[Question] Effect of mass of base and dummy links in urdf

[kbkartik]

Question

Currently in MassPropertiesCfg, I set mass equal to None. As per the documentation, it seems the density is computed automatically. When I fetch link masses using articulation.root_physx_view.current_masses(), the masses for base and dummy links are set to 1.0kg. How do these links affect physics behavior? Shouldn't the mass for these links be set close to 0?

[PeterL-NV]

When the mass property in MassPropertiesCfg is set to None, Isaac Lab computes the mass using the specified or default density along with the collision geometry of the link. If inertial data is also missing in the URDF, a default density is used, which results in a default computed mass - often 1.0 kg for base and dummy links.

Physics Behavior of 1.0 kg Links

• Dummy and base links with nonzero mass participate in physical simulation and can influence the robot's dynamics.
• If these links are not meant to impact the robot's dynamic behavior (for example, they exist only for structural reasons), setting their mass to 1.0 kg can introduce unintended effects like extra inertia or altered momentum response.

Should Dummy Links Have Near-Zero Mass?

• For most cases, dummy links and base links whose physical contribution should be negligible should be set to nearly zero mass (e.g., 0.001 kg), as this minimizes their effect while keeping the simulation numerically stable.
• However, setting mass exactly to zero is not allowed; a tiny positive value is best.
• The default of 1.0 kg is just a fallback - override this if accurate dynamics are required.