This paper combines the nonlinear Udwadia-Kalaba control approach with the Assumed Mode Method to model flexible structures and derives an attitude controller for a spacecraft. The study case of this paper is a satellite with four flexible cantilever beams attached to a rigid central hub. Two main topics are covered in this paper. The first one is the formulation of the equation of motion and the second one is the nonlinear controller design. The combination of these two techniques is able to provide a controller that damps the vibration of a flexible structure while achieving the desired rigid-motion state.

No abstract

This paper presents a control allocation solution for the technology demonstrator missionReFEx, which focuses on a vertical takeoff and horizontal landing strategy with autonomous navigation, online guidance, and controlled flight throughout the mission. The trajectory for the demonstration flight is aimed as one for a winged launch vehicle first stage: maintaining stability and control of the vehicle while reaching a predefined target. During the atmospheric phase the vehicle is stabilized by using an active aerodynamic control system which transforms inputs from the guidance and navigation systems into control commands for the individual actuators. In that sense, the control allocation subsystem translates commanded moments into commanded aerodynamic surface deflections. Due to the effect of modeling uncertainties, navigation errors, and underactuated regions, this subsystem needs to be robustified. The algorithm proposed in this paper addresses this challenge via a combination of the deflections required to trim the vehicle together with delta-deflections that aim at converging iteratively to the commanded moments. The combination of these two contributions is able to respond fast to state changes, compensate for modeling uncertainties and navigation errors, and provide a safe mode for the underactuated regions. The performance of the system is studied using a high-fidelity simulator. Nomenclature𝛼, 𝛽, 𝜇 = angle of attack, angle of sideslip, bank angle 𝜔 = angular rate of the body frame w.r.t the inertial frame q = dynamic pressure 𝐼 = moment of inertia 𝐶 𝑙 , 𝐶 𝑚 , 𝐶 𝑛 = aerodynamic coefficients for roll, pitch and yaw moments 𝐶 = aerodynamic coefficients vector, [𝐶 𝑙 , 𝐶 𝑚 , 𝐶 𝑛 ] 𝑇 Ma = Mach 𝜂 𝑆 , 𝜂 𝐴 = symmetric and asymmetric deflection of the canards 𝜁 = rudder deflection 𝛿 = deflection vector, [𝜂 𝑆 , 𝜂 𝐴 , 𝜁] 𝑇 𝑓 = generic function