flightgear/src/Instrumentation/mag_compass.cxx

// mag_compass.cxx - a magnetic compass.
// Written by David Megginson, started 2003.
//
// This file is in the Public Domain and comes with no warranty.

// This implementation is derived from an earlier one by Alex Perry,
// which appeared in src/Cockpit/steam.cxx

#ifdef HAVE_CONFIG_H
#  include <config.h>
#endif

#include <plib/sg.h>

#include "mag_compass.hxx"
#include <Main/fg_props.hxx>
#include <Main/util.hxx>


MagCompass::MagCompass ( SGPropertyNode *node )
    : _error_deg(0.0),
      _rate_degps(0.0),
      name("magnetic-compass"),
      num(0)
{
    int i;
    for ( i = 0; i < node->nChildren(); ++i ) {
        SGPropertyNode *child = node->getChild(i);
        string cname = child->getName();
        string cval = child->getStringValue();
        if ( cname == "name" ) {
            name = cval;
        } else if ( cname == "number" ) {
            num = child->getIntValue();
        } else {
            SG_LOG( SG_AUTOPILOT, SG_WARN, "Error in magnetic-compass config logic" );
            if ( name.length() ) {
                SG_LOG( SG_AUTOPILOT, SG_WARN, "Section = " << name );
            }
        }
    }
}

MagCompass::MagCompass ()
    : _error_deg(0.0),
      _rate_degps(0.0)
{
}

MagCompass::~MagCompass ()
{
}

void
MagCompass::init ()
{
    string branch;
    branch = "/instrumentation/" + name;

    SGPropertyNode *node = fgGetNode(branch.c_str(), num, true );
    _serviceable_node = node->getChild("serviceable", 0, true);
    _heading_node =
        fgGetNode("/orientation/heading-deg", true);
    _beta_node =
        fgGetNode("/orientation/side-slip-deg", true);
    _variation_node =
        fgGetNode("/environment/magnetic-variation-deg", true);
    _dip_node =
        fgGetNode("/environment/magnetic-dip-deg", true);
    _north_accel_node =
        fgGetNode("/accelerations/ned/north-accel-fps_sec", true);
    _east_accel_node =
        fgGetNode("/accelerations/ned/east-accel-fps_sec", true);
    _down_accel_node =
        fgGetNode("/accelerations/ned/down-accel-fps_sec", true);
    _out_node = node->getChild("indicated-heading-deg", 0, true);

    _serviceable_node->setBoolValue(true);
}

void
MagCompass::update (double delta_time_sec)
{
                                // algorithm from Alex Perry
                                // possibly broken by David Megginson

                                // don't update if it's broken
    if (!_serviceable_node->getBoolValue())
        return;

                                // jam on a sideslip of 12 degrees or more
    if (fabs(_beta_node->getDoubleValue()) > 12.0) {
        _rate_degps = 0.0;
        _error_deg = _heading_node->getDoubleValue() -
            _out_node->getDoubleValue();
        return;
    }

    double accelN = _north_accel_node->getDoubleValue();
    double accelE = _east_accel_node->getDoubleValue();
    double accelU = _down_accel_node->getDoubleValue() - 32.0; // why?

                                // force vector towards magnetic north pole
    double var = _variation_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
    double dip = _dip_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
    double cosdip = cos(dip);
    double forceN = cosdip * cos(var);
    double forceE = cosdip * sin(var);
    double forceU = sin(dip);

                                // rotation is around acceleration axis
                                // (magnitude doesn't matter)
    double accel = accelN * accelN + accelE * accelE + accelU * accelU;
    if (accel > 1.0)
        accel = sqrt(accel);
    else
            accel = 1.0;

                                // North marking on compass card
    double edgeN = cos(_error_deg * SGD_DEGREES_TO_RADIANS);
    double edgeE = sin(_error_deg * SGD_DEGREES_TO_RADIANS);
    double edgeU = 0.0;

                                // apply the force to that edge to get torques
    double torqueN = edgeE * forceU - edgeU * forceE;
    double torqueE = edgeU * forceN - edgeN * forceU;
    double torqueU = edgeN * forceE - edgeE * forceN;

                                // get the component parallel to the axis
    double torque = (torqueN * accelN +
                     torqueE * accelE +
                     torqueU * accelU) * 5.0 / accel;

                                // the compass has angular momentum,
                                // so apply a torque and wait
    if (delta_time_sec < 1.0) {
        _rate_degps = _rate_degps * (1.0 - delta_time_sec) - torque;
        _error_deg += delta_time_sec * _rate_degps;
    }
    if (_error_deg > 180.0)
        _error_deg -= 360.0;
    else if (_error_deg < -180.0)
        _error_deg += 360.0;

                                // Set the indicated heading
    _out_node->setDoubleValue(_heading_node->getDoubleValue() - _error_deg);
}

// end of altimeter.cxx