Variable speed scissored pair dual gimbal Control Moment Gyro for nano-satellites

Steyn, Douw (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The work presented in this thesis describes the development of an attitude control actuator for use in CubeSats. This actuator uses novel Control Moment Gyro technology to perform slew manoeuvres. The configuration consists of a symmetric pair of counter-rotating gyros mounted in scissoring dual gimbals. The outer gimbals are mechanically constrained using gears driven by a single stepper motor while the inner gimbals are individually actuated using two additional stepper motors. Three-axis control is achieved by changing the gimbal angles for roll and pitch manoeuvres and varying the wheel speed for yaw rotations. Electronics were designed to control the momentum wheel motors and the gimbal angle actuators. A mathematical model was derived from the mechanical design. The model was used in simulations where a slew manoeuvre was performed in each axis individually in order to match the Hardware-inthe- Loop test conditions. The average power usage, for the three-axis implementation, during a slew manoeuvre was measured at 420 milli-Watt with maximum of 556 milli- Watt. The volume required for the actuator is only 10 cm x 5 cm x 5 cm and has a mass of 260 g, making it suitable for CubeSat use. The attitude controllers tested include a Bang-Off-Bang and a Quaternion Feedback controller. The controllers are typically combined for large slew manoeuvres: The Bang-Off-Bang controller is used first and at the required final attitude, the Quaternion Feedback controller is enabled to accurately track the reference angle. Hardwarein- the-Loop tests were done on a low-friction air bearing platform for ground based attitude control demonstrations. Attitude knowledge was obtained from a MEMS inertial measurement unit and a laser pointer. Tests were repeated for various platform moments of inertia in order to empirically determine the expected pointing accuracy of the system. Similar tests were also performed using a conventional reaction wheel configuration to compare the performance. A maximum torque of 0.52 mNm can be achieved by this new actuator and a pointing accuracy of less than 0.2 degrees was demonstrated on the air bearing platform. Initial results show satisfactory performance to justify further development of a flight actuator module.

AFRIKAANSE OPSOMMING: Die doel van hierdie tesis is om ’n oriëntasie aktueerder te ontwikkel wat in CubeSats gebruik kan word. Hierdie aktueerder voer rotasie bevele uit deur gebruik te maak van 'n innoverende BeheerMoment Giroskoop (BMG) ontwerp. Die ontwerp bestaan uit ’n identiese paar van teenoor-roterende giroskope wat in dubbele rame gemonteer is. Beide buiterame roteer met dieselfde hoek in teenoorgestelde rigtings, sowel as die binnerame. Die buiterame is meganies gekoppel met ratte en roteer met behulp van ’n enkele stappermotor, terwyl die binnerame elkeen met hul eie stappermotor aangedryf word. Drie-as beheer word moontlik gemaak deur die rame te roteer vir rol en duik en die wielspoed te verander vir gier rotasies. Elektronika is ontwerp om die stapper en GS motors te beheer, waarna ’n wiskundige model vir die hardeware ontwerp ontwikkel is. Hierdie model is gebruik in simulasies om rotasies in elke as te toets en hardeware toetse is gedoen om die metings te bevestig. Die gemiddelde kragverbruik van die volledige ontwerp is 420 milli-Watt, met maksimum van 556 milli-Watt. Die stelsel is geskik vir nano-satelliete aangesien die volume slegs 10 cm x 5 cm x 5 cm is, en die massa 260 g is. Die rigting beheerdertegnieke wat getoets is sluit in ’n Aan-Af-Aan en ’n Kwarternioonterugvoerbeheerder. Hierdie word dikwels saam gebruik vir groot rotasies. Die Aan- Af-Aan beheerder word eerstens gebruik om die gewenste hoek te bereik, waarna daar na die Kwarternioon-terugvoer oorgeskakel word om die finale hoek akkuraat te volg. Hardeware toetse is uitgevoer deur van ’n luglaer, wat amper wrywinglose rotasies toelaat, gebruik te maak. Rigtingmetings was verkry met ’n MEMS inersieële sensor en ’n laserwyser. Deur sleurtoetse op die luglaer vir verskeie traagheidsmomente te herhaal, is ’n oriëntasie akkuraatheid van 0.2 grade behaal. Om die resultate te vergelyk, is dieselfde toetse ook uitgevoer op ’n enkele groot reaksiewiel. Die maksimum wringkrag wat die stelsel gedurende toetse behaal het, was 0.52 mNm. Hierdie resultate dui daarop dat die ontwerp genoeg voordele inhou om verdere ontwikkeling te doen en ’n vlugmodel te oorweeg.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98062
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