1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Linearization with a Constant Block in Matlab

  1. Sep 15, 2009 #1
    Hi All,

    1. The problem statement, all variables and given/known data
    I have modeled a simple spring mass system in Matlab and trying to use linmod to linearize the system.

    The system models a mass that hangs from ceiling with a spring and damper. So, the forces that act on the mass are gravity, spring force and damping force. I attached the Simulink model and a picture of the model to the question.

    Now, the problem is that, Matlab's linmod seems to completely ignore the gravity (constant block). I would like to know how should I change the model or simulation parameters to get correct results.

    2. Relevant equations

    state vector x is [h V]' (height and speed)
    output vector y is [h a] (height and acceleration)
    input vector is empty

    linmod('trim_linmod_test') gives following matrices:

    0.0000 1.0000
    -0.5000 -0.5000

    1.0000 0.0000
    -0.5000 -0.5000

    B,D empty because there are no inputs.


    y= C*x, hence a = -0.5 * h - 0.5*V

    On the other hand, if we simply look at the model we can see that
    a = (-5*h - 5*V + 9.81) / (m) ( m = 0.1, the gain block )

    So, obviously, the constant block is ignored.

    3. The attempt at a solution
    I can see that the state space equation of the form y=C*x can't really represent the gravity term. C is a 2x2 matrix but acceleration equation needs 3 terms.

    I thought of making gravity an input. This way state space equation will be y = C*x + D*u and there are enough terms to represent acceleration. But this doesn't really look logical to me.

    Thanks in Advance

    Attached Files:

  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted