Linear dynamic modeling of spacecraft with various flexible appendages and on-board angular momentums

We present here a method and some tools developed to build linear models of multi-body systems for space applications (typically satellites). The multi-body system is composed of a main body (hub) fitted with rigid and flexible appendages (solar panels, antennas, propellant tanks,...) and on-board angular momentums (flywheels, control moment gyros). Each appendage can be connected to the hub by a cantilever joint or a pivot joint. More generally, our method can be applied to any open mechanical chain. In our approach, the rigid six degrees of freedom (d.o.f) (three translational and three rotational) are treated all together. That is very convenient to build linear models of complex multi-body systems. Then, the dynamics model used to design AOCS, i.e. the model between forces and torques (applied on the hub) and angular and linear position and velocity of the hub, can be derived very easily. This model can be interpreted using block diagram representation

Codesign mechanics / attitude control for a simplified AOCS preliminary synthesis

This paper aims to present the advantages of a multibody modeling approach, adapted to all kinds of satellites. This approach gives not only a linearized satellite model at a nominal parametric configuration but also a linearized parameterized model available on a parametric range. Resulting dynamic models are representative of the couplings between different axes and the impact of flexibility. The two contributions of the paper concern the ability of these dynamic models to simulation purposes and the parameterization of the overall satellite model in terms of the inclination of a flexible appendage, useful for a parametric sensitivity study

L’identification bayesienne en vue de la retouche de correcteur

In this paper, a new controller adjustment procedure is proposed in order to ensure a better control of closed loop characteristics when the controller order is lower than the plant order. The basic idea is the close relation between identification quality and command insensitivity. The advantage of this method is that the full order model of the system is directly considered in association with its reduced controller. In this way, it is useless to compute a reduced model before adjustment : the result of the adjustment can be immediatly applied to the system. The method consists in defining closed loop noises in order to identify fictitiously modal characteristics of the controlled plant via a bayesian identification algorithm

Space Debris Removal using a Tether: A Model

Focused on the removal of space debris, this paper studies the modeling of a target satellite connected to a chaser satellite by a tether. All dynamic couplings between the flexible and rigid modes of the satellites are accounted for. The tether attached to both satellites is modeled as a massless spring when stretched and non-existent when compressed. The objective of this work is to model the behaviour of the three bodies: chaser, tether, and target in the orbital reference frame for arbitrary initial conditions of the target satellite. Simulations of the whole system including the chaser Attitude and Orbit Control System (AOCS) are performed to evaluate its ability to damp the debris tumbling motion and to tow the debris

Avionics/Control co-design for large flexible space structures

In this paper, a multi-model H1 synthesis scheme for fixed-structure controller design is developed and applied to the attitude control of a highly flexible earth-observation satellite. The particularity of the proposed approach is that the decision variables optimized by the fixed-structure Hinfinity solver include the structured controller parameters but also some parameters which characterize the avionics. Furthermore the proposed control scheme can be very easily adapted to a new configuration of sensors and thus can handle gyro or gyroless configurations. This way, various avionics configurations can be easily evaluated. The avionics characteristics for a given configuration and the control law can be simultaneously optimized avoiding time-consuming iterations between the definition of avionics and the design of the controller on the basis of the current avionics. The approach is applied on a earth observation satellite for two different study cases. The first one aims to design an improved controller in order to meet the nominal requirements with a poor avionics. The second ones aims to find a controller and an improved avionics to meet very challenging requirements

A Flexible Appendage Model for Use in Integrated Control/Structure Spacecraft Design

The study presents and validates a flexible appendage model to be used in an integrated control/structure spacecraft design. The integrated design methodology needs an accurate LFT representation of spacecraft flexible appendages so that parametric variations can be included. This requirement can be met using the Interconnected Flexible Appendage model studied in Perez et al. (2015). The model suitability is validated through the modeling of a real deployable boom, obtaining the same frequency modes and dynamical behavior

Forme standard de passage: une alternative à la synthèse multi-objectifs

This chapter presents a generalization of the Cross Standard Form (CSF) for a given plant and a given controller of arbitrary order (even, a two-degree of freedom compensator). The CSF is a canonical augmented standard plant whose H∞ or H2 unique optimal controller is a given controller, that is a solution to the general inverse optimal control problem. From the control design point of view, the general idea is to apply the CSF to a first controller in order to initialize an H∞ design procedure to handle complementary frequency-domain constraints or robustness specifications. An academic example and an industrial application in the field of aeronautics engineering are given

Two-input two-output port model for mechanical systems

This paper proposes a double input output port transfer to model complex mechanical systems composed of several sub-systems. The sub-structure decomposition is revisited from the control designer point of view. The objective is to develop modelling tools to be used for mechanical/control co-design of large space flexible structures involving various substructures (boom, links of robotic arm,...) connected one to each other through dynamics local (actuated) mechanisms inducing complex boundary conditions. The double input output port model of each substructure is a transfer where accelerations and external forces at the connection points are both on the model inputs and outputs. Such a model : * allows to the boundary conditions linked to interactions with the other substructures to be externalized outside the model, * is defined by the only substructure own dynamic parameters, * allows to build the dynamic model of the whole structure by just assembling the double port models of each substructure. The principle is first introduced on a single axis spring-mass system and then extented to the 6 degress-of-freedom case. This generalization uses the clamped-free substructure dynamic parameters such as finite element softwares can provide

Dynamic modeling and analysis of spacecraft with variable tilt of flexible appendages

This article describes a general framework to generate linearized models of satellites with large flexible appendages. The obtained model is parametrized according to the tilt of flexible appendages and can be used to validate an attitude control system over a complete revolution of the appendage. Uncertainties on the characteristic parameters of each substructure can be easily considered by the proposed generic and systematic multibody modeling technique, leading to a minimal LFT model. The uncertainty block has a direct link with the physical parameters avoiding non-physical parametric configurations. This approach is illustrated to analyze the attitude control system of a spacecraft fitted with a tiltable flexible solar panel. A very simple root locus allows the stability of the closed-loop system to be characterized for a complete revolution of the solar panel

Tuning of observer-based controllers

A particular postsynthesis problem is examined. Assume a first complex controller running on a given system. It turns out that a closed-loop mode is not sufficiently damped. Is it easy to know which adjustable combinations of controller parameters are the most relevant to master this problem? A Bayesian identification procedure is proposed to analyze the relevance of such a given parameter combination with respect to a given modal specification. One application, for which such a controller tuning is very interesting, is the adjustment of flight control laws during flight tests. In this practical context, the observer-based realization of the controller is used to derive an architecture suitable for its implementation, that is, an architecture in which physical tuning parameters can be easily highlighted. The results presented concern the lateral flight control law of a highly flexible aircraft that has been synthesized by a modern robust control approach