Task-space Control

The taskspace_control package provides task-space controllers for robotic manipulators using the ros2_control framework.

Package Overview

Each controller in the taskspace_control package accepts a setpoint defined by a pose $p \in SE(3)$ and a twist $\xi \in \mathbb{R}^6$, delivered as a PoseTwistSetpoint.msg. The taskspace_control_examples package provides an example setpoint publisher.

Running the Demos

Launch the demo robot system with the desired robot and controller

ros2 launch taskspace_control_examples robot_bringup.launch.py robot_type:=robot6R controller:=pose_controller

# robot_type (default "robot6R"): One of 'robot6R', 'robot7R

In another terminal, launch the control demo with the same controller

ros2 launch taskspace_control_examples pose_control_demo.launch.py robot_type:=robot6R controller:=pose_controller

# controller (default "pose_controller"):
#    One of 'pose_controller,
#            task_priority_controller,
#            as_nullspace_controller,
#            as_twist_decomposition_controller'

The redundancy resolution objectives, kinematic frames, and controller parameters can be modified in the configurations files of the examples package.

Controller Types

A list of available controller plugins can be found in the *_controller_plugins.xml of each package. The controllers are configured to work with hardware interfaces that accept a position and (optional) velocity command. A basic configuration for the controllers below can be found in the taskspace_control_examples package config.

Task-space Controllers

• taskspace_controllers/TaskspaceControllerBase

  The base task-space controller constructs the kinematic model of the robot. It also sets up some boilerplate functionality like a forward kinematics solver and the joint limits of the robot. All other controllers inherit from this class. Do not use this controller in practice.

• taskspace_controllers/PoseController

  A pose controller tracks a fully defined setpoint using a basic inverse Jacobian tracking controller.

Task-priority Controllers

If the robot is kinematically redundant (i.e. there are more than 6 joints) than a task priority controller can be used to achieve a secondary set of objectives in addition to tracking the setpoint. This is done through a nullspace projection using the robot's Jacobian. The redundancy resolution objective, or secondary priority task, is defined by a plugin that's loaded at runtime. See objective_plugins.xml for a list of provided plugins.

• task_priority_controllers/PoseController

  This controller tracks a fully defined setpoint and uses the redundant degrees-of-freedom to achieve the objective plugin.

Axially-symmetric Controllers

Axially-symmetric controllers inherit from the Task-priority controllers. These controllers treat the setpoint as a 5-DOF task with three position constraints (x, y, z) and two orientation constraints. The manipulator will track a provided position and align the eef_frame_axis with the setpoint_frame_axis (see: axially_symmetric_controller_parameters.yaml). The rotation about the z-axis of the tool is left as a redundant axis, with the orientation ultimately determined by a redundancy resolution objective (see Task-priority Controllers above).

• axially_symmetric_controllers/NullspaceController

  This controller uses the Jacobian nullspace projection for redundancy resolution.

• axially_symmetric_controllers/TwistDecompositionController

  This controller uses twist-decomposition for redundancy resolution.

Controller Diagnostics

The enable_diagnostics parameter provides a mechanism for diagnosing taskspace_controllers/PoseController and task_priority_controllers/PoseController. When set to true, the controllers publish a Diagnostic.msg that relays the following:

• Commanded joint state, feedback joint state, and joint state error
• Desired pose (controller setpoint), feedback pose, and pose error
• The magnitude of the translation and orientation error

The diagnostic publisher is disabled by default.