JSBSim源代码分析

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JSBSim是一个开源跨平台的飞行动力学模型（FDM）软件库，用于模拟航空航天飞行器的飞行动力学。 该库已被纳入飞行模拟软件包FlightGear和OpenEaagles。JSBSim可以独立运行，通过命令行参数指定飞行器和初始状态，进行简单情境下的飞行动力学仿真，也可以将JSBSim作为代码库，编程实现飞行器模型加载，设置输入，获得输出。下面将通过分析JSBSim源代码，研究其实现通用飞行动力学模型的方法。

入口分析

下面是JSBSim参考手册中的最简单实例，因JSBSim的不断开发，JSBSim参考手册中该编程实例有点过时，因此进行了少量修改。

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#include <FGFDMExec.h>
#include <sg_path.hxx>
 
using namespace std;

int main(int argc, char **argv)
{
  JSBSim::FGFDMExec FDMExec;
  bool result = true;
 
  FDMExec.LoadScript(SGPath::fromUtf8(argv[1]));
 
  while (result) result = FDMExec.Run();
}

从上述代码可知，调用JSBSim的主要方法是利用FGFDMExec类，通过实例化一个FGFDMExec类，就相当于获得了一个运行JSBSim仿真的工具箱，通过这个工具箱就可以调用JSBSim的大部分功能，实现我们要的仿真目标。同时FGFDMExec类通过加载外部飞机的XML脚本，实现飞行动力学模型的通用性。

JSBSim初始化流程

上述JSBSim最简仿真示例中已包含其初始化流程，采用图示如下：

JSBSim初始化流程

图1 JSBSim初始化流程

FGFDMExec初始化

FGFDMExec类在其构造函数中对各个模型进行初始化，具体代码在Allocate函数中：

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FGFDMExec::FGFDMExec(FGPropertyManager* root, unsigned int* fdmctr)
  : Root(root), RandomEngine(new default_random_engine), FDMctr(fdmctr)
{
  ...
  try {
    Allocate();
  } catch (const string& msg ) {
    cout << "Caught error: " << msg << endl;
    exit(1);
  }

  ...
}

Allocate函数代码如下：

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bool FGFDMExec::Allocate(void)
{
  bool result=true;

  Models.resize(eNumStandardModels);

  // First build the inertial model since some other models are relying on
  // the inertial model and the ground callback to build themselves.
  // Note that this does not affect the order in which the models will be
  // executed later.
  Models[eInertial]          = new FGInertial(this);

  // See the eModels enum specification in the header file. The order of the
  // enums specifies the order of execution. The Models[] vector is the primary
  // storage array for the list of models.
  Models[ePropagate]         = new FGPropagate(this);
  Models[eInput]             = new FGInput(this);
  Models[eAtmosphere]        = new FGStandardAtmosphere(this);
  ...

  // Assign the Model shortcuts for internal executive use only.
  Propagate = (FGPropagate*)Models[ePropagate];
  Inertial = (FGInertial*)Models[eInertial];
  Atmosphere = (FGAtmosphere*)Models[eAtmosphere];
  ...

  // Initialize planet (environment) constants
  LoadPlanetConstants();

  // Initialize models
  for (unsigned int i = 0; i < Models.size(); i++) {
    // The Input/Output models must not be initialized prior to IC loading
    if (i == eInput || i == eOutput) continue;

    LoadInputs(i);
    Models[i]->InitModel();
  }

  ...
  return result;
}

Allocate函数代码中需要注意LoadInputs函数，该函数决定各个子模型的初始化顺序，确定各个子模型的输入输出，具体代码如下：

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void FGFDMExec::LoadInputs(unsigned int idx)
{
  switch(idx) {
  case ePropagate:
    Propagate->in.vPQRidot     = Accelerations->GetPQRidot();
    Propagate->in.vUVWidot     = Accelerations->GetUVWidot();
    Propagate->in.DeltaT       = dT;
    break;
  case eInput:
    break;
  case eInertial:
    Inertial->in.Position      = Propagate->GetLocation();
    break;
  case eAtmosphere:
    Atmosphere->in.altitudeASL = Propagate->GetAltitudeASL();
    break;
  case eWinds:
    Winds->in.AltitudeASL      = Propagate->GetAltitudeASL();
    Winds->in.DistanceAGL      = Propagate->GetDistanceAGL();
    ...
    break;
  case eAuxiliary:
    Auxiliary->in.Pressure     = Atmosphere->GetPressure();
    Auxiliary->in.Density      = Atmosphere->GetDensity();
    ...
    break;
  case eSystems:
    // Dynamic inputs come into the components that FCS manages through properties
    break;
  case ePropulsion:
    Propulsion->in.Pressure         = Atmosphere->GetPressure();
    ...

    break;
  case eAerodynamics:
    Aerodynamics->in.Alpha     = Auxiliary->Getalpha();
    ...
    break;
  case eGroundReactions:
    // There are no external inputs to this model.
    GroundReactions->in.Vground         = Auxiliary->GetVground();
    ...
    break;
  case eExternalReactions:
    // There are no external inputs to this model.
    break;
  case eBuoyantForces:
    BuoyantForces->in.Density     = Atmosphere->GetDensity();
    ...
    break;
  case eMassBalance:
    MassBalance->in.GasInertia  = BuoyantForces->GetGasMassInertia();
    MassBalance->in.GasMass     = BuoyantForces->GetGasMass();
    ...
    break;
  case eAircraft:
    Aircraft->in.AeroForce     = Aerodynamics->GetForces();
    Aircraft->in.PropForce     = Propulsion->GetForces();
    ...
    break;
  case eAccelerations:
    Accelerations->in.J        = MassBalance->GetJ();
    Accelerations->in.Jinv     = MassBalance->GetJinv();
    ...
    break;
  default:
    break;
  }
}

FGFDMExec加载飞机配置

FGFDMExec的LoadScript函数在初始化时负责加载飞机配置，用于初始化各个子模型。

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bool FGFDMExec::LoadScript(const SGPath& script, double deltaT,
                           const SGPath& initfile)
{
  bool result;

  Script = new FGScript(this);
  result = Script->LoadScript(GetFullPath(script), deltaT, initfile);

  return result;
}

FGFDMExec运行

FGFDMExec的Run函数负责飞行动力学模型的计算，其代码如下：

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bool FGFDMExec::Run(void)
{
  bool success=true;

  ...

  // returns true if success, false if complete
  if (Script != 0 && !IntegrationSuspended()) success = Script->RunScript();

  for (unsigned int i = 0; i < Models.size(); i++) {
    LoadInputs(i);
    Models[i]->Run(holding);
  }

  ...

  return success;
}

FGFDMExec的Run函数将依次调用各个子模型的Run函数。

参考链接

  1. JSBSim编程实践之入门,by jackhuang.