Satellite Electrical Power System (EPS)

Transcription

1 Satellite Electrical Power System (EPS) Travis McCullar Joshua Stogsdill September 7th, 2007 Advisor : Dr. Jim Lumpp

2 Abstract The University of Kentucky satellite project, KySat, develops small satellites for low earth orbit experiments. One of the most critical aspects of satellite design is the electrical power system, or EPS. The EPS must provide stable power to all of the satellite components by regulating battery and solar cell power sources under extreme temperature variability. The problem is that EPS boards for satellites are very expensive due to lack of competition in the marketplace. It would greatly benefit the satellite program to both design and build a suitable EPS that would be more reasonably priced. For this project, the board currently used in the system made by Clyde Space (Clyde Space : Solutions for a New Age in Space) will be examined, along with EPS designs for other University satellite programs. After these designs are understood, a set of appropriate specifications will be generated. From these requirements, a mock up circuit, containing both digital and analog components, will be designed. After simulating the circuit and confirming correct operation, components for a prototype will be ordered and assembled. After rigorous terrestrial testing, the final test will be for the EPS to be placed in service aboard the 2008 KySat. The impact of a reasonably priced EPS is multifarious: Available funds could be redistributed to costly experimental equipment Financial feasibility for satellite programs in small Universities Increased competition in the marketplace sparks innovation A proper design of a satellite EPS must not only include the ability to provide reliable power from a set of rechargeable batteries, but also to charge the batteries from solar cells in such a way as to allow the batteries to be operational for as long a time as possible. Once these batteries have passed their useful lifetime, an important feature of our design will allow for the essential systems of the satellite to continue operation solely based on solar power.

3 Introduction The type of satellite employed by the KySat team is known as a CubeSat (Wiki : CubeSat), so named by the fact that the satellite itself is a cube of side length 10cm. Historically, this design was pioneered by Stanford University and California Polytechnic to allow for educational access to space science and exploration. The CubeSat standard is based around the stackable PC 104 (Wiki : PC104 Specifications) form factor circuit board. All electrical components of the satellite conform to this PC 104 standard. All CubeSat satellites require some type of Electrical Power System (EPS) to manage power flow within the satellite. The EPS is responsible for managing power input from solar cells, charging of on board batteries, and providing electrical power, at the required voltages, to all other satellite components. The EPS also provides services such as over current and under voltage protection, as well as supplying the main satellite microprocessor with telemetry data about its operations. Major limitations to EPS design include the inability Lithium Ion or Lithium Polymer batteries to charge at temperatures below a certain threshold and space/size limitations associated with the PC 104 form factor. The KySat team currently purchases their EPS from Clyde Space, a Scottish company which specializes in satellite power supplies of all sizes. We are attempting to produce our own design of an equivalent EPS to replace the Clyde Space EPS in future KySat satellites. Our main goal in producing our own EPS is to reduce cost and allow for more flexibility in overall satellite design.

4 Background The CubeSat program was first introduced in 1999 in a joint effort of Stanford University and California Polytechnic University by Professors Robert Twigg and Puig Suari (PolySat) respectively. The initial power systems designed for the first satellites are now used mostly as reference designs for smaller satellites (picosats), but there are two cases of larger satellites that provide us an ample amount of information on the proper design of an EPS, the Bogota EPS project and the Clyde Space design. The Bogota project was faced with the very same problem found in our design, efficient power system on a low budget. Their design was for the most part a success, and they launched their first satellite, the Libertad 1 (Wiki : Libertad 1)on April 17, 2007 from the Cosmodrome of Baikonur, Kazakhstan. The Libertad 1 EPS is a derivative of the KatySat design, a program adopted by Stanford University initially to interest a younger crowd in space exploration and experimentation. The Bogota project used the Rev 1.0 (basic EPS board of the KatySat design) as a reference to build the EPS that was launched on the Libertad 1. Clyde Space was founded by Craig Clark, who has more than eleven years of experience working in satellite technology, in Under him, Alex Lopez, is the senior engineer and principal designer of the flagship EPS board for Cubesats, and the current board used in the University of Kentucky satellite program, KySat (KySat). KySat is a joint program that enlists 5 different Universities internal to the state of Kentucky. Each of these participants takes a part in the design and construction of the satellites. So far KySat has completed one picosat conveniently named KySat 1. This satellite is due to launch early next year, and the construction of KySat 2 is currently under way. Impact Statement The EPS should be comparable to the industry standard, designed by Clyde Space, and competitive with other university systems implemented by Stanford University and the University of Bogota. The final product will consist of a unit that is sufficient for monitoring power flow on any CubeSat project, as well as an ability to send power related telemetry back to the user through the satellite s radio system. Due to high prices of other EPS devices on the market right now, a new system will allow other universities to begin satellite projects without dedicating a large portion of their budget to power monitoring. The EPS will meet the demands of potential users, as well as meet the environmental and safety standards necessary to be launched into space.

5 Technical Descriptions The specifications for the EPS are as follows: Dimensions cannot exceed 10cm by 10cm; vertical dimensions are flexible, but should be kept to a minimum A single PC104 connection is used to communicate with all other satellite systems; Six solar cell arrays will be connected as power input Three separately regulated, over voltage/over current protected busses a 5V, 6W; a 3.3V, 5W, and battery voltage (directly connected to batteries), limited at 10W Battery heaters must exist to maintain battery operability where possible; solar power will be routed to the heaters if the temperature is below battery charging/power providing threshold; Battery charging circuitry must be designed so as to maximize the performance of the batteries by charging/discharging in such a way as to prolong lifetime In the event of battery failure, the satellite must continue to operate with reduced functionality on solar power The satellite circuit board and batteries must be able to survive temperature swings from approximately 100 to 100 C. Solder melts readily at 180 C (Wiki : Solder), making a special solder unnecessary. Figure 1: Block Diagram / Flowchart

6 Block description: 1. The six respective solar cell arrays are connected to the EPS using connectors on the surface of the circuit board. 2. These connectors are placed in series to allow for maximum voltage and connected to regulation circuitry. The regulation circuitry will include a boost converter (Wiki : Boost Converter) to ramp up the voltage to necessary levels to power the satellite systems and to charge the batteries. 3. Once the solar cell power is sufficient to power the onboard microcontroller, it first checks the temperature of the batteries by monitoring the resistance of an internal thermistor. If they are warm enough to be charged, the microcontroller will start the charging cycle; otherwise, available power will be routed to heating elements to bring the batteries up to charging temperature. This temperature will be continually monitored and power will be routed as necessary to keep the batteries at charging temperature, typically between 0 to 45 C. 4. Once the batteries are at charging temperature, the microcontroller will start the charging cycle, provided the batteries pass all other qualifications (such as voltage above a lockout threshold) (Power Management in Portable Applications : Charging Lithium Ion/Lithium Polymer Batteries). 5. If the batteries do not respond after a period of time, the microcontroller will cease to attempt the charge for safety, and continue running on solar power.

7 Time Line Below is the tentative Gantt chart for the satellite EPS project. ID Task Name Duration Start Predecessors September 2007 October 2007 November 2007 Decem Project Proposal 6 days Thu 9/6/07 2 Word Document 5 days Thu 9/6/07 3 Pow erpoint Presentation 1 day Thu 9/13/ Research and Design 9.75 Thu 9/13/07 3 days 5 Voltage Regulation 2.5 days Sun 9/16/ Boost Converter 4 days Fri 9/14/ Battery Charge Controller 4 days Sat 9/15/ Communications 4 days Thu 9/13/ Software Simulation 3.31 days Thu 9/27/07 10 Boost Converter 1 day Thu 9/27/ Voltage Regulation 1 day Thu 9/27/ Ordering Components 11 days Tue 10/2/07 9 Travis McCullar Travis McCullar 13 Finding Components Online 4 hrs Tue 10/2/07 14 Placing Orders 1 hr Wed 10/3/ Waiting for Orders to Arrive 10 days Wed 10/3/ Bread Board Testing 7.25 Wed 12 days 10/17/07 17 Voltage Regulator 2 days Wed 10/17/07 18 Battery Charge Controller 3 days Wed 10/17/07 19 Boost Converter 3 days Thu 10/18/07 20 IC2 Communications 3 days Wed 10/17/07 21 PCB Production 1.56 Mon 10/29/07 16 days 22 Board Procurement 1 hr Mon 10/29/ Surface Mount Soldering of Main Board 4 hrs Tue 10/30/ Surface Mount Soldering of Battery Board 4 hrs Tue 10/30/ PCB Testing Wed 21 days 10/31/07 26 Battery Board 0.13 days Wed 10/31/07 27 Smoke Test 1 hr Wed 10/31/07 28 Main Board days Wed 10/31/07 29 Smoke Test 1 hr Wed 10/31/07 30 Boost Converter Test 1 day Wed 10/31/ Regulator Test 3 hrs Sun 11/4/ Battery Charging Test 2 days Tue 11/6/ Solar Cell Testing 4 hrs Mon 11/12/ Temperature Variability Testing 1 day Tue 11/13/ Communications w ith Other Satellite Sys 1 day Thu 11/15/ Final Presentation Preparation 3.63 Sun 11/18/07 25 days 37 Poster 1 day Mon 11/19/07 38 Word Document 1 day Mon 11/19/07 39 Prototype Demonstration 1 day Sun 11/18/07 40 Quality Check for Poster/Doc/Prototype 1 day Mon 11/19/07 Travis McCullar Travis McCullar Travis McCullar

8 Distribution of Effort From the Gantt chart above, particular aspects of the project have been divided by group member as detailed below. will be primarily in charge of the battery charging, testing, and management. Additional duties will include finding and ordering parts, as well as assuring quality on the final prototype, document, and poster. will be in charge of designing the communication logic to interface with satellite systems not present on the EPS. He will also be performing solar cell and temperature variability testing, as well as designing the final presentation poster. will design and test the boost converter, allowing the low voltage from the solar cells to provide the power necessary to charge the batteries and to operate satellite systems in the event of battery failure. He will also be the primary author of the final project document, and construct the main EPS circuit board. Travis McCullar will be designing, constructing and testing the voltage regulation circuitry. He will also be implementing the final prototype board for senior design day. Deliverables Deliverables will include a prototype EPS board, battery connector board, project document, and project poster. Summary of Project Ultimately, the goal of this project is to design and integrate a low cost electronic power system (EPS) for primary use in the KySat project. The EPS will convert solar energy into a usable means of electricity to power the microcontroller first and foremost. Secondly, it will regulate Li (ion/poly) battery charging and various bus voltages that are needed; the busses will also be protected from over voltage and over current incidence. Finally, due to the batteries operating inefficiency at low temperatures, the solar power will supply power to filaments beneath the batteries for heat. At the conclusion of our project we hope to make educational satellite research a little less expensive, thereby allowing a larger portion of the budget to blaze new trails in future educational space exploration.

9 Works Cited Clyde Space : Solutions for a New Age in Space. (n.d.). Retrieved September 10, 2007, from Clyde Space: space.com KySat. (n.d.). Retrieved September 10, 2007, from PolySat. (n.d.). Retrieved September 11, 2007, from Power Management in Portable Applications : Charging Lithium Ion/Lithium Polymer Batteries. (n.d.). Retrieved September 10, 2007, from Wiki : Boost Converter. (n.d.). Retrieved September 10, 2007, from Wiki : CubeSat. (n.d.). Retrieved September 10, 2007, from Wiki : Libertad 1. (n.d.). Retrieved September 10, 2007, from Wiki : PC104 Specifications. (n.d.). Retrieved Semptember 10, 2007, from Wiki : Solder. (n.d.). Retrieved September 11, 2007, from

10 Budget / Parts List Function Possible Suppliers Approximate Cost Expected Shipping Time Solar Cell Connectors DigiKey $1 each < 1 week Battery Charge Regulator (BCR) Microchip $5 each < 3 days Lithium Polymer Batteries PC 104 Board Clyde Space, Power Stream UK Local $8 each < 1 week Boost Converter Linear Technology $3 each < 1 week 3V/ 5V Voltage Regulators TC105 Microprocessor MSP430, PIC MCU Microchip $2 each < 1 week TI, Pic $10 < 1 week

11 Biographical Sketches is currently a student at the University of Kentucky in the University Scholars program, a program allowing graduate student status for senior undergraduates. As such, he currently works for Dr. Adams as an electronics teaching assistant, and previously as a research assistant in the area of computational electromagnetics while pursuing a Bachelor s and Master s degree in Electrical Engineering. Previous education includes an Associate s degree in Network Technology. Research interests include computational electromagnetics, audio electronics, and analog/digital controls. Joshua Stogsdill is currently a senior at the University of Kentucky pursuing a double major in Electrical Engineering and Computer Engineering with minors in Mathematics and Computer Science. He currently works part time in network administration and live sound production. Research interests include audio systems, communications systems, and networking. is an undergraduate senior at the University of Kentucky currently pursuing a Bachelor s degree in Electrical Engineering with minors in Mathematics and Computer Science. Experience in power systems includes an electromechanics, and other courses related to design and implantation include a digital logic class with lab and several computer architecture classes. Travis McCullar is an undergraduate senior at the University of Kentucky. He is currently pursuing a Bachelor of Science degree in Electrical Engineering. His experience in power systems include courses in electromechanics and a power systems lab. Other project related courses include electronics, digital logic, and C++ programming.