ROS 2 Clock

Learning Objectives

In this example, we will:

• Have a brief discussion on the ROS 2 Clock publisher and subscriber

• Publish simulation time to ROS 2 as a Clock message

• Subscribe to a ROS 2 Clock message

• Add a Clock Action Graph using the menu shortcut

Getting Started

Prerequisite

• Complete ROS 2 Installation.

• If using multiple systems, set the FASTRTPS_DEFAULT_PROFILES_FILE environment variable as per instructions in ROS 2 Installation before launching Isaac Sim, as well as any terminal where ROS messages will be sent or received, and ROS 2 Extension is enabled.

Note

In Windows 10 or 11, depending on your machine’s configuration, RViz2 might not open properly.

Simulation Time and Clock

For external ROS 2 nodes to synchronize with simulation time, a clock topic is usually used. Many ROS 2 nodes such as RViz2 use the parameter use_sim_time, which if set to True will indicate to the node to begin subscribing to the /clock topic and synchronizing to the published simulation time.

You can either set this parameter in a ROS 2 launch file or set the parameter using the following command in a new ROS 2-sourced terminal:

ros2 param set /node_name use_sim_time true

Make sure to replace /node_name with whatever node you are currently running. If setting using the terminal, the node must already be running first before setting the parameter.

Running ROS 2 Clock Publisher

1. Go to Window > Graph Editors > Action Graph to create an Action graph.

2. Add the following OmniGraph nodes into the Action graph:

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

• ROS 2 Context node to create a context using either the given Domain ID or the ROS_DOMAIN_ID environment variable.

• Isaac Read Simulation Time node to retrieve current simulation time. Note: By default the simulation time increases monotonically, meaning regardless of whether simulation is stopped and re-played, the time will continue incrementing. This is mainly to prevent issues that can arise with the time jumping back when simulation resets. You can set resetOnStop to True if you would like the clock to start from 0 every time simulation is reset.

• ROS 2 Publish Clock node to publish simulation time to the /clock topic.

  

3. Start RViz in a new ROS 2-sourced terminal.

   ros2 run rviz2 rviz2
   

   Humble

   Take note of the ROS Time and ROS Elapsed times listed in the bottom of the RViz window. These are currently displaying the wall time and, typically, match the Wall Time and Wall Elapsed fields.

4. In a new ROS 2-sourced terminal set the use_sim_time parameter to true for the RViz node. Ensure that simulation is stopped in Isaac Sim.

   ros2 param set /rviz use_sim_time true
   

   Humble

   Notice in RViz that ROS Time and ROS Elapsed are now both 0.

5. In Isaac Sim click Play.

   Humble

   In RViz, the ROS Time is now identical to the simulation time published from Isaac Sim over the /clock topic.

Publishing System Time

While publishing the simulation time is the most common workflow, there can be certain workflows that require certain messages to contain system time. To publish system time over the clock topic follow these steps:

1. Go to Window > Graph Editors > Action Graph to create an Action graph.

2. Add the following OmniGraph nodes into the Action graph:

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

• ROS2 Context node to create a context using either the given Domain ID or the ROS_DOMAIN_ID environment variable.

• Isaac Read System Time node to retrieve current system time.

• ROS2 Publish Clock node to publish simulation time to the /clock topic.

  

3. In Isaac Sim click Play. To observe the system timestamp published from Isaac Sim over the /clock topic run the following command in a ROS-sourced terminal:

   ros2 topic echo /clock
   

Camera Helper and RTX Lidar nodes

In upcoming tutorials you will observe the ROS 2 Camera Helper node and the ROS2 RTX Lidar Helper node. As both of these nodes automatically generate a sensor publishing pipeline, to use system timestamps for their publishers, ensure that their useSystemTime input field is set to True.

Running ROS 2 Clock Subscriber

1. Open a new stage. Go to Window > Graph Editors > Action Graph to create an Action graph.

2. Add the following OmniGraph nodes into the Action graph:

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

• ROS2 Context node to create a context using either the given Domain ID or the ROS_DOMAIN_ID environment variable.

• ROS2 Subscribe Clock node to subscribe to external timestamp data.

  

3. Start simulation by clicking Play. Select the ROS2 Subscribe Clock node inside the action graph to view its timeStamp output in the Property window. Verify that the timestamp is 0.

4. In a new ROS2-sourced terminal run the following command to manually publish a clock message once:

   ros2 topic pub  -t 1 /clock rosgraph_msgs/Clock "clock: { sec: 1, nanosec: 200000000 }"
   

   Verify that the timeStamp value in the ROS2 Subscribe Clock OmniGraph node changes to 1.2.

5. Change the previous command with different sec and nanosec values to observe those values being reflected in the timeStamp field of the ROS2 Subscribe Clock OmniGraph node.

Graph Shortcut

We provide a menu shortcut to build a clock graph within just a few clicks. Go to Tools > Robotics > ROS 2 OmniGraphs > Clock. If you don’t observe any ROS2 graphs listed, you need to enable the ROS2 bridge. A popup box will appear asking for the parameters needed to populate the graphs. Provide the graph path and click OK, verify that a graph publishing the simulated clock appears on the stage.

Summary

This tutorial covered:

1. Explanation for using the /clock topic and the use_sim_time ROS parameter for time synchronization.

2. Creating and using ROS2 Clock Publisher and Subscriber nodes.

Next Steps

Continue on to the next tutorial in our ROS2 Tutorials series, ROS 2 Publish Real Time Factor (RTF).