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Multidisciplinary Senior Design Conference Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York 14623 Project Number: P12408 NEAR SPACE SOLAR POWER CONDITIONING MODULE Stephen Giannotti Electrical Engineering Michael Hudak Computer Engineering Jacob Emenheiser Electrical Engineering Donald Lucas Mechanical Engineering William Dorney Electrical Engineering Abstract High altitude ballooning has been a method of scientific research for years and is a growing activity among electronic hobbyists. These balloons ascend into a near space environment at an altitude of approximately 100,000ft (30,000m) above the earth’s surface into the atmosphere. Each balloon is typically equipped with communications equipment, a GPS, and data acquisition hardware. Standard high-altitude balloon flights last less than 24 hours and are considered short-term flights. The goal of this project was to provide an efficient and reliable power supply module for a long duration flight high-altitude balloon payload. The Near Space Solar Power Conditioning Module (NSSPCM) utilizes the availability of sunlight to collect radiant energy and convert it into electrical energy. This system is designed to be a separate module from a main payload in the high-altitude balloon multi-component configuration. The module met or exceeded the majority of the design requirements. It supplied the required power, gathered accurate sensor data, communicated with the mock payload and was contained within a structural sound enclosure. Nomenclature NSSPCM: Near Space Solar Power Conditioning Module Payload: Module external to NSSPCM. Device that NSSPCM powers and communicates with. MPPT: Maximum Power Point Tracking: The Genasun solar battery charge controller. Introduction Electronics within a high-altitude balloon payload are generally powered solely by batteries. Once the battery is depleted, there is no other power system available. The NSSPCM project seeks to provide a means of charging a battery using solar energy to allow for longer missions with lower capacity batteries. This project was started from members’ work with the successful launch of RITCHIE-1; the RIT Amateur Radio Club’s High-Altitude Balloon project in May 2011. RITCHIE-1 was designed as a standard short-duration flight high-altitude balloon payload to provide telemetry data and photographs. The NSSPCM senior design project was conceived to create a system that could support a second iteration of RITCHIE-1. Design Process The NSSPCM design needed to provide a system that can harvest solar energy and convert it to electrical energy for storage and distribution. Electrical energy from the battery needs to be supplied across a 2m cable to an external payload. The payload requires a 12V supply with an average load of 2W for three days with a maximum Copyright © 2012 Rochester Institute of Technology Page 2 load of 5W for 6 minutes a day. The design also required the ability to monitor system inputs and outputs for data collection from sensor circuits. A microcontroller allows for system monitoring and control with configured circuitry and firmware. In addition to the power output, the NSSPCM needed to provide a communication output for data transfer. For structural support and protection from environmental conditions, a mechanical structure needed to be designed. It had to be lightweight, yet robust enough to withstand harsh environmental conditions throughout the length of the flight and survive a return descent. Module environmental conditions include an external temperature range of -40ᴼC to 45ᴼC, and the absence of convection due to a vacuum. The module must survive an impact velocity of at least 7 m/s to ensure adequate structural strength. Figure 1: NSSPCM High-Level System Design Solar Energy Harvesting Method The solar panel chosen had to produce adequate energy during the day to both power theload and charge the battery. In order to stay within the NSSPCM total weight requirement of 6 lbs, careful consideration was taken in choosing the type of solar panel. With the potential for adverse weather conditions, it was imperative that the solar panel be robust. In addition, the minimum solar panel voltage acceptable for this project was 12.5V. The culmination of criteria lead to the selection of a flexible thin film solar module. Under stress from the environment these solar panels can bend unlike rigid solar panels which may crack instead. Thin film is lighter weight and does not require the heavy case of a traditional solar panel. All of these requirements were met by the 1 meter (WSLE0240-24-ST-06-B) semiconductor type copper indium gallium selenide made by Ascent Solar. It has a nominal power of 24W, voltage (Vmax) of 16.7VDC and weighs 0.75 kg (1.65 lbs). Maximum Power Point Tracking Since the system power relies on the output of a solar panel, it was necessary to use a maximum power point tracking (MPPT) algorithm. Implementing a MPPT method allows for the system to intake the greatest power possible from the solar panel. For the scope of this project, it was decided to purchase a prefabricated MPPT battery charger module and integrate it into the system. The product chosen was the GV-5-LI-12.5V Low-Power Solar Charge Controller made by Genasun. Designing a custom built MPPT algorithm in either hardware or software was not feasible because of additional complexity that it would add to the project. Project P12408 Proceedings of the Multi-Disciplinary Senior Design Conference Page 3 Battery The combined daily requirements for the NSSPCM circuitry and load was determined to be a charge of 6.78 A-hr at 12V resulting in 81.36 W-hr of energy needed. The NSSPCM was designed to provide power to a load expecting 10 hours of light and 14 hours of darkness. In order to provide for greater power needs, a large safety margin of additional energy storage was deemed necessary. Since there were weight restrictions for the NSSPCM module, the type of battery used had to have a high power to weight ratio. The final choice was a 11.1V nominal voltage, 10.4 A-hr lithium-ion rechargeable battery pack made by Tenergy. At full capacity the battery will be 12.6V because lithium-ion cells increase in voltage as they are charged. This specific pack included an integrated protection board to properly balance each cell. System Circuitry The design of the system circuitry was dependant on the power requirements for the internal system and the payload. To power different components from a common source, 3.3VDC was declared to be the internal voltage supply. The output voltage supply was already determined from the engineering specifications to be 12VDC. Two pre-fabricated DC-DC converters were purchased and integrated into our design to provide the regulated voltage needs. For the 3.3VDC converter, a 2.64W (CC3-1203SF) DC-DC converter by TDK-Lambda was used. To provide 12VDC to the payload, a 20W (RPP20-2412SW) DC-DC converter by Recom was used. Design of the sensing circuitry to measure voltage and current required the balance of several important factors. The circuits first had to collect accurate sensor data and deliver it safely to the ADC pins of the microcontroller. Secondly, sensing of each value required a simple low power solution. Lastly, the sensing circuits could not interfere with the operation of other circuits in the module. These requirements were met by using precision sense resistors, high resistance voltage dividers and high accuracy temperature sensors (LM35). Each of the sense lines leading to the ADC pins of the microcontroller has a normally open analog switch for protection and a resistor and capacitor for noise filtering. The analog switches are controlled by the microcontroller, so that the pins will be protected any time the microcontroller does not have power. Printed Circuit Board Since the circuitry used surface mount and through-hole components, a two layer board was determined as the best option for a printed circuit board. A 6 inch by 8 inch board was designed to provide not only an interface for the system circuitry, but also a support for the MPPT and battery. The concept was to draw heat away from the active components in the circuits to keep the battery within the proper operating temperature range. Most of the bottom side of the board as well as the area under the battery and MPPT act as a combined ground plane and heat sink. This concept serves as a passive heating method illustrated in figure 2. As many of the surface mount components as possible were selected to be a size of 1206 to ensure they were large enough for hand assembly. The PCB layout and fabrication was provided by Elmgrove Technologies (EGT), a division of Photonamics, Inc. This donation to the project was critical to keep the project under budget. Figure 2: Printed Circuit Board Conceptual Heat Distribution Diagram Copyright © 2012 Rochester Institute of Technology Page 4 All small pitch components were placed first followed by the larger surface mount components and finally the through-hole components. Initially, the jumpers were not placed on the board so that the circuit could be powered and tested one section at a time. Microcontroller and Firmware The MSP430F2234 was selected because of its low power consumption, 12 ADC channels, and hardware UART for serial communication. This chip also has the capability of being programmed via a 2-wire spy-bi-wire interface from the MSP430G2 Launchpad, which is a low cost development board for Texas Instruments' value line series of microcontrollers. The MSP430F2234 has 512 bytes of RAM and 8 kilobytes of flash memory. The programming tool, Code Composer Studio, is based on Eclipse open source framework and is free for code sizes up to 16 KB. The MSP430 firmware is written in C. The major points of concern when designing the firmware were accuracy of the sensing circuits, the speed at which the sensors could be updated, and the speed at which the module status could be transmitted to a payload. The sensing circuits connect to analog switches which prevent damage to the microcontroller in the case that power is applied to the sensors, but the microcontroller is powered off. When power is applied and the microcontroller initializes, the switches are closed, enabling the sensors to be read by the ADC. Because the sensor values do not change at a fast rate, the sensors were read once per second allowing the microcontroller to remain in low power mode during idle time. All values of the sensors are stored as 16-bit unsigned integers in raw ADC format except for the temperature values which allow for negative values. Equations are provided for a human readable interface to convert the raw ADC format into the sensors’ actual values. Fault conditions were defined to indicate mission critical problems which can be detected and possibly corrected in flight. These include temperature out of range and low battery fault information sent to a payload for communication with the ground or decision making. During the design process, the use of the MSP430’s internal temperature sensor and voltage supply sensor was added. The supply voltage sensor was added to aid in the accurate conversion of raw ADC values to actual values. The firmware was also designed to be interrupt driven. This design approach allows the microcontroller to take advantage of low power modes as well as keeping the CPU free to work on needed tasks. The code was also designed to include multiple code blocks grouped by function. The blocks include clock functions, error checking functions, general purpose input/output functions, communication functions, sensor functions, and timer functions. Code relevant to each set of functions was contained within the include file. This allows for more flexibility and clarity of code implementation. Communication A simple and effective communication method was required for the NSSPCM to receive a query from a payload and transmit a response to a payload. Due to weight concerns of a communication cable, a minimum number of wires between the NSSPCM and a payload was required. RS-485 at the physical layer was chosen due to its resistance to noise (differential signaling), the ability to connect multiple transceivers to a communication bus running between different packages of a high altitude balloon assembly, and it's use only 3 wires. The use of a MAX3483 chip allowed for low power consumption while still supporting the RS-485 communication standard. This chip was easily interfaced with the UART of the MSP430F2234. Structural Design The main structure of the NSSPCM is constructed of aluminum framing. Aluminum provides the necessary strength while allowing the module to remain light weight. Aluminum also offers several options for easy manipulation which made that material an ideal choice. The overall structure of the module is represented in three basic parts: the enclosure, the cradle, and the panel mount. The interaction of each part is as follows. The enclosure is an expanded polystyrene case. This case measures externally 9 inches by 11 inches by 7.5 inches. The enclosure has a uniform thickness of 1.5 inches which provides the needed insulation to maintain a safe operating internal temperature for the duration of the flight. The enclosure is encased by the cradle. This network of aluminum strapping (1/16 inch thickness) allows for structural support of the enclosure as well as convenient means of mounting to other components. These two components together are attached by four (4) ½ inch hex-head machine bolts to the panel mount. The panel mount is a structure that outlines the solar panel and provides mounting points to attach the enclosure. Project P12408 Proceedings of the Multi-Disciplinary Senior Design Conference Page 5 The Enclosure The expanded polystyrene enclosure is used to protect and insulate the system electronics. The PCB is designed to be a snug fit within the enclosure to minimize effects of in-flight turbulence. The enclosure’s thickness of 1.5 inches provides the necessary insulation to protect the circuitry, and components from both freezing during the night and overheating during times of direct sunlight contact. To mitigate the effects of heating due to solar radiation, the enclosure is completely covered with aluminum foil. The foil acts as a reflective agent that will deflect or reflect the majority of the suns radiation. This effect coupled with the insulation provided and the circuitry design will allow the internal environment of the enclosure to remain relatively constant. The thickness also provides significant structural protection that eliminates the need for complete encapsulation using an aluminum structure. Aluminum strapping was used to provide minimal protection and generate mount points. The Cradle The cradle is constructed of 1/16th inch thick, ¾ inch width aluminum strapping. This strapping is easily manipulated into the needed shapes without extensive or complex machining. The thickness is sufficient as the frame will be supporting the enclosure in tensile loading while allowing the NSSPCM to remain as lightweight as possible. The enclosure containing the PCB has an estimated weight of 3.75 pounds and is well within the allowable limits of the material used. The unit is constructed of three (3) individual pieces of aluminum that were independently bent to fit the enclosure. The pieces were then welded into the structure pictured. This structure provides a firm press fit with the enclosure which is then secured in place by the attachment of the panel mount. Figure 3: Cradle Panel Mount The panel mount provides structural support to the flexible cell used. The outer frame is constructed of part 1/8th inch thickness aluminum strapping and part 1/16th inch thickness aluminum. These two parts are used to secure the panel on the frame. Holes through both the frame components as well as the panel’s edge allow for 20 machine bolts to be used to tighten the frame onto the panel. These bolts utilize lock-nuts to minimize the risk of in-flight loss due to vibration. The lower section (1/8 inch thickness) of the panel frame is welded to the shown cross members. These cross members (marked by blue triangles) are used as mounting point to the cradle. The outer cross members (marked by red diamonds) provide the mounting locations for the suspension wire that will be used to tether the NSSPCM to a high-altitude balloon. Figure 4: Solar Panel Frame Copyright © 2012 Rochester Institute of Technology Page 6 Results and Discussion Figure 5: Assembled NSSPCM The NSSPCM module structure was successfully assembled and correctly houses all components. The PCB fitted as anticipated into the enclosure and the lid was held on securely by the frame. During the drop test, the structure frame suffered significant permanent bending due to the load of the solar panel. All bending was able to be repaired without use of tools and did not adversely affect the overall integrity of the frame or enclosure. The fully constructed module weighs a total of 6 pounds, 10 ounces, which falls short of the requirement of 6 pounds. During assembly, testing and troubleshooting, some changes were required to the PBCA to ensure proper operation of the circuitry. The silk screen of the component internal temperature sensor U4 was mirrored on NSSPCM-PCB-001 REV 1.0. The part was placed in the opposite orientation from the silk screen to ensure the proper electrical connections and operation of the circuit. The output of the 12 volt DC-DC converter is electrically isolated from the input. The sensing circuitry on the output line was referenced to the output (-). Therefore, a connection between output (-) and system ground was required so that the sensing circuitry had the same reference as the ADC of the microcontroller. In the layout of NSSPCM-PCB-001 REV 1.0, the footprint of U6 was incorrect. The issue was resolved for this build by placing the part upside down on the PCB and connecting it in a dead-bug style. A subsystem test was conducted to analyze the amount of energy that could be collected in one day. Measurements were taken from 7:30 AM to approximately the time when the sun was highest in the sky (solar noon) 12:30 PM. These values were then extrapolated for the latter half of the day. The total energy collection for one day was calculated to be 118 W-hr which surpasses the required energy collection by over 36 W-hr. This test proves the solar panel is capable of powering our module. Figure 6: Solar Panel Test Power Output The NSSPCM met the power supply requirements of providing 12VDC to an average load of 2W and a max load of 5W. Engineering specifications require the output be protected from over current. This was to be accomplished by limiting current draw to 1 amp. During the design, it was decided not to fuse the output and rely on Project P12408 Proceedings of the Multi-Disciplinary Senior Design Conference Page 7 the overload protection within the Recom 12V DC-DC converter. The maximum rated output current of the 12V DC-DC converter is 1.67A and that does not meet our specification of 1A. When testing the overcurrent protection of the 12V DC-DC converter, a 5 ohm load was connected to cause the output to shut off at approximately 2 amps of output current. The DC-DC converter however did not shut off and supplied a measured 11.95VDC with a current of 2.29A (27.36W). The test was not continued because the system was already drawing approximately 3A from the battery which was more than expected to handle from the design. This downfall in design must be accounted for in future iterations. In its present configuration, all current draw considerations must be accounted for by the payload. To prove that the module would supply the required power over a 48-hour span, a longevity test was conducted. The solar panel was placed outdoors and the module was powered-on to supply 12V to the average load of 2W. The first test run lasted approximately 15.5 hours and was stopped due to adverse weather conditions. A second test iteration lasted approximately 64 hours. It was observed during both tests that the module adequately provided power to the load over the course of multiple light and dark cycles. From collected data, it was also observed that the battery was adequately charged throughout the day to provide enough power during periods of no sunlight. The PCB passive heating method was tested by measuring various spots on the PCB every 15 minutes for two hours while the module was in operation. The test results showed that the heating method worked according to design in a convection present environment. The temperature measurements showed an increase in temperature from the start of the test to the end of the test. The most important observation was that the bottom of the battery yielded the highest temperatures. This result is critical because the battery is the most temperature sensitive device in the module and requires a strict operating range The firmware was tested using a custom coded Diagnostic Utility containing a graphical user interface (GUI) written in Java. The purpose of the Diagnostic Utility was to act as a payload connected to the NSSPCM. The interface was able to issue commands to the NSSPCM and display results. The internal functions of the program relating to serial interface were C code from the NSSPCM microcontroller converted to Java code. The equations used to convert raw ADC values into actual measured values were included in the Diagnostic Utility. The interface provides a way for a user to interact with the NSSPCM, verify that the communication bus is active, and read the converted values of the sensors. The interface also reports the time taken from command issue to response returned for PING and GET_STATUS. All tested response times were below the 1 second specification. The average round trip time for a PING command is 810 ms and the average time for a GET_STATUS command is 874 ms which is within specification. The Diagnostic Utility can also save data at specified intervals to a comma separated value file which is useful for diagnostic data logging. Temperature and voltage sensor values reported by the Diagnostic Utility were accurate and exceeded specification. The largest challenge for the communication subsystem was timing. The Diagnostic Utility requires 200 ms between transmitting a request to the NSSPCM and beginning to listen for a response. This is dependent on the hardware from which the Diagnostic Utility is run. A manual 200 ms delay was applied in the executeMessage function to accommodate this. Additional 2 ms delays were required in the transmit Message function to allow for proper reception of bytes in the Diagnostic Utility. Originally, we had planned on using the MSP430 Launchpad’s UART to communicate with the NSSPCM. Using this hardware created two problems. The Launchpad’s maximum rated speed was 9600 baud instead of the intended 19200 baud. Because the communication speed still remained within specification, the timeouts and UART settings were reconfigured for 9600 baud. A second issue with the Launchpad was the lack of a request to send (RTS) signal from the UART which would enable the driver of the MAX3483 circuit and allow the Diagnostic Utility to transmit a request then return to receive mode. This was solved by propagating the transmit signal from the Diagnostic Utility through an MSP430G2231 microcontroller to the MAX3483 chip. The MSP430G2231 would detect the start bit of the UART transmission and enable the RS-485 driver for the duration of the transmission. The ability of the NSSPCM to transmit alerts to the payload was disabled due to communication conflicts. The Diagnostic Utility would cease to function if it attempted to transmit a query to the NSSPCM while it was simultaneously receiving an alert from the NSSPCM. This occurred because a collision avoidance mechanism was not implemented in the Diagnostic Utility communication code. Because alerts were not a specification, they could be disabled safely and the Diagnostic Utility continued to have the ability to receive fault information by polling the NSSPCM. Copyright © 2012 Rochester Institute of Technology Page 8 Conclusion and Recommendations There are several issues that would need to be addressed in a second iteration of this project. First, the following errors with the PCB need to be addressed: correct the silk screen of the internal temperature sensor U4, connect output(-) and system ground with a trace and correct the footprint of the 3.3V DC-DC converter U6. In order to allow another step in the circuit testing it would useful to add a jumper at the output of the 3.3V DC-DC so that the microcontroller circuits can remain unpowered while the power path is being tested. Finally, the connectors on the PCBA are very convenient for testing and troubleshooting, but they should be removed for dependability during flight. Regarding the mechanical structure, the next iteration it would be recommended that support braces are added to stiffen the junction between the cradle portion of the frame and the solar panel frame. Thicker aluminum stock would also eliminate a significant amount of deflection, but would dramatically increase the weight of the module. Using a more shallow enclosure will decrease the amount of building materials and in turn decrease weight. For future firmware development and testing, it is recommended that a USB to RS-485 converter be used instead of the MSP430 Launchpad. Using the converter, a higher baud rate can be achieved as well as eliminating the need for an additional microcontroller, driver chip and breadboard. Also, it is recommended using a different serial interface other than the javax.comm library. This library is no longer supported by Oracle and requires files to be installed on the computer to interface with the serial port. It would also be useful to add collision avoidance to the communication code of the Diagnostic Utility. To better test and investigate the NSSPCM passive heating method, a vacuum chamber needs to be used to eliminate the effect of convection. A vacuum chamber is needed to properly simulate the harsh conditions of a near space environment. In order to avoid damaging the payload, special care must be taken to limit the output power to within an acceptable range. This may be accomplished by replacing the 12V DC to DC converter or implementing a fuse. It is possible that the heat sink of the NSSPCM circuit board interfered with the operation of a thermal cut-off within the DC-DC converter and caused the device to stay on by dissipating the converter heat throughout the board. Further tests need to be conducted on the device independently to determine the cause of these results. After design, building, and testing the NSSPCM, it was determined that the device functioned properly and met the majority of the requirements. A second iteration of the project can take note of the specifications that were not met and further develop the design. Acknowledgements Faculty Guides Vince Burolla Leo Farnand Dr. Dorin Patru Additional Support Gordon Davies Project Sponsers RIT Multidisciplinary Senior Design Department RIT Electrical Engineering Department Elmgrove Technologies Special Thanks Jeff Lonneville – CEMA Laboratory Manager Elle Canales – Logo Designer Lowes Henrietta Bergquist Company Project P12408