ROS course 107 Make a client for rosblidge

environment

This article works in the following environment.

For installation, refer to ROS Course 02 Installation. Also, the program in this article has been uploaded to github. See ROS Course 11 git repository.

Overview

Last time I set up rosbridge_server and introduced an example of accessing from javascript of the browser. This time, use roslibpy to create a client in python that connects to rosbridge_server.

Installation

pip problem

This time we will use pip to install roslibpy. The trouble with pip (rather than python) is that there are various directories to install, and mixing them can be quite annoying. For example

• System area (/usr/local/lib/python2.7/) When you get pip with apt ・ When you get it with sudo pip
• User area (~ / .local / lib / python2.7 /) When entered with pip --user

This will pollute the environment and cause an error in pip when running the following command

I get an error after doing this

sudo apt install python-pip
pip install --upgrade pip

There are many problems with using apt and pip together, so it is better to install without using apt. If you have done the above, you can uninstall all pips with the following command.

Uninstall pip

sudo python -m pip uninstall pip
sudo apt autoremove python-pip

Command to execute

You can install roslibpy by executing the following.

Install roslibpy

curl -kL https://bootstrap.pypa.io/get-pip.py | sudo python
sudo pip install roslibpy
sudo pip install service_identity

Source code

web_lecture/scripts/rosbridge_client.py

#!/usr/bin/env python
# -*- coding:utf8 -*-
import time
from roslibpy import Message, Ros, Topic
import time

class rosbridge_client:
    def __init__(self):
        self.ros_client = Ros('127.0.0.1', 9090)
        print("wait for server")
        self.publisher = Topic(self.ros_client, '/cmd_vel', 'geometry_msgs/Twist')
        self.listener = Topic(self.ros_client, '/odom', 'nav_msgs/Odometry')
        self.listener.subscribe(self.callback)

        self.ros_client.on_ready(self.start_thread, run_in_thread=True)
        self.ros_client.run_forever()

    def callback(self, message):
        x = message["pose"]["pose"]["position"]["x"]
        y = message["pose"]["pose"]["position"]["y"]
        print(x, y)

    def start_thread(self):
        while True:
            if self.ros_client.is_connected:
                self.publisher.publish(Message({
                    'linear': {
                        'x': 0.5,
                        'y': 0,
                        'z': 0
                    },
                    'angular': {
                        'x': 0,
                        'y': 0,
                        'z': 0.5
                    }
                }))
            else:
                print("Disconnect")
                break
            time.sleep(1.0)

if __name__ == '__main__':
    rosbridge_client()

• Set up a connection to rosbridge_server with self.ros_client = Ros ('127.0.0.1', 9090).
• Determine if you are connected to rosbridge_server with self.ros_client.is_connected.
• Attempts to reconnect with self.ros_client.connect () even if it is broken.
• Write the message in json with Message () and publish it with self.publisher.publish ().
• Register the callback when subscribing with self.listener.subscribe (self.callback). You can pass the received message as an argument of the callback function and access it like x = message ["pose "] [" pose "] ["position "] ["x "].
• You can create a thread with self.ros_client.on_ready (self.start_thread, run_in_thread = True).

Run

Run sim

roslaunch web_lecture web_if.launch 

Run roslibpy client

roscd web_lecture/scripts && rospy_client.py`You can do it with.

Use Case

rosbridge is a tool for accessing ros from a system where ros is not originally installed. However, if you use this loosely coupled system, you can also build a multi-master system as shown in the figure below.

reference

roslibpy example Details of implementation

Link to table of contents page

Link to ROS course table of contents