从零开始学习PX4源码2(PX4姿态误差计算)

摘要

本节主要记录PX4姿态误差计算过程，欢迎批评指正。

1.源码

1.1源码路径

PX4-Autopilot/src/modules/mc_att_control/AttitudeControl/AttitudeControl.cpp

1.2源码程序

matrix::Vector3f AttitudeControl::update(const Quatf &q) const
{
	Quatf qd = _attitude_setpoint_q;

	// calculate reduced desired attitude neglecting vehicle's yaw to prioritize roll and pitch
	const Vector3f e_z = q.dcm_z();
	const Vector3f e_z_d = qd.dcm_z();
	Quatf qd_red(e_z, e_z_d);

	if (fabsf(qd_red(1)) > (1.f - 1e-5f) || fabsf(qd_red(2)) > (1.f - 1e-5f))
	{
		// In the infinitesimal corner case where the vehicle and thrust have the completely opposite direction,
		// full attitude control anyways generates no yaw input and directly takes the combination of
		// roll and pitch leading to the correct desired yaw. Ignoring this case would still be totally safe and stable.
		qd_red = qd;

	} else
	{
		// transform rotation from current to desired thrust vector into a world frame reduced desired attitude
		qd_red *= q;
	}

	// mix full and reduced desired attitude
	Quatf q_mix = qd_red.inversed() * qd;
	q_mix.canonicalize();
	// catch numerical problems with the domain of acosf and asinf
	q_mix(0) = math::constrain(q_mix(0), -1.f, 1.f);
	q_mix(3) = math::constrain(q_mix(3), -1.f, 1.f);
	qd = qd_red * Quatf(cosf(_yaw_w * acosf(q_mix(0))), 0, 0, sinf(_yaw_w * asinf(q_mix(3))));

	// quaternion attitude control law, qe is rotation from q to qd
	const Quatf qe = q.inversed() * qd;

	// using sin(alpha/2) scaled rotation axis as attitude error (see quaternion definition by axis angle)
	// also taking care of the antipodal unit quaternion ambiguity
	const Vector3f eq = 2.f * qe.canonical().imag();

	// calculate angular rates setpoint
	Vector3f rate_setpoint = eq.emult(_proportional_gain);

	// Feed forward the yaw setpoint rate.
	// yawspeed_setpoint is the feed forward commanded rotation around the world z-axis,
	// but we need to apply it in the body frame (because _rates_sp is expressed in the body frame).
	// Therefore we infer the world z-axis (expressed in the body frame) by taking the last column of R.transposed (== q.inversed)
	// and multiply it by the yaw setpoint rate (yawspeed_setpoint).
	// This yields a vector representing the commanded rotatation around the world z-axis expressed in the body frame
	// such that it can be added to the rates setpoint.
	if (std::isfinite(_yawspeed_setpoint))
	{
		rate_setpoint += q.inversed().dcm_z() * _yawspeed_setpoint;
	}

	// limit rates
	for (int i = 0; i < 3; i++)
	{
		rate_setpoint(i) = math::constrain(rate_setpoint(i), -_rate_limit(i), _rate_limit(i));
	}

	return rate_setpoint;
}