User_App_ROS_GazeboSimulationExample

2.2 Gazebo Simulation Example

Note: This tutorial assumes that you have completed the previous tutorials:

Overview

This application of ROS# illustrates the communication between Unity and ROS in which the real time simulation takes place in Gazebo on the ROS side. The following figure illustrates the general schema of the application.

General Picture

For simplicity, an existing Gazebo simulation is used to simulate the TurtleBot2 on the ROS side. The control signals are sent from Unity to ROS. Furthermore, the outcomes of ROS are captured by Unity for illustrative purposes.

Messages to be sent by Unity using ROS#:
/sensor_msgs/joy
Topics to be subscribed by Unity using ROS#:
/odom
/joint_states
/camera/rgb/image_raw/compressed

A joystick connected to Unity is used to control the TurtleBot2. Messages of the type /sensor_msgs/joy are sent to the rosbridge_websocket to be captured by ROS. The existing Gazebo simulation for TurtleBot2 requires messages of the type /geometry_msgs/twist on the ROS side. Therefore, the captured /sensor_msgs/joy messages are converted into /geometry_msgs/twist messages. This is accomplished by the ROS node joy_to_twist.

The Gazebo simulation publishes the topics /odom, /joint_states and /camera/rgb/image_raw/compressed, which are captured by Unity using rosbridge_websocket. Finally, the captured messages are linked to the URDF model in Unity.

Preparation

Setting up the Unity scene: Linking URDF to ROS#

Import the Unity scene GazeboSimulationScene.unity from here and the required Unity scene will be complete and ready to use.
OR: Create the Unity scene from scratch by reproducing the steps shown in this video to learn more about particular components and how they interact.
Make sure that you followed this tutorial. You should already have imported the URDF model to Unity.

Setting up ROS

Make sure that you followed this and this tutorial.
Place the folder gazebo_simulation_scene (from here) inside the src folder of your workspace and rebuild your workspace.
In the directory gazebo_simulation_scene/scripts make the file joy_to_twist.py executable by running

chmod +x joy_to_twist.py

Run the following command in your terminal:

$ roslaunch gazebo_simulation_scene gazebo_simulation_scene.launch

This will launch rosbridge_websocket, file_server, joy_to_twist, rqt_graph and a Gazebo simulation of the TurtleBot2 in the default turtlebot_world

Execution

As soon as all ROS nodes are launched, the robot in both simulations is ready to move.

Make sure that the joystick is connected to Unity, which can be verified by checking the Unity console.
When the Play button in pressed, the ROS terminal will show that a client has connected and subscribes to some topics:

[INFO] [1520503573.897354, 57.560000]: Client connected.  1 clients total.
[INFO] [1520503574.689767, 58.330000]: [Client 0] Subscribed to /camera/rgb/image_raw/compressed
[INFO] [1520503574.697262, 58.340000]: [Client 0] Subscribed to /joint_states
[INFO] [1520503574.736412, 58.380000]: [Client 0] Subscribed to /odom

After clicking the refresh button in the rqt_graph, a network similar to the following figure appears:
It can be seen here that the topic/sensor_msgs/joy is published by Unity, whereas the topics /odom, /joint_states and /camera/rgb/image_raw/compressed are subscribed by Unity using rosbridge_websocket.
Pressing the buttons of the controller will move the robot in Gazebo and in Unity. Furthermore, the camera image of the TurtleBot2 in Gazebo is projected onto a plane in Unity for illustrative purposes.