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sbasude

Assigning Coordinate System for Loading

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Hello,

 

I have been trying to assign a local coordinate system to a group of constraints and my model so that I can load it in a direction specified by the local coordinate system. Therefore, I believe I need to use the Mesh>Assign>Node Analysis System and BCs>Assign>Load Reference System for all my constraints and model nodes. However, when trying to assign the Node Analysis System, only "set displacement" works. "set reference" gives an error saying that "Nodes defining a system cannot be assigned to it". I have a feeling this is why my Optistruct simulation is quite bad. 

 

Interestingly, I tried the same method on a very simple box model in a separate model, and created a coordinate system using the nodes of the box. I was able to "Set reference" without a problem, and I could see a clear change in the OptiStruct displacements to the new local coordinate system. I am wondering why my other model does not allow me to set the reference.

 

Any advice is very appreciated.

altair help.PNG

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Set reference = set input system (CP field on GRID card), x y z of that grid refer to this local system.

Set displacement = set output system (CD field), ux uy uz will be computed in this system, so if you create constraints, D values following dofs checkbox will be done, too

 

 

"Nodes defining a system cannot be assigned to it". - you defined system "using nodes" and then assign it to CP, similar to case you want to hold your hair to lift you up!

 

Define system independently to nodes will be ok.

 

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Hi Tinh,

 

Thank you for your response, this makes sense now. What is strange to me is that in my other model in which I used the Geometry functions to create a meshed box, I was able to define the system using the box nodes and assign all my components to my system with no issues. 

 

Also, I cannot define the system independently of those nodes, because I am using landmarks on the bone to create an anatomy-based system.

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Hello Tinh,

 

See the box model attached. I have also attached my femur model. You can see that the femur moves in a strange way but the box movement moves predictably in an axis defined by the local system. Additionally, here is the page that I have used to define my displacements, for your reference (Altair Tips and Tricks).

 

Thanks

Krishna box 2.mvw Femur_Model_6_19.hm Femur Model 6_18.mvw

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Oh my god.

 

Do you know how to edit card?

Edit card of a system, if it is defined by nodes, you will see node IDs, otherwise you will see 3 vectors-this case system is independent. Check it to answer your self!

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Hahaha I am very new to this software... my apologies and thank you for your patience.

What is strange to me is how I used nodes to define both systems when I created them, and the fact that only my box model moves as expected. This is where I am having trouble figuring out how to solve the issue.

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Hi Tinh,

I figured out how to create a system that still uses nodes but does not directly reference them, by using Create>System>Axis Direction instead of Create>System>Node Reference. I can still pick points to define the system this way and it allowed me to assign reference+displacement to all nodes. However, this still didn't work. My model is instead twisting in strange directions. I even reconstructed the model to be sure.

edit: I checked the cards to be sure.

Femur_Model_6_25.fem

Edited by sbasude

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Tinh,

 

I think I have isolated the problem a bit futher. Please see the following link. Thank you again so much for your help.

 

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8 hours ago, sbasude said:

Hi Tinh,

I figured out how to create a system that still uses nodes but does not directly reference them, by using Create>System>Axis Direction instead of Create>System>Node Reference. I can still pick points to define the system this way and it allowed me to assign reference+displacement to all nodes. However, this still didn't work. My model is instead twisting in strange directions. I even reconstructed the model to be sure.

edit: I checked the cards to be sure.

Femur_Model_6_25.fem 40.64 MB · 0 downloads

 

Yes, that is definition by vectors

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