FreeCAD as pre-post processor for MBDyn

About the development of the FEM module/workbench.

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JamesLiu
Posts: 26
Joined: Sat Apr 28, 2018 6:23 am

Re: FreeCAD as pre-post processor for MBDyn

Post by JamesLiu »

Hey josegegas, would you please make a tutorial on cycloidal gearbox simulation? It is helpful to understand force interactions between disks and pins.

phpBB [video]
josegegas
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Location: New Zealand

Re: FreeCAD as pre-post processor for MBDyn

Post by josegegas »

JamesLiu wrote: Thu Jan 06, 2022 6:18 am Hey josegegas, would you please make a tutorial on cycloidal gearbox simulation? It is helpful to understand force interactions between disks and pins.

phpBB [video]
Hi. Sorry but what you see in the video is just an animation, not a MBD simulation. I basically just programmed a script to move the parts as they would in a real cycloidal gearbox.
JamesLiu
Posts: 26
Joined: Sat Apr 28, 2018 6:23 am

Re: FreeCAD as pre-post processor for MBDyn

Post by JamesLiu »


Hi. Sorry but what you see in the video is just an animation, not a MBD simulation. I basically just programmed a script to move the parts as they would in a real cycloidal gearbox.
OK, understood. Do you think the latest MBDyn WB is ready for this simulation? I want to try it myself as part of learning process.
josegegas
Posts: 241
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Location: New Zealand

Re: FreeCAD as pre-post processor for MBDyn

Post by josegegas »

JamesLiu wrote: Sat Jan 08, 2022 1:18 am

Hi. Sorry but what you see in the video is just an animation, not a MBD simulation. I basically just programmed a script to move the parts as they would in a real cycloidal gearbox.
OK, understood. Do you think the latest MBDyn WB is ready for this simulation? I want to try it myself as part of learning process.
It depends on what exactly you would like to model. It is for sure ready to model all the mechanism but not the contact between parts. Contact is not a very mature part yet. To model the motion you only need the motion equations of the gearbox, which can be found easily on Internet or derived from the gearbox design, and then use the existing joints... It would be interesting to model it as in this example:

https://www.sky-engin.jp/en/MBDynExampl ... /ex11.html

So you can get the input and output torque, and probably the efficiency as well, assuming some coefficients.

You may want to have a look at the "Wankel engine" example:

phpBB [video]


it is somehow similar to the cycloidal gearbox. You will find the example on GitLab.
JamesLiu
Posts: 26
Joined: Sat Apr 28, 2018 6:23 am

Re: FreeCAD as pre-post processor for MBDyn

Post by JamesLiu »


It depends on what exactly you would like to model. It is for sure ready to model all the mechanism but not the contact between parts. Contact is not a very mature part yet. To model the motion you only need the motion equations of the gearbox, which can be found easily on Internet or derived from the gearbox design, and then use the existing joints... It would be interesting to model it as in this example:


Thank you for the instruction! However, my primary interest is the contact force vector between cycloidal disk and pins.
carno
Posts: 12
Joined: Sun Jun 18, 2017 4:38 am

Re: FreeCAD as pre-post processor for MBDyn

Post by carno »

Hi There,

I am trying to model simple pendulum as per the link.
https://www.youtube.com/watch?v=6pIKi2hK-5M

Problem:
While creating structural node, I am getting, error as follows.

Code: Select all

<class 'AttributeError'>: 'Part.Solid' object has no attribute 'CenterOfGravity'
I tried different geometry also. There is something I am missing here. It is allowing me to go ahead and even completing the simulation until free fall. A new error starts while creating revolute joint.

Please let me know, where I am making mistake.

I am on 0.19.4. Is this the mistake?

Thanks in advance.
JamesLiu
Posts: 26
Joined: Sat Apr 28, 2018 6:23 am

Re: FreeCAD as pre-post processor for MBDyn

Post by JamesLiu »

carno wrote: Sat Mar 19, 2022 1:45 pm
I am on 0.19.4. Is this the mistake?
It works only for 0.20 above.
carno
Posts: 12
Joined: Sun Jun 18, 2017 4:38 am

Re: FreeCAD as pre-post processor for MBDyn

Post by carno »

Thanks JamesLiu.

I just downloaded 0.2 and checked. This error vanished and new error started. I will check further.

For information, the error is:

Code: Select all

Running the Python command 'MBdyn_AddStaticBody' failed:
Traceback (most recent call last):
  File "C:\Program Files\FreeCAD 0.20\Mod\freecad-mbdyn-dynamics-workbench-master\MBdynGui.py", line 1856, in Activated
    dyn.AddStaticBody(b[0])            
  File "C:\Program Files\FreeCAD 0.20\Mod\freecad-mbdyn-dynamics-workbench-master\dynamics.py", line 504, in AddStaticBody
    Dummybody(a, baseBody, baseBody.Label)
  File "C:\Program Files\FreeCAD 0.20\Mod\freecad-mbdyn-dynamics-workbench-master\dummybody.py", line 56, in __init__
    obj.addProperty("App::PropertyDistance","absolute center of mass X","Absolute center of mass","absolute center of mass X",1).absolute_center_of_mass_X = cmx

Invalid property name 'absolute center of mass X'
Addition of 'Rigid bodies' works fine.

Is this something related to Windows-10? I am on windows-10.

Thanks again ..
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Kunda1
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Re: FreeCAD as pre-post processor for MBDyn

Post by Kunda1 »

bumping thread (what's the status of this WB since we're close to a FreeCAD v0.20 release?)
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ChainSalad
Posts: 4
Joined: Wed Oct 07, 2020 8:34 pm

Re: FreeCAD as pre-post processor for MBDyn

Post by ChainSalad »

josegegas wrote: Wed Dec 22, 2021 4:32 pm The elastic contact example was set exactly as you described it. There is a structural force determined by the distance between the two nodes. It is basically an adaptation of this example:

https://www.sky-engin.jp/en/MBDynExampl ... /ex09.html

The "bouncing brick" is also the same, although there is one node at each corner of the brick, to which a structural force is applied. All the nodes are rigidly attached together.
Did you somehow select the node and the surface and then wrote an expression for the vertical force equal to the penetration multiplied by a stiffness, and zero when the penetration is negative, or is there a GUI button that I failed to find?

I think that this feature would greatly increase the real-world usability of MbDyn, as mechanisms often have catches and locks, all of which you cannot simulate without collisions.

I just had a look into the blender world: https://www.youtube.com/watch?v=V_QByJsZeRw
I've never used Blender, do you know how accurate the physics model behind this is?
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