1 // steam.cxx - Steam Gauge Calculations
3 // Copyright (C) 2000 Alexander Perry - alex.perry@ieee.org
5 // This program is free software; you can redistribute it and/or
6 // modify it under the terms of the GNU General Public License as
7 // published by the Free Software Foundation; either version 2 of the
8 // License, or (at your option) any later version.
10 // This program is distributed in the hope that it will be useful, but
11 // WITHOUT ANY WARRANTY; without even the implied warranty of
12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 // General Public License for more details.
15 // You should have received a copy of the GNU General Public License
16 // along with this program; if not, write to the Free Software
17 // Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
26 #if defined( FG_HAVE_NATIVE_SGI_COMPILERS )
27 # include <iostream.h>
32 #include <simgear/constants.h>
33 #include <simgear/math/sg_types.hxx>
34 #include <simgear/misc/props.hxx>
35 #include <Aircraft/aircraft.hxx>
36 #include <Main/bfi.hxx>
37 #include <NetworkOLK/features.hxx>
39 FG_USING_NAMESPACE(std);
41 #include "radiostack.hxx"
44 static bool isTied = false;
48 ////////////////////////////////////////////////////////////////////////
49 // Declare the functions that read the variables
50 ////////////////////////////////////////////////////////////////////////
52 // Anything that reads the BFI directly is not implemented at all!
55 double FGSteam::the_STATIC_inhg = 29.92;
56 double FGSteam::the_ALT_ft = 0.0; // Indicated altitude
57 double FGSteam::get_ALT_ft() { _CatchUp(); return the_ALT_ft; }
59 double FGSteam::the_ALT_datum_mb = 1013.0;
60 double FGSteam::get_ALT_datum_mb() { return the_ALT_datum_mb; }
62 void FGSteam::set_ALT_datum_mb ( double datum_mb ) {
63 the_ALT_datum_mb = datum_mb;
66 double FGSteam::get_ASI_kias() { return FGBFI::getAirspeed(); }
68 double FGSteam::the_VSI_case = 29.92;
69 double FGSteam::the_VSI_fps = 0.0;
70 double FGSteam::get_VSI_fps() { _CatchUp(); return the_VSI_fps; }
72 double FGSteam::the_VACUUM_inhg = 0.0;
73 double FGSteam::get_VACUUM_inhg() { _CatchUp(); return the_VACUUM_inhg; }
75 double FGSteam::the_MH_err = 0.0;
76 double FGSteam::the_MH_deg = 0.0;
77 double FGSteam::the_MH_degps = 0.0;
78 double FGSteam::get_MH_deg () { _CatchUp(); return the_MH_deg; }
80 double FGSteam::the_DG_err = 0.0;
81 double FGSteam::the_DG_deg = 0.0;
82 double FGSteam::the_DG_degps = 0.0;
83 double FGSteam::the_DG_inhg = 0.0;
84 double FGSteam::get_DG_deg () { _CatchUp(); return the_DG_deg; }
85 double FGSteam::get_DG_err () { _CatchUp(); return the_DG_err; }
87 void FGSteam::set_DG_err ( double approx_magvar ) {
88 the_DG_err = approx_magvar;
91 double FGSteam::the_TC_rad = 0.0;
92 double FGSteam::the_TC_std = 0.0;
93 double FGSteam::get_TC_rad () { _CatchUp(); return the_TC_rad; }
94 double FGSteam::get_TC_std () { _CatchUp(); return the_TC_std; }
97 ////////////////////////////////////////////////////////////////////////
98 // Recording the current time
99 ////////////////////////////////////////////////////////////////////////
102 int FGSteam::_UpdatesPending = 1000000; /* Forces filter to reset */
105 // FIXME: no need to use static
106 // functions any longer.
108 void FGSteam::update ( int timesteps )
112 fgTie("/steam/airspeed", FGSteam::get_ASI_kias);
113 fgTie("/steam/altitude", FGSteam::get_ALT_ft);
114 fgTie("/steam/altimeter-datum-mb",
115 FGSteam::get_ALT_datum_mb, FGSteam::set_ALT_datum_mb,
116 false); /* don't modify the value */
117 fgTie("/steam/turn-rate", FGSteam::get_TC_std);
118 fgTie("/steam/slip-skid", FGSteam::get_TC_rad);
119 fgTie("/steam/vertical-speed", FGSteam::get_VSI_fps);
120 fgTie("/steam/gyro-compass", FGSteam::get_DG_deg);
121 fgTie("/steam/vor1", FGSteam::get_HackVOR1_deg);
122 fgTie("/steam/vor2", FGSteam::get_HackVOR2_deg);
123 fgTie("/steam/glidescope1", FGSteam::get_HackGS_deg);
124 fgTie("/steam/adf", FGSteam::get_HackADF_deg);
125 fgTie("/steam/gyro-compass-error",
127 FGSteam::set_DG_err);
128 fgTie("/steam/mag-compass", FGSteam::get_MH_deg);
130 _UpdatesPending += timesteps;
137 /* tc should be (elapsed_time_between_updates / desired_smoothing_time) */
138 void FGSteam::set_lowpass ( double *outthe, double inthe, double tc )
142 { /* time went backwards; kill the filter */
145 { /* ignore mildly negative time */
149 { /* Normal mode of operation; fast approximation to exp(-tc) */
150 (*outthe) = (*outthe) * ( 1.0 - tc )
154 { /* Huge time step; assume filter has settled */
157 { /* Moderate time step; non linear response */
158 double keep = exp ( -tc );
159 // printf ( "ARP: Keep is %f\n", keep );
160 (*outthe) = (*outthe) * keep
161 + inthe * ( 1.0 - keep );
166 #define INHG_TO_MB 33.86388 /* Inches_of_mercury * INHG_TO_MB == millibars. */
168 // Convert air pressure to altitude by ICAO Standard Atmosphere
169 double pressInHgToAltFt(double p_inhg)
171 // Ref. Aviation Formulary, Ed Williams, www.best.com/~williams/avform.htm
172 const double P_0 = 29.92126; // Std. MSL pressure, inHg. (=10135.25 mb)
173 const double p_Tr = 0.2233609 * P_0; // Pressure at tropopause, same units.
174 const double h_Tr = 36089.24; // Alt of tropopause, ft. (=11.0 km)
176 // return (P_0 - p_inhg) * 1000.0; // ### crude approx. for low alt's
177 if (p_inhg > p_Tr) // 0.0 to 11.0 km
178 return (1.0 - pow((p_inhg / P_0), 1.0 / 5.2558797)) / 6.8755856e-6;
179 return h_Tr + log10(p_inhg / p_Tr) / -4.806346e-5; // 11.0 to 20.0 km
180 // We could put more code for higher altitudes here.
185 ////////////////////////////////////////////////////////////////////////
186 // Here the fun really begins
187 ////////////////////////////////////////////////////////////////////////
190 void FGSteam::_CatchUp()
191 { if ( _UpdatesPending != 0 )
192 { double dt = _UpdatesPending * 1.0 /
193 fgGetInt("/sim/model-hz"); // FIXME: inefficient
194 double AccN, AccE, AccU;
196 double d, the_ENGINE_rpm;
199 /**************************
200 There is the possibility that this is the first call.
201 If this is the case, we will emit the feature registrations
202 just to be on the safe side. Doing it more than once will
203 waste CPU time but doesn't hurt anything really.
205 if ( _UpdatesPending > 999999 )
206 { FGFeature::register_int ( "Avionics/NAV1/Localizer", &NAV1_LOC );
207 FGFeature::register_double ( "Avionics/NAV1/Latitude", &NAV1_Lat );
208 FGFeature::register_double ( "Avionics/NAV1/Longitude", &NAV1_Lon );
209 FGFeature::register_double ( "Avionics/NAV1/Radial", &NAV1_Rad );
210 FGFeature::register_double ( "Avionics/NAV1/Altitude", &NAV1_Alt );
211 FGFeature::register_int ( "Avionics/NAV2/Localizer", &NAV2_LOC );
212 FGFeature::register_double ( "Avionics/NAV2/Latitude", &NAV2_Lat );
213 FGFeature::register_double ( "Avionics/NAV2/Longitude", &NAV2_Lon );
214 FGFeature::register_double ( "Avionics/NAV2/Radial", &NAV2_Rad );
215 FGFeature::register_double ( "Avionics/NAV2/Altitude", &NAV2_Alt );
216 FGFeature::register_double ( "Avionics/ADF/Latitude", &ADF_Lat );
217 FGFeature::register_double ( "Avionics/ADF/Longitude", &ADF_Lon );
221 /**************************
222 Someone has called our update function and
223 it turns out that we are running somewhat behind.
224 Here, we recalculate everything for a 'dt' second step.
227 /**************************
228 The ball responds to the acceleration vector in the body
229 frame, only the components perpendicular to the longitudinal
230 axis of the aircraft. This is only related to actual
231 side slip for a symmetrical aircraft which is not touching
232 the ground and not changing its attitude. Math simplifies
233 by assuming (for small angles) arctan(x)=x in radians.
234 Obvious failure mode is the absence of liquid in the
235 tube, which is there to damp the motion, so that instead
236 the ball will bounce around, hitting the tube ends.
237 More subtle flaw is having it not move or a travel limit
238 occasionally due to some dirt in the tube or on the ball.
240 // the_TC_rad = - ( FGBFI::getSideSlip () ); /* incorrect */
241 d = - current_aircraft.fdm_state->get_A_Z_pilot();
243 set_lowpass ( & the_TC_rad,
244 current_aircraft.fdm_state->get_A_Y_pilot () / d,
247 /**************************
248 The rate of turn indication is from an electric gyro.
249 We should have it spin up with the master switch.
250 It is mounted at a funny angle so that it detects
251 both rate of bank (i.e. rolling into and out of turns)
252 and the rate of turn (i.e. how fast heading is changing).
254 set_lowpass ( & the_TC_std,
255 current_aircraft.fdm_state->get_Phi_dot ()
256 * RAD_TO_DEG / 20.0 +
257 current_aircraft.fdm_state->get_Psi_dot ()
258 * RAD_TO_DEG / 3.0 , dt );
260 /**************************
261 We want to know the pilot accelerations,
262 to compute the magnetic compass errors.
264 AccN = current_aircraft.fdm_state->get_V_dot_north();
265 AccE = current_aircraft.fdm_state->get_V_dot_east();
266 AccU = current_aircraft.fdm_state->get_V_dot_down()
268 if ( fabs(the_TC_rad) > 0.2 )
269 { /* Massive sideslip jams it; it stops turning */
271 the_MH_err = FGBFI::getHeading () - the_MH_deg;
273 { double MagDip, MagVar, CosDip;
274 double FrcN, FrcE, FrcU, AccTot;
275 double EdgN, EdgE, EdgU;
276 double TrqN, TrqE, TrqU, Torque;
277 /* Find a force vector towards exact magnetic north */
278 MagVar = FGBFI::getMagVar() / RAD_TO_DEG;
279 MagDip = FGBFI::getMagDip() / RAD_TO_DEG;
280 CosDip = cos ( MagDip );
281 FrcN = CosDip * cos ( MagVar );
282 FrcE = CosDip * sin ( MagVar );
283 FrcU = sin ( MagDip );
284 /* Rotation occurs around acceleration axis,
285 but axis magnitude is irrelevant. So compute it. */
286 AccTot = AccN*AccN + AccE*AccE + AccU*AccU;
287 if ( AccTot > 1.0 ) AccTot = sqrt ( AccTot );
289 /* Force applies to north marking on compass card */
290 EdgN = cos ( the_MH_err / RAD_TO_DEG );
291 EdgE = sin ( the_MH_err / RAD_TO_DEG );
293 /* Apply the force to the edge to get torques */
294 TrqN = EdgE * FrcU - EdgU * FrcE;
295 TrqE = EdgU * FrcN - EdgN * FrcU;
296 TrqU = EdgN * FrcE - EdgE * FrcN;
297 /* Select the component parallel to the axis */
298 Torque = ( TrqN * AccN +
300 TrqU * AccU ) * 5.0 / AccTot;
301 /* The magnetic compass has angular momentum,
302 so we apply a torque to it and wait */
304 { the_MH_degps= the_MH_degps * (1.0 - dt) - Torque;
305 the_MH_err += dt * the_MH_degps;
307 if ( the_MH_err > 180.0 ) the_MH_err -= 360.0; else
308 if ( the_MH_err < -180.0 ) the_MH_err += 360.0;
309 the_MH_deg = FGBFI::getHeading () - the_MH_err;
312 /**************************
313 This is not actually correct, but provides a
314 scaling capability for the vacuum pump later on.
315 When we have a real engine model, we can ask it.
317 the_ENGINE_rpm = controls.get_throttle(0) * 26.0;
319 /**************************
320 First, we need to know what the static line is reporting,
321 which is a whole simulation area in itself. For now, we cheat.
322 We filter the actual value by one second to
323 account for the line impedance of the plumbing.
325 double static_inhg = 29.92;
326 i = (int) FGBFI::getAltitude();
328 { static_inhg *= 0.707;
331 static_inhg *= ( 1.0 - 0.293 * i / 9000.0 );
332 set_lowpass ( & the_STATIC_inhg, static_inhg, dt );
335 NO alternate static source error (student feature),
336 NO possibility of blockage (instructor feature),
337 NO slip-induced error, important for C172 for example.
340 /**************************
342 ICAO standard atmosphere MSL pressure is 1013.25 mb, and pressure
343 gradient is about 28 ft per mb at MSL increasing to about 32 at
344 5000 and 38 at 10000 ft.
345 Standard altimeters apply the subscale offset to the output altitude,
346 not to the input pressure; I don't know exactly what pressure gradient
347 they assume for this. I choose to make it accurate at low altitudes.
348 Remember, we are trying to simulate a real altimeter, not an ideal one.
350 set_lowpass ( & the_ALT_ft,
351 pressInHgToAltFt(the_STATIC_inhg) +
352 (the_ALT_datum_mb - 1013.25) * 28.0, /* accurate at low alt. */
353 dt * 10 ); /* smoothing time 0.1 s */
355 /**************************
356 The VSI case is a low-pass filter of the static line pressure.
357 The instrument reports the difference, scaled to approx ft.
358 NO option for student to break glass when static source fails.
359 NO capability for a fixed non-zero reading when level.
360 NO capability to have a scaling error of maybe a factor of two.
362 the_VSI_fps = ( the_VSI_case - the_STATIC_inhg )
363 * 10000.0; /* manual scaling factor */
364 set_lowpass ( & the_VSI_case, the_STATIC_inhg, dt/6.0 );
366 /**************************
367 The engine driven vacuum pump is directly attached
368 to the engine shaft, so each engine rotation pumps
369 a fixed volume. The amount of air in that volume
370 is determined by the vacuum line's internal pressure.
371 The instruments are essentially leaking air like
372 a fixed source impedance from atmospheric pressure.
373 The regulator provides a digital limit setting,
374 which is open circuit unless the pressure drop is big.
375 Thus, we can compute the vacuum line pressure directly.
376 We assume that there is negligible reservoir space.
377 NO failure of the pump supported (yet)
379 the_VACUUM_inhg = the_STATIC_inhg *
380 the_ENGINE_rpm / ( the_ENGINE_rpm + 10000.0 );
381 if ( the_VACUUM_inhg > 5.0 )
382 the_VACUUM_inhg = 5.0;
385 > I was merely going to do the engine rpm driven vacuum pump for both
386 > the AI and DG, have the gyros spin down down in power off descents,
387 > have it tumble when you exceed the usual pitch or bank limits,
388 > put in those insidious turning errors ... for now anyway.
390 if ( _UpdatesPending > 999999 )
391 the_DG_err = FGBFI::getMagVar();
392 the_DG_degps = 0.01; /* HACK! */
393 if (dt<1.0) the_DG_err += dt * the_DG_degps;
394 the_DG_deg = FGBFI::getHeading () - the_DG_err;
396 /**************************
397 Finished updates, now clear the timer
401 // cout << "0 Updates pending" << endl;
406 ////////////////////////////////////////////////////////////////////////
407 // Everything below is a transient hack; expect it to disappear
408 ////////////////////////////////////////////////////////////////////////
411 double FGSteam::get_HackGS_deg () {
412 if ( current_radiostack->get_nav1_inrange() &&
413 current_radiostack->get_nav1_has_gs() )
415 double x = current_radiostack->get_nav1_gs_dist();
416 double y = (FGBFI::getAltitude() - current_radiostack->get_nav1_elev())
418 double angle = atan2( y, x ) * RAD_TO_DEG;
419 return (current_radiostack->get_nav1_target_gs() - angle) * 5.0;
426 double FGSteam::get_HackVOR1_deg () {
429 if ( current_radiostack->get_nav1_inrange() ) {
432 if ( current_radiostack->get_nav1_loc() ) {
433 // localizer doesn't need magvar offset
434 r = current_radiostack->get_nav1_heading()
435 - current_radiostack->get_nav1_radial();
437 r = current_radiostack->get_nav1_heading() - FGBFI::getMagVar()
438 - current_radiostack->get_nav1_radial();
441 r = current_radiostack->get_nav1_heading()
442 - current_radiostack->get_nav1_radial();
443 // cout << "Radial = " << current_radiostack->get_nav1_radial()
444 // << " Bearing = " << current_radiostack->get_nav1_heading()
447 if (r> 180.0) r-=360.0; else
448 if (r<-180.0) r+=360.0;
449 if ( fabs(r) > 90.0 )
450 r = ( r<0.0 ? -r-180.0 : -r+180.0 );
451 // According to Robin Peel, the ILS is 4x more sensitive than a vor
452 if ( current_radiostack->get_nav1_loc() ) r *= 4.0;
461 double FGSteam::get_HackVOR2_deg () {
464 if ( current_radiostack->get_nav2_inrange() ) {
467 if ( current_radiostack->get_nav2_loc() ) {
468 // localizer doesn't need magvar offset
469 r = current_radiostack->get_nav2_heading()
470 - current_radiostack->get_nav2_radial();
472 r = current_radiostack->get_nav2_heading() - FGBFI::getMagVar()
473 - current_radiostack->get_nav2_radial();
476 r = current_radiostack->get_nav2_heading()
477 - current_radiostack->get_nav2_radial();
478 // cout << "Radial = " << current_radiostack->get_nav1_radial()
479 // << " Bearing = " << current_radiostack->get_nav1_heading() << endl;
481 if (r> 180.0) r-=360.0; else
482 if (r<-180.0) r+=360.0;
483 if ( fabs(r) > 90.0 )
484 r = ( r<0.0 ? -r-180.0 : -r+180.0 );
493 double FGSteam::get_HackOBS1_deg () {
494 return current_radiostack->get_nav1_radial();
498 double FGSteam::get_HackOBS2_deg () {
499 return current_radiostack->get_nav2_radial();
503 double FGSteam::get_HackADF_deg () {
506 if ( current_radiostack->get_adf_inrange() ) {
507 r = current_radiostack->get_adf_heading() - FGBFI::getHeading();
509 // cout << "Radial = " << current_radiostack->get_adf_heading()
510 // << " Heading = " << FGBFI::getHeading() << endl;