# 1. Assemble a Simple Robot¶

Omniverse Isaac Sim’s GUI interface features are the same ones used in NVIDIA Omniverse™ USD Composer, an application dedicated to world-building. In this tutorial series, we will focus on the GUI functions that are most relevant to robotic uses. For more sophisticated general world creation, please check out Omiverse Create.

In this tutorial, we will rig a simple “robot” with three links and two revolute joints to introduce the basic concepts of joints and articulations. We will take the objects that were added to the stage in Add Simple Objects, and turn them into a mock mobile robot with rectangular body and two cylindrical wheels. While this step is not needed for robots that are imported from Importing your Onshape Document or URDF Importer, these are important concepts to understand for tuning your robots and assembling objects with articulations.

## 1.1. Learning Objectives¶

This tutorial details how to rig a two-wheel mobile robot. We will cover how to

• Organize stage tree hierarchy

• Add joints between two rigid bodies

• Add joint drives and joint properties

## 1.2. Getting Started¶

Prerequisites

1. If you are continuing from the Introductory Tutorials and have your own mock_robot.usd saved, open it using File> Open. Otherwise, load the asset provided in Isaac/Samples/Rigging/MockRobot/mock_robot_no_joints.usd. Do not load it as a reference since you will need to make permanent modifications to the file.

2. To add a joint between two bodies, first click on the parent body and then the child body. For our mock robot, we will first select body, then while holding Ctrl + Shift, select wheel_left. With both body highlighted, right-click and then select Create > Physics > Joints > Revolute Joint. A RevoluteJoint will appear under wheel_left on the stage tree. Rename it to wheel_joint_left.

3. Double check in the Property tab that body0 is the cube, and body1 is the cylinder.

4. Change the Axis of the joint to Y.

5. If you look closely, the joint’s two local frames are misaligned. To correct it, go to Local Rotation 0 and make x = 0 degrees. Next, go to Local Rotation 1 and make x = -90 degrees. The example here only shows an orientation misalignment, you may see translational misalignment depending on your robot.

6. Repeat steps 1-4 for the right wheel joint.

Before the joints were added, the three rigid bodies fell to the ground separately after pressing Play. Now that there are joints attached, the bodies will fall as if they are connected. You can drag the robot around by holding down the Shift key and clicking and dragging on any part of the robot in the viewport, and you can see that they move together like they are connected via revolute joints.

Adding the joint is only adding the mechanical connection. To be able to control and drive the joints, we need to add a joint drive API. Select both joints and click the + Add button in the Property tab, and select Physics > Angular Drive to add drive to both joints simultaneously.

• Position Control: for position controlled joints, set a high stiffness and relatively low or zero damping.

• Velocity Control: for velocity controller joints, set a high damping and zero stiffness.

For joints on a wheel, it makes more sense to be velocity controlled, so let’s set both wheels’ Damping to 1e4*and *Target Velocity to 200. If you are working with joints with limited range, those can be set in the Property tab, under the Raw USD Properties > Lower (Upper) Limit. Press Play to see our mock mobile robot drive off.

Even though directly driving the joints can move the robot, it is not the most computationally efficient way. Making things into articulations can achieve higher simulation fidelity, fewer joint errors, and can handle larger mass ratios between the jointed bodies. You can read more regarding the physics simulation behind it in Physics Core: Articulation. To turn a series of connected rigid bodies and joints into articulation, we need to set an articulation root to anchor the articulation tree. According to instructions on defining articulation trees in Physics Core: Articulation :

For a fixed-base articulation, add the Articulation Root Component either to: 1) the fixed joint that connects the articulation base to the world, or 2) an ancestor of the fixed joint in the USD hierarchy. The second option allows creating multiple articulations from a single root component added to the scene: Each descendant fixed joint will be defining an articulation base link. For a floating-base articulation, add the Articulation Root Component either to: 1) the root rigid-body link or 2) an ancestor of the root link in the USD hierarchy.

In our case, we will add the articulation root to the robot body. Select body on the tree, open + Add in the Property tab, and add Physics > Articulation Root.

The resulting robot should match the asset provided in Isaac/Samples/Rigging/MockRobot/mock_robot_rigged.usd.

## 1.6. Summary¶

In this tutorial, we covered how to connect rigid bodies using joints, add joint drive to control the joints, as well as turning a chain of joints into an articulation.

### 1.6.1. Next Steps¶

Continue on to Add Camera and Sensors to learn how to add a camera to the car.