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Wireless Temperature Sensor Final Design Report 4/15/2007 Tye Reid Greg Swanson Daren Berk James Wagoner Executive Summary The purpose of this project was to develop a wireless sensor system for future spacecraft and probes. Currently, the missions that do fly with sensors have them wired into the Thermal Protection System (TPS) of the spacecraft. Utilizing this architecture adds risk to the system due to the process of routing wires in the TPS and the difficulty of jettisoning the system after entry. Many current and past spacecraft engineers have decided not to fly embedded sensors in an effort to mitigate the risk of spacecraft failure during atmospheric entry. A wireless instrumentation system will solve these problems. The completed design of the wireless sensor system will be a multi-stage project spanning several years. Team ThermaSense’s focus was to research the current state of wireless transmission, design a wireless temperature sensor and then develop a prototype. To achieve this goal, off the self components were researched and purchased to create a table top demonstration that confirmed the success of the new wireless thermocouple system. Once the prototype system was operational it was then tested in the NASA X-Jet facility to simulate atmospheric entry. Three main wireless transmission types were researched for the system: RF, LED, and Near Field Magnetic Communication. After analyzing each type of transmission, RF was determined to be the best choice for the team’s needed application. Next, two main RF technologies were considered for the wireless system, Bluetooth and Zigbee. Since Zigbee is used for low power consumption situations, the team decided to use it for prototyping. After the Zigbee was chosen as the wireless transmission technology, the team designed a circuit to collect the data from the thermocouple sensors. Voltage signal from the thermocouple was connected to a cold junction correction chip that also does analog to digital signal conversion and amplification. This data was sent to a microcontroller that attaches an identification number and the time the data was recorded. The microcontroller is then programmed to send the data through an inverter to the Zigbee transmitter to be relayed to the Zigbee receiver. The receiver transfers the data to the 2 software created by the team via USB to a computer system. The software compiles the information and exports it to an Excel spreadsheet. For proof of design concept, testing was conducted in an X-Jet Chamber at NASA Ames test facility. Four thermocouples where embed at different depths in the LI-900 TPS material. Three Antennas were tested, the PCB antenna, the whip antenna, and the external antenna. Power settings were also tested at low, medium and high power. Testing proved that the best solution for the wireless transceivers was medium power with the whip antenna. The system designed by Team ThermaSense is proof that Zigbee technology is a feasible solution to NASA’s wired sensor problem. Initial analysis shows integrating sleep mode into the sensors design will allow for five to six years of operation from two AA batteries. The team’s final prototype weighs 0.2 lb reducing sensor system weight by an estimated 50%. Not only does the prototype reduce the weight of the spacecraft carrying the system, but it also reduces space required for the system. The next stage of this project will be to develop a flight test for the prototype, working towards the goal of implementation into NASA missions. A successful flight test will require a senor software update. Other areas of work needed to achieve implementation goal would be; powering the sensor, programming the sensor for advanced power savings capabilities, determining the transmitter’s interference tolerances, miniaturization, transmission through RF non-transparent material research, near field magnetic transmission research and finally a protective packaging for the wireless transmission circuit. 3 Table of Contents 1. Background and Concept 1.1 Background 6 1.2 Problem Definition 6 1.3 Concept Considered 7 1.4 Concept Selected 10 2. Product Description and Testing 2.1 Product Description 11 2.2 Calibration 12 2.3 Analytical Analysis 13 2.4 Testing at NASA Ames 15 3. Evaluation, Conclusions, and Recommended Work 3.1 Product Evaluation 15 3.2 Conclusions 16 3.3 Recommended Work 17 Support Material Drawings Appendix A Circuit Diagrams and Codes Appendix B Assembly and Operation Instructions Appendix C Math Models and Calculations Appendix D Testing Description and Analysis Appendix E DFMEA Appendix F Team Resumes Appendix G Project Timeline Appendix H 4 List of Figures Figure 1: Fundamental Transmitter circuit structure 8 Figure 2: Basic description of Orion Spacecraft Heat Shield 9 Figure 3: Final Transmitter Circuit Structure 10 Figure 4: Final Transmitter Prototype PCB 11 Figure 5: Top PCB Layout (Left), Bottom PCB Layout (Right) 12 Figure 6: % Error found in calibration testing 13 5 1. Background and Concept 1.1 Background Every space craft entering a planetary atmosphere needs a Thermal Protection System (TPS). The TPS must endure severe heat loads, which requires an understanding of atmospheric properties, vehicle aerodynamics, TPS material properties and the physics of the entry environment. NASA and other space agencies would like to collect temperature, pressure, heat flux, radiation, and recession measurements on flight tests and flight missions in order to verify TPS design and to aid in the characterization of physical and chemical phenomena in the entry environment. Currently the missions that do fly with thermocouples have them wired into the TPS of the spacecraft. Utilizing this architecture adds risk to the TPS system due to the process of routing wires in the shield and the difficulty of jettisoning the system after entry. Many current and past spacecraft engineers have decided not to fly embedded sensors within the TPS in an effort to mitigate the risk of spacecraft failure during entry. A wireless instrumentation system could collect the required measurements needed for scientists and engineers to improve future spacecraft design while lowering the overall risk of incrementing entry vehicles. 1.2 Problem Definition NASA would like to develop a wireless sensor system for future spacecraft and probes, and has contracted our team, ThermaSense, to initiate the design process. The design of the wireless sensor system is a multi-stage project spanning several years. The ThermaSense team’s focus was to research and understand the current state of wireless transmission. To achieve this goal the team has been asked to design a wireless temperature sensor and develop a prototype. The project’s main focus was on establishing wireless communication between the sensor and a computer for data acquisition with a secondary focus on the actual temperature sensor and the data acquisition. This project required the team to design a wireless system using off the self components and to create a table top demonstration that confirmed the success of the new 6 wireless thermocouple system. To complete this goal the team researched the current state of wireless communication architecture and decided on specific components. Once designed and perfected the new wireless architecture could be extended to many other different types of TPS mounted sensors such as velocity and pressure sensors. 1.3 Concepts Considered With the basic understanding of the obstacles pertaining to the thermal wireless project, research was completed to identify the different elements needed to complete a wireless system that is to be embedded in the spacecraft. The team’s initial knowledge led us to a system where a thermocouple was attached to a device for cold junction correction. The system was attached to the transmission wireless device and the data was transferred to the receiving wireless device and sent to the data acquisition system. After researching thermocouples themselves, the team learned that not only does the thermocouple voltage need to be corrected, but it should also be amplified to allow accurate temperature readings. To amplify the voltage, the team inserted an amplifier into the system after the cold junction correction and before the wireless transmitter. This described system setup will work for a basic table top demonstration without any interference, but since noise interference is a major obstacle of this project, the team researched different ways to protect the thermocouple signal from outside interference. One of the best ways the team found to reduce susceptibility to noise interference was by adding an analog to digital converter to the system on the transmission side. This addition changed the thermocouple signal to a digital signal, in which the wireless transmission was many times less susceptible to interference. At this point in the design, the team believed the system described above would be the final product. Actual testing with a prototype proved the system required a microcontroller. The microcontroller was needed to provide a clock signal for certain 7 chips and also covert between different interfaces, i.e. UART to SPI. The addition of the microcontroller gave the team our final system. Thermocouple Cold Junction Correction Chip Amplification Xbee Wireless Transmitter Analog to Digital Conversion Figure 1: Fundamental Transmitter circuit structure Another main part of this project was to research different types of wireless architectures that were available for use in the system. Research showed that there were three main architectures: radio frequency, light emission and near field magnetic communication. Before looking at each architecture in respect to the specific project scenario, the team researched and developed a decision matrix for each of the types of wireless transmission in order to develop an understanding of the capabilities and restraints. The decision matrix (shown in Appendix 2) evaluated each architecture by; size, power consumption, ability to transmit power, transmission distance, requiring line of sight, noise immunity and cost. Based on these characteristics alone, each architecture is capable of transmitting the thermocouple signal for a table top demonstration in an interference free facility. This will not be the environment of the final product, so the team judged the wireless architectures mainly on noise immunity and power consumption. When looking at the requirements of these two specifications, near field magnetic communication is the best wireless option. After obtaining a description of the Orion Spacecraft Heat Shield Structure (Figure 2) the team had a new understanding of the obstacles they had to overcome for a successful design. 8 Figure 2: Basic description of Orion Spacecraft Heat Shield With this new knowledge the team realized the real challenge was to communicate through the many layers of the spacecraft. Light emission was ruled out since solid structures in between the transmitter and receiver could not have holes drilled in them. The two options left were radio frequency and near field magnetic coupling. The team spent some time in the lab experimenting with the magnetic communication through aluminum, which is one of the most troublesome layers with in the Orion construction. This rough experiment demonstrated that it would be very difficult to transmit a signal through aluminum. The team then researched the possibility of transmitting a radio frequency through the aluminum layer and found the probability of success to be a little less than using the magnetic communication process. Due to this research, near field magnetic communication seemed to be best choice until the team was informed that the transmission distance was larger than the range of magnetic communication. In addition we learned that there was no commercial availability of the wireless architecture or this purpose. This knowledge made radio frequency, although higher in noise susceptibility, the best choice for our system overall. The team continued to work with the near field 9 magnetic communication for the possibility of other wireless transmission within the system that might need its specific capabilities. 1.4 Concept Selection Taking into consideration the specifications defined during the problem definition phase (shown in Appendix 3) and the results of our initial research, the circuit structure shown in Figure 1, was modified to look like the structure in the following figure. Thermocouple Cold Junction Correction Chip Xbee Wireless Reciever Pickaxe Microcontroller Bit Inverter Xbee Wireless Transmitter Figure 3: Final Transmitter Circuit Structure The first change was the inclusion of four thermocouples instead of one connected to the circuit. With four thermocouples, the circuit met our specification and also better meets NASA’s needs by giving them the ability to imbed more thermocouples into the TPS material. In order to incorporate four thermocouples, the team also had to change to four CJC chips. It was decided that the CJC chip would also incorporate the amplification, A/D conversion, and the voltage to temperature calculations. This eliminated the amplifier and A/D converter shown in Figure 1 and also reduced the chance that noise could enter the system. No other changes were needed to the Structure shown in Figure 1, after this 10 change because the microcontroller has the ability to read 4 signals and pass them along to the transmitter. 2. Product Description and Testing 2.1 Product Description The transmitter PCB is made up of seven IC’s (integrated circuits) and four headers. Starting from left to right there is first the terminal block. This terminal block is connected to four cold junction correction IC’s. These IC’s correct the voltage potential read coming in from the thermocouples connected to the terminal block for room temperature. The corrected temperature data is passed to the PicAxe microcontroller using SPI protocol. The PixAxe microcontroller converts the temperature data to the correct resolution and outputs the data to the bit inverter using UART serial protocol. The bit inverter inverts the bits and feeds it to the X-Bee transceiver. The X-Bee transmits the temperature data wirelessly to the receiver connected to the computer. The receiver sends the temperature data down the USB cable and the software displays the temperature data on the screen. This process is shown in the code for the microcontroller itself. Refer to Appendix B for the code. The code for the software can also be seen in Appendix B. 11 Figure 4: Final Transmitter Prototype PCB The temperature software ThermaSense.exe was written in Visual Basic 6.0. The PicAxe microcontroller code was written in the PicAxe Programming editor. This editor can be downloaded from the PicAxe website, or borrowed on a disk from Joe Plummer. The PCBs were laid out in EAGLE Layout Editor, which is a free download from the Eagle layout website. The Layout files are available from the project website. The circuit traces are shown below in Figure 5. Figure 5: Top PCB Layout (Left), Bottom PCB Layout (Right) 2.2 Calibration The CJC correction chips are factory calibrated to be accurate to with-in +/- 4 degrees Celsius through the complete range of temperatures added to a +/- .2% reading error. 12 Once the temperature data enters the CJC, the only other possible error in data received would be a dropped packet. To test the factory calibration a controlled test was set up and conducted. A voltage from a DC source inputted into the thermocouple inputs caused our circuit to read out a temperature. Comparing this temperature against standard type K thermocouple tables showed the error that the circuit had. Shown below in Figure 6, are the results of the temperature calibration test and the accuracy of the circuit in a typical room environment. % Error for Range of Temperatures 1.4 1.2 % Error 1 0.8 0.6 0.4 0.2 0 -0.2 0 500 1000 1500 2000 Temperature (F) Figure 6: % Error found in calibration testing This temperature calibration test table shows a maximum of a +/- 1.2 % error within the range of the temperatures to be read. This error is over the manufacturers maximum rated error of +/- .8 % error at maximum reading. The differences in max error can be accounted by several things. The first is the circuit layout. Because there is a thermocouple header between the CJC chip and the thermocouple a temperature difference is possible and the circuit reads another thermocouple voltage. The second is a programming error where the CJC reads off a temperature in Celsius and rounded off decimals turn into significant errors when converted to Fahrenheit. 13 2.3 Analytical Analysis Attached in Appendix D is a TK Solver sheet for the analytical analysis. For the analytical analysis, a nodal method was used. The nodal analysis was one dimensional going through the block. The one inch thick block was divided into 10 nodes. On the heated side, the heat used was a heat flux. This method was the best assumption since a heat flux sensor was used in the actual experiment. The only other heat transfer for the first node, was heat transfer to the second node. Everything that was not transferred to the second node was stored in the node, increasing temperature. All of the nodes in the middle of the block had terms for the heat transfer between the nodes on either side of that particular node. Anything that was not transferred was stored in that node. The final node had convection to the surrounding environment. Since the testing occurred in a vacuum, the convective coefficient was assumed to be low. The coefficient used was 5 W/(m2*C). Using the heat flux calculated by the heat flux sensor, the temperatures in the block was an order of magnitude larger than what was found in the experiment. A heat flux of 4800 W/m2 produced temperatures similar to the ones found during testing. The heat flux sensor measured a heat flux of 580000W/m2. This value is much bigger than 4800 W/m2. Several factors contributed to the larger measured heat flux. First a Cold Walled Gardon gage was the measurement device. Presently a Hot Walled Gardon gage is not available for the X-Jet. The Cold Walled Gardon gauge can produce values 20-30% higher than the true value. Additionally, the heat flux sensor was not measured at the same distance that the heat shield was tested in the X-Jet. The sensor was closer to the jet than the heat shield was during testing. The distance between the jet and the material has an exponential effect of the heat flux and even small distances can have a large effect on the heat flux measured. 14 Even with the known errors, the remaining difference between the two values is too significant. Another possible error that could be affecting the values is calibration. The X-Jet is a small facility in which only a few technicians can operate. The heat flux sensor might not have been fully calibrated prior to testing at the site. It is also possible that the analytical analysis is not accurate. Presently the difference between the temperature values of the tested version and the calculated version is so vastly different that nothing can be discerned. The analytical analysis is correct in one aspect. The nodes response to time produced the same graph just on a different magnitude of order. This means that our analytical analysis set-up can accurately simulate the tests but that there is an error in the input heat flux. The conclusion of the analytical analysis is that since the graphs of the analytical analysis and the testing were similar the step response is accurate. The reaction of the heat shield can be simulated. Unfortunately, since the temperatures were so different, the temperatures cannot be verified. 2.4 Testing at NASA Ames For proof of concept, testing was conducted in an X-Jet Chamber at NASA Ames test facility. An X-Jet chamber consists of a vacuum chamber and plasma torch as a heat source. Four thermocouples were embedded at different depths in the LI-900 material. The thermocouples were then attached to the wireless prototype to transmit back to the computer. Many tests were conducted at Ames test facility. Three Antennas were tested; the PCB antenna, the whip antenna, and the external antenna. Power settings were also tested along with the antennas. It was found that the best solution for the wireless transceivers was medium power with the whip antenna. In conclusion, the wireless link was found to be strong with system recovery under heavy EMI noise, making our prototype a successful solution for wireless sensors. For more information and a complete analysis of all tests, see Appendix E. 15 3. Evaluation, Conclusions, and Recommended Work 3.1 Product Evaluation The wireless TPS sensor system was initially specified to transmit data from the sensor to a data collection system once a second. The wireless system was to be powered by about 9 volts and have a system life of 2 years. The size of the system was specified to be 5” x 4” x 1” and weigh 16 oz. The system was specified to use thermocouple sensors attached to 3 – 4 communication channels. Temperatures recorded by the system were to be accurate to +/- 5 degrees Celsius. Finally the system was required to have medium susceptibility to electromagnetic interference. ThermaSense’s final product’s recorded temperature data to the data collection system ten times a second. The system in final design required 3.3 volts for operation; the team implemented two AA batteries to supply this energy. The current consumed by the system was measured to be 49 mA during continuous transmission. Given this measurement a rough estimate for system life expectancy is 8 hours, this time span does not meet the system life specification. To overcome the issue the system can be programmed into sleep when not transmitting. The system only needs to transmit for five to six minutes and can be programmed into a sleep mode for the rest of the time, which will only consumes a few micro amps, and means the system could roughly last five to six years. The final system weighs only 3.2 oz and is scaled to be 3.1” x 2.9” x .75”. This size and weight is a large improvement over the original requirements. The final prototyped system included 4 thermocouple sensor communication channels. Calibration testing was completed outside an electrical interference environment using a precise mV source. Using the ASTM table given for a type K thermocouple the mV source was used to compare measured temperature to ASTM standard. Calibration testing on the final system resulted in a +/- 1 percent temperature reading from the 16 thermocouples. Electromagnetic interference testing was conducted at NASA Ames’ XJet facility. The system transmitter was set for medium power using a whip antenna. These tests concluded that it has low susceptibility to electromagnetic interference; this was the main accomplishment of our system. 3.2 Conclusions The system designed by this team is proof Zigbee technology is a feasible solution to NASA’s wired sensor problem. Zigbee design goals for low power and low bandwidth were tested and achieved with ThermSense’s wireless TPS sensor system. Initial analysis shows integrating sleep mode into the design will allow for five to six years of operation from two AA batteries. The designed system will allow probes to travel to nearby planets such as Mars, Venus, and Mercury. The team’s final prototype weighs 0.2 lb reducing sensor system weight by an estimated 50%. Not only does this reduce the weight of the spacecraft carrying the system, but it also reduces space required for the system. With the tested ability of the final prototype to overcome electromagnetic interference, the system will now be miniaturized and further developed by next years senior design team. Using the thermocouple system as proof of concept, the wireless system should be expanded to all different types of sensors useful to embed within TPS material. Once these steps are complete the system will be flown on a probe mission to ensure performance and reliability. 3.2 Recommended Work This project’s goal is a flight test and implementation into NASA missions. In order to achieve this goal, additional areas of work would be; powering the sensor, programming the sensor for advanced power savings capabilities, determining the transmitter’s interference tolerances, miniaturization, transmission through RF non-transparent 17 material research, and near field magnetic transmission research and implementation studies. The next stage of this project would be to develop a flight test for the system as we work to achieve NASA’s goal of implementation into missions . A successful flight test will require a software update. Currently, if the receiver gets corrupted data the software crashes and needs to be reset before data can be recorded. New software will need to overcome this issue as software will not be able to reset during a flight test and will also need to recover from lost packets and general signal disruption. Most likely the stage of circuits may need a different sensor connected other than a thermocouple, requiring the transmitter side of the circuit to be decoupled from the thermocouple side. If decoupling is needed, work towards a standard transmitter circuit and a sensor connection strategy is recommended. This change would make it easier to connect the transmitter to any data source as long as that source transmits a standard set of data (time, sensor ID, and reading). Additional work needed for a successful flight test is the protective packaging of the circuit. The environment that the circuit is exposed during testing and flight will vary depending on the test and packaging that can resist many environmental factors would be needed. The environment would likely involve thermal extremes, vibrations, and radiation which all could damage the circuit and disrupt the signal. These are next steps and additional work recommended for next year but the list is not all inclusive, as other issues, parameters, requirements and ideas may be uncovered as the project progresses. Other work that fits into the scope of the project and propels the project towards the final goal would be worthwhile and should be pursued. 18