Double-stage active control of micro-vibrations for high accuracy pointing missions
Authors:   Isae Supaero, Toulouse, France, Sanfedino Francesco , Alazard Daniel , Guercio Nicola , Deslaef Nicolas 
Published: January 2024

With the development of the next generation of Earth observation and science Space missions, there is an increasing trend towards highly performing payloads. This trend is leading to increased detector resolution and sensitivity, as well as longer integration time which directly drive pointing requirements to higher stability and lower line-of-sight (LOS) jitter [1]. Such instruments typically come with stringent pointing requirements and constraints on attitude and rate stability over an extended frequency range well beyond the attitude control system bandwidth, by entailing micro-vibration mitigation down to the arcsecond (arcsec) level or less [2]. Micro-vibrations are defined in [3] as low-level vibrations causing a distortion of the LOS during on-orbit operations of mobile or vibratory parts.
However, in order to guarantee high pointing performance, it is necessary to entirely characterise the transmission path between the micro-vibration source and the payload. The earlier the model is available, the easier it is to meet the stringent pointing requirements, by designing appropriate control strategies. The main difficulties encountered in Space system characterisation are both the impossibility to correctly identify the system on ground due to the presence of gravity and the consideration of all possible system uncertainties. Several uncertainties are in fact determined by: manufacture imperfections of structures and mechanisms, evolution of the system during the mission (i.e. material exposition to Space environment, mass and inertia variation due to ejected ergol), misknowledge of sensors/actuator dynamics. Uncertainty quantification is the preliminary step to be accomplished before designing robust control laws which provide a certificate on the closed-loop system stability and performance.
The increased need in pointing performance together with the use of lighter and flexible structures directly come with the need of a robust pointing performance budget from the very beginning of the mission design. An extensive understanding of the system physics and its uncertainties is then necessary in order to push control design to the limits of performance and constrains the choice of the set of sensors and actuators.
An analytical methodology to model all flexible elements and mechanisms of a scientific satellite and its optical payload in a multi-body framework is presented. In particular the Two-Input Two-Output Ports approach is used to propose novel models for a reaction wheel assembly including its imbalances and two kinds of actuators to control the line-of-sight: an FSM and a set of PMAs. This approach allows the authors to assemble a complex industrial spacecraft where detailed finite element models can be easily included as well. All these feature are available in the Satellite Dynamics Toolbox Library (SDTlib) [4].
Since in this framework an uncertain Linear Parametric-Varying system can be directly derived by including all possible configurations and uncertainties of the plant, two novel robust active control strategies are proposed to mitigate the propagation of the microvibrations to the LOS error. A first one consists in synthesising an observer of the LOS error by blending the low-frequency measurements of the LOS directly provided by a CCD camera and the accelerations measured in correspondence of the most flexible optical elements (primary and secondary mirrors of a space telescope) together with the accelerations measured on a passive isolator placed at the base of the payload. An FSM then uses this information to mitigate the pointing error. In order to obtain even tighter micro-vibration attenuation, a second stage of active control was proposed as well. This strategy consists in measuring the accelerations of the payload isolator and actuating six PMAs attached to the same isolator. Thanks to this double-stage active control strategy, the propagation of the micro-vibrations induced by the RWs and SADMs is finely reduced on a very large frequency band. In particular, a reduction of the pointing error to 10 arcsec is guaranteed at low frequency approximatively 1 rad/s) with a progressive reduction of the jitter until 40 marcsec for higher frequencies where micro-vibration sources act.
This application finally allows the authors to demonstrate the interest of the proposed modelling approach, that is able to finely capture the dynamics of a complex industrial benchmark by including all possible uncertainties in a unique LFT model. This modular framework, which permits to easily build and design a multi-body flexible structure, is in fact conceived in order to perfectly fit with the modern robust control theory. In this way the authors demonstrate how to push the control design to the limits of achievable performance, which is fundamental in the preliminary design phases of systems with very challenging pointing requirements.
The present work synthesises the results obtained in an ESA Open Invitations to Tender initiative “Line of Sight Stabilization Techniques (LOSST)” performed together with Thales Alenia Space, Cannes, France (Contract NO. 1520095474 / 02) [5].
[1] C. Dennehy, O. S. Alvarez-Salazar, Spacecraft Micro-Vibration: A Survey of Problems, Experiences, Potential Solutions, and Some Lessons497
Learned, Technical Report NASA/TM-2018-220075, NASA, 2018.
[2] A. J. Bronowicki, Vibration isolator for large space telescopes, Journal of Spacecraft and Rockets 43 (2006) 45–53.
[3] A. Calvi, N. Roy, E. Secretariat, ECSS-E-HB-32-26A Spacecraft Mechanical Loads Analysis Handbook, Technical Report, ESA, 2013.
[4] Alazard, Daniel, and Francesco Sanfedino. "Satellite dynamics toolbox for preliminary design phase." 43rd Annual AAS Guidance and Control Conference. Vol. 30. 2020.
[5] Sanfedino, F., Thiébaud, G., Alazard, D., Guercio, N., & Deslaef, N. (2022). Advances in fine line-of-sight control for large space flexible structures. Aerospace Science and Technology, 130, 107961.