Active braking feature (for multicopter) about tgy

[lakeroe]

Answer from SimonK per mail:

Active braking described in this way would require closed loop drive
within the ESC, since there would need to be a controller to apply the
brakes based on timing feedback. As such, the system would have to be in
a control mode by speed, since otherwise we would not know when to brake.

I have thought about adding this, but I have two concerns: The difficulty
involved in properly calibrating the maximum RPM (and thus scaling of the
input) with full expected load, and timing interactions with such a
system and a similar loop already present in the flight controller. There
would need to be tunables for the controller and the input scaling,
making the system quite complicated and likely difficult to tune.

Ultimately, complementary PWM implements the complementary side of
applying power and braking. The only thing that prevents it from being
completely symmetric is perhaps that MOTOR_BRAKE=1 is stronger than the
equivalent of what would be 0% duty cycle with COMP_PWM=1. I was thinking
of adding this mode, but, in a way, it already works with the flight
controller keeping the output range above idle when mixing motor outputs.

Regardless, the output of such a speed controller is effectively what the
flight controller is already trying to achieve. So, I'm not yet convinced
that adding an additional loop will actually help. The only difference is
that such a system could reduce under/over-shoot of the motor speed
before the resulting thrust can be measured, but I'm not sure this is
actually what is wanted for stable flight. Perhaps it allows to work
separately with the frequency response of the motor bell/propeller versus
that of the frame.