Hardware interface for Dynamixels and ROS control

ROS control is a framework to design software control loops in ROS (Robot Operating System) where the controller code is decoupled from the actual hardware.

This piece of software provides a hardware interface for ROS control. Its aim is to allow generic software controllers to control a set of Dynamixel actuators.

We are using it for our robots and developing it for this purpose. Version 0.1.0 is clearly no final code but it works for the regular needs.

Documentation : http://www.resibots.eu/dynamixel_control_hw/

Features

• simple, ROS-style control interface for your dynamixel-based robot
• not specific to a given number or set of actuators
• position and velocity control
• works with both version of Dynamixel protocol (1 and 2)
• uses radians uniformly over all Dynamixel models (no need to worry about ticks-to-angle conversion)
• you can set an offset for each actuator’s position
• set names for your dynamixels in a configuration file

Limitations

We are currently working on the following features:

• joint limits not implemented yet
• add support for the more exotic control modes (including current and multi-turn)
• it would be great to offer a service to reset one actuator after an overload error

Installation and usage

dynamixel_control_hw depends on the libdynamixel library. Please install it, it’s light and quick.

Then, you’ll need to store the installation path of libdynamixel in the LIBDYNAMIXEL environment variable.

Install the ros package ros_control. In apt-get, it is called ros-YourDistro-ros-control (where YourDistro must be changed by the ROS distribution’s name, like indigo or jade).

Clone the dynamixel_control_hw repository in your catkin workspace’s source directory and run a catkin_make.

Parameters

For sample configuration files, see the ones in config/. Here are the accepted parameters.

• serial_interface: path to the USB to serial interface for example “/dev/ttyUSB0”
• baudrate: baud-rate for the serial communication with actuators (in bauds)
• loop_frequency: frequency at which the control loop will run (in Hz)
• cycle_time_error_threshold: how much delay is tolerated on the control loop (in s)
• read_timeout: timeout on the reception of replies from the servos (in s)
• read_timeout: (for the scan only) timeout on the reception of replies from the servos (in s)
• servos: object which keys are the name of the joints and which values contain:
  • id (required): actuator’s ID to its name (the one used in the controller list and in URDF)
  • offset: correction to be applied to the angle of the joint (in rad)
  • command_interface: the command mode (velocity or position)
  • max_speed: maximal allowed velocity (rad/s), for now, works only for joints in position control
• default_command_interface: if no command_interface is defined for a joint, this value is used instead

Testing the hardware interface

If you want to use the sample launch files or to use one of the default controllers, please install with apt-get:

• ros-YourDistro-ros-controllers and
• ros-YourDistro-joint-state-publisher.

Have a look at the launch/sample.launch file. It will by default launch two feed-forward only controllers (one position and one velocity) and a virtual controller that publishes the states of the two actuators.

Before starting it, check config/sample.yaml for the id parameters, the serial_interface and baudrate settings. Once you are sure that it’s correct, you can roslaunch dynamixel_control_hw sample.launch. By looking at the available topics, you should find two, for the commands the joints, and one for the joint state.

Troubleshooting read time

If the reading time seems too long, check the value of the USB latency timer. On ubuntu this value is retrieved with this command cat /sys/bus/usb-serial/devices/ttyUSB0/latency_timer. It is the time, in milliseconds, for which the device driver buffers data before making it available.

You can change this timer with the command echo 4 | sudo tee /sys/bus/usb-serial/devices/ttyUSB0/latency_timer which sets it to 4 ms for the device /dev/ttyUSB0.

Alternative software

If you know of an other software offering similar functionalities to this one, feel free to open an issue so that we can add it here.

Support and contact

This software is developed as part of the ResiBots project. We do our best to keep it free of bug and to implement relevant features. Should you face an issue or have a suggestion, please open an issue.

Projects using this software

• ResiBots

License

Unless stated otherwise, the license for this repository is CeCILL-C (see LICENSE and LICENSE.fr).

Main changes to the code

In reverse chronological order:

• allow an actuator to be inverted (angle and speed)
• send velocity limits to the actuators, if they are in position mode
• rename the node executable so that the suffixes reflect the protocl version
• make plugins for this hardware interface to be included in combined hardware interfaces
• implement joint limits (parameterised through URDF or param server)
• breaking change: the configuration parameter syntax has changed; see sample configuration files
• velocity control mode integrated
• replace c-style code with std::chrono for time-related code
• replace std::map with std::unordered_map when possible (hypothetical performance gain)
• remove all dependencies to boost
• max speeds are now defined in radians per seconds
• this package now supports both protocols 1 and 2 of the Dynamixels
  the new node names are dynamixel_control_hw_v1 and dynamixel_control_hw_v2