Node - Simple Mover

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

The goal of the simple_mover node is to command each joint in the simple arm and make it swing between -pi/2 to pi/2 over time.

Topics

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

Build and Run

Before you can run the simple_mover node, you have to compile the C++ script.

Modifying CMakeLists.txt

In order for catkin to generate the C++ libraries, you must first modify simple_arm’s CMakeLists.txt.

CMake is the build tool underlying catkin, and CMakeLists.txt is a CMake script used by catkin. If you’re familiar with the concept of makefiles, this is similar.

Navigate to the package CMakeLists.txt file and open it:

$ cd /home/shengchen/workspace/SimpleArm/catkin_ws/src/simple_arm/
$ gedit CMakeLists.txt

a. find_package()

First, ensure that the find_package() macro lists std_msgs, message_generation, and controller_manager as required packages. The find_package() macro should look as follows:

find_package(catkin REQUIRED COMPONENTS
        std_msgs
        message_generation
        controller_manager
)

As the names might imply, the std_msgs package contains all of the basic message types, and message_generation is required to generate message libraries for all the supported languages (cpp, lisp, python, javascript). The contoller_manager is another package responsible for controlling the arm.

b. include directories, executable file, link libraries, and dependencies

Now, add the following block of code at the bottom of the file:

include_directories(include ${catkin_INCLUDE_DIRS})

add_executable(simple_mover src/simple_mover.cpp)
target_link_libraries(simple_mover ${catkin_LIBRARIES})
add_dependencies(simple_mover simple_arm_generate_messages_cpp)

These instructions ask the compiler to include the directories, executable file, link libraries, and dependencies for your C++ code:

add_executable(node_name sourcecode_directory)

Creates the executable simple_mover file.

target_link_libraries(node_name ${catkin_LIBRARIES})

This will add all the linked libraries to the compiler.

add_dependencies(node_name package_name_generate_messages_cpp)

Generates message headers for this package before you can use them.

Keep in mind that you should always include these instructions whenever you want to write a C++ ROS node. For more information about CMakeLists.txt check out the CMakeLists.txt page on the ROS wiki.

Building the Package

Now that you have included specific instructions for your compiler, let’s build the package:

$ cd /home/workspace/catkin_ws/
$ catkin_make

Running simple_mover

Asuming that your workspace has recently been built, you can launch simple_arm as follows:

$ cd /home/workspace/catkin_ws/
$ source devel/setup.bash
$ roslaunch simple_arm robot_spawn.launch

Once the ROS Master, Gazebo, and all of our relevant nodes are up and running, we can finally launch simple_mover. To do so, open a new terminal and type the following commands:

$ cd ~/workspace/SimpleArm/catkin_ws
$ source devel/setup.bash
$ rosrun simple_arm simple_mover