Robust motion control strategies for hydrofoil-equipped naval vessels considering scaling effects.

Sammanfattning: Hydrofoil-equipped naval vessels are not new. The technique appeared at the beginning of the 20th century and there are still new developments within the field. The main principle of hydrofoils is to use fluid dynamics to generate lift much like an aircraft and lift the hull of the craft out of the water, thus reducing drag while traversing the water surface. At KTH, Royal Institute of Technology a new craft called FoilCart has been built using this principle. Due to the design, the motion control resembles that of an inverted pendulum which is a classical control problem with many ways of reaching stable behavior. Previous studies have focused on pitch actuation, however the FoilCart incorporates roll actuation through hydrofoils as well which opens up new possibilities on movement. The nature of this inverted pendulum is therefore also made more complex by being in several degrees of freedom, rotating and translating around multiple axes. The idea is to assess the impact this has on overall robustness and stability by studying possible coupling effects between roll and pitch. The thesis also aims to study possible scaling effects when the physical parameters of the prototype are changed. This is done as an important assessment of the craft as an autonomous drone as the craft may be used for sea mapping purposes in which energy usage efficiency is essential. Multiple control algorithms are considered, classic PID as well as linear quadratic methods and later on used and tested through simulation in Simscape Multibody as means of verification. Further strategies on motion control are developed to increase stability and robustness by extending the amount of controllable variables. Results show that the coupling effects are minimal when the system is disturbed and suggests that the rather simplified control design is feasible, this however can only be said when moving about the equilibrium point as a linearization is done. Concerning scaling effects the craft needs a faster response of control that puts higher requirements on hardware when scaled down. For example with relatively slow servos to actuate the system, it exhibits oscillatory behavior. Also measurement noise is amplified to a higher extend which may cause unwanted behavior.