SYSTEMS INTEGRATION AND STABILIZATION OF A CUBESAT

Transcription

1 SYSTEMS INTEGRATION AND STABILIZATION OF A CUBESAT Tyson Kikugawa Department of Electrical Engineering University of Hawai i at Manoa Honolulu, HI ABSTRACT A CubeSat is a fully functioning satellite, confined to a box that is ten centimeters on each side and can fit in the palm of your hand. Just as the conventional satellites currently orbiting Earth, the CubeSat is comprised of several subsystems that play a crucial role in carrying out the small satellite s mission. These subsystems, including communication, power, and data handling must be able to work together as one system in order for the mission to be a success. Systems integration exists to ensure this compatibility among subsystems. In addition, depending on the mission, the CubeSat may need some form of stabilization to keep it from spinning too freely in space. The basic ideas of this have also been investigated. INTRODUCTION Satellites play a crucial role in everyday life, from relaying phone calls to protecting the security of an entire nation. These satellites, however, are very large, extremely expensive, and take years to develop. This is where small satellites, such as the CubeSat, have big advantages over their larger counterparts. Small satellites cost much less per unit, so many more can be fabricated for the price of one. Since they are small, they are easier to build and can be produced at a much faster rate. Obviously, one cannot fit all the hardware of a large satellite in the small box that is a CubeSat. Instead, by having many CubeSats, one can construct a network of small satellites that can accomplish the same tasks one large satellite can. Also, should one of these CubeSats in the network some how fail, the network can be reconfigured to pick up the slack. Unfortunately, for the larger satellites, should something go wrong, the entire mission is at risk of failure because of a single fallout. The basic subsystems that make up the internals of a CubeSat are the telemetry, tracking and control (TTC), power generation and distribution (PGD), data command and handling (DCH), and the payload. The contents of the payload depend on the requirements of the mission. Another subsystem that can be included if deemed necessary is attitude determination and control (ADC), which is the stabilizing of the CubeSat and the basics will be discussed later. Each subsystem plays a crucial role in keeping the satellite up and running properly. The main role of the TTC subsystem is the communication link between the satellite and the ground station. The role of PGD is to ensure there is enough power being produced so that all the components of the satellite can operate at the appropriate times. The DCH subsystem is in charge of the overall flow of operations of the satellite, which includes gathering, storing, and processing information from the various subsystems and passing the information on accordingly. Each payload subsystem is specific to the mission that the CubeSat is designed to carry out. The purpose of the payload can vary from temperature tests, image acquisition, or even a biological study. Aside from the mission needs, there are also several constraints that apply to all CubeSats and is discussed next. 46

2 CUBESAT STANDARDS The CubeSat standards are limitations and requirements that CubeSat developers are advised to follow when designing and constructing their satellite. The standards were created by the California Polytechnic State University who has also created the launcher that will hold and release the CubeSats once they have reached orbit height. The standards serve as a means to allow the CubeSat launcher to be universal to hold and launch any CubeSat. The launcher is known as the P-POD (Poly Picosatellite Orbit Deployer). The P-POD is basically a rectangular box with a large spring inside. On one face, there is a hatch door where the CubeSat will enter and exit. When a CubeSat is placed in the P-POD, it compresses the spring which, at launch time, will push the CubeSats out into orbit. The P-POD is where the size constraints of the CubeSat is derived from, and the size of one CubeSat is 10 cm x 10 cm x 10cm and weight limit of 1 kg. However, if a single unit size is too small for the parts, a team can opt to increase the size to 1.5, 2, or even 3 units (three is the limit because the P-POD can only hold three single unit CubeSats). By increasing the size of the desired CubeSat, the weight limit also increases. This can be determined by simply taking the single CubeSat limit of 1 kg, and multiplying it by the factor of increase (i.e. if using a 1.5 sized CubeSat, the weight limit is 1.5 kg). As for the housing size increase, only the length is affected. The height and width of the CubeSat remains the same, and like the weight, the length increase is by the same factor of increase in comparison to the single unit CubeSat. So a 1.5 CubeSat, means a 10 cm x 10 cm x 15 cm size limit. Separation springs and switches may also be vital to a mission. The CubeSat, powered by electricity and made of metal, needs a mechanical or electrical means of determining when it has been released into space and is clear to begin its operations. This is where the separation switch would come into play. When the CubeSat is in the P-POD, it will be pressed up against another CubeSat, so by designing the switch to extend slightly out of the structure, it will be pressed down. When the CubeSat has been released, the switch will no longer be registered as being pushed and the satellite can then activate. The springs, on the other hand, help the CubeSat to separate itself from the adjacent CubeSat(s). The suggested placement of the springs is in opposite corners of the point of contact with the other CubeSats. Electrical requirements include a pull before flight pin. The purpose of this pin is for those who would like to launch their satellite with their batteries charged. Since the satellites, once inspected, would be sitting on a shelf as they await launch, it would be beneficial to have a mechanism that will disconnect the batteries from the rest of the components so that the batteries do not slowly discharge. The pin would be pulled before they are placed in the P-POD for flight (hence the name), and the separation switches would take over from that point. This brings up another requirement, which is the CubeSat must be powered down while in the P-POD so it will not interfere with the primary payload. ADDITIONAL SYSTEMS INTEGRATION CONCERNS Aside from making sure the CubeSat will meet the standards, systems integration also has a few other issues to address. Keeping the size limit in mind, systems integration needs to make sure that all the parts will fit within the confines of the housing. The internal layout is crucial in the success of the CubeSat. Since this needs to be solved prior to ordering the parts, it would be 47

3 ideal to have some sort of model of the housing and parts so that a layout can be planned and tinkered with to ensure everything will fit. As mentioned before, systems integration needs to ensure all the subsystems are able to come together as one system. They needs a means of connecting with each other and systems integration will need to determine how these interconnections will be made. This is done when the layout and printed circuit boards are being designed. These connectors are very important since they are the means of which the subsystems will be communicating with one another, and a poor selection could result in a system failure down the road. Once parts have been received, each subsystem should be testing the parts specific to their subsystem separately, keeping in mind what is needed of them so that their parts will be able to cooperate with the other subsystems. After each part and subsystem has been developed and tested, the integration process begins. A suggested method would be to test only a few pairings/groupings of subsystems at a time, then start to put it all together and finish off with a complete system test. This final test procedure could go as far as simulating the actual flight process as it goes into its orbit. *The camera and Fox will be modified to be lighter (approx 150g total). PCB weight is estimated from a previous project PCB boards. APPLICATION The University of Hawaii CubeSat team is jointly working on a CubeSat with an electrical engineer from a local engineering firm. The primary mission of this CubeSat is image acquisition for disaster mitigation purposes. Therefore, the primary payload will be an on board camera. Also included in the payload, is a global positioning system (GPS) and a nano-inertial measurement unit (nimu). These two devices will be used for geo-referencing the images taken so the location of the image can be readily determined. The engineering firm has been placed in Table 1. Weight Budget Part Weight (in grams) MHX2400 (radio) Structure Battery pack GPS Antenna GPS Receiver PCB boards ~ Fox board ~90.72 Camera ~ nimu Solar cells ~47 Miscellaneous ~400 Total charge of developing this system, while the duty for UH is to develop the system bus (TTC, PGD, and DCH subsystems, and the structure). This iteration will be built as an engineering model, so certain aspects that may be important for a flight ready system, will be left out. Due to the large number of parts, a one and a half sized CubeSat will be used. The first matter (and easiest) of business is the weight budget. Since a 1.5 unit cube can be allotted 1.5 kg of total weight, it would be good to make sure it will not exceed that weight or it could devastating (or it would just require selecting different, smaller parts). Table 1 shows the current weight situation the UH CubeSat. The other major constraint set by the 1.5 unit cube, is the 10 cm x 10 cm x 15 cm size limit. Figure 1 shows the layout of the parts being used within the CubeSat housing (drawn by UH mechanical engineering graduate student, Lance Yoneshige). There is sufficient room for the connectors and wire (not shown) which will be needed to connect each of the circuit boards. There will be five separate circuit boards in the 48

4 CubeSat. Two full boards, one for the PGD subsystem circuits, and the other that will also have PGD components, but it will be shared with the TTC part, the MHX2400 radio from Microhard. On the other side of the satellite, there will be two half boards, one for the nimu and the other Figure 1. Internal Layout Top View. (Courtesy Lance Yoneshige) -1- Axis camera -2- nimu board -3- Fox board -4- GPS antenna -5- GPS receiver -6- battery box -7- PGD board -8- TTC/PDG board for the Fox board (the central processor). Finally, there will be a mini board, that will contain just the GPS receiver and it sits perpendicularly on the PGD board. As far as the structure is concerned, it will contain a separation switch, but due to cost concerns, the springs will be left out (as mentioned earlier, the project is only an engineering model). Fortunately, the original structure design had springs, so if needed, a quick adjustment can be made to include the springs. It has been manufactured by 3V, a company in Tacoma, Washington, and except for a few adjustments (due to late design changes), it is in good shape and up to specifications. The project has not reached the level of any system testing thus far due to unforeseen setbacks, but the individual subsystem development has been moving fairly well and it should not be too large of a task to integrate the subsystems together. STABILIZATION Stabilization is a daunting task for any small satellite. Although, the design and development of any stabilization system is quite complicated no matter the size of the satellite, it becomes a greater problem for a CubeSat because of it size and weight limitations. For larger satellites, it might be easier to design because it has a lot of room and a much greater weight budget to put bigger, better measurement equipment. By having more of these devices, it could make it slightly easier to implement such a system. The CubeSat on the other hand, does not have this luxury. It can only house a limited amount of stabilization equipment while still requiring room for the rest of the subsystems. As the name suggests, the purpose of the stabilization subsystem, or attitude determination and control (ADC), is to be able to determine the orientation of the satellite in space and be able to redirect the way the satellite is facing to a more desirable direction. This application could be useful in a CubeSat such as the one described above, but due to the difficulty of the design and the fact it is simply an engineering model, there will be no ADC included in it. 49

5 DISTURBANCES IN SPACE While in orbit, the CubeSat will be subject to certain environmental torques that will cause it to spin freely. Environmental torques are external forces that could cause the satellite to rotate in certain directions. The first of these torques is the gravitational field. The force of gravity, though much smaller in space, still has an effect on the satellite. Gravity exerts its greatest force on an object on the object s center of mass. If the center of mass is not in the geometric center of the satellite, gravity will favor the side that the center of mass of the satellite is closer to, and it will cause the satellite to rotate in that direction. The Earth s magnetic field is also an important one. Even without magnets in the satellite, the magnetic field can still have an effect on the satellite. The residual magnetic field of the satellite that is produced by the magnetic moment of the satellite, eddy currents, and the hysteresis effect on the materials of the satellite. Another possible disturbance is solar radiation. This comes from the electromagnetic radiation from the sun. The effect of this disturbance is related to the orbit height, and the geometry of the surface that is being hit. A disturbance that will normally affect low orbits, is a sort of aerodynamic torque. This torque is a result of the molecules in the upper atmosphere colliding with the sides of the satellite, and like the effect of solar radiation, the effect of this torque relies on the geometry of the satellite faces being hit. Sometimes certain devices in the satellite can produce disturbances to the satellite itself and needs to be considered (depending on the device, this could be desirable and is used to actually control the satellite instead of cause it to venture off target). An example of an undesirable torque produced by the satellite upon itself is the sloshing of a liquid. This case is usually for micro-thrusters, where a liquid is sometimes used a thrusting fuel and as it sloshes around in its container, could jolt the satellite. PASSIVE STABILIZATION The idea of stabilization is to take advantage of these torques to steer the satellite in a desired direction. There are two types of stabilization; passive and active. Passive stabilization is a method which interacts with the environmental torques to reorient itself. There are two primary ways to implement this, and that is to use the gravitational field and/or magnetic field of the Earth. The simplest way to use the magnetic field is to include magnets in the satellite and setting them up in such way that it will line up with Earth s magnet. Hysteresis rods may also be included to damp the rotation of the satellite and get the satellite to become more and more stabilized. Using the gravitational field is also a fairly simple idea (again, the idea is simple, but the implementation and design could get very messy), and can be done by offsetting the center of mass so that gravity will pull the side that the center of mass is closest to, down toward Earth. This can be done by extending a gradient boom, which is simply a mass extended out to shift the center of gravity towards that mass. ACTIVE STABLIZATION Active stabilization is the type of stabilization that requires the satellite to actively take part in changing its own orientation. This is done by utilizing data taken from on-boards sensors and then using some type of torque producing mechanism to adjust the satellite to an orientation 50

6 that is practical to the mission. These on-board sensors include magnetometers that measure the direction of the magnetic field relative to the satellite; accelerometers that measure the direction of acceleration of the satellite; sun sensors, that determine the location of the sun relative to the satellite; and gyroscope that measure the rotation of the satellite and the direction it is facing relative to an origin. In terms of torquers used, some common selections are micro-thrusters, that use liquid or gas, and shoot out a small amount to force rotation in the opposite direction of the thrust; electromagnets, which is simply a coil of wire that has a current passed through it (this follows the phenomenon that a current flowing through a wire coil acts as a magnet); and momentum wheels that spin within the satellite and use rotational motion effects to redirect the rotation of the satellite. DISCUSSION/CONCLUSION Systems integration, though not a subsystem, playas much more crucial role in the later development of the satellite than in the beginning. If the group working on the project is smaller, it is much easier to coordinate all the parts and interconnections between the individual subsystems. However, as the CubeSat group size increases, it becomes much more difficult to coordinate, in which the role of systems integration greatly increases, since they will need to make sure each subsystem is getting what they need from the other systems. Passive and active stabilization each have their pros and cons, and it is up to the developer to select the method that is best for their mission. But some key points should be noted, although active gives the satellite more control over itself, it does require a much greater dedication to the development of this type of system. It requires much more code and design for such a system to be successful. Also, active stabilization would put more pressure on the PGD subsystem to produce more power because of the added requirements of the sensors and activating the torquers. Of course, if the team has the resources and manpower to do so, it may be an advantage to have an active system. ACKNOWLEDGEMENTS I would like to thank my mentor, Dr. Wayne Shiroma, for his support; the Hawai i Space Grant Consortium, for funding my project and their aid in our trips to Colorado and Utah for small satellite workshops; and the entire University of Hawaii CubeSat team. REFERENCES CubeSat Documents For CubeSat Developers. California Polytechnic State University CubeSat website. < CubeSat Workshop Aug Utah State University. Murakami, B. (2005) The Design, Fabrication and Testing of a Nanosatellite: A How To Approach. Student Paper, University of Hawaii-Manoa. Wertz, James R., Spacecraft Attitude Determination and Control,