Creating transfer function with a variable gain "k"

Let's say we have a variable gain "k" in series with H(s)=(s*0.5-1)/(s*(s+1)^2), how may I get the complete transfer function with the variable K in matlab out of this? I don't want to use simulink.

Answer 1

[Robert]

This problem was never really solved and now i have the same one. How do you handle this? You create a transfer function with tf function but it does not allow for adding gain?

Makes no sense to me

Answer 2

[Yamac Tan]

I have the same problem right now but I cannot find an exact solution. I have a closed loop negative feedback model with the static gain as "K" on the upper branch. I need to show "Input/Output = Transfer Function" with the variable "K". Still makes no sense.

[Paul Kroemer]

I have the same Problem right now, so i wanted to ask if you found a solution for it? Thanks for your help.

[Alpay Taskir]

try to use command '' zpk'' guys.

Answer 3

[Paulo Silva]

K=1:10
sys=K*H(s) %replace H(s) with your transfer function

sys(1) is your system with gain K=1 and so on until sys(10) -> K=10

you can also use functions like the step

step(sys) %this will plot your system response to a step for each gain K

[Helge]

Thanks for answering.

I understand you, but I want the final transfer function to be with K as an variable(constant) visible in the transfer funtion. For eksample T(s)= K(1-s)/s^3+s^2+s+K.

This is because i want to use the final transfer funtion to do further calculations manually.

[Paulo Silva]

So you want to find the K values that satisfy your project requirements?

[Helge]

Yes, I want to manualy set up an Routh Hurwitz table and solve the given transfer function with the variable K inside it. By completing the RH table i want to find out what K (gain) is requiered to design a stable system.

Because of this I need the complete transfer function including the constant K to solve the RH-table.

[Paulo Silva]

If you have the symbolic toolbox take a look at this

http://www.mathworks.com/matlabcentral/answers/6355-how-do-i-get-the-coefficients-of-this-symbolic-expression

[Paulo Silva]

For your example

syms K s

T=K*(1-s)/s^3+s^2+s+K

pretty(T)

Answer 4

[Prajith S A]

s = tf('s'); % Assign variable s, as variable of the transfer function

sys = (s+1)/(sˆ2+2*s+1)

s = zpk('s');

sys = (s+1)/(sˆ2+2*s+1) % Returns transfer function in factored form