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()
{
	// reset the flight alignment flag so that the mag fields will be
	//  re-initialised next time we achieve flight altitude
	if (!_control_status_prev.flags.in_air && _control_status.flags.in_air) {
		_control_status.flags.mag_aligned_in_flight = false;
	}

	// check mag state observability
	checkYawAngleObservability();
	checkMagBiasObservability();
	checkMagHeadingConsistency();

	if (_mag_bias_observable || _yaw_angle_observable) {
		_time_last_mov_3d_mag_suitable = _time_delayed_us;
	}

	if (_params.mag_fusion_type == MagFuseType::NONE) {
		stopMagFusion();
		stopMagHdgFusion();
		return;
	}

	// stop mag (require a reset before using again) if there was an external yaw reset (yaw estimator, GPS yaw, etc)
	if (_mag_decl_cov_reset && (_state_reset_status.reset_count.quat != _state_reset_count_prev.quat)) {
		ECL_INFO("yaw reset, stopping mag fusion to force reinitialization");
		stopMagFusion();
		resetMagCov();
	}

	magSample mag_sample;

	if (_mag_buffer && _mag_buffer->pop_first_older_than(_time_delayed_us, &mag_sample)) {

		if (mag_sample.reset || (_mag_counter == 0)) {
			// sensor or calibration has changed, reset low pass filter (reset handled by controlMag3DFusion/controlMagHeadingFusion)
			_control_status.flags.mag_fault = false;

			_state.mag_B.zero();
			resetMagCov();

			_mag_lpf.reset(mag_sample.mag);
			_mag_counter = 1;

		} else {
			_mag_lpf.update(mag_sample.mag);
			_mag_counter++;
		}

		const bool starting_conditions_passing = (_params.mag_fusion_type != MagFuseType::NONE)
				&& checkMagField(mag_sample.mag)
				&& (_mag_counter > 5) // wait until we have more than a few samples through the filter
				&& (_control_status.flags.yaw_align == _control_status_prev.flags.yaw_align) // no yaw alignment change this frame
				&& (_state_reset_status.reset_count.quat == _state_reset_count_prev.quat) // don't allow starting on same frame as yaw reset
				&& isNewestSampleRecent(_time_last_mag_buffer_push, MAG_MAX_INTERVAL);

		// 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 & GeoDeclinationMask::USE_GEO_DECL)
		    && (PX4_ISFINITE(_mag_inclination_gps) && PX4_ISFINITE(_mag_declination_gps) && PX4_ISFINITE(_mag_strength_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;
		}

		controlMag3DFusion(mag_sample, starting_conditions_passing, _aid_src_mag);
		controlMagHeadingFusion(mag_sample, starting_conditions_passing, _aid_src_mag_heading);

	} else if (!isNewestSampleRecent(_time_last_mag_buffer_push, 2 * MAG_MAX_INTERVAL)) {
		// No data anymore. Stop until it comes back.
		stopMagHdgFusion();
		stopMagFusion();
	}
}

bool Ekf::checkHaglYawResetReq() const
{
#if defined(CONFIG_EKF2_TERRAIN)
	// 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.yaw_align && !_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;
		return above_mag_anomalies;
	}
#endif // CONFIG_EKF2_TERRAIN

	return false;
}

void Ekf::resetMagStates(const Vector3f &mag, bool reset_heading)
{
	// reinit mag states
	const Vector3f mag_I_before_reset = _state.mag_I;
	const Vector3f mag_B_before_reset = _state.mag_B;

	// reset covariances to default
	resetMagCov();

	// 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 expected 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);

		// mag_B: reset
#if defined(CONFIG_EKF2_GNSS)
		if (isYawEmergencyEstimateAvailable()) {

			const Dcmf R_to_earth = updateYawInRotMat(_yawEstimator.getYaw(), _R_to_earth);
			const Dcmf R_to_body = R_to_earth.transpose();

			// mag_B: reset using WMM and yaw estimator
			_state.mag_B = mag - (R_to_body * mag_earth_pred);

			ECL_INFO("resetMagStates using yaw estimator");

		} else if (!reset_heading && _control_status.flags.yaw_align) {
#else
		if (!reset_heading && _control_status.flags.yaw_align) {
#endif
			// mag_B: reset using WMM
			const Dcmf R_to_body = quatToInverseRotMat(_state.quat_nominal);
			_state.mag_B = mag - (R_to_body * mag_earth_pred);

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

		// mag_I: reset, skipped if no change in state and variance good
		_state.mag_I = mag_earth_pred;

		if (reset_heading) {
			resetMagHeading(mag);
		}

		// earth field was reset to WMM, skip initial declination fusion
		_mag_decl_cov_reset = true;

	} else {
		// mag_B: reset
		_state.mag_B.zero();

		// Use the magnetometer measurement to reset the field states
		if (reset_heading) {
			resetMagHeading(mag);
		}

		// mag_I: use the last magnetometer measurements to reset the field states
		_state.mag_I = _R_to_earth * mag;
	}

	if (!mag_I_before_reset.longerThan(0.f)) {
		ECL_INFO("initializing mag I [%.3f, %.3f, %.3f], mag B [%.3f, %.3f, %.3f]",
			 (double)_state.mag_I(0), (double)_state.mag_I(1), (double)_state.mag_I(2),
			 (double)_state.mag_B(0), (double)_state.mag_B(1), (double)_state.mag_B(2)
			);

	} else {
		ECL_INFO("resetting mag I [%.3f, %.3f, %.3f] -> [%.3f, %.3f, %.3f]",
			 (double)mag_I_before_reset(0), (double)mag_I_before_reset(1), (double)mag_I_before_reset(2),
			 (double)_state.mag_I(0), (double)_state.mag_I(1), (double)_state.mag_I(2)
			);

		if (mag_B_before_reset.longerThan(0.f) || _state.mag_B.longerThan(0.f)) {
			ECL_INFO("resetting mag B [%.3f, %.3f, %.3f] -> [%.3f, %.3f, %.3f]",
				 (double)mag_B_before_reset(0), (double)mag_B_before_reset(1), (double)mag_B_before_reset(2),
				 (double)_state.mag_B(0), (double)_state.mag_B(1), (double)_state.mag_B(2)
				);
		}
	}

	// record the start time for the magnetic field alignment
	if (_control_status.flags.in_air) {
		_control_status.flags.mag_aligned_in_flight = true;
		_flt_mag_align_start_time = _time_delayed_us;
	}
}

void Ekf::checkYawAngleObservability()
{
	if (_control_status.flags.gps) {
		// Check if there has been enough change in horizontal velocity to make yaw observable
		// Apply hysteresis to check to avoid rapid toggling
		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()
{
	// 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)(_time_delayed_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 = _time_delayed_us;
}

void Ekf::checkMagHeadingConsistency()
{
	if (fabsf(_mag_heading_innov_lpf.getState()) < _params.mag_heading_noise) {
		if (_yaw_angle_observable) {
			// yaw angle must be observable to consider consistency
			_control_status.flags.mag_heading_consistent = true;
		}

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

bool Ekf::checkMagField(const Vector3f &mag_sample)
{
	_control_status.flags.mag_field_disturbed = false;

	if (_params.mag_check == 0) {
		// skip all checks
		return true;
	}

	bool is_check_failing = false;
	_mag_strength = mag_sample.length();

	if (_params.mag_check & static_cast<int32_t>(MagCheckMask::STRENGTH)) {
		if (PX4_ISFINITE(_mag_strength_gps)) {
			if (!isMeasuredMatchingExpected(_mag_strength, _mag_strength_gps, _params.mag_check_strength_tolerance_gs)) {
				_control_status.flags.mag_field_disturbed = true;
				is_check_failing = true;
			}

		} else if (_params.mag_check & static_cast<int32_t>(MagCheckMask::FORCE_WMM)) {
			is_check_failing = true;

		} else {
			constexpr float average_earth_mag_field_strength = 0.45f; // Gauss
			constexpr float average_earth_mag_gate_size = 0.40f; // +/- Gauss

			if (!isMeasuredMatchingExpected(mag_sample.length(), average_earth_mag_field_strength, average_earth_mag_gate_size)) {
				_control_status.flags.mag_field_disturbed = true;
				is_check_failing = true;
			}
		}
	}

	const Vector3f mag_earth = _R_to_earth * mag_sample;
	_mag_inclination = asinf(mag_earth(2) / fmaxf(mag_earth.norm(), 1e-4f));

	if (_params.mag_check & static_cast<int32_t>(MagCheckMask::INCLINATION)) {
		if (PX4_ISFINITE(_mag_inclination_gps)) {
			const float inc_tol_rad = radians(_params.mag_check_inclination_tolerance_deg);
			const float inc_error_rad = wrap_pi(_mag_inclination - _mag_inclination_gps);

			if (fabsf(inc_error_rad) > inc_tol_rad) {
				_control_status.flags.mag_field_disturbed = true;
				is_check_failing = true;
			}

		} else if (_params.mag_check & static_cast<int32_t>(MagCheckMask::FORCE_WMM)) {
			is_check_failing = true;

		} else {
			// No check possible when the global position is unknown
			// TODO: add parameter to remember the inclination between boots
		}
	}

	if (is_check_failing || (_time_last_mag_check_failing == 0)) {
		_time_last_mag_check_failing = _time_delayed_us;
	}

	return ((_time_delayed_us - _time_last_mag_check_failing) > (uint64_t)_min_mag_health_time_us);
}

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

void Ekf::resetMagHeading(const Vector3f &mag)
{
	// use mag bias if variance good (unless configured for HEADING only)
	Vector3f mag_bias{0.f, 0.f, 0.f};
	const Vector3f mag_bias_var = getMagBiasVariance();

	if ((mag_bias_var.min() > 0.f) && (mag_bias_var.max() <= sq(_params.mag_noise))) {
		mag_bias = _state.mag_B;
	}

	// calculate mag heading
	// rotate the magnetometer measurements into earth frame using a zero yaw angle
	const Dcmf R_to_earth = updateYawInRotMat(0.f, _R_to_earth);

	// the angle of the projection onto the horizontal gives the yaw angle
	const Vector3f mag_earth_pred = R_to_earth * (mag - mag_bias);

	// calculate the observed yaw angle and yaw variance
	const float declination = getMagDeclination();
	float yaw_new = -atan2f(mag_earth_pred(1), mag_earth_pred(0)) + declination;
	float yaw_new_variance = math::max(sq(_params.mag_heading_noise), sq(0.01f));

	ECL_INFO("reset mag heading %.3f -> %.3f rad (bias:[%.3f, %.3f, %.3f], declination:%.1f)",
		 (double)getEulerYaw(_R_to_earth), (double)yaw_new,
		 (double)mag_bias(0), (double)mag_bias(1), (double)mag_bias(2),
		 (double)declination);

	// update quaternion states and corresponding covarainces
	resetQuatStateYaw(yaw_new, yaw_new_variance);
	_mag_heading_last_declination = declination;

	_time_last_heading_fuse = _time_delayed_us;

	_mag_heading_innov_lpf.reset(0.f);
	_control_status.flags.mag_heading_consistent = true;
}

float Ekf::getMagDeclination()
{
	// set source of magnetic declination for internal use
	if (_control_status.flags.mag_aligned_in_flight) {
		// Use value consistent with earth field state
		return atan2f(_state.mag_I(1), _state.mag_I(0));

	} else if (_params.mag_declination_source & GeoDeclinationMask::USE_GEO_DECL) {
		// use parameter value until GPS is available, then use value returned by geo library
		if (PX4_ISFINITE(_mag_declination_gps)) {
			return _mag_declination_gps;
		}
	}

	// otherwise use the parameter value
	return math::radians(_params.mag_declination_deg);
}