The design and implementation of a stellar gyroscope for accurate angular rate estimation on CubeSats

Calitz, Nico (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Until recently, small form factor satellites (such as CubeSats) relied almost exclusively on micro electromechanical system (MEMS) gyroscopes for attitude propagation purposes. Unfortunately, the nature of MEMS gyros is such that they exhibit a measure of bias drift. This drift must be compensated for, a task for which stellar gyros have proved to be exceptionally useful. Stellar gyros are satellite subsystems capable of inferring three-axis attitude propagation based on the displacement of a series of stars between successive image frames. Their design is analogous to that of star trackers, using many of the same hardware designs and algorithms. When used in combination with MEMS solutions, stellar gyros provide not only a means for drift compensation, but also a measure of functional redundancy with regard to attitude propagation. This thesis presents the design and implementation of stellar gyroscope algorithms capable of operating alongside existing orientation algorithms on traditional star tracker hardware. The CubeStar star tracker module is used as development platform. The proposed stellar gyro solution retains CubeStar’s existing star extraction algorithms, while investigating alternative methods for star centroiding in addition to the existing centre of gravity (CoG) approach. A dynamic proximity based matching algorithm is suggested to determine star correspondence between image frames. Finally, various well established estimation algorithms are considered for the purpose of rate determination, including singular value decomposition (SVD), Davenport’s q-Method and weighted least-squares (WLS). An initial evaluation of the proposed algorithms is made based on simulations in the MATLAB environment. Simulation results are confirmed through means of practical tests, performed on a simulated night sky in a controlled environment. With a focus on low angular rates, results suggest reliable operation up to ±1 deg/s in all three axes of rotation. As expected for stellar imaging solutions, angular rates estimated in both cross-boresight axes are almost an order of magnitude more accurate than the corresponding estimates in the boresight axis itself.

AFRIKAANSE OPSOMMING: Mikrosatelliete, soos CubeSats, het tot onlangs byna uitsluitlik op mikro elektromeganiese (MEMS) vibrerende struktuur giroskope staatgemaak vir die meet van hoeksnelhede. Ongelukkig is die aard van MEMS giroskope sodanig dat hierdie metings afsette toon wat al hoe verder van hul werklike waardes verskuif. Daar moet gekompenseer word vir hierdie verskuiwing, ’n taak waarvoor stergiroskope besonder geskik is. Sterrebeeld gebaseerde giroskope (of bloot gewoon stergiroskope) is satelliet substelsels wat daartoe in staat is om ’n satelliet se oriëntasie in drie dimensies te propageer deur gebruik te maak van die verplasing van ’n reeks sterre tussen twee opeenvolgende beelde. Hulle ontwerp in terme van beide hardeware en algoritmes is soortgelyk aan dié van stervolger kameras. Stergiroskope kan ook saam met MEMS toestelle gebruik word. Hulle verskaf beide ’n metode om te kompenseer vir verskuiwings in MEMS metings sowel as ’n funksionele alternatief met betrekking tot hoekafskatting. Hierdie tesis beskryf die ontwerp en implementering van ster giroskoop algoritmes wat in staat is om hand-in-hand met bestaande oriëntasie algoritmes op tradisionele ster volger hardeware te funksioneer. Die CubeStar stervolger module is as ontwikkelings platform gebruik. Die beoogde stergiroskoop ontwerp behou CubeStar se bestaande ster ontginnings algoritmes. Verskeie metodes benewens die bestaande swaartepunt benadering word wel ondersoek vir die bepaling van ster sentroïedes. Die korrespondensie tussen opeenvolgende sterbeelde word bepaal deur middel van ’n dinamiese nabyheid gebaseerde passings algoritme. Ten slotte word verskeie algoritmes oorweeg vir die afskatting van ’n satelliet se hoeksnelhede. Dit sluit in enkelvoud waarde ontbinding (SVD), Davenport se q-metode en ’n geweegte kleinste kwadraat (WLS) benadering. Die voorgestelde algoritmes is ge-evalueer op grond van simulasies in die MATLAB omgewing. Praktiese toetse is uitgevoer op ’n gesimuleerde sterrebeeld om simulasie resultate te bevestig. Met ’n fokus op lae hoeksnelhede dui resultate op betroubare afskatting teen hoeksnelhede van tot ±1 grade/s rondom al drie rotasie-asse. Soos verwag van ster kameras is die hoekafskattings rondom die transversale asse ’n orde meer akkuraat as die ooreenstemmende afskattings rondom die optiese as.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98059
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