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AHRS: normalize the ge vector in drift correction, and use barometer 

The normalisation ensures the error term scales uniformly with
different accelerations.

The barometer is used for vertical acceleration estimation
master
Andrew Tridgell 13 years ago
parent
dae1a57dc5
commit
3a41ad8e7c
6 changed files with 157 additions and 170 deletions
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  1. 6
      libraries/AP_AHRS/AP_AHRS.h
  2. 274
      libraries/AP_AHRS/AP_AHRS_DCM.cpp
  3. 27
      libraries/AP_AHRS/AP_AHRS_DCM.h
  4. 12
      libraries/AP_AHRS/AP_AHRS_Quaternion.cpp
  5. 6
      libraries/AP_AHRS/examples/AHRS_Test/AHRS_Test.pde
  6. 2
      libraries/AP_AHRS/examples/DCM_Test/DCM_Test.pde

6
libraries/AP_AHRS/AP_AHRS.h
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@ -14,6 +14,7 @@ @@ -14,6 +14,7 @@
#include <AP_Compass.h>
#include <AP_GPS.h>
#include <AP_IMU.h>
#include <AP_Baro.h>
#if defined(ARDUINO) && ARDUINO >= 100
#include "Arduino.h"
@ -27,7 +28,8 @@ public: @@ -27,7 +28,8 @@ public:
// Constructor
AP_AHRS(IMU *imu, GPS *&gps):
_imu(imu),
_gps(gps)
_gps(gps),
_barometer(NULL)
{
// base the ki values by the sensors maximum drift
// rate. The APM2 has gyros which are much less drift
@ -40,6 +42,7 @@ public: @@ -40,6 +42,7 @@ public:
// Accessors
void set_fly_forward(bool b) { _fly_forward = b; }
void set_compass(Compass *compass) { _compass = compass; }
void set_barometer(AP_Baro *barometer) { _barometer = barometer; }
// Methods
virtual void update(void) = 0;
@ -94,6 +97,7 @@ protected: @@ -94,6 +97,7 @@ protected:
// IMU under us without our noticing.
IMU *_imu;
GPS *&_gps;
AP_Baro *_barometer;
// true if we can assume the aircraft will be flying forward
// on its X axis

274
libraries/AP_AHRS/AP_AHRS_DCM.cpp
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@ -70,22 +70,11 @@ AP_AHRS_DCM::update(void) @@ -70,22 +70,11 @@ AP_AHRS_DCM::update(void)
void
AP_AHRS_DCM::matrix_update(float _G_Dt)
{
// _omega_integ_corr is 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;
// this is a replacement of the DCM matrix multiply (equation
// 17), with known zero elements removed and the matrix
// operations inlined. This runs much faster than the original
// version of this code, as the compiler was doing a terrible
// job of realising that so many of the factors were in common
// or zero. It also uses much less stack, as we no longer need
// two additional local matrices
_omega = _gyro_vector + _omega_I;
Vector3f r = _omega * _G_Dt;
// add in P correction terms
Vector3f r = (_omega + _omega_P + _omega_yaw_P) * _G_Dt;
_dcm_matrix.rotate(r);
}
@ -97,14 +86,9 @@ AP_AHRS_DCM::matrix_update(float _G_Dt) @@ -97,14 +86,9 @@ AP_AHRS_DCM::matrix_update(float _G_Dt)
void
AP_AHRS_DCM::reset(bool recover_eulers)
{
if (_compass != NULL) {
_compass->null_offsets_disable();
}
// reset the integration terms
_omega_I.zero();
_omega_P.zero();
_omega_integ_corr.zero();
_omega.zero();
// if the caller wants us to try to recover to the current
@ -116,12 +100,6 @@ AP_AHRS_DCM::reset(bool recover_eulers) @@ -116,12 +100,6 @@ AP_AHRS_DCM::reset(bool recover_eulers)
// otherwise make it flat
_dcm_matrix.from_euler(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
}
}
/*
@ -245,60 +223,19 @@ AP_AHRS_DCM::normalize(void) @@ -245,60 +223,19 @@ AP_AHRS_DCM::normalize(void)
}
// yaw drift correction using the compass
void
AP_AHRS_DCM::drift_correction_compass(float deltat)
// produce a yaw error value. The returned value is proportional
// to sin() of the current heading error in earth frame
float
AP_AHRS_DCM::yaw_error_compass(void)
{
if (_compass == NULL ||
_compass->last_update == _compass_last_update) {
// slowly degrade the yaw error term so we cope
// gracefully with the compass going offline
_drift_error_earth.z *= 0.97;
return;
}
Vector3f mag = Vector3f(_compass->mag_x, _compass->mag_y, _compass->mag_z);
if (!_have_initial_yaw) {
// this is our first estimate of the yaw,
// or the compass has come back online after
// no readings for 2 seconds.
//
// construct a DCM matrix based on the current
// roll/pitch and the compass heading.
// First ensure the compass heading has been
// calculated
_compass->calculate(_dcm_matrix);
// now construct a new DCM matrix
_compass->null_offsets_disable();
_dcm_matrix.from_euler(roll, pitch, _compass->heading);
_compass->null_offsets_enable();
_have_initial_yaw = true;
_field_strength = mag.length();
_compass_last_update = _compass->last_update;
return;
}
float yaw_deltat = 1.0e-6*(_compass->last_update - _compass_last_update);
_compass_last_update = _compass->last_update;
// keep a estimate of the magnetic field strength
_field_strength = (_field_strength * 0.95) + (mag.length() * 0.05);
// get the mag vector in the earth frame
Vector3f rb = _dcm_matrix * mag;
// normalise rb so that it can be directly combined with
// rotations given by the accelerometers, which are in 1g units
rb *= yaw_deltat / _field_strength;
rb.normalize();
if (rb.is_inf()) {
// not a valid vector
return;
return 0.0;
}
// get the earths magnetic field (only X and Y components needed)
@ -308,17 +245,88 @@ AP_AHRS_DCM::drift_correction_compass(float deltat) @@ -308,17 +245,88 @@ AP_AHRS_DCM::drift_correction_compass(float deltat)
// calculate the error term in earth frame
Vector3f error = rb % mag_earth;
// setup the z component of the total drift error in earth
// frame. This is then used by the main drift correction code
_drift_error_earth.z = error.z*70; //constrain(error.z, -0.4, 0.4);
//TODO: figure out proper error scaling instead of using 70
return error.z;
}
// produce a yaw error value using the GPS. The returned value is proportional
// to sin() of the current heading error in earth frame
float
AP_AHRS_DCM::yaw_error_gps(void)
{
return sin(ToRad(_gps->ground_course * 0.01) - yaw);
}
// yaw drift correction using the compass or GPS
// this function prodoces the _omega_yaw_P vector, and also
// contributes to the _omega_I.z long term yaw drift estimate
void
AP_AHRS_DCM::drift_correction_yaw(float deltat)
{
bool new_value = false;
float yaw_error;
float yaw_deltat;
if (_compass && _compass->use_for_yaw()) {
if (_compass->last_update != _compass_last_update) {
yaw_deltat = (_compass->last_update - _compass_last_update) * 1.0e-6;
_compass_last_update = _compass->last_update;
if (!_have_initial_yaw) {
float heading = _compass->calculate_heading(_dcm_matrix);
_dcm_matrix.from_euler(roll, pitch, heading);
_omega_yaw_P.zero();
_have_initial_yaw = true;
}
new_value = true;
yaw_error = yaw_error_compass();
}
} else if (_gps && _gps->status() == GPS::GPS_OK) {
if (_gps->last_fix_time != _gps_last_update &&
_gps->ground_speed >= GPS_SPEED_MIN) {
yaw_deltat = (_gps->last_fix_time - _gps_last_update) * 1.0e-3;
_gps_last_update = _gps->last_fix_time;
if (!_have_initial_yaw) {
_dcm_matrix.from_euler(roll, pitch, ToRad(_gps->ground_course*0.01));
_omega_yaw_P.zero();
_have_initial_yaw = true;
}
new_value = true;
yaw_error = yaw_error_gps();
}
}
if (!new_value) {
// we don't have any new yaw information
// slowly decay _omega_yaw_P to cope with loss
// of our yaw source
_omega_yaw_P.z *= 0.97;
return;
}
// 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);
_error_yaw_sum += fabs(_drift_error_earth.z);
// update the proportional control to drag the
// yaw back to the right value
_omega_yaw_P = error * _kp_yaw.get();
// don't update the drift term if we lost the yaw reference
// for more than 2 seconds
if (yaw_deltat < 2.0) {
// also add to the I term
_omega_I_sum.z += error.z * _ki_yaw * yaw_deltat;
}
_error_yaw_sum += fabs(yaw_error);
_error_yaw_count++;
}
// perform drift correction. This function aims to update _omega_P and
// _omega_I with our best estimate of the short term and long term
// gyro error. The _omega_P value is what pulls our attitude solution
@ -332,23 +340,15 @@ void @@ -332,23 +340,15 @@ void
AP_AHRS_DCM::drift_correction(float deltat)
{
Vector3f error;
Vector3f gps_velocity;
bool nogps=false;
Vector3f velocity;
uint32_t last_correction_time;
if(_omega.length() < ToRad(20))
_omega_I += _omega_I_delta * deltat;
// perform yaw drift correction if we have a new yaw reference
// vector
drift_correction_compass(deltat);
// scale the accel vector so it is in 1g units. This brings it
// into line with the mag vector, allowing the two to be combined
_accel_vector *= (deltat / _gravity);
drift_correction_yaw(deltat);
// integrate the accel vector in the earth frame between GPS readings
_ra_sum += _dcm_matrix * _accel_vector;
_ra_sum += _dcm_matrix * (_accel_vector * deltat);
// keep a sum of the deltat values, so we know how much time
// we have integrated over
@ -362,92 +362,84 @@ AP_AHRS_DCM::drift_correction(float deltat) @@ -362,92 +362,84 @@ AP_AHRS_DCM::drift_correction(float deltat)
// not enough time has accumulated
return;
}
nogps=true;
gps_velocity.zero();
velocity.zero();
_last_velocity.zero();
last_correction_time = millis();
} else {
if (_gps->last_fix_time == _ra_sum_start) {
// we don't have a new GPS fix - nothing more to do
return;
}
gps_velocity = Vector3f(_gps->velocity_north(), _gps->velocity_east(), 0);
velocity = Vector3f(_gps->velocity_north(), _gps->velocity_east(), 0);
if (_barometer != NULL) {
// Z velocity is down
velocity.z = - _barometer->get_climb_rate();
}
last_correction_time = _gps->last_fix_time;
}
// see if this is our first time through - in which case we
// just setup the start times and return
if (_ra_sum_start == 0) {
_ra_sum_start = last_correction_time;
_gps_last_velocity = gps_velocity;
_last_velocity = velocity;
return;
}
// get the corrected acceleration vector in earth frame. Units
// are 1g
// are m/s/s
Vector3f ge;
if(!nogps) {
ge = Vector3f(0, 0, -1.0) + ((gps_velocity - _gps_last_velocity)/_gravity)/_ra_deltat;
}
else {
ge = Vector3f(0, 0, -1.0);
float v_scale = 1.0/(_ra_deltat*_gravity);
ge = Vector3f(0, 0, -1.0) + ((velocity - _last_velocity) * v_scale);
// calculate the error term in earth frame.
ge.normalize();
_ra_sum.normalize();
if (_ra_sum.is_inf() || ge.is_inf()) {
// the _ra_sum length is zero - we are falling with
// no apparent gravity. This gives us no information
// about which way up we are, so treat the error as zero
error = Vector3f(0,0,0);
} else {
error = _ra_sum % ge;
}
// calculate the error term in earth frame
error = (_ra_sum/_ra_deltat % ge)/ge.length();
_error_rp_sum += error.length();
_error_rp_count++;
// extract the X and Y components for the total drift
// error. The Z component comes from the yaw source
// we constrain the error on each axis to 0.2
// the Z component of this error comes from the yaw correction
_drift_error_earth.x = error.x; //constrain(error.x, -0.2, 0.2);
_drift_error_earth.y = error.y;// constrain(error.y, -0.2, 0.2);
//_drift_error_earth.z = error.z;
// convert the error term to body frame
error = _dcm_matrix.mul_transpose(_drift_error_earth);
error = _dcm_matrix.mul_transpose(error);
// we now want to calculate _omega_P and _omega_I. The
// _omega_P value is what drags us quickly to the
// accelerometer reading.
_omega_P = error * _kp;
_omega_I_delta = (error * _ki) / _ra_deltat;
// the _omega_I is the long term accumulated gyro
// error. This determines how much gyro drift we can
// handle.
//_omega_I_delta_sum += error * _ki * _ra_deltat;
//_omega_I_sum_time += _ra_deltat;
// if we have accumulated a gyro drift estimate for 15
// seconds, then move it to the _omega_I term which is applied
// on each update
/*if (_omega_I_sum_time > 5) {
// limit the slope of omega_I on each axis to
// the maximum drift rate reported by the sensor driver
//float drift_limit = _gyro_drift_limit * _ra_deltat * 2;
_omega_I_delta_sum /= _omega_I_sum_time;
_omega_I_delta_sum.x =_omega_I_delta_sum.x; //constrain(_omega_I_delta_sum.x, -drift_limit, drift_limit);
_omega_I_delta_sum.y =_omega_I_delta_sum.y; //constrain(_omega_I_delta_sum.y, -drift_limit, drift_limit);
_omega_I_delta_sum.z =_omega_I_delta_sum.z; //constrain(_omega_I_delta_sum.z, -drift_limit, drift_limit);
_omega_I_delta = _omega_I_delta_sum;
_omega_I_delta_sum.zero();
_omega_I_sum_time = 0;
}*/
// accumulate some integrator error
_omega_I_sum += error * _ki * _ra_deltat;
_omega_I_sum_time += _ra_deltat;
if (_omega_I_sum_time >= 5) {
// limit the rate of change of omega_I to the hardware
// reported maximum gyro drift rate. This ensures that
// short term errors don't cause a buildup of omega_I
// beyond the physical limits of the device
float change_limit = _gyro_drift_limit * _omega_I_sum_time;
_omega_I_sum.x = constrain(_omega_I_sum.x, -change_limit, change_limit);
_omega_I_sum.y = constrain(_omega_I_sum.y, -change_limit, change_limit);
_omega_I_sum.z = constrain(_omega_I_sum.z, -change_limit, change_limit);
_omega_I += _omega_I_sum;
_omega_I_sum.zero();
_omega_I_sum_time = 0;
}
// zero our accumulator ready for the next GPS step
_ra_sum.zero();
_ra_deltat = 0;
_ra_sum_start = last_correction_time;
// remember the GPS velocity for next time
_gps_last_velocity = gps_velocity;
// these sums support the reporting of the DCM state via MAVLink
_error_rp_sum += Vector3f(_drift_error_earth.x, _drift_error_earth.y, 0).length();
_error_rp_count++;
// remember the velocity for next time
_last_velocity = velocity;
}

27
libraries/AP_AHRS/AP_AHRS_DCM.h
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@ -18,9 +18,13 @@ public: @@ -18,9 +18,13 @@ public:
{
_dcm_matrix.identity();
// base the ki values on the sensors drift rate
_ki = _gyro_drift_limit * 5;
_kp = .13;
// these are experimentally derived from the simulator
// with large drift levels
_ki = 0.0087;
_ki_yaw = 0.01;
_kp = 0.4;
_kp_yaw.set(0.4);
}
// return the smoothed gyro vector corrected for drift
@ -44,6 +48,7 @@ public: @@ -44,6 +48,7 @@ public:
private:
float _kp;
float _ki;
float _ki_yaw;
bool _have_initial_yaw;
// Methods
@ -52,9 +57,9 @@ private: @@ -52,9 +57,9 @@ private:
void check_matrix(void);
bool renorm(Vector3f const &a, Vector3f &result);
void drift_correction(float deltat);
void drift_correction_old(float deltat);
void drift_correction_compass(float deltat);
void drift_correction_yaw(float deltat);
float yaw_error_compass();
float yaw_error_gps();
void euler_angles(void);
// primary representation of attitude
@ -63,11 +68,12 @@ private: @@ -63,11 +68,12 @@ private:
Vector3f _gyro_vector; // Store the gyros turn rate in a vector
Vector3f _accel_vector; // current accel vector
Vector3f _omega_P; // accel Omega Proportional correction
Vector3f _omega_P; // accel Omega proportional correction
Vector3f _omega_yaw_P; // proportional yaw correction
Vector3f _omega_I; // Omega Integrator correction
Vector3f _omega_integ_corr; // Partially corrected Gyro_Vector data - used for centrepetal correction
Vector3f _omega_I_sum;
float _omega_I_sum_time;
Vector3f _omega; // Corrected Gyro_Vector data
Vector3f _omega_I_delta;
// state to support status reporting
float _renorm_val_sum;
@ -84,13 +90,10 @@ private: @@ -84,13 +90,10 @@ private:
// state of accel drift correction
Vector3f _ra_sum;
Vector3f _gps_last_velocity;
Vector3f _last_velocity;
float _ra_deltat;
uint32_t _ra_sum_start;
// estimate of the magnetic field strength
float _field_strength;
// current drift error in earth frame
Vector3f _drift_error_earth;
};

12
libraries/AP_AHRS/AP_AHRS_Quaternion.cpp
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@ -100,13 +100,7 @@ void AP_AHRS_Quaternion::update_IMU(float deltat, Vector3f &gyro, Vector3f &acce @@ -100,13 +100,7 @@ void AP_AHRS_Quaternion::update_IMU(float deltat, Vector3f &gyro, Vector3f &acce
// our quaternion is bad! Reset based on roll/pitch/yaw
// and hope for the best ...
renorm_blowup_count++;
if (_compass) {
_compass->null_offsets_disable();
}
q.from_euler(roll, pitch, yaw);
if (_compass) {
_compass->null_offsets_enable();
}
return;
}
SEq_1 *= norm;
@ -263,9 +257,7 @@ void AP_AHRS_Quaternion::update_MARG(float deltat, Vector3f &gyro, Vector3f &acc @@ -263,9 +257,7 @@ void AP_AHRS_Quaternion::update_MARG(float deltat, Vector3f &gyro, Vector3f &acc
// our quaternion is bad! Reset based on roll/pitch/yaw
// and hope for the best ...
renorm_blowup_count++;
_compass->null_offsets_disable();
q.from_euler(roll, pitch, yaw);
_compass->null_offsets_disable();
return;
}
SEq_1 *= norm;
@ -311,12 +303,10 @@ void AP_AHRS_Quaternion::update(void) @@ -311,12 +303,10 @@ void AP_AHRS_Quaternion::update(void)
if (!_have_initial_yaw && _compass &&
_compass->use_for_yaw()) {
// setup the quaternion with initial compass yaw
_compass->null_offsets_disable();
q.from_euler(0, 0, _compass->heading);
q.from_euler(0, 0, _compass->calculate_heading(0,0));
_have_initial_yaw = true;
_compass_last_update = _compass->last_update;
gyro_bias.zero();
_compass->null_offsets_enable();
}
// get current IMU state

6
libraries/AP_AHRS/examples/AHRS_Test/AHRS_Test.pde
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@ -110,7 +110,6 @@ void setup(void) @@ -110,7 +110,6 @@ void setup(void)
compass.set_orientation(MAG_ORIENTATION);
if (compass.init()) {
Serial.printf("Enabling compass\n");
compass.null_offsets_enable();
ahrs.set_compass(&compass);
} else {
Serial.printf("No compass detected\n");
@ -126,6 +125,7 @@ void loop(void) @@ -126,6 +125,7 @@ void loop(void)
static uint16_t counter;
static uint32_t last_t, last_print, last_compass;
uint32_t now = micros();
float heading = 0;
if (last_t == 0) {
last_t = now;
@ -135,7 +135,7 @@ void loop(void) @@ -135,7 +135,7 @@ void loop(void)
if (now - last_compass > 100*1000UL &&
compass.read()) {
compass.calculate(ahrs.get_dcm_matrix());
heading = compass.calculate_heading(ahrs.get_dcm_matrix());
// read compass at 10Hz
last_compass = now;
#if WITH_GPS
@ -155,7 +155,7 @@ void loop(void) @@ -155,7 +155,7 @@ void loop(void)
ToDeg(drift.x),
ToDeg(drift.y),
ToDeg(drift.z),
compass.use_for_yaw()?ToDeg(compass.heading):0.0,
compass.use_for_yaw()?ToDeg(heading):0.0,
(1.0e6*counter)/(now-last_print));
last_print = now;
counter = 0;

2
libraries/AP_AHRS/examples/DCM_Test/DCM_Test.pde
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@ -95,7 +95,6 @@ void setup(void) @@ -95,7 +95,6 @@ void setup(void)
compass.set_orientation(MAG_ORIENTATION);
if (compass.init()) {
Serial.printf("Enabling compass\n");
compass.null_offsets_enable();
dcm.set_compass(&compass);
}
}
@ -115,7 +114,6 @@ void loop(void) @@ -115,7 +114,6 @@ void loop(void)
last_t = now;
compass.read();
compass.calculate(dcm.get_dcm_matrix());
dcm.update_DCM();
delay(20);
counter++;

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