px4_autopilot/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()
{
	// handle undefined behaviour
	if (_params.mag_fusion_type > MAG_FUSE_TYPE_NONE) {
		return;
	}

	// When operating without a magnetometer and no other source of yaw aiding is active,
	// yaw fusion is run selectively to enable yaw gyro bias learning when stationary on
	// ground and to prevent uncontrolled yaw variance growth
	if (_params.mag_fusion_type == MAG_FUSE_TYPE_NONE) {
		if (noOtherYawAidingThanMag())
		{
			_is_yaw_fusion_inhibited = true;
			fuseHeading();
		}
		return;
	}

	checkMagFieldStrength();

	// 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;
	}

	if (_control_status.flags.mag_fault || !_control_status.flags.tilt_align) {
		stopMagFusion();
		return;
	}

	_mag_yaw_reset_req |= otherHeadingSourcesHaveStopped();
	_mag_yaw_reset_req |= !_control_status.flags.yaw_align;
	_mag_yaw_reset_req |= _mag_inhibit_yaw_reset_req;

	if (noOtherYawAidingThanMag() && _mag_data_ready) {
		// 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:
			selectMagAuto();
			break;

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

		case MAG_FUSE_TYPE_3D:
			startMag3DFusion();
			break;
		}

		if (_control_status.flags.in_air) {
			checkHaglYawResetReq();
			runInAirYawReset();
			runVelPosReset(); // TODO: review this; a vel/pos reset can be requested from COG reset (for fixedwing) only

		} else {
			runOnGroundYawReset();
		}

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

		checkMagDeclRequired();
		checkMagInhibition();

		runMagAndMagDeclFusions();
	}
}

bool Ekf::noOtherYawAidingThanMag() const
{
	// If we are using external vision data or GPS-heading for heading then no magnetometer fusion is used
	return !_control_status.flags.ev_yaw && !_control_status.flags.gps_yaw;
}

void Ekf::checkHaglYawResetReq()
{
	// 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.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;
		const bool above_mag_anomalies = (getTerrainVPos() - _state.pos(2)) > mag_anomalies_max_hagl;
		_mag_yaw_reset_req = _mag_yaw_reset_req || above_mag_anomalies;
	}
}

void Ekf::runOnGroundYawReset()
{
	if (_mag_yaw_reset_req && isYawResetAuthorized()) {
		const bool has_realigned_yaw = canResetMagHeading()
					       ? resetMagHeading(_mag_lpf.getState())
					       : false;

		if (has_realigned_yaw) {
			_mag_yaw_reset_req = false;
			_control_status.flags.yaw_align = 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.
			if (_mag_inhibit_yaw_reset_req) {
				_mag_inhibit_yaw_reset_req = false;
				// Zero the yaw bias covariance and set the variance to the initial alignment uncertainty
				P.uncorrelateCovarianceSetVariance<1>(12, sq(_params.switch_on_gyro_bias * FILTER_UPDATE_PERIOD_S));
			}
		}
	}
}

bool Ekf::canResetMagHeading() const
{
	return !isStrongMagneticDisturbance() && (_params.mag_fusion_type != MAG_FUSE_TYPE_NONE);
}

void Ekf::runInAirYawReset()
{
	if (_mag_yaw_reset_req && isYawResetAuthorized()) {
		bool has_realigned_yaw = false;

		if (canRealignYawUsingGps()) { has_realigned_yaw = realignYawGPS(); }
		else if (canResetMagHeading()) { has_realigned_yaw = resetMagHeading(_mag_lpf.getState()); }

		if (has_realigned_yaw) {
			_mag_yaw_reset_req = false;
			_control_status.flags.yaw_align = true;
			_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.
			if (_mag_inhibit_yaw_reset_req) {
				_mag_inhibit_yaw_reset_req = false;
				// Zero the yaw bias covariance and set the variance to the initial alignment uncertainty
				P.uncorrelateCovarianceSetVariance<1>(12, sq(_params.switch_on_gyro_bias * FILTER_UPDATE_PERIOD_S));
			}
		}

	}
}

void Ekf::runVelPosReset()
{
	if (_velpos_reset_request) {
		resetVelocity();
		resetHorizontalPosition();
		_velpos_reset_request = false;
	}
}

void Ekf::selectMagAuto()
{
	check3DMagFusionSuitability();
	canUse3DMagFusion() ? startMag3DFusion() : startMagHdgFusion();
}

void Ekf::check3DMagFusionSuitability()
{
	checkYawAngleObservability();
	checkMagBiasObservability();

	if (isMagBiasObservable() || isYawAngleObservable()) {
		_time_last_mov_3d_mag_suitable = _imu_sample_delayed.time_us;
	}
}

void Ekf::checkYawAngleObservability()
{
	// Check if there has been enough change in horizontal velocity to make yaw observable
	// Apply hysteresis to check to avoid rapid toggling
	_yaw_angle_observable = _yaw_angle_observable
				? _accel_lpf_NE.norm() > _params.mag_acc_gate
				: _accel_lpf_NE.norm() > 2.0f * _params.mag_acc_gate;

	_yaw_angle_observable = _yaw_angle_observable
				&& (_control_status.flags.gps || _control_status.flags.ev_pos); // Do we have to add ev_vel here?
}

void Ekf::checkMagBiasObservability()
{
	// 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;
	}

	_yaw_delta_ef = 0.0f;
	_time_yaw_started = _imu_sample_delayed.time_us;
}

bool Ekf::canUse3DMagFusion() const
{
	// 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
	return _control_status.flags.mag_aligned_in_flight
	       && ((_imu_sample_delayed.time_us - _time_last_mov_3d_mag_suitable) < (uint64_t)2e6);
}

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));
}

void Ekf::checkMagInhibition()
{
	_is_yaw_fusion_inhibited = shouldInhibitMag();
	if (!_is_yaw_fusion_inhibited) {
		_mag_use_not_inhibit_us = _imu_sample_delayed.time_us;
	}

	// If magnetometer use has been inhibited continuously then a yaw reset is required for a valid heading
	if (uint32_t(_imu_sample_delayed.time_us - _mag_use_not_inhibit_us) > (uint32_t)5e6) {
		_mag_inhibit_yaw_reset_req = true;
	}
}

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)
	       || isStrongMagneticDisturbance();
}

void Ekf::checkMagFieldStrength()
{
	if (_params.check_mag_strength) {
		_control_status.flags.mag_field_disturbed = _NED_origin_initialised
							    ? !isMeasuredMatchingGpsMagStrength()
							    : !isMeasuredMatchingAverageMagStrength();

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

bool Ekf::isMeasuredMatchingGpsMagStrength() const
{
	constexpr float wmm_gate_size = 0.2f; // +/- Gauss
	return isMeasuredMatchingExpected(_mag_sample_delayed.mag.length(), _mag_strength_gps, wmm_gate_size);
}

bool Ekf::isMeasuredMatchingAverageMagStrength() const
{
	constexpr float average_earth_mag_field_strength = 0.45f; // Gauss
	constexpr float average_earth_mag_gate_size = 0.40f; // +/- Gauss
	return isMeasuredMatchingExpected(_mag_sample_delayed.mag.length(),
					  average_earth_mag_field_strength,
					  average_earth_mag_gate_size);
}

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

void Ekf::runMagAndMagDeclFusions()
{
	if (_control_status.flags.mag_3D) {
		run3DMagAndDeclFusions();
	} else if (_control_status.flags.mag_hdg) {
		fuseHeading();
	}
}

void Ekf::run3DMagAndDeclFusions()
{
	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();

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

bool Ekf::otherHeadingSourcesHaveStopped()
{
    // detect rising edge of noOtherYawAidingThanMag()
    bool result = noOtherYawAidingThanMag() && _non_mag_yaw_aiding_running_prev;

    _non_mag_yaw_aiding_running_prev = !noOtherYawAidingThanMag();

    return  result;
}