[Question] Contact Sensor Behavior in Custom Dexterous Hand (Isaac Sim 4.5)

[xuxuezhou]

Hi, I’m currently working on a dexterous hand project in Isaac Lab and using the contact sensor to retrieve interaction forces between the robot and objects. I observed some unexpected behaviors and would appreciate clarification regarding the following issues. I’m using Isaac Sim 4.5.

1. Scope of enable_self_collision: Are adjacent link collisions ignored?
   According to discussion #1428:

enabled_self_collisions ignores adjacent link collisions. This is the current, expected behavior but is going to be changed in a future release.

However, I could not find any documentation or changelog confirming whether this behavior has been updated in Isaac Sim 4.5.
To verify this, I tested the following setup:

• A dexterous hand is fixed in the air.
• An object is dropped and lands on the hand.
• The contact sensor is configured as: contact_forces = ContactSensorCfg( prim_path="{ENV_REGEX_NS}/Robot/palm_link", update_period=0.0, history_length=0, debug_vis=True, filter_prim_paths_expr=["{ENV_REGEX_NS}/object"], )

The results are as follows:

• Using the Allegro Hand from Isaac Lab with enable_self_collision=True:
  The outputs of force_matrix_w and net_forces_w are both consistent with the visualization and as expected.
• Using my own Leap Hand USD (converted from URDF), with enable_self_collision=True:
  net_forces_w shows non-zero values even when no external contact is present.
• Using Leap Hand with enable_self_collision=False:
  force_matrix_w and net_forces_w are again consistent and reasonable.

This suggests there are self-collisions between links in my Leap Hand USD.
Moreover, the red contact markers indicate contact between palm_link and adjacent links.

Can you confirm that adjacent link collisions are still counted when enable_self_collision=True in Isaac Sim 4.5?
If not, is there an API or debugging utility to identify which links are involved in a given contact?

Also, I’m curious why the Allegro Hand does not exhibit self-collision issues regardless of the enable_self_collision setting. I suspect this may be due to differences in URDF or USD structure. I created my USD file using the official Leap Hand URDF (https://v1.leaphand.com/) and the following command:

./isaaclab.sh -p scripts/tools/convert_urdf.py ~/git/leap_right/robot.urdf \ source/isaaclab_assets/data/Robots/LEAPHand/leaphand.usd \ --joint-target-type position --headless --fix-base

Any insight into why this happens would be very helpful!

2. Contact Sensor Position and Coverage: Is manual adjustment possible?

I’ve noticed that force_matrix_w sometimes outputs all zeros even when an object is clearly in contact with palm_link.

Here’s my contact sensor visualization configuration:

CONTACT_SENSOR_MARKER_CFG = VisualizationMarkersCfg( markers={ "contact": sim_utils.SphereCfg( radius=0.02, visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(1.0, 0.0, 0.0)), ), "no_contact": sim_utils.SphereCfg( radius=0.02, visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(0.0, 1.0, 0.0)), visible=False, ), }, )

During visualization, the red contact marker does not appear in the center of the palm but between two fingers. This leads me to wonder:

• Is the contact sensor possibly misaligned with the visual/physical geometry of palm_link?
• Is there any way to manually adjust the sensor's position or coverage area?
• Does the sphere radius in SphereCfg have any relation to the actual sensing area?

Thanks in advance!

[RandomOakForest]

Thank you for posting this set of questions. I'll move the post to our Discussions section for follow up. In the meantime, here is a summary, still under review, with main points to consider.

1. Adjacent Link Collisions with enable_self_collision

Confirmed behavior in Isaac Sim 4.5:
Adjacent link collisions are still counted when enable_self_collision=True.

Your Leap Hand exhibits self-collisions because:

• The Allegro Hand USD likely includes predefined collision filtering (e.g., via CollisionPropertiesCfg or joint-level exclusions), which your converted Leap Hand lacks ¹².
• LEAP Hand's URDF-to-USD conversion may not preserve collision groups. The URDF converter (convert_urdf.py) doesn't auto-generate collision filters for adjacent links ².

Debugging utilities:

• Use PhysX Visual Debugger (PVD) to visualize collision pairs in real-time ³.
• Check pxr.PhysxSchema.PhysxContactReportAPI on prims to inspect active collision pairs programmatically ⁴.

2. Allegro Hand vs. LEAP Hand Behavior

Why Allegro Hand works correctly:
The Allegro Hand USD includes optimized collision groups that explicitly exclude adjacent links, even with enable_self_collision=True ¹. This is a design choice in its asset configuration, not inherent engine behavior.

Solution for LEAP Hand:
Add collision filters in your USD to exclude adjacent links:

# Example for palm_link and adjacent finger links
collision_filters = [
    ("palm_link", "index_proximal_link"),
    ("palm_link", "thumb_proximal_link"),
    # Add all adjacent pairs
]

Apply these via PhysicsSchema.CollisionGroupAPI ⁴³.

3. Contact Sensor Positioning and Coverage

Key insights:

• Sensor position is defined by prim_path and defaults to the origin of the specified link. Use translation in ContactSensorCfg to offset it ⁵⁶:

ContactSensorCfg(
    prim_path="{ENV_REGEX_NS}/Robot/palm_link",
    translation=(0, 0, 0.05),  # Adjust for palm center
    ...
)

• Coverage radius is controlled by radius (not SphereCfg). The visualization marker's radius is independent of sensing range ³:

ContactSensorCfg(radius=0.1)  # Increase to cover entire palm

• Zero-force outputs occur when:

1. Contacts fall outside the radius.
2. Forces are below min_threshold (default=0.0001) ³.
3. The sensor is misaligned (verify via debug_vis=True).

Recommendations

1. Fix LEAP Hand self-collisions:
   • Explicitly filter adjacent links in your USD using collision groups.
   • Alternatively, set enable_self_collision=False if self-contacts aren't needed.
2. Optimize contact sensor:
   • Increase radius to cover the palm geometry.
   • Adjust min_threshold if small forces are relevant.
   • Use translation to center the sensor on the palm.
3. Verify USD conversion:
   • Inspect the generated USD for LEAP Hand to ensure collision meshes and joints are correctly defined. The Allegro Hand's USD uses optimized collision primitives ¹, while URDF conversions may retain redundant geometry.

For reference, the Allegro Hand's collision setup can be examined in the USD files at /Robots/AllegroHand in Isaac Sim's asset library ¹.

Footnotes

1. https://docs.isaacsim.omniverse.nvidia.com/4.5.0/assets/usd_assets_robots.html ↩ ↩² ↩³ ↩⁴

2. https://ar5iv.labs.arxiv.org/html/2309.06440 ↩ ↩²

3. https://docs.omniverse.nvidia.com/isaacsim/latest/advanced_tutorials/tutorial_advanced_isaac_contact_sensor.html ↩ ↩² ↩³ ↩⁴

4. https://docs.isaacsim.omniverse.nvidia.com/4.5.0/sensors/isaacsim_sensors_physics_contact.html ↩ ↩²

5. https://isaac-sim.github.io/IsaacLab/main/source/overview/sensors/contact_sensor.html ↩

6. https://isaac-sim.github.io/IsaacLab/main/source/overview/core-concepts/sensors/contact_sensor.html ↩

[xuxuezhou]

Thanks for your prompt reply. However, I still have some follow-up questions.

1. I am unable to open these two links: https://docs.omniverse.nvidia.com/isaacsim/latest/advanced_tutorials/tutorial_advanced_isaac_contact_sensor.html, https://isaac-sim.github.io/IsaacLab/main/source/overview/sensors/contact_sensor.html. Would you kindly suggest the tutorial to use PVD to visualize collision pairs?
2. In Isaaclab, there's no arguments including translation, radius, or min_threshold, therefore I cannot directly modify them in ContactSensorCfg(). Would you suggest ways to modify these parameters? https://github.com/isaac-sim/IsaacLab/blob/752be19bade88c1f1e2a06a1ca6519baafbba216/source/isaaclab/isaaclab/sensors/contact_sensor/contact_sensor_cfg.py