[Question] Identifying Contacts on different regions of a Rigid Body

[george-nehma]

Question

I have a rigid body which has 4 legs, and I would like to assign a contact sensor to the foot of each of these legs. Now being a RigidBody would be preferable because the body as a whole is rigid and nothing really moves so making the body as part of an Articulation doesn't make sense (and I have had issues with using the correct joints so that when contact is made the body doesnt just separate).

That being said currently I set up my robot as a RigidBody, my contact sensor like:
contact_forces: ContactSensorCfg = ContactSensorCfg( prim_path="/World/envs/env_.*/Robot/MainBody", update_period=0.0, track_air_time = True, debug_vis=True, history_length=5, filter_prim_paths_expr=["/World/ground"]

and initialise it in the environment with self._contact_forces = ContactSensor(self.cfg.contact_forces) and then self.scene.sensors["contact_forces"] = self._contact_forces.

Does anyone know of a method that would work for this problem?

[zxh123abc]

Hi George, I’m currently trying to reproduce your work from “ Data-Driven Linearization of Relative Orbital Motion: An
Unconstrained Alternative to Clohessy-Wiltshire” and I’ve sent you an email with my code and questions. Would you mind checking if you received it? Thanks a lot!”

[PeterL-NV]

To identify contacts on different regions (like each foot) of a RigidBody, you should create contact sensors for each region/foot. Each contact sensor is typically attached to a specific prim (the foot), or configured with a filter to listen only to contact events for a designated region.

Assigning Contact Sensors by Region

• Each foot should be a separate prim or an identifiable substructure in your USD asset. Normally, robot feet are individual prims (e.g., /Robot/LF_FOOT, /Robot/RF_FOOT, etc.) even if the whole robot is mainly a single rigid body.
• Use the contact sensor interface to create a sensor for each foot prim.

[george-nehma]

@PeterL-NV Thanks for the reply. I have each leg in the USD as a Xform (with no rigid body underneath it) as shown in the screenshot of my tree, and now i create the sensor like:

contact_sensor: ContactSensorCfg = ContactSensorCfg(
        prim_path="/World/envs/env_.*/Robot/MainBody/.*_LEG", 
        update_period=0.0, 
        track_air_time = True,
        debug_vis=True,
        history_length=5,
        filter_prim_paths_expr=["/World/ground"]
    )

and keep the same initialise as my original post. But I get this kind of error

2025-10-02T19:06:55Z [104,285ms] [Error] [omni.physx.tensors.plugin] Pattern '/World/envs/env_*/Robot/MainBody/(FL_LEG|FR_LEG|BL_LEG|BR_LEG)' did not match any rigid bodies

2025-10-02T19:06:55Z [104,285ms] [Error] [omni.physx.tensors.plugin] Provided pattern list did not match any rigid bodies

2025-10-02T19:06:55Z [104,285ms] [Error] [omni.physx.tensors.plugin] Pattern '/World/envs/env_*/Robot/MainBody/(FL_LEG|FR_LEG|BL_LEG|BR_LEG)' did not match any rigid contact for filters

2025-10-02T19:06:55Z [104,285ms] [Error] [omni.physx.tensors.plugin] /World/ground
2025-10-02T19:06:55Z [104,285ms] [Error] [omni.physx.tensors.plugin] Provided patterns for sensor and filters did not match any rigid contact entries

I think I might be defining the 'LEGs' incorrectly in the USD, or need a rigid body underneath them?

[PeterL-NV]

Contact sensors cannot directly function on an Xform prim if there is no rigid body or collider API present. Contact sensors in Isaac Sim and Isaac Lab rely on the PhysX Contact Report API, which only works for physical objects—specifically rigid bodies or articulated links that have collision and physics enabled.

[george-nehma]

@PeterL-NV ,

Forgive me for not understanding. I just added some cylinders under the .*_LEG Xforms, and then added the Colliders Preset to the _LEG Xform prims. I then pointed the contact sensor to prim_path="/World/envs/env_.*/Robot/MainBody/.*_LEG", but i get the error

Sensor at path '/World/envs/env_.*/Robot/MainBody/.*_LEG' could not find any bodies with contact reporter API.
HINT: Make sure to enable 'activate_contact_sensors' in the corresponding asset spawn configuration.

When I create my Robot i use: robot: RigidObjectCfg = LUNAR_LANDER_CFG.replace(prim_path="/World/envs/env_.*/Robot").

How would you suggest I proceed to get this working?

[PeterL-NV]

Contact sensors only works if the target prim (e.g., your .*_LEG node) is a rigid body or an articulated link with collision and physics enabled. Merely adding a collider to an Xform without making it a rigid body is not sufficient. Your prim_path in ContactSensorCfg should point to the actual rigid body geometry, i.e., the cylinder collider(s) under each leg, not just the parent Xform if it’s not itself a rigid/physically active prim.

The error hints at another requirement: when you define your robot asset (e.g., with LUNAR_LANDER_CFG), you must enable contact sensor activation using the correct flag.

robot: RigidObjectCfg = LUNAR_LANDER_CFG.replace(
    prim_path="/World/envs/env_.*/Robot",
    activate_contact_sensors=True,
)

[YuHoChau]

Hello @PeterL-NV,

Could you please provide some insights if we want to simulate a dense tactile array using IsaacLab (through the Contact sensors), as far as we know that IsaacLab seems does not support any robots that has more than 256 links, otherwise it will has some internal CUDA errors. The known issues have been reported in linchangyi1/LocoTouch#2 and #855.

Thank you very much for your time.