Pressure Sensor Testing

We tested the MPX5100A pressure sensor by placing the device in the same room as an analogue barometer and reading both measurements, and by placing the device outside and looking up the barometric pressure of our area online. Our results are below.

	Expected	Measured	%-error
Weather.com	100.6 kPa	100.0 kPa	0.6 %
Classroom Barometer	94.8 kPa	98.8 kPa	4.2 %

We feel that the percent difference is higher when compared to the analogue barometer because the analogue barometer is old and probably hasn’t been calibrated in a while. We are still confident in our results to send the sensor up with our payload

MicroTrack

                In order to be able to find the payload after it has completed its flight, we will be using a Micro-Trak GPS system, shown below.

The Micro-Trak AIO takes a GPS signal and sends it to APRS receiving towers, which then send the data online where we can see its position. To configure the Micro-Trak, all we had to do was plug it into the USB port on a computer and open up the TinyTrak3 computer program.

We did not have to do any further configuring on the Micro-Trak, but did use the software to find the devices call sign so it could be used to find the GPS signal online. So once we had the call sign, we took the Micro-Trak outside with a computer, turned on the device, then searched for the signal using the website aprs.fi. The signal was found almost immediately and showed us to be right where we were. Below is a picture of the website showing our signal location.

Flight Termination

                Team Weathered Warriors will be using a flight termination unit to ensure the flight of our weather balloon ends when we need it to. The primary cut down system for the flight is the balloon’s expansion as the altitude rises.  As altitude increases the air density will decrease, this will cause the balloon to expand in volume. Upon reaching our target altitude of 100,000 feet, the balloon will expand past its maximum volume and pop due to the pressure difference between the inside and outside of the balloon. If this system fails for any reason, we’ll be using a secondary flight termination unit. This is also in accordance with FAA regulations.

                The flight termination unit (FTU) that we’ve built and tested will use current running through a high resistance nichrome wire. The current will cause the wire to heat up dramatically. This nichrome wire will be wrapped around the kite string which connects the payload to the balloon. The heat will sever the kite string and the payload will fall back to Earth.

                To do this, we’ve employed a second arduino and a relay circuit. The arduino is programmed to send a current through a pin after a specific amount of time has passed. For this flight, the ascent time will be about 90 minutes.  To test this system we built a circuit which used a relay to light an LED with a 9 volt battery. After the secondary arduino had been powered for 10 seconds, the relay switch was flipped and the secondary was closed. This allowed the LED to light up using the relay circuit battery only. Once this had been tested, the LED was replaced with a small segment of nichrome wire. The relay circuit worked as anticipated and the 9 volt battery caused the wire to heat up to high enough temperature to melt the kite string we will be using in our flight. 

Aerocomm AC4490LR-1000M

                The communications system our balloon utilizes are the Laird Technologies Aerocomm AC4490LR-1000M radios. The radios operate on 900MHz frequencies and have a max operating range of 40 miles with a clear line of sight. The radios also require a low amount of power to operate at their full potential. The radios are small and lightweight. These traits make them ideal for our high altitude weather balloon. Since the radios use amateur bands of radio frequencies to operate, two of our group members have obtained our amateur radio licenses so that we may legally use the radios for our project.

                The group is using a AC4490 developer's kit to make the configuration of the radios easier. The software that comes with the dev kit is designed for easy configuration of the radios and allows the users to set them up in any way you'd need. Our set up is fairly simple. A radio (the client) will be wired to the PCB in our payload and an antenna connected to the radio will poke its way through the payload wall which should provide an easy line of site to the ground station. The ground station will consist of our other radio (the server) connected to a laptop which will be receiving signals from the balloon. The picture below shows the server radio wired to a board which connects to our ground station laptop via USB cable.

                The software which Laird has provided as a configuration utility can be rather clunky at times. The program is susceptible to random freezing and will often decide to stop reading the radios altogether without actually freezing up. A restart of the program or computer we were using the software on would fix these issues. The radios our team was using had never been written to before. The radios come factory set with a 56700 baud rate. The radios had to be read with this rate initially and then written to our baud rate of 9600. This little fact that isn't in the manual took a while to solve.

                Despite these hiccups the software allowed us to configure one radio as the client and one as the server as well as address them to each other. Through several trial and error attempts we were able to use the manual to correctly wire the radio to our arduino and send a test message of "Hello!" several hundred times. From here,  it was a matter of wiring the radio in with the rest of our sensors and main arduino to see if the radios sent all the data we needed them to.

                This caused a few unforeseen problems. The pin which activates the long range mode, pin 11, of the radio needs exactly 3.3V to function. The arduino wasn't providing pin 11 with enough potential when all the other sensors were connected. We got around this by powering this specific pin separately using an external power source and a voltage divider. The nice thing about the potentiometer being used as a voltage divider is that it is adjustable. So if things suddenly change on the day of launch and we need more or less voltage, we can simply change it. The code for our sensors took a little touching up to get the right information and timing to come across through the radio, but we have configured these radios so that the balloon payload will send the measurements to the ground station in real time.