm

An Experimental Study of Station Keeping on an Underactuated ASV

Dynamic positioning is an important application for marine vehicles that do not have the luxury of anchoring or mooring themselves. Such vehicles are usually large and have arrays of thrusters that allow for controllability in the sway as well as the surge and yaw axes. Most smaller boats however, are underactuated and do not possess control in the sway direction. This makes the control problem significantly more challenging. We address the station keeping problem for a small autonomous surface vehicle (ASV) with significant windage. The vehicle is required to hold station at a given position. We describe the design of a weighted controller that uses wind feed-forward to complement a Line-Of-Sight guidance controller to achieve satisfactory performance under slow-varying moderate wind conditions. We test the control system in simulation and in field trials with a twin-propeller ASV. Experiments show that the controller works very well in moderate wind conditions allowing the ASV to keep station with a position error of approximately one vehicle length