Transfer of a msg to gazebo from ROS to control the simulated turtlebot3 [closed]

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Hey,

I am new to ROS and Gazebo and just try to understand how I can control and command the turtlebot3. I followed the instructions of the common tutorial website and everything went nice. I also built up new worlds etc. Also, when I roslaunch the gazebo world and the corresponding dqn file (for Reinforcement Learning and controlling), a cmd_vel topic is used which is published by the dqn node and subscribed by Gazebo.

Node Overview: C:\fakepath\Bildschirmfoto vom 2019-06-12 13-28-26.png

Here I get two questions. The topic only contains data for linear and angular position

self.pub_cmd_vel = rospy.Publisher('cmd_vel', Twist, queue_size=5)

Twist contains:

marcel@marcel-macbookpro:~$ rosmsg show geometry_msgs/Twist
geometry_msgs/Vector3 linear
  float64 x
  float64 y
  float64 z
geometry_msgs/Vector3 angular
  float64 x
  float64 y
  float64 z

Anyway, this topic is filled with velocity informations and subscribed by gazebo to control the robot:

def cmd_vel(self, action):
        max_angular_vel = 1.5
        ang_vel = ((self.action_size - 1)/2 - action) * max_angular_vel * 0.5

        vel_cmd = Twist()
        vel_cmd.linear.x = 0.15
        vel_cmd.angular.z = ang_vel
        self.pub_cmd_vel.publish(vel_cmd)

My two big questions are now, how can Twist be filled with velocity information and how is gazebo using this information to control the turtlebot. I mean, there is no node between the dqn_node and gazebo that translates those linear and angular information in roation for the wheels i.e.. Is Gazebo doing this by itself?

Thanks for your help!