Articulate a Basic Robot

Learning Objectives

After completing this tutorial, you will have learned:

• How to organize the stage tree hierarchy
• How to create joints between rigid bodies
• How to configure joint drives (drive properties)
• How to build efficient physics simulations by adding articulations
• How to control robot movement using the Articulation Velocity Controller

Getting Started

Prerequisites

• Complete Tutorial 2: Assemble a Simple Robot before starting this tutorial.

Starting from a Checkpoint Asset

If you have not completed the previous tutorial, you can start from this tutorial by loading the mock_robot_no_joints asset from the Samples > Rigging > MockRobot folder in the Content tab at the bottom right of the screen. When loading, open it as a File (not as a reference). Since this tutorial makes direct changes to the asset, it must be opened as a file rather than a reference.

Estimated Time

Approximately 15-20 minutes.

Overview

In this tutorial, you will connect the robot body and wheels assembled in the previous tutorial using joints, configure articulations to create a functioning robot, and finally add a velocity controller to learn how to control the robot during simulation.

Adding Joints

Joints are mechanisms for physically connecting two rigid bodies. Here we use Revolute Joints to enable wheel rotation.

Creating a Scope for Joints

First, create a Scope to organize and store the joints.

1. Right-click World (or mock_robot if using the mock_robot_no_joints asset) in the stage tree and select Create > Scope.
2. Rename the created Scope to Joints.

Creating the Left Wheel Joint

1. Select the body Xform in the stage tree, then hold Ctrl and select the wheel_left Xform.

   Selection order matters

   The first selected object becomes Body 0 (parent: fixed side), and the second selected object becomes Body 1 (child: rotating side). Always select the body first, then the wheel.

2. Right-click and select Create > Physics > Joints > Revolute Joint.

3. Right-click the RevoluteJoint added to the stage tree, select Rename, and rename it to wheel_joint_left.
4. Verify the following in the Property tab:
   • Body 0: The body's Cube is set
   • Body 1: The wheel_left's Cylinder is set

Configuring the Joint Rotation Axis

Configure the joint axis so that the wheels rotate in the correct direction. Since there is a 90-degree transformation difference between the body and the cylinder, the Local Rotation needs to be adjusted.

1. With wheel_joint_left selected, check the Property tab.
2. Set Local Rotation 0 as follows:
   • X: 0.0, Y: 0.0, Z: 0.0
3. Set Local Rotation 1 as follows:
   • X: -90.0, Y: 0.0, Z: 0.0
4. Change Axis to Y.

Creating the Right Wheel Joint

Create the right wheel joint following the same procedure as the left wheel.

1. Select the body Xform in the stage tree, then hold Ctrl and select the wheel_right Xform.
2. Right-click and select Create > Physics > Joints > Revolute Joint.
3. Rename it to wheel_joint_right.
4. Configure Local Rotation and Axis the same way as the left wheel.

Organizing the Joints

1. Drag and drop the created wheel_joint_left and wheel_joint_right into the Joints Scope created earlier to organize them.

   

Adding Joint Drives

Joint drives provide driving force to joints. Here we add drives to control the wheels by angular velocity (rotation speed).

1. Select both wheel_joint_left and wheel_joint_right in the stage tree while holding the Ctrl key.
2. Click the + Add button in the Property tab.
3. Select Physics > Angular Drive.
4. In the added Angular Drive section, set the following parameters:
   • Damping: 1e4 (10000)
   • Target Velocity: 200 (rad/s)

Difference between Damping and Stiffness

• Setting Damping enables velocity control. Driving force is applied toward the target velocity.
• Setting Stiffness enables position control. Driving force is applied toward the target angle.
• For continuous rotation like wheels, use Damping and keep Stiffness at 0.

Adding Articulation

Articulation is a mechanism for treating multiple rigid bodies and joints as a single robot unit. Configuring this significantly improves simulation accuracy and performance.

1. Select World (or mock_robot if using the mock_robot_no_joints asset) (the parent Xform of the entire robot) in the stage tree.
2. Click the + Add button in the Property tab.
3. Select Physics > Articulation Root.

Effects of Articulation Root

Setting an Articulation Root converts the connected rigid bodies and joints into an efficient articulation structure. This provides the following benefits:

• Improved simulation accuracy: Fewer joint errors occur
• Better mass ratio handling: Stable simulation even for robots with large mass ratios
• Improved computational efficiency: Physics calculations are processed more efficiently

Adding a Controller

Finally, add a Velocity Controller to control robot movement during simulation.

Creating a Scope for the Controller

1. Right-click World (or mock_robot if using the mock_robot_no_joints asset) in the stage tree and select Create > Scope.
2. Rename the created Scope to Graphs.

Adding the Velocity Controller

1. Select Tools > Robotics > Omnigraph Controllers > Joint Velocity from the menu bar.
2. When the dialog appears, configure the following:
   • Robot Prim: Click + Add and select the Prim with the Articulation Root API (World, or mock_robot if using the mock_robot_no_joints asset).
   • Graph Path: Enter /World/Graphs/Velocity_Controller (or mock_robot/Graphs/Velocity_Controller if using the mock_robot_no_joints asset).
3. Click OK to create the controller.

Testing in Simulation

1. Press Play to start the simulation.
2. Select JointCommandArray inside Graphs > velocity_controller in the stage tree.
3. Change the input0 and input1 values displayed in the Property tab to verify that you can independently control the speed of the left and right wheels.

Note about units

The controller uses radians (rad/s), but USD drive properties display angles in degrees. Be aware of this difference when setting velocity and position values.

Summary

This tutorial covered the following topics:

1. Organizing the stage tree hierarchy using Scope
2. Connecting the body and wheels with Revolute Joints
3. Configuring angular velocity control with Angular Drive
4. Improving simulation accuracy and performance by adding an Articulation Root
5. Controlling robot movement with Velocity Controller

Reference Asset

The completed robot is similar to the mock_robot_rigged asset found in the Samples > Rigging > MockRobot folder in the Content tab at the bottom right of the screen.

Next Steps

Proceed to the next tutorial, "Add Camera and Sensors", to learn how to attach camera sensors to a robot.