Nuts and Volts - July/August 2018

APPROACHING THE FINAL FRONTIER

The RaspPi Zero is the warm blob (yellow rectangle) on the right side of the figure. The smaller warm blob at the lower left I think is where I held the PCB before taking this image. Seeing this makes me wonder if the heat generated by the Pi means batteries inside the airframe won’t need a hand warmer.

The Raspberry Pi runs hotter than the rest of the flight computer PCB (printed circuit board) or PICAXE. Based on a recommendation of one of my near space friends, Mark N9XTN, I took a thermal image of the powered-up flight computer (Figure A).

The second command gets repeated four times (once for each analog measurement). The last line of code is executed once since there’s only one GPGGA sentence to store.

seconds before reading another GPS sentence. As a result, I programmed it to read the GPS every second and then throw away four sequential sentences.

 d0 = (b1*256)+b0 combine the two
bytes to make a word of data
 textd0 = str(d0) convert binary
data into text
 text = textcount+”,” append a comma to
 the end of the string
 text = text+textd0 append  the first
 data to the record count
 text = text+GPSSentence append the GPGGA
 sentence to the data to store

Storing Data

Analog sensor data is binary by its nature, but the GPS data is ASCII text. These data types must be converted into the same format before they can be combined together and stored in the SD card. Since I’ll use Excel to analyze the data, I choose to convert all the data into ASCII. First, however, both bytes from the analog-to-digital conversion must be combined into a word before it can be converted into ASCII text (a string variable). The commands to do this are: After creating the string of data that the Pi collected, it’s time to write the text to the file stored on the SD card. The Python commands to open the file, append the new data, and close the file look like this:

 file = open(Logfile,”a+”) open the log file
and append data
 file.write(text) write the data into
the log file
 file.close() close the log file

After converting each analog voltage into text, it’s time to concatenate them together along with the GPS sentence. The commands to do that are shown next. Note that the first line of code adds a comma to the string so that Excel knows where to divide the text data into its proper fields (I store the flight computer’s data as a comma delimited file).

It’s taken me a good month to get to this point. I think it’s worth it, however, even if just for the learning opportunity. Next time, I’ll talk about the camera that I found for the Raspberry Pi and how I plan to integrate it into the flight computer.