This paper discusses our ongoing research into assessing the accuracy of inertial measuring units (IMU). We used three different IMU’s varying in price and performance. The utilization of the different models was explored over the recent August 21st, 2017 full solar eclipse high-altitude ballooning live stream project. The IMU’s were used on the ground tracking station for orientation. Understanding the precision and accuracy of different units was paramount to the ground station’s functionality. The results of our findings are presented in this paper; distinct configurations, including being near metallic materials and how that affected the inertial measuring unit’s performance are discussed

Gahl, Trevor

Mciver, Carter

English

Via Sapientiae: The Institutional Repository at DePaul University

______________________________________ 1 Undergraduate Student, Mechanical Engineering, Montana State University, Bozeman, MT. 59717 cartermciver@yahoo.com (845)-489-5522 2 Graduate Student, Montana State University, Bozeman, MT. 59717  Positioning and Testing of Inertial Measuring Units Carter McIver1 and Trevor Gahl2 Montana State University, Bozeman, Montana 59717  This paper discusses our ongoing research into assessing the accuracy of inertial measuring units (IMU). We used three different IMU’s varying in price and performance. The utilization of the different models was explored over the recent August 21st, 2017 full solar eclipse high-altitude ballooning live stream project. The IMU’s were used on the ground tracking station for orientation. Understanding the precision and accuracy of different units was paramount to the ground station’s functionality. The results of our findings are presented in this paper; distinct configurations, including being near metallic materials and how that affected the inertial measuring unit’s performance are discussed. I. Introduction A. Project Overview This research was done as a component of the 2017 full solar eclipse NASA live stream project. The goal was to stream live footage of the eclipse from 60,000 to 80,000 feet to the NASA website. In addition to our team, there were 55 other teams nationwide that used our project as a foundation to live-stream their own eclipse video. A key component of the project was the affordability, this was integral in allowing other teams a chance to participate in the project and as such was capped at $4,300 per system. To meet this budget the project configuration consisted of a series of payloads containing a variety of commercially available electronics and camera systems strung underneath latex weather balloons. The video systems consisted of a raspberry pi and, depending on the configuration, one or eight cameras in addition to a power board and batteries. Still image payloads consisted of similar hardware, but with modified firmware. Transmission frequencies to the ground station consisted of a  combination of RFD 900MHz for still images and Ubiquiti Wifi 5.8 GHz for video. An overview of the system can be seen in Figure 1.  Figure 1: Flow chart of overall project Ref. [1]. In addition to the payload systems, the project included a ground based system that was used to track the balloon in addition to being able to send and receive transmissions. The Ubiquiti Wifi band of the video transmission had a ±2.8° acceptance angle, therefore the pointing accuracy of the ground station needed to meet or exceed the accepted beam angle of the antenna.  It can be deduced how important the accuracy of the IMU was for the system. The ground station consisted of a variety of components which can be seen below in Figure 2. The IMU and GPS systems were used during initial setup to ensure that the absolute position and orientation of the directional antennas could be derived. To do this the IMU communicated with an Arduino Uno that was equipped with a GPS shield. After a calibration sequence performed during every instance of the system setup, the GPS and IMU would return a combination of Euler vectors and global position. After the data is obtained, it gets passed through to the ground station computer through serial communication. The GPS data is used in conjunction with a lookup table to  determine the magnetic declination of the systems location. This data is then used to calculate the absolute magnetic bearing of the ground station system. Throughout the duration of the flight one of the payloads communicated it’s GPS location through the Iridium network. Data received was passed from an XML packet into a database maintained on the Montana State University campus. Once every three seconds the ground station computer would access the database to receive the positional data of the balloon. After receiving the data, the computer used the Haversine function to calculate the bearing and elevation between the ground station position and the position of the balloon.    Figure 2: Ground station flow chart. B. IMU Overview An inertial measuring unit (IMU) is a system that can measure linear and angular motion through a combination of gyroscopes, accelerometers and magnetometers. A simplified reference can be seen below in Figure 3 below. There are two standard configurations to attaching an IMU. The first is, a fixed position (strapdown), the second is being gimbaled. The IMU will report a variety of information including but not limited to; velocity, acceleration, and magnetic fields. These data points can be used to calculate relative or absolute position. Most low cost IMU’s are used to give relative position, this is due to the sensitivity restriction of the magnetometers in use. When paired with a GPS, systems can account for declination to find true north in addition to magnetic north. While it’s possible to use a microcontroller to calculate the positional data, some of the algorithms can be too intensive, so for the project, the calculations are handled by the ground station computer. There are two main mathematical methods used, the first is quaternions, which prevents gimbal lock while the second, Euler vectors, does not. Since the ground station was designed to be stationary, Euler vectors were used since gimbal lock was not a concern. One of the most  recent studies on the functionality of inertial measuring units was done by Frank Van Graas from Ohio University in 2013 Ref. [2].   Figure 3: Function of gyroscope Ref. [3]. C. Research in Question  The research done in this paper started when we noted loss of transmission of the payloads at shorter than anticipated distances and that the antenna on the ground station did not appear to be pointing in the correct direction. This was discovered when the values began changing drastically when the IMU was placed further away from the 80/20 bar (6105-T5 aluminum alloy) that it is mounted to and near the servo motors that are on the ground station. The purpose of this research was to find a solution to the interferences and develop a configuration that optimized the performance of the ground station. This was done by experimenting with different IMU models and implementing an assortment of mechanical configurations. The IMUs that were tested were the BNO055 Bosch IMU mounted on an Adafruit breakout board (existing IMU on system), the DC-4E from Sparton Industries, and the VN-100s from Vectornav Technologies. This information would be able to assist similar configurations and problems faced with IMU positioning.  II. Materials and Methods A. IMU models tested The first potential problem that was explored was if the bearing from the BNO055 was accurate. The procedure used to test the IMU consisted of mapping out known locations with a GPS receiver the unit used was a  Trimble Geo 7x receiver. Then the data was processed using the program Pathfinder Office, through this we could set up a control area to do testing. Through this we confirmed the accuracy of the BNO055 to ±1.5°.  The next test was to see if a more expensive unit with a higher accuracy would increase the performance of the system. The models that were tested alongside the BNO055, was the DC-4EP and the VN-100. Each varying in price and accuracy which can be found in the Figure 4 below. This test was using signal strength of the Ubiquiti modem and compass bearing as a reference. Through this process we tested all three units and compared the data. All three IMU’s were within 1° of each other, the price of the BNO055 was a fraction of the cost with comparable results. Since, our system had a beam acceptance angle of ±2.8°, we ended up staying with the BNO055 IMU.      Figure 4: Tested IMU Data. B. Interference from 80/20 bar Even though the 80/20 bar is comprised of a non-magnetic alloy it was observed that the IMU reported a bearing that was influenced by being in close proximity to the bar. This effect was significant, leading to errors as large as 8°. The 6105-T5 aluminum alloy consists of aluminum, magnesium, and silicon, there are trace amounts of iron, manganese, copper, chromium, nickel and zinc Ref. [4]. Normally an IMU is capable of being calibrated around the soft-iron effects presented by these materials. However, the discrepancy was noticed too late in the project for a code fix to be implemented, thus requiring a hardware modification to minimize the discrepancies.  Model Manufacturer Cost per Unit Accuracy BNO055 Adafruit $35  ±1.5° DC-4EP  Sparton $700  ±.3° VN-100 Vectornav $1,000  ±.2°  Figure 5: Original IMU bracket. There were two main brackets that were created to reposition the IMU away from the bar to solve this problem. The original bracket set flat on the 80/20 bar and on the same axis refer to Figure 5 above.  Both brackets that were constructed stood 6-inchs off the rod, anything less, interference was detected and anything more was excessive. Another design concern was that the IMU needed to be able to remove from the bar for calibration. The first bracket was created for rigidity and resilience. It was constructed out of a rail-to-tube holder, 6-inch ABS rod and a CAD designed mount which was 3D printed on a Formlabs Form 2 SLA 3D printer at 100 microns. The bracket extends out from the ABS rod to remain in the same axis as the 80/20 bar, this can be seen in Figure 6 below. During ground station testing we received non-variable readings from the IMU which, in turn gave a good signal to the payloads. Making the first bracket a viable solution.   Figure 6: First new IMU bracket. The second bracket that was constructed consisted of a piece of acrylic sheet which was CNC laser cut and a 3D mount which was printed on a Ultimaker 2+ FDM printer at a 400-micron resolution, this can be viewed below in Figure 7.   Figure 7: Second new IMU bracket. The reason for the construction of the second bracket was cost considerations and manufacturing times. The second bracket was 40 times less expensive and could be manufactured 19 times faster using the FDM printer and CNC laser cutter. The complexity of the first IMU mount resulted in inaccuracies when printed on the FDM printer so we were forced to use the SLA printer. In Figure 8 below there is a full price and time break down. This was taken into strong consideration with the project nearing its conclusion. The first bracket also required ordering parts which had limited availability. With over 55 teams this would not be a feasible option. The first bracket is the one that we used and the second is the one we shipped out to the other teams, who wanted it.  Figure 8: Price and Time breakdown of first and second bracket. First IMU BracketParts Cost per unit Time to make6" ABS Rod $3.57 1wk shippingRail-tube holder $34.10 1wk shippingIMU printed mount $12.00 7 hoursTotal $49.77 1wk, 7hoursSecond IMU BracketParts Cost per unit Time to makeAcrylic sheet 3mm $0.73 2minsIMU printed mount $0.50 20minsTotal $1.23 22mins C.  Interference from Servo Motors  The IMU bracket had the ability to be mounted anywhere on the 80/20 bar. When it was placed closer to the servo motors it began to experience a ±2° variability. In theory, the frequency of the servos was one of the harmonics of the clock on the BNO055 which was causing interference and leading to inaccurate measurements. To counteract this, we designed Faraday cages for both servo motors (tilt and pan). The choice of metallic material was between mumetal and 1060 aluminum alloy. Mumetal is slightly better at shielding than Aluminum above 1MHz Ref. [5]. We needed shielding below 1Mhz; the clock on the BNO055 max frequency was 400kHz with this we decided to go with Aluminum, also considering cost and availability. For the construction of the Faraday cages .025inch 1060 aluminum alloy was used, below in figure 9 you can see the cages mounted on the system. The cages were grounded to the base plate which was ground using a copper rod. Aluminum flashing tape was used to cover seems and holes for components.  Figure 9: Faraday Cages on Tilt and Pan Servo Motors.    III. Conclusion The purpose of this research was to find the best IMU for our system, to find solutions to the interference caused by the 80/20 bar and servo motors that would optimize the performance of our ground station. This can be implemented in future projects that are using IMU’s with similar issues. In part II section A, we tested three different IMU units varying in price and performance. After testing them in a controlled area we concluded that the BNO055 was the best unit for our project. The price of the unit being $35 compared to the two other units which were more accurate but started at 20 times the price of the BNO055. The accuracy of the BNO055 was within ±1.5°, which was within our acceptable error of ±2.8°. In part II section B, the main goal was to develop a solution to the 80/20 bar interference. The BNO055 IMU is capable of being calibrated around the soft-iron effects presented by these materials. However, the issue was noticed too late in the project for a code fix to be implemented. We developed two different brackets that solved the problem equally as well. The first new IMU bracket was more resilient but, was more costly and timely to construct. We ended up using the first bracket on our system and the second was available to the other teams. In conclusion of this section, raising the IMU away from the metallic material by 6 inches decreased variability by 8°.  In part II section C, the main goal was to eliminate the interference caused by the servo motors that was causing a 1-2° variability. The solution was to construct Faraday cages out of 1060 aluminum alloy with a .025-inch thickness. Measuring the differential of the Faraday cage addition to the system was challenging since the BNO055 had a ±1.5° accuracy. Keeping that in mind we decided that it was worth keeping the cages on the system considering they were not affecting it in a negative way. Design information, including part files and drawings are available upon request for the Faraday cages and IMU brackets. Funding Sources Montana Space Grant Consortium grant, funded by NASA, grant number NNX15AJ19H. Ronald E. McNair Post Baccalaureate Achievement Program, funded by the U.S. Department of Education, grant number P217A130148.   Acknowledgements Montana Space Grant Consortium Ronald E. McNair Post Baccalaureate Achievement Program BOREALIS High-Altitude Ballooning Lab Department of Mechanical and Electrical Engineering  Special thanks to Randy Larimer PE and Dr. Berk Knighton for all their contributions, and assistance. References [1] “About the Eclipse Ballooning Project,” Eclipse Ballooning Project Available: http://eclipse.montana.edu/about/. [2]  Graas, F. V., “Workshop on GNSS Data Application to Low Latitude Ionospheric Research,” International Centre for Theoretical Physics, Jun. 2013. [3] Gyroscopes Theory - DutchOps.com powered Available: http://www.dutchops.com/Portfolio_Marcel/Articles/Instruments/Gyroscopic_Instruments/Theory_Gyroscopes.htm. [4] “Technical Data for Aluminum Extrusion,” Technical Data for Aluminum Extrusions - T-Slot Aluminum Details | Faztek Available: http://www.faztek.net/technical.html. [5] Loya, S., & H. (2016). Analysis of Shielding Effectiveness in the Electric Field and Magnetic Field and Plane Wave for Infinite Sheet Metals. International Journal of Electromagnetics and Applications. doi:10.5923/j.ijea.20160602.02   

Positioning and Testing of Inertial Measuring Units

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Positioning and Testing of Inertial Measuring Units

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