Vivian Meazza:

I attach 2 new files and a diff file for the associated changes to add a
“fluxgate compass” to the instrument inventory. Whist this outputs
essentially the same data as /orientation/heading-magnetic-deg, it has to
be powered, and can be made to fail. I also followed Roy’s suggestion to
generate the error properties for this instrument here rather than in
xmlauto.xml.

When this instrument is included in cvs, I intend to use it in the Hunter,
A4F Seahawk and KC135. After a bit more research, it might be appropriate
for the Spitfire and Hurricane as well. AJ would also like to use it for his
Lightning model.

src/Instrumentation/Makefile.am

@@ -15,6 +15,7 @@ libInstrumentation_a_SOURCES = \
         gps.cxx gps.hxx \
         gyro.cxx gyro.hxx \
         heading_indicator.cxx heading_indicator.hxx \
+        heading_indicator_fg.cxx heading_indicator_fg.hxx \
 	kr_87.hxx kr_87.cxx \
 	kt_70.cxx kt_70.hxx \
         mag_compass.cxx mag_compass.hxx \

src/Instrumentation/heading_indicator_fg.cxx

@@ -0,0 +1,160 @@
+// heading_indicator_fg.cxx - a flux_gate compass.
+// Based on the vacuum driven Heading Indicator Written by David Megginson, started 2002.
+//
+// Written by Vivian Meazza, started 2005.
+//
+// This file is in the Public Domain and comes with no warranty.
+
+#include <simgear/compiler.h>
+#include STL_IOSTREAM
+#include STL_STRING
+#include <sstream>
+
+#include "heading_indicator_fg.hxx"
+#include <Main/fg_props.hxx>
+#include <Main/util.hxx>			    
+
+
+HeadingIndicatorFG::HeadingIndicatorFG ( SGPropertyNode *node )
+    :
+    name("heading-indicator-fg"),	 
+    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_INSTR, SG_WARN, "Error in flux-gate heading-indicator config logic" );
+            if ( name.length() ) {
+                SG_LOG( SG_INSTR, SG_WARN, "Section = " << name );
+            }
+        }
+    }
+}
+
+HeadingIndicatorFG::HeadingIndicatorFG ()
+{
+}
+
+HeadingIndicatorFG::~HeadingIndicatorFG ()
+{
+}
+
+void
+HeadingIndicatorFG::init ()
+{
+    string branch;
+    branch = "/instrumentation/" + name;
+    
+	_heading_in_node = fgGetNode("/orientation/heading-deg", true);
+    SGPropertyNode *node = fgGetNode(branch.c_str(), num, true );
+    _offset_node = node->getChild("offset-deg", 0, true);
+    _serviceable_node = node->getChild("serviceable", 0, true);
+	_error_node = node->getChild("heading-bug-error-deg", 0, true);
+	_nav1_error_node = node->getChild("nav1-course-error-deg", 0, true);
+    _heading_out_node = node->getChild("indicated-heading-deg", 0, true);
+    _last_heading_deg = (_heading_in_node->getDoubleValue() +
+                         _offset_node->getDoubleValue());
+	_electrical_node = fgGetNode("/systems/electrical/outputs/DG", true);
+}
+
+void
+HeadingIndicatorFG::bind ()
+{
+    std::ostringstream temp;
+    string branch;
+    temp << num;
+    branch = "/instrumentation/" + name + "[" + temp.str() + "]";
+
+    fgTie((branch + "/serviceable").c_str(),
+          &_gyro, &Gyro::is_serviceable, &Gyro::set_serviceable);
+    fgTie((branch + "/spin").c_str(),
+          &_gyro, &Gyro::get_spin_norm, &Gyro::set_spin_norm);
+	
+}
+
+void
+HeadingIndicatorFG::unbind ()
+{
+    std::ostringstream temp;
+    string branch;
+    temp << num;
+    branch = "/instrumentation/" + name + "[" + temp.str() + "]";
+
+    fgUntie((branch + "/serviceable").c_str());
+    fgUntie((branch + "/spin").c_str());
+}
+
+void
+HeadingIndicatorFG::update (double dt)
+{
+                                // Get the spin from the gyro
+	 _gyro.set_power_norm(_electrical_node->getDoubleValue());
+
+	_gyro.update(dt);
+    double spin = _gyro.get_spin_norm();
+
+                                // No time-based precession	for a flux gate compass
+	                            // We just use offset to get the magvar
+
+	double offset = _offset_node->getDoubleValue();
+	   
+                                // TODO: movement-induced error
+
+                                // Next, calculate the indicated heading,
+                                // introducing errors.
+    double factor = 0.01 / (spin * spin * spin * spin * spin * spin);
+    double heading = _heading_in_node->getDoubleValue();
+
+                                // Now, we have to get the current
+                                // heading and the last heading into
+                                // the same range.
+    while ((heading - _last_heading_deg) > 180)
+        _last_heading_deg += 360;
+    while ((heading - _last_heading_deg) < -180)
+        _last_heading_deg -= 360;
+
+    heading = fgGetLowPass(_last_heading_deg, heading, dt/factor);
+    _last_heading_deg = heading;
+
+	heading += offset;
+    while (heading < 0)
+        heading += 360;
+    while (heading > 360)
+        heading -= 360;
+
+    _heading_out_node->setDoubleValue(heading);
+
+	                             // calculate the difference between the indicacted heading
+	                             // and the selected heading for use with an autopilot
+	static SGPropertyNode *bnode
+        = fgGetNode( "/autopilot/settings/heading-bug-deg", false );
+	double diff = 0;
+	if ( bnode ){
+		diff = bnode->getDoubleValue() - heading;
+		if ( diff < -180.0 ) { diff += 360.0; }
+		if ( diff > 180.0 ) { diff -= 360.0; }
+		_error_node->setDoubleValue( diff );
+	}   
+	                             // calculate the difference between the indicated heading
+	                             // and the selected nav1 radial for use with an autopilot
+	SGPropertyNode *nnode
+        = fgGetNode( "/instrumentation/nav/radials/selected-deg", true );
+	double ndiff = 0;
+	if ( nnode ){
+		ndiff = nnode->getDoubleValue() - heading;
+		if ( ndiff < -180.0 ) { ndiff += 360.0; }
+		if ( ndiff > 180.0 ) { ndiff -= 360.0; }
+		_nav1_error_node->setDoubleValue( ndiff );
+	}   
+
+
+}
+
+// end of heading_indicator_fg.cxx

src/Instrumentation/heading_indicator_fg.hxx

@@ -0,0 +1,69 @@
+// heading_indicator.hxx - a vacuum-powered heading indicator.
+// Written by David Megginson, started 2002.
+//
+// This file is in the Public Domain and comes with no warranty.
+
+
+#ifndef __INSTRUMENTS_HEADING_INDICATOR_FG_HXX
+#define __INSTRUMENTS_HEADING_INDICATOR_FG_HXX 1
+
+#ifndef __cplusplus
+# error This library requires C++
+#endif
+
+#include <simgear/props/props.hxx>
+#include <simgear/structure/subsystem_mgr.hxx>
+
+#include "gyro.hxx"
+
+
+/**
+ * Model an electicallym-powered fluxgate compass
+ *
+ * Input properties:
+ *
+ * /instrumentation/"name"/serviceable
+ * /instrumentation/"name"/spin
+ * /instrumentation/"name"/offset-deg
+ * /orientation/heading-deg
+ * /systems/electrical/outputs/DG
+ *
+ * Output properties:
+ *
+ * /instrumentation/"name"/indicated-heading-deg
+ */
+class HeadingIndicatorFG : public SGSubsystem
+{
+
+public:
+
+    HeadingIndicatorFG ( SGPropertyNode *node );
+    HeadingIndicatorFG ();
+    virtual ~HeadingIndicatorFG ();
+
+    virtual void init ();   
+    virtual void bind (); 
+    virtual void unbind ();
+    virtual void update (double dt);
+
+private:
+
+    Gyro _gyro;
+    double _last_heading_deg;
+
+    string name;
+    int num;
+    
+    SGPropertyNode_ptr _offset_node;
+    SGPropertyNode_ptr _heading_in_node;
+    SGPropertyNode_ptr _serviceable_node;
+    SGPropertyNode_ptr _heading_out_node;
+	SGPropertyNode_ptr _electrical_node;
+	SGPropertyNode_ptr _error_node;
+	SGPropertyNode_ptr _nav1_error_node;
+
+
+    
+};
+
+#endif // __INSTRUMENTS_HEADING_INDICATOR_HXX

src/Instrumentation/instrument_mgr.cxx

@@ -30,6 +30,7 @@
 #include "encoder.hxx"
 #include "gps.hxx"
 #include "heading_indicator.hxx"
+#include "heading_indicator_fg.hxx"
 #include "kr_87.hxx"
 #include "kt_70.hxx"
 #include "mag_compass.hxx"
@@ -124,6 +125,9 @@ bool FGInstrumentMgr::build ()
         } else if ( name == "heading-indicator" ) {
             set_subsystem( "instrument" + temp.str(), 
                            new HeadingIndicator( node ) );
+        } else if ( name == "heading-indicator-fg" ) {
+            set_subsystem( "instrument" + temp.str(), 
+                           new HeadingIndicatorFG( node ) );
         } else if ( name == "KR-87" ) {
             set_subsystem( "instrument" + temp.str(), 
                            new FGKR_87( node ) );

src/Main/fg_init.cxx

@@ -1578,6 +1578,13 @@ bool fgInitSubsystems() {
     // model or control parameters are set
     fgAircraftInit();   // In the future this might not be the case.
 
+    ////////////////////////////////////////////////////////////////////
+    // Initialize the aircraft systems and instrumentation (before the
+    // autopilot.)
+    ////////////////////////////////////////////////////////////////////
+
+    globals->add_subsystem("instrumentation", new FGInstrumentMgr);
+    globals->add_subsystem("systems", new FGSystemMgr);
 
     ////////////////////////////////////////////////////////////////////
     // Initialize the XML Autopilot subsystem.
@@ -1699,10 +1706,6 @@ bool fgInitSubsystems() {
 	*dispatcher);
 	     //globals->get_subsystem("Traffic Manager")->init();
 
-    globals->add_subsystem("instrumentation", new FGInstrumentMgr);
-    globals->add_subsystem("systems", new FGSystemMgr);
-
-
 
     ////////////////////////////////////////////////////////////////////
     // Initialize the cockpit subsystem

src/Main/main.cxx

@@ -830,6 +830,8 @@ static void fgIdleFunction ( void ) {
                                     globals->get_time_params()->getJD() );
         double var = globals->get_mag()->get_magvar() * SGD_RADIANS_TO_DEGREES;
         fgSetDouble("/instrumentation/heading-indicator/offset-deg", -var);
+        fgSetDouble("/instrumentation/heading-indicator-fg/offset-deg", -var);
+
 
         // airport = new ssgBranch;
         // airport->setName( "Airport Lighting" );