Degraded Planary Tracking Control of an Omnidirectional Vectored-Thruster Aerostat

Abstract

The problem of horizontal-plane tracking control of an omni-directional, four vectored-thruster aerostat when subjected to actuator failure is considered. The actuator failures result in the aerostat becoming underactuated, so it can only effect surge force and pure yaw moment about the body centre. To achieve accurate position control in the horizontal plane, direct position control is used instead of heading control.This mode of controller is called degraded tracking control in contrast to full authority control of the overactuated four vectored-thruster aerostat. This degraded tracking controller uses commanded yaw rate to track lateral position, and yaw moment to eliminate lateral position error, therefore yaw angle is not directly controlled. To guarantee the stability of the yaw motion, a Virtual Reference Point (VRP) tracking strategy is proposed, where the VRP is used instead of the body center (BC) in position tracking. The VRP generates a negative compensated force in the surge direction which makes the side-force and yaw moment have the same sign and so ensure that the aerostat is in a stable tracking configuration. Meanwhile the VRP also decreases the transmission ratio of commanded yaw rate to commanded lateral velocity, making the aerostat's yaw motion vary slowly during transitional phase, so steady position tracking is obtained.

A transformed proportional-integral-derivative controller for a multi-vectored propeller aerostat with independent actuator magnitude and rate saturations

“…If we assume slight excursions in the roll, pitch, yaw, and heave, then the simplified horizontal plane 2-DOF dynamics used for the controller design are described by 27 (m + m 11 ) _ u = (m + m 22 )vr +…”

“…External forces and moments are induced by gravity, buoyancy, fluid inertia force, aerodynamics, and thrusters. The following dynamics equation can be constructed in the body-fixed frame through force analysis 26 , 27 :…”