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Rosworkshop

Let’s understand ROS framework

ROS nodes

Every process in ROS consider as nodes and depending upon the behaviors of these nodes they are catogarised into

Master node
Publisher/Subscriber nodes
Service nodes
action nodes

In the above Master Node is a unquine node which take care of logs of all other types of nodes and all the history of other nodes at a time there can be only one Master Node running in the section

so to visualise the above we can you ROS native package turtle_sim.

open terminal

Ctrl+Alt+t

Run master node aka roscore
```
roscore
```
open new terminal
```
rosrun turtlesim turtlesim_node
```
open new terminal
```
rqt_graph
```
open new terminal
```
rosrun turtlesim turtle_teleop_key
```
use arrow key to move the turtle

In the gui window open for rqt_graph we have two oval shape objects with text in it and arrow connecting them above the arrow some text is written these are known as Topics

for debugging

use rqt_graph or use the below to check all the node are running
```
rosnode list
```
which outputs
```
/rosout
/teleop_turtle
/turtlesim
```

ROS Topics

Topics are data vector name given by the user to using ROS message(ROSmsg) structures topics may be of native ROSmsg or an User defined ROSmsg which is a combination of native ROSmsgs

ROS message

As disscused above the structure in which ROS topic publish data is in ROSmsg either you can use native ROSmsg structure or you can create your own ROSmsgs with combiations of native ROSmsgs.

To view the pulished data in topic for above example

open new terminal and type
```
rosmsg list
```
To show structure of ROSmsg-simple
```
rosmsg show std_msgs/Float32
```
```
 float32 data
```
above is simple rosmsg struture std_msgs is the stuture containor and Float32 is data formats ‘data’ is the variable
To show structure of ROSmsg-complex
```
rosmsg show geometry_msgs/Twist
```
which will output
```
 geometry_msgs/Vector3 linear
float64 x
float64 y
float64 z
geometry_msgs/Vector3 angular
float64 x
float64 y
float64 z
```
Let us view the cmd_vel topic

To view the pulished data in topic for above example open new terminal and type
```
rostopic echo /turtle1/cmd_vel
```
here /turtle1/cmd_vel is the topic name which has ROSmsg structure

for debugging

use rqt_graph or use the below to check all the node are running

rostopic list

which outputs

/rosout
/rosout_agg
/turtle1/cmd_vel
/turtle1/color_sensor
/turtle1/pose

Back to:Chapter 1

Next to:Chapter 3

Let’s understand ROS framework

ROS nodes

ROS Topics

ROS message

Let us view the cmd_vel topic