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Homework Help: Get transfer function from block diagram

  1. Oct 15, 2014 #1
    Hello guys!

    1 & 2:
    I would like to get some help to take everything in A and find transfer function at B and C.

    I can't find these answer...


    I know how to go from b) to c) .. we only multiply by E(s) and B(s). But I can't find b) at first.

    Thanks so much!
  2. jcsd
  3. Oct 15, 2014 #2


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    Staff: Mentor

    Is there any more detail you can assist with this? For example, where is Vm located? Where is ΩM?

    Your F(s) expression seems incomplete.
  4. Oct 15, 2014 #3
    I think omega m is wm.

    Here is mechanical drawing from where my block diagram came. Sorry it is in french.

    a) should link motor electric supply Vm to speed of shaft in rotation. (Wm)


  5. Oct 16, 2014 #4


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    Staff: Mentor

    If after couple of days you still have no help with this, I suggest that you attach a translation of the description associated with it.

    You checked F(s) as I suggested?
  6. Oct 16, 2014 #5
    F(s) is ok. Here is why (this is a list of all the parameter): La = 0.0H


    • Why this table, everything that can help find a solution is here.
  7. Oct 16, 2014 #6
    Here is a translation:

    We want to know position x(t) of mass m with the help of a DC moteur and a gear system. All system elements are shown on picture 2.

    a) With the use of block diagram (first picture I posted) and parameters (in my last picture), find transfer function of every system elements. Consider initial condition to zero.

    b) Simplify the block diagram found in (a) to obtain the transfer function between the power supply of the motor and the speed of rotation of its shaft.

    c) From the transfer functions found in (b), find one that connects the motor electric supply to the displacement of the mass x (t). Determine the order of the system.
  8. Oct 16, 2014 #7
    Where are you stuck? Show your work.
  9. Oct 16, 2014 #8
    Well I know how to get transfer function when I got Input and Output. (I only simplify my diagram). But I don't know how to get my transfer function when it is not between input and output. (There's no output..)

    Like this:
  10. Oct 16, 2014 #9
    I'll try to simplify my block diagram tonight. Right now, I'm at work.
  11. Oct 16, 2014 #10
    I'm not sure I understand what you mean. In (b), for instance, you're asked to find the transfer function ##\frac{\omega_M(s)}{v_M(s)}##, i.e. ##v_M(s)## is the input and ##\omega_M(s)## is the output.

    In other words, you have to simplify the diagram so you have:
    ##v_M \rightarrow##some transfer function##\rightarrow \omega_M##
  12. Oct 16, 2014 #11
    I don't know where to split my diagram to get wm as an output. What are the transfer function useless, etc.
  13. Oct 16, 2014 #12
    What does it mean, in a diagram, for a signal to be an output? How can you tell that ##\omega_M## isn't an output in the first diagram you posted?
  14. Oct 16, 2014 #13
    In my idea an output is like my last picture y(Z), when there is nothing after it.
  15. Oct 16, 2014 #14
    That depends on your point of view. Would it help if you drew an arrow out from ##\omega_M## in your first diagram? There's nothing more to the right of that either.
  16. Oct 16, 2014 #15
    Yeah you're right.. I'll try tonight!
  17. Oct 16, 2014 #16
    If you take the view that ##\omega_M## is the output, then it's helpful to realize that there's then just a simple feedback loop around H(s), which you can simplify using the rules for blocks in series, parallel and negative feedback.
  18. Oct 16, 2014 #17


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    Gold Member

    Just use algrebra!!!
    My guess, you are trying to use formulas, just derive the equation yourself.

    Lets start by defining Wm.
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