Heli AutoTune: fix rate and accel limiting

[bnsgeyer]

The original implementation of this relied on the attitude controller to provide the rate and accel limiting. The rate limiting worked but it can cause square wave behavior as it clips the rate at the rate limit. As for the accel limiting, the attitude controller wasn’t designed with the feedforward turned off to limit accel. It actually reduces the angle P to reduce acceleration.

So a different approach was taken. Using a sinusoidal input makes it easy to limit the acceleration and rate without causing any clipping. The attitude amplitude was modified based on the first and second derivative of the amplitude sinusoid chirp. This ensures the desired rate and acceleration are limited. However, the target values maybe smaller or larger based on the error and angle P. In most cases I have seen the target rates are lower.

This has been tested in SITL.