The design should be pretty simple. One AHRS board, one Arduino, and one helicopter interface board. The interface board will be stacked on top of the AHRS and the Arduino stacked on top of the interface board. Both sets will mate in rows of headers. Both the AHRS and interface board have mounting holes to connect to the enclosure. The bottom of the interface board is a pretty complete ground plane. It will sit like a lid on the shielded enclosure and the exposed ground plane will contact the shield.
With that away, time to turn to the next steps:
- Port AHRS code from MATLAB to the embedded system
- Write Arduino code to talk to servos, AHRS, and UART
- Build rig to constrain helicopter to rotations about CG
I think step 3 won't be as hard as it sounds. If I tie 3 strings to the helicopter skids (or 4...4 is redundant, but may be easier to implement due to symmetry) and anchor those strings to the ground such that, when taught, the strings point towards the helicopter CG, then for small angles the helicopter's constrained motion will approximate rotation about the CG. Depending on how much extra lift the helicopter is generating (there will need to be extra lift to keep the strings taught), there will be some restoring moment in yaw. But using this setup and running a frequency sweep in the cyclic inputs should be sufficient for system identification.
The other option is to put a straight gain on the feedback, control that gain with channel 5 (the "gyro gain" knob on the transmitter), and fly it and hope for the best :)
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