Finn Ankersen

Robust Attitude Control Design for the BIOMASS Satellite (Earth Explorer Core Mission Candidate)

Abstract The purpose of this paper is to describe the current activities, results to date, and future activities of the European Space Agency (ESA) Robust AOCS technology program in support to the Phase A of BIOMASS, candidate as Earth Explorer Core Mission 7. Due to the specificity of the chosen BIOMASS configuration this activity is driven by the expected interaction between the large flexible reflector antenna structure and its attitude control system. Currently ESA has developed a technology program to enable the capabilities of integrating the structural sizing and control system design in order to avoid interactions problems. The objective is the development of an Integrated Modeling, Control and Analysis framework IMCA: it incorporates uncertainty modeling via LFTs, robustness analysis via the Structured Singular Value μ and various robust control synthesis techniques such as H ∞ and μ methods. This framework results as natural multivariable extensions of the classical Bode frequency domain techniques. It shall be integrated with a structural design loop into an unified computational framework to exploit control structures interactions in order to increase the spacecraft capabilities, such as better pointing stability, and to improve the overall design when compared to the traditional approach. In the execution of this program two parallel activities will be presented, respectively one by Astrium Limited, UK, and the other by ThalesAlenia Space (TAS), Italy.

Mechanical-Attitude Controller Co-design of Large Flexible Space Structures

This paper provides an overview on the application and advantages of novel sub-structuring techniques in defining the multi-flexible body dynamics of an open mechanical chain. It employs single and double input-output port models that represent the six Degree Of Freedom (DOF) dynamics of terminal and intermediate appendages in the chain, with one and two connection points respectively. The interface that links the Finite Element Model (FEM) modal analysis output of the sub-structure to its parametric model in SIMULINK has also been explained. Also an analytical double port model of the six DOF dynamics of a uniform beam element with two connection points, called super-element, has been derived.

Integrated Structure/Control Optimisation Applied to the BIOMASS Earth Observation Mission

A new approach of an integrated structure/control co-design methodology is developed based on the recognition that a high degree of coupling exists between the control and structural disciplines in the control of flexible space structures. A unified computational framework is developed gathering methodologies and tools coming from robust control theory and advanced worst case analysis techniques together with mechanical modelling engineering tools. Within this environment, design iterations consist in updating critical control and structure design variables by assessing controlled performance while minimizing structural mass. The optimisation process utilises a Differential Evolution algorithm. Multiobjective optimisation is also supported highlighting the compromise between mechanical and control objectives. The Linear Fractional Transformation formalism provides an uncertain representation of the spacecraft dynamics which is considered during the controller synthesis and analysis processes together, managed in the H∞/μ setting. Traditional Monte-Carlo simulations evaluate the robust performance of the controller design whilst optimisation-based worst-case analysis has been implemented to increase the efficiency of the worst-case extraction. This paper presents the work supported by the European Space Agency in the scope of the robust AOCS technology program initiated to support the BIOMASS mission; a candidate for the Earth Explorer Core 7 missions.

An Integrated Analytical/Numerical Framework for Verication and Validation of Attitude Control Systems for Flexible Satellites

Modern geostationary telecommunication satellites typically employ large antenna and solar arrays which can generate flexible modes close to the bandwidth of the attitude control system. Combined with uncertainties in mass and inertias and effects such as fuel sloshing in large tanks without membranes, such dynamics represent a particular problem for accurate attitude control. This paper describes a framework for assessing the effects of these uncertain dynamics on the stability and performance of attitude control systems, using a combination of analytical and numerical tools. We develop detailed Linear Fractional Transformation (LFT)-based models of the uncertainties present in a modern telecom satellite and apply -analysis to these models in order to generate robustness guarantees. We validate these models and results by cross-checking them against worst-case analysis results produced by global optimisation algorithms applied to the original system model. The proposed integrated analytical/numerical framework is shown to provide more reliable results, and to be significantly more efficient, than standard Monte-Carlo simulations. I. Introduction A key challenge in the design of modern telecom satellites is the assessment of the effects of flexible modes generated by large appendages on the satellite’s attitude control system. Both the frequency and damping of these flexible modes are very difficult to predict accurately, and are typically only specified to lie within certain minimum and maximum bounds. The problem is compounded by the possibility of interactions with other sources of uncertainty in the system, in particular the effects of fuel sloshing in large tanks that are required in order to reach and maintain geostationary orbits. Attitude control systems for telecom satellites are generally designed using simplified models which do not include all sources of uncertainty and variation present in the system. This necessitates a formal process of verification and validation which assess the stability and performance properties of the controller when implemented on a high-fidelity simulation model. The standard approach to this problem adopted by satellite manufacturers is to combine tests on configurations that (based on engineering judgement) are considered likely to be problematic, with extensive Monte Carlo simulation campaigns in order to accumulate statistical confidence in the robustness properties of the controller. Such campaigns can have significant computational and cost overheads for manufacturers, since very large numbers of simulations are required in order to provide strong statistical guarantees of robustness. 1 In addition, there is now accumulating evidence that such campaigns can fail to reliably assess worst-case behaviour, especially for systems which are subject to flexible dynamics. 2–5

Preliminary AOCS Design for Pointing Budget Assessment of the BIOMASS Candidate Earth Explorer Core Mission

Abstract This paper presents the preliminary assessment of the attitude control design and performance for a baseline concept of the Biomass mission, a candidate for ESAs next Earth Explorer Core Missions. This satellite concept, as conceived at the end of the Phase 0 studies, has a 20-meter P-band antenna, for which the impact of high flexibility and uncertainty must be assessed. As a result, a rapid prototyping procedure has been established to assess the feasibility of various performance and robustness requirements at an early stage of the satellites development. This has been done by incorporating a covering uncertainty model to reflect the structural dynamics with associated tolerances. This allows the use of a control design solely based on rigid dynamics, resulting in a low-order control solution, for which coverage is demonstrated by means of multivariable robust performance and stability margins expressed in the structured singular value metrics. Using non-linear simulations, we demonstrate the suitability of the robust design approach to the selected satellite concept.

Enhanced Linear Fractional Transformation: a Matlab Toolbox for Space System Modeling and Controller Analysis and Synthesis

Abstract Through an example of orbital rendezvous between a flexible spacecraft and a rigid target, this paper presents the Enhanced Linear Fractional Transformation Toolbox (ELFTT) developed under ESA contract in order to support advanced design, validation and verification processes of space system control loop. ELFTT will be distributed within the ESA Member States under an Open Source-like license. It is built on (and fully compatible with) Matlab Robust Control Toolbox RCT. It is dedicated to build, verify, manage Linear Fractional Transformations LFT specialized for space applications. On the one hand, it contains a library of generic LFT models relevant for space applications including typical uncertain actuators models (reaction wheels, thrusters. . .) uncertain dynamics models (flexible spacecraft, rendezvous dynamics. . .), uncertain sensors models (star trackers, gyrometer. . . ), etc. On the other hand, it implements new Matlab functions dedicated to manage the LFT models and to support robust controller synthesis and analysis.

A Matching Pursuit Algorithm Approach to Chaser-Target Formation Flying Problems with Linear Time-Invariant Dynamics

Abstract In this article we present a new approach to satellite formation flying problems involving two spacecraft, one of which (the “chaser”) is actively controlling its relative position with respect to the other (the “target”). The goal of this controller is to use the minimum number of thruster firings in order to keep the chaser following its reference position within a certain tolerance. We show an approach based on the Matching Pursuit algorithm (MP), which proves to provide a solution to this problem in the open-loop case. A feedback version of the controller can be obtained by turning the method into a Model Predictive Control (MPC) strategy, thanks to the fact that the computational cost of the method is small. An example of application is shown at the end for a mission scenario inspired by the PROBA3 mission, which is currently under development.

Position Control Design and Validation Applied to ATV During Docking to ISS

Abstract The European cargo transfer vehicle ATV will perform an autonomous docking to the International Space Station (ISS). This mission represents from position control point of view a very complex problem, that gather huge constraints on performances but also on safety and validation level. This paper shall focus on the design and validation of the ATV position control loop used for final approach and it is organised as follows. The general problem of ATV to ISS docking is first described, and especially the different Guidance, Navigation and Control (GNC) functions requirements as well as the environmental perturbations likely to disturb it. In a second part, discrete H∞ control approach is applied to the position control loop. Methodology, design and tuning are then fully described and illustrated with ATV application. At last, frequencial validation is presented. Then, GNC algorithms performances are presented to show their compliance with the requirements.

Accurate Modes Design of the Darwin Precursor Formation Flying Demonstration Mission

Abstract In preparation for planet detection ESA mIssIon Darwin, a range of demonstration activities allow to study the concepts and technology needed to achieve the required performance. The considered precursor mission is a fully operational demonstration mission that aims both at validating the overall metrology chain and manoeuvres feasibility and at performing imagery of science objects by aperture synthesis. This precursor mission consists in three spacecraft, two telescope flyers and one hub combiner, allowing to constitute an interferometric arm. To achieve science imaging requirements, the GNC nominal mode shall ensure Optical Path Difference accuracy of 23nm (1σ) and relative pointing accuracy of both telescopes line of sight of 62mas (1σ). This paper presents the GNC design to drive the constellation from coarse formation (based on RF and STR measurements) to the ultimate performance mode (scientific mode, based on fringe tracker OPD measurements and either ODL (Optical Delay Line) or µN FEEPs thrusters actuation).

Integrated Control and Structure design framework for spacecraft applied to Biomass satellite.

Abstract This paper presents a research activity on the design of an Integrated Control Structure framework enabling to ensure co-jointly structure sizing and robust control design for flexible satellite. This work has been supported by the European Space Agency in the scope of the robust AOCS technology program initiated to support Biomass phase A mission. An Integrated Modeling, Control and Analysis Framework (IMCA) has been developed and exploited to optimize the articulated arm of Biomass satellite reflector while guaranteeing the existence of a controller fulfilling mission requirements. AOCS and Mechanical engineering tools have been merged into a single and unified multilevel optimization process where a spacecraft structure parameter set is iteratively and automatically updated to minimize the overall structure mass. The control design problem has been formulated and managed in the H ∞ /μ setting and the optimization process executed using Differential Evolution algorithm.