Posted 2022-04-16Updated 2026-05-11Note2 minutes read (About 239 words)

Study note for ROS subscriber.

Model of Topic

Publisher(turtlesim)
- –[Message(geometry_msgs::Twist)]–>
Topic (/turtle1/pose)
- –[Message(geometry_msgs::Twist)]–>
Subscriber(Pose Listener)

Programming realization

How to realize a subscriber

initialize ROS node
subscribe required topic
loop wait the msgs from topic, after receiving msgs, enter callback function
implement msgs processing in the callback function

Code

Also we put this node into function package learning_topic
Create velocity_publisher.cpp file in learning_topic/src:

#include <ros/ros.h>
#include <turtlesim/Pose.h>

void poseCallback(const turtlesim::Pose::ConstPtr& msg)
{
    ROS_INFO("Turtle pose: x:%0.6f, y:%0.6f, ",msg->x,msg->y);
}

int main(int argc, char *argv[])
{
    ros::init(argc, argv, "pose_subscriber");
    ros::NodeHandle n;
    ros::Subscriber pose_sub = n.subscribe("/turtle1/pose",10,poseCallback）
    //queue also first in first thrown
    //loop waiting for callback function, keep the endless loop
    ros::spin();
    return 0;
}

Note

Callback function must be sufficient, or newer data will be blocked

Configure the compile run of subscriber

Steps same as publisher
Add two lines to learning\_topic/CMakeLists.txt (in build part):

1 2	add_executable(velocity_publisher src/pose_subscriber.cpp) target_link_libraries(pose_subsriber ${catkin_LIBRARIES})

Compile

$ cd ~/ROS_study/catkin_ws
$ catkin_make
$ source ./devel/setup.zsh
$ roscore
$ rosrun turtlesim turtlesim_node
$ rosrun learning_topic velocity_publisher
$ rosrun learning_topic pose_subscriber

The output of pose_subscriber:

...
[ INFO] [1650184199.757606156]: Turtle pose: x:3.100077, y:7.501662,
[ INFO] [1650184199.773327854]: Turtle pose: x:3.101826, y:7.493855,
[ INFO] [1650184199.790252785]: Turtle pose: x:3.103601, y:7.486054,
...

Posted 2022-04-15Updated 2026-05-11Note3 minutes read (About 378 words)

ROS-Publisher

Study note for ROS publisher.

Model of Topic

Publisher(turtle Velocity)
- –[Message(geometry_msgs::Twist)]–>
Topic (/turtle1/cmd_vel)
- –[Message(geometry_msgs::Twist)]–>
Subscriber(turtlesim)

Programming realization

Take turtlesim as the example.
We write a program acting like a publisher and sending controlling messages.

Create package

1 2	$ cd catkin_ws/src $ catkin_create_pkg learning_topic roscpp rospy std_msgs geometry_msgs turtlesim

can see:

1 2	$ ls CMakeLists.txt include package.xml src

How to realize a publisher?

Initialize ROS node
register node info from ROS Master, including name of the topic and msg type
create message data
publish message at a specific frequency

Code

create velocity_publisher.cpp file in learning_topic/src

#include <ros/ros.h>
#include <geometry_msgs/Twist.h>

int main(int argc, char *argv[])
{
    //init the node
    ros::init(argc,argv,"velocitypublisher");

    //create node handle
    ros::NodeHandle n;

    //create a publisher, who publish topic names '/turtle1/cmd_vel', message type 'geometry_msgs:Twist', queue length 10
    ros::Publisher turtle_vel_pub = n.advertise<geometry_msgs::Twist>("/turtle1/cmd_vel",10);

    //set loop rate
    ros::Rate loop_rate(10);
    while (ros::ok()) {
        //initialize geometry_msgs:Twist msg
        geometry_msgs::Twist vel_msg;
        vel_msg.linear.x = 0.5;
        vel_msg.angular.z = 0.2;

        //publish msgs
        turtle_vel_pub.publish(vel_msg);
        ROS_INFO("Publish turtle velocity command at %0.2f m/s, %0.2f rad/s",
                vel_msg.linear.x,vel_msg.angular.z);

        //delay
        loop_rate.sleep();
    }
    return 0;
}

What is queue length?

msgs can be published very fast while the data transmission rate is limited, so the queue act as a buffer
first in, first thrown, keeping the remained msgs in the queue relatively new

Configure the compile rule of publisher

Steps:

set the code need to be complied and the generated excutable file
set the link library

Add two lines to learning\_topic/CMakeLists.txt (in build part):

1 2	add_executable(velocity_publisher src/velocity_publisher.cpp) target_link_libraries(velocity_publisher ${catkin_LIBRARIES})

Compile

$ cd ~/ROS_study/catkin_ws
$ catkin_make
$ source ./devel/setup.zsh
$ roscore
$ rosrun turtlesim turtlesim_node
$ rosrun learning_topic velocity_publisher

Then, you can see turtle:

And outputs:

...
[ INFO] [1650088100.376299177]: Publish turtle velocity command at 0.50 m/s, 0.20 rad/s
[ INFO] [1650088100.476312726]: Publish turtle velocity command at 0.50 m/s, 0.20 rad/s
[ INFO] [1650088100.576304995]: Publish turtle velocity command at 0.50 m/s, 0.20 rad/s
...

Posted 2022-04-12Updated 2026-05-11Note3 minutes read (About 505 words)

ROS Orgmode Note

Study note for ROS
* Core Concept
** Node and ROS Master
*** Node
implement functions for purposes
support various language
can have thousands of nodes
*** ROS Master
mange the registion and name of various nodes
support(trace and record) communication between different nodes
for nodes to find each other
provide parameter server(store global variables...)[[about parameter]]
** Communication between nodes
|                    | Topic               | Service        |
|--------------------+---------------------+----------------|
| sync               | n                   | y              |
| model              | publish/subscribe   | service/client |
| protocol           | ROSTCP/ROSUDP       | ROSTCP/ROSUDP  |
| response           | n                   | y              |
| buffer             | y                   | n              |
| real-time          | weak                | strong         |
| nodes relationship | multi/one to multi  | one to multi   |
| scene              | data transformation | logic process  |
*** Topic
Node(publisher)->Topic(/example)&Message_Tyoe(std_msgs/String)->other ROS nodes(subscriber)
Message: define the data type and struct of data, defined by .msg file
async
*** Service
C/S model(Client/Server)
client send requests, server return response data
defined by .srv file, defines the structure of the requests and response
    ROS node(Service Server)
    ^  |response
request|  v
    ROS node(Service Client)
sync
** Parameter
<<about parameter>>
talker -> ROS master : setParam("foo",1)
listener -> ROS master: getParam("foo")
ROS master ->listener: {"foo",1}

multiple variable dictionary
for storing static nonbinary configuration, not suitable for dynamic
** File system
*** package
Basic unit in ROS, include the src of nodes, cfg, data def
*** package manifest
record the basic information of packages
*** Meta packages
orgnize multi packages for one common purpose
* Commond Line Tools
** rqt_graph
visualize tool
display the calculate process graph
** rosnode
display node information
*** rosnode list
list all node
default node /rosout
*** rosnode info /turtle_sim
show info of a node
** rostopic
usage like rosnode(list,info)
*** rostopic pub /../.. data_structure data 
publish message as a topic
add -r num to repeat sending at rate num/s
** rosmsg
*** rosmsg show data_structure
show the data structure
** rosservice 
| commond addition       |                                   |
|------------------------+-----------------------------------|
| list                   | list all service the node provide |
| call service_name data |                                   |
** rosbag
| command addition        |                                               |        |                 |
|-------------------------+-----------------------------------------------+--------+-----------------|
| record -a -O cmd_record | record the topic data in the system           | -a:all | -O as a package |
| play cmd_record.bag     | play the recorded data of previous simulation |        |                 |
|                         |                                               |        |                 |
* setup workspace and function package
** workspace
*** overview
| src     | source space  |                         |
| build   | build space   | no need to touch        |
| devel   | develop space | script, executable file |
| install | install sapce |                         |

work space folder/
  src/
    CmakeLists.txt
    package_1/
      CmakeList.txt
      package.xml
      ...
    package_n/
      CmakeList.txt
      package.xml
      ...
  build/
    CATKIN_IGNORE
  devel/
    bin/
    etc/
    include/
    lib/
    share/
    .catkin
    env.bash
    setup.bash
    setup.sh
    ...
  install/
    bin/
    etc/
    include/
    lib/
    share/
    .catkin
    env.bash
    setup.bash
    setup.sh
    ...
*** setup
**** establish ws
mkdir -p ./catkin_ws/src
cd ./catkin_ws/src
catkin_init_workspace
**** compile ws
cd ./catkin_ws/
catkin_make -DPYTHON_EXECUTABLE=/usr/bin/python3
catkin_make install
**** set env variables
source devel/setup.bash
**** check env variables
echo $ROS_PACKAGE_PATHH
** fucntion package
*** setup
| establish package | cd ./src                                                                   |                                       |
|                   | catkin_create_pkg <pkg_name> [depend1] [depend2].. (rospy,roscpp,std_msgs) |                                       |
| source env path   | source devel/setup.bash                                                    | make sure that ros can detect the pkg |

*** package.xml
| label                        | description                                         |
|------------------------------+-----------------------------------------------------|
| <name>                       |                                                     |
| <version>                    |                                                     |
| <description>                |                                                     |
| <maintainer email="">        | maintainer's name and email                         |
| <license>                    |                                                     |
| <buildtool depend>           | usually catkin                                      |
| <build_depend>/<exec_depend> | package dependencies ,usually roscpp rospy std_msgs |
*** CmakeList.txt
TODO

Model of Topic

Programming realization

Code

Configure the compile run of subscriber

Compile

Model of Topic

Programming realization

Create package

Code

Configure the compile rule of publisher

Compile

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