addTopic

Add ROS 2 topic for time-synchronization of messages

Since R2026a

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Syntax

addTopic(syncObj,topic)

Description

Use the addTopic function to add ROS 2 topics from the network for time synchronization to a ros2topicsynchronizer object. To add a topic, its message type must contain the std_msgs/header field. To initiate time synchronization after adding all the desired topics, use the synchronize object function.

addTopic(syncObj,topic) adds the topic with the name topic from the ROS 2 network to the ros2topicsynchronizer object syncObj. You can use this syntax multiple times with the same ros2topicsynchronizer object to add up to nine topics for time synchronization.

example

Examples

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Open Live Script

This example shows how to use the ros2topicsynchronizer object to get sets of time-synchronized messages from multiple ROS 2 topics publishing at different frequencies.

Read all the messages from the ROS 2 bag file. The bag file contains odometry and laser scan messages which are published at different rates. The odometry messages are published at 25 Hz and the laser scan messages are published at 5 Hz. 

unzip("ros2_netwrk_bag.zip");
bagReader = ros2bagreader(fullfile(pwd,"ros2_netwrk_bag"));
odomMsgs = readMessages(select(bagReader,Topic="/odom"));
scanMsgs = readMessages(select(bagReader,Topic="/scan"));

Create two publishers /odom and /scan, which can publish odometry and laserscan messages.

publishNode = ros2node("publisherNode");
odomPub = ros2publisher(publishNode,"/odom","nav_msgs/Odometry",Durability="transientlocal",Depth=10);
scanPub = ros2publisher(publishNode,"/scan","sensor_msgs/LaserScan",Durability="transientlocal",Depth=10);

Create a ros2topicsynchronizer object which uses approximate epsilon time policy to time-synchronize messages. This means that sets of messages with timestamps between an epsilon tolerance are considered to be time-synchronized. Also, to ensure that messages from topics published at higher frequencies are not lost while waiting for those published at lower frequencies, specify the queue size as at least twice the highest frequency.

Use the syncPolicySettings structure to specify:

  • Epsilon tolerance — 0.05 seconds

  • Queue size — 50

Specify the syncOdomScan function as the callback to execute each time a set of time-synchronized messages become available. The syncOdomScan function prints the individual timestamps of odometry and laser scan messages in each time-synchrnized set.

syncPolicySettings = struct("MessageQueueSize",uint32(50),"Epsilon",0.05);
syncObj = ros2topicsynchronizer("ApproximateEpsilonTime",SyncCallback=@syncOdomScan,PolicySettings=syncPolicySettings);

Add the two topics /odom and /scan for time-synchronization using the addTopic object function.

addTopic(syncObj,"/odom")
addTopic(syncObj,"/scan")

Initiate time-synchronization using the synchronize object function.

synchronize(syncObj)

Before publishing messages, create separate ros2rate objects for each topic to control publishing messages at different frequencies.

odomRate = ros2rate(publishNode,25);
scanRate = ros2rate(publishNode,5);

Start publishing messages on the /odom and /scan topics.

j = 1;
i = 1;

nextOdomPubTime = odomRate.TotalElapsedTime;
nextScanPubTime = scanRate.TotalElapsedTime;

while i <= numel(odomMsgs) || j <= numel(scanMsgs)
    currentTime = odomRate.TotalElapsedTime;
    if currentTime >= nextOdomPubTime && i <= numel(odomMsgs)
        send(odomPub,odomMsgs{i});
        i = i+1;
        nextOdomPubTime = nextOdomPubTime + odomRate.DesiredPeriod;
    end

    if currentTime >= nextScanPubTime && j <= numel(scanMsgs)
        send(scanPub,scanMsgs{j});
        j = j+1;
        nextScanPubTime = nextScanPubTime + scanRate.DesiredPeriod;
    end

    waitfor(odomRate);
end

You can also view the latest set of time-synchronized odometry and laser scan messages using the LatestSynchronizedMessages property.

syncMsgs = syncObj.LatestSynchronizedMessages;
syncMsgs{1}
ans = struct with fields:
       MessageType: 'nav_msgs/Odometry'
            header: [1×1 struct]
    child_frame_id: 'base_footprint'
              pose: [1×1 struct]
             twist: [1×1 struct]


syncMsgs{2}
ans = struct with fields:
        MessageType: 'sensor_msgs/LaserScan'
             header: [1×1 struct]
          angle_min: 0
          angle_max: 6.2832
    angle_increment: 0.0175
     time_increment: 0
          scan_time: 0
          range_min: 0.1200
          range_max: 3.5000
             ranges: [360×1 single]
        intensities: [360×1 single]

The syncOdomScan function receives two input arguments odomMsg and scanMsg. These input arguments correspond to a time-synchronized set of odometry and scan messages.

function syncOdomScan(odomMsg,scanMsg)
    t1 = double(odomMsg.header.stamp.sec) + 1e-9*double(odomMsg.header.stamp.nanosec);
    t2 = double(scanMsg.header.stamp.sec) + 1e-9*double(scanMsg.header.stamp.nanosec);
    fprintf("OdomMsgTime: %.3f ScanMsgTime: %.3f\n",t1,t2);
end

Input Arguments

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ROS 2 topic synchronizer, specified as a ros2topicsynchronizer object. The ros2topicsynchronizer manages the time synchronization of messages across all the added topics.

Name of the topic to add for time synchronization, specified as a string scalar or character vector.

Messages in the specified topic must contain the std_msgs/header field.

Version History

Introduced in R2026a

See Also

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