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AP_NavEKF : Enable calc of wind velocity when not using airspeed sensing

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This patch also cleans up the logic associated with use of the synthetic
sideslip measurement so that it can never be used for a non fly-forward
vehicle type

This patch adds functionality that initialises the wind-speed vector to the
reciprocal of the ground speed vector, and scaled to 6 m/s. On average this gives
a better initial wind velocity estimate on launch by assuming:

a) launch will be into wind
b) wind speed is equal to global average

It also helps prevent a headwind causing initial underestimation of airspeed
causing high autopilot gains and limit cycles on climb-out, until first
turn when the EKF is able to estimate the wind.
This commit is contained in:
priseborough 2014-04-10 18:05:49 +10:00 committed by Andrew Tridgell
parent df192a9efd
commit 188bea6bab
2 changed files with 40 additions and 5 deletions
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39
libraries/AP_NavEKF/AP_NavEKF.cpp
View File

@ -761,9 +761,10 @@ void NavEKF::SelectBetaFusion()
// synthetic sidelip fusion only works for fixed wing aircraft and relies on the average sideslip being close to zero
// it requires a stable wind estimate for best results and should not be used for aerobatic flight
// we fuse synthetic sideslip measurements if:
// (we are not using a compass OR (we are dead-reckoning position AND using airspeed)) AND not on the ground AND enough time has lapsed since our last fusion AND
// (we have not fused magnetometer data on this time step OR the immediate fusion flag is set)
if ((!use_compass() || (!posHealth && useAirspeed())) && !onGround && ((IMUmsec - BETAmsecPrev) >= _msecBetaAvg) && (!magFusePerformed || fuseMeNow)) {
// we are a fly forward vehicle type AND NOT using a full range of sensors with healthy position
// AND NOT on the ground AND enough time has lapsed since our last fusion
// AND (we have not fused magnetometer data on this time step OR the immediate fusion flag is set)
if (_ahrs->get_fly_forward() && !(use_compass() && useAirspeed() && posHealth) && !onGround && ((IMUmsec - BETAmsecPrev) >= _msecBetaAvg) && (!magFusePerformed || fuseMeNow)) {
FuseSideslip();
BETAmsecPrev = IMUmsec;
}
@ -2709,6 +2710,14 @@ void NavEKF::OnGroundCheck()
if (!onGround && prevOnGround && !use_compass()) {
alignYawGPS();
}
// If we are flying a fly-forward type vehicle without an airspeed sensor and exiting onGround
// we set the wind velocity to the reciprocal of the velocity vector and scale states so that the
// wind speed is equal to the 6m/s. This prevents gains being too high at the start
// of flight if launching into a headwind until the first turn when the EKF can form a wind speed
// estimate
if (!onGround && prevOnGround && !useAirspeed() && _ahrs->get_fly_forward()) {
setWindVelStates();
}
}
prevOnGround = onGround;
}
@ -2795,9 +2804,9 @@ void NavEKF::CopyAndFixCovariances()
}
}
}
// if we flying and are not using airspeed and are not using synthetic sideslip measurements, we want all the off-diagonals for the wind
// if we have a non-forward flight vehicle type and no airspeed sensor, we want the wind
// states to remain zero and want to keep the old variances for these states
else if (!useAirspeed() && use_compass()) {
else if (!useAirspeed() && !_ahrs->get_fly_forward()) {
// copy calculated variances we want to propagate
for (uint8_t i=0; i<=13; i++) {
P[i][i] = nextP[i][i];
@ -3133,6 +3142,26 @@ void NavEKF::alignYawGPS()
}
}
// This function is used to do a forced alignment of the wind velocity states so that they are set to the reciprocal of
// the ground speed and scaled to 6 m/s. This is used when launching a fly-forward vehicle without an airspeed sensor
// on the assumption that launch will be into wind and 6 is representative global average at height
// http://maps.google.com/gallery/details?id=zJuaSgXp_WLc.kTBytKPmNODY&hl=en
void NavEKF::setWindVelStates()
{
float gndSpd = sqrtf(sq(states[4]) + sq(states[5]));
if (gndSpd > 4.0f) {
// set the wind states to be the reciprocal of the velocity and scale to 6 m/s
float scaleFactor = 6.0f / gndSpd;
states[14] = - states[4] * scaleFactor;
states[15] = - states[5] * scaleFactor;
// reinitialise the wind state covariances
zeroRows(P,14,15);
zeroCols(P,14,15);
P[14][14] = 64.0f;
P[15][15] = P[14][14];
}
}
// return the transformation matrix from XYZ (body) to NED axes
void NavEKF::getRotationBodyToNED(Matrix3f &mat) const
{

6
libraries/AP_NavEKF/AP_NavEKF.h
View File

@ -248,6 +248,12 @@ private:
// force alignment of the yaw angle using GPS velocity data
void alignYawGPS();
// Forced alignment of the wind velocity states so that they are set to the reciprocal of
// the ground speed and scaled to 6 m/s. This is used when launching a fly-forward vehicle without an airspeed sensor
// on the assumption that launch will be into wind and 6 is representative global average at height
// http://maps.google.com/gallery/details?id=zJuaSgXp_WLc.kTBytKPmNODY&hl=en
void setWindVelStates();
// initialise the earth magnetic field states using declination and current attitude and magnetometer meaasurements
// and return attitude quaternion
Quaternion calcQuatAndFieldStates(float roll, float pitch);