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This tutorial will show you how to use the Arbotix-M Robocontroller board to control DYNAMIXEL servos with ROS.

1. Flash the Arbotix board

Before you can use the Arbotix board with ROS, you need to flash a proper firmware. Follow the Quick Start Guide from TrossenRobotics to learn how to program the board with Arduino IDE.

If you have successfully programmed the example sketches, open and upload the following sketch:

File -> Sketchbook -> ArbotiX Sketches -> ros

The rest of this tutorial can be done in any machine that runs ROS and have an Internet connection. It can be Leo Rover

or your computer

2. Install Arbotix ROS driver

The driver for Arbotix board is distributed in arbotix package. It can be installed from official repository just by typing:

sudo apt install ros-[DISTRO]-arbotix

replace [DISTRO] with the ROS distribution you are using (If you are on LeoOS, type: melodic).

3. Set Dynamixel IDs and test the servos

Make sure your setup looks like this:

Then, type in your terminal:

arbotix_terminal

arbotix_terminal [PORT]

 sudo chmod 666 /dev/ttyS[N]

A terminal prompt should appear. Type help for the list of commands.

Connect the Dynamixel you want to set id to. Then type ls to see id of connected servo and mv [source] [target] to change it. For example, when the servo has ID 1 and we want to set it to 2, just type:

mv 1 2

You can also read and change some parameters to test servo movement with get and set commands. For example, to read current position of servo with ID 1, type:

get pos 1

To move the servo to position 300 (the range is between 0 and 1023), type:

set pos 1 300

To set the goal speed of the servo to half of the maximum, type:

set spd 1 512

The servo should start moving continuously in one direction.

4. Test the ROS driver

We will create a package that tests the arbotix driver. Let's start by creating a catkin workspace (if you don't have one yet):

mkdir -p ~/ros_ws/src && cd ~/ros_ws/
catkin init
catkin config --extend /opt/ros/melodic

and an empty package:

cd ~/ros_ws/src
catkin create pkg arbotix_test --catkin-deps arbotix_python

Inside the package, create config/test.yaml file with the following content:

port: /dev/ttyUSB0
rate: 15
joints: {
    dynamixel1: {id: 1},
    dynamixel2: {id: 2},
    dynamixel3: {id: 3},
    dynamixel4: {id: 4},
    dynamixel5: {id: 5}
}

And launch/test.launch with the following:

<launch>
  <node name="arbotix_driver_test" pkg="arbotix_python" type="arbotix_driver" output="screen">
    <rosparam file="$(find arbotix_test)/config/test.yaml" command="load" />
  </node>
</launch>

Build the package and source the result space:

cd ~/ros_ws
catkin build
source devel/setup.bash

Now, run the launch file, by typing:

roslaunch arbotix_test test.launch

You should see new topics spawned (check with rostopic list):

/joint_states
/dynamixel1/command
/dynamixel2/command
/dynamixel3/command
/dynamixel4/command
/dynamixel5/command

And new services (check with rosservice list):

/dynamixel1/enable
/dynamixel1/relax
/dynamixel1/set_speed
/dynamixel2/enable
...

To shortly summarize the features:

• /dynamixelX/command topic will enable the torque and move the servo with ID X to a specified position if published to. The position is specified in radians.

• /dynamixelX/enable service will just enable the torque on the servo.

• /dynamixelX/relax service will relax the servo (disable the torque).

• /dynamixelX/set_speed service will set the speed the servo will move with when it receives a new command. The speed is specified in radians per second.

• The current joint positions and velocities are published to /joint_states topic.

We can try to test the features using rostopic and rosservice tools.

Assuming you have connected the servo with ID 1, start by setting the speed to a safe value (0.2 rad/s in this case):

rosservice call /dynamixel1/set_speed 0.2

Move servo to a default neutral value:

rostopic pub /dynamixel1/command std_mgs/Float64 -- 0.0

Relax joint:

rosservice call /dynamixel1/relax