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What are the general forms of first and second order dynamic systems?

  1. Apr 11, 2014 #1
    What is the general form of a first and second order dynamic system, in laplace and time space?

    I have some of the information, but a lot of it is missing and I can't find a clear answer on the web.

    The general form of a second order dynamic system is:

    $$\frac{d^2x(t)}{dt^2}+2\zeta\omega_n\frac{dx(t)}{dt}+\omega_n^2x(t)=f(t)\:\:\:\:\:\:\:[1]$$

    where

    x(t) - is the output, e.g current
    f(t) - is the input, e.g a voltage signal
    zeta - is the damping coefficient
    wn - is the natural frequency


    An example of a second order dynamic system is a RLC circuit. If a resistor, capacitor, and inductor are connected to a power source with voltage at time t equal to f(t), the summed voltage across the three components always equals f(t) at any time t.

    $$Ri(t)+\frac{1}{C}\int_0^ti(t)dt+L\frac{d\,i(t)}{d\,t}=f(t)\:\:\:\:\:\:[2]$$

    So for example if f(t) is a step input from 0 to 6V, the equation will be:

    $$Ri(t)+\frac{1}{C}\int_0^ti(t)dt+L\frac{d\,i(t)}{d\,t}=6\:\:\:\:\:\:\:[3]$$

    If this equation is rearranged so that it is in the general form shown in equation [1], then you will be able to find out the natural frequency of the system (omega_n) and the damping coefficient of the system (zeta).

    The general form of a first order dynamic system in laplace space is:

    $$G(s)=\frac{k(s+a)}{s+b}\:\:\:\:\:\:\:[4]$$

    where

    G(s) - is the transfer function (output/input)
    k - is the "gain" of the system
    a - is the zero of the system, which partially determines transient behavior
    b - is the pole of the system, which determines stability and settling time

    An example of a first order dynamic system is a RC circuit. The voltage across the resistor plus the voltage across the capacitor equal the voltage of the source f(t) at any time t.

    $$Ri(t)+\int_0^ti(t)dt=f(t)\:\:\:\:\:\:\:[5]$$

    So again, for a step input from 0 to 6V, the equation will be:

    $$Ri(t)+\int_0^ti(t)dt=6\:\:\:\:\:\:\:[6]$$

    In laplace space this is

    $$RI(s)+\frac{1}{Cs}I(s)=\frac{6}{s}\:\:\:\:\:\:\:[7]$$

    rearrange to general form:

    $$I(s)*(Rs+\frac{1}{c})=6\:\:\:\:\:\:\:[8]$$

    $$I(s) =\frac{6}{Rs+\frac{1}{c}}\:\:\:\:\:\:\:[9]$$

    $$I(s) =\frac{\frac{6}{R}}{s+\frac{1}{Rc}}\:\:\:\:\:\:\:[10]$$

    Since the input was the step input 6, you can see that the steady state gain is equal to 6/R, the pole is equal to 1/RC, and there are no zeros. You could do the inverse laplace transform of this equation to find out the instantaneous current i(t) at any time t.

    Now my questions:

    - I have shown the general form of a first order dynamic system in laplace space, and the general form of a second order dynamic system in time space, but what is the general form of a first order system in time space, and a second order system in laplace space? I have definitely seen these written somewhere but I can't find them in my notes or online.

    - How would you go from equation [3], to the general form (time space) as shown in equation [1]?

    Any answer is much appreciated, even if it only answers one of the questions, or partially answers a question. Thanks for reading!
     
    Last edited: Apr 11, 2014
  2. jcsd
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