ODE4 is an explicit solver, so it tends to be less stable when the time step is "large". You trying to reduce the time step is definitely heading in the right direction.
At the same time, ode4 may not be the ideal solver for a Simscape network. One common practice to run Simscape in fixed step is to turn on the local solver using fixed time step. You can use a same value as you have for the main model. You can set up the local solver in the Solver Configuration block in your Simscape network. Backward Euler usually works fine. More details here: https://www.mathworks.com/help/physmod/simscape/ug/setting-up-solvers-for-physical-models.html#bsjpduw Backward Euler is an implicit solver, so it will perform iteration during each time step. Depending on your need/requirement, you may want to limit the number of iterations in the block settings as well.
I would suggest you try things in this order:
- Simulink solver: variable auto; Simscape local solver: off. This will be the RIGHT answer.
- Simulink solver: variable auto; Simscape local solver: backward Euler, no limit on iterations; make sure you get RIGHT answer.
- Simulink solver: variable auto; Simscape local solver: backward Euler, put limit on iterations and gradually reduce the number to say 5 (or whatever small number that your model can take and produce RIGHT answer).
- Simulink solver: low order fixed step solver (ode2, e.g.); everything else the same. Play with time steps. Check results and stability. Change time step in the local solver together with the SL solver (unless you have a good reason not to).
- Increase the order of Simulink solver. You'll need to strike some balance here. Given a desired level of accuracy, high order solvers can usually tolerate a larger time step than low order solvers, BUT they can be less stable at the same step size (aka blow up more easily). The "best" choice depends on the type of problem and the resulting equations you have.
Hope you can get to step 5 with no problem. If you still have problem and can share the model, I can take a look and let you know what I find.
