gz topic not publishing

Question

I have a fresh installation of Ubuntu 22.04 in a VM VirtualBox, fully updated. I compiled Gazebo Harmonic from source code and followed the "Building your own robot" tutorial creating the building_robot.sdf and then move the vehicle with the keyboard...

The problem is that when, from another terminal I type the command:

gz topic -t "/cmd_vel" -m gz.msgs.Twist -p "linear: {x: 0.5}, angular: {z: 0.05}"

the message does not reach Gazebo and the vehicle does not move. But if you keep on sending the same command about 10 - 12 times, all of a sudden the vehicle starts to move...!

This is my diff-drive configuration:

        <plugin
            filename="gz-sim-diff-drive-system"
            name="gz::sim::systems::DiffDrive">
            <left_joint>left_wheel_joint</left_joint>
            <right_joint>right_wheel_joint</right_joint>
            <wheel_separation>1.2</wheel_separation>
            <wheel_radius>0.4</wheel_radius>
            <odom_publish_frequency>1</odom_publish_frequency>
            <topic>cmd_vel</topic>
        </plugin>
    </model>                

Can someone pls explain why this is so? What am I missing or doing wrong?

Thanks in advance for any suggestions,

Marco

Answer 1

I've recreated your issue on the lastest harmonic and I indeed see the same.

The steps to recreate it (for reference for others:

Started the sim with (I used the finished version which is called moving_robot.sdf)

gz sim moving_robot.sdf

And only angular

gz topic -t "/cmd_vel" -m gz.msgs.Twist -p "linear: {x: 0.5}"

Indeed no response! Eventhough it is clearly subscribed when I check with

$gz topic --info -t /model/vehicle_blue/cmd_vel
No publishers on topic [/model/vehicle_blue/cmd_vel]
Subscribers [Address, Message Type]:
  tcp://172.19.0.1:40489, google.protobuf.Message

Apparent solution

However, when I started the gazebo and the world with -r, it does respond to the twist commands! I don't see any difference in the above subcription test either

gz sim -r moving_robot.sdf

I've tried this on multiple Ubuntu installs and reinstalled harmonic multiple times both from binaries and source and on different types of model so I see the same thing. So perhaps its a bug that we have found. I'll see where the best place is to report it, but probably the gz-sim repo?

Update

It's not responding because it's paused and the -r starts gazebo immediately upon startup. There is even a note in the tutorial and I have missed it as well at first.