DiffDrive

Differential drive controller which can be attached to a model with any number of left and right wheels. more

Gazebo

<plugin
    filename="gz-sim-diff-drive-system"
    name="gz::sim::systems::DiffDrive">
    <left_joint>left_wheel_joint</left_joint>
    <right_joint>right_wheel_joint</right_joint>
    <wheel_separation>1.2</wheel_separation>
    <wheel_radius>0.4</wheel_radius>
    <odom_publish_frequency>1</odom_publish_frequency>
    <topic>cmd_vel</topic>
    <odom_topic>odom</odom_topic>
    <frame_id>odom</frame_id>
    <child_frame_id>base_footprint</child_frame_id>
</plugin>

• tf_topic: Custom topic on which this system will publish the transform from frame_id to child_frame_id. This element is optional, and the default value is /model/{name_of_model}/tf.
• frame_id: Custom frame_id field that this system will use as the origin of the odometry transform in both the gz.msgs.Pose_V message and the gz.msgs.Odometry message. This element if optional, and the default value is {name_of_model}/odom. child_frame_id: Custom child_frame_id that this system will use as the target of the odometry trasnform in both the gz.msgs.Pose_V message and the gz.msgs.Odometry message. This element if optional, and the default value is {name_of_model}/{name_of_link}.

child_frame_id

According to REP-105 (coordinate frames for mobile platforms):

• header.frame_id = "odom" (or "map" if you have localization).
• child_frame_id = "base_link" (or base_footprint depending on your URDF).

Rule of Thumb - header.frame_id = where the robot is located (world reference). - child_frame_id = what part of the robot is moving (usually base_link).

Twist from gazebo cli

gz topic -t "/cmd_vel" -m gz.msgs.Twist -p "linear: {x: 0.5}, angular: {z: 0.05}"

---

ROS

bridge file

diff-drive tf topic name

the default tf topic name is : /model/{name_of_model}/tf we control the tf name using tf_topic tag check the bride yaml tf part for correct mapping

#sync sim and ros
- ros_topic_name: "/clock"
  gz_topic_name: "/clock"
  ros_type_name: "rosgraph_msgs/msg/Clock"
  gz_type_name: "gz.msgs.Clock"
  direction: GZ_TO_ROS

# gz topic published by DiffDrive plugin
- ros_topic_name: "odom"
  gz_topic_name: "odom"
  ros_type_name: "nav_msgs/msg/Odometry"
  gz_type_name: "gz.msgs.Odometry"
  direction: GZ_TO_ROS

# gz topic published by DiffDrive plugin
- ros_topic_name: "tf"
  gz_topic_name: "tf"
  ros_type_name: "tf2_msgs/msg/TFMessage"
  gz_type_name: "gz.msgs.Pose_V"
  direction: GZ_TO_ROS

# gz topic subscribed to by DiffDrive plugin
- ros_topic_name: "cmd_vel"
  gz_topic_name: "cmd_vel"
  ros_type_name: "geometry_msgs/msg/Twist"
  gz_type_name: "gz.msgs.Twist"
  direction: ROS_TO_GZ

Twist From ROS

ros2 topic pub /cmd_vel geometry_msgs/msg/Twist \
"{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}"

Rviz

• The base of the red arrow mark the robot position
• The arrow direction display the robot heading

TF tree

The tf tree include tf from joint_states

#joint state
- ros_topic_name: "joint_states"
  gz_topic_name: "joint_states"
  ros_type_name: "sensor_msgs/msg/JointState"
  gz_type_name: "gz.msgs.Model"
  direction: GZ_TO_ROS

---

Usgae

Rqt has nice plugin robot steering to send Twist message

sudo apt install ros-jazzy-rqt-robot-steering

The nav_msgs/Odometry message contains two main fields:

• pose: robot position + orientation in some reference frame (usually odom)
• twist: robot linear & angular velocities in body reference frame

The important detail is that the twist is expressed in the robot’s local frame (the FRD/FLU convention,

Twist

The twist message expressed in the robot's local frame (FRD/FLU) REP-103 defines the reference frame for mobile base (differential drive) as FLU (Forward, Left, Up):

• x-axis: point forward
• y-axis: point left
• z-axis: point up