ROS

Create ROS package

# create workspace
mkdir -p catkin_ws/src
 
# create virtualenv
cd catkin_ws/
python3 -m virtualenv venv
# activate python venv
source venv/bin/activate
 
# build workspace
catkin build
 
# create package
cd src/
catkin_create_pkg pkg dependencies

ROS Elements

Node

A node is an executable that uses ROS to communicate with other nodes.

Message

ROS data type used when subscribing or publishing to a topic.

.msg files are simple text files that describe the fields of a ROS message. They are used to generate source code for messages in different languages.

# Add build and runtime dependence in package.xml
<build_depend>message_generation</build_depend>
<exec_depend>message_runtime</exec_depend>
 
# Add build and runtime dependence in CMakeLists.txt
find_package(catkin REQUIRED COMPONENTS
   ...
   message_generation
)
catkin_package(
  ...
  CATKIN_DEPENDS message_runtime ...
  ...)
 
# Add msg file
add_message_files(
  FILES
  Num.msg
)
 
# uncomment generate_messages

Service

An .srv file describes a service. It is composed of two parts: a request and a response. The two parts are separated by ”---“.

Topic

Nodes can publish messages to a topic as well as subscribe to a topic to receive messages.

Master

Name service for ROS.

Others

ROS equivalent of stdout/stderr. Master + rosout + parameter server.

Q&A

How to avoid ros node duplication?

1. rosrun

# set different namespace
rosrun turtlesim turtlesim_node __ns:=/xxx
# set different alternative name
rosrun turtlesim turtlesim_node __name:=t1

1. roslaunch

set different name (ns) to node.

1. code

# anonymous will add timestamp behind the original node name
rospy.init_node("turtle_sim", anonymous=True)

How to avoid ros topic duplication?

1. rosrun

# rename /cmd_vel topic to /turtle1/cmd_vel
rosrun teleop_twist_keyboard teleop_twist_keyboard.py /cmd_vel:=/turtle1/cmd_vel

1. roslaunch

<launch>
    <node pkg="turtlesim" type="turtlesim_node" name="t1" />
    <node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="key">
      <!-- remap topic -->
      <remap from="/cmd_vel" to="/turtle1/cmd_vel" />
    </node>
</launch>

1. code

# init and start node with name "hello" and namespace "xxx"
 
# global topic name
# /chatter
pub = rospy.Publisher("/chatter", String, queue_size=1000)
 
# relative topic name
# /xxx/chatter
pub = rospy.Publisher("chatter",String,queue_size=1000)
 
# private topic name
# /xxx/hello/chatter
pub = rospy.Publisher("~chatter",String,queue_size=1000)