Clear Filters
Clear Filters

Please help. The first input argument of the "tf" command cannot be a string.

16 views (last 30 days)
clear, clc
G1=tf([1],[1 0 5])
G2=tf([10],[1])
G3=tf('k')
G4=tf([1 1],[1 0])
H1=tf([1 0],[1])
H2=tf([1 5],[1])
TR1=feedback(G1,H1,-1)
T1=series(TR1,G2)
TR2=feedback(T1,H2,-1)
T2=series(TR2*G3)
Error using tf
Invalid syntax. The first input argument of the "tf" command cannot be a string.
  9 Comments
Melissa
Melissa on 23 Nov 2023
Variable K doesn't a specific value, I want multiply K other variables like TR2
It's a unknown variable that can be replace with X or other letter

Sign in to comment.

Answers (3)

Sulaymon Eshkabilov
Sulaymon Eshkabilov on 23 Nov 2023
Besides of using 'k' for tf, there is another syntax error in T2. See the corrected core here:
clear, clc
G1=tf([1],[1 0 5])
G1 = 1 ------- s^2 + 5 Continuous-time transfer function.
G2=tf([10],[1])
G2 = 10 Static gain.
G3=tf('s')
G3 = s Continuous-time transfer function.
G4=tf([1 1],[1 0])
G4 = s + 1 ----- s Continuous-time transfer function.
H1=tf([1 0],[1])
H1 = s Continuous-time transfer function.
H2=tf([1 5],[1])
H2 = s + 5 Continuous-time transfer function.
TR1=feedback(G1,H1,-1)
TR1 = 1 ----------- s^2 + s + 5 Continuous-time transfer function.
T1=series(TR1,G2)
T1 = 10 ----------- s^2 + s + 5 Continuous-time transfer function.
TR2=feedback(T1,H2,-1)
TR2 = 10 --------------- s^2 + 11 s + 55 Continuous-time transfer function.
T2=series(TR2,G3) % T2=series(TR2*G3) leads to "Not enough input arguments"
T2 = 10 s --------------- s^2 + 11 s + 55 Continuous-time transfer function.

Sulaymon Eshkabilov
Sulaymon Eshkabilov on 23 Nov 2023
If you want to use a user prompt then it should be in this way (Note that just numerical input prompt won't work):
Try:
G3N = 1 1
G3D = 1 2 3
enter numerator coefficients: 1 1
enter denominiator coefficients: 1 2 3
G3N = input('enter numerator coefficients: ', 's' );
G3D = input('enter denominiator coefficients: ', 's');
G3 = tf(str2num(G3N), str2num(G3D))
G3 =
s + 1
-------------
s^2 + 2 s + 3

Sam Chak
Sam Chak on 24 Nov 2023
Variable K doesn't a specific value, I want multiply K other variables like TR2...
From the root locus, it appears that the closed-loop system system will be stable for any value of the gain parameter K.
G1 = tf([1],[1 0 5]);
G2 = tf([10],[1]);
% G3 = tf('k') % <-- it seems that you want to multiply k with TR2 at the end
H1 = tf([1 0],[1]);
H2 = tf([1 5],[1]);
TR1 = feedback(G1, H1);
T1 = series(TR1, G2);
rlocus(T1*H2)
% T2 = series(TR2*G3)

Categories

Find more on Dynamic System Models in Help Center and File Exchange

Products

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!