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Transfer Functions in MatLab

  1. Feb 7, 2008 #1
    I am trying to find the transfer function for a simple RL circuit. At my school they don't use version 7, and I was able to find it with this code:
    NUM = 20e3;
    DEN = [1 20e3];
    sys = tf(NUM, DEN)

    That found the transfer function for the RL series circuit with a 20K resistor and a Inductor with a value of 500mH. The transfer function was [tex] \frac{20000}{s+20000}[/tex].

    When I try this same code with MatLab version 7, it says that tf is not an undefined function. How do I find the Transfer Function with version 7?
    Last edited: Feb 7, 2008
  2. jcsd
  3. Feb 7, 2008 #2
    It's not that version 7 doesn't support transfer functions. You need the right toolboxes. Type help tf. You will get the path where the m file is located. From this you should be able to tell what toolbox you need. I presume you need the control systems toolbox.
  4. Feb 7, 2008 #3
    when i type help tf it says:
    tf.m not found.

    How do I get it?
  5. Feb 7, 2008 #4
    I found it on the mathworks website, but you have to buy it. Do you know where I can find this toolbox for free?
  6. Feb 7, 2008 #5
    I'm sure you can figure out how this might be done.
  7. Feb 8, 2008 #6
    I have tried. How about you tell me if you know?
  8. Mar 5, 2008 #7
    Still can't figure out how to get the new toolboxes. Could someone please help?
  9. Mar 5, 2008 #8
    OK. So I found the toolbox I need. It is called the control system toolbox. PLEASE help me figure out how to get it!!!!
  10. Mar 6, 2008 #9

    Dr Transport

    User Avatar
    Science Advisor
    Gold Member

    you have to purchase those toolboxes, thay are easy to download then install.
  11. Jul 27, 2008 #10

    Are you using Matlab 7 on campus?

    If so, I imagine your school has a license for the control toolbox. You should check in the "licenses" folder in the main Matlab directory.

    In that folder there should be either a .dat or .lic file, which needs to be up-to-date in order for you to get all the toolboxes your school has licensed. Check with your campus computing support, and ask them specifically about what I've described.

    This advice comes pretty late, I know, but I'd had problems similar to yours before, so I thought I'd register and give my two cents.
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