pyomo.dae extract integrator timesteps from CasADi

[adowling2]

Summary

When using the DAE simulator (e.g., CasADi), extract the adaptive time steps from cvodes or idas integrator.

Rationale

For stiff systems, it is increadibly helpful to get the adaptive timesteps. These timesteps can then be used to place collocation points.

Description

ChatGPT 5o suggested this is easy to do with CasADi 3.5 or later using the monitor option:

opts = {"t0": 0, "tf": 10, "monitor": ["t", "x"]}
F = ca.integrator("F", "idas", dae, opts)
res = F(x0=x0, z0=z0, p=p)
print(F.stats()["monitor"]["t"])

I have not tried this code sample yet, but I think it may just require a few modifications here: https://github.com/Pyomo/pyomo/blob/613f2ad1f64d8dead2ff15fd9c33a6b591b9bb93/pyomo/dae/simulator.py#L998

Additional information

@djlaky what do you think?