[Question] PhysX error: PxRigidBody::setMassSpaceInertiaTensor(): components must be non-negative during training my own quadruped URDF

[xiaoxiaconfidence]

Question

When importing a custom quadruped robot URDF file into Isaac Lab, I encounter repeated PhysX errors during environment initialization:

✅ URDF Validation Passed:

• All mass values > 0
• All inertia diagonal elements > 0
• Inertia matrices are positive definite
• Joint axes properly aligned with coordinate axes
• URDF syntax validation passed

(env_isaaclab6) admin2@admin2-Legion-R9000P-ARX8:~/sunshine/Isaaclab$ python scripts/rsl_rl/base/train.py --task RobotLab-Isaac-Velocity-Rough-JRQ-v0 --num_envs 2000

2025-07-21 12:11:16 [30,150ms] [Error] [omni.physx.plugin] PhysX error: PxRigidBody::setMassSpaceInertiaTensor(): components must be non-negative, FILE /builds/omniverse/physics/physx/source/physx/src/NpRigidBodyTemplate.h, LINE 459
2025-07-21 12:11:17 [31,589ms] [Warning] [omni.kit.notification_manager.manager] PhysX error: PxRigidBody::setMassSpaceInertiaTensor(): components must be non-negative

[RandomOakForest]

Thanks for posting this. That's interesting. I'll review with the PhysX team.

[RandomOakForest]

Following up, I'll move this to our Discussions for follow up. Can you confirm you have included

import_config = ImportConfig()
import_config.set_import_inertia_tensor(True)

in your import script?

[Univers-rover]

I also encountered the same problem when I was training my robot in Isaac Lab 2.3.1.

System is ubuntu 22.04, and envirpment is uv

Error is:

2026-01-27T09:15:27Z [40,062ms] [Error] [omni.physx.plugin] PhysX error: PxRigidBody::setMass(): Mass must be non-negative!, FILE /builds/omniverse/physics/physx/source/physx/src/NpRigidBodyTemplate.h, LINE 424
2026-01-27T09:15:27Z [40,062ms] [Error] [omni.physx.plugin] PhysX error: PxRigidBody::setMassSpaceInertiaTensor(): Components must be non-negative, FILE /builds/omniverse/physics/physx/source/physx/src/NpRigidBodyTemplate.h, LINE 459
2026-01-27T09:15:27Z [40,069ms] [Warning] [omni.physx.fabric.plugin] FabricManager::initializePointInstancer mismatch 

My robot is also converted from URDF, and UrdfConverterCfg is set as follows:

urdf_converter_cfg = UrdfConverterCfg(
    asset_path=urdf_path,
    usd_dir=os.path.dirname(dest_path),
    usd_file_name=os.path.basename(dest_path),
    fix_base=False,
    merge_fixed_joints=False,
    root_link_name="base_link",
    force_usd_conversion=True,
    self_collision=False,
    make_instanceable=True,
    replace_cylinders_with_capsules=False,
    joint_drive=UrdfConverterCfg.JointDriveCfg(
        gains=UrdfConverterCfg.JointDriveCfg.PDGainsCfg(
            stiffness=args_cli.joint_stiffness,
            damping=args_cli.joint_damping,
        ),
        target_type=args_cli.joint_target_type,
    ),
)

The URDF I use can be checked through the following Python script:

#!/usr/bin/env python3

import xml.etree.ElementTree as ET
from pathlib import Path

import numpy as np


def check_inertia_positive_definite(ixx, ixy, ixz, iyy, iyz, izz):
    I = np.array([[ixx, ixy, ixz], [ixy, iyy, iyz], [ixz, iyz, izz]])

    # check symmetry
    if not np.allclose(I, I.T):
        return (False,)

    # check if all eigenvalues are positive
    eigenvals = np.linalg.eigvals(I)
    if np.all(eigenvals > 0):
        return True, f"Eigenvalues: {eigenvals}"
    else:
        return False, f"Eigenvalues: {eigenvals} (exist non-positive values)"


def check_axis_alignment(axis_xyz):
    x, y, z = axis_xyz
    # normalize the vector
    norm = np.sqrt(x**2 + y**2 + z**2)
    if norm < 1e-6:
        return False, "Zero vector"

    x_norm, y_norm, z_norm = x / norm, y / norm, z / norm

    # check if aligned with standard axes (allow small errors)
    tolerance = 1e-3
    axes = {
        (1, 0, 0): "X axis",
        (0, 1, 0): "Y axis",
        (0, 0, 1): "Z axis",
        (-1, 0, 0): "-X axis",
        (0, -1, 0): "-Y axis",
        (0, 0, -1): "-Z axis",
    }

    for (ax, ay, az), name in axes.items():
        if (
            abs(x_norm - ax) < tolerance
            and abs(y_norm - ay) < tolerance
            and abs(z_norm - az) < tolerance
        ):
            return True, name

    return False, f"Not aligned ({x_norm:.4f}, {y_norm:.4f}, {z_norm:.4f})"


def check_urdf(urdf_path):
    """Check URDF file"""
    tree = ET.parse(urdf_path)
    root = tree.getroot()

    issues = []
    warnings = []

    # check all link's mass and inertia matrix
    print("=" * 80)
    print("Check mass and inertia matrix")
    print("=" * 80)

    for link in root.findall("link"):
        link_name = link.get("name")
        inertial = link.find("inertial")

        if inertial is None:
            warnings.append(f"Link '{link_name}' has no inertia property")
            continue

        # check mass
        mass_elem = inertial.find("mass")
        if mass_elem is None:
            issues.append(f"Link '{link_name}': no mass definition")
            continue

        mass = float(mass_elem.get("value", 0))
        if mass <= 0:
            issues.append(f"Link '{link_name}': mass value <= 0: {mass}")
        elif mass < 0.001:
            warnings.append(f"Link '{link_name}': mass value is too small: {mass}")

        # check inertia matrix
        inertia_elem = inertial.find("inertia")
        if inertia_elem is None:
            issues.append(f"Link '{link_name}': no inertia matrix definition")
            continue

        ixx = float(inertia_elem.get("ixx", 0))
        ixy = float(inertia_elem.get("ixy", 0))
        ixz = float(inertia_elem.get("ixz", 0))
        iyy = float(inertia_elem.get("iyy", 0))
        iyz = float(inertia_elem.get("iyz", 0))
        izz = float(inertia_elem.get("izz", 0))

        # check diagonal elements
        diagonal_ok = True
        if ixx <= 0:
            issues.append(f"Link '{link_name}': ixx <= 0: {ixx}")
            diagonal_ok = False
        if iyy <= 0:
            issues.append(f"Link '{link_name}': iyy <= 0: {iyy}")
            diagonal_ok = False
        if izz <= 0:
            issues.append(f"Link '{link_name}': izz <= 0: {izz}")
            diagonal_ok = False

        # check if inertia matrix is positive definite
        if diagonal_ok:
            is_pd, msg = check_inertia_positive_definite(ixx, ixy, ixz, iyy, iyz, izz)
            if not is_pd:
                issues.append(
                    f"Link '{link_name}': inertia matrix is not positive definite - {msg}"
                )

        print(f"\nLink: {link_name}")
        print(f"  Mass: {mass}")
        print(f"  Inertia matrix diagonal elements: ixx={ixx}, iyy={iyy}, izz={izz}")
        if diagonal_ok:
            is_pd, msg = check_inertia_positive_definite(ixx, ixy, ixz, iyy, iyz, izz)
            print(f"  Positive definite: {'✓' if is_pd else '✗'} {msg}")

    # check joint axis alignment
    print("\n" + "=" * 80)
    print("Check joint axis alignment")
    print("=" * 80)

    for joint in root.findall("joint"):
        joint_name = joint.get("name")
        joint_type = joint.get("type")

        # fixed joint type does not need to check axis
        if joint_type == "fixed":
            continue

        axis_elem = joint.find("axis")
        if axis_elem is None:
            issues.append(f"Joint '{joint_name}': no axis definition")
            continue

        xyz_str = axis_elem.get("xyz", "0 0 1")
        try:
            xyz = [float(x) for x in xyz_str.split()]
            if len(xyz) != 3:
                issues.append(f"Joint '{joint_name}': xyz format error: {xyz_str}")
                continue
        except:
            issues.append(f"Joint '{joint_name}': xyz parsing failed: {xyz_str}")
            continue

        is_aligned, msg = check_axis_alignment(xyz)
        if not is_aligned:
            issues.append(f"Joint '{joint_name}': axis not aligned - {msg}")

        print(f"\nJoint: {joint_name} (type: {joint_type})")
        print(f"  Axis: xyz=({xyz[0]}, {xyz[1]}, {xyz[2]})")
        print(f"  Aligned: {'✓' if is_aligned else '✗'} {msg}")

    # summary
    print("\n" + "=" * 80)
    print("Summary")
    print("=" * 80)

    if issues:
        print(f"\nFound {len(issues)} issues:")
        for i, issue in enumerate(issues, 1):
            print(f"  {i}. {issue}")
    else:
        print("\n✓ No issues found")

    if warnings:
        print(f"\nFound {len(warnings)} warnings:")
        for i, warning in enumerate(warnings, 1):
            print(f"  {i}. {warning}")

    return len(issues) == 0


if __name__ == "__main__":
    urdf_path = Path("urdf_path")

    if not urdf_path.exists():
        print(f"Error: file not exists: {urdf_path}")
        exit(1)

    print(f"Check URDF file: {urdf_path}\n")
    success = check_urdf(urdf_path)

    if success:
        print("\n✓ All checks passed!")
        exit(0)
    else:
        print("\n✗ Found issues, please fix them and check again")
        exit(1)