How can I do summation in transfer function?

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larry liu
larry liu 2022-3-25
评论： larry liu 2022-3-27
s = tf('s');
sys_Foster = tf(R_Foster(1)/(1+R_Foster(1)*C_Foster(1)*s)+R_Foster(2)/(1+R_Foster(2)*C_Foster(2)*s)+R_Foster(3)/(1+R_Foster(3)*C_Foster(3)*s));
[N_tf,D_tf] = tfdata(sys_Foster);
If I have a transfer function like the code show
How can I perform sys_Foster not only summation for 3 terms but i terms, where i is size(R_Foster)
I have tried the code below
for i = 1:size(R_Foster)
sys_Foster_i(i) = tf(R_Foster(i)/(1+R_Foster(i)*C_Foster(i)*s))
end
sys_Foster = sum(sys_Foster_i)
but got wrong answer.

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采纳的回答

Paul
Paul 2022-3-25
Ran in:
One approach
R_Foster = 1:3;
C_Foster = 11:13;
sys_Foster = tf(0)
sys_Foster = 0 Static gain.
for ii = 1:3
sys_Foster = sys_Foster + tf(R_Foster(ii),[R_Foster(ii)*C_Foster(ii) 1])
end
sys_Foster = 1 -------- 11 s + 1 Continuous-time transfer function. sys_Foster = 46 s + 3 ------------------ 264 s^2 + 35 s + 1 Continuous-time transfer function. sys_Foster = 2586 s^2 + 268 s + 6 ------------------------------- 10296 s^3 + 1629 s^2 + 74 s + 1 Continuous-time transfer function.
sys_Foster
sys_Foster = 2586 s^2 + 268 s + 6 ------------------------------- 10296 s^3 + 1629 s^2 + 74 s + 1 Continuous-time transfer function.
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Paul
Paul 2022-3-25
Ran in:
Now I'm very confused as to what the goal actually is. In this answer I showed how to use a loop to add three first order transfer functions together using the Control System Toolbox functions. In this comment I showed how to create the same tf object but adding together the first order transfer functions using the Symbolic Toolbox to get the same result. Maybe you want to use the Symbolic Toolbox but in a loop?
R_Foster = sym(1:3);
C_Foster = sym(11:13);
syms s
H(s) = sym(0);
for ii = 1:numel(R_Foster)
H(s) = H(s) + R_Foster(ii)/(1+R_Foster(ii)*C_Foster(ii)*s);
end
H(s) % this is a symfun object
ans = 
[num,den] = numden(H(s));
H(s) = num/den % still a symfun object
H(s) = 
H(s) = expand(num)/expand(den) % still a symfun object
H(s) = 
If none of this solves the problem, please be more clear about the problem actually is. Do you want a Symbolic result or a Control System Toolbox result? Does the result need to be in a specific format? Etc.
larry liu
larry liu 2022-3-27
@Paul Thanks a lot! I got the answer I want. You really solve my problem. Really aprreciate.

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