Do you know about density unit?

Hisamichi · May 22, 2024, 3:43am

I am trying to set the material density of my robot.
I know I can set it in Rigid Body Material, but I don’t know what the units are.
Is it kg/cm^3?

Thanks.

AlesBorovicka · May 22, 2024, 8:47am

As defined in the schema:

github.com

PixarAnimationStudios/OpenUSD/blob/release/pxr/usd/usdPhysics/massAPI.h#L212


      
          /// In the context of rigid body physics, density indirectly results in 
          /// setting mass via (mass = density x volume of the object). How the 
          /// volume is computed is up to implementation of the physics system.
          /// It is generally computed from the collision approximation rather than
          /// the graphical mesh. In the case where both density and mass are 
          /// specified for the same object, mass has precedence over density. 
          /// Unlike mass, child's prim's density overrides parent prim's density 
          /// as it is accumulative. Note that density of a collisionAPI can be also
          /// alternatively set through a PhysicsMaterialAPI. The material density
          /// has the weakest precedence in density definition. Note if density is
          /// 0.0 it is ignored. Units: mass/distance/distance/distance.
          ///
          /// | ||
          /// | -- | -- |
          /// | Declaration | `float physics:density = 0` |
          /// | C++ Type | float |
          /// | \ref Usd_Datatypes "Usd Type" | SdfValueTypeNames->Float |
          USDPHYSICS_API
          UsdAttribute GetDensityAttr() const;
          
          /// See GetDensityAttr(), and also

The density is in the units your layer is, so if your MPU (metersPerUnit) and KGPU (kilogramsPerUnit) are 1 then its kg/m^3, if your layer is in cm (MPU = 0.01) its kg/cm^3.
Regards,
Ales

user56551 · May 22, 2024, 5:12pm

Does density get scaled when isaacsim automatically applies its unit conversion fix? Or does it stay related to the layer?

AlesBorovicka · May 23, 2024, 7:04am

That depends what conversion happens. If metrics assembler kicks in it will apply the units scale correction, but since omni.metrics.assembler.physics extension is not enabled by default it would not correct density. If enabled and also the reference is not using scene graph instancing (we cant write there) the density gets corrected.

There are also inbuild tools in Isaac sim for units correction, those should update density.

system · June 6, 2024, 7:04am

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