Hi, I'm running Ubuntu 18.04, melodic.

I'm very new to ROS in general (1 week new), so hopefully you guys can be a bit more patient. So anyway, I wrote a Python publisher for my (differential drive) car. It supposed to send the joint state of my two wheels to RVIZ, and in RVIZ, I supposed to see my car moving with the set speed coming from python side.

It didn't happen. I think maybe I'm missing something, or misunderstood something? If you guys can point that out, it would be great!

Here's my Python side:

Hi, I'm very new to ROS in general (1 week new), so hopefully you guys can be a bit more patient. So anyway, I wrote a Python publisher for my (differential drive) car. It supposed to send the joint state of my two wheels to RVIZ, and in RVIZ, I supposed to see my car moving with the set speed coming from python side.

It didn't happen. I think maybe I'm missing something, or misunderstood something? If you guys can point that out, it would be great!

Here's my Python side:

#!/usr/bin/env python

import rospy
from std_msgs.msg import Header
from sensor_msgs.msg import JointState

def talker():
    pub = rospy.Publisher('joint_states', JointState, queue_size=10)
    rospy.init_node('joint_state_publisher', anonymous = True)
    rate = rospy.Rate(10) # 10hz
    while not rospy.is_shutdown():
        jointState_msg = JointState()
        jointState_msg.header = Header()
        jointState_msg.header.stamp = rospy.Time.now()
        jointState_msg.name = ['left_wheel_holder_to_left_wheel', 'right_wheel_holder_to_right_wheel']
        jointState_msg.position = []
        jointState_msg.velocity = [1, 1]
        jointState_msg.effort = []

        pub.publish(jointState_msg)

        rate.sleep()

if __name__ == '__main__':
try:
    talker()
except rospy.ROSInterruptException:
    pass

Here's my launch file:

<launch>
    <param name="robot_description" textfile="$(find rover_project)/rover_description/urdf/rover.urdf"/>

    <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />

    <node name="rviz" pkg="rviz" type="rviz" args="-d $(find rover_project)/rover_description/launch/rviz_config.rviz" />

    <node pkg="robot_state_publisher" type="robot_state_publisher" name="rob_st_pub">
        <param name="use_gui" value="True"/>
    </node>
    <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher">
        <param name="use_gui" value="True"/>
    </node>

</launch>

And most importantly, and this is where I think went wrong, my urdf file:

<?xml version="1.0"?>

<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="Rover Platform">

  <material name="blue">
    <color rgba="0 0 0.8 1"/>
  </material>

  <material name="grey">
    <color rgba="0.2 0.2 0 1"/>
  </material>   

  <xacro:macro name="default_origin">
    <origin rpy="0 0 0" xyz="0 0 0"/>
  </xacro:macro>


  <link name="base_link">
    <visual>
      <geometry>
        <box size="1.0 1.0 0.2"/>
      </geometry>
      <xacro:default_origin />
      <material name="blue"/>

    </visual>

    <collision>
      <geometry>
        <box size="1.0 1.0 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="5"/>
      <inertia ixx="0.83333" ixy="0.0" ixz="0.0" iyy="0.433333" iyz="0.0" izz="0.433333"/>
    </inertial>

  </link>

  <link name="right_wheel_holder_link">

    <visual>
      <xacro:default_origin />
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>

    <collision>
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="1"/>
      <inertia ixx="0.011" ixy="0.0" ixz="0.0" iyy="0.0067" iyz="0.0" izz="0.011"/>
    </inertial>

  </link>

  <link name="left_wheel_holder_link">

    <visual>
      <xacro:default_origin />
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>

    <collision>
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="1"/>
      <inertia ixx="0.011" ixy="0.0" ixz="0.0" iyy="0.0067" iyz="0.0" izz="0.011"/>
    </inertial>

  </link>

  <link name="right_wheel_link">

    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <cylinder length="0.05" radius="0.25"/>
      </geometry>
      <material name="grey"/>
    </visual>

    <collision>
      <geometry>
        <cylinder length="0.05" radius="0.25"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="1"/>
      <inertia ixx="0.00021" ixy="0.0" ixz="0.0" iyy="0.0" iyz="0.0" izz="0.00021"/>
    </inertial>

  </link>

  <link name="left_wheel_link">

    <visual>
        <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <cylinder length="0.05" radius="0.25"/>
      </geometry>
      <material name="grey"/>
    </visual>

    <collision>
      <geometry>
        <cylinder length="0.05" radius="0.25"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="1"/>
      <inertia ixx="0.00021" ixy="0.0" ixz="0.0" iyy="0.0" iyz="0.0" izz="0.00021"/>
    </inertial>

  </link>

  <joint name="base_to_right_wheel_holder" type="fixed">
    <parent link="base_link"/>
    <child link="right_wheel_holder_link"/>
    <origin rpy="0 0 0" xyz="0.4 0.2 -0.2"/>
  </joint>

  <joint name="base_to_left_wheel_holder" type="fixed">
    <parent link="base_link"/>
    <child link="left_wheel_holder_link"/>
    <origin rpy="0 0 0" xyz="-0.4 0.2 -0.2"/>
  </joint>

  <joint name="left_wheel_holder_to_left_wheel" type="continuous">
    <axis xyz="0 0 1"/>
    <parent link="left_wheel_holder_link"/>
    <child link="left_wheel_link"/>
    <origin rpy="0 -1.57075 0" xyz="-0.15 0 0"/>
  </joint>

  <joint name="right_wheel_holder_to_right_wheel" type="continuous">
    <axis xyz="0 0 1"/>
    <parent link="right_wheel_holder_link"/>
    <child link="right_wheel_link"/>
    <origin rpy="0 1.57075 0" xyz="0.15 0 0"/>
  </joint>

  <transmission>
    <type>transmission_interface/SimpleTransmission</type>
    <joint name="left_wheel_holder_to_left_wheel">
      <hardwareInterface>EffortJointInterface</hardwareInterface>
    </joint>
    <actuator name="motor_left">
      <hardwareInterface>EffortJointInterface</hardwareInterface>
      <mechanicalReduction>1</mechanicalReduction>
    </actuator>

    <joint name="right_wheel_holder_to_left_wheel">
      <hardwareInterface>EffortJointInterface</hardwareInterface>
    </joint>
    <actuator name="motor_right">
      <hardwareInterface>EffortJointInterface</hardwareInterface>
      <mechanicalReduction>1</mechanicalReduction>
    </actuator>

  </transmission>


</robot>

I also did not modify my CMakeLists file, I wonder if I have to though...