Urdf import fixed joint not merged and collision mesh not visible

[AndreaRossetto]

Description

Hi,

1. In the older version if I import a urdf model the bodies connected with the fixed joint they were merged into a single body in the new version of Isaacsim 5.1 this not true. How can I do it?

2. In the menu the collision visualization is enabled for all so I can see also the ground plane collision but the collision mesh of the imported urdf are not shown.

In the video you can se the two issue... the body parts "Full_base_link" and "body" are linked together with a fixed joint but are not merged into one single body

Video.del.2025-11-26.11-49-07.webm

Isaac Sim version

5.1

Operating System (OS)

ununtu 24

GPU Name

RTX4070

GPU Driver and CUDA versions

Driver 580.95.05 Cuda 13.0

Logs

No response

Additional information

No response

[AndreaRossetto]

I also tried in this way but the fixed joint are not merged

from isaacsim.asset.importer.urdf import _urdf

urdf_interface = _urdf.acquire_urdf_interface()

# Configure the settings for importing the URDF file
status, import_config = omni.kit.commands.execute("URDFCreateImportConfig")

import_config.merge_fixed_joints = True
import_config.convex_decomp = False
import_config.import_inertia_tensor = True
import_config.fix_base = False
import_config.distance_scale = 1.0

status, prim_path = omni.kit.commands.execute(
    "URDFParseAndImportFile",
    urdf_path="URDF PATH",
    import_config=import_config,
    get_articulation_root=True,
)

[vick-yu]

Thanks for the report and for sharing the script snippet.

Are you able to reproduce both issues by enabling merge_fixed_joints in standalone_examples/api/isaacsim.asset.importer.urdf/urdf_import.py and importing carter.urdf there?

Also, to help narrow this down, could you confirm which Isaac Sim version was the last one where:

1. fixed joints were correctly merged, and
2. the collision meshes for the imported URDF were visible?

[AndreaRossetto]

The last version where the fixed joints were correctly merged was the 4.5.0 and the collision meshes were visible. I can't find the path for the urdf_import.py

In my urdf file the joint are made in this way:

<joint name = "base_link" type = "fixed">
        <origin xyz = "0.0 0.0 0.0" rpy = "0 0 0"/>
        <parent link = "Body"/>
        <child link = "BatteryHolder"/>
        <axis xyz = "1.0 0.0 0.0"/>
</joint>

[vick-yu]

standalone_examples/api/isaacsim.asset.importer.urdf/urdf_import.py is available in both 5.1.0 and 4.5.0. Could you please import carter.urdf using that script in both versions with import_config.merge_fixed_joints = True, then share screenshots showing how the fixed joints and collision meshes differ between the two? This will help narrow down whether the regression is specific to your URDF or reproducible with a reference asset.

[AndreaRossetto]

In issacSim 5.1

This is with merge False

This is with merge True

[vick-yu]

This is with merge True

Could you share your modified urdf_import.py for this import test? I still see chassis_link in the stage even with import_config.merge_fixed_joints = True enabled on carter.urdf in 5.1.0.

[AndreaRossetto]

from isaacsim import SimulationApp

# URDF import, configuration and simulation sample
kit = SimulationApp({"renderer": "RaytracedLighting", "headless": False})
import omni.kit.commands
from isaacsim.core.prims import Articulation
from isaacsim.core.utils.extensions import get_extension_path_from_name
from pxr import Gf, PhysicsSchemaTools, PhysxSchema, Sdf, UsdLux, UsdPhysics

# Setting up import configuration:
status, import_config = omni.kit.commands.execute("URDFCreateImportConfig")
import_config.merge_fixed_joints = True
import_config.convex_decomp = False
import_config.import_inertia_tensor = True
import_config.fix_base = False
import_config.distance_scale = 1.0

# Get path to extension data:
extension_path = get_extension_path_from_name("isaacsim.asset.importer.urdf")
# Import URDF, prim_path contains the path the path to the usd prim in the stage.
status, prim_path = omni.kit.commands.execute(
    "URDFParseAndImportFile",
    urdf_path=extension_path + "/data/urdf/robots/carter/urdf/carter.urdf",
    import_config=import_config,
    get_articulation_root=True,
)
# Get stage handle
stage = omni.usd.get_context().get_stage()

# Enable physics
scene = UsdPhysics.Scene.Define(stage, Sdf.Path("/physicsScene"))
# Set gravity
scene.CreateGravityDirectionAttr().Set(Gf.Vec3f(0.0, 0.0, -1.0))
scene.CreateGravityMagnitudeAttr().Set(9.81)
# Set solver settings
PhysxSchema.PhysxSceneAPI.Apply(stage.GetPrimAtPath("/physicsScene"))
physxSceneAPI = PhysxSchema.PhysxSceneAPI.Get(stage, "/physicsScene")
physxSceneAPI.CreateEnableCCDAttr(True)
physxSceneAPI.CreateEnableStabilizationAttr(True)
physxSceneAPI.CreateEnableGPUDynamicsAttr(False)
physxSceneAPI.CreateBroadphaseTypeAttr("MBP")
physxSceneAPI.CreateSolverTypeAttr("TGS")

# Add ground plane
PhysicsSchemaTools.addGroundPlane(stage, "/groundPlane", "Z", 1500, Gf.Vec3f(0, 0, -0.25), Gf.Vec3f(0.5))

# Add lighting
distantLight = UsdLux.DistantLight.Define(stage, Sdf.Path("/DistantLight"))
distantLight.CreateIntensityAttr(500)

'''# Get handle to the Drive API for both wheels
left_wheel_drive = UsdPhysics.DriveAPI.Get(stage.GetPrimAtPath("/carter/joints/left_wheel"), "angular")
right_wheel_drive = UsdPhysics.DriveAPI.Get(stage.GetPrimAtPath("/carter/joints/right_wheel"), "angular")

# Set the velocity drive target in degrees/second
left_wheel_drive.GetTargetVelocityAttr().Set(150)
right_wheel_drive.GetTargetVelocityAttr().Set(150)

# Set the drive damping, which controls the strength of the velocity drive
left_wheel_drive.GetDampingAttr().Set(15000)
right_wheel_drive.GetDampingAttr().Set(15000)

# Set the drive stiffness, which controls the strength of the position drive
# In this case because we want to do velocity control this should be set to zero
left_wheel_drive.GetStiffnessAttr().Set(0)
right_wheel_drive.GetStiffnessAttr().Set(0)'''

# Start simulation
omni.timeline.get_timeline_interface().play()
# perform one simulation step so physics is loaded and dynamic control works.
kit.update()
art = Articulation(prim_path)
art.initialize()

if not art.is_physics_handle_valid():
    print(f"{prim_path} is not an articulation")
else:
    print(f"Got articulation ({prim_path})")

# perform simulation
for frame in range(300000):
    kit.update()

# Shutdown and exit
omni.timeline.get_timeline_interface().stop()
kit.close()

I tried the code with my model I still see all the bodies connected with fixed joint as separete parts

[vick-yu]

In issacSim 5.1

This is with merge False

This is with merge True

I also see the "Got articulation (/carter/chassis_link)" message when using import_config.merge_fixed_joints = True. Could you attach the full log files for the two cases?

[AndreaRossetto]

Here there are the logs:

log_merge_True

log_merge_False

[vick-yu]

Thanks for the report and for sharing the details.

In earlier Isaac Sim versions the URDF importer supported merging links connected by fixed joints, but in the current importer this behavior is no longer supported and merge_fixed_joints = True is not applied. This means fixed-joint chains will be kept as separate links in the imported asset, which matches the current, expected behavior of the tool.