Qb50 Uydusunun Yönelim Belirleme Ve Kontrol Sisteminin Entegrasyonu Ve Testleri

Abstract

BeEagleSat bir Avrupa Birliği FP7 projesi olan QB50 kapsamında geliştirilen 2 birim küp uydudur. Bu proje iki ve üç birimlik 50 küp uydu yardımıyla alt termosfer katmanının incelenmesini amaçlar. Dünyanın farklı üniversiteleri tarafından gerçekleştirilen çalışmalar ile üretilmekte olan bu uydular, Belçika von Karman Enstitüsü başkanlığındaki proje birliği tarafından sağlanan 3 faydalı yükten birini seçmek ve taşımakla görevlidir. Atmosferin en üst katmanı olan termosfer'in hakkında yerinde ölçümler ile en az araştırma yapılan bölümüdür. Yüksek eliptiklikte birkaç uydunun ve zaman zaman fırlatılan sonda roketlerinin dakikalar mertebesinde geçtiği bu bölümden alınan bilgiler yeterli düzeyde değildir. Bu eksikliği gidermek üzere, QB50 projesi ile hedeflenen 3-6 aylık görev süresi boyunca 50 uyduyla termosferin deniz seviyesinden 200-380 km yükseklikteki kısmı hakkında yerinde çok noktadan eş zamanlı ölçümler ile daha ayrıntılı bilgiler elde edilecek ve daha hassas atmosfer modellerinin geliştirilmesine olanak sağlanacaktır. Bu tez çalışmasında BeEagleSat uydusunda kullanılan ve QB50 projesi kapsamında Güney Afrika Cumhuriyeti ve İngiltere ekipleri tarafından geliştirilen yönelim belirleme ve kontrol sisteminin (YBKS-ADCS) testlerini gerçekleştirmeyi ve uydumuza entegrasyonu gerçekleştirilmiştir. YBKS temel olarak iki görevi üstlenir: fırlatmanın ardından yörüngeye yerleştirme sırasında oluşan dönme hareketlerini sönümlenmesi ve uydunun istenen yöne doğrultulması. Benzer şekilde, QB50 görevleri gereği ayrıntılandırılmış özel YBKS isterleri vardır ve bunlar 3 farklı ana yük için farklıdır: m-NLP taşıyan küp uydular 200 km'ye kadar doğrultma kesinliğini 15° (±5° hata payı ile) sağlamalıdır ve küp uydular yörüngeye yerleştirilmelerinden 3 gün içeresinde 50 derece/sn' ye varan tip-off oranlarını sönümleyebilmelidir. Bu koşulların sağlanabilmesi için yüksek doğrulukla yönelim belirlemesi ve 3 eksende kontrol yapılması gereklidir. Bu koşullar göz önünde bulundurularak QB50 proje birliği tarafından QB50 ADCS geliştirilmiş ve isteyen ekiplerin kullanımına sunulmuştur. İTÜ ekibine de bir adet uyduda kullanılmak üzere tahsis edilmiştir. Birimin alınması sonrasında öncelikle QB50 tarafından belirtilen temel işlerlik testleri gerçekleştirilmiştir. Ancak bu sağlık testleri uzay ortamı benzetiminde yapılmadığından ve sensör/eyleyicilerin çalışma doğrulukları hakkında yeterli bilgi vermediğinden ek testlerin yapılması planlanmıştır. Ek testlerin ilki ısıl vakum testleridir. USTTL'de bulunan ısıl Vakum Odasında (IVO/TVAC) gerçekleştirilecek bu testler yardımıyla QB50-ADCS'nin uzayın ısıl vakum ortamında beklenildiği şekilde çalışıp çalışmadığı denetlenecektir. Ayrıca, manyetik eyleyicilerin ve momentum tekerinin performanslarının incelenmesi de hedeflenmiştir. Bu amaçla, homojen manyetik alan oluşturan Helmholtz bobinlerinden meydana gelen Helmholtz kafesi ve uydunun kütlesi ve test sisteminin kararlılık gereksinimleri göz önüne alınarak tasarlanacak basit bir havalı yatak aracılığıyla momentum tekerinin ve manyetik eyleyicilerin testleri gerçekleştirilecektir. Ayrıca, tasarlanacak bu test sistemleri ileride tarafımızdan geliştirilecek bir küp uydu YBKS sisteminde de kullanılabilecek alt yapıyı oluşturacaktır.

ADC systems basically detumble the satellites after deployment and point them in some desired directions. Furthermore some missions can have specific requirements needs to be fulfilled for accomplishment. In QB50 case it is specified as the CubeSats carrying the m-NLP SU shall have an attitude control with pointing accuracy of 15 and pointing knowledge of 5 from its initial launch altitude down to at least 200 km and the CubeSat shall be able to recover from tip-off rates of up to 50 deg / sec within 3 days. To meet these attitude requirements of the QB50 mission QB50 ADCS is chosen. In this study, QB50 ADCS health check procedure has conducted QB50 ADCS. Tests are conducted to verify of each component's functionality, power consumption and communication with others via UI called CubeSupport. QB50 ADCS consists of three main components which are named with their functions: CubeComputer, CubeSense and CubeControl. First step of the process is the confirmation of CubeComputer's which is a onboard computer but in this system ADCS computer functions. ACP (Attitude Control Processor) which is main part of the CubeComputer, connection is primary for a working ADC system is done without any problem. The second step is to verify communication between CubeSense and CubeComputer whose purpose to control of sensors whose measurements are initiative for attitude determination process. Although the bundle has 3 attiude determination measurements which is sun vector, nadir pointing and magnetic field data; CubeSense has only Nadir sensor and Sun sensor on it. Both sensors are CMOS camera's with 190 degrees FOV lenses. Because of sensor's own requirement Sun sensor has neutral density filter to reduce intensity. These sensors functions depend on light existence besides amount of light. Therefore, it is expected Sun and Nadir sensors don't make detection without a proper stimulus. For testing the sensors, phone flash light is used as stimulant. To test the Nadir sensor a simulator which consists of basically a card box with a paper covered 16cm diameter circular cutout which center is in the bore-sight is used. When the flash light is being moved in the positive X-axis direction, measurement of angle around Y-axis (elevation) increases as expected. Then the movement of the light is reverse, decrease in elevation detected. Again the same process is done in the positive Y-axis direction, measurement of angle around X-axis (azimuth) increases in Sun sensor but decreases in Nadir sensor as expected. Lastly, the movement of the light is reverse, decrease in Sun sensor but increase in Nadir sensor in azimuth detected. The third step of the test is on CubeControl which could be examined in 2 sub groups: CubeControl Signal and CubeControl Motor. CubeControl Signal group contains its own MCU (micro controller unit), magnetometer, coarse Sun sensors, magnetic torquers and finally GPS receiver (will be integrated). CubeControl group has also a MCU its own, rate sensor and reaction wheel. After verifying CubeControl Signal MCU communicates with no error, tests of the sensors and actuators are conducted. First phase of this step magnetometer checks. By rotating magnetometer, it's been assured that we can read negative and positive fields in three directions and the vector sizes between 10,000 and 55,000 in normal conditions via UI. The second phase is to check the coarse Sun sensors tested whether they give reasonable outputs with exposure to the light, in this case also used phone flash is used light source. The third phase is checking magnetic fields' direction induced by magnetorquers. In all three axis, verified that the compass which positioned in each working torquer, direction north pole points in the same axis direction. The fourth phase starts with verification of CubeControl MCU communication is error-free. The rate sensor data is +/- 1 deg/s as expected and any tilt in Y- axis gives reasonable outputs. The final phase is on testing reaction wheel. The wheel speed increase/decrease reaches 2000 rpm +/- 200 rpm in 10 seconds condition is met. Consequently, an ADC system that satisfies the strict attitude requirements of the QB50 mission is chosen. QB50 ADCS health check procedure has conducted without errors which indicates that the system functions properly and ready to go on TVAC , Helmholtz Cage and Spherical Airbearing tests. But for the two latter tests it is needed to design and produce test systems before testing. After all tests are done, it is finally integration phase. For Helmholtz cage and spherical airbearing tests first step is to build these systems. This thesis is aims to also prepare design prosedures and program codes of these systems. An spherical air bearing test bed is used to stimulate weightlessness for the satellite. Basically, a spherical air bearing consists of 3 parts, first part is the base of the system which has orifices to transmit the pressurized air to the bearing, second part is the sphere (normally semi-sphere is used in many designs but in this case it is a full sphere) and a compressor with a regulator. But to test the ADCS this sphere is designed hollow in order to put the whole satellite in it and to make the system rotate in 3 axis without limitations. The design of the bearing calculations (diameters of the orifices, needed pressure, compressor power consumption) are coded in Matlab and added to appendix. And to verify the rotation of the satellite, a imu board is added to the system, which is called IMUduino. IMUduino is a 10dof system which has gyro, accelerometer, magnetometer and altimeter. It is chosen beacuse of it is an Arduino, it is easy and open source. So an open source code is used to program the IMUduino and successful results are got. To visualize the outputs a Matlab code is done to import the serial port data and to create graphics of them. This codes are included to the appendix of the thesis. Helmholtz coils is a system which creates homogeneous magnetic fields through coils. Altough the QB50 satellite has magnetic coils in three axis. Because of this reason, BeEagleSat needs a three axis magnetic modeller which can be possible if a three axis Helmholtz coil groups which is also called "Helmholtz cage". In a Helmholtz cage design, three main phase exists. First phase is to program a code that calculates the magnetic field of a point which is given by altitude, latitude and longitude information of the satellite position. This code consists of mainly 3 components. SGP4 orbit propagator and perturbation modeller, transformation of the position data for IGRF needs, IGRF the magnetic field calculator. First component is SGP4 (Simplified Perturbations Model) which is a perturbation modeller which includes drag, Earth's shape effects, gravitation effects of moon and Sun and also radiation. In this code it is used as an orbit propagator which initials with TLE (Two Line Elements) . TLE information gives details about the satellite orbital elements and the time of the information is created. The TLE data and SGP4 code calculates the position of the satellite in XYZ (km) in TEME (True Equator Mean Equniox) frame. Although to get the altitude, latitude and longitude information it is needed to transform the position in to ECEF (Earth Centered Earth Fixed) frame. After getting the suitable position information in the needed frame, this information is converted in to altitude, latitude and longitude data which is requisite for magnteic field modelling called IGRF. IGRF (International Geomagnetic Reference Field) is model of magnetic field calculator around the World which consists of both mathematical models of magnetic field and magnetic measurings of spacecrafts. In this thesis IGRF code is not used, instead of a Matlab Aerospace Toolbox function "igrf11magm" function is used, which inputs decimal year, height (altitude), latitude, longitude and outputs some magnetic field information, but in this thesis it is used just mangnetic field vector information output. All these codes are included to the appendix of the thesis. Second phase of the Helmholtz cage design is to calculations. A code which calcultes and visualize magnetic field created by the Helmholtz cage, depending on radius of the coil, number of turns and current. Third phase is modelling the Helmholtz cage with finite element method. For this process it is used Comsol multiphysiscs modelling software. With this process a 3D magnetic field visualization of the cage is provided. At the end of these phases, Helmholtz cage procedure needs building one, calibration and the power control of the system with a code. But this is beyond of the scope of this thesis. To sum up, this thesis basically is on, health checks of the QB50 satellite BeEagleSat, designing procedures and coding the programs of a Helmholtz Cage and an air bearing test bed which is needed before building phase.