Degraded Planary Tracking Control of an Omnidirectional Vectored-Thruster Aerostat

Abstract

The problem of horizontal-plane tracking control of an omnidirectional, four vectored-thruster aerostat subjected to actuator failure is considered. The actuator failures result in the aerostat’s becoming underactuated, so that it can affect only surge force and pure yaw moment about the body center. To achieve accurate position control in the horizontal plane, direct position control is used instead of heading control. This mode of control 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, in which 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 thus ensures that the aerostat is in a stable tracking configuration. In addition, the VRP decreases the transmission ratio of commanded yaw rate to commanded lateral velocity, making the aerostat’s yaw motion vary slowly during the transitional phase so that steady position tracking is obtained.