Queries about my differential drive project

Hello, community!

First of all, I would like to ask if I can do my project in constructsim because I do not have a laptop or desktop with Linux to do simulation?

Let me move on to my main question, Ι have a question about the project.
I saw the course on the basic kinematics of a mobile robot, but up to the point of a differential drive section 3.3. I was able to find the equations that give the wheel speeds (right and left) as you describe in 3.2, I also saw exercise 3.2 with its solution, but I was not able to understand it to modify my template code according to the solution.

Below are the tests I did in the code to print the values Vl and Vr.

#!/usr/bin/env python

Test ROS Node

import rospy, roslib
import operator
import time
import sys
#import pigpio
from math import sin, cos, atan2, pi, sqrt
import numpy as np
from std_msgs.msg import Float64
from geometry_msgs.msg import Twist

class TestNode():
    """Test node for templating"""
    def __init__(self):

        #ROS subscriber
        self.sub = rospy.Subscriber('/cmd_listener', Twist, self._callback, queue_size=1)
        self.twist_vels = Twist()
        self.vr = Float64()
        self.vl = Float64()
        self.rate = rospy.Rate(1)

    def _callback(self, msg):
        L = 0.4  # L = 40cm
        R = 0.05 # R = 5cm
        self.twist_vels = msg
        self.vr = ((2*self.twist_vels.linear.x) + (self.twist_vels.angular.z*L))/(2*R)
        self.vl = ((2*self.twist_vels.linear.x) - (self.twist_vels.angular.z*L))/(2*R)
        print(self.vr, self.vl)
    def publish(self):
        msg2pub = String()
        msg2pub.data = 'test_msg'
        while not rospy.is_shutdown():


    def turn_off_node(self):
        Execute here the code when the node is terminated Ctrl + c

if __name__ == '__main__':
    #Initialize the node

    #Create test node object
    tn = TestNode()
    #Handle shutdown

Eventually, I could not see the speed values, what mistake did I make in the code?

Thanks in advance!

Hello @vasileios.tzimas ,

Yes, you can do any project using TheConstruct platform. Remember that rosjects can only have up to 2GB of files, but it is enough for a big variety of projects.

I noticed you are mentioning about sections of the course. Are you running a similar simulation in the rosject? Are you running inside the course environment?


No, I tried the following code in my school lab on a Raspberry Pi with Ubuntu.

I understand.

In that case you must make sure you have messages arriving to the topic /cmd_listener, since you are expecting to have these values printed in the _callback method, right?

You can check that by using the commands:

  • rostopic list
  • rostopic echo

Do you have a node publishing to this topic?

It would be great if you can create a rosject where I can reproduce your environment and share with me. Could you do that? It would be easier to check your code


Yes, I do, dear. Let me think about how to do that! Give me some time!

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I think I created it right

Hello, did you see the rosject?

I have created a package named mobile_robot_diff_drive

Hi @vasileios.tzimas ,

Yes, I got it. But I see a script slightly different than the one you posted in the first moment.
You mention that you couldn’t see the calculated speeds, let me show you the result I got here using your rosject and maybe you can point me out what is missing. Here it is possible to see the calculated speeds being published in a second terminal like I show in the video below:

What was the expected result up to this point?


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Yes yes… I managed to see the result with the launch file.
If you look now I did it last night.
But is there a way to combine the two python files into one and just run that? That is, you do not need a launch file.

I understand.

If you want to keep both nodes in a single file, I would not recommend and I don’t think it is possible, as far as I know. This is the reference I found related to that: Is it possible to launch multiple nodes from one python script? - ROS Answers: Open Source Q&A Forum

If you need both nodes in the same file to launch at the computer startup, I suggest to look on how to do that using roslaunch:

Is that the case?

If you don’t need both nodes, but a single node that publishes and subscribes, yes this is totally possible. For example, from pub_vels.py to calc_speed.py, you would need to define the publisher:

pub = rospy.Publisher('/twist_vels', Twist, queue_size=1)

to the class and you can use inside the same loop, no problem at all. The thing is that you would need a logic if you want to setup different publishing rates to each one.

Is the topic published by pub_vels.py being subscribed by another node? The robot for example? If not, you could get rid of this topic and include the logic inside calc_speed.py, for example