Aerospace vehicles often encounter challenging mission scenarios. These represent critical phases that determine the satisfactory achievement of the mission objectives, and, sometimes, the outcome of the entire mission. This research project is intended to develop and apply an effective methodology for real-time guidance and control of aerospace vehicles, based on nonlinear control theory and techniques. Potential challenging scenarios include (a) precise orbit control for low-altitude space vehicles dedicated to monitoring or surface mapping, (b) docking maneuvers between two cooperating or non-cooperating spacecraft, and (c) safe planetary landing of unmanned vehicles. High-fidelity dynamical modeling represents a crucial issue for investigating the critical phases of aerospace missions. In this research project, the environment and vehicle parameters are being identified, together with the related uncertainties. Constraints and quantization of control actuators introduce further complexity and are being included in the dynamical modeling through original geometric and analytic approaches. Moreover, this research aims at selecting the representations and governing equations (for trajectory and attitude) that are most advantageous in relation to the specific application. In fact, a suitable choice of variables offers a deep insight on the natural dynamical evolution of the aerospace system of interest and can facilitate considerably the design of the nonlinear control algorithm. With this regard, sliding mode control appears as a very promising option among the nonlinear techniques proposed in recent years. The performance of the nonlinear guidance and control algorithms being developed in this research will be tested using high-fidelity modeling of both the environment and the aerospace vehicle, with the final objective of demonstrating their accuracy and robustness, even in the presence of large deviations from nominal flight conditions.
High-fidelity dynamical modeling represents a crucial issue for investigating critical phases of aerospace missions. In this research project, the environment and vehicle parameters are being identified, together with the related uncertainties, which are described stochastically. Constraints and quantization of control actuators, which are inevitable features of real systems, introduce further complexity, and are included in the dynamical modeling. Moreover, several options exist for the governing equations of translational and attitude motion of aerospace vehicles. In fact, the motion of their mass center can be described using spherical coordinates, orbit elements, or equinoctial elements. For attitude dynamics either the Eulerian or the Lagrangian or the Hamiltonian description can be employed, as well as alternative mixed or hybrid approaches [1,2]. With respect to previous work, this research aims at selecting the representations and governing equations (for trajectory and attitude) that are most advantageous in relation to the specific application. In fact, a suitable choice of coordinate system offers a deep insight on the natural dynamical evolution of the aerospace system of interest and can facilitate considerably the design of the nonlinear control algorithm.
Three aerospace mission scenarios are being considered.
1. Orbit control in perturbed dynamical environments, such as low-altitude orbits. Placing a spacecraft in low Earth orbits allows dedicated monitoring tasks over specific target regions, interested by natural disasters or anthropic activities of various types. Moreover, low-altitude satellites that orbit different celestial bodies are certainly useful for providing a high-resolution mapping of their surface. However, orbit perturbations are proven to lead the spacecraft to impact in a few weeks if the orbit altitude is not sufficiently high [3]. Low-thrust propulsion can increase the spacecraft lifetime, and this research aims at proving that effective throttle level and pointing direction can both be determined using nonlinear control algorithms.
2. Docking maneuvers of spacecraft. Orbit rendezvous and docking includes several phases, starting from selection of proper initial conditions. The last, most challenging phase is the docking maneuver, which ends with the safe connection of the two space vehicles. This maneuver involves both translational and attitude motion. A further challenge may be represented by a non-cooperating spacecraft (e.g., a decommissioned satellite or orbit debris, which could be even captured and removed). Impulsive and low thrust propulsion are being considered and compared in similar mission scenarios, with the intent of proving that nonlinear control techniques can provide both the throttle level and the thrust pointing direction needed to perform a successful docking.
3. Safe planetary landing. Since the dawn of spaceflight, safe landing represented a crucial phase, for both robotic and human missions. In the present research project, the focus is on three-dimensional soft touchdown for unmanned vehicles, also in the presence of wind shear, unlike what appeared in recent work [4]. This challenging dynamical scenario involves again both translational and attitude motion, and is, by definition, a single-attempt maneuver. This circumstance implies that the nonlinear control system must be designed with enhanced properties of effectiveness and robustness.
For each application, Monte Carlo campaigns are being performed, in the presence of stochastic deviations from nominal flight conditions. Actuation constraints and quantization are addressed as well, by investigating original analytical and geometric approaches. The outcomes coming from applying the nonlinear control algorithms are being evaluated with reference to two main properties: (a) accuracy, i.e. precise satisfaction of the boundary conditions, and (b) robustness, related to the system performance in very unfavorable conditions.
In summary, originality of the proposed research project resides in (a) high-fidelity modeling of the environment and the aerospace vehicle, including selection of proper analytical descriptions of translational and rotational motion, and (b) development and application of suitable nonlinear control techniques for real-time guidance and control of aerospace vehicles in some specific challenging scenarios, in the presence of nonnominal flight conditions.
*References
[1] Schaub H., Junkins J. L., Analytical Mechanics of Space Systems, AIAA Education Series, 2009, pp. 289-432
[2] Meirovitch L., Methods of Analytical Dynamics, McGraw-Hill, 1970, pp. 122-169
[3] Pontani M., Di Roberto R., Graziani F., "Lunar Orbit Dynamics and Maneuvers for Lunisat Missions," Acta Astronautica, Vol. 149, 2018, pp. 111-122
[4] Harl N., Balakrishnan S. N., "Reentry Terminal Guidance Through Sliding Mode Control," Journal of Guidance, Control, and Dynamics, Vol. 33, No. 1, 2010, pp. 186-199