Exercise 3.2 in ros in 5 days with python

i cant see where to publish the messages and got really confused . at last when i saw the solution it was publishing to /cmd_vel topic which is not mentioned in exercise.

Hi Ashish,
can you please clarify what you mean? I have checked the exercise 3.2 of the course of ROS in 5 days Python and there is nothing related to /cmd_vel (actually the robot used is an industrial arm, so doesn’t receive any speed command).

Please clarify what is your problem so I can help you solve it

after example 3.7, In Exercise 3.2 there is nowhere written to publish into ‘/cmd_vel’ topic for movement of bb8 robot in circular path.
but ‘/cmd_vel’ topic is used in the solution given.

you might have seen example 3.2.
but was asking about exercise 3.2.
hope you got this!

Ok, Ashish, I got it. Yes I was confused by example 3.2

Now I see what you mean. The /cmd_vel topic is not mentioned in the execise because at that point of the course, you should have already understood that in order to move a ROS robot, you have to publish a Twist message in a speed topic. Which speed topic? That is something that you should find for every robot.

Usually, for many robots, the speed topic is the /cmd_vel, but that is not mandatory, so you have to use a rostopic list and rostopic info <topic> to identify which topic you need to use for moving the robot.

The exercise mentions that you have to move the robot. The rest is up to you to figure out how to do it. Please believe me, you need to do that effort and exercise in order to really understand ROS (that is why we do not put exercises that are just copy/paste).

I don’t know if you did the previous exercises, but the previous ones were created in order to build you that knowledge up to that point. All the exercises build on top of the previous ones. So please, do not skip any exercise, because they are there with a purpose.


I haven’t skipped any of them but was not sure about using this ‘/cmd_vel’ topic for this exercise.
Thanks a lot for helping me out!

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