Publishing joint velocity with joint_state_publisher

edit

I'm new to ROS. Currently I am following a tutorial on how to publish joint states of your robot: link here. I'm trying to apply this to my own robot. I'm able to publish joint positions and visualize them in Rviz without any problem.

However, when I try publishing joint velocities from sensor_msgs. It doesn't work. If I don't publish joint positions, but only joint velocities, the program does not run. There are missing links and joints (apart from my base link) in the tf tree. And if I do publish the joint positions, my joint velocities get overwritten by the joint positions and the robot does not move.

Any suggestions on this issue? Thanks in advance

Edit: My code has the same structure as in this tutorial.

while (ros::ok()) {
    //update joint_state
    joint_state.header.stamp = ros::Time::now();
    joint_state.name.resize(6);
    joint_state.position.resize(6);
    joint_state.name[0] ="joint1";
    joint_state.position[0] = joint1_trans;
    joint_state.name[1] ="joint2";
    joint_state.position[1] = joint2_rot;
    joint_state.name[2] ="joint3";
    joint_state.position[2] = joint3_rot;
    ...
}

which are published by: joint_pub.publish(joint_state);

Now, I'm trying to set the velocities in a similar manner: joint_state.velocity[0] = ...:

joint_state.name.resize(6);
joint_state.position.resize(6);
joint_state.velocity.resize(6);
joint_state.name[0] ="joint1";
joint_state.position[0] = joint1_trans;
joint_state.velocity[0] = joint1_vel;
joint_state.name[1] ="joint2";
joint_state.position[1] = joint2_rot;
joint_state.velocity[1] = joint2_vel;
...

but this does not work. The joint_state.position[0] overwrites this and nothing happens in Rviz. If I leave out joint_state.position[0], my robot gets messed up in Rviz.

rostopic echo /joint_states gives the following:

header: 
  seq: 375
  stamp: 
    secs: 1571488752
    nsecs: 533079554
  frame_id: ''
name: [joint1, joint2, joint3, joint4, joint5, joint6]
position: [0.0, 0.0, 0, 0.0, 0.0, 0.0]
velocity: [0.2, 0.2, 0.0, 0.0, 0.0, 0.0]
effort: []

There you can see my messages are coming through, but these velocities are not getting displayed by Rviz. Because the joint position stays on 0. Is there a way to only send joint.velocity, and also getting the joint.position updated and displayed correctly in Rviz?