src/Instrumentation/vertical_speed_indicator.cxx

   1 // vertical_speed_indicator.cxx - a regular VSI.
   2 // Written by David Megginson, started 2002.
   3 //
   4 // Last change by E. van den Berg, 17.02.1013
   5 //
   6 // This file is in the Public Domain and comes with no warranty.
   7
   8 #ifdef HAVE_CONFIG_H
   9 #  include "config.h"
  10 #endif
  11
  12 #include <simgear/constants.h>
  13 #include <simgear/math/interpolater.hxx>
  14
  15 #include "vertical_speed_indicator.hxx"
  16 #include <Main/fg_props.hxx>
  17 #include <Main/util.hxx>
  18
  19 //** NOTE: do not change these values. If you change one of them the others need to be changed too */
  20 //** these values calibrate the VSI at SL. */
  21 #define Vol_casing 1.25e-4          //m3
  22 #define A_orifice 7.853982e-9       //m2
  23 #define Factor_cal 189.145628       //-
  24
  25 using std::string;
  26
  27 VerticalSpeedIndicator::VerticalSpeedIndicator ( SGPropertyNode *node )
  28     : _casing_pressure_Pa(101325),
  29       _name(node->getStringValue("name", "vertical-speed-indicator")),
  30       _num(node->getIntValue("number", 0)),
  31       _static_pressure(node->getStringValue("static-pressure", "/systems/static/pressure-inhg")),
  32       _static_temperature(node->getStringValue("static-temperature", "/environment/temperature-degc"))
  33 {
  34 }
  35
  36 VerticalSpeedIndicator::~VerticalSpeedIndicator ()
  37 {
  38 }
  39
  40 void
  41 VerticalSpeedIndicator::init ()
  42 {
  43     string branch;
  44     branch = "/instrumentation/" + _name;
  45
  46     SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );
  47     _serviceable_node = node->getChild("serviceable", 0, true);
  48     _pressure_node = fgGetNode(_static_pressure.c_str(), true);
  49     _temperature_node = fgGetNode(_static_temperature.c_str(), true);
  50     _speed_fpm_node = node->getChild("indicated-speed-fpm", 0, true);
  51     _speed_mps_node = node->getChild("indicated-speed-mps", 0, true);
  52     _speed_kts_node = node->getChild("indicated-speed-kts", 0, true);
  53
  54     reinit();
  55 }
  56
  57 void
  58 VerticalSpeedIndicator::reinit ()
  59 {
  60     // Initialize at ambient conditions
  61     double casing_pressure_inHg = _pressure_node->getDoubleValue();
  62     _casing_pressure_Pa =  casing_pressure_inHg * SG_INHG_TO_PA;
  63     double casing_temperature_C = _temperature_node->getDoubleValue();
  64     double casing_temperature_K = casing_temperature_C + 273.15;
  65     _casing_density_kgpm3 = _casing_pressure_Pa / (casing_temperature_K * SG_R_m2_p_s2_p_K);
  66     _casing_airmass_kg = _casing_density_kgpm3 * Vol_casing;
  67     _orifice_massflow_kgps = 0.0;
  68 }
  69
  70 void
  71 VerticalSpeedIndicator::update (double dt)
  72 {
  73     if (_serviceable_node->getBoolValue()) {
  74         double pressure_inHg = _pressure_node->getDoubleValue() ;
  75         double pressure_Pa = pressure_inHg * SG_INHG_TO_PA;
  76         double Fsign = 0.;
  77         double orifice_mach = 0.0;
  78
  79 // This is a thermodynamically correct model of a mechanical vertical speed indicator:
  80 // It represents an aneroid in a closed (constant volume) casing with the aneroid internal pressure = static pressure
  81 // The casing has an orifice to static pressure
  82 // the mass flow through the orifice is calculated using compressible aerodynamics (but adiabatic and of course a perfect gas)
  83 // using the pressure in the casing and static pressure
  84 //
  85 // sadly at very low flows (small VS) in conjunction with the fact discrete timesteps (dt) are used, a numerical instability is formed.
  86 // this is counteracted by setting the massflow 0 at very small pressure differentials
  87 // this causes a small funny jump of your VSI when passing through 0...cannot be helped!
  88 //
  89 // also note the calibration is only valid for 0ft, so at higher altitudes, the vertical speed is not correct, but would indicate as a real mechanical VSI.
  90 // Only use for conventional mechanical VSI-s. Dont use in an Air Data Computer.
  91 //
  92 // (...and it is supposed to lag!)
  93
  94         _casing_airmass_kg = _casing_airmass_kg - _orifice_massflow_kgps * dt;
  95         double new_density_kgpm3 = _casing_airmass_kg / Vol_casing;
  96         _casing_pressure_Pa = _casing_pressure_Pa * pow(new_density_kgpm3 / _casing_density_kgpm3 , SG_gamma);
  97         double casing_temperature_K = _casing_pressure_Pa / (new_density_kgpm3 * SG_R_m2_p_s2_p_K);
  98
  99         if( _casing_pressure_Pa - pressure_Pa > 0.0 ) {
 100             Fsign = 1.0;         //outflow, pos VS
 101         } else {
 102             Fsign = -1.0;        //inflow, neg VS
 103         }
 104
 105         if( fabs(_casing_pressure_Pa - pressure_Pa) < 0.01 ) {
 106             orifice_mach = 0.0;
 107         } else {
 108             orifice_mach = sqrt(fabs (2.0*SG_cp_m2_p_s2_p_K / (SG_gamma * SG_R_m2_p_s2_p_K) * ( pow(pressure_Pa / _casing_pressure_Pa ,(SG_gamma-1)/SG_gamma ) -1 ) ) );
 109         }
 110
 111         _orifice_massflow_kgps = Fsign * _casing_pressure_Pa / sqrt(casing_temperature_K) * sqrt(SG_gamma/SG_R_m2_p_s2_p_K) * orifice_mach * pow(1+(SG_gamma-1)/2*orifice_mach*orifice_mach,-(SG_gamma+1)/(2*(SG_gamma-1))) * A_orifice;
 112
 113         double vs_fpm = Fsign * sqrt( fabs( pressure_Pa - _casing_pressure_Pa ) ) * Factor_cal;
 114         double vs_kts = vs_fpm / 60 * SG_FPS_TO_KT;
 115         double vs_mps = vs_fpm / 60 * SG_FEET_TO_METER;
 116
 117         _speed_fpm_node
 118           ->setDoubleValue(vs_fpm);
 119         _speed_kts_node
 120           ->setDoubleValue(vs_kts);
 121         _speed_mps_node
 122           ->setDoubleValue(vs_mps);
 123
 124         _casing_density_kgpm3 = new_density_kgpm3;
 125
 126     }
 127 }
 128
 129 // end of vertical_speed_indicator.cxx