Modeling and stability of balloon-borne gondolas with coupled pendulum-torsion dynamics

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

Linear Fractional Transformation Modeling of Multibody Dynamics Around Parameter-Dependent Equilibrium

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

FIREBall-2: advancing TRL while doing proof-of-concept astrophysics on a suborbital platform

Here we discuss advances in UV technology over the last decade, with an emphasis on photon counting, low noise, high efficiency detectors in sub-orbital programs. We focus on the use of innovative UV detectors in a NASA astrophysics balloon telescope, FIREBall-2, which successfully flew in the Fall of 2018. The FIREBall-2 telescope is designed to make observations of distant galaxies to understand more about how they evolve by looking for diffuse hydrogen in the galactic halo. The payload utilizes a 1.0-meter class telescope with an ultraviolet multi-object spectrograph and is a joint collaboration between Caltech, JPL, LAM, CNES, Columbia, the University of Arizona, and NASA. The improved detector technology that was tested on FIREBall-2 can be applied to any UV mission. We discuss the results of the flight and detector performance. We will also discuss the utility of sub-orbital platforms (both balloon payloads and rockets) for testing new technologies and proof-of-concept scientific ideasComment: Submitted to the Proceedings of SPIE, Defense + Commercial Sensing (SI19

FIREBall-2: The Faint Intergalactic Medium Redshifted Emission Balloon Telescope

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 September 22nd, 2018 from Fort Sumner, New Mexico. The flight was the culmination of a complete redesign of the spectrograph from the original FIREBall fiber-fed IFU to a wide-field multi-object spectrograph. The flight was terminated early due 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, 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 EMCCD, and an aspheric grating. The FIREBall-2 team is rebuilding the payload for another flight attempt in the Fall of 2021, delayed from 2020 due to COVID-19.Comment: 23 Pages, 14 Figures, Accepted for Publication in Ap

Robust integrated control/structure co-design for stratospheric balloons

Stratospheric balloons offer cost-effective platforms for optical payloads in the context of astronomy missions. During the 2018 flight of the Faint Intergalactic medium Redshifted Emission Balloon (FIREBall) experiment, the moon light was scattered from the surface of the balloon and re-directed into the telescope which resulted in degraded optical performance. To reduce this parasite effect, it is sought to increase the length of the fight train. However, this change in the mechanical design significantly modifies the dynamics of the system and the pointing performance must not be altered. In this purpose, a robust integrated control/structure co-design method is proposed. After deriving a Linear Fractional Transformation (LFT) model of the system, the co-design is tackled as a multi-objective, structured, robust H2/H∞ problem that is solved with a non-smooth optimization algorithm to maximize the train's length under constraints of pointing performance. By optimizing in a single iteration the controllers along with the structural parameter with regard to the worst-case configurations of the uncertain parameters, time-consuming procedures requiring not only to iterate between control and mechanical design, but also to analyze the robustness based on Monte-Carlo simulations, are avoided

Linear Fractional Transformation modeling of multibody dynamics around parameter-dependent equilibrium

This paper 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 in 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 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

Robust line-of-sight pointing control on-board a stratospheric balloon-borne platform

This paper addresses the lack of a general methodology for the controller synthesis of an optical instrument on-board a stratospheric balloon-borne platform, such as a telescope or siderostat, to meet pointing requirements that are becoming more and more stringent in the context of astron- omy missions. Most often in the literature, a simple control structure is chosen, and the control gains are tuned empirically based on ground testings. However, due to the large dimensions of the balloon and the flight chain, experimental set-ups only involve the pointing system and the platform, whereas flight experience shows that the pointing performance is essentially limited by the rejection of the natural pendulum-like oscillations of the fully deployed system. This obser- vation justifies the need for a model that predicts such flight conditions that cannot be replicated in laboratory, and for an adequate methodology addressing the line-of-sight controller design. In particular, it is necessary to ensure robust stability and performance to the parametric uncertain- ties 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 dynam- ical model of the complete system is proposed, based on a multibody approach and accounting for parametric uncertainties with Linear Fractional Transformations. The comparison with flight data shows that the frequency content of the platform’s motion is accurately predicted. Then, the robust control of the line-of-sight is tackled as a H∞ problem that allows to reach the performance objectives in terms of disturbance rejection, control bandwidth and actuators limitations

Vers un système de positionnement stellaire de jour et à basse altitude : défis et premiers résultats.

International audienceSince the emergence of electronic image sensors, stellar pictures can be easily processed by computers to identify stars and measure angles : distance between light spots on the same scene or angle above the visible horizon, like the sextant does for celestial navigation.Adding an inclinometer on the camera axis, the computer is now able to calculate the attitude angles (course, trim and list) in a local coordinate system and the position (latitude, longitude and even altitude). The last coordinate, time, may be measured by observing angular distances on the celestial sphere between stars located near the orbit of a satellite (Moon or LEO). If the sensor is on a vehicle, gyroscopes are necessary to smooth the rotations or vibrations.Going from this appealing theory to the reality of a daytime stellar positioning system hides many challenges such as detecting stars by day, at low altitude or in bad weather. If the level of accuracy increases, many sources of error emerge, e.g. sky background, diffraction limit, vertical deflection.Many prototypes or manufactured products have shown the potential of a stellar positioning system thanks to the maturity of technologies like inertial sensors and silicon CMOS detectors. Most of these applications are designed for altitude operations such as : • yield the attitude angles of satellites, stratospheric balloons, high altitude aircraft or missiles ; • adjust the inertial system of civil aircraft ; • measure satellite parameters from immobile ground telescopes in altitude ; • but rarely for positioning.The last challenge lies in tracking stars from a moving vehicle during daytime at low altitude : sea or land. The lower layers of atmosphere add numerous perturbations, forcing to work by day in infrared band, where silicon sensitivity ends and InGaAs sensitivity starts (or other technologies).This paper aims at looking for a compromise between detector technologies, stars detectability, duration of availability and accuracy of results for a stellar positioning system on a moving vehicle on the ground or at sea.A first parameter is the transmission of the atmosphere : when wavelength increases, scattering decreases but thermal emissions in infrared submerge starlight. Some bands obscure it but transmit photons from molecular resonance fluorescence of sunlight. A rigorous simulation of radiative transfer with local weather parameters is necessary to figure out the best band of transmission.Secondly, the number of observable stars in each band, with a given magnitude or flux, increases with the wavelength… except in R (0.6-0.7μm) and I (0.7-1.0μm) bands where we have few data in star catalogs : the magnitudes of many stars have been calculated by comparing their known data and their spectral type, confirming the tendency.Lastly, silicon technology for light detection is well developed thanks to public cameras and smartphones : the size and number of pixels on a sensor allow a high accuracy on angular distance measurements. In the opposite, Short Wave InfraRed detectors (SWIR: 1.0–2.5 μm) suffer bigger pitch and poor definition. But the potential of SWIR detectors remains high, and their performances are improving quickly. Betting on this ascending technology readiness, daytime star tracking tests have been realized : simulations are confirmed, and new challenges occur

End-to-end ground calibration and in-flight performance of the FIREBall-2 instrument

International audienceThe payload of the Faint Intergalactic Redshifted Emission Balloon (FIREBall-2), the second generation of the FIREBall instrument (PI: C. Martin, Caltech), has been calibrated and launched from the NASA Columbia Scientific Balloon Facility in Fort Sumner, New Mexico. FIREBall-2 was launched for the first time on the September 22, 2018, and the payload performed the very first multi-object acquisition from space using a multi-object spectrograph. Our performance-oriented paper presents the calibration and last ground adjustments of FIREBall-2, the in-flight performance assessed based on the flight data, and the predicted instrument's ultimate sensitivity. This analysis predicts that future flights of FIREBall-2 should be able to detect the HI Lyα resonance line in galaxies at z ∼ 0.67, but will find it challenging to spatially resolve the circumgalactic medium