cassander's profile - activity

The rospy API docs have a list of modules that are part of the rospy library. For someone wanting to write nodes using rospy, which modules in the api docs should he/she be concerned with. The reason I ask is that some of those modules are marked internal only. Does that mean people writing nodes need not be concerned with them?

I have a few questions regarding the turtlebot_gazebo demo.

1 > Does the physics simulator gazebo limit the frame rate at 25 fps?

2 > What happens when a node publishes messages to a topic faster than they are being consumed? Do the messages wait in a queue to be consumed?

3 > And is it possible to have a computation graph where nodes are written using different client libraries?

4 > Could you also please tell what is the meaning of the term RMS Error in gazebo?

5 > How can a node consume and produce messages to the same topic? Is it some kind of closed feedback loop control for that particular node?

Thanks and regards.

I have the turtlesim node running at /ajsim/turtle1 It is subscribing to the topic /ajsim/turtle1/command_velocity

I started another node called turtle_teleop_key which publishes the topic /turtle1/command_velocity

The problem is that the node in /ajsim/turtle1 cannot subscribe to the topic at /turtle1/command_velocity

I tried the command, [code] rosrun turtlesim turtle_teleop_key command_velocity:=ajsim/turtle1/command_velocity [/code]

but it doesn't work and the wiki is a little unclear about how the change the namespace. Could someone please tell me how I can make sure they are in the same namespace?

Is there a way to refresh the graph being shown on rxgraph? Because sometimes, it seems like it is getting stuck or fails to update itself and refresh the updated ROS node network.

I have 2 nodes running. I stopped one node and restarted it again. rxgraph still seems to show the 'graph node' depicting the earlier ROS node. When I hover the mouse over it, it simply says "Communication with node failed"..

But ofcourse both the ROS nodes are working fine. rxgraph simply can't seem to pick up the changes. Are there any refresh shortcuts that I might have missed?

Thanks and regards.

Hi. I am interested in using ROS for some of my projects, but at the moment I do not have any equipment to work with. The robots that ROS supports at the moment are a little beyond my budget. And to be honest, I want to be confident in my knowledge of ROS before I go about spending money on a robot to use with ROS.

So I was wondering if, it is possible to learn how to use ROS without access to any physical hardware other than a desktop computer? Like for example, would I be able to write programs in C++ or Python to control virtual robots and simulate them in ROS? That way, I can learn ROS and when I do eventually get access to physical hardware, I can transfer the code to it and control it..

I hope I haven't been too vague. Thanks.

I understand that ROS queues all incoming messages until nodes are able to service those callbacks and that this queue length is specified in the constructor of a subscriber.

But what I would like to know is, what happens if the subscriber specifies a queue length that is bigger than what ROS can support? Does the limit depend on some setting in ROS or does it merely depend on the amount of RAM available on the system?

From some of the other question I realized it is possible to remap the namespace of node on the command line using rosrun. But it is not working for me..

The syntax i tried is ,

rosrun somepkg somenode:="new_namespace/somenode.py"

and many other variations of it.

Could someone please tell me what is the correct way to remap a node to another name space on the command line while using rosrun?

At the fundamental level, can it be said that ROS is an efficient message passing system, since essentially, that is what it is doing and managing for the most part.. If that is the case, is it possible to use ROS for creating a powerful computing cluster instead of using it only for robotics?

It seems like I can interface a ROS runtime graph with an arduino using rosserial. But I am not sure which Arduino to get. I wouldn't want to get an arduino that rosserial couldn't support... I am mostly worried about the memory requirements. Because running rosserial could be memory intensive in certain cases.

So my question is..should I go with the cheapest available arduino or should I go with the one that has the most RAM? My ultimate goal is to interface ROS with an arduino so that I can send and recieve signals or message between a ROS runtime graph and the arduino..

The turtlesim node offers a service called "/reset" that allows me to reset the position of the turtle. The type of message accepted by the "/reset" service is of type

std_srvs.Empty

I tried to do,

import std_srvs
from std_srvs.srv import Empty

but it didn't work. I kept getting error messages saying std_srvs couldn't be found. I then tried adding 'std_srvs' as one of the dependencies in the manifest file of the package. Now the node was able to access std_srvs but not 'Empty'. Finally I had to do,

roscp std_srvs Empty mypackage/srv

and then rosmake and finally it worked.

Could someone please tell me, why I wasn't able to access std_srv.Empty before copying it to the srv folder in my package?

Thanks

I would like an affordable beginner's robot that can be used for learning and exploring ROS. The ones that are compatible with ROS mentioned on the ROS website, are a little out of my budget at the moment. Are there any cheaper alternatives?

A simple mobile platform would be sufficient to begin with and then I can add more components as my proficiency with ROS increases. I tried search for other similar questions on the site, but it turned up naught...

The iRobot create would not be an option because I am from outside the US.. unless, they changed the rules that allows foreigners to buy iRobot creates..

Thanks

from std_msgs.msg import String

when ROS encounters a statement like the one above, where does it search, to obtain String class? I check $PYTHONPATH and it doesn't seem like the class exists in a file located in that path. I searched the ROS package std_msgs..but it only shows a folder msg that has the String.msg file in it. I don't see any module called 'msg' in which I expect to find the 'String' class..

Could someone please tell me whats going on? Am I missing something fundamental or are things really twisted a little bit.. ?

Thanks

Is there a difference between rosmake and make? The way I understand it, rosmake is for building and updating the dependencies of a package, where these dependencies could be other ros packages.

rosdep is for installing dependencies outside the realm of ROS... More like system dependencies for building a ros package.

Then what is the purpose of running "make" in the directory of a ros package?

Thanks