Linear quadratic integrator: adding the integrator on a state

Hi to everyone!

I'm having an issue tuning an lqi, in particular:

I want to add an integrator on a state, hence I use this state as an output and use the matlab lqi function. The error I'm having is the following:

Error using ss/lqi (line 115)

Cannot compute the stabilizing Riccati solution S

for the LQR design.

This could be because:

* R is singular,

* [Q N;N' R] needs to be positive definite,

* The E matrix in the state equation is singular.

Has someone had the same problem?

I don't think the problem is in the model

Thanks in advance