Mirko Leomanni

Explicit Model Predictive Control Approach for Low-Thrust Spacecraft Proximity Operations

The key role of autonomous systems in future space missions has made model predictive control a very attractive guidance and control technique. However, the capability of low-power spacecraft processors to handle the real-time computational load of this technique still needs to be fully established, especially for complex control problems. This paper introduces a method to improve the computational efficiency of model predictive control when applied to the problem of autonomous rendezvous and proximity maneuvering using low-thrust propulsion. To ensure safe trajectories in this scenario, a long control horizon is required and the control problem must be solved at a relatively fast sampling rate. The proposed design addresses such requirements by parameterizing the thrust profile with a set of Laguerre functions. In this setting, the number of control variables can be made significantly smaller than the length of the control horizon, as opposed to standard design methods. By exploiting this property, in co...

Autonomous Low-Earth-Orbit Station-Keeping with Electric Propulsion

is based on an extended Kalman filter, employing gyro, star-tracker, and Global Positioning System measurements. Lyapunov-based and proportional–derivative feedback laws are used for orbit and attitude control, respectively. The performance of the proposed electric propulsion system and guidance, navigation, and control solution is evaluated on a low-Earth-orbit mission.

All-Electric Spacecraft Precision Pointing Using Model Predictive Control

Recent technological advances in the development of electric microthrusters pave the way to the successful diffusion of all-electric spacecraft. Since pointing accuracy is a key requirement for space platforms, suitable control systems accounting for the peculiarity of electric on/off actuators have to be devised. In this paper, an attitude control system (ACS) for spacecraft equipped with cold gas and electrothermal xenon microthrusters is presented. The number of thruster firings, which has a key impact on the thruster lifetime, is explicitly taken into account in the control design phase. By adopting a model predictive control (MPC) approach, a cost functional including both fuel consumption and number of firing cycles is minimized at each time step, within a receding horizon scheme. The effectiveness of the proposed ACS is validated on a sample GEO mission and its performance is compared with different control laws involving on/off actuators.

An MPC-based attitude control system for all-electric spacecraft with on/off actuators

Pointing accuracy is a key requirement in communication satellites and Earth observation missions. Attitude control systems must guarantee tracking of the reference attitude and angular rate, while accounting for mission performance indexes such as fuel consumption and actuator wear. In this paper, an MPC-based attitude control scheme is proposed for an all-electric spacecraft using cold gas and resistojet thrusters as on/off actuators for attitude control. This technology imposes restrictions on the number of thruster firings, which are explicitly taken into account in the MPC formulation and suitably traded-off with fuel consumption. The performance of the proposed attitude control system is demonstrated on a GEO mission and compared with other control schemes involving on/off actuators.

Minimum switching control for systems of coupled double integrators

This paper studies the minimum switching control problem for a system of coupled double integrators with on-off input signals, in the presence of a constant disturbance term. This type of problem is relevant to a variety of applications in which the number of transitions of on-off actuators must be minimized, in order to prevent actuator wear. Two solutions are presented in terms of steady state limit cycles. The first one provides an analytic upper bound to the maximum number of transitions per input signal. The second solution exploits the relative phases of the trajectories of the state variables, thus providing a less conservative upper bound. Additionally, a control law is presented, which steers the system in finite time to the previously derived limit cycles. The proposed techniques are demonstrated on a spacecraft attitude control application.

Minimum switching limit cycle oscillations for systems of coupled double integrators

In this paper, we study the limit cycle oscillations of multiple double integrators with coupled dynamics, subject to a constant disturbance term and switching inputs. Such systems arise in a variety of control problems where the minimization of both fuel and number of input transitions is a key requirement. The problem of finding the minimum switching limit cycle, among all the fuel-optimal solutions satisfying given state constraints, is addressed. Starting from well known results available for a single double integrator, two suboptimal solutions are provided for the multivariable case. First, an analytic upper bound on the number of input switchings is derived. Then, a less conservative numerical solution exploiting the additional degrees of freedom provided by the phases of the limit cycles is presented. The proposed techniques are compared on two simulation examples.

SPACECRAFT LOCALIZATION VIA ANGLE MEASUREMENTS FOR AUTONOMOUS NAVIGATION IN DEEP SPACE MISSIONS

Abstract This paper addresses the problem of spacecraft localization based on angular measurements, for deep space missions. The dynamic model of the spacecraft accounts for several perturbing effects, such as Earth and Moon gravitational field asymmetry and errors associated with the Moon ephemerides. The measurement process is based on elevation and azimuth of Moon and Earth with respect to the spacecraft reference system. Distance measurements are not employed. Position and velocity of the spacecraft are estimated by using both the Extended Kalman Filter (EKF) and the Unscented Kalman Filter (UKF). The performance of the filters are evaluated on an example of Earth-to-Moon transfer mission.

Nonlinear orbit control with longitude tracking

The growing level of autonomy of unmanned space missions has attracted a significant amount of research in the aerospace field towards feedback orbit control. Existing Lyapunov-based controllers can be used to to transfer a spacecraft between two elliptic orbits of given size and orientation, but do not consider the stabilization of the spacecraft phase angle along the orbit, which is a key requirement for application to formation flying missions. This paper presents a control law based on the orbital element parametrization, which is able to track a given true longitude (i.e. a reference phase angle), in addition to the parameters describing the reference orbit shape and orientation. A numerical simulation of an orbital rendezvous demonstrates the effectiveness of the proposed approach.

Minimum switching control for spacecraft precision pointing with on/off actuators

Maintaining the spacecraft attitude precisely aligned to a given orientation, while rejecting a persistent disturbance, using on/off actuators, is crucial for missions involving electric propulsion spacecraft. The objective is to enforce an oscillating attitude motion about the setpoint, so as to simultaneously minimize both the propellant consumption and the switching frequency of the control system. This paper evaluates the feasibility of a recently proposed feedback control law for this problem. This techniques is able to track both the period and the phase of periodic oscillations along the rotational axes, which is instrumental to minimize the switching frequency in the presence of input coupling. Two simulation case studies of a low Earth orbit missions are considered, showing that the proposed approach can effectively deal with both constant and time-varying disturbance torques.

Minimum Switching Thruster Control for Spacecraft Precision Pointing

Maintaining the attitude of a spacecraft precisely aligned to a given orientation is crucial for commercial and scientific space missions. The problem becomes challenging when on/off thrusters are employed instead of momentum exchange devices due to, e.g., wheel failures or power limitations. In this case, the attitude control system must enforce an oscillating motion about the setpoint, so as to minimize the switching frequency of the actuators, while guaranteeing a predefined pointing accuracy and rejecting the external disturbances. This paper develops a three-axis attitude control scheme for this problem, accounting for the limitations imposed by the thruster technology. The proposed technique is able to track both the period and the phase of periodic oscillations along the rotational axes, which is instrumental to minimize the switching frequency in the presence of input coupling. Two simulation case studies of a geostationary mission and a low Earth orbit mission are reported, showing that the proposed controller can effectively deal with both constant and time-varying disturbance torques.