Changing state variables used in JSBSim linearization model

[viniciusrottolo]

Hello everyone,

I'm currently working with JSBSim and I'm trying to understand how to modify the state variables used in the linearization model.

Currently, the state vector used for linearization is composed of

['Vt', 'Alpha', 'Theta', 'Q', 'Beta', 'Phi', 'P', 'Psi', 'R', 'Latitude', 'Longitude', 'Alt'].

However, I would like to change it to the standard set of variables

[u, v, w, p, q, r, X, Y, Z, phi, theta, psi].

Could anyone guide me on how to make this modification? Is it possible to customize the state variables for linearization in JSBSim?

Thank you in advance for your help!

[seanmcleod]

How are you planning on using the linearization model?

Depending on your use case, you do realize that all the states you're looking for can be derived from the existing state vector right?

[bcoconni]

@viniciusrottolo You could add new classes inheriting from FGStateSpace::Component for each of the variable you need. Similar to the class FGStateSpace::Theta below:

jsbsim/src/math/FGStateSpace.h

Lines 402 to 419 in e26d03b

	class Theta : public Component
	{
	public:
	Theta() : Component("Theta","rad") {};
	double get() const
	{
	return m_fdm->GetPropagate()->GetEuler(2);
	}
	void set(double val)
	{
	m_fdm->GetIC()->SetFlightPathAngleRadIC(val-m_fdm->GetIC()->GetAlphaRadIC());
	//m_fdm->GetIC()->SetThetaRadIC(val);
	}
	double getDeriv() const
	{
	return m_fdm->GetAuxiliary()->GetEulerRates(2);
	}
	};

and modify the constructor FGLinearization::FGLinearization to use the components that you have created (replace each occurrence of ss.x.add(new FGStateSpace::***))

jsbsim/src/initialization/FGLinearization.cpp

Lines 28 to 63 in e26d03b

	FGStateSpace ss(fdm);
	ss.x.add(new FGStateSpace::Vt);
	ss.x.add(new FGStateSpace::Alpha);
	ss.x.add(new FGStateSpace::Theta);
	ss.x.add(new FGStateSpace::Q);
	// get propulsion pointer to determine type/ etc.
	auto engine0 = fdm->GetPropulsion()->GetEngine(0);
	FGThruster * thruster0 = engine0->GetThruster();
	if (thruster0->GetType()==FGThruster::ttPropeller)
	{
	ss.x.add(new FGStateSpace::Rpm0);
	// TODO add variable prop pitch property
	// if (variablePropPitch) ss.x.add(new FGStateSpace::PropPitch);
	int numEngines = fdm->GetPropulsion()->GetNumEngines();
	if (numEngines>1) ss.x.add(new FGStateSpace::Rpm1);
	if (numEngines>2) ss.x.add(new FGStateSpace::Rpm2);
	if (numEngines>3) ss.x.add(new FGStateSpace::Rpm3);
	if (numEngines>4) {
	std::cerr << "more than 4 engines not currently handled" << std::endl;
	}
	}
	ss.x.add(new FGStateSpace::Beta);
	ss.x.add(new FGStateSpace::Phi);
	ss.x.add(new FGStateSpace::P);
	ss.x.add(new FGStateSpace::Psi);
	ss.x.add(new FGStateSpace::R);
	ss.x.add(new FGStateSpace::Latitude);
	ss.x.add(new FGStateSpace::Longitude);
	ss.x.add(new FGStateSpace::Alt);
	ss.u.add(new FGStateSpace::ThrottleCmd);
	ss.u.add(new FGStateSpace::DaCmd);
	ss.u.add(new FGStateSpace::DeCmd);
	ss.u.add(new FGStateSpace::DrCmd);