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joint_trajectory_controller

Controller for executing joint-space trajectories on a group of joints. The controller interpolates in time between the points so that their distance can be arbitrary. Even trajectories with only one point are accepted. Trajectories are specified as a set of waypoints to be reached at specific time instants, which the controller attempts to execute as well as the mechanism allows. Waypoints consist of positions, and optionally velocities and accelerations.

Parts of this documentation were originally published in the ROS 1 wiki under the CC BY 3.0 license. Citations are given in the respective section, but were adapted for the ROS 2 implementation. [1]

Hardware interface types

Currently, joints with hardware interface types position, velocity, acceleration, and effort (defined here) are supported in the following combinations as command interfaces:

  • position

  • position, velocity

  • position, velocity, acceleration

  • velocity

  • effort

This means that the joints can have one or more command interfaces, where the following control laws are applied at the same time:

  • For command interfaces position, the desired positions are simply forwarded to the joints,

  • For command interfaces acceleration, desired accelerations are simply forwarded to the joints.

  • For velocity (effort) command interfaces, the position+velocity trajectory following error is mapped to velocity (effort) commands through a PID loop if it is configured (Details about parameters).

This leads to the following allowed combinations of command and state interfaces:

  • With command interface position, there are no restrictions for state interfaces.

  • With command interface velocity:

    • if command interface velocity is the only one, state interfaces must include position, velocity .

  • With command interface effort, state interfaces must include position, velocity.

  • With command interface acceleration, state interfaces must include position, velocity.

Further restrictions of state interfaces exist:

  • velocity state interface cannot be used if position interface is missing.

  • acceleration state interface cannot be used if position and velocity interfaces are not present.”

Example controller configurations can be found below.

Other features

  • Realtime-safe implementation.

  • Proper handling of wrapping (continuous) joints.

  • Robust to system clock changes: Discontinuous system clock changes do not cause discontinuities in the execution of already queued trajectory segments.

Using Joint Trajectory Controller(s)

The controller expects at least position feedback from the hardware. Joint velocities and accelerations are optional. Currently the controller does not internally integrate velocity from acceleration and position from velocity. Therefore if the hardware provides only acceleration or velocity states they have to be integrated in the hardware-interface implementation of velocity and position to use these controllers.

The generic version of Joint Trajectory controller is implemented in this package. A yaml file for using it could be:

controller_manager:
  ros__parameters:
    joint_trajectory_controller:
    type: "joint_trajectory_controller/JointTrajectoryController"

joint_trajectory_controller:
  ros__parameters:
    joints:
      - joint1
      - joint2
      - joint3
      - joint4
      - joint5
      - joint6

    command_interfaces:
      - position

    state_interfaces:
      - position
      - velocity

    state_publish_rate: 50.0
    action_monitor_rate: 20.0

    allow_partial_joints_goal: false
    open_loop_control: true
    constraints:
      stopped_velocity_tolerance: 0.01
      goal_time: 0.0
      joint1:
        trajectory: 0.05
        goal: 0.03

Preemption policy [1]

Only one action goal can be active at any moment, or none if the topic interface is used. Path and goal tolerances are checked only for the trajectory segments of the active goal.

When an active action goal is preempted by another command coming from the action interface, the goal is canceled and the client is notified. The trajectory is replaced in a defined way, see trajectory replacement.

Sending an empty trajectory message from the topic interface (not the action interface) will override the current action goal and not abort the action.

Description of controller’s interfaces

References

(the controller is not yet implemented as chainable controller)

States

The state interfaces are defined with joints and state_interfaces parameters as follows: <joint>/<state_interface>.

Legal combinations of state interfaces are given in section Hardware Interface Types.

Commands

There are two mechanisms for sending trajectories to the controller:

Both use the trajectory_msgs/msg/JointTrajectory message to specify trajectories, and require specifying values for all the controller joints (as opposed to only a subset) if allow_partial_joints_goal is not set to True. For further information on the message format, see trajectory representation.

Actions [1]

<controller_name>/follow_joint_trajectory [control_msgs::action::FollowJointTrajectory]

Action server for commanding the controller

The primary way to send trajectories is through the action interface, and should be favored when execution monitoring is desired.

Action goals allow to specify not only the trajectory to execute, but also (optionally) path and goal tolerances. When no tolerances are specified, the defaults given in the parameter interface are used (see Details about parameters). If tolerances are violated during trajectory execution, the action goal is aborted, the client is notified, and the current position is held.

The action server returns success to the client and continues with the last commanded point after the target is reached within the specified tolerances.

Subscriber [1]

<controller_name>/joint_trajectory [trajectory_msgs::msg::JointTrajectory]

Topic for commanding the controller

The topic interface is a fire-and-forget alternative. Use this interface if you don’t care about execution monitoring. The goal tolerance specification is not used in this case, as there is no mechanism to notify the sender about tolerance violations. If state tolerances are violated, the trajectory is aborted and the current position is held. Note that although some degree of monitoring is available through the ~/query_state service and ~/state topic it is much more cumbersome to realize than with the action interface.

Publishers

<controller_name>/controller_state [control_msgs::msg::JointTrajectoryControllerState]

Topic publishing internal states with the update-rate of the controller manager

Services

<controller_name>/query_state [control_msgs::srv::QueryTrajectoryState]

Query controller state at any future time

Further information

Footnote