ardupilot/libraries/AP_DCM/AP_DCM.cpp

/*
	APM_DCM.cpp - DCM AHRS Library, fixed wing version, for Ardupilot Mega
		Code by Doug Weibel, Jordi Mu<EFBFBD>oz and Jose Julio. DIYDrones.com

	This library works with the ArduPilot Mega and "Oilpan"

	This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

        Methods:
				update_DCM()	: Updates the AHRS by integrating the rotation matrix over time using the IMU object data
				get_gyro()			: Returns gyro vector corrected for bias
				get_accel()		: Returns accelerometer vector
				get_dcm_matrix()	: Returns dcm matrix

*/
#include <AP_DCM.h>

// this is the speed in cm/s above which we first get a yaw lock with
// the GPS
#define GPS_SPEED_MIN 300

// this is the speed in cm/s at which we stop using drift correction
// from the GPS and wait for the ground speed to get above GPS_SPEED_MIN
#define GPS_SPEED_RESET 100

void
AP_DCM::set_compass(Compass *compass)
{
	_compass = compass;
}


// run a full DCM update round
// the drift_correction_frequency is how many steps of the algorithm
// to run before doing an accelerometer drift correction step
void
AP_DCM::update_DCM(uint8_t drift_correction_frequency)
{
	float delta_t;
	Vector3f accel;

	// tell the IMU to grab some data
	_imu->update();

	// ask the IMU how much time this sensor reading represents
	delta_t = _imu->get_delta_time();

	// Get current values for gyros
	_gyro_vector  = _imu->get_gyro();

	// accumulate some more accelerometer data
	accel = _imu->get_accel();
	_accel_sum += accel;
	_drift_correction_time += delta_t;

	// Integrate the DCM matrix using gyro inputs
	matrix_update(delta_t);
	if (_dcm_matrix.is_nan()) {
		SITL_debug("NaN matrix\n");
	}

	// add up the omega vector so we pass a value to the drift
	// correction averaged over the same time period as the
	// accelerometers
	_omega_sum += _omega_integ_corr;

	// Normalize the DCM matrix
	normalize();

	// see if we will perform drift correction on this call
	_drift_correction_count++;

	if (_drift_correction_count == drift_correction_frequency) {
		// calculate the average accelerometer vector over
		// this time
		float scale = 1.0 / _drift_correction_count;
		_accel_vector = _accel_sum * scale;
		_accel_sum.zero();

		// calculate the average omega value over this time
		_omega_smoothed = _omega_sum * scale;
		_omega_sum.zero();

		// Perform drift correction
		drift_correction(_drift_correction_time);

		_drift_correction_time = 0;
		_drift_correction_count = 0;
	}

	// paranoid check for bad values in the DCM matrix
	check_matrix();

	// Calculate pitch, roll, yaw for stabilization and navigation
	euler_angles();
}

// update the DCM matrix using only the gyros
void
AP_DCM::matrix_update(float _G_Dt)
{
	// Used for _centripetal correction (theoretically better than _omega)
	_omega_integ_corr = _gyro_vector + _omega_I;
	// Equation 16, adding proportional and integral correction terms
	_omega = _omega_integ_corr + _omega_P;

	float tmpx = _G_Dt * _omega.x;
	float tmpy = _G_Dt * _omega.y;
	float tmpz = _G_Dt * _omega.z;

	// this is an expansion of the DCM matrix multiply, with known
	// zero elements removed
	_dcm_matrix.a.x += _dcm_matrix.a.y * tmpz  - _dcm_matrix.a.z * tmpy;
	_dcm_matrix.a.y += _dcm_matrix.a.z * tmpx  - _dcm_matrix.a.x * tmpz;
	_dcm_matrix.a.z += _dcm_matrix.a.x * tmpy  - _dcm_matrix.a.y * tmpx;
	_dcm_matrix.b.x += _dcm_matrix.b.y * tmpz  - _dcm_matrix.b.z * tmpy;
	_dcm_matrix.b.y += _dcm_matrix.b.z * tmpx  - _dcm_matrix.b.x * tmpz;
	_dcm_matrix.b.z += _dcm_matrix.b.x * tmpy  - _dcm_matrix.b.y * tmpx;
	_dcm_matrix.c.x += _dcm_matrix.c.y * tmpz  - _dcm_matrix.c.z * tmpy;
	_dcm_matrix.c.y += _dcm_matrix.c.z * tmpx  - _dcm_matrix.c.x * tmpz;
	_dcm_matrix.c.z += _dcm_matrix.c.x * tmpy  - _dcm_matrix.c.y * tmpx;
}


// adjust an accelerometer vector for known acceleration forces
void
AP_DCM::accel_adjust(Vector3f &accel)
{
	float veloc;
	// compensate for linear acceleration, limited to 1g
	float acceleration = _gps->acceleration();
	acceleration = constrain(acceleration, 0, 9.8);
	accel.x -= acceleration;

	// compensate for centripetal acceleration
	veloc = _gps->ground_speed / 100;

	// We are working with a modified version of equation 26 as
	// our IMU object reports acceleration in the positive axis
	// direction as positive

	// Equation 26 broken up into separate pieces
	accel.y -= _omega_smoothed.z * veloc;
	accel.z += _omega_smoothed.y * veloc;
}

/*
  reset the DCM matrix and omega. Used on ground start, and on
  extreme errors in the matrix
 */
void
AP_DCM::matrix_reset(bool recover_eulers)
{
	if (_compass != NULL) {
		_compass->null_offsets_disable();		
	}

	// reset the integration terms
	_omega_I.x = 0.0f;
	_omega_I.y = 0.0f;
	_omega_I.z = 0.0f;
	_omega_P = _omega_I;
	_omega_integ_corr = _omega_I;
	_omega_smoothed = _omega_I;
	_omega = _omega_I;

	// if the caller wants us to try to recover to the current
	// attitude then calculate the dcm matrix from the current
	// roll/pitch/yaw values
	if (recover_eulers && !isnan(roll) && !isnan(pitch) && !isnan(yaw)) {
		rotation_matrix_from_euler(_dcm_matrix, roll, pitch, yaw);
	} else {
		// otherwise make it flat
		rotation_matrix_from_euler(_dcm_matrix, 0, 0, 0);
	}

	if (_compass != NULL) {
		_compass->null_offsets_enable();	// This call is needed to restart the nulling
		// Otherwise the reset in the DCM matrix can mess up
		// the nulling
	}
}

/*
  check the DCM matrix for pathological values
 */
void
AP_DCM::check_matrix(void)
{
	if (_dcm_matrix.is_nan()) {
		//Serial.printf("ERROR: DCM matrix NAN\n");
		SITL_debug("ERROR: DCM matrix NAN\n");
		renorm_blowup_count++;
		matrix_reset(true);
		return;
	}
	// some DCM matrix values can lead to an out of range error in
	// the pitch calculation via asin().  These NaN values can
	// feed back into the rest of the DCM matrix via the
	// error_course value.
	if (!(_dcm_matrix.c.x < 1.0 &&
	      _dcm_matrix.c.x > -1.0)) {
		// We have an invalid matrix. Force a normalisation.
		renorm_range_count++;
		normalize();

		if (_dcm_matrix.is_nan() ||
		    fabs(_dcm_matrix.c.x) > 10) {
			// normalisation didn't fix the problem! We're
			// in real trouble. All we can do is reset
			//Serial.printf("ERROR: DCM matrix error. _dcm_matrix.c.x=%f\n",
			//	   _dcm_matrix.c.x);
			SITL_debug("ERROR: DCM matrix error. _dcm_matrix.c.x=%f\n",
				   _dcm_matrix.c.x);
			renorm_blowup_count++;
			matrix_reset(true);
		}
	}
}

/**************************************************/
bool
AP_DCM::renorm(Vector3f const &a, Vector3f &result)
{
	float	renorm_val;

	// numerical errors will slowly build up over time in DCM,
	// causing inaccuracies. We can keep ahead of those errors
	// using the renormalization technique from the DCM IMU paper
	// (see equations 18 to 21).

	// For APM we don't bother with the taylor expansion
	// optimisation from the paper as on our 2560 CPU the cost of
	// the sqrt() is 44 microseconds, and the small time saving of
	// the taylor expansion is not worth the potential of
	// additional error buildup.

	// Note that we can get significant renormalisation values
	// when we have a larger delta_t due to a glitch eleswhere in
	// APM, such as a I2c timeout or a set of EEPROM writes. While
	// we would like to avoid these if possible, if it does happen
	// we don't want to compound the error by making DCM less
	// accurate.

	renorm_val = 1.0 / a.length();

	// keep the average for reporting
	_renorm_val_sum += renorm_val;
	_renorm_val_count++;

	if (!(renorm_val < 2.0 && renorm_val > 0.5)) {
		// this is larger than it should get - log it as a warning
		renorm_range_count++;
		if (!(renorm_val < 1.0e6 && renorm_val > 1.0e-6)) {
			// we are getting values which are way out of
			// range, we will reset the matrix and hope we
			// can recover our attitude using drift
			// correction before we hit the ground!
			//Serial.printf("ERROR: DCM renormalisation error. renorm_val=%f\n",
			//	   renorm_val);
			SITL_debug("ERROR: DCM renormalisation error. renorm_val=%f\n",
				   renorm_val);
			renorm_blowup_count++;
			return false;
		}
	}

	result = a * renorm_val;
	return true;
}

/*************************************************
Direction Cosine Matrix IMU: Theory
William Premerlani and Paul Bizard

Numerical errors will gradually reduce the orthogonality conditions expressed by equation 5
to approximations rather than identities. In effect, the axes in the two frames of reference no
longer describe a rigid body. Fortunately, numerical error accumulates very slowly, so it is a
simple matter to stay ahead of it.
We call the process of enforcing the orthogonality conditions <EFBFBD>renormalization<EFBFBD>.
*/
void
AP_DCM::normalize(void)
{
	float error;
	Vector3f t0, t1, t2;

	error = _dcm_matrix.a * _dcm_matrix.b; 						// eq.18

	t0 = _dcm_matrix.a - (_dcm_matrix.b * (0.5f * error));		// eq.19
	t1 = _dcm_matrix.b - (_dcm_matrix.a * (0.5f * error));		// eq.19
	t2 = t0 % t1;					                // c= a x b // eq.20

	if (!renorm(t0, _dcm_matrix.a) ||
	    !renorm(t1, _dcm_matrix.b) ||
	    !renorm(t2, _dcm_matrix.c)) {
		// Our solution is blowing up and we will force back
		// to last euler angles
		matrix_reset(true);
	}
}


/**************************************************/
void
AP_DCM::drift_correction(float deltat)
{
	float error_course = 0;
	Vector3f accel;
	Vector3f error;
	float error_norm = 0;
	const float gravity_squared = (9.80665*9.80665);
	float yaw_deltat = 0;

	accel = _accel_vector;

	// if enabled, use the GPS to correct our accelerometer vector
	// for centripetal forces
	if(_centripetal &&
	   _gps != NULL &&
	   _gps->status() == GPS::GPS_OK) {
		accel_adjust(accel);
	}


	//*****Roll and Pitch***************

	// calculate the z component of the accel vector assuming it
	// has a length of 9.8. This discards the z accelerometer
	// sensor reading completely. Logs show that the z accel is
	// the noisest, plus it has a disproportionate impact on the
	// drift correction result because of the geometry when we are
	// mostly flat
	float zsquared = gravity_squared - ((accel.x * accel.x) + (accel.y * accel.y));
	if (zsquared < 0) {
		_omega_P.zero();
	} else {
		if (accel.z > 0) {
			accel.z = sqrt(zsquared);
		} else {
			accel.z = -sqrt(zsquared);
		}

		error =  _dcm_matrix.c % accel;

		// error is in m/s^2 units
		// Limit max error to limit max omega_P and omega_I
		error_norm = error.length();
		if (error_norm > 2) {
			error *= (2 / error_norm);
		}

		// scale the error for the time over which we are
		// applying it
		error *= deltat;

		// calculate the new proportional offset
		_omega_P = error * _kp_roll_pitch;

		// we limit the change in the integrator to the
		// maximum gyro drift rate on each axis
		float drift_limit = ToRad(_gyro_drift_rate) * deltat / _ki_roll_pitch;
		error.x = constrain(error.x, -drift_limit, drift_limit);
		error.y = constrain(error.y, -drift_limit, drift_limit);
		error.z = constrain(error.z, -drift_limit, drift_limit);

		// update gyro drift estimate
		_omega_I += error * _ki_roll_pitch;
	}

	// these sums support the reporting of the DCM state via MAVLink
	_error_rp_sum += error_norm;
	_error_rp_count++;

	// yaw drift correction

	if (_compass && _compass->use_for_yaw() &&
	    _compass->last_update != _compass_last_update) {
		if (_have_initial_yaw) {
			// Equation 23, Calculating YAW error
			// We make the gyro YAW drift correction based
			// on compass magnetic heading
			error_course = (_dcm_matrix.a.x * _compass->heading_y) - (_dcm_matrix.b.x * _compass->heading_x);
			yaw_deltat = 1.0e-6*(_compass->last_update - _compass_last_update);
			_compass_last_update = _compass->last_update;
		} else {
			// this is our first estimate of the yaw,
			// construct a DCM matrix based on the current
			// roll/pitch and the compass heading, but

			// first ensure the compass heading has been
			// calculated
			_compass->calculate(_dcm_matrix);

			// now construct a new DCM matrix
			_compass->null_offsets_disable();
			rotation_matrix_from_euler(_dcm_matrix, roll, pitch, _compass->heading);
			_compass->null_offsets_enable();
			_have_initial_yaw = true;
			_compass_last_update = _compass->last_update;
		}
	} else if (_gps && _gps->status() == GPS::GPS_OK &&
		   _gps->last_fix_time != _gps_last_update) {
		// Use GPS Ground course to correct yaw gyro drift
		if (_gps->ground_speed >= GPS_SPEED_MIN) {
			if (_have_initial_yaw) {
				float course_over_ground_x = cos(ToRad(_gps->ground_course/100.0));
				float course_over_ground_y = sin(ToRad(_gps->ground_course/100.0));
				// Equation 23, Calculating YAW error
				error_course = (_dcm_matrix.a.x * course_over_ground_y) - (_dcm_matrix.b.x * course_over_ground_x);
				yaw_deltat = 1.0e-3*(_gps->last_fix_time - _gps_last_update);
				_gps_last_update = _gps->last_fix_time;
			} else  {
				// when we first start moving, set the
				// DCM matrix to the current
				// roll/pitch values, but with yaw
				// from the GPS
				if (_compass) {
					_compass->null_offsets_disable();
				}
				rotation_matrix_from_euler(_dcm_matrix, roll, pitch, ToRad(_gps->ground_course));
				if (_compass) {
					_compass->null_offsets_enable();
				}
				_have_initial_yaw =  true;
				error_course = 0;
				_gps_last_update = _gps->last_fix_time;
			}
		} else if (_gps->ground_speed >= GPS_SPEED_RESET) {
			// we are not going fast enough to use GPS for
			// course correction, but we won't reset
			// _have_initial_yaw yet, instead we just let
			// the gyro handle yaw
			error_course = 0;
		} else {
			// we are moving very slowly. Reset
			// _have_initial_yaw and adjust our heading
			// rapidly next time we get a good GPS ground
			// speed
			error_course = 0;
			_have_initial_yaw = false;
		}
	}

	if (yaw_deltat == 0 || error_course == 0) {
		// nothing to do
		return;
	}

	// Equation 24, Applys the yaw correction to the XYZ rotation of the aircraft
	error = _dcm_matrix.c * error_course;

	// Adding yaw correction to proportional correction vector.
	_omega_P += error * _kp_yaw;

	// limit maximum gyro drift
	float drift_limit = ToRad(_gyro_drift_rate) * yaw_deltat / _ki_yaw;
	error.z = constrain(error.z, -drift_limit, drift_limit);

	// add yaw correction to integrator correction vector, but
	// only for the z gyro. We rely on the accelerometers for x
	// and y gyro drift correction. Using the compass for x/y drift
	// correction is too inaccurate, and can lead to incorrect builups in
	// the x/y drift
	_omega_I.z += error.z * _ki_yaw;

	_error_yaw_sum += error_course;
	_error_yaw_count++;
	//Serial.print("*");
}


/**************************************************/
void
AP_DCM::euler_angles(void)
{
	calculate_euler_angles(_dcm_matrix, &roll, &pitch, &yaw);

	roll_sensor 	= degrees(roll)  * 100;
	pitch_sensor 	= degrees(pitch) * 100;
	yaw_sensor 	= degrees(yaw)   * 100;

	if (yaw_sensor < 0)
		yaw_sensor += 36000;
}

/* reporting of DCM state for MAVLink */

// average accel_weight since last call
float AP_DCM::get_accel_weight(void)
{
	return 1.0;
}

// average renorm_val since last call
float AP_DCM::get_renorm_val(void)
{
	float ret;
	if (_renorm_val_count == 0) {
		return 0;
	}
	ret = _renorm_val_sum / _renorm_val_count;
	_renorm_val_sum = 0;
	_renorm_val_count = 0;
	return ret;
}

// average error_roll_pitch since last call
float AP_DCM::get_error_rp(void)
{
	float ret;
	if (_error_rp_count == 0) {
		return 0;
	}
	ret = _error_rp_sum / _error_rp_count;
	_error_rp_sum = 0;
	_error_rp_count = 0;
	return ret;
}

// average error_yaw since last call
float AP_DCM::get_error_yaw(void)
{
	float ret;
	if (_error_yaw_count == 0) {
		return 0;
	}
	ret = _error_yaw_sum / _error_yaw_count;
	_error_yaw_sum = 0;
	_error_yaw_count = 0;
	return ret;
}