[advanced ROS chapter] Lecture 4: duplicate names in ROS (nodes, topics and parameters)

【ROS Advanced 】 Lesson four ROS The problem of double names in English （ node 、 Topics and parameters ）

Preface

In the last blog post , We have to ROS The file management system is analyzed in hierarchy , The workspace coverage and the communication between different hosts are studied , This section focuses on changing the direction , To consider ROS There are many duplicate names in the system , for example Duplicate node name 、 Topic name 、 Parameter duplicate name etc. .

One 、 Duplicate node name

1. Introduction to the problem

• Concept ： stay ROS Node is the most basic concept in the system , When creating a node , For example, using C++ Language , You need to specify API Define the node name , If there is a node with the same name , Then there will be problems when calling ;
• Physical truth ： stay ROS When the node with the same name is started , The existing nodes will be closed directly , The tips are as follows ：

[ WARN] [1578812836.351049332]: Shutdown request received.
[ WARN] [1578812836.351207362]: Reason given for shutdown: [new node registered with same name]

• Solutions ：

1. using namespace std ： add prefix
2. Name remapping ： Rename

For the above two strategies , There are three ways to achieve this ：rosrun command 、launch Documentation and programming , This article will analyze and introduce one by one

2. resolvent

1）rosrun

• Set namespace ：rosrun Function package name The node name __ns:=/ New node roll call
  Examples are as follows ：

rosrun turtlesim turtlesim_node __ns:=/xxx
rosrun turtlesim turtlesim_node __ns:=/yyy

• Name remapping ：rosrun Function package name The node name __name:= New node roll call
  Examples are as follows ：

rosrun turtlesim  turtlesim_node __name:=t1 |  rosrun turtlesim   turtlesim_node /turtlesim:=t1
rosrun turtlesim  turtlesim_node __name:=t2 |  rosrun turtlesim   turtlesim_node /turtlesim:=t2

2）launch

• In the previous launch File parsing tutorial , We mentioned that node Two attributes of a tag ,name and ns, By setting these two properties, you can easily implement the above two strategies ;

<launch>

    <node pkg="turtlesim" type="turtlesim_node" name="t1" />
    <node pkg="turtlesim" type="turtlesim_node" name="t1" ns="xxx"/>

</launch>

3）c++

• Set namespace ：

std::map<std::string, std::string> map;
map["__ns"] = "xxxx";
ros::init(map,"test_node");

• Name remapping ： After giving the original node name Add time stamp

ros::init(argc,argv,"test_node",ros::init_options::AnonymousName);

Two 、 Topic name

1. Introduction to the problem

• Compared to nodes ,ROS Of course, there is also the problem of duplicate names , But it will be more complicated , Because in practical application , The setting of topic name is relatively flexible , It needs to be modified flexibly .
• Solutions ： The same as the node name , The basic strategy is still Remapping and prefixes The two methods , The difference is that there are many kinds of prefixes , It can be divided into the following three categories ：

1. Global prefix ： Reference resources ROS System , Level with namespace
2. Relative prefix ： Reference namespace , Level with node name
3. Private prefix ： Reference node name , Under the node name

• Case study ： In the test of the solution , Suppose the scenario is as follows ：
  Use the keyboard to control the function pack , Used to control turtle movement , The instructions are as follows ：

sudo apt install ros-noetic-teleop-twist-keyboard
rosrun teleop_twist_keyboard teleop_twist_keyboard.py
rosrun turtlesim turtlesim_node

• problem ： The former ( The control node ) The topic is cmd_vel, the latter ( According to the node ) yes /turtle1/cmd_vel

2. resolvent

1）rosrun

• The name remapping method is mainly used , Modify the topic name conveniently through the following instructions
  rosrun Function package name The node name Topic name := New words title

Examples are as follows ：

rosrun teleop_twist_keyboard teleop_twist_keyboard.py /cmd_vel:=/turtle1/cmd_vel
rosrun turtlesim turtlesim_node
//  Or vice versa 
rosrun teleop_twist_keyboard teleop_twist_keyboard.py
rosrun turtlesim turtlesim_node /turtle1/cmd_vel:=/cmd_vel

2）launch

• Again , adopt launch In file node Medium remap Topic remapping can be realized

<launch>

    <node pkg="turtlesim" type="turtlesim_node" name="t1" />
    <node pkg="teleop_twist_keyboard" type="teleop_twist_keyboard.py" name="key">
        <remap from="/cmd_vel" to="/turtle1/cmd_vel" />
    </node>
    
	<--  notes ： The opposite solution  -->

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

</launch>

3）c++

• about C++ Solutions for , Mainly adopts add prefix The way to , And different names , Prefixes added are different ：

0. Premise ： Set the node name to tsnode, Add the space prefix when starting the node ：xxx
1. Global name ：

// With / The name at the beginning , Independent of node name 
ros::Publisher pub = nh.advertise<std_msgs::String>("/chatter",1000);
// result ：/chatter

2. Relative name ：

// Not / The name at the beginning , Refer to the workspace to determine the topic name 
ros::Publisher pub = nh.advertise<std_msgs::String>("chatter",1000);
// result ：xxx/chatter

3. Private name ：

// With ~ The name at the beginning , Add workspace and node name prefix 
ros::NodeHandle nh("~");
ros::Publisher pub = nh.advertise<std_msgs::String>("chatter",1000);
// result ：/xxx/tsnode/chatter

3、 ... and 、 Parameter duplicate name

1. Introduction to the problem

• Concept ： Different from before , Parameter names can also be overwritten by duplicate names , But most of the solutions are add prefix The way to ;
• terms of settlement ： When generating parameters , In essence, it will have a certain prefix , Prefix types are three of the same topic , This article will introduce ;

2. resolvent

1）rosrun

• use rosrun Start node , Can be set by the following command ：

rosrun  Function package name   The node name  _ Parameter name := Parameter values 

• example ： Automatic addition Node prefix （ Private model ）：

rosrun turtlesim turtlesim_node _A:=100
//  View the corresponding parameter information 
/turtlesim/A

2）launch

• The two methods ：

1. node Set parameters outside the label ： Global properties

<launch>

    <param name="p1" value="100" />
    <--  result ：/p1 -->

</launch>

2. node Set parameters in the label ： Private nature

<launch>

    <node pkg="turtlesim" type="turtlesim_node" name="t1">
        <param name="p2" value="100" />
    </node>
     <--  result ：/t1/p2 -->

</launch>

3）c++

• about C++ Programmatically , adopt API：param::set perhaps NodeHandle Realization ：

• paramset：

ros::param::set("/set_A",100); // overall situation , It has nothing to do with namespace and node name 

ros::param::set("set_B",100); // relative , Reference namespace 

ros::param::set("~set_C",100); // private , Reference namespace and node name 

// result 
/set_A
/xxx/set_B
/xxx/yyy/set_C

• NodeHandle：

ros::NodeHandle nh;
nh.setParam("/nh_A",100); // overall situation , It has nothing to do with namespace and node name 

nh.setParam("nh_B",100); // relative , Reference namespace 

ros::NodeHandle nh_private("~");
nh_private.setParam("nh_C",100);// private , Reference namespace and node name 

// result 
/nh_A
/xxx/nh_B
/xxx/yyy/nh_C

summary

• Statement ： The blog section of this section refers to CSDN User zhaoxuzuo ROS course , From the next blog ROS The advanced tutorial will step into the study of functional components , We start with TF Change the toolkit and start again .