Prevents randomizing mass to zero or less (#4060)

# Description

This PR prevents users from accidentally randomizing the mass of rigid
body to small and negative by clamping at 1e-6 kg.

Fixes #518 

## Type of change

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applicable. -->

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [ ] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [ ] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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---------
Signed-off-by: Kelly Guo <kellyg@nvidia.com>
Co-authored-by: Kelly Guo <kellyg@nvidia.com>

source/isaaclab/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.50.1"
+version = "0.50.2"
 
 # Description
 title = "Isaac Lab framework for Robot Learning"

source/isaaclab/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.50.2 (2025-11-21)
+~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Prevent randomizing mass to zero in :meth:`~isaaclab.envs.mdp.events.randomize_mass_by_scale` to avoid physics errors.
+
+
 0.50.1 (2025-11-25)
 ~~~~~~~~~~~~~~~~~~~
 

source/isaaclab/isaaclab/envs/mdp/events.py

@@ -323,6 +323,12 @@ class randomize_rigid_body_mass(ManagerTermBase):
                 "Randomization term 'randomize_rigid_body_mass' does not support operation:"
                 f" '{cfg.params['operation']}'."
             )
+        if cfg.params.get("min_mass") is not None:
+            if cfg.params.get("min_mass") < 1e-6:
+                raise ValueError(
+                    "Randomization term 'randomize_rigid_body_mass' does not support 'min_mass' less than 1e-6 to avoid"
+                    " physics errors."
+                )
 
     def __call__(
         self,
@@ -333,6 +339,7 @@ class randomize_rigid_body_mass(ManagerTermBase):
         operation: Literal["add", "scale", "abs"],
         distribution: Literal["uniform", "log_uniform", "gaussian"] = "uniform",
         recompute_inertia: bool = True,
+        min_mass: float = 1e-6,
     ):
         # resolve environment ids
         if env_ids is None:
@@ -360,6 +367,7 @@ class randomize_rigid_body_mass(ManagerTermBase):
         masses = _randomize_prop_by_op(
             masses, mass_distribution_params, env_ids, body_ids, operation=operation, distribution=distribution
         )
+        masses = torch.clamp(masses, min=min_mass)  # ensure masses are positive
 
         # set the mass into the physics simulation
         self.asset.root_physx_view.set_masses(masses, env_ids)