Obiroy,

1: your OutputResponse syntax is wrong. I do not understand what you are trying to do. OutputResponse 2nd argument is an input that is independent of the states. For example, this would work:

    out = OutputResponse[nsys, {UnitStep[t], UnitStep[t]}, {t, 0, 300}];

to get a plot of all the outputs:

    Plot[out, {t, 0, 300}]

You have the inputs as a function of the states. MMA cannot interpret that properly and thinks that those are new variables that you have introduced. If you really intended to do that, you should add those state connections into the model using the various functions for that (i.e. SystemsModelSeriesConnect, SystemsModelFeedbackConnect, etc.)

2: I do not understand why you are using interpFN at all. The best way to make your assignment is

    {u1, u2, u3, u4, u5, u6, u7, u8} = sol;

3: you can use If in the input such as:

    out = OutputResponse[
       nsys, {If[t >= 100, Sin[t], 0], UnitStep[t]}, {t, 0, 300}];

Regards,

Neil