Demonstrative Example

A typical Tool has two major parts: an online interactive software app for fast and efficient solutions of a particular engineering problem, and a brief review of relevant background materials. In a Tool, you can

  • Click About the Tool to see what the Tool does;
  • Click Tutorial to see how to use the Tool and what the Tool delivers; and
  • Click Subject Review to read the background materials, including fundamental theories, commonly used formulas, and some references
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    standard form of first order dynamic system

    Transfer Function



    Initial Condition


    External Loads

    Load Type

    r(t) = I0δ(t)

    Amplitude (I0):

    Plot Option

    Number of points: (≤1000)

    Range of time (t): 0 to

    About the Tool

    The tool DC-01 plots time response of a first-order system subject to input and initial disturbance. Three types of inputs are considered: impulse, step and ramp inputs.

    In addition, a tutorial on how to use the tool in computation is provided and a subject review on fundamental theories and useful formulas is presented.


    Example: A first-order system

    The input in the tool is

    After clicking "Run", the results will be shown in another page. The responses of the first-order system are

    The analytical results are also shown as follows.

    Time Constant, τ = 0.5

    Free Response with Initial Condition y(0) = 0.2

    yFree = y0*e-t/τ


    y0 = 0.2; τ = 0.5

    Forced Response with Initial Condition y(0) = 0

    yForced = Term 1 + Term 2


    Term 1 = -2*e-2*t

    Term 2 = 2*δ(t), with δ(t) being the delta function

    Subject Review

    Time Response of First-Order Dynamic Systems

    First-order dynamic systems are commonly-seen models in various engineering applications. Examples are diverse, including rotating shaft-bearing systems, RC circuits, DC motors, liquid-level systems and thermal systems. First-order systems are usually described by transfer functions in two forms: standard form and general form.

    1. First-Order Systems in Standard Form

    The standard form of transfer function representation of a first-order system is

    where R(s) and Y(s) are the input and output of the system, respectively; s is the Laplace transform parameter; and T is the time constant of the system. The transfer function G0(s) has a pole at s = -1/T, and it does not have any zeros at all. By Eq. (1), the governing differential equation of the system is

    In the previous equations,

    The free response of the system is described by

    where y0 is the initial value of the system output. The free response of the system thus is obtained by solving the previous equation and it is given by

    The forced response of the system is described by

    where zero initial disturbance has been prescribed. The solution of Eq. (5) can be obtained by several methods. One method is Laplace transform method

    Listed below is forced response in three cases of inputs.

    (a) Impulse response: given an impulsive input

    where δ(t) is the Dirac delta function, the system response is

    (b) Step response: given a step (constant) input

    the system response is given by

    The steady state response of the system (the final value of the system output) in this input case is

    By Eqs. (8) and (9)

    which indicates that the system response at t = T is 63.2% of its final value. Equation (10) can be used to identify time constant T.

    (c) Ramp response: given a ramp input

    the system response is

    Define an error function as the difference between the input and output

    It can be shown that the steady-state error of the system subject to a ramp input is

    2. First-Order Systems in General Form

    The transfer function representation for general first-order systems is

    The transfer function has a pole at s = -a0/a1, and a zero at s = -b0/b1 if b1 is nonzero. For a0 being nonzero, the above transfer function can be written as

    where the time constant

    The free response of the system is described by

    and it has the same form as Eq. (4), with the time constant given in Eq. (16).

    Assume that b1 is nonzero. The forced response of the system in the s domain (Laplace transform domain) can be expressed as

    which indicates that the output of a first-order system in general form can be expressed by that of the corresponding system in standard form. For instance, given a step input r(t) = r0, the forced response in the s domain is

    By Eqs. (7) and (8), inverse Laplace transform of Y(s) given in Eq. (19) yields the time response

    which is a linear combination of impulse response and step response of the standard system (1).

    3. Total Response

    For a first-order system subject to an input and initial disturbance, its total response is the summation of the free response and forced response. For instance, consider a system subject to a ramp input and initial disturbance, which is described by

    The total response, by Eqs. (4) and (11) is given by


    1. R.C. Dorf and R.H. Bishop, 2010, Modern Control Systems, 12th edition, Prentice Hall.

    2. K. Ogata, 2009, Modern Control Engineering, 5th Edition, Prentice Hall.

    3. N.S. Nise, 2010, Control Systems Engineering, 5th Edition, Wiley.