Transfer function 'tf'

Hi, in Matlab R2015b, I used the commando ' tf ' for the transfer function. When I run the script where I used it, I get an error message ' undefined function or variable 'tf' '. How do we get or create this function 'tf' ? I thought it was an inherent function of matlab. Thank you.

Answer 1

[Walter Roberson]

http://www.mathworks.com/help/control/ref/tf.html Control System Toolbox

http://www.mathworks.com/help/signal/ref/tf.html Signal Processing Toolbox

http://www.mathworks.com/help/mpc/ref/tf.html Model Predictive Control Toolbox

You need one of those three.

[Aurore Lecharlier]

Dear, Thank you. I however still don't manage to use the tf function. Here is my script (see annexe)

[Aurore Lecharlier]

Do I need to downlaod an extra toolbox (like e.g. Simulink) ? I apologize for my unknowledge, I use Matlab for academic purposes.

[Walter Roberson]

The syntax you are using indicates that you expect to be using the Control System Toolbox. You need to have that optional toolbox installed and licensed. If the command

which tf

shows you a tf function but indicates "no license available" then you have the software installed but no license for it. If the command

   ver

shows you the Control System toolbox but "which" said it did not exist, then you have a license for it but do not have it installed. If it did not show up for "which" or "ver" then you do not have it installed and you do not have it licensed.

If you are using an Academic license then it is possible that your institution has already arranged licenses for you; you would need to check with your IT people about that. If you are already licensed then you should go back to the MATLAB installer and request to install the toolbox (it is not installed by default.)

[Evan Zhang]

Hi, I'm having a similar issue in Matlab 2020a, where

which tf

returns 'tf' not found

but

ver

shows that both dsp toolbox and signal processing toolbox are installed. I have even found a tf.m file in the toolbox files, but it appears to be only comment where the code should be.

[Petar Vukovic]

Same problem with my MATLAB 2020a

[Akella Kartik]

I have the isseu in 2021a as well. I installed both Signal Processing Toolbox and the DSP System Toolbox, but still I get this:

Unrecognized function or variable 'tf'. for tf and

'tf' not found. for which tf

[Jerry Maravilla]

Try the control system tool box. If that doesn't work you can just use matlab's online editor. Doesn't fix the issue but you'll be able to run your code.

[Anish Mitra]

Note that the tf command in the Signal Processing Toolbox is for converting a digital filter to a transfer function. Similarily the Model Predictive Control Toolbox command is for converting an MPC controller to a transfer function.

If you want to create a transfer function model by specifying the numerator and denominator (see code below), then you need a Control System Toolbox license.

tf - Transfer function model - MATLAB

sys = tf([1 2],[1 4 3])

sys = 
      s + 2
  -------------
  s^2 + 4 s + 3
 
Continuous-time transfer function.