ROS2 Clock
Learning Objectives
In this example, we will
Have a brief discussion on the ROS2 Clock publisher and subscriber
Publish simulation time to ROS2 as a Clock message
Subscribe to a ROS2 Clock message
Add a Clock Action Graph using the menu shortcut
Getting Started
Important
Make sure to source your ROS 2 installation from the terminal before running Isaac Sim. If sourcing ROS 2 is a part of your bashrc
then Isaac Sim can be run directly.
Prerequisite
Complete ROS and ROS 2 Installation.
Set the
FASTRTPS_DEFAULT_PROFILES_FILE
environment variable per instructions in ROS and ROS 2 Installation before launching Isaac Sim, as well as any terminal where ROS messages will be sent or received, and ROS2 Extension is enabled.
Note
In Windows 10 or 11, depending on your machine’s configuration, RViz2 may not open properly.
Simulation Time and Clock
In order for external ROS2 nodes to synchronize with simulation time, a clock topic is usually used. Many ROS2 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 ROS2 launch file or set the parameter using the following command in a new ROS2-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
Warning
ROS 2 Foxy is no longer tested or supported. This may result in potential issues when ROS 2 Foxy is used in conjunction with Isaac Sim 4.2 or later.
Go to Create -> Visual Scripting -> Action Graph to create an Action graph.
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 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 may aries 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.ROS2 Publish Clock node to publish simulation time to the
/clock
topic.
Start RViz in a new ROS2-sourced terminal.
ros2 run rviz2 rviz2
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 should match the Wall Time and Wall Elapsed fields.
Run the following command in a new ROS2-sourced terminal and take note of the subscribers listed.
ros2 node info /rviz
In a new ROS2-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
Notice in RViz that ROS Time and ROS Elapsed are now both 0.
Run the following command and take note of the subscribers listed. The RViz window should now be subscribed to the
/clock
topic.ros2 node info /rviz
In Isaac Sim click Play.
In RViz, the ROS Time is now identical to the simulation time published from Isaac Sim over the
/clock
topic.RViz, will now synchronize to the time published from Isaac Sim over the
/clock
topic.
Publishing System Time
While publishing the simulation time is the most common workflow, there may be certain workflows that require certain messages to contain system time. To publish system time over the clock topic follow these steps:
Go to Create -> Visual Scripting -> Action Graph to create an Action graph.
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.
In Isaac Sim click Play. To see 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
You have already seen the ROS2 Camera Helper node in a previous tutorial and you will see the ROS2 RTX Lidar Helper node in a following tutorial. As both of these nodes automatically generate a sensor publishing pipeline, to use system timestamps for their publishers, ensure that their useSystemTime
input is set to True.
Running ROS 2 Clock Subscriber
Open a new stage. Go to Create -> Visual Scripting -> Action Graph to create an Action graph.
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.
Start simulation by clicking Play. Select the ROS2 Subscribe Clock node inside the action graph to view its
timeStamp
output in the Property window. The timestamp should be currently 0.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 }"
The
timeStamp
value in the ROS2 Subscribe Clock OmniGraph node should change to 1.2.Change the previous command with different sec and nanosec values to see 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 Isaac Utils > Common Omnigraphs > ROS2 Clock. (If you don’t see any ROS2 graphs listed, you need to first 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”, a graph publishing the simulated clock should appear on the stage.
Summary
This tutorial covered
Explanation for using the
/clock
topic and theuse_sim_time
ROS parameter for time synchronization.Creating and using ROS2 Clock Publisher and Subscriber nodes.
Next Steps
Continue on to the next tutorial in our ROS2 Tutorials series, RTX Lidar Sensors, to learn how to add a RTX Lidar sensor to the Turtlebot3.