MATLAB feedback function gives wrong result for positive feedback system

[nand_1]

Hi All,

Suppose I have the following system:

A = 1 / (S + 1) (open loop)

B = S / (2S + 1) (feedback path)

I know that the positive feedback closed loop system will thus be: A / (1 - AB), and entering this manually in MATLAB (as well as doing it by hand), I get:


2 s^2 + 3 s + 1
-----------------------
2 s^3 + 4 s^2 + 3 s + 1

However, when I use the following command:

sys = feedback(A,B,+1)

I get the following:

2 s + 1
---------------
2 s^2 + 2 s + 1


Is there something I'm missing?

Regards,

 

[kreil]

$$\frac{2s^2+3s+1}{2s^3+4s^2+3s+1} = \frac{(2s+1)(s+1)}{(2s^2+2s+1)(s+1)} = \frac{2s+1}{2s^2+2s+1}$$

 

[nand_1]

Thanks Kreil, realized this later on after the post. Thought it was an issue with the way i was using the feedback function.