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EKF: protect declination fusion from badly conditioned earth field estimates

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Fusion with large initial magnetometer biases errors can result in the the NE earth field states reducing in magnitude and effectively flipping sign.

EKF: Move declination state limiting into a separate function

EKF: Limit NE mag states after each 3-axis mag fusion

EKF: Fix bug in mag field strength look-up scale factor
This commit is contained in:
Paul Riseborough
2019-01-31 09:53:57 -05:00
committed by Daniel Agar
parent 7bddbd1cc5
commit 3accab1ac5
6 changed files with 71 additions and 4 deletions
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EKF/mag_fusion.cpp
+59 -1
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@@ -419,6 +419,8 @@ void Ekf::fuseMag()
// apply the state corrections
fuse(Kfusion, _mag_innov[index]);
// constrain the declination of the earth field states
limitDeclination();
}
}
}
@@ -797,6 +799,10 @@ void Ekf::fuseDeclination(float decl_sigma)
float magN = _state.mag_I(0);
float magE = _state.mag_I(1);
// minimum horizontal field strength before calculation becomes badly conditioned (T)
float h_field_min = 0.001f;
// observation variance (rad**2)
float R_DECL = sq(decl_sigma);
// Calculate intermediate variables
@@ -804,7 +810,7 @@ void Ekf::fuseDeclination(float decl_sigma)
float t3 = magN*magN;
float t4 = t2+t3;
// if the horizontal magnetic field is too small, this calculation will be badly conditioned
if (t4 < 1e-4f) {
if (t4 < h_field_min*h_field_min) {
return;
}
float t5 = P[16][16]*t2;
@@ -922,5 +928,57 @@ void Ekf::fuseDeclination(float decl_sigma)
// apply the state corrections
fuse(Kfusion, innovation);
// constrain the declination of the earth field states
limitDeclination();
}
}
void Ekf::limitDeclination()
{
// get a reference value for the earth field declinaton and minimum plausible horizontal field strength
// set to 50% of the horizontal strength from geo tables if location is known
float decl_reference;
float h_field_min = 0.001f;
if (_params.mag_declination_source & MASK_USE_GEO_DECL) {
// use parameter value until GPS is available, then use value returned by geo library
if (_NED_origin_initialised) {
decl_reference = _mag_declination_gps;
h_field_min = fmaxf(h_field_min , 0.5f * _mag_strength_gps * cosf(_mag_inclination_gps));
} else {
decl_reference = math::radians(_params.mag_declination_deg);
}
} else {
// always use the parameter value
decl_reference = math::radians(_params.mag_declination_deg);
}
// do not allow the horizontal field length to collapse - this will make the declination fusion badly conditioned
// and can result in a reversal of the NE field states which the filter cannot recover from
// apply a circular limit
float h_field = sqrtf(_state.mag_I(0)*_state.mag_I(0) + _state.mag_I(1)*_state.mag_I(1));
if (h_field < h_field_min) {
if (h_field > 0.001f * h_field_min) {
float h_scaler = h_field_min / h_field;
_state.mag_I(0) *= h_scaler;
_state.mag_I(1) *= h_scaler;
} else {
// too small to scale radially so set to expected value
_state.mag_I(0) = 2.0f * h_field_min * cosf(_mag_declination);
_state.mag_I(1) = 2.0f * h_field_min * sinf(_mag_declination);
}
h_field = h_field_min;
}
// do not allow the declination estimate to vary too much relative to the reference value
const float decl_tolerance = 0.5f;
const float decl_max = decl_reference + decl_tolerance;
const float decl_min = decl_reference - decl_tolerance;
const float decl_estimate = atan2(_state.mag_I(1) , _state.mag_I(0));
if (decl_estimate > decl_max) {
_state.mag_I(0) = h_field * cosf(decl_max);
_state.mag_I(1) = h_field * sinf(decl_max);
} else if (decl_estimate < decl_min) {
_state.mag_I(0) = h_field * cosf(decl_min);
_state.mag_I(1) = h_field * sinf(decl_min);
}
}