ROS 2 Migration: Skid Steer drive

Overview

This pages describes the changes for skid steer drive plugin in gazebo_plugins for ROS 2, including a migration guide. The skid steer drive is integrated into diff drive.

From ROS 2, gazebo_ros_diff_drive can be made to behave like skid steer drive by specifying extra joints and kinematic properties. This way, a single plugin works for both differential drive and skid steer.

Summary

• All SDF parameters are now snake_cased
• Use remapping argument to change default topics (cmd_vel and odom)
• Skid steer drive is no longer limited to 4 wheels. Any number of wheels can be specified using num_wheel_pairs. The number of wheels including left and right will be double the num_wheel_pairs.
• To specify the left joints, (instead of leftFrontJoint and leftRearJoint) left_joint has to be specified num_wheel_pairs of times. The first left_joint would be taken as the front one. The second left_joint would be the second from front and so on.
• Similarly right joints have to be specified.
• Instead of a common wheelSeparation, each wheel pair can be assigned a different wheel_separation. wheel_separation can be specified num_wheel_pairs times. The first wheel_separation refers to the front most wheel pair. The second wheel_separation refers to the second from front and so on.
• Similarly, different wheel_diameters can be specified.
• Instead of broadcastTF, publish_wheel_tf and publish_odom_tf can be used to specify independently whether to publish 'wheels TF' or 'odom TF'.

SDF parameters

ROS 1	ROS 2
odometryFrame	odometry_frame
updateRate	update_rate
torque	max_wheel_torque
covariance_x	covariance_x
covariance_y	covariance_y
covariance_yaw	covariance_yaw
commandTopic	<ros><remapping>cmd_vel:=custom_cmd_vel</remapping></ros>
odometryTopic	<ros><remapping>odom:=custom_odom</remapping></ros>
robotBaseFrame	robot_base_frame
wheelSeparation	wheel_separation
wheelDiameter	wheel_diameter
broadcastTF	publish_wheel_tf / publish_odom_tf
leftFrontJoint	1st left_joint
rightFrontJoint	1st right_joint
leftRearJoint	2nd left_joint
rightRearJoint	2nd right_joint

Example Migration

ROS1

    <model name='vehicle'>
      ...

      <joint name='left_wheel_joint0' type='revolute'>
        ...
      </joint>

      <joint name='right_wheel_joint0' type='revolute'>
        ...
      </joint>

      <joint name='left_wheel_joint1' type='revolute'>
        ...
      </joint>

      <joint name='right_wheel_joint1' type='revolute'>
        ...
      </joint>

      <plugin name='skid_steer_drive' filename='libgazebo_ros_skid_steer_drive.so'>
        <updateRate>500</updateRate>

        <leftFrontJoint>left_wheel_joint0</leftFrontJoint>
        <rightFrontJoint>right_wheel_joint0</rightFrontJoint>
        <leftRearJoint>left_wheel_joint1</leftRearJoint>
        <rightRearJoint>right_wheel_joint1</rightRearJoint>

        <wheelSeparation>1.25</wheelSeparation>
        <wheelDiameter>0.3</wheelDiameter>

        <torque>20</torque>

        <commandTopic>cmd_demo</commandTopic>
        <odometryTopic>odom_demo</odometryTopic>
        <odometryFrame>odom_demo</odometryFrame>
        <broadcastTF>true</broadcastTF>
      </plugin>

    </model>

ROS2

    <model name='vehicle'>
      ...

      <joint name='left_wheel_joint0' type='revolute'>
        ...
      </joint>

      <joint name='right_wheel_joint0' type='revolute'>
        ...
      </joint>

      <joint name='left_wheel_joint1' type='revolute'>
        ...
      </joint>

      <joint name='right_wheel_joint1' type='revolute'>
        ...
      </joint>

      <plugin name='skid_steer_drive' filename='libgazebo_ros_diff_drive.so'>
        <ros>
          <!-- Set namespace -->
          <namespace>/demo</namespace>

          <!-- Remap default topics -->
          <remapping>cmd_vel:=cmd_demo</remapping>
          <remapping>odom:=odom_demo</remapping>
        </ros>

        <!-- Update rate -->
        <update_rate>500</update_rate>

        <!-- Number of wheel pairs -->
        <num_wheel_pairs>2</num_wheel_pairs>

        <!-- wheels0 -->
        <left_joint>left_wheel_joint0</left_joint>
        <right_joint>right_wheel_joint0</right_joint>

        <!-- wheels1-->
        <left_joint>left_wheel_joint1</left_joint>
        <right_joint>right_wheel_joint1</right_joint>

        <!-- kinematics -->
        <wheel_separation>1.25</wheel_separation>
        <wheel_separation>1.25</wheel_separation>

        <wheel_diameter>1.0</wheel_diameter>
        <wheel_diameter>0.6</wheel_diameter>

        <!-- limits -->
        <max_wheel_torque>20</max_wheel_torque>
        <max_wheel_acceleration>1.0</max_wheel_acceleration>

        <!-- output -->
        <publish_odom>true</publish_odom>
        <publish_odom_tf>true</publish_odom_tf>
        <publish_wheel_tf>true</publish_wheel_tf>

        <odometry_frame>odom_demo</odometry_frame>
        <robot_base_frame>chassis</robot_base_frame>

      </plugin>

    </model>