İtü Psat Iı: Yönelim Kontrolüne Sahip Nano Uydular İçin Yüksek Yeterlilikli Platform Geliştirilmesi

Abstract

Günümüzde uzay teknolojilerine fazlasıyla ilgi görülmektedir. İlk uzaya gönderilen uydudan bugüne gelişen teknoloji ile uzaktan algılama, iletişim, bilimsel deney, navigasyon ve keşif gibi bir çok farklı amaçta kullanılmak üzere uydular tasarlanmaktadır. Bu tasarımlar için de büyük yatırımlar yapılmaktadır ve bu yatırımlar sayesinde sadece uzay teknolojileri değil bir çok mühendislik alanında gelişim sağlanmıştır. İlk geliştirilen uydu yapılarının büyük boyutlu ve maliyetli olması sebebiyle özel amaçlar için daha özel yapılar kullanılması istenmektedir. Bu özel yapılar önceki uydulara göre daha kompakt ve amaca göre değişmekle birlikte daha küçük boyutlarda olmuştur. Boyutların ve ağırlığın değişmesindeki bir diğer etken ise fırlatma maliyetinin azaltılmasıdır. Büyük boyutlu uyduların fırlatma maliyeti yüksek olmasından dolayı fırlatıldıktan sonra oluşabilecek bir hatada küçük boyutlu uydulara göre daha fazla paranın çöpe atılması demektir. Bir öğrenci eğitim projesi olarak başlayan küp uydu tasarım projeleri şimdi farklı amaçlarla gelişerek yüksek öncelikli bir konu haline gelmiştir. Bir çok kişi ve kurum, bu konu hakkında çalışmaları teşvik etmektedir, TÜBİTAK da ülkemizde bu kurumlardan biri olmaktadır. TÜBİTAK tarafından desteklenen projemizde, İTÜ bünyesinde İTÜ pSAT I sonrasında tasarlanan ikinci küp uydu tasarımı gerçekleştirilmiştir. İTÜ pSAT II’de ilk projeden edinilen tecrübe ile yüksek yeterlilikli uçuş sistemi (bus) mimarisi geliştirilmiş, daha hassas yönelim belirleme ve kontrol sistemini (YBKS) temel alan sistem tasarlanmış ve üretilmiştir. İçerdiği sistemler dışında nano uydu temel tasarım kriterleri göz önüne alınarak tasarlanan İTÜ pSAT II’nin modüler yapısı da özgün bir tasarımdır. Bu tasarımlar ve geliştirilen sistemler; uydu içerisinde istenilen çalışmaya göre değiştirilebilir alt sistemleri olmasını sağlaması sebebiyle projenin amacı olan yüksek yeterlilikli nano uydu platformu elde edilmiştir. Ayrıca bu platformun uçuşa hazır halde olup olmadığını sınamak adına testleri yapılmıştır ve başarıyla sonuçlanmıştır. Bu tez içerisinde İTÜ pSAT II adına yaptılan sistem mühendisliği ve testler anlatılmıştır. Buna ek olarak projemizde yer alan diğer arkadaşlar tarafından yapılmış olan, YBKS ve bus sistemlerine ek olarak uydunun modüler yapısı alt sistemleri özetlenmiştir.

Space technologies are very popular at present time. Since the first satellite sent to space with the developing technologies, many satellites have been designed for different purposes such as remote sensing, communication, scientific experiments, navigation and reconnaissance. Major investments have been done for these designs and it has been achieved many improvements in the field of engineering for sake of these investments. More private structures have been requested for specific purposes because, first developed satellites were very big and expensive. These special structures have been more compact than the previous satellites, and have been smaller depending on the purpose. Another factor about changes in the dimensions and weight is to reduce launching cost. It means to waste money in case of any failure after launching of satellites due to the high cost of launching of large-sized satellites. Cubic satellite design projects, which began as student educational projects now have become a matter of high priority. Many people and institutions are encouraging these studies. TUBITAK is also one of those institutions in our country. This, ITU pSAT II, is second cube satellite design project supported by TUBITAK in ITU after designing ITU pSAT I which had been realized in the same way. Beside single satellite operations, it is expected in near future that groups of pico-, nano- and micro-satellites (satellite constellations) will be working as a group on orbit to perform space science and service missions. Satellite constellations superiority in risk distributing risk, higher target area visit frequency, distributed measurement capability and backup ability make it an ideal choice for missions such as measurement of earth s magnetic field alterations and providing lower-cost global communication services. The most explicit properties of cubesats are weight and size. Cube shaped satellites must be less than 1 kg and its size must be approximately 10 *10*10 cm that is called one unit -1U- cubesat. Correspondingly, 2U cubesats have 20*10*10 cm size and 3U’s have 30*10*10 cm size. Their weights change as 1U 0-1.3kg, 2U 0-3kg. Also 3U cubesats weight are between 0 to 4kg . This standardization makes easy compatibility between developers. Towards this end, in this thesis it is presented that the design and analysis of a modular 3U structure that gives provides the much needed flexibility to the satellite designers. Small satellite concept is present since the outbreak of the space age. The need for smaller systems was constantly in minds and the technological development is always in that way, making it smaller and efficient. Miniaturization of the spacecraft elements and advancements in the micro-electro-mechanical systems lead us to a new idea. Small satellites started to find new application areas like remote sensing, scientific experiments and even communications. The most remarkably advantage of a small satellite arises in the mission cost trades; design, testing, launch and operation costs are pulled down by a significant margin. For example, since they are lighter, they can be launched as a secondary or tertiary payload by a launch vehicle, meaning that a sharp fall in the launch expenditures. High-qualified flight system architecture (bus) was developed with the experience of the first project, the system based on more sensitive navigation and control system (ADCS) was designed and produced. ITU-pSAT II ADCS system consists of three distinct hardware layers integrating sensors, actuators, and ADCS computer to achieve high precision control. The ADCS computer system has Blackfin BF537 processor module as a core. Its compatibility with MATLAB/Simulink and powerful features makes the processor a perfect candidate for ADCS computer. Procession of incoming sensor data with embedded software on-board and production of necessary control signals are handled by Blackfin. Satellite has low cost ADIS16405 board including three axis gyroscope with internal magnetic field sensor for inertial angular velocity measurements. Furthermore, Honeywell HMR 3300 external magnetometer placed end of the boom mechanism, and Silonex SLCD-61N8 sun sensor has been installed to ADCS system for multiple and precise measurement capacity. In addition to those sensors, optionally, in house developed star tracker and/or SGR-05U GPS receiver can be used for precise attitude calculations and accurate orbital position and velocity measurements. The sensor package of the project also includes an in-house developed star tracker as an experimental device. The actuator layer consists of a redundant assembly of in-house developed reaction wheels, magnetic-torquer coils and an experimental set of micro-pulse plasma thrusters (uPPTs). It is provided the design features of ITUpSAT II, which is aimed at providing a capable bus for such future missions. Specifically, in comparison to the existing on-market pico/nano-satellite buses, ITUpSAT II bus provides not only higher computational power and data link capabilities but also precise orbit determination through its GPS receiver. To cope up with different requirements across a wide range of scientific missions, an indigenous ADCS for precise attitude control is developed. Embedded within the ADCS is a suite of attitude determination and controller algorithms with different operation modes as to fulfill the pointing accuracy needs depending on the mission. Furthermore, the modular subsystem structure enables ADCS and S-Band data link to serve as a standalone payload computer. A filter is designed and embedded within the ADCS computer to carry out the asynchronous fusion of filtered sensor data with outputs from the orbit and attitude propagation algorithms. The filtered and fused state data is fed to a fault- tolerant and reconfigurable control layer. The control layer is divided into different operation modes control strategies on the actual spacecraft operation modes such as de-tumbling or high precision attitude control for purposes such as image capturing. To test the actual behavior of the control system elements during execution of real mission of the spacecraft including failure scenarios, we have developed a software and hardware in- the-loop test system (SIL and HIL). The software and hardware-in-the-loop system consists of an in-house developed air bearing table (to simulate attitude dynamics) and Helmholtz Coil system (to emulate Earth s magnetic field) in addition to the computer rack, which provides high-precision orbit, attitude and environment “truth-model” propagation with 3D orbit and attitude visualization. Overall architecture is suitable for testing under number of possible hardware failure scenarios and control modes, and greatly aids the development process of the ADCS and bus components. Except the systems included in the system, modular structure of ITU pSAT II that is designed by considering basic design criteria of nanosatellite is also unique. All the side faces can be opened using slide through top and bottom caps to simplify accessibility to the interior of the spacecraft and easy assembly of payloads and components. The design is fully compliant with Cubesat Design Specifications. All the structural components are made of Aluminum 7075 T6 as it provides cheaper access and easier production in comparison to the other Aluminum grades and metals available for space usage. The modular structure is reconfigurable towards various different mission scenarios as the internal payload volume is maximized to 2U units. In addition, the payload components and boards can be placed vertically depending on size and the need for access to the side panels. Other driving design factors are to meet the criteria for successful launch since the rockets accelerate the spacecraft to the high velocities required for injection into orbit. The wide range of vibration frequencies is transmitted through the spacecraft structure assures the structure will survive. An iterative procedure is followed to conclude the design. Finite element analyses of the structure is handled for the given launch environment-loading conditions. Sufficient nanosatellite platform that was the purpose of the project is obtained because this design of the satellite and the systems developed which are replaceable in it. Structural tests also performed on our satellite. It is necessary to perform the required tests to the structure of satellite before launch that are specified by PSLV. These tests important because structural problems must be seen before launch. In testing, the aim is to guarantee that the spacecraft performs satisfactorily under the environmental conditions during flight. Moreover, a factor of safety shall be used to assure that the environmental loads during flight will not exceed the qualification test levels. Although a satellite is also affected by the structural loading during the life on its orbit, the most serious forces that act on the satellite appear at the period of launching phase. Therefore, the vibration test scenario is formed using the vibration levels of India’s Polar Satellite Launch Vehicle, which are given in PSLV manual. During the launch and trajectory of PSLV, ITUpSAT II will be subjected to various dynamic loads. Firstly quasi static load is applied on satellite to simulate the launch vehicle’s acceleration during launch. After that shock tests are performed because that shocks occur at heat shield seperation. Also resonance survey, sinusoidal vibration and random vibration tests applied onto all three axes. Thermal cycling test and vacuum test performed after vibration tests. These tests performed according to the information provided by ISIS and according to the examination of the orbit characteristics of ITUpSAT II. In conclusion this thesis includes ITU pSAT II’s systems engineering and tests performed on this satellite. In addition to this, satellite bus systems, ADCS and modular structure of the satellite, which is studied by project group, is described in this thesis.