Controlling robots with ros2_control
ROS nodes are easy to conceptualize as pure functional components that process input topics and generate output topics. However, the end goal in robotics is often not just to perform some computation, but to control real physical hardware such as a robot arm.
Interfacing with hardware peripherals imposes additional challenges, including:
- networking and communication beyond the ROS Middleware (RMW) publisher / subscriber mechanism
- hardware-specific control interfaces and requirements
- safety and reliability around controller and hardware limits
- potential real-time requirements for deterministic control performance
The ros2_control project defines an open standard for writing controllers and hardware interfaces with the aim of
simplifying the integration of new and existing robot hardware within a ROS-based applications through hardware
abstraction. It specifies the following key concepts.
Controller Manager
The idea behind hardware abstraction is to hide the additional complexity of hardware-specific software behind a standardized API. The Controller Manager is a process that handles hardware interfaces and controllers and passes messages between them in a standard format. In brief, the Controller Manager allows controllers in the ROS 2 graph to read the state of a robot and to write a command to the robot in a real-time loop.
State interfaces
State interfaces provide the current state of the hardware. This is most commonly defined as the robot joint positions, and may also include joint velocities, joint torques, other available sensor data, and general purpose digital inputs and outputs (GPIO).
Command interfaces
Command interfaces represent the desired control action that a hardware should execute. The nature of a command interface depends on the actuators, but commonly includes joint positions, velocities or torques. For real-time control, velocity and torque are the preferred command modes. For other actuators such as robot grippers, the command interface might instead specify a gripper finger distance or force through a GPIO.
Hardware interfaces
Communication protocols and message standards vary between robot brands. Hardware interfaces, also known as hardware components or hardware plugins, are software packages that implement robot-specific communication drivers. They are responsible for reading data from the robot and translating them into the standard state interface format for the Controller Manager. They are also responsible for translating command interfaces from the Controller Manager into real robot control actions.
Controllers
As might be expected, controllers are responsible for calculating and sending desired control actions to the Controller Manager via the standardized command interfaces. They also have access to state interfaces from the Controller Manager to use as feedback in closed-loop control.
Controllers are otherwise similar to ROS 2 lifecycle components; they have parameters, can publish or subscribe to messages on the ROS 2 network to interact with other processes in the application, and can be dynamically loaded, activated, deactivated or unloaded.
Read more about ros2_control
in the official ros2_control documentation.