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How to make a spring mass damper system in Multiphysics

4 REPLIES 4
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Message 1 of 5
9msl1
1451 Views, 4 Replies

How to make a spring mass damper system in Multiphysics

Hello

 

I'm new to autodesk simulation and I'm trying to make a simple spring mass damper system for my thesis project.  

 

As shown in  the figure, the masses will be connected to springs which will provide 3 natural freqeuncies and mode shapes.

 

My problem is that I can create rigid masses in Autosdesk simulation or use spring elements.  Every time I set the element types accordingly and mesh, they return to brick elements.  To make matters worse, when I set the masses as "rigid elemetn type" there is no input for a mass and therefore I can' get any mode shape or natural frequency data from the system.

 

How can I solve the problem?

 

Thanks very much!

 

 

4 REPLIES 4
Message 2 of 5
ilyas_drawbridge
in reply to: 9msl1

Hi,

 

Sometimes, the definition of the terms used in the software is not what we expect it to be. In this case, rigid element is actually an artificial element drawn inside the geometry to transfer loads from non-contacting surfaces. It is a line element and not a 3D element.

 

Back to your case, I recommend modeling the springs as lines set as spring elements and applying point mass at the nodes. Setup > Loads > Weight. I have no idea how to include damping in modal analysis.

 

Regards

Ilyas


Mass Spring.png

Message 3 of 5
AstroJohnPE
in reply to: 9msl1

 

Hi 9msl1

 

Out of curiosity, what are you meshing? ("I set the element types accordinglyand mesh, they return to brick elements") It sounds like you have created a solid model (Inventor, Fusion, Solidworks, etc.), probably of the masses and springs, transferred the solid model to Simulation Mechanical, and are meshing it. So whatever you have in the solid model represents "the real thing", and therefore you would not want to use any element type other than brick (or tet).

 

Otherwise, if you want to create an idealized engineering model, you can draw 3 lines and add 3 weights (weight = mass*gravity) as Ilyas described, and define the lines to be truss elements (assuming you only need axial stiffness). Recall that the axial stiffness of a truss or rod-like bar is k=A*E/L. So once you choose the length L, you are free to use any combination of A*E that gives the desired stiffness.

 

Ilyas was correct in that the natural frequency analysis types do not have the capability to include damping. I do not know why this is the case, but usually you want to know the displacement versus time due to some load when damping is involved, and the natural frequency (in its simplest case) is not related to applied loads. What you can do is use one of the other analysis types, such as a frequency response or time history, and include the load and damping. See the Example titled "Ground Motion - Vibrating Support" in the online help ("Start & Learn > Learn > Help" in Simulation 2014).

 

Also, can you provide some more details about your thesis? I am curious.

Message 4 of 5
9msl1
in reply to: AstroJohnPE

Hello

 

I was doing the exact thing you were saying "meshing the solid model" and tryign to set some of the components to springs.

 

I'm modelling the vibrations of a cyrostat to improve upon its design.  My supervisor wanted me to approximate the system as a simple system to see if autodesk could obtain the same answers that I could with pen and paper

 

Btw, your solution worked perfectly but I used spring elements instead of trusses for convienience.

 

Thank you very much!!  You have no idea how much this helps me

 

 

Message 5 of 5
AstroJohnPE
in reply to: 9msl1

 

Spring elements? I didn't remember that the software had those!

 

Now that I have learned something new for the day, I am going home. 😉

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