04. Simple Mover

You will now go through the process of implementing your first ROS node in python.
This node is called simple_mover . As it’s name implies, this node only has one responsibility, and that is to command joint movements for simple_arm .

To do so, it must publish joint angle command messages to the following topics:

| Topic Name | /simple_arm/joint_1_position_controller/command |
|-|-|
| Message Type | std_msgs/Float64 |
| Description | Commands joint 1 to move counter-clockwise, units in radians |

|**Topic Name ** | /simple_arm/joint_2_position_controller/command |
|-|-|
| Message Type | std_msgs/Float64 |
| Description | Commands joint 2 to move counter-clockwise, units in radians |

Note : If you no longer have the catkin workspace from the previous lesson, you can download a copy of it here . Alternately, If you’d prefer to skip to the punch, you can download the entire, complete simple_arm package from here .

Adding the scripts directory

In order to create a new node in python, you must first create the scripts directory within the simple_arm package, as it does not yet exist. 

$ cd ~/catkin_ws/src/simple_arm/
$ mkdir scripts

Creating a new script

Once the scripts directory has been created, executable scripts can be added to the package. However, in order for rosrun to find them, their permissions must be changed to allow execution. Let’s add a simple bash script that prints “Hello World” to the console. 

$ cd scripts
$ echo '#!/bin/bash' >> hello
$ echo 'echo Hello World' >> hello

After setting the appropriate execution permissions on the file, rebuilding the workspace, and sourcing the newly created environment, you will be able to run the script. 

$ chmod u+x hello
$ cd ~/catkin_ws
$ catkin_make
$ source devel/setup.bash
$ rosrun simple_arm hello

And there you have it! You have now added a script

Creating the empty simple_mover node script

To create the simple_mover node script, you will must simply follow the same basic routine introduced a moment ago.

sh $ cd ~/catkin_ws/src/simple_arm $ cd scripts $ touch simple_mover $ chmod u+x simple_mover
You can now edit the empty simple_mover script with your favorite text editor.

Let’s write the code!

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