otto_hardware

Hardware interface for connecting ros2_control controllers to a VEX V5 brain + motors.

URDF Plugin Name:otto_hardware/OmniWheelhardware

omni_wheel_hardware

Hardware interface system component.

CallBack Return Methods:

• on_init()
  • runs before the hardware is actually connected to
  • loads parameters from URDF ros2_control hardware tag
    • serial_port (default: '/dev/ttyUSB0' )
    • baud (default: 115200)
  • checks configuration per joint against expected configuration in order to catch early errors:
    • number and type of state interfaces
    • number and type of command interfaces
• on_activate()
  • activates HW by opening a serial connection with seriallib
• on_deactivate()
  • deactivates HW by closing the connection with seriallib

State Interface Methods:

• export_state_interfaces()
  • position of the wheels using encoders
  • velocity of the wheels as reported by the hardware

Command Interface Methods:

• export_command_interfaces()
  • velocities are being sent from the controller to the hardware

Return Type Methods:

• read()
  • Read both position and velocity state interfaces from the connected hardware and update both state interfaces
• write()
  • Write velocity commands to the connected hardware in the form of the MotorVelocity struct shown below:

struct MotorVelocities {
  uint16_t magic; // 0xFEFA
  double fl; // forward left wheel
  double fr; // forward right wheel
  double bl; // back left wheel
  double br; // back right wheel
};

URDF/xacro configuration

Example ros2_control tag for this hardware interface + the omni wheel drive controller from ros2_controllers:

  <ros2_control name="DriveSystem" type="system">
        <hardware>
            <plugin>otto_hardware/OmniWheelHardware</plugin>
            <param name='serial_port'>/dev/ttyUSB0</param>
            <param name='baud_rate'>115200</param>
        </hardware>
        <joint name="wheel_1">
            <command_interface name="velocity"/>
            <state_interface name="position"/>
            <state_interface name="velocity"/>
        </joint>
        <joint name="wheel_2">
            <command_interface name="velocity"/>
            <state_interface name="position"/>
            <state_interface name="velocity"/>
        </joint>
        <joint name="wheel_3">
            <command_interface name="velocity"/>
            <state_interface name="position"/>
            <state_interface name="velocity"/>
        </joint>
        <joint name="wheel_4">
            <command_interface name="velocity"/>
            <state_interface name="position"/>
            <state_interface name="velocity"/>
        </joint>
    </ros2_control>

Documentation + References

• ros2_control API
• Serialib GitHub Repository
• ros2_control_demos/example_2