ahrs_float_utils.h

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/*
 * Copyright (C) 2009 Felix Ruess <felix.ruess@gmail.com>
 * Copyright (C) 2009 Antoine Drouin <poinix@gmail.com>
 *
 * This file is part of paparazzi.
 *
 * paparazzi is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2, or (at your option)
 * any later version.
 *
 * paparazzi is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with paparazzi; see the file COPYING.  If not, write to
 * the Free Software Foundation, 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

#ifndef AHRS_FLOAT_UTILS_H
#define AHRS_FLOAT_UTILS_H

#include "math/pprz_algebra_float.h"
#include "subsystems/ahrs/ahrs_magnetic_field_model.h"

#include "std.h" // for ABS

static inline void ahrs_float_get_euler_from_accel_mag(struct FloatEulers *e, struct FloatVect3 *accel,
    struct FloatVect3 *mag)
{
  /* get phi and theta from accelerometer */
  const float phi   = atan2f(-accel->y, -accel->z);
  const float cphi = cosf(phi);
  const float theta = atan2f(cphi * accel->x, -accel->z);

  /* get psi from magnetometer */
  /* project mag on local tangeant plane */
  const float sphi   = sinf(phi);
  const float ctheta = cosf(theta);
  const float stheta = sinf(theta);
  const float mn = ctheta * mag->x + sphi * stheta * mag->y + cphi * stheta * mag->z;
  const float me =     0. * mag->x + cphi          * mag->y - sphi          * mag->z;
  float psi = -atan2f(me, mn) + atan2(AHRS_H_Y, AHRS_H_X);
  if (psi > M_PI) { psi -= 2.*M_PI; } if (psi < -M_PI) { psi += 2.*M_PI; }
  EULERS_ASSIGN(*e, phi, theta, psi);
}

static inline void ahrs_float_get_quat_from_accel(struct FloatQuat *q, struct FloatVect3 *accel)
{
  /* normalized accel measurement */
  struct FloatVect3 acc_normalized = *accel;
  float_vect3_normalize(&acc_normalized);

  /* check for 180deg case */
  if (ABS(acc_normalized.z - 1.0) < 5 * FLT_MIN) {
    QUAT_ASSIGN(*q, 0.0, 1.0, 0.0, 0.0);
  } else {
    /*
     * axis we want to rotate around is cross product of accel and reference [0,0,-g]
     * normalized: cross(acc_normalized, [0,0,-1])
     * vector part of quaternion is the axis
     * scalar part (angle): 1.0 + dot(acc_normalized, [0,0,-1])
     */
    q->qx = - acc_normalized.y;
    q->qy = acc_normalized.x;
    q->qz = 0.0;
    q->qi = 1.0 - acc_normalized.z;
    float_quat_normalize(q);
  }
}

static inline void ahrs_float_get_quat_from_accel_mag(struct FloatQuat *q, struct FloatVect3 *accel,
    struct FloatVect3 *mag)
{

  /* the quaternion representing roll and pitch from acc measurement */
  struct FloatQuat q_a;
  ahrs_float_get_quat_from_accel(&q_a, accel);

  /* and rotate to horizontal plane using the quat from above */
  struct FloatRMat rmat_phi_theta;
  float_rmat_of_quat(&rmat_phi_theta, &q_a);
  struct FloatVect3 mag_ltp;
  float_rmat_transp_vmult(&mag_ltp, &rmat_phi_theta, mag);

  /* heading from mag -> make quaternion to rotate around ltp z axis*/
  struct FloatQuat q_m;

  /* dot([mag_n.x, mag_n.x, 0], [AHRS_H_X, AHRS_H_Y, 0]) */
  float dot = mag_ltp.x * AHRS_H_X + mag_ltp.y * AHRS_H_Y;

  /* |v1||v2| */
  float norm2 = sqrtf(SQUARE(mag_ltp.x) + SQUARE(mag_ltp.y))
                * sqrtf(SQUARE(AHRS_H_X) + SQUARE(AHRS_H_Y));

  // catch 180deg case
  if (ABS(norm2 + dot) < 5 * FLT_MIN) {
    QUAT_ASSIGN(q_m, 0.0, 0.0, 0.0, 1.0);
  } else {
    /* q_xyz = cross([mag_n.x, mag_n.y, 0], [AHRS_H_X, AHRS_H_Y, 0]) */
    q_m.qx = 0.0;
    q_m.qy = 0.0;
    q_m.qz = mag_ltp.x * AHRS_H_Y - mag_ltp.y * AHRS_H_X;
    q_m.qi = norm2 + dot;
    float_quat_normalize(&q_m);
  }

  // q_ltp2imu = q_a * q_m
  // and wrap and normalize
  float_quat_comp_norm_shortest(q, &q_m, &q_a);
}

#endif /* AHRS_FLOAT_UTILS_H */