I have had some trouble with my Pixhawk saying it is armed, but the motors not spinning, or the ESC’s losing calibration and then not spinning together when it is armed.
I believe that I now have a solution to this, and I would like to share it. In my case I am using Hobbywing Platinum Pro 40A ESC’s with a serial and ground connection but no power to the servo rail. Receiving power from a separate 6s power source then the Pixhawk.
It has to do with the order that you plug things in basically. If you plug in the ESC power, then wait for the motors to start beeping, then plug in the flight controller and arm normally then it always starts up properly. If you plug them in at around the same time and try to arm it before the motors are done beeping, then this is when (I think) you can lose the calibration on the ESC’s. I have been unplugging them in the reverse order, pixhawk first then the esc power. So far since doing this I haven’t had a failure to start, or a loss of calibration.
Further, if you do lose ESC calibration, I was having to break out my flashing tool and reset the ESC’s one at a time before I could re-calibrate. For whatever reason the normal steps didn’t work after the first time. I discovered last night though that if you put the pixhawk into calibration mode (throttle up, pixhawk on, pixhawk off, pixhawk on, pixhawk makes a tone, press the safety button). and THEN provide power to your ESC’s with the throttle still up, then they will always calibrate. Make sure your PROPS ARE OFF FOR THIS STEP!!!
I hope this helps someone, let me know if there are any questions.