Unclear errors when working with ROS

1. The first place i am giving to a bunch of ROS with Gazebo (this is a simulator). The script is as follows: by the launch file the gazebo starts with the needed world, the robot model are spawned there, and then the controllers are started. You sit in the console, start the launch, look rostopic list – everything is fine, but when trying to listen any topic (rostopic echo /joint_states) – silence.
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Ros loses messages

Let’s take the simplest example (eg, Python’s) and update it slightly to our needs:

rospy.init_node('test')
pub = rospy.Publisher('hello', String, queue_size=10)
rate = rospy.Rate(250) # 250hz
for i in range(10):
    pub.publish("hello world"+str(i))
    rate.sleep()

And it seems to be clear what to expect in the subscriber (lets just output to the console that what came), but we can see:
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Rospy import dancing

I would like to share a story about how I killed a couple of hours recently. I write on rospy a small architecture to work with a robot. Since there are no good examples and I’m not an expert in the python, without thinking twice, I added the path to the project (let it be ~/work/src/coolp) in PYTHONPATH. Then I added empty __init__.py directly to the root of proper packages ( eg. coolp/control/__ init__.py next to the scripts that you want to import coolp/control/control_arm.py) and now at any place where I need this script – from control.control_arm import LeftArm.

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ROS impression

I have to tell you right away that I’ve been working with ROS for about 3 months what is not much but fairly enough. What I’ve sensed:

Advantages:

  • Idea. The basis of ROS is a set of nodes that communicate with each other such as a graph. In case of death of one node the others continue to communicate. In the real world this is not uncommon and thus the robot’s vitality rises in times. Well what of it if the robot’s hand has disconnected when it is not used now, because hand will be rebooted by the time of need.

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