Control Marine Vehicles with Azimuth Thrusters using Convex Constrained Quadratic Programming

Abstract

Azimuth thrusters are widely used for controlling marine vehicles, especially, for dynamic positioning and hovering purposes. However, including azimuth thruster makes control allocation a nonlinear non-convex problem which is commonly solved using nonlinear programming methods, simplified by paring azimuth thrusters (e.g., two thrusters will always move at the same angle), or locally linearized using approximation equations such as Taylor series expansions and polynomial functions. In this paper, a new approach is presented to modify the azimuth thruster control allocation problem into a convex quadratic problem with a new force decomposition and linear first-order inequality constraints. As a result, the complexity of the control allocation increases linearly with respect to the number of azimuth thrusters, allowing it to be implemented on the marine vehicles with increased numbers of azimuth thrusters controlled independently and can be solved using constrained quadratic programming solvers. Case studies has been presented to validate the proposed method on simulated Autonomous Underwater Vehicles (AUVs) with two and four azimuth thrusters configured with different azimuth angle limits (±45, ±90, and ±135 degrees). The results shows excellent control performance of the proposed approach in controlling multiple states (surge, pitch, yaw, depth and sway) simultaneously, even when experiencing a cross-track ocean current. Recommendation on hardware implementation is also discussed for real world platform integration.