px4_autopilot/EKF/control.cpp

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/**
 * @file control.cpp
 * Control functions for ekf attitude and position estimator.
 *
 * @author Paul Riseborough <p_riseborough@live.com.au>
 *
 */

#include "ekf.h"

void Ekf::controlFusionModes()
{
	// Determine the vehicle status
	calculateVehicleStatus();

	// Get the magnetic declination
	calcMagDeclination();

	// Check for tilt convergence during initial alignment
	// filter the tilt error vector using a 1 sec time constant LPF
	float filt_coef = 1.0f * _imu_sample_delayed.delta_ang_dt;
	_tilt_err_length_filt = filt_coef * _tilt_err_vec.norm() + (1.0f - filt_coef) * _tilt_err_length_filt;

	// Once the tilt error has reduced sufficiently, initialise the yaw and magnetic field states
	if (_tilt_err_length_filt < 0.005f && !_control_status.flags.tilt_align) {
		_control_status.flags.tilt_align = true;
		_control_status.flags.yaw_align = resetMagHeading(_mag_sample_delayed.mag);
	}

	// optical flow fusion mode selection logic
	// to start using optical flow data we need angular alignment complete, and fresh optical flow and height above terrain data
	if ((_params.fusion_mode & MASK_USE_OF) && !_control_status.flags.opt_flow && _control_status.flags.tilt_align
	    && (_time_last_imu - _time_last_optflow) < 5e5 && (_time_last_imu - _time_last_hagl_fuse) < 5e5) {
		// If the heading is not aligned, reset the yaw and magnetic field states
		if (!_control_status.flags.yaw_align) {
			_control_status.flags.yaw_align = resetMagHeading(_mag_sample_delayed.mag);
		}

		// If the heading is valid, start using optical flow aiding
		if (_control_status.flags.yaw_align) {
			// set the flag and reset the fusion timeout
			_control_status.flags.opt_flow = true;
			_time_last_of_fuse = _time_last_imu;

			// if we are not using GPS and are in air, then we need to reset the velocity to be consistent with the optical flow reading
			if (!_control_status.flags.gps) {
				// calculate the rotation matrix from body to earth frame
				matrix::Dcm<float> body_to_earth(_state.quat_nominal);

				// constrain height above ground to be above minimum possible
				float heightAboveGndEst = fmaxf((_terrain_vpos - _state.pos(2)), _params.rng_gnd_clearance);

				// calculate absolute distance from focal point to centre of frame assuming a flat earth
				float range = heightAboveGndEst / body_to_earth(2, 2);

				if (_in_air && (range - _params.rng_gnd_clearance) > 0.3f && _flow_sample_delayed.dt > 0.05f) {
					// calculate X and Y body relative velocities from OF measurements
					Vector3f vel_optflow_body;
					vel_optflow_body(0) = - range * _flow_sample_delayed.flowRadXYcomp(1) / _flow_sample_delayed.dt;
					vel_optflow_body(1) =   range * _flow_sample_delayed.flowRadXYcomp(0) / _flow_sample_delayed.dt;
					vel_optflow_body(2) = 0.0f;

					// rotate from body to earth frame
					Vector3f vel_optflow_earth;
					vel_optflow_earth = body_to_earth * vel_optflow_body;

					// take x and Y components
					_state.vel(0) = vel_optflow_earth(0);
					_state.vel(1) = vel_optflow_earth(1);

				} else {
					_state.vel.setZero();
				}
			}
		}

	} else if (!(_params.fusion_mode & MASK_USE_OF)) {
		_control_status.flags.opt_flow = false;
	}

	// GPS fusion mode selection logic
	// To start use GPS we need angular alignment completed, the local NED origin set and fresh GPS data
	if ((_params.fusion_mode & MASK_USE_GPS) && !_control_status.flags.gps) {
		if (_control_status.flags.tilt_align && (_time_last_imu - _time_last_gps) < 5e5 && _NED_origin_initialised
		    && (_time_last_imu - _last_gps_fail_us > 5e6)) {
			// If the heading is not aligned, reset the yaw and magnetic field states
			if (!_control_status.flags.yaw_align) {
				_control_status.flags.yaw_align = resetMagHeading(_mag_sample_delayed.mag);
			}

			// If the heading is valid, reset the positon and velocity and start using gps aiding
			if (_control_status.flags.yaw_align) {
				resetPosition();
				resetVelocity();
				_control_status.flags.gps = true;
				_time_last_gps = _time_last_imu;
			}
		}

	}  else if (!(_params.fusion_mode & MASK_USE_GPS)) {
		_control_status.flags.gps = false;
	}

	// handle the case when we are relying on GPS fusion and lose it
	if (_control_status.flags.gps && !_control_status.flags.opt_flow) {
		// We are relying on GPS aiding to constrain attitude drift so after 10 seconds without aiding we need to do something
		if ((_time_last_imu - _time_last_pos_fuse > 10e6) && (_time_last_imu - _time_last_vel_fuse > 10e6)) {
			if (_time_last_imu - _time_last_gps > 5e5) {
				// if we don't have gps then we need to switch to the non-aiding mode, zero the veloity states
				// and set the synthetic GPS position to the current estimate
				_control_status.flags.gps = false;
				_last_known_posNE(0) = _state.pos(0);
				_last_known_posNE(1) = _state.pos(1);
				_state.vel.setZero();

			} else {
				// Reset states to the last GPS measurement
				resetPosition();
				resetVelocity();
			}
		}
	}

	// Handle the case where we have rejected height measurements for an extended period
	// This excessive vibration levels can cause this so a reset gives the filter a chance to recover
	// After 10 seconds without aiding we reset to the height measurement provided the data is fresh
	if ((_time_last_imu - _time_last_hgt_fuse > 10e6) && (_time_last_imu - _time_last_baro < 5e5)) {
		// Reset vertical position and velocity states to the last measurement
		resetHeight();
	}

	// handle the case when we are relying on optical flow fusion and lose it
	if (_control_status.flags.opt_flow && !_control_status.flags.gps) {
		// We are relying on flow aiding to constrain attitude drift so after 5s without aiding we need to do something
		if ((_time_last_imu - _time_last_of_fuse > 5e6)) {
			// Switch to the non-aiding mode, zero the veloity states
			// and set the synthetic position to the current estimate
			_control_status.flags.opt_flow = false;
			_last_known_posNE(0) = _state.pos(0);
			_last_known_posNE(1) = _state.pos(1);
			_state.vel.setZero();
		}
	}

	// 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
	if (_params.mag_fusion_type == MAG_FUSE_TYPE_AUTO) {
		if (!_control_status.flags.armed) {
			// use heading fusion for initial startup
			_control_status.flags.mag_hdg = true;
			_control_status.flags.mag_2D = false;
			_control_status.flags.mag_3D = false;

		} else {
			if (_control_status.flags.in_air) {
				// if transitioning into 3-axis fusion mode, we need to initialise the yaw angle and field states
				if (!_control_status.flags.mag_3D) {
					_control_status.flags.yaw_align = resetMagHeading(_mag_sample_delayed.mag);
				}

				// use 3D mag fusion when airborne
				_control_status.flags.mag_hdg = false;
				_control_status.flags.mag_2D = false;
				_control_status.flags.mag_3D = true;

			} else {
				// use heading fusion when on the ground
				_control_status.flags.mag_hdg = true;
				_control_status.flags.mag_2D = false;
				_control_status.flags.mag_3D = false;
			}
		}

	} else if (_params.mag_fusion_type == MAG_FUSE_TYPE_HEADING) {
		// always use heading fusion
		_control_status.flags.mag_hdg = true;
		_control_status.flags.mag_2D = false;
		_control_status.flags.mag_3D = false;

	} else if (_params.mag_fusion_type == MAG_FUSE_TYPE_2D) {
		// always use 2D mag fusion
		_control_status.flags.mag_hdg = false;
		_control_status.flags.mag_2D = true;
		_control_status.flags.mag_3D = false;

	} else if (_params.mag_fusion_type == MAG_FUSE_TYPE_3D) {
		// if transitioning into 3-axis fusion mode, we need to initialise the yaw angle and field states
		if (!_control_status.flags.mag_3D) {
			_control_status.flags.yaw_align = resetMagHeading(_mag_sample_delayed.mag);
		}

		// always use 3-axis mag fusion
		_control_status.flags.mag_hdg = false;
		_control_status.flags.mag_2D = false;
		_control_status.flags.mag_3D = true;

	} else {
		// do no magnetometer fusion at all
		_control_status.flags.mag_hdg = false;
		_control_status.flags.mag_2D = false;
		_control_status.flags.mag_3D = false;
	}

	// if we are using 3-axis magnetometer fusion, but without external 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 recommneded to prevent problem if the vehicle is static for extended periods of time
	if (_control_status.flags.mag_3D && (!_control_status.flags.gps || (_params.mag_declination_source & MASK_FUSE_DECL))) {
		_control_status.flags.mag_dec = true;

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

	// Control the soure of height measurements for the main filter
	_control_status.flags.baro_hgt = true;
	_control_status.flags.rng_hgt = false;
	_control_status.flags.gps_hgt = false;


	// Placeholder for control of wind velocity states estimation
	// TODO add methods for true airspeed and/or sidelsip fusion or some type of drag force measurement
	if (false) {
		_control_status.flags.wind = false;
	}

	// Store the status to enable change detection
	_control_status_prev.value = _control_status.value;
}

void Ekf::calculateVehicleStatus()
{
	// determine if the vehicle is armed
	_control_status.flags.armed = _vehicle_armed;

	// record vertical position whilst disarmed to use as a height change reference
	if (!_control_status.flags.armed) {
		_last_disarmed_posD = _state.pos(2);
	}

	// Transition to in-air occurs when armed and when altitude has increased sufficiently from the altitude at arming
	bool in_air = _control_status.flags.armed && (_state.pos(2) - _last_disarmed_posD) < -1.0f;

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

	// Transition to on-ground occurs when disarmed or if the land detector indicated landed state
	if (_control_status.flags.in_air && (!_control_status.flags.armed || !_in_air)) {
		_control_status.flags.in_air = false;
	}
}