ardupilot/libraries/AP_Airspeed/Airspeed_Calibration.cpp

/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
/*
 *   auto_calibration.cpp - airspeed auto calibration

 * Algorithm by Paul Riseborough 
 *
 */

#include <AP_HAL.h>
#include <AP_Math.h>
#include <AP_Common.h>
#include <AP_Airspeed.h>

extern const AP_HAL::HAL& hal;

// constructor - fill in all the initial values
Airspeed_Calibration::Airspeed_Calibration(const AP_Vehicle::FixedWing &parms) :
    P(100,   0,         0,
      0,   100,         0,
      0,     0,  0.000001f),
    Q0(0.01f),
    Q1(0.0000005f),
    state(0, 0, 0),
    DT(1),
    aparm(parms)
{
}

/*
  initialise the ratio
 */
void Airspeed_Calibration::init(float initial_ratio)
{
    state.z = 1.0 / sqrtf(initial_ratio);
}

/*
  update the state of the airspeed calibration - needs to be called
  once a second

  On an AVR2560 this costs 1.9 milliseconds per call
 */
float Airspeed_Calibration::update(float airspeed, const Vector3f &vg)
{
    // Perform the covariance prediction
    // Q is a diagonal matrix so only need to add three terms in
    // C code implementation
    // P = P + Q;
    P.a.x += Q0;
    P.b.y += Q0;
    P.c.z += Q1;
    
    // Perform the predicted measurement using the current state estimates
    // No state prediction required because states are assumed to be time
    // invariant plus process noise
    // Ignore vertical wind component
    float TAS_pred = state.z * pythagorous3(vg.x - state.x, vg.y - state.y, vg.z);
    float TAS_mea  = airspeed;
    
    // Calculate the observation Jacobian H_TAS
    float SH1 = sq(vg.y - state.y) + sq(vg.x - state.x);
    if (SH1 < 0.000001f) {
        // avoid division by a small number
        return state.z;
    }
    float SH2 = 1/sqrtf(SH1);

    // observation Jacobian
    Vector3f H_TAS(
        -(state.z*SH2*(2*vg.x - 2*state.x))/2,
        -(state.z*SH2*(2*vg.y - 2*state.y))/2,
        1/SH2);
    
    // Calculate the fusion innovaton covariance assuming a TAS measurement
    // noise of 1.0 m/s
    // S = H_TAS*P*H_TAS' + 1.0; % [1 x 3] * [3 x 3] * [3 x 1] + [1 x 1]
    Vector3f PH = P * H_TAS;
    float S = H_TAS * PH + 1.0f;
    
    // Calculate the Kalman gain
    // [3 x 3] * [3 x 1] / [1 x 1]
    Vector3f KG = PH / S; 
    
    // Update the states
    state += KG*(TAS_mea - TAS_pred); // [3 x 1] + [3 x 1] * [1 x 1]
    
    // Update the covariance matrix
    Vector3f HP2 = H_TAS * P;
    P -= KG.mul_rowcol(HP2);
    
    // force symmetry on the covariance matrix - necessary due to rounding
    // errors
	float P12 = 0.5f * (P.a.y + P.b.x);
	float P13 = 0.5f * (P.a.z + P.c.x);
	float P23 = 0.5f * (P.b.z + P.c.y);
	P.a.y = P.b.x = P12;
	P.a.z = P.c.x = P13;
	P.b.z = P.c.y = P23;

    // Constrain diagonals to be non-negative - protects against rounding errors
    P.a.x = max(P.a.x, 0.0f);
    P.b.y = max(P.b.y, 0.0f);
    P.c.z = max(P.c.z, 0.0f);

    state.x = constrain_float(state.x, -aparm.airspeed_max, aparm.airspeed_max);
    state.y = constrain_float(state.y, -aparm.airspeed_max, aparm.airspeed_max);
    state.z = constrain_float(state.z, 0.5f, 1.0f);

    return state.z;
}


/*
  called once a second to do calibration update
 */
void AP_Airspeed::update_calibration(const Vector3f &vground)
{
    if (!_autocal) {
        // auto-calibration not enabled
        return;
    }

    // set state.z based on current ratio, this allows the operator to
    // override the current ratio in flight with autocal, which is
    // very useful both for testing and to force a reasonable value. 
    float ratio = constrain_float(_ratio, 1.0f, 4.0f);

    _calibration.state.z = 1.0 / sqrtf(ratio);

    // calculate true airspeed, assuming a airspeed ratio of 1.0
    float dpress = get_differential_pressure();
    float true_airspeed = sqrtf(dpress) * _EAS2TAS;

    float zratio = _calibration.update(true_airspeed, vground);

    if (isnan(zratio) || isinf(zratio)) {
        return;
    }

    // this constrains the resulting ratio to between 1.0 and 4.0
    zratio = constrain_float(zratio, 0.5f, 1.0f);
    _ratio.set(1/sq(zratio));
    if (_counter > 60) {
        if (_last_saved_ratio > 1.05f*_ratio || 
            _last_saved_ratio < 0.95f*_ratio) {
            _ratio.save();
            _last_saved_ratio = _ratio;
            _counter = 0;
        }
    } else {
        _counter++;
    }
}

// log airspeed calibration data to MAVLink
void AP_Airspeed::log_mavlink_send(mavlink_channel_t chan, const Vector3f &vground)
{
    mavlink_msg_airspeed_autocal_send(chan,
                                      vground.x,
                                      vground.y,
                                      vground.z,
                                      get_differential_pressure(),
                                      _EAS2TAS,
                                      _ratio.get(),
                                      _calibration.state.x,
                                      _calibration.state.y,
                                      _calibration.state.z,
                                      _calibration.P.a.x,
                                      _calibration.P.b.y,
                                      _calibration.P.c.z);
}