Hi @FreyJo ,
Same problem happens with F^3. F^3 is differentiable, can have positive and negative values. For me, it seems to be an initial value issue. If the initial value of the input is 0, then nothing happens. But if you change slightly the inital value, then it works.
Maybe this is because if you linearize wrt the input variable, since the derivative is 0 at the inital point, the system behaves like an autonomous system, not controllable anymore. That would explain why it works when changing slightly the inital value.
I used F=sign(F).*abs(F) just to provide a simple example that should work. In my specific case, I need u.*abs(u), which is differentiable at u=0 (but not twice differentiable). It represents an aerodynamic friction force, proportional to the velocity squared. My workaround is using an initial value other than 0, but I don’t really understand why the solver cannot deal with it.