Home brew data logger and flight controller

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Indiana

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Hi all, been a long time since I've posted.

I needed a project to keep the ol' gray matter pliable, so I designed a data logging flight controller. It reads acceleration, absolute air pressure and temperature (haven't added the temp sensor yet). The circuit is built around a Microchip PIC18F4550.

I'm using a 15bit A2D converter to read the sensors. Currently I'm taking data at 50Hz, though I should be able to go up to 200Hz. Data is saved to an on-board 1Mbit eeprom. The data is transferred to the PC with USB using a pretty basic (so far) C# program.

I'm currently using a very simple algorithm to detect apogee using the pressure sensor. I used a very conservative setting this flight to make absolutely certain that it had reached apogee. I don't have much test data yet, and I was uncertain how smooth the pressure curve would be during the flight. If I've done all my math right, it detected apogee at 443' and set the deployment charge at 441'.

The blurry picture below shows the connection point for an external battery and two "high current" drivers used for air-starts and recovery charge.

I did it mostly for the fun of it and the mental exercise, lord knows I could have bought one cheaper!

Finally got to test launch today. Here are some pics. My video helper managed to miss most the flight.



The board:







The rocket is a BT-60 of my own design:


The first flight data, 443' AGL:

Raw data:


Alt plot. The ejection charge really wallops the air pressure sensor:




The scene of the crime. Wild success or epic failure is mere moments away!
 

RandyM

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The suspense is killing me. Tell us more! :D
 

Indiana

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The suspense is killing me. Tell us more! :D
Not much more to say. It ripped off the pad, the software detected the launch, allowing the code to start watching for apogee. After weather-cocking ever so slightly, it traced a slow, graceful arc as it went over the top, and with the beautiful precision of electronic control, it fired the ejection charge just as it started to fall.
 

RandyM

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Guess I need to learn how to read better. :D

From the last pic I thought you were flying again with a different result.

Glad to hear it went well. Hope to see it in person sometime in the near future.
 

bobkrech

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How are you isolating the altimeter from the ejection pressure?

Bob
 

Indiana

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The e-bay is separated from the parachute bay by a 1-1/2" balsa joiner. The wires to the ejection charge are passed through a 1/8" hole. I have a resistor in one of those wires to limit current in the ignitor firing circuit. The resistor is covered in shrink wrap and fits very snugly in the hole. I thought this would be sufficient to isolate the chambers.

This is my first experience with electronics in rockets, so I have much to learn. I am assuming that the disturbance is mechanical displacement of the sensor's diaphram and I plan to try to verify that on the bench.

Suggestions though, are quite welcome.
 
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RandyM

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My first suggestion is to switch to plywood instead of balsa, for strength if nothing else. It a little heavier, but on that size rocket, I doubt you'll notice much of a decrease in performance. The other thing would be to put a connector of some kind that the wires from the altimeter would run to. That could then be sealed with something like silicone for a better seal.
 

sylvie369

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The original question was about protecting/isolating the electronics from the pressure wave, but you'll also want to be concerned about keeping the BP residue from the ejection charge away from the electronics - it's corrosive.

Beautiful work, by the way.
 
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