xArm7 gripper jitters / falls apart after pressing Play in Isaac Sim 5.1.0

Omniverse Isaac Sim
,
1

Isaac Sim Version

[✅] 5.1.0
5.0.0
4.5.0
4.2.0
4.1.0
4.0.0
4.5.0
2023.1.1
2023.1.0-hotfix.1
Other (please specify):

Operating System

Ubuntu 24.04
[✅] Ubuntu 22.04
Ubuntu 20.04
Windows 11
Windows 10
Other (please specify):

GPU Information

  • Model: GeForce RTX 4090 Laptop
  • Driver Version: 570.195.03

Topic Description

Detailed Description

I’m trying to use a UFactory xArm7 with the official xArm two-finger gripper in Isaac Sim 5.1.0. The robot description comes from the official xarm_ros2 repo. I generate the URDFs with xacro:

  • Arm + gripper: from xarm_ros2/xarm_description/urdf
  • Gripper-only: from xarm_ros2/xarm_description/urdf/gripper/gripper.urdf.xacro → xarm_gripper_only.urdf

I can import the full arm+gripper URDF into Isaac Sim, and I can also import the gripper alone as a separate URDF.

The problem:

  • In edit mode, the gripper looks correct (all links aligned).
  • As soon as I press Play, the gripper links start moving relative to each other in a strange way, as if the joints are not properly constrained.
  • The fingers and knuckles drift/jitter and never settle, even when I import the gripper only with a fixed base (no arm attached).
  • This happens both when:
    • importing the full xarm7_with_gripper URDF, and
    • importing xarm_gripper_only.urdf by itself.

I’ve attached a short video showing the behavior: the gripper is static in edit mode, but when I start simulation, the fingers/knuckles start “floating” and wiggling around instead of staying as a rigid mechanism.

I’m not sure if this is:

  • a known issue with the xArm gripper URDF + PhysX,
  • a problem with the way mimic joints are imported, or
  • something wrong in my import / physics configuration.

I’m trying to get a stable, physics-driven gripper that I can use for grasp-and-place experiments.

Steps to Reproduce

  1. Clone and build xarm_ros2 (official repo)

  2. Generate gripper URDF

  3. Start Isaac Sim 5.1.0 and import the URDF

  4. Press play

Error Messages

For the plain import (no edits), there are no critical errors beyond standard mesh resolution messages (which I fixed by using absolute paths).

Screenshots or Videos

Additional Information

What I’ve Tried

  • Importing arm + gripper URDF directly → same issue.
  • Importing gripper only as a separate URDF (fixed base) → same issue.
  • Inspecting the URDF:
    • drive_joint is the main DOF.
    • left_inner_knuckle_joint, right_inner_knuckle_joint, left_finger_joint, right_finger_joint, and right_outer_knuckle_joint are mimic joints of drive_joint.
    • The kinematic structure is a tree (no actual closed loop in the URDF); the parallel motion is encoded via Mimic, not via extra constraints.
  • Attempted fixes in Isaac Sim:
    • Tried Exclude From Articulation on various joints (inner knuckles, fingers), but this conflicts with PhysxMimicJointAPI → reverted.
    • Disabled collisions on some of the small linkage links (inner/outer knuckles) and kept only fingertip colliders.
    • Turned off self-collision for the gripper articulation root.
    • Reduced stiffness and increased damping on drive_joint’s drive.
  • Despite these attempts, the gripper still does not behave like a rigid mechanism; links visibly “float” relative to each other under physics.

Related Issues

I’ve read the “Rig Closed Loop Structures” tutorial for Robotiq and other hands, but my URDF is not actually a closed articulation in the same way, and the mimic joints make it tricky to apply the same workflow directly.

Additional Context

This might be a simple issue for others, but it has bothered me for quite a while. Previously I avoided it by switching to the Panda robot that comes with Isaac Sim, but now I really want to use the xArm7 + gripper imported from URDF so that my Isaac Sim and ROS2 setups match.

I’d be very grateful for any guidance or tips, Thank you very much for any help! 🙏

3

Did you check the URDF file is complete? From your video seems some joints are not correctly configured or are missing.