Dynamically Attach Two Rigid Bodies at Runtime in IsaacLab

[bikcrum]

Hi everyone,

I'm working with a manipulator arm in Isaac Lab, and I would like to dynamically attach its end-effector to a rigid body (a box) at a specific location on the box's surface. The goal is for the box to drive the motion of the arm i.e., when the box moves, the end-effector (and consequently the entire arm and its base) should move along with it. Note that the base of the manipulator is its articulation root.

One approach is to redesign the arm such that the box is a permanent child link of the end-effector, but this doesn’t allow for runtime flexibility. I’m looking for a solution that supports attaching and detaching the end-effector during simulation.

Another idea I considered was simulating a magnetic-like attachment using external forces. But this seems nontrivial as applying force only to the end-effector towards the box isn’t sufficient; I would also need a counteracting force from the box towards the end-effector to avoid unrealistic behavior. Modeling this bidirectional force interaction accurately appears quite complex.

Is there a better or more recommended way to do it?

Thanks!

[RandomOakForest]

Thanks for posting this. A possible way to approach this would be as follows.

1. Fixed Joint Creation
   Create a FixedJoint between the end-effector and the box at runtime using the PhysX API. This binds their motion without altering the articulation hierarchy:

from omni.physx import get_physx_scene_interface

def attach_objects(end_effector_prim, box_prim, local_pose_ee, local_pose_box):
    physx_scene = get_physx_scene_interface()
    joint_prim = physx_scene.create_fixed_joint(
        prim_path="/World/Joints/Attachment",
        body0=end_effector_prim.GetPath(),
        body1=box_prim.GetPath(),
        local_pose0=local_pose_ee,
        local_pose1=local_pose_box,
        enable_collision=False  # Disable collision between attached bodies
    )
    return joint_prim

• local_pose_ee/local_pose_box: Specify attachment points relative to each body's frame.

2. Dynamic Detachment
   To detach, delete the joint and re-enable collisions:

def detach_objects(joint_prim, end_effector_prim, box_prim):
    physx_scene = get_physx_scene_interface()
    physx_scene.remove_joint(joint_prim.GetPath())
    # Re-enable collisions if needed
    physx_scene.set_collision_enabled(end_effector_prim.GetPath(), box_prim.GetPath(), True)

3. Motion Propagation
   The box’s movement will drive the arm via the fixed joint. Ensure the box is either:
   • Kinematically controlled (directly set its position/rotation).
   • Dynamically simulated with forces applied to it.

---

Key Considerations

• Articulation Stability: Avoid modifying the arm’s articulation structure (e.g., reparenting prims) during simulation, as this can destabilize the physics.
• Collision Handling: Disable collisions between attached bodies to prevent internal forces from disrupting the joint.
• Performance: Fixed joints are lightweight and suitable for frequent attach/detach cycles.

References

• Dynamically Attach Two Rigid Bodies at Runtime in IsaacLab #2472
• https://forums.developer.nvidia.com/t/proper-way-to-create-an-arm-with-a-tool-changer-in-isaac-sim/311718
• https://forums.developer.nvidia.com/t/dynamically-create-attachments-at-runtime/322378