Module 3 : Stability analysis of discrete time systems

Lecture 1 : Stability Analysis of closed loop system in z-plane

Example 2:   The characteristic equation is

$ P(z)= z^{4}-1.2z^{3}+0.07z^{2}+0.3z-0.08=0$
Thus, $ a_{0}=1$     $ a_{1}=-1.2$     $ a_{2}=0.07$    $ a_{3}=0.3$      $ a_{4}=-0.08$

We will now check the stability conditions.

1. $ \vert a_{n}\vert = \vert a_{4}\vert = 0.08 < a_{0}=1 \Rightarrow $ First condition is satisfied.

2. $ P(1)=1-1.2+0.07+0.3-0.08+0.09>0 \Rightarrow $ Second condition is satisfied.

3. $ P(-1)= 1+1.2+0.07-0.3-0.08=1.89>0 \Rightarrow$ Third condition is satisfied.

Next we will construct the Jury Table.

Jury Table

$ b_{1}=-0.0756$        

Rest of the elements are also calculated in a similar fashion. The elements are $ b_{1}=-0.0756$     $ b_{0}=-0.204$     $ c_{2}=0.946$     $ c_{1}=-1.184$     $ c_{0}=0.315$. One can see
$ \vert b_{3}\vert=0.9936>\vert b_{0}\vert=0.204$
$ \vert c_{2}\vert=0.946>\vert c_{0}\vert=0.315$

All criteria are satisfied. Thus the system is stable.