Joint Command Topic Based System

The Joint Command Topic Based System implements a ros2_control hardware_interface::SystemInterface supporting command and state interfaces through the ROS topic communication layer. It publishes commands as control_msgs/JointCommand messages, one topic per interface type. Only position, velocity and effort command interfaces are supported, anything else is ignored with a warning.

ros2_control urdf tag

The joint_command_topic_hardware_interface has a few ros2_control urdf tags to customize its behavior.

Parameters

• joint_commands_topic: (default: “/robot_joint_commands”). Base topic for the joint command topics. Example: <param name="joint_commands_topic">/my_topic_joint_commands</param>.
• joint_states_topic: (default: “/robot_joint_states”). Example: <param name="joint_states_topic">/my_topic_joint_states</param>.
• trigger_joint_command_threshold: (default: 1e-5). Used to avoid spamming the joint command topic when the difference between the current joint state and the joint command is smaller than this value, set to -1 to always send the joint command. Example: <param name="trigger_joint_command_threshold">0.001</param>.
• sum_wrapped_joint_states: (default: “false”). Used to track the total rotation when the position values reported on the joint_states_topic wrap from 2pi to -2pi when rotating in the positive direction. (Isaac Sim only reports joint states from 2pi to -2pi) Only position states are affected. Example: <param name="sum_wrapped_joint_states">true</param>.

Mimic joints

Often used with parallel grippers. The relation is declared in the URDF, not with ros2_control params:

<joint name="joint2" type="revolute">
    <mimic joint="joint1" multiplier="-2" offset="0"/>
    ...
</joint>

Then mark the joint with mimic="true" and give it state interfaces only. Its states come from the mimicked joint instead of the joint_states_topic:

<joint name="joint2" mimic="true">
    <state_interface name="position"/>
    <state_interface name="velocity"/>
</joint>

offset only applies to position. velocity and acceleration use multiplier alone.

Example

        <ros2_control name="name" type="system">
            <hardware>
              <plugin>joint_command_topic_hardware_interface/JointCommandTopicSystem</plugin>
              <param name="joint_commands_topic">/topic_based_joint_commands</param>
              <param name="joint_states_topic">/topic_based_joint_states</param>
            </hardware>
            <joint name="joint_1">
                <command_interface name="position"/>
                <command_interface name="velocity"/>
                <state_interface name="position">
                  <param name="initial_value">0.0</param>
                </state_interface>
                <state_interface name="velocity"/>
            </joint>
            ...
        </ros2_control>

Topics

Each write() publishes one control_msgs/JointCommand per interface type, on topics derived from joint_commands_topic:

• <joint_commands_topic>/position: joints with a driven position command.
• <joint_commands_topic>/velocity: joints with a driven velocity command.
• <joint_commands_topic>/effort: joints with a driven effort command.

joint_names and values line up, interface_name repeats the interface. With the defaults, a joint with position and velocity commands publishes to /robot_joint_commands/position and /robot_joint_commands/velocity.

Command interfaces that no controller writes to hold NaN. Those joints are dropped from the message, and if nothing drives an interface type at all its topic stays quiet. Declaring a command interface is not enough to get traffic on the matching topic.

Nothing is published while the summed state-command difference stays at or below trigger_joint_command_threshold, which is the normal case once a controller has converged. Set it to -1 to publish every cycle.

QoS

Command publishers are reliable with a depth of 1, so don’t count on them to buffer.

The joint_states_topic subscription uses SensorDataQoS (best effort, depth 5), which any joint state publisher can match.