1 // inst_vertical_speed_indicator.cxx
2 // -- Instantaneous VSI (emulation calibrated to standard atmosphere).
4 // Started September 2004.
8 // This program is free software; you can redistribute it and/or
9 // modify it under the terms of the GNU General Public License as
10 // published by the Free Software Foundation; either version 2 of the
11 // License, or (at your option) any later version.
13 // This program is distributed in the hope that it will be useful, but
14 // WITHOUT ANY WARRANTY; without even the implied warranty of
15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
16 // General Public License for more details.
18 // You should have received a copy of the GNU General Public License
19 // along with this program; if not, write to the Free Software
20 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
28 #include <simgear/math/interpolater.hxx>
30 #include "inst_vertical_speed_indicator.hxx"
31 #include <Main/fg_props.hxx>
32 #include <Main/util.hxx>
35 // Altitude based on pressure difference from sea level.
37 // See http://www.pdas.com/programs/atmos.f90, the tables match exactly
38 // environment.cxx (Standard 1976).
40 // - at 27900 m the level is 20 km :
41 // geopotential altitude 27.9 x 6369 / ( 27.9 + 6369) = 27.778 km.
42 // - deltah = 27.778 - 20 = 7.778 km and tbase 216.65 degK.
43 // - delta = 5.403295E-2 x exp( -34.163195 x 7.778 / 216.65 ) = 0.016.
44 // - pressure = (1 - delta ) x 29.92 = 29.44 inhg.
45 // - to construct the tables, round delta to 3 digits.
47 // altitude ft, pressure step inHG
48 static double pressure_data[][2] = {
49 { 0.00, 3.33 }, // guess !
87 // pressure difference inHG, altitude ft
88 static double altitude_data[][2] = {
123 { 29.62, 100393.70 },
129 #define SEA_LEVEL_INHG 29.92
131 // A higher number means more responsive.
132 #define RESPONSIVENESS 5.0
134 // External environment
135 #define MAX_INHG_PER_S 0.0002
138 InstVerticalSpeedIndicator::InstVerticalSpeedIndicator ( SGPropertyNode *node ) :
139 _name(node->getStringValue("name", "inst-vertical-speed-indicator")),
140 _num(node->getIntValue("number", 0)),
141 _internal_pressure_inhg( SEA_LEVEL_INHG ),
142 _internal_sea_inhg( SEA_LEVEL_INHG ),
143 _speed_ft_per_s( 0 ),
144 _pressure_table(new SGInterpTable),
145 _altitude_table(new SGInterpTable)
148 for ( i = 0; pressure_data[i][0] != -1; i++)
149 _pressure_table->addEntry( pressure_data[i][0], pressure_data[i][1] );
151 for ( i = 0; altitude_data[i][0] != -1; i++)
152 _altitude_table->addEntry( altitude_data[i][0], altitude_data[i][1] );
156 InstVerticalSpeedIndicator::~InstVerticalSpeedIndicator ()
158 delete _pressure_table;
159 delete _altitude_table;
163 void InstVerticalSpeedIndicator::init ()
165 SGPropertyNode *node = fgGetNode("/instrumentation", true)->getChild(_name, _num, true);
168 node->getNode("serviceable", true);
170 fgGetNode("/sim/freeze/master", true);
172 // A real IVSI is operated by static pressure changes.
173 // It operates like a conventional VSI, except that an internal sensor
174 // detects load factors, to momentarily alters the static pressure
176 // It appears lag free at subsonic speed; at high altitude indication may
177 // be less than 1/3 of actual conditions.
179 fgGetNode("/environment/pressure-inhg", true);
181 fgGetNode("/environment/pressure-sea-level-inhg", true);
183 fgGetNode("/sim/speed-up", true);
185 node->getNode("indicated-speed-fps", true);
187 node->getNode("indicated-speed-fpm", true);
190 void InstVerticalSpeedIndicator::reinit ()
192 // Initialize at ambient pressure
193 _internal_pressure_inhg = _pressure_node->getDoubleValue();
194 _speed_ft_per_s = 0.0;
195 _internal_sea_inhg = _sea_node->getDoubleValue();
199 void InstVerticalSpeedIndicator::update (double dt)
201 if (_serviceable_node->getBoolValue())
203 // avoids hang, when freeze
204 if( !_freeze_node->getBoolValue() && std::numeric_limits<double>::min() < fabs(dt))
206 double pressure_inhg = _pressure_node->getDoubleValue();
207 double sea_inhg = _sea_node->getDoubleValue();
208 double speed_up = _speed_up_node->getDoubleValue();
210 // must work by speed up
216 // limit effect of external environment
217 double rate_sea_inhg_per_s = ( sea_inhg - _internal_sea_inhg ) / dt;
219 if( rate_sea_inhg_per_s > - MAX_INHG_PER_S && rate_sea_inhg_per_s < MAX_INHG_PER_S )
221 double rate_inhg_per_s = ( pressure_inhg - _internal_pressure_inhg ) / dt;
223 // IVSI determines alone the current altitude, without altimeter setting.
224 // Altimeter setting is 29.92 above 10000 or 18000 ft.
225 // Below this level, the slope is slightly wrong.
226 double altitude_ft = _altitude_table->interpolate( SEA_LEVEL_INHG - pressure_inhg );
227 double slope_inhg = _pressure_table->interpolate( altitude_ft );
230 double last_speed_ft_per_s = _speed_ft_per_s;
233 _speed_ft_per_s = - rate_inhg_per_s * 2952.75591 / slope_inhg;
236 _speed_ft_per_s = fgGetLowPass( last_speed_ft_per_s, _speed_ft_per_s,
237 dt * RESPONSIVENESS );
240 _speed_node->setDoubleValue( _speed_ft_per_s );
241 _speed_min_node->setDoubleValue( _speed_ft_per_s * 60.0 );
244 _internal_pressure_inhg = pressure_inhg;
245 _internal_sea_inhg = sea_inhg;
250 // end of inst_vertical_speed_indicator.cxx