dynamixel-controller

This is a HardwareInterface for ros2_control for dynamixel actuators from ROBOTIS. Its mainly developed for and with the XL series of actuators, so other dynamixels may or may not work. Open up a issue when you encounter problems. Pull requests are welcome too.

Build

Make sure the DynamixelSDK for ROS2 is installed. Clone it in your workspace and checkout the branch ros2:

git clone https://github.com/ROBOTIS-GIT/DynamixelSDK.git -b ros2
cd DynamixelSDK/

finally build with colcon colcon build

Configure Dynamixel motor parameters via URDF

Add the serial_port, baud_rate, and id parameters to the ros2_control part of your .urdf / .xarco files.

<ros2_control name="my_robot" type="system">
      <hardware>
        <plugin>dynamixel_ros2_control/DynamixelHardwareInterface</plugin>
        <param name="serial_port">/dev/ttyUSB0</param>
        <param name="baud_rate">4000000</param>
      </hardware>
      <joint name="joint1">
        <param name="id">10</param>
        <param name="p_gain">1120.0</param>
        <param name="i_gain">170.0</param>
        <param name="d_gain">3600.0</param>   
        <!--param name="reboot_on_error">true</param-->
        <command_interface name="position"/>
        <!--command_interface name="velocity"/-->
        <state_interface name="position"/>
        <state_interface name="velocity"/>
        <state_interface name="effort"/>
      </joint>
      ...

Optional parameters

When reboot_on_error is "true" the motor automaticaly reboots on mechanical overload error (it still loses torque for a brief moment, but that should be enogh to let the motor recover the fault). p_gain, i_gain and d_gain set the PID gain values of the motor.

Run a test

You need the urdf of the robot with the ros2_control xml added. After starting the control nodes and activating the Joint Trajectory Controller you can use ros2cli commands to test the actuators:

ros2 topic pub /joint_trajectory_controller/joint_trajectory trajectory_msgs/msg/JointTrajectory '{header: {stamp: {sec: '"$(date '+%s + 6' | bc)"'}}, joint_names: ["joint1"], points: [{positions: [0.0, ], velocities: [0, ], time_from_start: {sec: 0}}]}' --once

The example launch file expects the urdf in its robot description package and a robot bringup package with the controller.yaml (example for that is also in this package).

USB Latency tuning

To increase the frequency the actuators can be read and be written to via usb you can decrease the usb latency. On Linux you can do that with the following command (if the dynamixel serial device is on ttyUSB0):

sudo chmod o+w /sys/bus/usb-serial/devices/ttyUSB0/latency_timer
echo 1 > /sys/bus/usb-serial/devices/ttyUSB0/latency_timer

This change however gets lost on reboot. You can create a systemd-service to automatically do this on reboot:

[Unit]
Description=lower usb latency timer

[Service]
Type=oneshot
User=root
ExecStart=/usr/bin/bash -c '/usr/bin/echo 1 > /sys/bus/usb-serial/devices/ttyUSB0/latency_timer'

[Install]
WantedBy=multi-user.target

(Filepath /etc/systemd/system/usb-latency.service) and enable with:

sudo systemctl enable usb-latency.service --now

Credits

This is partly based on the work of Youtalk (GitHub link), who made a dynamixel ros2_control driver based on the official dynamixel_workbench packages. I swaped out the dynamixel_workbench parts with my own implementation, that only uses the dynamixelSDK.