Planetary landing: modelling and control of the propulsion descent

In the propulsive phase, after parachute release, of planetary landing like Mars or Moon, horizontal motion is obtained by tilting the axial thrust, so that it aligns either to the negative velocity vector (gravity turn) or to the requested acceleration vector. The latter strategy is assumed here, as it allows pinpoint landing. As such, tilt angles (pitch and yaw) become proportional to the horizontal acceleration. Instead of designing a hierarchical guidance and control in which horizontal acceleration becomes the attitude control target, a unique control system can be designed based on the fourth order dynamics from angular acceleration to position. The paper shows that the combined dynamics can be (quasi) input-state linearized except the nonlinear factor of the tilt angles (the axial thrust imposed by vertical braking). The paper shows that control design around the reference trajectory (tilt and position) given by the guidance can exploit the quasi linearization, but tracking error stability must be proved in presence of a not stabilizable external disturbance. The paper is restricted to closed-loop control strategies, and their effectiveness is proved through Monte Carlo simulations

Embedded Model Control For Mars Terminal Descent Phase

The objective of this thesis is to provide an exhaustive (as much as possible) description of Guidance Navigation and Control systems (GNC) for the propulsive terminal phase of mars landing scientific missions, and to define a unified GNC architecture capable of solving soft landing, and pin-point landing problems. Accordingly, a model-based control system design methodology will be employed throughout. Such a design technique, called Embedded Model Control (EMC), is hinged on the construction of a finite dimensional Discrete-Time (DT) State Equation (SE), called Embedded Model (EM), describing the landing vehicle motion dynamics. The EM is embedded in the Digital Control Unit (DCU), acting as the core of the GNC algorithms. In this chapter , the current scenario of mars exploration technologies will be briefly introduced. Then a brief review of key missions will be done, particularly attention will be take into landing missions and its control system requirements for the terminal decent phase, giving to the reader a fast overview of the past and actual control requirements for mars landing exploration missions

Guidance and control for the propulsion phase of planetary landing

: In the propulsion phase of planetary landing, horizontal motion is obtained by tilting and aligning the axial thrust either to the opposite of the velocity vector or to the requested acceleration vector. The second strategy is assumed here, as it allows free horizontal motion and is preliminary to achieve accurate landing. Instead of designing a hierarchical guidance and control in which horizontal acceleration becomes the attitude reference, a unique control system is designed based on a fourth-order state equation per degree-of-freedom from the angular acceleration to the position coordinate. Following the Embedded Model Control methodology, a unique discrete-time state equation (the embedded model) is derived and employed by guidance, navigation and control. Here only guidance and control are outlined. The whole guidance, navigation and control algorithm has been tested on a high-fidelity descent simulator. The results of Monte Carlo runs for assessing performance versus requirements that are typical of accurate landing are presented and discussed

Orbit and attitude control for gravimetry drag-free satellites

The paper outlines orbit and attitude control problems of a long-distance (>100 km) two-satellite formation for the Earth gravity monitoring. Modelling and control design follows the Embedded Model Control and shows how disturbance dynamics and rejection are mandatory to solve such problems. Orbit and attitude control can be treated separately except in the thrust dispatching law (not treated here) because of an all-propulsion actuation. Orbit and attitude control split into three sub-problems to be designed in a hierarchical way. In both cases the inner loop is a wide-band drag-free control aiming to zero the linear non gravitational accelerations in the orbit control and the total angular acceleration in the attitude case. Drag-free demands for disturbance measurement and rejection by means of a specific disturbance dynamics and observer. The orbit outer loops are altitude and distance control to meet formation requirements. The attitude outer loops are in charge of rejecting the residual drag-free bias and drift, which demands a narrow band control suitable for star tracker measurements, and of aligning the optical axes of each satellite, which demands accurate sensor and wide bandwidth. Simulated and experimental results are provided

PROPULSIVE GUIDANCE AND CONTROL FOR PLANETARY LANDING

The paper describes a reference path-tracking algorithm for the compensation of the atmospheric and aerodynamic dispersion during the atmospheric entry of a low lift-to-drag interplanetary vehicle. The paper focuses on the longitudinal control. Lateral control is briefly mentioned. The algorithm follows the Embedded Model Control methodology and is based on the realtime estimation and cancellation of the causes that deviate the vehicle path from the reference trajectory. The real-time control modulates the vertical component of the lift in order to drive the vehicle fourth-order longitudinal dynamics. To simplify the control structure, longitudinal dynamics is decomposed in a series of two second-order dynamics. The upstairs dynamics (flight path angle and altitude) is commanded by the lift vertical component, the downstairs dynamics (velocity and downrange) is driven by altitude modulation. Arranging the control algorithm in a hierarchical manner becomes straightforward. Control algorithms have been tested by Monte Carlo simulations on a high fidelity six degrees-of-freedom simulator showing that the control approach provides acceptable residual dispersion at the parachute deployment point

LONGITUDINAL PATH-TRACKING DURING ATMOSPHERIC PLANETARY ENTRY

The paper describes a reference path-tracking algorithm for the compensation of the atmospheric and aerodynamic dispersion during the atmospheric entry of a low lift-to-drag interplanetary vehicle. The paper focuses on the longitudinal control. Lateral control is briefly mentioned. The algorithm follows the Embedded Model Control methodology and is based on the realtime estimation and cancellation of the causes that deviate the vehicle path from the reference trajectory. The real-time control modulates the vertical component of the lift in order to drive the vehicle fourth-order longitudinal dynamics. To simplify the control structure, longitudinal dynamics is decomposed in a series of two second-order dynamics. The upstairs dynamics (flight path angle and altitude) is commanded by the lift vertical component, the downstairs dynamics (velocity and downrange) is driven by altitude modulation. Arranging the control algorithm in a hierarchical manner becomes straightforward. Control algorithms have been tested by Monte Carlo simulations on a high fidelity six degrees-of-freedom simulator showing that the control approach provides acceptable residual dispersion at the parachute deployment poin

(1-Adamantyl)(3-aminophenyl)methanone

In the crystal sructure of the title compound, C(17)H(21)NO, the molecular packing is stabilized by intermolecular N-H center dot center dot center dot O hydrogen bonds and additional weak N-H center dot center dot center dot pi interactions, forming chains that propagate along the b axis. Conjugation of the carbonyl group and the benzene ring is rather attenuated due to a twisting of the carbonyl group from the plane of the benzene ring [torsion angle = 27.1 (2)degrees]

Design and verification of the European ExoMars EDL demonstrator

Paper AIAA 2011-634

Embedded Model Control urges disturbance modelling and rejection

Editors: Ji-Feng Zhang, Qian-Chuan Zha