PX4-Autopilot/src/modules/ekf2/EKF/mag_control.cpp

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/**
 * @file mag_control.cpp
 * Control functions for ekf magnetic field fusion
 */

#include "ekf.h"
#include <mathlib/mathlib.h>

void Ekf::controlMagFusion()
{
	bool mag_data_ready = false;

	magSample mag_sample;

	if (_mag_buffer) {
		mag_data_ready = _mag_buffer->pop_first_older_than(_imu_sample_delayed.time_us, &mag_sample);

		if (mag_data_ready) {
			_mag_lpf.update(mag_sample.mag);
			_mag_counter++;

			// if enabled, use knowledge of theoretical magnetic field vector to calculate a synthetic magnetomter Z component value.
			// this is useful if there is a lot of interference on the sensor measurement.
			if (_params.synthesize_mag_z && (_params.mag_declination_source & MASK_USE_GEO_DECL)
			    && (_NED_origin_initialised || PX4_ISFINITE(_mag_declination_gps))
			   ) {
				const Vector3f mag_earth_pred = Dcmf(Eulerf(0, -_mag_inclination_gps, _mag_declination_gps)) * Vector3f(_mag_strength_gps, 0, 0);
				mag_sample.mag(2) = calculate_synthetic_mag_z_measurement(mag_sample.mag, mag_earth_pred);
				_control_status.flags.synthetic_mag_z = true;

			} else {
				_control_status.flags.synthetic_mag_z = false;
			}

			_control_status.flags.mag_field_disturbed = magFieldStrengthDisturbed(mag_sample.mag);
		}
	}

	// If we are on ground, reset the flight alignment flag so that the mag fields will be
	// re-initialised next time we achieve flight altitude
	if (!_control_status.flags.in_air) {
		_control_status.flags.mag_aligned_in_flight = false;
		_num_bad_flight_yaw_events = 0;
	}

	checkYawAngleObservability();
	checkMagBiasObservability();

	if (_mag_bias_observable || _yaw_angle_observable) {
		_time_last_mov_3d_mag_suitable = _imu_sample_delayed.time_us;
	}

	if (_params.mag_fusion_type >= MAG_FUSE_TYPE_NONE
	    || _control_status.flags.mag_fault
	    || !_control_status.flags.tilt_align) {

		stopMagFusion();
		return;
	}

	if (mag_data_ready && !_control_status.flags.ev_yaw && !_control_status.flags.gps_yaw) {

		const bool mag_enabled_previously = _control_status_prev.flags.mag_hdg || _control_status_prev.flags.mag_3D;

		// Determine if we should use simple magnetic heading fusion which works better when
		// there are large external disturbances or the more accurate 3-axis fusion
		switch (_params.mag_fusion_type) {
		default:

		// FALLTHROUGH
		case MAG_FUSE_TYPE_AUTO:
			// Use of 3D fusion requires an in-air heading alignment but it should not
			// be used when the heading and mag biases are not observable for more than 2 seconds
			if (_control_status.flags.mag_aligned_in_flight
			    && ((_imu_sample_delayed.time_us - _time_last_mov_3d_mag_suitable) < (uint64_t)2e6)
			   ) {
				startMag3DFusion();

			} else {
				startMagHdgFusion();
			}

			break;

		case MAG_FUSE_TYPE_INDOOR:

		/* fallthrough */
		case MAG_FUSE_TYPE_HEADING:
			startMagHdgFusion();
			break;

		case MAG_FUSE_TYPE_3D:
			startMag3DFusion();
			break;
		}

		const bool mag_enabled = _control_status.flags.mag_hdg || _control_status.flags.mag_3D;

		const bool declination_changed = _control_status.flags.mag_hdg && (fabsf(_mag_heading_last_declination - getMagDeclination()) > math::radians(1.f));

		bool mag_fuse_recent = !isTimedOut(_time_last_mag_heading_fuse, (uint64_t)5e6) || !isTimedOut(_time_last_mag_3d_fuse, (uint64_t)5e6);

		if (!_control_status.flags.yaw_align
		    || !mag_fuse_recent
		    || declination_changed
		    || haglYawResetReq()
		    || (!mag_enabled_previously && mag_enabled)) {

			runYawReset();
		}

		if (_control_status.flags.mag_hdg) {
			_mag_heading_last_declination = getMagDeclination();
		}

		if (!_control_status.flags.yaw_align) {
			// Having the yaw aligned is mandatory to continue
			return;
		}

		checkMagDeclRequired();

		_is_yaw_fusion_inhibited = shouldInhibitMag();

		runMagAndMagDeclFusions(mag_sample.mag);
	}
}

bool Ekf::haglYawResetReq() const
{
	// We need to reset the yaw angle after climbing away from the ground to enable
	// recovery from ground level magnetic interference.
	if (_control_status.flags.in_air && !_control_status.flags.mag_aligned_in_flight) {
		// Check if height has increased sufficiently to be away from ground magnetic anomalies
		// and request a yaw reset if not already requested.
		static constexpr float mag_anomalies_max_hagl = 1.5f;

		if ((getTerrainVPos() - _state.pos(2)) > mag_anomalies_max_hagl) {
			return true;
		}
	}

	return false;
}

void Ekf::runYawReset()
{
	bool has_realigned_yaw = false;

	if (_control_status.flags.gps && _control_status.flags.fixed_wing) {
		has_realigned_yaw = realignYawGPS();
	}

	if (!has_realigned_yaw) {
		has_realigned_yaw = resetMagHeading();
	}

	if (has_realigned_yaw) {
		_control_status.flags.yaw_align = true;

		if (_control_status.flags.in_air) {
			_control_status.flags.mag_aligned_in_flight = true;
		}

		// Handle the special case where we have not been constraining yaw drift or learning yaw bias due
		// to assumed invalid mag field associated with indoor operation with a downwards looking flow sensor.
		bool mag_fuse_recent = !isTimedOut(_time_last_mag_heading_fuse, (uint64_t)5e6) || !isTimedOut(_time_last_mag_3d_fuse, (uint64_t)5e6);

		if (!mag_fuse_recent) {
			// Zero the yaw bias covariance and set the variance to the initial alignment uncertainty
			P.uncorrelateCovarianceSetVariance<1>(12, sq(_params.switch_on_gyro_bias * _dt_ekf_avg));
		}

		// reset
		_time_last_mag_heading_fuse = _time_last_imu;
	}
}

void Ekf::checkYawAngleObservability()
{
	// calculate a filtered horizontal acceleration with a 1 sec time constant

	// Calculate an earth frame delta velocity
	const Vector3f corrected_delta_vel = _imu_sample_delayed.delta_vel - _state.delta_vel_bias;
	const Vector3f corrected_delta_vel_ef = _R_to_earth * corrected_delta_vel;

	const float alpha = 1.0f - _imu_sample_delayed.delta_vel_dt;
	_accel_lpf_NE = _accel_lpf_NE * alpha + corrected_delta_vel_ef.xy();

	// Check if there has been enough change in horizontal velocity to make yaw observable
	// Apply hysteresis to check to avoid rapid toggling
	if (_control_status.flags.gps) {
		if (_yaw_angle_observable) {
			_yaw_angle_observable = _accel_lpf_NE.norm() > _params.mag_acc_gate;

		} else {
			_yaw_angle_observable = _accel_lpf_NE.norm() > _params.mag_acc_gate * 2.f;
		}

	} else {
		_yaw_angle_observable = false;
	}
}

void Ekf::checkMagBiasObservability()
{
	// calculate a yaw change about the earth frame vertical
	const Vector3f delta_angle = _imu_sample_delayed.delta_ang - _state.delta_ang_bias;

	const float spin_del_ang_D = delta_angle.dot(Vector3f(_R_to_earth.row(2)));
	float yaw_delta_ef = spin_del_ang_D;

	// Calculate filtered yaw rate to be used by the magnetometer fusion type selection logic
	// Note fixed coefficients are used to save operations. The exact time constant is not important.
	_yaw_rate_lpf_ef = 0.95f * _yaw_rate_lpf_ef + 0.05f * spin_del_ang_D / _imu_sample_delayed.delta_ang_dt;

	// check if there is enough yaw rotation to make the mag bias states observable
	if (!_mag_bias_observable && (fabsf(_yaw_rate_lpf_ef) > _params.mag_yaw_rate_gate)) {
		// initial yaw motion is detected
		_mag_bias_observable = true;

	} else if (_mag_bias_observable) {
		// require sustained yaw motion of 50% the initial yaw rate threshold
		const float yaw_dt = 1e-6f * (float)(_imu_sample_delayed.time_us - _time_yaw_started);
		const float min_yaw_change_req = 0.5f * _params.mag_yaw_rate_gate * yaw_dt;
		_mag_bias_observable = fabsf(yaw_delta_ef) > min_yaw_change_req;
	}

	_time_yaw_started = _imu_sample_delayed.time_us;
}

void Ekf::checkMagDeclRequired()
{
	// if we are using 3-axis magnetometer fusion, but without external NE aiding,
	// then the declination must be fused as an observation to prevent long term heading drift
	// fusing declination when gps aiding is available is optional, but recommended to prevent
	// problem if the vehicle is static for extended periods of time
	const bool user_selected = (_params.mag_declination_source & MASK_FUSE_DECL);
	const bool not_using_ne_aiding = !_control_status.flags.gps;
	_control_status.flags.mag_dec = (_control_status.flags.mag_3D && (not_using_ne_aiding || user_selected));
}

bool Ekf::shouldInhibitMag() const
{
	// If the user has selected auto protection against indoor magnetic field errors, only use the magnetometer
	// if a yaw angle relative to true North is required for navigation. If no GPS or other earth frame aiding
	// is available, assume that we are operating indoors and the magnetometer should not be used.
	// Also inhibit mag fusion when a strong magnetic field interference is detected or the user
	// has explicitly stopped magnetometer use.
	const bool user_selected = (_params.mag_fusion_type == MAG_FUSE_TYPE_INDOOR);

	const bool heading_not_required_for_navigation = !_control_status.flags.gps
			&& !_control_status.flags.ev_pos
			&& !_control_status.flags.ev_vel;

	return (user_selected && heading_not_required_for_navigation) || _control_status.flags.mag_field_disturbed;
}

bool Ekf::magFieldStrengthDisturbed(const Vector3f &mag_sample) const
{
	if (_params.check_mag_strength
	    && ((_params.mag_fusion_type <= MAG_FUSE_TYPE_3D) || (_params.mag_fusion_type == MAG_FUSE_TYPE_INDOOR && _control_status.flags.gps))) {

		if (PX4_ISFINITE(_mag_strength_gps)) {
			constexpr float wmm_gate_size = 0.2f; // +/- Gauss
			return !isMeasuredMatchingExpected(mag_sample.length(), _mag_strength_gps, wmm_gate_size);

		} else {
			constexpr float average_earth_mag_field_strength = 0.45f; // Gauss
			constexpr float average_earth_mag_gate_size = 0.40f; // +/- Gauss
			return !isMeasuredMatchingExpected(mag_sample.length(), average_earth_mag_field_strength, average_earth_mag_gate_size);
		}
	}

	return false;
}

bool Ekf::isMeasuredMatchingExpected(const float measured, const float expected, const float gate)
{
	return (measured >= expected - gate)
	       && (measured <= expected + gate);
}

void Ekf::runMagAndMagDeclFusions(const Vector3f &mag)
{
	if (_control_status.flags.mag_3D) {
		run3DMagAndDeclFusions(mag);

	} else if (_control_status.flags.mag_hdg) {
		// Rotate the measurements into earth frame using the zero yaw angle
		Dcmf R_to_earth = shouldUse321RotationSequence(_R_to_earth) ? updateEuler321YawInRotMat(0.f, _R_to_earth) : updateEuler312YawInRotMat(0.f, _R_to_earth);

		Vector3f mag_earth_pred = R_to_earth * (mag - _state.mag_B);

		// the angle of the projection onto the horizontal gives the yaw angle
		// calculate the yaw innovation and wrap to the interval between +-pi
		float measured_hdg = wrap_pi(-atan2f(mag_earth_pred(1), mag_earth_pred(0)) + getMagDeclination());

		float innovation = wrap_pi(getEulerYaw(_R_to_earth) - measured_hdg);

		float obs_var = fmaxf(sq(_params.mag_heading_noise), 1.e-4f);

		// Update the quaternion states and covariance matrix
		if (fuseYaw(innovation, obs_var)) {
			_time_last_mag_heading_fuse = _time_last_imu;
		}
	}
}

void Ekf::run3DMagAndDeclFusions(const Vector3f &mag)
{
	// For the first few seconds after in-flight alignment we allow the magnetic field state estimates to stabilise
	// before they are used to constrain heading drift
	const bool update_all_states = ((_imu_sample_delayed.time_us - _flt_mag_align_start_time) > (uint64_t)5e6)
			&& !_control_status.flags.mag_fault && !_control_status.flags.mag_field_disturbed;

	if (!_mag_decl_cov_reset) {
		// After any magnetic field covariance reset event the earth field state
		// covariances need to be corrected to incorporate knowledge of the declination
		// before fusing magnetomer data to prevent rapid rotation of the earth field
		// states for the first few observations.
		fuseDeclination(0.02f);
		_mag_decl_cov_reset = true;
		fuseMag(mag, update_all_states);

	} else {
		// The normal sequence is to fuse the magnetometer data first before fusing
		// declination angle at a higher uncertainty to allow some learning of
		// declination angle over time.
		fuseMag(mag, update_all_states);

		if (_control_status.flags.mag_dec) {
			fuseDeclination(0.5f);
		}
	}
}

bool Ekf::resetMagStates()
{
	bool reset = false;

	// reinit mag states
	const bool mag_available = (_mag_counter != 0) && isRecent(_time_last_mag, 500000);

	// if world magnetic model (inclination, declination, strength) available then use it to reset mag states
	if (PX4_ISFINITE(_mag_inclination_gps) && PX4_ISFINITE(_mag_declination_gps) && PX4_ISFINITE(_mag_strength_gps)) {
		// use predicted earth field to reset states
		const Vector3f mag_earth_pred = Dcmf(Eulerf(0, -_mag_inclination_gps, _mag_declination_gps)) * Vector3f(_mag_strength_gps, 0, 0);
		_state.mag_I = mag_earth_pred;

		ECL_DEBUG("resetting mag I to [%.3f, %.3f, %.3f]", (double)_state.mag_I(0), (double)_state.mag_I(1), (double)_state.mag_I(2));

		if (mag_available) {
			const Dcmf R_to_body = quatToInverseRotMat(_state.quat_nominal);
			_state.mag_B = _mag_lpf.getState() - (R_to_body * mag_earth_pred);

			ECL_DEBUG("resetting mag B to [%.3f, %.3f, %.3f]", (double)_state.mag_B(0), (double)_state.mag_B(1), (double)_state.mag_B(2));

		} else {
			_state.mag_B.zero();
		}

		reset = true;

	} else if (mag_available && !magFieldStrengthDisturbed(_mag_lpf.getState())) {
		// Use the last magnetometer measurements to reset the field states

		// calculate initial earth magnetic field states
		_state.mag_I = _R_to_earth * _mag_lpf.getState();
		_state.mag_B.zero();

		ECL_DEBUG("resetting mag I to [%.3f, %.3f, %.3f]", (double)_state.mag_I(0), (double)_state.mag_I(1), (double)_state.mag_I(2));

		reset = true;
	}

	if (reset) {
		resetMagCov();

		if (mag_available) {
			// record the start time for the magnetic field alignment
			_flt_mag_align_start_time = _imu_sample_delayed.time_us;
			_control_status.flags.mag_aligned_in_flight = true;
			_time_last_mag_heading_fuse = _time_last_imu;
			_time_last_mag_3d_fuse = _time_last_imu;
		}

		return true;
	}

	return false;
}