Attempting to stabilize a MIMO TECS controller in Matlab/Simulink

Hello,

I am working on the implementation of a MIMO TECS controller to a 3DoF model for a buisness jet aircraft, following the work of Lambregts et alii.

In my project, the inner loop controllers were designed thanks to the systune tool; you can find out more here. Starting from this, I have implemented the TECS core algorithm

in this way

The tuning of the gains was accomplished manually. At first, I have tuned the proportional controllers and then the integral ones. Then, I have proceeded to tune the proportional gains Kh and Kv in the Altitude and Airspeed normalized command generator

The value of these two latter gains was selected in order to obtain a decupled acceleration and flight path angle response.

The issue that now I am facing is that the TECS is unstable. I have some unstable poles/zeros, as you can see from here

Do you have any advice/example/lecture that could help me in improving the stability of the closed-loop system without violating the requirements imposed by the TECS algorithm? Probably the tuning was wrong?

Thank you.