Running Hardware Components Asynchronously

The ros2_control framework allows to run hardware components asynchronously. This is useful when some of the hardware components need to run in a separate thread or executor. For example, a sensor takes longer to read data that affects the periodicity of the controller_manager control loop. In this case, the sensor can be run in a separate thread or executor to avoid blocking the control loop.

Parameters

The following parameters can be set in the ros2_control hardware configuration to run the hardware component asynchronously:

• is_async: (optional) If set to true, the hardware component will run asynchronously. Default is false.

• thread_priority: (optional) The priority of the thread that runs the hardware component. The priority is an integer value between 0 and 99. The default value is 50.

Note

The thread priority is only used when the hardware component is run asynchronously. When the hardware component is run asynchronously, it uses the FIFO scheduling policy.

Examples

The following examples show how to use the different hardware interface types synchronously and asynchronously with ros2_control URDF. They can be combined together within the different hardware component types (system, actuator, sensor) (see detailed documentation) as follows

For a RRBot with multimodal gripper and external sensor:

<ros2_control name="RRBotSystemMutipleGPIOs" type="system">
  <hardware>
    <plugin>ros2_control_demo_hardware/RRBotSystemPositionOnlyHardware</plugin>
    <param name="example_param_hw_start_duration_sec">2.0</param>
    <param name="example_param_hw_stop_duration_sec">3.0</param>
    <param name="example_param_hw_slowdown">2.0</param>
  </hardware>
  <joint name="joint1">
    <command_interface name="position">
      <param name="min">-1</param>
      <param name="max">1</param>
    </command_interface>
    <state_interface name="position"/>
  </joint>
  <joint name="joint2">
    <command_interface name="position">
      <param name="min">-1</param>
      <param name="max">1</param>
    </command_interface>
    <state_interface name="position"/>
  </joint>
  <gpio name="flange_digital_IOs">
    <command_interface name="digital_output1"/>
    <state_interface name="digital_output1"/>    <!-- Needed to know current state of the output -->
    <command_interface name="digital_output2"/>
    <state_interface name="digital_output2"/>
    <state_interface name="digital_input1"/>
    <state_interface name="digital_input2"/>
  </gpio>
</ros2_control>
<ros2_control name="MultimodalGripper" type="actuator" is_async="true" thread_priority="30">
  <hardware>
    <plugin>ros2_control_demo_hardware/MultimodalGripper</plugin>
  </hardware>
  <joint name="parallel_fingers">
    <command_interface name="position">
      <param name="min">0</param>
      <param name="max">100</param>
    </command_interface>
    <state_interface name="position"/>
  </joint>
</ros2_control>
<ros2_control name="RRBotForceTorqueSensor2D" type="sensor" is_async="true">
  <hardware>
    <plugin>ros2_control_demo_hardware/ForceTorqueSensor2DHardware</plugin>
    <param name="example_param_read_for_sec">0.43</param>
  </hardware>
  <sensor name="tcp_fts_sensor">
    <state_interface name="fx"/>
    <state_interface name="tz"/>
    <param name="frame_id">kuka_tcp</param>
    <param name="fx_range">100</param>
    <param name="tz_range">100</param>
  </sensor>
  <sensor name="temp_feedback">
    <state_interface name="temperature"/>
  </sensor>
  <gpio name="calibration">
    <command_interface name="calibration_matrix_nr"/>
    <state_interface name="calibration_matrix_nr"/>
  </gpio>
</ros2_control>

In the above example, the following components are defined:

• A system hardware component named RRBotSystemMutipleGPIOs with two joints and a GPIO component that runs synchronously.

• An actuator hardware component named MultimodalGripper with a joint that runs asynchronously with a thread priority of 30.

• A sensor hardware component named RRBotForceTorqueSensor2D with two sensors and a GPIO component that runs asynchronously with the default thread priority of 50.