Robotics System Toolbox: How to create a new publisher for a topic for an existing node?

Hi all,

I've some codes below to connect to gazebo (turtlebot);

>> rosinit('http://192.168.2.87:11311')
Initializing global node /matlab_global_node_77319 with NodeURI http://192.168.2.126:62581/

>> rosnode list
/gazebo
/laserscan_nodelet_manager
/matlab_global_node_77319
/mobile_base_nodelet_manager
/robot_state_publisher
/rosout

>> rostopic list
/camera/depth/camera_info                              
/camera/depth/image_raw                                
/camera/depth/points                                   
/camera/parameter_descriptions                         
/camera/parameter_updates                              
/camera/rgb/camera_info                                
/camera/rgb/image_raw                                  
/camera/rgb/image_raw/compressed                       
/camera/rgb/image_raw/compressed/parameter_descriptions
/camera/rgb/image_raw/compressed/parameter_updates     
/clock                                                 
/gazebo/link_states                                    
/gazebo/model_states                                   
/gazebo/parameter_descriptions                         
/gazebo/parameter_updates                              
/gazebo/set_link_state                                 
/gazebo/set_model_state                                
/joint_states                                          
/mobile_base/commands/motor_power                      
/mobile_base/commands/reset_odometry                   
/mobile_base/commands/velocity                         
/mobile_base/events/bumper                             
/mobile_base/events/cliff                              
/mobile_base/sensors/imu_data                          
/odom                                                  
/rosout                                                
/rosout_agg                                            
/tf   

>> rosnode info /matlab_global_node_77319
Node: [/matlab_global_node_77319]
URI: [http://192.168.2.126:62581/]

Publications (1 Active Topics): 
 * /rosout

Subscriptions (1 Active Topics): 
 * /clock

When I tried to create a publisher for a topic as below (using ‘rospublisher’), it works:

    >> robot = rospublisher('/mobile_base/commands/velocity')

    robot = 

      Publisher with properties:

             TopicName: '/mobile_base/commands/velocity'
            IsLatching: 1
        NumSubscribers: 0
           MessageType: 'geometry_msgs/Twist'

As shown below, the topic is added to the publishers list:

>> rosnode info /matlab_global_node_77319
Node: [/matlab_global_node_77319]
URI: [http://192.168.2.126:62581/]

Publications (3 Active Topics): 
 * /mobile_base/commands/velocity
* /odom
* /rosout

Subscriptions (1 Active Topics): 
 * /clock

Services (0 Active):

Now, I add a new node:

>> node_1 = robotics.ros.Node('node_1', 'http://192.168.2.87:11311');
>> rosnode info node_1
Node: [/node_1]
URI: [http://192.168.2.126:62966/]

Publications (1 Active Topics): 
 * /rosout

Subscriptions (1 Active Topics): 
 * /clock

Services (0 Active):

And add the same topic to publish but using another code (robotics.ros.publisher). It works too, as shown below:

    >> robotics.ros.Publisher(node_1,'/mobile_base/commands/velocity');
    >> rosnode info node_1
    Node: [/node_1]
    URI: [http://192.168.2.126:62966/]

    Publications (2 Active Topics): 
     * /mobile_base/commands/velocity
    * /rosout

    Subscriptions (1 Active Topics): 
     * /clock

    Services (0 Active):

However, now when I tried to use (robotics.ros.publisher) on the first node (/matlab_global_node_77319), it doesn’t work:

>> tPub = robotics.ros.Publisher(matlab_global_node_77319,'/mobile_base/commands/motor_power');
Undefined function or variable
'matlab_global_node_77319'.

>> tPub = robotics.ros.Publisher(matlab_global_node_77319,'/mobile_base/commands/motor_power','kobuki_msgs/MotorPower');
Undefined function or variable
'matlab_global_node_77319'.

>> tPub = robotics.ros.Publisher(/matlab_global_node_77319,'/mobile_base/commands/motor_power','kobuki_msgs/MotorPower');
tPub = robotics.ros.Publisher(/matlab_global_node_77319,'/mobile_base/commands/motor_power','kobuki_msgs/MotorPower');
                               |
Error: Unexpected MATLAB operator.

However, if I used the same code on the new node, and publish a new topic, it works:

>> robotics.ros.Publisher(node_1,'/mobile_base/commands/motor_power')

ans = 

    Publisher with properties:

           TopicName: '/mobile_base/commands/motor...'
          IsLatching: 1
      NumSubscribers: 1
         MessageType: 'kobuki_msgs/MotorPower'

>> rosnode info /node_1
Node: [/node_1]
URI: [http://192.168.2.126:62966/]

Publications (2 Active Topics): 
 * /mobile_base/commands/motor_power
* /rosout

Subscriptions (1 Active Topics): 
 * /clock

Services (0 Active):

How do I publish a topic for a node that already exist?

Answer 1

[Sebastian Castro]

I don't believe that the "official" toolbox syntax is meant to play together with the "robotics.ros.Node" stuff that's hidden away in the background.

If you start a ROS master using rosinit , it doesn't create a robotics.ros.Node object in the base workspace. This is where your questions comes in, in that you can't find an object with that name.

That being said, if you use the rospublisher function, this publisher will by default be associated with the /matlab_global_node_#### node that was created.

So, it seems that if you create a node using rosinit , you must create the publisher using rospublisher . If you create a node using robotics.ros.Node , you must create the publisher using robotics.ros.Publisher .

- Sebastian

[James Ang]

I see...thanks Sebastian. This is helpful. Appreciate it.