Dynamixel Interface: A Fast, Scalable Dynamixel Driver

This package aims to provide a scalable and easily configurable interface for controlling many dynamixel’s across multiple serial devices.

Features:

• Fast C++ implementation capable of controlling 10’s of dynamixels at very high (>100Hz) rates.
• Easily configure all dynamixels in a single yaml file
• Synchronous across multiple serial ports (using threaded IO)
• All dynamixel states/commands are exchanged via a single sensor_msgs/JointState message
• Supports position, velocity and current* control modes.
  • In position control mode, can supply profile velocities as well

*current control not available on all models.

Currently the following series of motors are supported:

• AX
• RX
• DX
• EX
• MX
• XM
• PRO
• PRO+
• P

INSTALLATION

Requirements

• Ubuntu 18.04 LTS and ROS Melodic; or
• Ubuntu 20.04 LTS and ROS Noetic

Dependencies

• dynamixel_sdk

Building from source

mkdir -p catkin_ws/src
cd catkin_ws/src
git clone https://github.com/csiro-robotics/dynamixel_interface.git
cd ..
rosdep install --from-paths src --ignore-src -r -y
catkin build

USAGE NOTES

For detailed instructions, consult the tutorials in the subdirectory of this package:

• Tutorial 1: Using the controller
• Tutorial 2: Multi Port Communications

Serial Access

For serial communications, give the user access to the dialout group

sudo usermod -a -G dialout $USER

Operation

To use the dynamixel_interface_controller to operate servos:

• Create a controller_config.yaml with the necessary parameters for each servo (see the tutorials as well as the example in the config/ folder for an example and explanation of the various configuration options)
• Launch the dynamixel_interface_controller_node, loading in the custom config file (see the example in the launch/ folder)

During operation:

• The controller communicates using the sensor_msgs::JointState message.
• Current joint states are published to /joint_states
• Commands should be sent to /desired_joint_states
• For each control mode
  • Position Control: only position and velocity are considered in the command message, velocity controls the moving speed to the goal position of the dynamixel. If no velocity is specified the default is used
  • Velocity Control: only the velocity value is considered

  • Torque Control: only the effort value is considered

  • Note that all non-empty arrays must have the same length as the array of joint_names, otherwise the command is invalid

A note on units:

• Positions are in radians
• Velocities are in radians/second
• Effort is in mA of current

IMPROVING DRIVER PERFORMANCE

This driver is designed with goal of achieveing the maximum possible loop rate for a given configuration of dynamixels. To that end, there are three key optimisations that we recommend :

Baud rate

The baud rate used to communicate on the bus directly affects the maximum achieveable loop rate on the bus. A higher loop rate will result in faster update rates but can reduce the noise the bus can tolerate, for most applications the recommended baud rate is the maximum of 3Mbps.

Return delay time

The return delay time is a parameter on the dynamixels that sets the amount of delay in microseconds before they respond to a packet, by default this value is quite large (250, which equates to 500 microseconds). Lowering this value will reduce the total round-trip time for any comms on the bus. It is recommended to set this value very low (tested at 5, or 10 microseconds).

Both the above values will need to be configured PRIOR TO OPERATION using the dynamixel wizard available from robotis.

FTDI DRIVER LATENCY FOR SERIAL COMMUINICATIONS

Since 16.04, the default latency_timer value for the ftdi usb driver was changed from 1ms to 16ms. This value represents how long the kernel driver waits before passing data up to the user application. It significantly affects the speed the driver will be able to communicate with the dynamixels, resulting in very low achievable loop rates. To correct this, a script is included that can change a serial port into ‘low latency’ mode (a latency_timer value of 1), which requires the installation of the setserial command:

sudo apt install setserial

then, run the script (no roscore required) with:

rosrun dynamixel_interface set_lowlatency_mode.sh <port>

If setserial is not available as a command on your system, the value for the device can be set directly in the kernel:

echo "1" | sudo tee /sys/bus/usb-serial/devices/<device>/latency_timer

Authors

• Thomas Molnar
• Ryan Steindl
• Marisa Bucolo
• Benjamin Tam
• Jacob Oestreich

License

This project is licensed under the CSIRO Open Source Software Licence Agreement (variation of the BSD / MIT License) - see the LICENSE file for details.

Contributing

Thank you for being interested in contributing. We encourage the community to create new features and to fix bugs. If it is a new feature, please create a new issue via the Issue Tracker and describe the intended feature so that we can discuss the implementation. Else, search for the existing issue and comment on your proposed work. Please fork the repo, work on your changes and then send a pull request.

Issues

Please report bugs using Issue Tracker or contact us via email shc-support@csiro.au.