I am building self-balancing robot using modified code from YABR (Arduino project - Your Arduino Balancing Robot (YABR) - Part 3 - explaining the code - YouTube), although I use android app for remote control using Bluetooth instead of nunchuck and also not the same hardware and other parts:
2 NEMA 17 (12 V 0,5A 0,44Nm)
2 90x10 mm wheels
8 AA batteries(~ 12 V supply for stepper motor)
All in all, my project is very similar (software,hardware, placement of parts) to this one Self Balancing Robot Using Arduino – Android Control - YouTube.
After many tries of changing PID values, I haven't found proper ones and my robot is still falling down.
I checked pitch angle calculation and it seems to be good, stepper motors also behaved as expected when I used simple programs for them with different step sizes (in self-balancing robot I use 0,45 deg. step size).