Check attitude control integrator resets

[LorenzMeier]

Based on personal flight experience and user reports we might want to improve the integrator resets / anti-windup on the multicopter attitude control. Essentially we need to cover these cases:

• If the controller output already reaches its limit based on the output of the P and D parts, the I part should not integrate (anti-windup)
• If the output of the I controller reaches a certain absolute limit, it should stop to integrate. E.g. we don't want to have more than 0.3 (on a -1..1 scale) due to the I-term so that I cannot completely govern the P term
• If we know the system can't be moving, we should not integrate. For a multicopter throttle < 20%-25% is a very good indicator that we can't be moving and should not be integrating.

These guards may or may not be present, but we should make sure that all three are effective.

[DrTon]

Yes, I need to think more about AUW. Already implemented:

• If full output is more than -1/1 don't integrate
• If |full output| > throttle (will result saturation after mixing) don't integrate

[LorenzMeier]

The do-not-integrate-on-low-throttle part is probably the essential one we should add as soon as possible for roll, pitch and yaw. The position controller should also not integrate in that state.

[DrTon]

Just reviewed my code, already imlemented, but hardcoded values maybe need tuning:

• Don't integrate on low throttle (< 0.1): https://github.com/PX4/Firmware/blob/beta/src/modules/mc_att_control/mc_att_control_main.cpp#L661
• Don't integrate if full output (P+I+D, not only P+D) saturated: https://github.com/PX4/Firmware/blob/beta/src/modules/mc_att_control/mc_att_control_main.cpp#L663
• Limit max integral component (max = 0.5): https://github.com/PX4/Firmware/blob/beta/src/modules/mc_att_control/mc_att_control_main.cpp#L666
• Don't integrate if full output (P+I+D, not only P+D) > integral limit: https://github.com/PX4/Firmware/blob/beta/src/modules/mc_att_control/mc_att_control_main.cpp#L667

It's not clear for me what can we improve here. Maybe replace full output by P+D? I can increase min throttle from 0.1 to 0.2 and decrease max integral from 0.5 to 0.3.

[DrTon]

This commit may help:
7d80f05