ardupilot/libraries/AP_AHRS/AP_AHRS_MPU6000.cpp

/*
	AP_AHRS_MPU6000.cpp

	AHRS system using MPU6000's internal calculations

	Adapted for the general ArduPilot AHRS interface by Andrew Tridgell

	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.
*/
#include <FastSerial.h>
#include <AP_AHRS.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

// the limit (in degrees/second) beyond which we stop integrating
// omega_I. At larger spin rates the DCM PI controller can get 'dizzy'
// which results in false gyro drift. See
// http://gentlenav.googlecode.com/files/fastRotations.pdf
#define SPIN_RATE_LIMIT 20

void
AP_AHRS_MPU6000::init()
{
	_mpu6000->dmp_init();
	push_gains_to_dmp();
	push_offsets_to_ins();
};

// run a full MPU6000 update round
void
AP_AHRS_MPU6000::update(void)
{
	float delta_t;

	// tell the IMU to grab some data.  is this necessary?
	_imu->update();

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

	// convert the quaternions into a DCM matrix
	_mpu6000->quaternion.rotation_matrix(_dcm_matrix);
	
	// we run the gyro bias correction using gravity vector algorithm
	drift_correction(delta_t);

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

// wrap_PI - ensure an angle (expressed in radians) is between -PI and PI
// TO-DO: should remove and replace with more standard functions
float AP_AHRS_MPU6000::wrap_PI(float angle_in_radians)
{
  if( angle_in_radians > M_PI ){
    return(angle_in_radians - 2*M_PI);
  }
  else if( angle_in_radians < -M_PI ){
    return(angle_in_radians + 2*M_PI);
  }
  else{
    return(angle_in_radians);
  }
}

// Function to correct the gyroX and gyroY bias (roll and pitch) using the gravity vector from accelerometers
// We use the internal chip axis definition to make the bias correction because both sensors outputs (gyros and accels)
// are in chip axis definition
void AP_AHRS_MPU6000::drift_correction( float deltat )
{
	float errorRollPitch[2];

	// Get current values for gyros
	_accel_vector = _imu->get_accel();

	// We take the accelerometer readings and cumulate to average them and obtain the gravity vector
	_accel_filtered += _accel_vector;
	_accel_filtered_samples++;

	_gyro_bias_from_gravity_counter++;
	// We make the bias calculation and correction at a lower rate (GYRO_BIAS_FROM_GRAVITY_RATE)
	if( _gyro_bias_from_gravity_counter == GYRO_BIAS_FROM_GRAVITY_RATE ){
		_gyro_bias_from_gravity_counter = 0;

		_accel_filtered /= _accel_filtered_samples;  // average

		// Adjust ground reference : Accel Cross Gravity to obtain the error between gravity from accels and gravity from attitude solution
		// errorRollPitch are in Accel LSB units
		errorRollPitch[0] = _accel_filtered.y * _dcm_matrix.c.z + _accel_filtered.z * _dcm_matrix.c.x;
		errorRollPitch[1] = -_accel_filtered.z * _dcm_matrix.c.y - _accel_filtered.x * _dcm_matrix.c.z;

		errorRollPitch[0] *= deltat * 1000;
		errorRollPitch[1] *= deltat * 1000;

		// we limit to maximum gyro drift rate on each axis
		float drift_limit = _mpu6000->get_gyro_drift_rate() * deltat / _gyro_bias_from_gravity_gain;  //0.65*0.04 / 0.005 = 5.2
		errorRollPitch[0] = constrain(errorRollPitch[0], -drift_limit, drift_limit);
		errorRollPitch[1] = constrain(errorRollPitch[1], -drift_limit, drift_limit);

		// We correct gyroX and gyroY bias using the error vector
		_gyro_bias[0] += errorRollPitch[0]*_gyro_bias_from_gravity_gain;
		_gyro_bias[1] += errorRollPitch[1]*_gyro_bias_from_gravity_gain;
		
		// TO-DO: fix this.  Currently it makes the roll and pitch drift more!
		// If bias values are greater than 1 LSB we update the hardware offset registers
		if( fabs(_gyro_bias[0])>1.0 ){
			//_mpu6000->set_gyro_offsets(-1*(int)_gyro_bias[0],0,0);
			//_mpu6000->set_gyro_offsets(0,-1*(int)_gyro_bias[0],0);
			//_gyro_bias[0] -= (int)_gyro_bias[0];  // we remove the part that we have already corrected on registers...
		}
		if (fabs(_gyro_bias[1])>1.0){
			//_mpu6000->set_gyro_offsets(-1*(int)_gyro_bias[1],0,0);
			//_gyro_bias[1] -= (int)_gyro_bias[1];
		}

		// Reset the accelerometer variables
		_accel_filtered.x = 0;
		_accel_filtered.y = 0;
		_accel_filtered.z = 0;
		_accel_filtered_samples=0;
	}

	// correct the yaw
	drift_correction_yaw();
}


/*
  reset the DCM matrix and omega. Used on ground start, and on
  extreme errors in the matrix
 */
void
AP_AHRS_MPU6000::reset(bool recover_eulers)
{
	// 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)) {
		_dcm_matrix.from_euler(roll, pitch, yaw);
	} else {
		// otherwise make it flat
		_dcm_matrix.from_euler(0, 0, 0);
	}
}

// push offsets down from IMU to INS (required so MPU6000 can perform it's own attitude estimation)
void
AP_AHRS_MPU6000::push_offsets_to_ins()
{
	// push down gyro offsets (TO-DO: why are x and y offsets are reversed?!)
	_mpu6000->set_gyro_offsets_scaled(((AP_IMU_INS*)_imu)->gy(),((AP_IMU_INS*)_imu)->gx(),((AP_IMU_INS*)_imu)->gz());
	((AP_IMU_INS*)_imu)->gx(0);
	((AP_IMU_INS*)_imu)->gy(0);
	((AP_IMU_INS*)_imu)->gz(0);

	// push down accelerometer offsets (TO-DO: why are x and y offsets are reversed?!)
	_mpu6000->set_accel_offsets_scaled(((AP_IMU_INS*)_imu)->ay(), ((AP_IMU_INS*)_imu)->ax(), ((AP_IMU_INS*)_imu)->az());
	//((AP_IMU_INS*)_imu)->ax(0);
	//((AP_IMU_INS*)_imu)->ay(0);
	//((AP_IMU_INS*)_imu)->az(0);
}

void
AP_AHRS_MPU6000::push_gains_to_dmp()
{
	uint8_t gain;
	if( _kp.get() >= 1.0 ) {
		gain = 0xFF;
	}else if( _kp.get() <= 0.0 ) {
		gain = 0x00;
	}else{
		gain = (uint8_t)((float)0xFF * _kp.get());
	}

	_mpu6000->dmp_set_sensor_fusion_accel_gain(gain);
}

// produce a yaw error value. The returned value is proportional
// to sin() of the current heading error in earth frame
float
AP_AHRS_MPU6000::yaw_error_compass(void)
{
	Vector3f mag = Vector3f(_compass->mag_x, _compass->mag_y, _compass->mag_z);
	// get the mag vector in the earth frame
	Vector3f rb = _dcm_matrix * mag;

	rb.normalize();
	if (rb.is_inf()) {
		// not a valid vector
		return 0.0;
	}

	// get the earths magnetic field (only X and Y components needed)
	Vector3f mag_earth = Vector3f(cos(_compass->get_declination()),
				      sin(_compass->get_declination()), 0);

	// calculate the error term in earth frame
	Vector3f error = rb % mag_earth;

	return error.z;
}

//
// drift_correction_yaw - yaw drift correction using the compass
//
void
AP_AHRS_MPU6000::drift_correction_yaw(void)
{
	static float yaw_last_uncorrected = HEADING_UNKNOWN;
	static float yaw_corrected = HEADING_UNKNOWN;
	float yaw_delta = 0;
	bool new_value = false;
	float yaw_error;
	static float heading;

	// we assume the DCM matrix is completely uncorrect for yaw
	// retrieve how much heading has changed according to non-corrected dcm
	if( yaw_last_uncorrected != HEADING_UNKNOWN ) {
		yaw_delta = wrap_PI(yaw - yaw_last_uncorrected);	// the change in heading according to the gyros only since the last interation
		yaw_last_uncorrected = yaw;
	}

	// initialise yaw_corrected (if necessary)
	if( yaw_corrected != HEADING_UNKNOWN ) {
		yaw_corrected = yaw;
	}else{
		yaw_corrected = wrap_PI(yaw_corrected + yaw_delta);	// best guess of current yaw is previous iterations corrected yaw + yaw change from gyro
		_dcm_matrix.from_euler(roll, pitch, yaw_corrected);		// rebuild dcm matrix with best guess at current yaw
	}

	// if we have new compass data
	if( _compass && _compass->use_for_yaw() ) {
		if (_compass->last_update != _compass_last_update) {
			_compass_last_update = _compass->last_update;
			heading = _compass->calculate_heading(_dcm_matrix);
			if( !_have_initial_yaw ) {
				yaw_corrected = heading;
				_have_initial_yaw = true;
				_dcm_matrix.from_euler(roll, pitch, yaw_corrected);		// rebuild dcm matrix with best guess at current yaw
			}
			new_value = true;
		}
	}

	// perform the yaw correction
	if( new_value ){
		yaw_error = yaw_error_compass();
		// the yaw error is a vector in earth frame
		//Vector3f error = Vector3f(0,0, yaw_error);

		// convert the error vector to body frame
		//error = _dcm_matrix.mul_transpose(error);

		// Update the differential component to reduce the error (it<69>s like a P control)
		yaw_corrected += wrap_PI(yaw_error * _kp_yaw.get());		// probably completely wrong

		// rebuild the dcm matrix yet again
		_dcm_matrix.from_euler(roll, pitch, yaw_corrected);
	}
}

// calculate the euler angles which will be used for high level
// navigation control
void
AP_AHRS_MPU6000::euler_angles(void)
{
	_dcm_matrix.to_euler(&roll, &pitch, &yaw);
	//quaternion.to_euler(&roll, &pitch, &yaw);	// cannot use this because the quaternion is not correct for yaw drift

	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 error_roll_pitch since last call
float AP_AHRS_MPU6000::get_error_rp(void)
{
	// not yet supported with DMP
	return 0.0;
}

// average error_yaw since last call
float AP_AHRS_MPU6000::get_error_yaw(void)
{
	// not yet supported with DMP
	return 0.0;
}