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In order to control the Ackermann steering joints, we need to define the gazebo_ros2_control parameters to send commands. Unlike the DIFFERENTIAL drive robot which could use a simple Gazebo plugin to control movement, the Gazebo Ackermann steering plugin can only be used to publish odometry information from the wheels but currently does not work for control for the Humble ROS Distro.

Table of Contents

XACRO

ros2_control

Now it’s time to create controllable joints for the robot. First, we are going to change all the wheel joints from fixed back to continuous. Also, change the steer joints from fixed to revolute. This will break your tf2 tree in RVIZ, but that’s okay because we are going to be adding Gazebo plugins to fix this.

Navigate to your hunter_steer.xacro, and change steer_joint from fixed to revolute. Next, change your wheel_joint from fixed to continuous. Navigate to your hunter_motors.xacro, and change your wheel_joint from fixed to continuous.

Next, we are going to be creating a new XACRO called hunter_se_gazebo.xacro. Open your hunter_se_gazebo.xacro file and paste the following code:

<?xml version="1.0"?>
<robot name="hunter_se_gazebo" xmlns:xacro="<http://wiki.ros.org/xacro>">
		<xacro:macro name="steering_control"
        params="
        name
        steer_limlow
        steer_limhigh">
        <joint name="${name}_steer_joint">
            <command_interface name="position">
                <param name="min">${steer_limlow}</param>
                <param name="max">${steer_limhigh}</param>
            </command_interface>
            <state_interface name="position" />
            <state_interface name="velocity" />
        </joint>
    </xacro:macro>
    <xacro:macro name="velocity_control"
        params="
        name
        velocity_limlow
        velocity_limhigh">
        <joint name="${name}_wheel_joint">
            <command_interface name="velocity">
                <param name="min">${velocity_limlow}</param>
                <param name="max">${velocity_limhigh}</param>
            </command_interface>
            <state_interface name="position" />
            <state_interface name="velocity" />
            <state_interface name="effort" />
        </joint>
    </xacro:macro>
</robot>

Next, navigate to your hunter_se.xacro file and create the gazebo_ros2_control plugin call, ros2_control tags and the macro calls:

	<xacro:include filename="$(find hunter_se_description)/models/xacro/hunter_se_gazebo.xacro" />

		<gazebo>
        <plugin name="gazebo_ros2_control" filename="libgazebo_ros2_control.so">
            <robotNamespace>${robot_namespace}</robotNamespace>
            <parameters>$(find hunter_se_description)/models/config/controllers.yaml</parameters>
        </plugin>
		</gazebo>
		<ros2_control name="GazeboSystem" type="system">
        <hardware>
            <plugin>gazebo_ros2_control/GazeboSystem</plugin>
            <robotNamespace>${robot_namespace}</robotNamespace>
        </hardware>
        <xacro:steering_control
            name="center"
            steer_limlow="${-max_steer}"
            steer_limhigh="${max_steer}" />
        <xacro:steering_control
            name="front_right"
            steer_limlow="${-max_steer}"
            steer_limhigh="${max_steer}" />
        <xacro:steering_control
            name="front_left"
            steer_limlow="${-max_steer}"
            steer_limhigh="${max_steer}" />
        <xacro:velocity_control
            name="rear_right"
            velocity_limlow="-7.5"
            velocity_limhigh="10.0" />
        <xacro:velocity_control
            name="rear_left"
            velocity_limlow="-7.5"
            velocity_limhigh="10.0" />
        <xacro:velocity_control
            name="front_right"
            velocity_limlow="-7.5"
            velocity_limhigh="10.0" />
        <xacro:velocity_control
            name="front_left"
            velocity_limlow="-7.5"
            velocity_limhigh="10.0" />
    </ros2_control>

Examine the code

First, we define macros that set the command and state parameters. These tags define how the joint is able to be controlled, and what information the joint should return to the joint_states topic. For the steer joint we define the command interface to be a position interface and want it to return both the position and velocity information. For the rear wheel joints, we set the command interface to be a velocity interface and want it to return the position, velocity, and effort information.

<aside> ⚠️ All joints must return position information for them to be visible in RVIZ.

</aside>

		<xacro:macro name="steering_control"
        params="
        name
        steer_limlow
        steer_limhigh">
        <joint name="${name}_steer_joint">
            <command_interface name="position">
                <param name="min">${steer_limlow}</param>
                <param name="max">${steer_limhigh}</param>
            </command_interface>
            <state_interface name="position" />
            <state_interface name="velocity" />
        </joint>
    </xacro:macro>
    <xacro:macro name="velocity_control"
        params="
        name
        velocity_limlow
        velocity_limhigh">
        <joint name="${name}_wheel_joint">
            <command_interface name="velocity">
                <param name="min">${velocity_limlow}</param>
                <param name="max">${velocity_limhigh}</param>
            </command_interface>
            <state_interface name="position" />
            <state_interface name="velocity" />
            <state_interface name="effort" />
        </joint>
    </xacro:macro>

Next, we add the file inclusion tag, plugin import, and macro calls to our hunter_se.xacro file.

Within the gazebo_ros2_control plugin inclusion, the <parameters> file is set to controllers.yaml, we will create this file next to define our controller types.

		<xacro:include filename="$(find hunter_se_description)/models/xacro/hunter_gazebo.xacro" />

		<gazebo>
        <plugin name="gazebo_ros2_control" filename="libgazebo_ros2_control.so">
            <namespace>${robot_namespace}</namespace>
            <parameters>$(find hunter_se_description)/models/config/controllers.yaml</parameters>
        </plugin>
    </gazebo>

Next, ros2_control tags are created and the macros defined earlier are placed within them. It is important that these ros2_control tags are only called on once, otherwise, the latest call will override earlier calls. The plugin type is set to the gazebo_ros2_control/GazeboSystem plugin.