Implementation of a Hardware Interface for simple Ethercat module integration with ros2_control and building upon IgH EtherCAT Master for Linux.
EtherCAT provides applications with the capacity of reliable, real-time communication between systems and is therefore a common industrial standard. In order to simplify the development/deployment of new application using EtherCAT modules, the ethercat_driver allows to combine them with ros2_control. This driver proposes a generic ways to parametrise and assemble Hardware Interfaces based on EtherCAT modules that can be defined using parameter files.
Installation steps for IgH EtherCAT Master for Linux and the driver can be found here.
The ethercat_driver is designed in such a way that each EtherCAT Master is defined in ros2_control as a specific Hardware Interface. This is done in the ros2_control description file, where the EtherCAT driver is loaded as Hardware Interface and linked to an EtherCAT Master by its ID:
<ros2_control name="mySystem" type="system">
<hardware>
<plugin>ethercat_driver/EthercatDriver</plugin>
<param name="master_id">0</param>
<param name="control_frequency">100</param>
</hardware>
</ros2_control>NOTE: As in the current implementation of ros2_control there is no information about the system update frequency, it needs to be passed here as parameter. This is only needed systems that include EtherCAT modules that use the Distributed Clock.
In this driver, the EtherCAT Slave modules are defined as Plugins and can be parametrized in the ros2_control description file :
<ec_module name="ECModule">
<plugin>ethercat_plugins/ECModule</plugin>
<param name="alias">0</param>
<param name="position">1</param>
</ec_module>All modules have alias and position parameters that specify their address in the EtherCAT Bus topology. Additional parameters can be specified depending on the purpose of the module. A list of implemented modules and their parameters can be found here.
In ros2_control the access to resources within a system from a controller is done by means of Hardware Resources. For this purpose state_interface and command_interface tags need to be defined and associated with the module functionalities.
Also, for better understanding of the overall system, the pupropose of the used modules need to be clearly identified and sorted into the following types:
<joint>: logical component actuated by at least one actuator with read-write capacity.<sensor>: logical component to read-only states from system.<gpio>: logical component for general purpose IO systems.
NOTE: These components have the possibility to include parameters that will be used to link particular states and commands to the slave module input/outputs.
Here is an example of a gpio resource built using 2 EtherCAT IO modules, where digital commands are mapped on ports 4 and 6 of the digital output module and analog states are read from ports 1 and 4 of the analog input module:
<gpio name="myGPIO">
<command_interface name="dig_output.1"/>
<command_interface name="dig_output.2"/>
<state_interface name="dig_output.1"/>
<state_interface name="dig_output.2"/>
<state_interface name="ana_input.1"/>
<state_interface name="ana_input.2"/>
<ec_module name="EL3104">
<plugin>ethercat_plugins/Beckhoff_EL3104</plugin>
<param name="alias">0</param>
<param name="position">1</param>
<param name="ai.1">ana_input.1</param>
<param name="ai.4">ana_input.2</param>
</ec_module>
<ec_module name="EL2008">
<plugin>ethercat_plugins/Beckhoff_EL2008</plugin>
<param name="alias">0</param>
<param name="position">2</param>
<param name="do.4">dig_output.2</param>
<param name="do.6">dig_output.1</param>
</ec_module>
</gpio>NOTE : To send commands to gpio resources, a generic controller was developed and can be found here.
Parts of the driver are based on the implementation of SimplECAT.
ICube Laboratory, University of Strasbourg, France
Maciej Bednarczyk: m.bednarczyk@unistra.fr, @github: mcbed