General Nonlinear Systems 

Just like affine systems, the general nonlinear systems can also be specified in multiple ways and be converted to other systems models.

A system specified using an ODE.

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X NonlinearStateSpaceModel[ m x''[t] + c[\[ScriptX]] x'[t] + k[x] == F[t], x[t], F[t], x[t], t]

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A system specified using its components.

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X NonlinearStateSpaceModel[{{Subscript[f, 1][Subscript[x, 1], Subscript[ x, 2], Subscript[u, 1]], Subscript[f, 2][Subscript[x, 1], Subscript[x, 2], Subscript[u, 1]]}, {Subscript[h, 1][Subscript[x, 1], Subscript[x, 2], Subscript[u, 1]]}}, {Subscript[x, 1], Subscript[x, 2]}, {Subscript[u, 1]}]

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Systems obtained from other systems models.

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X NonlinearStateSpaceModel /@ {StateSpaceModel[{{{Subscript[a, 11], Subscript[a, 12]}, {Subscript[a, 21], Subscript[a, 22]}}, {{Subscript[b, 11]}, {Subscript[b, 21]}}, {{Subscript[c, 11], Subscript[c, 12]}}, {{0}}}, {Subscript[x, 1], Subscript[x, 2]}, SamplingPeriod ->None, SystemsModelLabels -> None], TransferFunctionModel[{{{1}}, (-2) s + s^2}, s]}

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X NonlinearStateSpaceModel[ AffineStateSpaceModel[ {{Subscript[f, 1][Subscript[x, 1], Subscript[ x, 2]], Subscript[f, 2][Subscript[x, 1], Subscript[x, 2]]}, {{ Subscript[g, 11][Subscript[x, 1], Subscript[x, 2]] }, {Subscript[g, 21][Subscript[x, 1], Subscript[x, 2]] }}, {Subscript[h, 1][Subscript[x, 1], Subscript[x, 2]]}, {{ 0 }}}, {Subscript[x, 1], Subscript[x, 2]}, {Subscript[u, 1]}, {Automatic}, Automatic , SamplingPeriod -> None] ]

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A linear NonlinearStateSpaceModel is exactly converted to linear systems models.

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X nssm = NonlinearStateSpaceModel[ {{u + Subscript[x, 1] + Subscript[x, 2], u + Subscript[x, 1]}, {Subscript[x, 1]}}, {Subscript[x, 1], Subscript[x, 2]}, {u}, {Automatic}, Automatic , SamplingPeriod -> None];

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X {StateSpaceModel[nssm], TransferFunctionModel[nssm]}

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An input-linear system is exactly converted to an AffineStateSpaceModel.

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X AffineStateSpaceModel[ NonlinearStateSpaceModel[ {{u + Subscript[x, 1] + Subscript[x, 2] + Subscript[x, 1] Subscript[x, 2], u + Subscript[x, 1] + \!\(\*SubsuperscriptBox[\(x\), \(2\), \(2\)]\)}, {Subscript[x, 1]}}, {Subscript[x, 1], Subscript[x, 2]}, {u}, {Automatic}, Automatic , SamplingPeriod -> None]]

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In general, conversions to other systems models are approximate.

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X nssm = NonlinearStateSpaceModel[ {{u + u^2 + Subscript[x, 1] + Subscript[x, 2] + Subscript[x, 1] Subscript[x, 2], u + Subscript[x, 1] + \!\(\*SubsuperscriptBox[\(x\), \(2\), \(2\)]\)}, {Subscript[x, 1]}}, {Subscript[x, 1], Subscript[x, 2]}, {u}, {Automatic}, Automatic , SamplingPeriod -> None];

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X {StateSpaceModel[nssm], TransferFunctionModel[nssm], AffineStateSpaceModel[nssm]}

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