Open_scout_ROS

4. ROS 4.1. Understanding ROS Initially I have studied what is ROS and why it is very helpful for robotics. It helps me to understand why simulation is needed and why simulation of robot will help us to debug our problem. It is open source platform and we can use packages to reduce time in building software from scratch. Because of more robotics packages available in opensource, It will be helpful for our open scout project to reduce time of building software from scratch if we extend this project in future use. Vision packages, gazebo simulation, rviz and more robotics tools can be used if the project gets bigger. 4.2 Understanding ROS Topic publisher, subscriber and ROS Node ROS topic is used to communicate messages from one node to another node. Ros topic pub is used to publish messages from command line. I have tested publishing string message in one terminal and echoing the same string message by using rostopic echo. I have tested how much the rate of message is being published by giving values of 1, 5, 10 seconds. I have used the command line and created python file to test this scenarios. rostopic echo -n 1 /topic_name to test the string message being published. rostopic list used to check active topic. Similarly tried with rosnode to see current active node. 4.3 Creating ROS node Publisher node and subscriber node is created in python file with topic name “chatter” to check the string message is being published and received. This nodes are tested since I was making a bridge between MQTT and ROS. 4.4 Understanding MQTT protocol MQTT is light weight protocol used for connections with remote locations and it exchanges data between devices and server. Arduino command velocity messages are sent to MQTT server and MQTT server will send it to ROS node. 4.5 Understanding MQTT publisher and subscriber Mosquitto pub command is used to test in command line to check how MQTT protocol works. Similarly python file has been created for publisher and subscriber to test communication between subscriber and publisher. 4.6 Creating Bridge between ROS and MQTT We have to create a bridge between ROS and MQTT to communicate each other. The protocol used by mqtt bridge is ROS message. JSON (or message pack) is used to serialize messages from ROS for MQTT, while message pack is used to deserialize MQTT messages for ROS topic. So MQTT messages ought to be compatible with ROS messages. (For message conversion, we employ rosbridge library internal message conversion) Tested by sending some messages from MQTT to ROS and ROS to MQTT. Fig 2. MQTT ROS bridge Source: http://wiki.ros.org/mqtt_bridge 4.7 Receiving Arduino Command velocity from MQTT to ROS Arduino will send command velocity message on MQTT server. Tested sending of command velocity on command line by using mosquito pub command. MQTT message will be deserialized and stored according to Twist ROS message. mqtt_ros/receive_arduino_msg topic has been used to receive message from MQTT. 4.8 Sending Joystick Command Velocity to Arduino from ROS to MQTT ROS will send the Joystick command velocity to Arduino. This has been tested by sending command velocity “x”:data.linear.x , “z”:data.angular.z. Linear velocity is used to move robot forward and angular velocity is used to turn the robot. ros_mqtt/cmd_vel used to publish message from ROS to MQTT.