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

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.
 

[Dyuman Joshi]

'k' is not an accepted variable to define a transfer function variable.

Reference - https://in.mathworks.com/help/control/ref/tf.html#mw_b9bcdab0-1636-4131-916a-78888032098e

[Melissa]

clear, clc
G1=tf([1],[1 0 5])
G2=tf([10],[1])
G3=tf('s')
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 InputOutputModel/series

Not enough input arguments.

Now it's the answer at the command window

[Walter Roberson]

In particular, when you use tf() with just a single character parameter, the only two supported characters are 's' and 'z' . The s and p and ^-1 possibilities only apply if you use 'Variable' option.

Note: It is not possible to use the Control System Toolbox to create a transfer function that has an undetermined parameter. You cannot use Control System to construct something along the lines of

s^2 + k*s + 1
-------------
5*s^3 - 3 * s + 2

where k is an undetermined coefficient.

The closest that Control System Toolbox has to that, is the possibility of putting in tuning variables. Tuning variables always have a particular value at any one time, but the code is constructed to make it easy to change the value (including changing it under program control.) No facility to "reason" about the form of the equations is available.

If you have a system like the one I showed above, with an undetermined coefficient that is not the s or z, then you need to use the Symbolic Toolbox

[Walter Roberson]

T2=series(TR2*G3)

series constructs control systems in series. Each system to be placed in series must be given as a seperate parameter, such as

T2=series(TR2,G3)

[Melissa]

So insted of

G3= tf('s')

Should I put:

G3=input('put values here')

?

That's the doubt, or how can I replace it?

[Walter Roberson]

What @Sulaymon Eshkabilov posted is potentially fine.

It is not clear to us what you are trying to do. Do you want the user to enter coefficients of a transfer function? If so then it would be common to prompt for numerator and denominator separately, something like

G3N = input('enter numerator coefficients: ');
G3D = input('enter denominiator coefficients: ');
G3 = tf(G3N, G3D);

[Star Strider]

I am not certain what you want to do with respect to:

G3 = tf('s')

The usual approach is something like this —

s = tf('s');

then:

G1 = tf([1],[1 0 5])

G1 =
 
     1
  -------
  s^2 + 5
 
Continuous-time transfer function.

G2 = tf([10],[1])

G2 =
 
  10
 
Static gain.

G3 = 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 =
 
       10 s
  ---------------
  s^2 + 11 s + 55
 
Continuous-time transfer function.

figure
stepplot(T2)
grid

figure
bodeplot(T2)
grid

I added the plots out of interest.

.

[Melissa]

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

[Walter Roberson]

@Melissa

As I indicated earlier, the Control System Toolbox has absolutely no ability to work with variables with unknown value. I discussed what is available in https://www.mathworks.com/matlabcentral/answers/305339-how-to-create-a-transfer-function-with-gain-k#comment_395202

Answer 1

[Sulaymon Eshkabilov]

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.

[Melissa]

Thank you :)

Answer 2

[Sulaymon Eshkabilov]

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

Answer 3

[Sam Chak]

Hi @Melissa

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)