Characterization of pod motion during balloon flight

Research Group: Derek Hansen, Sarah Zeitler, and Al-Amin Yusuf

Launch: Whitworth Spring 2016

''Looking at the violent motion and force on pods during balloon flight has yet to been quantified, thus the goal of this project was to understand and have a base line data of the acceleration that occurs during the balloon burst. In order to fully understand this information, a LCP microcontroller was sent in a high altitude balloon with a ADLX345 Accelerometer to record the acceleration in the x, y, and z direction. This data was then written to a SD card for study upon returning to earth. The captured and recorded data was analyzed so that teams who send pods into near space in the future can have an approximation of the force that will be exerted on the lines securing the pods, up to 8G, as well as understand some of the motion that occurs during the decent.''

Background

 * The stratosphere is a region in the earth’s atmosphere approximately sixteen kilometers above the surface of the earth. The particle density is very low here, and it has become common practice to launch scientific high altitude balloons into this region as it has very little weather.   The stratosphere is just above the troposphere, the region of the atmosphere in which weather exists and life flourishes.1  In between the stratosphere and the troposphere is a region called the tropopause, which is a barrier between the chaotic air of the troposphere and the calm of the stratosphere.


 * When a high-altitude balloon is launched, it passes from the troposphere to the stratosphere, roughly sixteen to fifty kilometers above the earth. High Altitude Balloons are made of latex and typically filled with helium to provide lift.  As the weather around the balloon dissipates in the stratosphere, air density on the outside decreases, causing the balloon to expand until eventually it bursts.2   This burst has been described as violent but with very few details as to what this actually means.  A multitude of scientists have experienced difficulties such as losing payloads containing data likely due to burst. Despite a large number of balloon flights recording such difficulties, there has been little research to see what is actually happening at the moment of burst and therefore people can only speculate the best way to prepare Near Space experiments for the consequences.


 * With this in mind, this research group set out to categorize and quantify what exactly is happening to the payload at balloon burst in terms of acceleration and force exerted on the payloads. Thus, the goal of this research will be to track the motion of the payload in a High Altitude Balloon flight, retrieve the sent pods, and analyze the flight data including the changes in the force of gravity on the payloads as they ascend and descend. Data analysis of the magnitude of gravity and rotation of the pod on different axes will be used to determine the effect of the atmosphere and gravity on balloon as it bursts.


 * In order to accomplish this task, this group has decided to program Mbed systems, using    C++ on the ARMbed Online Open Source Library. This system will allow the programming of the ADXL345 to gather the different accelerations in the X,Y, and Z directions. Then, as the flight takes place it can write to a file the data being collected as to what is happening in terms of gravity.

Mechanical Design
The battery pack along with the breadboards were tightly secured to wooden boards through means of zip ties and screws. Holes were drilled into the wooden board and screws were threaded through the holes and tightened with nuts. Zip ties running horizontally were tightly placed with barely any interference to the circuit to provide rigidity and compactness to the entire system. Also, this provided ease in inserting the circuits into various pods. The entirety of the wooden board with the circuits were then fitted securely into the pods by either zip tying the entire board vertically (perpendicular to the first zip ties) or by screwing the wooden boards firmly into the Styrofoam of the pods. Each microcontroller system was placed with similar layouts and can be seen below.

Above is a layout of how the breadboard is attached to a wooden backing strapped in with zip ties to secure and stabilize during the flight and the decent. On the right is the layout of how the balloon will look as it goes into space with the specific devices in each module.





Devices To Be Used

 * In order to test such an idea, this research group is proposing to measuring the acceleration and direction of movement of pods being sent up with a high altitude balloon. This is to be accomplished using accelerometers and  located in multiple pods across the payload with the LPC controller and the ADLX345 accelerometer.

Electrical Design
In order to characterize the motion of fight, there had to be consistent readings of the acceleration and movement of the pods during the entire flight. In order to obtain this, it was decided to use an accelerometer and SD reader card attached to a LCP microcontroller. The first thing that need to be considered was the required power for a circuit containing all of these components. Having the right input voltage to powers all components for the required length of the flight was of utmost importance. After testing, it was decided that an input voltage of 6V supplied from 4 AA batteries/cells was sufficient to perform the task. The four batteries were arranged in a battery pack with a switch to control the supply of power. The battery pack was connected to the “voltage-in” and “ground” of the breadboard.

Schematic Layout
Below is a view of the schematic diagram describes the layout of the electrical wiring connecting all of thr control to the microcontroller, in order for it to read the correct accelerations for the duration of the flight.

Breadboard Layout
Bellow there is a layout of our breadboard. Which gives you a detailed look at how all of the pins where connected.

Software
This code was written to record the acceleration of the pods every second, for the first hour. Then after the first half hour it was changed to take readings every tenth of a second so that at the balloon burst we would collect more data to fully understand what motion was occurring. To access or download this code visit https://developer.mbed.org/users/dhansen17/code/Tail_Whip_Project/.

Code Flow
 Define and make all the connections and functions Write the data to a file Run a loop: Read data and increment the half-hour variable every half hour. if after first hour begins to take readings every tenth of a second. Write data to file Repeat.  

The following code kept track of time and also took care of the reading in the accelerations as the flight was in progress, as well as during the balloon burst and fall.

Validation

 * The sensors were tested to make sure they were recording data. For example, the accelerometer was validated by turning it on, and allowing it to run for the duration of the estimated flight. This allowed the researchers to ensure that the code was effective, the devices where reading the correct data, and that errors in electrical design had been avoided.


 * The battery source was tested by being turned on and left on while powering the device. This was confirmed as adequate when the battery lasted over two hours and was still powering the controller when it was turned off after the validation experiment.  The same battery was then used the following week for a similar test, confirming that the battery pack with fresh batteries will supply ample power for the duration of the flight.

Calibration

 * The accelerometer was calibrated by taking the data produced when the device was at rest for a period of three hours just like how our flight shall occur. It was then compared to the collected data with the expected value for acceleration, using 9.806 m/s^2 for the z direction and 0 meters per square seconds for the x and y direction. The magnitude of this data was then put into the following graphs for use when analyzing data after launch.

The first set of data from the calibration of Microcontroller A was fairly close to the expected value of acceleration at 9.16m/s^2 simply 0.64m/s^2 in difference as can be seen below.


 * The second accelerometer that was calibrated, Microcontroller B, had a smaller variance from the expected gravitational acceleration with a difference of 0.41 m/s^2 as seen below.




 * The last calibration that calculated was for Microcontroller C. This data was the closest calibration to what the expected acceleration was with only a difference of 0.2 m/s^2 also seen below.



Data and Analysis

 * The data collected was similar to what was expected. Results were obtained and viewed after launch for all three of the microcontrollers that were sent in the balloon's payloads, each in a separate pods.  The SD card came out of the reader mid-balloon burst for microcontroller A, so data for that particular accelerometer is shortened.  Each of the accelerometers measured increased acceleration at the point of burst, which is clearly seen in the graphs posted.  Even with the accelerometer that became disconnected, Microcontroller A, there was an observed magnitude of acceleration 3.16 times the calibrated standard for the gravity before the SD came out.  This was located in the second pod from the top on the payload.  In Microcontroller B, there was a peak increase in magnitude of acceleration of 7.84 times the calibrated standard.  In Microcontroller C, there was a peak increase in magnitude of acceleration of 4.68 times the calibrated standard.  This then in turn increased force on the payload dramatically as well.  The movement continued for the duration of the descent, gradually tapering off.  The increase in acceleration was in the X,Y, and Z directions, but for simplistic comparison purposes, magnitudes were used.







Conclusion

 * The purpose of this experiment was to examine the motion and intensity of motion for a payload attached to a High Altitude Balloon at the moment of burst. The results demonstrated in this experiment reflect the idea that the movement is indeed very violent at the moment of burst.  Which will can help us as we try and solve different problems for future flights.


 * The first problem it can help us solve is taking measures to secure both the pod and the experiments within the pod, considering they are crucial to increased likelihood of a successful launch and retrieval. Payloads should be able to withstand  at least an eight fold increase in acceleration.  They should be well tested pre-launch, to avoid any post launch disaster.


 * There are many possible opportunities for further studies with this project. First, it would be ideal to spend more time examining and applying the data already collected.  Might the increased motion have affected other experiments in unplanned ways?  What does the acceleration over altitude look like?  Does the acceleration look clearly different in the troposphere and the stratosphere?  These are all questions worth asking, and they can be answered by spending time with the data already collected.  Time was not a luxury this research group was afforded, but with it, other conclusions might be able to be drawn.  Another opportunity for further study might be to use a gyroscope.  This would provide data of motion in an additional dimension and would be useful for calculating additional forces put upon the pod.  A video camera might be able to capture the visual occurrence as burst as well.


 * The hope of this experiment was to enable future research groups to be able to collect data better. Maybe through the research done, groups might be able to recognize the importance of stability, and with this stability be able to discover valuable information that can benefit life on earth for everyone.