This paper investigates state-of-the-art modeling and control techniques for the robust line-of-sight pointing control of an optical payload on-board a stratospheric balloon, to meet stringent ...pointing requirements in the context of astronomy missions. Previous experience shows that the pointing performance of such systems is essentially limited by the rejection of the natural pendulum-like oscillations of the flight chain. This observation justifies the need for a model that accurately predicts such flight conditions that cannot be replicated in laboratory, and for a systematic methodology addressing the line-of-sight controller design to reject these flexible dynamics excited by wind disturbances. Moreover, it is sought to ensure robust stability and performance to the parametric uncertainties inherent to balloon-borne systems, such as complex balloon’s properties or release of ballast throughout the flight, especially since experimental validation is limited. In this paper, a first dynamical model of the complete system is proposed, based on Lagrangian mechanics; the comparison with flight data show that the frequency content of the platform’s motion is accurately predicted. Then, a multibody approach is applied to derive a second model taking into account the parametric uncertainties with the Linear Fractional Transformation representation. Finally, the control of the line-of-sight is tackled as a robust, structured
H
2
/
H
∞
problem that improves the performance with regard to traditional PID controller. A
μ
-sensitivity analysis is also proposed to identify the most sensitive parameters and verify the performance.
The Polarized Instrument for Long-wavelength Observation of the Tenuous interstellar medium (
PILOT
) is a balloon-borne experiment that aims to measure the polarized emission of thermal dust at a ...wavelength of 240µm (1.2 THz). The
PILOT
experiment flew from Timmins, Ontario, Canada in 2015 and 2019 and from Alice Springs, Australia in April 2017. The in-flight performance of the instrument during the second flight was described in
1
. In this paper, we present data processing steps that were not presented in
1
and that we have recently implemented to correct for several remaining instrumental effects. The additional data processing concerns corrections related to detector cross-talk and readout circuit memory effects, and leakage from total intensity to polarization. We illustrate the above effects and the performance of our corrections using data obtained during the third flight of
PILOT
, but the methods used to assess the impact of these effects on the final science-ready data, and our strategies for correcting them will be applied to all
PILOT
data. We show that the above corrections, and in particular that for the intensity to polarization leakage, which is most critical for accurate polarization measurements with
PILOT
, are accurate to better than 0.4% as measured on Jupiter during flight#3.
This brief proposes a new linear fractional transformation (LFT) modeling approach for uncertain linear parameter-varying (LPV) multibody systems with parameter-dependent equilibrium. Traditional ...multibody approaches, which consist of building the nonlinear model of the whole structure and linearizing it around equilibrium after a numerical trimming, do not allow to isolate parametric variations with the LFT form. Although additional techniques, such as polynomial fitting or symbolic linearization, can provide an LFT model, they may be time-consuming or miss worst case configurations. The proposed approach relies on the trimming and linearization of the equations at the substructure level, before assembly of the multibody structure, which allows to only perform operations that preserve the LFT form throughout the linearization process. Since the physical origin of the parameters is retained, the linearized LFT-LPV model of the structure exactly covers all the plants, in a single parametric model, without introducing conservatism or fitting errors. An application to the LFT-LPV modeling of a robotic arm is proposed; in its nominal configuration, the model obtained with the proposed approach matches the model provided by the software Simscape Multibody, but it is enhanced with parametric variations with the LFT form; a robust LPV synthesis is performed using MATLAB robust control toolbox to illustrate the capacity of the proposed approach for control design.
The objective of this paper is to fill the gap in literature on an exhaustive coupled pendulum-torsion model for balloon-borne systems. The development of such a model is required to explain the ...unexpected oscillatory behavior recorded on the flight data of scientific balloon-borne missions and more particularly the performance degradation due to the coupling of pendulum and azimuth dynamics through the azimuth control loop, which is classically designed using a decoupled torsion model. First, a complete dynamic model of balloon-borne systems is derived. The proposed model is applied to the Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall) experiment and validated by flight data. Then, the stability issue raising from the commonly neglected coupling assumption is investigated. Sufficient stability conditions are presented by using positivity properties. Based on the FIREBall model, it is finally shown how the azimuth control can destabilize the pendulum dynamics, and how the proposed model can be used during preliminary design phases to size a flight chain and the associated control system to prevent this instability.
The Faint Intergalactic Medium Redshifted Emission Balloon (FIREBall) is a mission designed to observe faint emission from the circumgalactic medium of moderate-redshift (z ∼ 0.7) galaxies for the ...first time. FIREBall observes a component of galaxies that plays a key role in how galaxies form and evolve, likely contains a significant amount of baryons, and has only recently been observed at higher redshifts in the visible. Here we report on the 2018 flight of the FIREBall-2 Balloon telescope, which occurred on 2018 September 22 from Fort Sumner, New Mexico. The flight was the culmination of a complete redesign of the spectrograph from the original FIREBall fiber-fed integral field unit to a wide-field multiobject spectrograph. The flight was terminated early owing to a hole in the balloon, and our original science objectives were not achieved. The overall sensitivity of the instrument and telescope was 90,000 LU, due primarily to increased noise from stray light. We discuss the design of the FIREBall-2 spectrograph, including modifications from the original FIREBall payload, and provide an overview of the performance of all systems. We were able to successfully flight-test a new pointing control system, a UV-optimized, delta-doped, and coated electron multiplying CCD, and an aspheric grating. The FIREBall-2 team is rebuilding the payload for another flight attempt in the fall of 2021, delayed from 2020 as a result of COVID-19.
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