How to simplify transfer function？

Hi guys

I have used the function s=tf('s') to ceate a transfer function, but  I find that Matlab do not simplify the tf at all.

for example.

s=tf('s');
A=1/(1+s);B=A/(1+A);

the result is 1+s/(s^2+3*s+2)

What I want is 1/(2+s)

Anyone know that how to simplify the tf in Matlab?

Thank you!

Answer 1

[Thomas]

Try

s=tf('s');
A=1/(1+s);B=A/(1+A);
minreal(B)

[tqy]

Thanks a lot.

But in another case, I have to set the tolerance so that I can get what I want.

I think Matlab only keeps the default effective bit of the a number.

[Tim]

Looks like a solution to my problem, however I need to find variables from a rewritten tf.

I have a tf G like below

    lsys = ss(A B C D); % were all matrices are 3x3
    G = tf(lsys);
from which I am intersted in rewriting G(2,2)

G(2,2)=

    1.422e-05 s + 4.696e-06
    -----------------------
     s^2 + 21.76 s + 1.216
into

      a * (b*s + 1)
    -----------------
     (c*s +1)*(d*s+1)
to yield constants a b c d
MatLab does not accept 

minreal(G(2,2));

any suggestions?

Answer 2

[Artur Araujo]

You could try H = zpk(G(2,2))

[Zuhair Abduljabbar]

Thanks