Gravity Gradient Disturbance Research Articles

Fault-tolerant sliding mode satellite attitude stabilization using magneto-Coulombic torquers

This paper presents a sliding mode attitude control scheme for satellites actuated by magneto-Coulombic torquers. A terminal sliding mode control with modified super twisting algorithm is designed considering the averaged dynamics of the magneto-Coulombic system to ensure fast convergence with negligible chattering in the presence of process noise and gravity gradient disturbances. It is shown that the proposed actual time varying super twisting control gets reduced to the averaged control. The same control scheme is thereafter extended to the situation under the failure of one of the three Coulomb shell pairs and it is shown that even if the actuator fails along one of the axes, the system can still be stabilized and forced to reach the origin in finite time. Global stability of the averaged attitude control system with finite time convergence is mathematically proved under the proposed control law. Simulations are carried out to validate the theoretical results under process noise and destabilizing gravity gradient. The proposed modified super twisting controller is shown to outperform a finite time sliding mode controller.

Concurrent attitude and orbit control for Deorbiter CubeSats

This paper details a concurrent attitude and orbit control method for a debris-removing nanosatellite, called deobriter CubeSat, during the rendezvous and synchronization maneuver with an uncontrollable tumbling debris object. The CubeSat is designed based on the utilization of an eight-unit form factor and commercially-available components with substantial space heritage, and is intended for the removal of sizable debris objects in low-Earth orbit. In particular, a low-thrust propulsion system is used for orbit control, as well as three reaction wheels allowing for a three-axis attitude control. Since the thruster can only produce force in one direction in the body frame, the spacecraft is considered to be underactuated. The controller employs the reaction wheels and the thruster to simultaneously rendezvous and synchronize the attitude of the CubeSat with the tumbling debris object, allowing for a concurrent attitude and position tracking. Detailed derivation of the concurrent controller is discussed, the effects of high-order derivatives are analyzed, and the stability of the system is proved. Simulation scenarios are created for two different thruster operation modes, namely, unsaturated thrust force and continuously-saturated thrust force, in order to verify the performance of the controller, as well as its robustness against gravity gradient disturbance torque and gravitational perturbation force.

Gravity Gradient Effect on a LEO Satellite with an Elliptic Orbit and Unsymmetrical Mass Properties

The study presented in this paper investigates the dynamic behavior of a low earth orbit (LEO) satellite due to gravity gradient disturbances when the product of inertia terms of the spacecraft and the orbit eccentricity are considered. It is shown that the classical stable solution for the gravity gradient disturbance becomes unstable if at least one product of inertia term exceeds a critical value. Dimensional analysis technique is used to develop the significant dimensionless groups which were used to correlate the data generated from the response of the coupled roll, pitch and yaw dynamics. Based on the findings of these groups, stability maps are developed to predict the influence of the product of inertia terms on the long-term behavior of the spacecraft attitude dynamics. The developed stability maps are verified numerically and successful prediction of a spacecraft stability condition due to gravity gradient disturbance is achieved.

Adaptive Robust Tracking for Flexible Spacecraft in Presence of Disturbances

The paper deals with trajectory tracking for a flexible spacecraft, subject to a gravity-gradient disturbance, under parameter uncertainties. The controls are gas jets and reaction wheels, and the measured variables describe the attitude and angular velocity of the rigid part. The flexible dynamics is treated as an additional disturbance acting on a rigid structure. First, an adaptive control is designed with only the gravity-gradient disturbance acting on the spacecraft; second, it is proved to be effective also in the presence of disturbance due to the flexibility, provided that appropriate robustness conditions on the controller gains are satisfied. These conditions use partial knowledge of the parameters describing the elastic dynamics. Simulations show the good performance of such control scheme and demonstrate its applicability even in the presence of input saturation.

A Re-Entry Capsule Control System Design for Microgravity Experiments

Abstract CARINA is a new retrievable unmanned capsule designed by ALENIA SPAZIO under ASI (Agenzia Spaziale Italiana) contract. This paper deals with the operative attitude acquisition phase and the operative phase of its life cycle. In the operative phase the control system must assure microgravity requirements. Two different type of actuators are used for the two different phases: 20N hydrazine thrusters for the first, magnetic coils and a reaction wheel for the second, and, consequently, two control systems techniques are used. The former is based on a new discrete time modeling of the system due to the PWM-like behaviour of the thrusters and results in a discrete controller able to guarantee robustness against parameter uncertainties. The latter is a scheduling technique based on a LOR approach and guarantees the fulfilling of microgravity requirements. All the simulations, performed on the ESA-MIDAS dynamics simulator, take into account atmospheric drag, solar pressure and gravity gradient disturbances, and show the effectiveness of the adopted approach.