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Connecting to Dynamixel bus
Description: This tutorial describes how to connect and examine raw feedback from Robotis Dynamixel servos.Keywords: robotis,dynamixel,connecting,feedback
Tutorial Level: BEGINNER
Next Tutorial: Creating meta controller
Step 1: Create a package
If the Dynamixel Controllers driver is not installed open the terminal and type:
sudo apt-get install ros-<distro>-dynamixel-controllers
Before we begin we need to create a package that has proper dependencies and will contain all the files we create in this tutorial. Note: before executing command listed below make sure that the current directory is in one of the directories of the ROS_PACKAGE_PATH environment variable. Refer to the environment tutorials for how to best create a workspace and overlay directory.
# You should have created this in the Creating a Workspace Tutorial cd ~/catkin_ws/src catkin_create_pkg my_dynamixel_tutorial dynamixel_controllers std_msgs rospy roscpp
Step 1: Create a package
Before we begin we need to create a package that has proper dependencies and will contain all the files we create in this tutorial. Note: before executing command listed below make sure that the current directory is in one of the directories of the ROS_PACKAGE_PATH environment variable. Refer to the environment tutorials for how to best create a workspace and overlay directory.
roscreate-pkg my_dynamixel_tutorial dynamixel_controllers roscd my_dynamixel_tutorial
Step 2: Create a launch file for the manager node
We assume that the Dynamixel servos are connected to /dev/ttyUSB0 serial port.
First we need to start up the controller manager that will connect to the motors and publish raw feedback data (e.g. current position, goal position, error, etc.) at a specified rate. The easiest way to do that is to write a launch file that will set all necessary parameters. Let's copy and paste the following text into a controller_manager.launch file.
1 <!-- -*- mode: XML -*- -->
2
3 <launch>
4 <node name="dynamixel_manager" pkg="dynamixel_controllers" type="controller_manager.py" required="true" output="screen">
5 <rosparam>
6 namespace: dxl_manager
7 serial_ports:
8 pan_tilt_port:
9 port_name: "/dev/ttyUSB0"
10 baud_rate: 1000000
11 min_motor_id: 1
12 max_motor_id: 25
13 update_rate: 20
14 </rosparam>
15 </node>
16 </launch>
Note: Make sure you set your baud rate correctly. If you are using RX-28 motors, it should be set to 57142. If you have further problems, see if your user account needs to be added to the dialout group and if the USB port needs to be made writeable.
Now if we execute:
roslaunch my_dynamixel_tutorial controller_manager.launch
We should see the status output similar to the following:
[INFO] [WallTime: 1295282870.051953] pan_tilt_port: Pinging motor IDs 1 through 25... [INFO] [WallTime: 1295282870.103676] pan_tilt_port: Found 4 motors - 4 AX-12 [4, 5, 6, 13], initialization complete.
Naturally the output depends on what motors are connected to the USB bus.
Step 3: Examine motor feedback
Now the controller manager is publishing feedback on /motor_states/pan_tilt_port topic. First, let's check that the topic is there:
rostopic list
The output should look similar to the one shown below:
/motor_states/pan_tilt_port /rosout /rosout_agg
One of the topic in the list should be the /motor_states/pan_tilt_port topic. Let's see what kind of information is published:
rostopic echo /motor_states/pan_tilt_port
In this case we have four AX-12+ motors connected:
motor_states:
motor_states:
-
timestamp: 1351555461.28
id: 4
goal: 517
position: 527
error: 10
speed: 0
load: 0.3125
voltage: 12.4
temperature: 39
moving: False
-
timestamp: 1351555461.28
id: 5
goal: 512
position: 483
error: -29
speed: 0
load: 0.0
voltage: 12.6
temperature: 40
moving: False
-
timestamp: 1351555461.28
id: 6
goal: 511
position: 516
error: 5
speed: 0
load: 0.0
voltage: 12.3
temperature: 41
moving: False
-
timestamp: 1351555461.28
id: 13
goal: 256
position: 256
error: 0
speed: 0
load: 0.0
voltage: 12.3
temperature: 38
moving: False
---That's it! In the next tutorial you'll learn how to connect to several motors.
