The bulk of the code in this project for an autopilot for soaring UAVs stems from two separate sources:
The Attitude and Heading Reference System (AHRS), which is used for interpreting the IMU data stream through sensor fusion, is a cut-down and simplified version of Peter Bartz's splendid Razor AHRS.
The rest of the BirdBrain code is, for the most part, the result of radically reducing DIY Drones' somewhat bloated ArduPilot v2.7.1 code-base to a minimum, keeping only the functions considered strictly necessary for this particular application as a basic autopilot for non-powered gliders. This meant, among other things, that everything having to do with GPS, various obsolete types of sensors, track-following, turnpoint navigation, throttle management, automatic take-offs and landings or egg-dropping (sic), etc, was left out.
Some notable "user-interface" features of our application:
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If A0 and A1 pins are jumpered (connected) on start-up, BirdBrain goes into a special sensor calibration mode. Note that this is only useful while being connected to a computer with two-way serial communication so that the sensor test and calibration sketches from the Razor project tutorial on github may be used.
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To store a suitable default heading value in volatile memory before a flight, the model is simply pointed in the desired direction and a connected momentary switch held for a couple of seconds until the confirmation LED lights up.
The various system tasks are scheduled to run at different speeds, either 50Hz, 10Hz or 3.33 Hz:
- The inner (fastest) loop includes the AHRS routines, vario-reading, the stabilisation PID and servo-driving.
- The middle loop involves the navigation PID and navigation mode selection.
- In the outer loop the state of the R/C control switch is checked and its corresponding control mode selected.
Link to project description: http://xneb.org/mabe/xnebmisc/home/birdbrain.html or https://sites.google.com/view/xneb-project-logs/home/birdbrain