Ackermann Controller

Learning Objectives

In this example, we will drive a forklift by subscribing to a AckermannDriveStamped message through the ROS network. We will learn to

• Setup Articulation and Ackermann controllers to a forklift

• Setup the robot to be driven by a ROS AckermannDriveStamped message

Getting Started

Prerequisite

• The ackermann_msgs ROS package is required to run this sample. To install this package run the following command:

  sudo apt install ros-noetic-ackermann-msgs
  

• This tutorial requires isaac_tutorials ROS package which is provided in IsaacSim-ros_workspaces repo. Complete ROS and ROS 2 Installation to make sure the ROS Noeitc workspace is built and the environment is setup correctly.

• ROS Bridge is enabled.

• roscore is running.

Ackermann Controller and Drive Setup

1. In a new stage, create the Flat Grid environment by going to Create -> Isaac -> Environments -> Flat Grid.

2. Add the Forklift C robot by going to Create -> Isaac -> Robots -> Forklift -> Forklift C.

3. Create a new action graph by navigating to Window > Visual Scripting > Action Graph and then in the Action Graph panel, select New Action Graph.

4. Now add the following nodes to the graph and connect them as shown:

   • On Playback Tick node to execute other graph nodes every simulation frame.

   • Isaac Compute Odometry Node node to compute the robot’s current linear speed.

   • Ackermann Controller node to compute individual wheel steering angles and wheel speed.

   • ROS1 Subscribe AckermannDrive node to subscribe to Ackermann drive commands.

     

   • In the Property tab for the Isaac Compute Odometry Node:

     • Add the Forklift prim (ie. /Forklift) to its chassisPrim input field. This node calculates the position of the robot relative to its start location. Its linear velocity output will be fed into the Ackermann Controller node.

   • In the Property tab for the ROS1 Subscribe AckermannDrive node:

     • Ensure the topicName input field is set to ackermann_cmd.

   • In the Property tab for the Ackermann Controller node set inputs as shown in the table below:

     Input Field	Value
     invertSteeringAngle	True
     maxWheelRotation	0.69813
     maxWheelVelocity	20.0
     trackWidth	0.82
     turningWheelRadius	0.255
     useAcceleration	True
     wheelBase	1.65

5. Add the following nodes to the graph and modify them as explained:

   • Articulation Controller node to manipulate the wheels of the forklift. In the Property tab:

     • Add the Forklift prim (ie. /Forklift) to its targetPrim input field.

     • For the jointNames input field:

       • Click the Add Element button and enter left_back_wheel_joint in the text field that appears.

       • click the Add Element button again and enter right_back_wheel_joint in the text field that appears.

   • Make Array node to create an array of wheel rotation velocities to feed into the articulation controller. In the Property tab:

     • Under the input0 field click the plus icon to add a second input.

   • Now connect the nodes as shown:

     

6. Add another set of these nodes to the graph and modify them as explained:

   • Articulation Controller node to manipulate the steering angle of the forklift wheels. In the Property tab:

     • Add the Forklift prim (ie. /Forklift) to its targetPrim input field.

     • For the jointNames input field:

       • Click the Add Element button and enter left_rotator_joint in the text field that appears.

       • click the Add Element button again and enter right_rotator_joint in the text field that appears.

   • Make Array node to create an array of wheel angles to feed into the articulation controller. In the Property tab:

     • Under the input0 field click the plus icon to add a second input.

   • Now connect the nodes as shown:

     

7. Hit Play in Isaac Sim to start simulation.

8. In a new terminal, ensure your Isaac Sim ROS workspace is sourced and run the following node to start publishing Ackermann commands.

   rosrun isaac_tutorials ros_ackermann_publisher.py
   

   The forklift should move similarly to the one below:

   

Summary

This tutorial covered

1. Creating and setting up Ackermann Controller node and with articulation controller nodes.

2. Adding a ROS1 AckermannDriveStamped subscriber node which feeds commands into the Ackermann Controller node.

Next Steps

Continue on to the next tutorial in our ROS Tutorials series, ROS 1 to ROS 2 to learn how to interface with ROS2 nodes in Isaac Sim from ROS1.