Hi. I want creat a tranfer function have parameter,but i don't know how to do it.help me pls

     a*s + 3 
-------------- 
s^2 + b*s + c

Answer 1

[Robert U]

Hi super bad,

have a look into the matlab function "tf":

https://de.mathworks.com/help/control/ref/tf.html

Kind regards,

Robert

[super bad]

hello bro: I want to ask about parameter a,b,c.

[Robert U]

Hi super bad,

asume you need to write a function with parameters a, b, and c that returns you a transfer function H(a,b,c) in Laplace domain:

 function [ H ] = make_my_tf( a,b,c )
 %MAKE_MY_TF creates a transfer function from parameters a,b, and c. It is
 %restricted to systems of the fixed form below and returns the transfer function in
 %Laplace domain.
 %
 % fixed transfer function: H(a,b,c) = ( a*s + 3 ) / (s^2 + b*s + c)

    H = tf([a 3],[1 b c]);

 end

Kind regards,

Robert

[Robert U]

Just saw the discussion below. My answer comprises a function file. Star Strider gave it already as anonymous function.

Kind regards,

Robert

[Walter Roberson]

MATLAB cannot create a transfer function that displays like

a s + 3 
-------------- 
s^2 + b s + c

with literal characters 'a' and 'b' and 'c' showing up, except as a symbolic expression like Star Strider shows.

If particular numeric values are to be substituted, so it might for example show up as

5 s + 3 
-------------- 
s^2 - 2 s + 17.1

then that can be done with the methods Robert or Star Strider show.

Answer 2

[Star Strider]

If you are creating a ‘tunable’ system, see the documentation on the realp (link) function.

[super bad]

No bro: I want creat tranfer function in command window of matlab . My problem is parameter (a b c)

[Star Strider]

Your question is ambiguous. It lacks details necessary to provide a specific solution.

Try this:

sys_fcn = @(a,b,c) tf([a 3], [1 b c]);
sys = sys_fcn(1,2,3);

bode(sys)

[super bad]

I want declare a b c in numeric form, and use them to set the transfer function.

[Star Strider]

Try this:

a = 1;
b = 2;
c = 3;
sys = tf([a 3], [1 b c]);

bode(sys)

[super bad]

sorry bro: it not true. you try it. a,b,c represents any number

    a s + 3 
-------------- 
s^2 + b s + c

[Star Strider]

I did try it, it is true, and I tested all the code I posted here to be certain it works (in R2017b) before I posted it. I am out of ideas for solving your ambiguous problem.

Good luck!

[super bad]

thank bro so much.But I still can not do it, do not know why. hihi...i use matlab V65.

[Star Strider]

The only documentation I can find for V6.5 is for V6.5.2: LTI Models (link), probably the 2005 version. It seems that you should be able to use the last code I posted.

My last effort (before I completely give up):

sys_fcn = inline('tf([a 3],[1 b c])', 'a','b','c');
sys = sys_fcn(1,2,3);

This works in 2017b. I do not have access to V6.5, so I cannot test it with your version. MathWorks folks may have to solve your problem.

[super bad]

Your ways are not right for me. My teacher wants to have the end result:

a s + 3 
-------------- 
s^2 + b s + c

[Star Strider]

You never mentioned that as a specific requirement before.

What you want to do is not possible other than in the Symbolic Math Toolbox:

syms a b c s 
sys = (a*s + 3)/(s^2 + b*s + c)
pretty(sys)

     a s + 3
  ------------
   2
  s  + b s + c

Stopping here.

Again, Good luck!

[super bad]

If it's so easy then I'm not up here asking .

Answer 3

[Walter Roberson]

Please post the actual question, including any requirements as to version restrictions or toolbox restrictions.

See https://www.mathworks.com/matlabcentral/answers/271698-software-request-which-works-with-symbolic-transfer-function and https://www.mathworks.com/matlabcentral/answers/305339-how-to-create-a-transfer-function-with-gain-k#answer_236890