Previously the rate controller disabled updating the integral below 20%
throttle. This is not ideal for several reasons:
- some racers already hover with 20% throttle.
- for acro it is important to always keep the integral enabled, it has a
noticeable effect on flight performance.
The meaning of the yaw weight changed with #8003:
- before, the yaw weight decreased with increasing tilt angle error, so
it was mostly 1
- now, it is constant and depends on the tuning gains (around 0.4 by default)
It means that #8003 reduced the feedforward term, and we get the closer
behavior as before with this change.
It also reduces coupling between different parameters.
- On initialization _v_att_sp got filled with zeros
leaving invalid quaternions
- While not armed mc_pos_control did not publish any
attitude setpoint which makes no sense
- The attitude control just uses the data in _v_att_sp
if it was (ever) updated or not
While operating on exactly normalized float quaternions
it can aparently still happen that one of the elements
gets just slightly above 1 or below -1 and hence out of
the domain of the acosf and asinf functions resulting in
NaN. The constrain function uses stricly smaller/bigger
comparisons and catches all tested cases.
- Delete left over identity matrix.
- Corner case with a zero element when using the signum function:
We always need a sign also for zero.
- Corner case with arbitrary yaw but and 180 degree roll or pitch:
Reduced attitude control calculation always rotates around roll
because there's no right choice when neglecting yaw. In that small
corner case it's better to just use full attitude contol and hence
rotate around the most efficient roll/pitch combination.
According to the paper the quaternion controller is built on
the yaw weight represents the ratio between the roll/pitch and
the yaw attitude control time constant. It also states that as a
thumb rule a value of ~0.4 works alright for most multicopter
platforms. The default attitude gains of PX4 which were determined
independent of the paper from experimental results have a ratio of
2.8/6.5 = 0.43 which matches.
Because the parameter does not make sense from a control theory
perspective. Either you have a gain with the unit 1/s or an inverse
gain or time constant with the unit s. But the time constant parameter
was neither bound to any exact unit nor did it apply instead of a gain.
Rather it adjusted multiple gains from rate and attitude control
according to an arbitrary scale. This can only by accident lead to
good tuning.
to prioritize yaw compared to roll and pitch by combining
the shortest rotation to achieve a total thrust vector with
the full attitude respecting the desired yaw
not by scaling down the control output with the gains
tested on at least 5 different vehicles, including AeroFC. The values
should be conservative, good setups (with low vibrations) can increase
these values even further.
increasing IMU_GYRO_CUTOFF allows for better tuning gains (increased P).
Beat Küng