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Homework Help: Find the Differential Equation Relating Input Voltage and Output Voltage

  1. Nov 10, 2012 #1
    Hey all,

    I'm stuck on a dynamic systems question, it's attached as a jpeg

    I started off by writing nodal equations for each node:

    Node 1: 1/R1(ei-eA)=C1D(eA-eo)+1/R2(eA-eB)

    Node 2:1/R2(eA-eB)=C2D(eB)

    I know that I have to isolate for ei and eo but I'm really confused with the D^2 that's in the answer. Any help is greatly appreciated! :)

    Attached Files:

  2. jcsd
  3. Nov 10, 2012 #2


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    Staff: Mentor

    Write the node equations in the state space domain (Laplace); The impedance of a capacitor is given by 1/(s*C). The state variable 's' translates to the differential operator D → ##\frac{d}{dt}## in the time domain.
  4. Nov 10, 2012 #3
    It's the second derivative DDy -> D2y

    For example, if you took the derivative of your second node equation:

    1/R2(eA-eB) = C2D(eB)
    D(1/R2(eA-eB)) = D(C2D(eB))
    1/R2 D(eA-eB) = C2 D2(eB)
  5. Nov 11, 2012 #4
    Ohhhh okay that makes sense. Took a while to wrap my head around it but I got the right answer in the end. Thanks! :)
  6. Nov 11, 2012 #5
    While I'm at it, I'm having trouble with another question like the one above, but this time it's a mechanical system. It's the jpeg below.

    I started off with the writing equations for each mass

    Mass 1: K1x1 + DB(x) + k2(x1-x2) = 0

    Mass 2: m2D^2x = DB(x1-x2) + k2(x1-x2)

    First off, can someone confirm that those equations are right?

    After that, I'm supposed to sub in the given values and get everything in terms of fa(t) and x1, but I feel like my equations are wrong because it's not working out.

    Thanks for looking!

    Attached Files:

  7. Nov 11, 2012 #6


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    Staff: Mentor

    You've used variables x, x1, and x2 in your equations. I can see x1 and x2 indicated on your diagram, but what is x?

    The method that I usually prefer for these sorts of problems is to convert the mechanical system into its equivalent electrical analog. Then I have a available all the analytical apparatus for electrical circuits (including circuit simulators). Your problem "translates" into:


    Note that potentials at V1 and V2 will represent the velocities of the masses. In the Laplace domain, position x1(s) can be found by integrating velocity V1(s). Integration is trivially accomplished by dividing by the Laplace state variable: x1(s) = V1(s)/s.

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