[SOLVED] Consistency of units in Sofa

[Noura]

Hello,

my question is, as other people asked before, about units in SOFA. I read the discussion on a previous topic, but I’m still not sure that I did it the correct way after getting some confusing results in my simulation.

Let’s assume that my scene contains a soft-body cuboid with dimensions of 40mm*30mm*5mm, and and Young’s modulus of 1.5 MPa.
I would like to apply a constant force of 60N. on a certain point (and let’s say in Y direction).

I set Young’s modulus as
youngModulus="1.5e6"

Should I set the force in Newton directly like that

<ConstantForceField indices="0" totalForce="0 60 0"/>

or should I give the Young’s modulus in different units?

Thanks in advance,

Noura

[Hugo]

Hi Noura,

Good to ask the question, since as you know it is an important point to understand.
If the file mesh you load has values between 0 and 40, then your file is in mm. Your Young’s modulus should be set in MPa, therefore youngModulus="1.5".

Your dimension are in mm, your stress in MPa. Since MPa = N / mm^2 you can set your force in Newton.

Best,

Hugo

[Noura]

Hi Hugo,

I see. Thanks!
This means that I have to set youngModulus in MPa. and the force in Newton.

You also mentioned:

If the file mesh you load has values between 0 and 40

Non, they are not! The shape is not necessarily translated to the origin (0,0,0). The bounding box of the cuboid could be for instance [200 100 50 , 240 130 55]. Still, the dimensions remain 40*30*5 mm. Right? Should I care about the coordinates as well?

Thanks,

Noura

[Hugo]

Hi @noura

You understood it well, you have to set youngModulus in MPa. and the force in Newton.

Regarding the file, indeed, whatever the translation, the dimension are in mm.

I will therefore close the topic if this is fine for you.
Best,

Hugo

[Noura]

Fine for me. Marked as resolved!
Noura

[Zahra]

Dear @Hugo.

I’m not sure if it is OK to ask my question here, but as it is directly related, I’am asking it here.

In reply to Noura, you have mentioned that “If the file mesh you load has values between 0 and 40, then your file is in mm.” This confused me. So why 40? What if the dimension of the loaded mesh be around 70?

I have another question here, is applying a constant force of 60 Newton to a node of a soft tissue realistic and reasonable?

Thanks for your guidance,
Zahra

[Hugo]

Hi @zahra

What is the range of the values within your mesh file?
What is the object you are aiming at modeling?

Hugo

[Zahra]

Dear @Hugo,

The object I’m currently working with is a liver. The range of the values of the current mesh file is 8.5*5.1*2.3

Best,
Zahra

[Hugo]

Hi @zahra

These are inches if you consider the size of a human liver.
You therefore need to scale it in meters if you want to use the International Unit System (e.g. Newton for forces)

Hugo

[Zahra]

Hi @Hugo,

I was considering it in Centimeters, thank you for correcting me.

Best,
Zahra

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