ROS_Control

In general, there are several types of control software. Some of them are available out of the box whereas some others are those you may write by yourselves. ros_control is among the out-of-the-box control software you can directly use (so that you don’t need to write one from scratch) which is compatible with third-party packages built on top of ROS (such as ros_navigation and MoveIt). Hence, ros_control offers at least two advantages:

• out-of-the-box solution
• compatibility for third-party ROS packages

Historically, ros_control started from pr2_controller_manager developed by Willow Garage specifically for PR2 robot only. In late 2012, Willow Garage and hiDOF started developing ros_control as a robot-agnostic control software. The development of ros_control was later done by PAL Robotics and the community.

A presentation about ros_control can be found here: video and slides.

ros_control has several main components:

• ros_controllers: consisting of several types of controllers one can use.
• controller_manager: managing all the controllers
• hardware_interface: providing a standard interface between controller(s) and the robot hardware through the hardware interface node.

Besides the aforementioned components, the following components also exist in ros_control:

• transmissions_interface: supports several transmission methods such as reducer, differential, and fourbar linkage.
• joint_limits_interface: defining joint limits.

If one would like to use ros_control with a real robot, he/she should provide the following customized components:

• hardware interface node (this is a piece of code/software customized to someone’s robot).
• a real robot (with actuators/motors and sensors).

Now let’s talk about some details of the ros_control main components. The following is the wiki page of ros_control: http://wiki.ros.org/ros_control

1 ros_controllers

There are several types of controllers available in ros_control:

>position_controllers:

• joint_position_controller: sets one joint position at once.
• joint_group_position_controller: sets multiple joint positions at once.

>velocity_controllers:

• joint_velocity_controller: sets one joint velocity at once.
• joint_group_velocity_controller: sets multiple joint velocities at once.

>effort_controllers: commands the desired force/torque to joints.

• joint_effort_controller
• joint_position_controller
• joint_velocity_controller

>joint_trajectory_controller: is used to control a trajectory-following motion. It has spline interpolation capability.

• position_controller
• velocity_controller
• effort_controller
• position_velocity_controller
• position_velocity_acceleration_controller

>diff_drive_controller: is used to control differential-drive mobile robot.

>ackermann_steering_controller: is used to control ackermann-steering mobile robot.

>forward_command_controller

>gripper_action_controller

There are also read-only controllers (hence they actually just perform sensor state reporting; they are actually not controllers):

>joint_state_controller: publishes the joint state as “sensor_msgs/JointState” data type.

>force_torque_sensor_controller: publishes force/torque sensor reading as “geometry_msgs/Wrench” data type.

>imu_sensor_controller: publishes IMU state as “sensor_msgs/Imu” data type.

2 controller_manager

It has capability to manage the controllers such as list, load, unload, start, stop, spawn, switch, kill. You can run the controller_manager in several ways:

• CLI commands
• running the controller_manager by using a launch file
• graphically running the controller_manager by using RQT Plugin

3 hardware_interface

The following two hardware interfaces are the most commonly used:

• Joint Command Interface
• Joint State Interface

Beside these two hardware interfaces, there are also some other hardware interfaces available in ros_control:

• Actuator State Interface
• Actuator Command Interface
• PosVelJointInterface
• PosVelAccJointInterface
• Force-torque Sensor Interface
• IMU Sensor Interface

4 transmissions

Transmissions can be conveniently specified in URDF such as the following:

5 joint_limits

The joint limits can be specified by using either URDF or YAML file.

Specifying joint limits in URDF looks like this:

Specifying joint limits in a YAML file looks like this:

IMPORTANT

Notice that a joint can use more than one controller, BUT NOT AT THE SAME TIME.

Other useful links:

• https://github.com/ros-controls/ros_control/wiki
• https://sir.upc.edu/projects/rostutorials/10-gazebo_control_tutorial/index.html#