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Building Robot Hardware Interfaces with ros2_control

Building Robot Hardware Interfaces with ros2_control

A robot software stack needs a clean boundary between high-level controllers and physical hardware. ros2_control provides this abstraction by separating hardware interfaces from controllers and exposing standardized state and command interfaces.

The framework's controller manager runs the main read-update-write control loop, while hardware components expose state and command interfaces to controllers. Hardware plugins can be dynamically loaded through pluginlib. ros2_control documentation

Architecture 

ROS 2 Application
       ↓
Controller
       ↓
Controller Manager
       ↓
ros2_control Hardware Interface
       ↓
Motor / Sensor Driver
       ↓
Physical Robot
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1. Understand State and Command Interfaces

A state interface represents information read from hardware:

joint1/position
joint1/velocity
joint1/effort
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A command interface represents values sent to hardware:

joint1/position
joint1/velocity
joint1/effort
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The exact interfaces depend on the actuator and controller design. Hardware interface types

2. Describe Hardware in URDF

A simplified configuration can look like:

<ros2_control name="MyRobotSystem" type="system">
  <hardware>
    <plugin>my_robot_hardware/MyRobotHardware</plugin>
    <param name="port">/dev/robot</param>
  </hardware>

  <joint name="left_wheel_joint">
    <command_interface name="velocity"/>
    <state_interface name="position"/>
    <state_interface name="velocity"/>
  </joint>

  <joint name="right_wheel_joint">
    <command_interface name="velocity"/>
    <state_interface name="position"/>
    <state_interface name="velocity"/>
  </joint>
</ros2_control>
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The URDF tells ros2_control which hardware plugin to load and which interfaces the robot exposes.

3. Create a Hardware Plugin

Create a package:

ros2 pkg create --build-type ament_cmake my_robot_hardware
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A system hardware component generally derives from:

hardware_interface::SystemInterface
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The framework provides lifecycle hooks and methods such as:

on_init()
on_configure()
on_activate()
on_deactivate()
read()
write()
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The exact implementation depends on your hardware. The official hardware-component guide documents the required interfaces and lifecycle. Writing a hardware component

4. Implement Initialization

A simplified implementation starts with:

hardware_interface::CallbackReturn
MyRobotHardware::on_init(
    const hardware_interface::HardwareInfo & info)
{
    if (SystemInterface::on_init(info) !=
        hardware_interface::CallbackReturn::SUCCESS) {
        return hardware_interface::CallbackReturn::ERROR;
    }

    // Read parameters and initialize driver state.

    return hardware_interface::CallbackReturn::SUCCESS;
}
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Use initialization to validate configuration and prepare the hardware driver.

5. Implement the Read Cycle

The read() method retrieves current hardware state.

hardware_interface::return_type
MyRobotHardware::read(
    const rclcpp::Time &,
    const rclcpp::Duration &)
{
    // Read encoders and update state values.
    return hardware_interface::return_type::OK;
}
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Conceptually:

Encoder
   ↓
Motor Driver
   ↓
read()
   ↓
State Interfaces
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6. Implement the Write Cycle

The write() method sends controller commands:

hardware_interface::return_type
MyRobotHardware::write(
    const rclcpp::Time &,
    const rclcpp::Duration &)
{
    // Send command interface values to motors.
    return hardware_interface::return_type::OK;
}
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The complete control cycle is:

read hardware
     ↓
controller update
     ↓
write hardware
     ↓
repeat
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7. Export the Plugin

Hardware components are dynamically loaded through pluginlib. A simplified export is:

#include <pluginlib/class_list_macros.hpp>

PLUGINLIB_EXPORT_CLASS(
    my_robot_hardware::MyRobotHardware,
    hardware_interface::SystemInterface)
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You also need the plugin description and appropriate CMake configuration.

8. Build the Package 

colcon build --packages-select my_robot_hardware
source install/setup.bash
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Check available hardware components:

ros2 control list_hardware_components
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You can also inspect interfaces:

ros2 control list_hardware_interfaces
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The ros2_control CLI provides commands for controllers, hardware components, and hardware interfaces. ros2_control CLI

9. Test Without Real Hardware

Use mock hardware or simulation first.

Controller
   ↓
Mock Hardware
   ↓
State Feedback
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This allows you to validate controller behavior before connecting motors.

10. Design for Real-Time Behavior

The hardware interface participates in the robot's control loop, so avoid unnecessary blocking operations.

Prefer:

Fast read()
Fast write()
Predictable memory behavior
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Move non-real-time work such as verbose logging, diagnostics, and network management away from the critical loop.

The controller manager documentation specifically emphasizes minimizing jitter in the main control loop. Controller Manager

11. Add Error Handling

Hardware can fail.

Handle:

  • Motor communication timeout
  • Encoder failure
  • Invalid commands
  • Over-temperature
  • Communication loss
  • Emergency stop

A hardware failure should transition the system to a known safe state rather than leaving the actuator uncontrolled.

Conclusion

ros2_control provides a reusable boundary between robot hardware and controllers. A well-designed hardware plugin hides vendor-specific communication while exposing clean state and command interfaces.

Once this layer is stable, the same controller architecture can often be used with simulated, mock, or physical hardware.

Useful Links

Website: www.v-modal.com
SDK Flutter: https://github.com/v-modal/vmodal_sdk_flutter
SDK Android: https://github.com/v-modal/vmodal_sdk_android
Discord: https://discord.gg/K72z28KUx

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