How to "attach" two objects to the robot?

[anarnuri]

Question: Attaching two objects to a dual-arm robot — only one attachment respected in planning

Hi all,

I’m working with a custom dual-arm robot and trying to grasp and transport two different objects simultaneously, one with each arm.

I’m able to successfully plan simultaneous dual end-effector (EE) goals, and both grasps execute as expected. However, I’m running into an issue once the objects are attached.

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Problem description

• Each object is attached to a different gripper link:
  • Right object → right_gripper_base_link
  • Left object → left_gripper_base_link
• Both attached objects are added as extra links with collision spheres and are included in the collision configuration.
• During motion planning:
  • The planner respects collisions for only one attached object, depending on which EE link is specified.
  • If ee_link is set to the right gripper, the right attached object is collision-aware, but the left attached object is ignored.
  • If I switch ee_link to the left gripper, the behavior flips.

In other words, only one attached object is ever considered for collision avoidance, even though both objects are attached to the same robot and listed in collision_link_names.

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Question

What is the correct way to attach multiple objects to different links on the same robot so that both attachments are respected during motion planning and obstacle avoidance?

Is this expected behavior due to using a single ee_link, or is there additional configuration required to make both attached objects fully collision-aware?

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Current configuration (excerpt)

robot_cfg:
  kinematics:
    use_usd_kinematics: False

    urdf_path: "/home/richtech/Desktop/rm_65_fixed_gripper_right_hand_only/DEX_dual_arms.urdf"
    asset_root_path: "/home/richtech/Desktop/rm_65_fixed_gripper_right_hand_only"
    usd_path: "/home/richtech/Desktop/rm_65_fixed_gripper_right_hand_only/DEX_dual_arms_flattened.usd"
    usd_robot_root: "/rm65_stationary"

    base_link: "base_body"
    ee_link: "right_gripper_base_link"

    link_names:
      - right_gripper_base_link
      - left_gripper_base_link

    collision_link_names:
      - left_base_link
      - left_link_1
      - left_link_2
      - left_link_3
      - left_link_4
      - left_link_5
      - left_link_6
      - left_gripper_base_link
      - left_hand_narrow1_Link
      - left_hand_wide1_Link
      - attached_object_left

      - right_base_link
      - right_link_1
      - right_link_2
      - right_link_3
      - right_link_4
      - right_link_5
      - right_link_6
      - right_gripper_base_link
      - right_hand_narrow1_Link
      - right_hand_wide1_Link
      - attached_object_right

    extra_links:
      attached_object_right:
        parent_link_name: right_gripper_base_link
        link_name: attached_object_right
        joint_type: FIXED

      attached_object_left:
        parent_link_name: left_gripper_base_link
        link_name: attached_object_left
        joint_type: FIXED