Automatic Station Keeping of Small Twin Screw Boats

This work details the development of tools and controllers for station keeping
control of twin screw vessels. A fundamental analysis is conducted of the dynamics of
twin screw displacement hull vessels and their actuator systems, where the response
characteristics and maneuverability are quantified through a series of full scale trials
conducted in different environmental conditions while recording the environmental
conditions, actuator states, and geodetic and inertial measurements. The data from these
maneuvers were repeatable from run to run and thus provide valuable benchmarks for
several maneuvers and the measured actuator response provides valuable set points of
performance characteristics/limitations for control development. A comprehensive
general simulation of small twin screw displacement hull boats is developed as a tool to
estimate ship and actuator responses in support of developing and tuning of control
systems. The model and computer simulation is capable of modeling a wide range of the
surface vessels, including their actuators and environmental conditions. This model
proved to be accurate, when compared to the sea trial data, and model estimates have rms velocity errors for the various steady maneuvers of 1.2-4.6% for surge, 12.6-17.9% for
sway, and 7.6-20.2% for yaw.
A path following station keeping controller is developed that uses Lyapunov
stability analysis to determine the path the vessel should follow to effectively eliminate
position error. This controller showed good performance for several different
environmental conditions. Encouraged by these finding, three additional station keeping
control methodologies are developed for twin screw surface ships. All four of these
controllers are examined for their robustness to environmental conditions, as well as their
sensitivity to sensor precision, sensor update rates, and actuator limitations. All
controllers are evaluated in sea state 4 yielding rms position errors from 3.3 to 16.2 m,
the rms surge and sway accelerations are under 0.62 m/s , and the engine shifting
frequencies are between 0.011 and 0.145 Hz. These four controllers are then tested over a
wide range of environmental conditions, sensor precisions and update rates, and actuator
response rates. The results from these tests give quantitative data that will aid in selecting
the appropriate controller for a specific application, and will assist in selecting
appropriate sensors.