Unrealistic simulation of a soft finger: unit issue

[softrobotsfan]

Hi everyone! I'm a new enthusiast of SOFA and I'm quite impressed by how powerful this framework is.
I am trying to simulate a soft finger. Despite that the final goal is to use the 'pneumatic actuator' included in the Soft Robots plugin, I am currently stuck with a simple "gravity only" simulation (no actuation at all).

The cad models used to produce the required meshes are designed in millimeters.
For mass I use 'kg' and for the material Young's Modulus 'N/mm^2' (MPa).
The finger's Young's Modulus is set to 1.0 (1 MPa), the Poisson Ratio to 0.4 and the mass to 0.033 kg.

To define the simulation scene I employ a '.pyscn' file.
As you can see in the attached pictures the result is unrealistic.
It seems that there is a problem with the units used for the simulation.

However, I have already checked older questions (here and here) and I can't figure out what I'm doing wrong.

Could you please suggest a way to address the mentioned issue?
You can find attached the simulation files (SOFA version = 19.06.99 due to compatibility issues with the Soft Robots plugin).

Thank you in advance

gravity_only_test.zip

[hugtalbot]

Hey @softrobotsfan
welcome on the SOFA GitHub discussion! Thanks for your interest and your positive feedback!

You did very well take care of units. However, I think I mis-explained this in the previous posts.
By using a mesh in mm, your LENGTH unit is now mm. You also have seconds for TIME and kg for MASS.

I think the error comes from the Young's Modulus which unit is : MASS / (TIME²⋅LENGTH)

• In the IS unit it corresponds to ➡️ kg / ( s²⋅m) = [kg⋅m/s²] / m² = N / m² = Pa
• with your units it corresponds to ➡️ kg / ( s²⋅mm) = [kg⋅mm/s²] / mm² = mN / mm² = kPa

Then youngModulus=1000 should give you the expected behavior. Note that the forces computed would therefore be in mN.
Does this makes sense?

Best wishes,

Hugo

[hugtalbot]

@softrobotsfan used 9806 from what is in the script @TeoREN0217
which makes sense since the unit of the gravitational acceleration is: LENGTH/TIME^2 --> using mm for LENGTH and s for TIME we have: mm/s^2. Earth gravitational acceleration therefore becomes: 9810 mm/s^2

[TeoREN0217]

Thank you for your reply so fast! With setting 9810, if I want 1MPa youngModules and 7g Mass, should I set it 1000 and 0.007? Is that right? I tried it but it looks to heavy for my 7g model.

[hugtalbot]

Firstly @TeoREN0217 what I wrote earlier assumes your mesh is in millimeters and that other units are IS units.
To know which value of Young's modulus to set, define the basic units:

• MASS
• TIME
• LENGTH
  then you will be able to define which value to set.

[TeoREN0217]

Thanks a lot!

[RickYang1]

May I ask if the units of all parameters related to LENGTH will change when LENGTH becomes mm, such as massDensity ➡️ Kg/mm ^ 2. Also, why does the unit of LENGTH change to mm when the load grid values are between 0 and 40? Will the units of time and mass change with the load grid size.