{"id":1256,"date":"2024-08-14T18:34:01","date_gmt":"2024-08-14T16:34:01","guid":{"rendered":"https:\/\/erdalpekel.de\/?p=1256"},"modified":"2024-08-14T19:11:29","modified_gmt":"2024-08-14T17:11:29","slug":"add-basic-suction-gripper-to-panda-in-omniverse-isaac","status":"publish","type":"post","link":"https:\/\/erdalpekel.de\/?p=1256","title":{"rendered":"Add basic suction gripper to Panda in Omniverse Isaac"},"content":{"rendered":"\n

In this blog post, we will add a suction gripper to the Panda robotic arm in NVIDIA’s Omniverse Isaac simulation environment. In our latest contribution, we added the Panda robotic arm to Isaac Sim and controlled it with MoveIt! via ros_bridge<\/code>. We will continue this series by modifying the arm’s model and by adding the suction gripper functionality. The resulting robotic arm model will be able to grip rigid bodies with its gripper in the simulation. <\/p>\n\n\n\n

Blender<\/h2>\n\n\n\n

We will start by creating a suction gripper 3d model with Blender. The model will be fairly simple: We will create a simple cube and attach it to the last link of the Panda robotic arm. We start by resizing the default cube that appears when Blender is started. We choose 0.1 by 0.1 by 0.04 meters as it fits perfectly to Panda’s last link with these dimensions. Then, we move the cube’s coordinate system origin along its z-axis by 0.02 m to the bottom. Finally, we save the model as .stl<\/code> file to the franka_description<\/code> ROS repository at meshes\/collision<\/code> and meshes\/visual<\/code>.<\/p>\n\n\n\n

\n
\"Default
Default Cube<\/figcaption><\/figure>\n\n\n\n
\"Scale
Scale Cube to Desired Dimensions<\/figcaption><\/figure>\n\n\n\n
\"Zoom
Zoom to View Smaller Cube<\/figcaption><\/figure>\n\n\n\n
\"Open
Open Cube Transform Window<\/figcaption><\/figure>\n\n\n\n
\"Move
Move Cube Center to Bottom<\/figcaption><\/figure>\n\n\n\n
\"Move
Move Cube Center to Bottom – Change Angle<\/figcaption><\/figure>\n\n\n\n
\"Export
Export Cube as .stl file<\/figcaption><\/figure>\n<\/figure>\n\n\n\n

Robot Model<\/h2>\n\n\n\n

We will add the cube that we exported from Blender in to Panda in its model file panda_arm.urdf<\/code>. We will accomplish this by first adding the gripper link as panda_eef<\/code> and pointing to the cube .stl<\/code> file. panda_joint_eef<\/code> connects links panda_link8<\/code> and panda_eef<\/code> to each other as fixed joint. Additionally, we will create a tip link panda_tip<\/code> for easier configuration of the suction gripper in Isaac Sim and also easier planning with MoveIt!.<\/p>\n\n\n\n

<link name=\"panda_eef\">\n  <visual>\n    <geometry>\n      <mesh filename=\"package:\/\/franka_description\/meshes\/visual\/gripper.stl\" \/>\n    <\/geometry>\n  <\/visual>\n  <collision>\n    <geometry>\n      <mesh filename=\"package:\/\/franka_description\/meshes\/collision\/gripper.stl\" \/>\n    <\/geometry>\n  <\/collision>\n<\/link>\n<joint name=\"panda_joint_eef\" type=\"fixed\">\n  <origin rpy=\"0 0 0\" xyz=\"0 0 0\" \/>\n  <parent link=\"panda_link8\" \/>\n  <child link=\"panda_eef\" \/>\n<\/joint>\n<link name=\"panda_tip\" \/>\n<joint name=\"panda_joint_tip\" type=\"fixed\">\n  <origin rpy=\"0 1.57 0\" xyz=\"0 0 0.04\" \/>\n  <parent link=\"panda_eef\" \/>\n  <child link=\"panda_tip\" \/>\n<\/joint><\/code><\/pre>\n\n\n\n
We can visualize the resulting robot model in RViz by using the urdf_tutorial<\/code> repository. Copy the robot model panda_arm.urdf<\/code> to the urdf_tutorial<\/code> repository and start the display script with the model as parameter:<\/p>\n\n\n\n
ros2 launch urdf_tutorial display.launch.py model:=urdf\/panda_arm.urdf<\/code><\/pre>\n\n\n\n
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\"Panda
Panda Robot Model in RViz<\/figcaption><\/figure>\n\n\n\n
\"Panda
Panda Coordinate Transforms<\/figcaption><\/figure>\n\n\n\n
\"Panda
Panda Coordinate Transforms – eef<\/figcaption><\/figure>\n\n\n\n
\"Panda
Panda Coordinate Transforms – tip<\/figcaption><\/figure>\n\n\n\n
\"Panda
Panda Coordinate Transforms – tip – Close Up<\/figcaption><\/figure>\n<\/figure>\n\n\n\n
We can see that the new links and the suction gripper cube were inserted correctly into the robot model. The x-axis of the final link points towards the last link or inwards. This is important as the the suction gripper in Isaac Sim is configured in x-axis direction by default.<\/p>\n\n\n\n
It is also important to reset the joint effort limits in the robot model file in order to eliminate weird behavior of the robotic arm after the importing the robot URDF into Isaac Sim. We will set the maximum effort and velocity values to 0<\/code> and the Isaac Sim will allow the maximum value for these parameters in the simulation. Finally, we will remove the safety_controller tags as those are not necessary in Isaac Sim:<\/p>\n\n\n\n
<joint name=\"panda_joint7\" type=\"revolute\">\n  <origin rpy=\"1.5707963267948966 0 0\" xyz=\"0.088 0 0\" \/>\n  <parent link=\"panda_link6\" \/>\n  <child link=\"panda_link7\" \/>\n  <axis xyz=\"0 0 1\" \/>\n  <limit effort=\"0\" lower=\"-2.8973\" upper=\"2.8973\" velocity=\"0\" \/>\n<\/joint><\/code><\/pre>\n\n\n\n
Isaac Sim<\/h2>\n\n\n\n
Now that we have prepared Panda arm’s robot model we can import it using the same routine as in our previous blog post<\/a>. There, we first imported the URDF file franka_description\/panda_arm.urdf<\/code> using Isaac Sim’s URDF importer. In a subsequent step, we modified the resulting .usda<\/code> file to move the arm’s articulation to the root XForm panda<\/code>. Below is a gallery illustrating the URDF import process in Isaac Sim:<\/p>\n\n\n\n
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\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n
\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n
\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n
\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n
\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n\n\n\n
\"\"\n\t\t\t\n\t\t\t\t\n\t\t\t<\/svg>\n\t\t<\/button><\/figure>\n
Importing a robot URDF model into Isaac Sim<\/figcaption><\/figure>\n\n\n\n
We will also add a ground plane and a cube to our scene.usda<\/code> file. We will attempt to pick the cube with the suction gripper. It is important to mention, that we added rigid body properties, enabled collisions and most importantly, added a small mass. If no mass is specified, the suction mechanism will not work in our testing.<\/p>\n\n\n\n
def Cube \"Cube\" (\n    prepend apiSchemas = [\"PhysicsRigidBodyAPI\", \"PhysxRigidBodyAPI\", \"PhysicsCollisionAPI\", \"PhysxCollisionAPI\", \"PhysicsMassAPI\"]\n)\n{\n    float3[] extent = [(-0.5, -0.5, -0.5), (0.5, 0.5, 0.5)]\n    float physics:mass = 0.000001\n    bool physics:collisionEnabled = 1\n    bool physics:kinematicEnabled = 0\n    bool physics:rigidBodyEnabled = 1\n    double size = 1\n    quatd xformOp:orient = (1, 0, 0, 0)\n    double3 xformOp:scale = (0.1, 0.1, 0.2)\n    double3 xformOp:translate = (0.5, 0, 0.1)\n    uniform token[] xformOpOrder = [\"xformOp:translate\", \"xformOp:orient\", \"xformOp:scale\"]\n}<\/code><\/pre>\n\n\n\n
In our extension code, we will modify the ui_builder.py<\/code> file to add a button that closes the suction gripper. In scenario.py<\/code>, we will initialize the suction gripper with the new init_gripper()<\/code> method:<\/p>\n\n\n\n
def init_gripper(self):\n    self._dc = dc.acquire_dynamic_control_interface()\n    self.sgp = Surface_Gripper_Properties()\n    self.sgp.d6JointPath = \"\/World\/panda\/panda_tip\/d6FixedJoint\"\n    self.sgp.parentPath = \"\/World\/panda\/panda_tip\"\n    self.sgp.offset = dc.Transform()  # Offset the transform to the base of the Cone\n    self.sgp.offset.p.x = -0.001\n    self.sgp.offset.r = [1.0, 0, 0, 0]\n    self.sgp.gripThreshold = 0.02\n    self.sgp.forceLimit = 1.0e3\n    self.sgp.torqueLimit = 1.0e3\n    self.sgp.bendAngle = 7.5\n    self.sgp.stiffness = 1.0e4\n    self.sgp.damping = 1.0e3\n    self.sgp.retryClose = True\n\n    self.surface_gripper = Surface_Gripper(self._dc)\n    self.surface_gripper.initialize(self.sgp)<\/code><\/pre>\n\n\n\n
We can see a few important aspects of the surface gripper in this method:<\/p>\n\n\n\n