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Four bar linkage odd behaviour? (Simscape Multibody)

  1. Jul 31, 2016 #1
    Dear community,

    I posted this thread in Mathworks Simscape Community two days ago or so. And I have decided to record a video and post all the information in this forum. This is my first time, so hello and thanks for reading!

    Last month I was playing around with Simscape Multibody and I realised something. They have a "four bar linkage" preset to simulate its physical behaviour, so I changed a bit the initial positions and I found this:

    I have simulated the same mechanism in Autodesk Inventor and It behaves as I expected (as a pendulum). I supposed that the simmetric position in this case should be the equilibrium position, the attractor.

    Then, I decided to create a new simulation in Matlab and this very weird situation popped up. But I do not see why, how, where... I mean, It goes far beyond my understanding.

    Some data are:

    -Connector Link (Yellow bar):
    -Density: 1000 kg/m^3
    -Volume: 10x2x0.5 cm^3

    -Rocker Link (Red bar):
    -Density: 100 kg/m^3
    -Volume: 25x2x0.5 cm^3

    -Crank Link (Blue bar):
    -Density: 100 kg/m^3
    -Volume: 25x2x0.5 cm^3

    -No friction.
    -No external foces.
    -Just gravitation acting.
    -State targets (initial positions): specified in the video. Sorry about the size. Please watch full screen in 1080.

    I would appreciate your comments on this issue, please, I really need it...

    Thanks again!
  2. jcsd
  3. Aug 1, 2016 #2


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    Gold Member

    So, what exactly is the issue? Looks to me like the simulation is not reflecting reality due to some minuscule unbalance in forces.
  4. Aug 1, 2016 #3
    Thanks for your feedback Mech_Engineer, really appreciate it.

    I have refused the possibility you mention because, when initial positions are eliminated (see 1:28 in the embedded video) the linkage behaves as expected. It oscillates around the equilibrium position (the attractor), the simmetric position. I cannot believe that just by changing its initial position, the physical behaviour could vary in such a different way...

    I have simulated dynamically the linkage in Autodesk Inventor, and it behaves as shown in the video below:

    It definitely looks better... I refuse to believe that a change in initial position might affect (in this way) the dynamics of the mechanism.

    To double-check the dynamics of this mechanism, I have simulated in Matlab the mechanism and I have come across the following result:

    My results are quite similar to Simscape Multibody (similarly wrong?).
    As you can see (adjusting the resolution to 1080p), it seems there is an attractor when the angle on the upper-left is set to 0.7 Rad, the mechanism shows a minimal displacement. It seems that this position is its equilibrium position, but this is definitely wrong, as it is supposed to be the simmetric position.

    I feel quite lost in this regard... Sometimes physical problems are quite frustrating and mind blowers. This one is blowing my self-esteem! XD

    Thanks a lot for your time, hope anyone can help me...
  5. Aug 1, 2016 #4
    Solved in Mathworks Community (link to the thread).

    Thanks to everyone
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