Solver options for Sim in System Identification Toolbox

For a project, I want to be able to simulate a continuous-time system with a fix-step ODE and then estimate the system parameters using the System Identification Toolbox.

However, I cannot arrive at a good fit when estimating the parameters on data created by a simple deterministic system using an ODE, but I will get a 100% fit when the data is created by the same system simulated using the build-in Sim function. "compare" won't even give me a 100% fit if I fix the estimation parameters to the parameter values used for the simulation.

After looking into the issue I figured that the problem stems from how Sim solves the underlying equations -- which I cannot find any documentation on.

Through some tests (see the two attached scripts) I found that sim probably uses the same adaptive solution find as simulink, and hence that Sim chooses its solver based on system dynamic and system input behavior.

My question is then: How do I know what solve Sim is using, and can I somehow fix the solver option?

Answer 1

[soeren graabaek]

I found that 'idnlgrey' [1] let you specify the simulation options.

[1]: https://se.mathworks.com/help/ident/ref/idnlgrey.html