I'm not an expert on Simulink models, but it looks to me like the issue is the fitting function you're using has the wrong concavity; you can see it especially at the end of the data where it fits the step-point incorrectly. Can you adjust this?
Some other suggestions:
- Does your yellow line have a better fit than the orange line? If so, the solver might have just not found an optimum. Try seeding the initial values of the parameters from the orange parameters instead.
- You didn't mention what solver you were using, but you can try "focusing" some components of the data by weighting the maximum points more heavily using a weighting function.
- Try using a different solver entirely; perhaps a local, non-parametric one would work?
