Sofa.Unit : handling your simulation units

[hugtalbot]

A first discussion about how to manage the units of your simulation was open. The recent SOFA v26.06 release (#622) introduced a new Python utility to manage units of your simulation. This Python project provides a class defining the unit system of your scene while allowing the user to define each parameter in any units.

Required import

from Sofa.Units.Definitions import s, m, mm, dm, N, g, kg, kPa  #here I specify the units I want to use
from Sofa.Units.UnitSystem import MechanicalUnitSystem

Declare your scene units

In case you want to make sure all units of your scene are expressed using the same base units (length, time and mass units) you can now specify these units as follows:

# Here, we know the scene units are second for time, mm for length (e.g. because the file we import is in mm) and g for mass
scene_unit = MechanicalUnitSystem(s, mm, g)

Define the scene parameters in any unit

For all scene parameters, you can now define them using any unit. The utility can scale these parameter units the scene units declared above!

rootNode.dt = scene_unit(0.01, s)
rootNode.gravity = [0, scene_unit(-9.81, N/kg), 0]

...

mechanicalModel.addObject("TetrahedronFEMForceField", name="elastic_material_law", template="Vec3", poissonRatio=0.3, youngModulus=scene_unit(5000, kPa))
mechanicalModel.addObject("MeshMatrixMass", name="mass", template="Vec3,Vec3", massDensity=scene_unit(3e3, kg/m**3))

As you can see, units can be composed naturally using multiplication, division and power operation to create a so called derived unit.

Finally, every unit type can then be scaled to create new faction of basic unit (like mm which is 1.0e-3*m). You might for instance want to create your own scaled unit, :

from Sofa.Units.Core import ScaledUnit
bar = ScaledUnit(Pa, 1e5)

We hope this utility will help you making the units of your scene more robust!
Big up to @bakpaul