Why my - tf2sym - command is not working?

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Enes Emre Öztürk
Enes Emre Öztürk 2023-11-19
回答： Star Strider 2023-11-19
Hello, i am trying to do my homework but my tf2sym command is not working.
İt says "Unrecognized function or variable 'tf2sym'."
What can i do about it?
I looked to add-ons and i have all toolboxes currently, it shows. Please can someone help me?
Gs=tf(1,[1 10 27 18]);
Gss= tf2sym(Gs)

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回答（2 个）

Sam Chak
Sam Chak 2023-11-19
Ran in:
Hi @Enes Emre Öztürk,
I guess you want to convert the transfer function to state-space nodel.
Gs = tf(1, [1 10 27 18])
Gs = 1 ------------------------ s^3 + 10 s^2 + 27 s + 18 Continuous-time transfer function.
%% Method 1: Using tf2ssm()
[A, B, C, D] = tf2ssm(1, [1 10 27 18])
A = 3×3
-10 -27 -18 1 0 0 0 1 0
B = 3×1
1 0 0
C = 1×3
0 0 1
D = 0
%% Method 2: Directly using ss() function
Gss = ss(Gs)
Gss = A = x1 x2 x3 x1 -10 -6.75 -2.25 x2 4 0 0 x3 0 2 0 B = u1 x1 0.25 x2 0 x3 0 C = x1 x2 x3 y1 0 0 0.5 D = u1 y1 0 Continuous-time state-space model.
Star Strider
Star Strider 2023-11-19
Ran in:
If you want to conovert that transfer function to a symbolic variable, try this —
Gs=tf(1,[1 10 27 18]);
Gss= tf2sym(Gs)
Gss = 
whos Gss
Name Size Bytes Class Attributes Gss 1x1 8 sym
function symtf = tf2sym(sys)
TF = tf(sys);
n = TF.Numerator;
d = TF.Denominator;
s = sym('s');
symtf = poly2sym(n,s) / poly2sym(d,s);
end
I can find no reference to any function called ‘tf2sym’ so I created one.
.

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