POWER GENERATION AND DISTRIBUTION SYSTEM DESIGN FOR THE LEONIDAS CUBESAT NETWORK

Transcription

1 POWER GENERATION AND DISTRIBUTION SYSTEM DESIGN FOR THE LEONIDAS CUBESAT NETWORK Justin M. Akagi Department of Electrical Engineering University of Hawai`i at Manoa Honolulu, HI ABSTRACT The Power Generation and Distribution (PGD) system design of the University of Hawaii Small Satellite Program s Phase III, Ho`okele (Way Finder), is described. The design progression of the PGD system is presented to provide a working reference for small satellite developers. The system is described using the fundamental blocks: Power Generation, Energy Storage and Power Management. Power generation and system-level power management are discussed from both hardware and software standpoints. The final design of the PGD system for Phase III is also presented. INTRODUCTION The University of Hawaii s Small Satellite Program was created with the intent of promoting high-technology research with the State of Hawaii. By creating a university-level satellite program, undergraduate and graduate students will have the opportunity to work on realworld engineering projects. One of the major advantages of these student-driven satellite projects is the cost small satellites may be developed for a fraction of the cost required to develop larger, conventional satellite systems. In addition, small satellites are designed to support experimental payloads, and may be completed within a much shorter period of time. The ability to support experimental payloads also offers an excellent opportunity for student developers to work collaboratively with the scientific community and high-tech industry. TECHNICAL OVERVIEW The Power Generation and Distribution System is responsible for generating and supplying power for the satellite bus and payload. Regardless of the specific design requirements, the basic building blocks for any small-satellite power system are essentially the same. Energy received from the solar cells is used to power the system during sun-on periods, and to recharge the battery pack for sun-off periods. During solar eclipse, the battery is used as the primary power source. The main power line (connected to the solar cells and battery) feeds into a number of DC-DC power converters, which provides the necessary supply voltages for the satellite s electronics. A battery monitor is also integrated into this system to measure sustainable power consumption and facilitate in power management. Fig. 1 shows a basic block diagram of the power system. 1

2 Figure 1: Basic block diagram of PGD system. The basic building blocks within this block diagram are present in any autonomous small-satellite power system. Although every autonomous satellite system has these basic building blocks, each system requires unique power components and circuitry. Specific component selection and circuit design is dependent on a number of system/payload requirements and hardware specifications. POWER GENERATION Spectrolab improved triple junction solar cells were selected based on their highefficiency (26.8%) characteristics [1]. The solar cells are connected as pairs in series, with all of the pairs connected in parallel. This layout is necessary because of the dimensions of the small satellite (10x10x15 cm). Each of the square faces of the satellite (10x10 cm) could fit two solar cells, and each of the elongated faces (10x15 cm) could fit four cells. (The two solar cells in each series pair would be located on the same side.) This design ensures that regardless of the satellite s orientation, both solar cells in every series pair would receive the same amount of solar energy. However, after specifying this solar cell layout, it was determined that external hardware would require a considerable amount of space on the two square faces, and no solar cells could be placed on either of these two faces. Fig. 2 shows the finalized solar cell layout. The solar cell circuit configuration is shown in Figure 3. of solar cells (on the same face) are connected in series, and all of the Figure 2: 2D Solar Cell Layout. Solar cells labeled as SCxx. Empty blocks represent space occupied by other system hardware. Pairs pairs 2

3 are connected in parallel. Figure 3: Solar Cell Circuit Connections. Bypass diodes are built into the solar cells to protect from series connection power drain. Blocking diodes are integrated to prevent power drain caused by unpowered pairs. ENERGY STORAGE Small-satellite developers must also consider the issue of energy storage. During periods of orbit where the satellite is in view of the sun, the power system uses the readily available solar energy. However, during periods of solar eclipse, the satellite must use previously stored energy. To maximize the system s energy storage capacity, lithium-ion batteries are used for our satellite system. Fig. 4 shows a visual comparison of the energy densities of lithium-ion (Li-ion), nickelmetal hydride (NiMH), and nickel cadmium (Ni-Cd) batteries. As the figure shows, Li-ion batteries have a greater energy density (both gravimetric and volumetric) than Ni-Cd and NiMH batteries. Figure 4: Comparison of Energy Density vs. Battery Type (Li-Ion, Ni-Cad, Ni-MH) [2] Based on refined battery characteristic requirements, the UR18650F lithium-ion battery was specified for energy storage for the power system. Based on power requirements, a 2-cell battery pack (4200mAh) was selected. Some of the parameters that affect battery selection include: operating temperature range, maximum load current, charge/discharge cycles, timeaveraged depth of discharge, and power management schemes. 3

4 POWER MANAGEMENT The power requirements for Ho`okele have been modified at least four times due to changes in the payload system design. Although the experimental payload is not a part of the main satellite bus, the power system must be designed to satisfy the specified payload power requirements. Payload system redesign and corresponding power requirement modifications led to many intermediate design changes. Due to the numerous design changes, the high-level design of the PGD system was made modular, so that the power system circuit could be easily modified to accommodate any power requirement changes. These modules may also prove useful for our subsequent satellite projects, in which they may be easily integrated into the power systems (based on design requirements). Table 1 shows a list of the on-board electronic components with corresponding supply voltage and load current requirements. Table 1: Satellite Power Requirements The power converter design for the PGD system is shown in Fig. 5. The converter circuits are designed to satisfy the power requirements specified in Table 1. A 3.3V, 600 ma regulated source [9] powers the GPS unit (Payload), and the I 2 C to 1-wire bridge (PGD). A 5.0V, 1.2 A source powers the satellite s primary microprocessor board, a Linux-based embedded system, and the Ethernet camera (Payload). A 5.4V, 1.2A source supplies the transceiver (TTC) with the required power, and a 6.5V, 300mA source powers the nimu (Payload). 4

5 Figure 5: Power Converter Design Layout After designing the power converter layout, a management scheme needed to be created to manage the limited on-board power supply. Unlike many earth-bound systems, there are no power outlets that the satellite can plug into to satisfy its power needs. Instead, mechanical and electronic switches are required to manage the power consumption of the on-board systems. In addition to the power-switching network, a battery monitor is necessary to facilitate in battery management during normal operation of the satellite. Fig 6 shows the complete power system design, with power management devices. Figure 6: Complete PGD System Design with Integrated Power Management Devices 5

6 During the satellite s normal operation, on-board electronic components (camera, transceiver and nimu) are powered on and off to avoid unnecessarily high power consumption. This power-switching design is necessary to ensure that sufficient power levels are maintained throughout solar eclipse periods. The power switches are implemented in hardware using Fairchild Semiconductor P-channel MOSFETs. The MOSFET is capable of regulating current from the source (input) to the drain (output) using a logic level control signal (gate). When the control signal is set high (5 V), the power switch cuts off power to the electronic component; when it is pulled low (0 V), the current may flow from source to drain. Power management decisions for normal operation flow are based on the current battery status, and will be performed by the satellite s primary microprocessor. The DS2780 battery monitor is capable of performing battery voltage, current flow, temperature, and battery capacity measurements. The telemetry data from the battery monitor is used to facilitate satellite operation flow decisions (e.g.: if there is enough power to operate electronic devices or perform user-defined missions). CONCLUSION The third phase of our Small Satellite Program is reaching its completion goal. In the last few months, our undergraduate team has done a good job designing all of the satellite subsystems. Each of the groups, though nominally different, needed to work together to integrate the entire system. The Power Generation and Distribution System has been a huge task (individually, and for system integration), but our group has done good work designing the system to ensure that it satisfies all of the satellite requirements. ACKNOWLEDGEMENTS I have been working on University of Hawai`i Small Satellite projects for four years, starting with Phase I of the program. The practical experience provided by this satellite research has been an excellent opportunity for me. I would like to thank NASA, the University of Hawai`i Space Grant Consortium, and all the other sponsors of our Small Satellite Program. I would also like to thank Dr. Wayne Shiroma for working so hard to establish the Small Satellite Program at the University of Hawai`i, and for providing his technical experience and guidance throughout the lifetime of the program. REFERENCES [1] Spectrolab webpage. [2] Sanyo Energy (USA) Corporation webpage. [3] ACME Systems webpage. [4] Maxim/Dallas webpage. [5] Microhard Systems Inc. webpage. [6] MEMSense webpage. [7] Garmin International Inc. webpage. [8] Axis Communications webpage. [9] Linear Technology webpage. 6