ROS2笔记-2：第一个在Gazebo中能动的例子

写在前面

第一个在Gazebo中能动的例子 是指对我来说。在Gazebo中显示，第二个要求是能动。
这个目标对我来说并不容易。可能是我没有装实体机的原因。
如果在MobaStrem中，当然不需要考虑X-windows的问题，但如果在cursor中，就是一个大困难。

X-windows 与cursor

在学习新技术时，我一般需要cursor.
怎么说呢，我不想看书。不是我不想看，一般来说，也没有有书可以看。
再者，人脑容量是有限的，我们需要用一种哲学模式，来描述世界，我们需要在学习一种技术之前，利用哲学窠臼，来思考，如果是我们自己来实现，应当如何做。

所以，我一般都是放弃深度思考，反复与cursor磨合，看它如何进展和不断地犯错，来思考。
但是对X-windows的支持，cursor做得不好。这样一来，我就无法搞清楚是vmware带来的问题还是cursor的问题。
虽然，用手工来执行，也是一个方法，但那并不是个好主意。

所以，想来想去想了一晚上，才想到直接装到ubuntu上。真的是，这么个简单的答案，我一直没想到。
在ubuntu上，我又来到基本的目标，就是机器人能动。

＃ 部分笔记
因为ubuntu 的桌面，这几年没怎么用。
所以，也不想装输入法，就英文吧。蹩脚，乱，是难免的。

Simple ROS2 Robot

A simple differential drive robot implementation in ROS2 Humble with Gazebo simulation. This project serves as a learning platform for ROS2 and Gazebo integration.

Project Structure

simple_robot_ws/
├── src/
│   └── simple_robot/
│       ├── simple_robot/
│       │   ├── launch/
│       │   │   └── robot_gazebo.launch.py
│       │   ├── robot.urdf.xacro
│       │   └── square_movement.py
│       ├── package.xml
│       └── setup.py

Features

• Simple differential drive robot with two wheels
• Gazebo simulation integration
• Square movement pattern demonstration
• ROS2 Humble compatible

Dependencies

• ROS2 Humble
• Gazebo
• Python 3.10
• Required ROS2 packages:
  • gazebo_ros_pkgs
  • gazebo_ros2_control
  • robot_state_publisher
  • xacro

Installation

1. Clone this repository into your ROS2 workspace:

cd ~/ros2_ws/src
git clone <repository-url> simple_robot

2. Install dependencies:

sudo apt-get update
sudo apt-get install -y ros-humble-gazebo-ros-pkgs ros-humble-gazebo-ros2-control ros-humble-robot-state-publisher ros-humble-xacro

3. Build the package:

cd ~/ros2_ws
colcon build --symlink-install

Usage

1. Source the workspace:

source install/setup.bash

2. Launch the robot in Gazebo:

ros2 launch simple_robot robot_gazebo.launch.py

3. In a new terminal, run the square movement node:

source install/setup.bash
ros2 run simple_robot square_movement

Robot Description

The robot consists of:

• A blue rectangular base (0.3m x 0.3m x 0.1m)
• Two black wheels (radius: 0.05m, width: 0.04m)
• Differential drive control system

Movement Pattern

The robot follows a square pattern:

1. Moves forward for 5 seconds at 0.2 m/s
2. Turns 90 degrees at 0.5 rad/s
3. Repeats the pattern

Customization

You can modify:

• Robot dimensions in robot.urdf.xacro
• Movement speed and pattern in square_movement.py
• Launch parameters in robot_gazebo.launch.py

后面我会把这个代码放到github.

解释

the main source code files in detail to help you understand ROS2 concepts:

1. First, let’s look at the URDF file (robot.urdf.xacro):

---

Read file: /data/myrobot/simple_robot_ws/src/simple_robot/simple_robot/robot.urdf.xacro

---

Let me explain the key components of the URDF file:

1. Properties and Parameters:

<xacro:property name="PI" value="3.1415926535897931"/>
<xacro:property name="robot_width" value="0.3"/>

• These are reusable variables that define robot dimensions
• Makes it easy to modify robot size by changing one value

2. Links (Physical parts of the robot):

<link name="base_link">
    <visual>...</visual>
    <collision>...</collision>
    <inertial>...</inertial>
</link>

• visual: Defines how the robot looks in simulation
• collision: Defines collision geometry for physics
• inertial: Defines mass and inertia properties

3. Joints (Connections between links):

<joint name="base_right_wheel_joint" type="continuous">
    <parent link="base_link"/>
    <child link="right_wheel"/>
    <origin xyz="0 ${-(robot_width/2 + wheel_width/2)} 0" rpy="${PI/2} 0 0"/>
    <axis xyz="0 0 1"/>
</joint>

• type="continuous": Allows unlimited rotation (for wheels)
• parent and child: Defines which links are connected
• origin: Position and orientation of the joint
• axis: Axis of rotation

4. Gazebo Plugin (Simulation interface):

<gazebo>
    <plugin name="differential_drive" filename="libgazebo_ros_diff_drive.so">
        <left_joint>base_left_wheel_joint</left_joint>
        <right_joint>base_right_wheel_joint</right_joint>
        ...
    </plugin>
</gazebo>

• Connects ROS2 to Gazebo simulation
• Handles differential drive control
• Publishes odometry information

Now, let’s look at the Python node that controls the robot:

---

Read file: /data/myrobot/simple_robot_ws/src/simple_robot/simple_robot/simple_robot/square_movement.py

---

Let me explain the Python node:

1. Node Initialization:

def __init__(self):
    super().__init__('square_movement')
    self.publisher = self.create_publisher(Twist, '/cmd_vel', 10)
    self.timer = self.create_timer(0.1, self.timer_callback)

• Creates a ROS2 node named ‘square_movement’
• Creates a publisher for Twist messages on ‘/cmd_vel’ topic
• Sets up a timer that calls timer_callback every 0.1 seconds

2. Movement Control:

def timer_callback(self):
    current_time = self.get_clock().now()
    elapsed_time = (current_time - self.state_start_time).nanoseconds / 1e9
    
    msg = Twist()
    
    if self.state == 'forward':
        msg.linear.x = 0.2  # Move forward at 0.2 m/s
        msg.angular.z = 0.0
        if elapsed_time >= 5.0:  # After 5 seconds
            self.state = 'turn'
    elif self.state == 'turn':
        msg.linear.x = 0.0
        msg.angular.z = 0.5  # Turn at 0.5 rad/s
        if elapsed_time >= 3.14:  # After ~90 degrees
            self.state = 'forward'

• Uses a state machine to control robot movement
• linear.x: Forward/backward velocity
• angular.z: Rotational velocity
• Switches between forward and turning states

3. Main Function:

def main(args=None):
    rclpy.init(args=args)
    node = SquareMovement()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

• Initializes ROS2
• Creates and runs the node
• Handles cleanup on shutdown

Finally, let’s look at the launch file:

---

Read file: /data/myrobot/simple_robot_ws/src/simple_robot/simple_robot/launch/robot_gazebo.launch.py

---

The launch file is responsible for starting all necessary components:

1. URDF Processing:

xacro_file = os.path.join(pkg_dir, 'robot.urdf.xacro')
robot_description_config = xacro.process_file(xacro_file)
robot_description = {'robot_description': robot_description_config.toxml()}

• Loads and processes the URDF file
• Converts Xacro macros into XML

2. Gazebo Launch:

gazebo_launch = IncludeLaunchDescription(
    PythonLaunchDescriptionSource([os.path.join(
        get_package_share_directory('gazebo_ros'), 'launch', 'gazebo.launch.py')]),
)

• Starts the Gazebo simulator

3. Robot State Publisher:

robot_state_publisher = Node(
    package='robot_state_publisher',
    executable='robot_state_publisher',
    output='screen',
    parameters=[robot_description]
)

• Publishes robot joint states
• Required for visualization and control

4. Robot Spawning:

spawn_entity = Node(
    package='gazebo_ros',
    executable='spawn_entity.py',
    arguments=['-entity', 'simple_robot',
              '-topic', 'robot_description'],
    output='screen'
)

• Spawns the robot in Gazebo
• Uses the processed URDF description

Key ROS2 Concepts Demonstrated:

1. Nodes: Independent processes (square_movement.py)
2. Topics: Communication channels (‘/cmd_vel’)
3. Messages: Data structures (Twist)
4. Launch System: Starting multiple nodes
5. URDF/Xacro: Robot description
6. Gazebo Integration: Simulation
7. Parameters: Configuration values
8. Publishers/Subscribers: Communication pattern

运行的效果

启动

root@ros:/data/myrobot/simple_robot_ws# source install/setup.bash && ros2 launch simple_robot robot_gazebo.launch.py
[INFO] [launch]: All log files can be found below /root/.ros/log/2025-04-06-12-30-45-332217-ros-4626
[INFO] [launch]: Default logging verbosity is set to INFO
[INFO] [gzserver-1]: process started with pid [4634]
[INFO] [gzclient-2]: process started with pid [4636]
[INFO] [robot_state_publisher-3]: process started with pid [4638]
[INFO] [spawn_entity.py-4]: process started with pid [4640]
[robot_state_publisher-3] [WARN] [1743913845.758133319] [kdl_parser]: The root link base_link has an inertia specified in the URDF, but KDL does not support a root link with an inertia.  As a workaround, you can add an extra dummy link to your URDF.
[robot_state_publisher-3] [INFO] [1743913845.758202925] [robot_state_publisher]: got segment base_link
[robot_state_publisher-3] [INFO] [1743913845.758239006] [robot_state_publisher]: got segment left_wheel
[robot_state_publisher-3] [INFO] [1743913845.758242357] [robot_state_publisher]: got segment right_wheel
[spawn_entity.py-4] [INFO] [1743913845.891193009] [spawn_entity]: Spawn Entity started
[spawn_entity.py-4] [INFO] [1743913845.891440305] [spawn_entity]: Loading entity published on topic robot_description
[spawn_entity.py-4] [INFO] [1743913845.892783074] [spawn_entity]: Waiting for entity xml on robot_description
[spawn_entity.py-4] [INFO] [1743913846.020159987] [spawn_entity]: Waiting for service /spawn_entity, timeout = 30
[spawn_entity.py-4] [INFO] [1743913846.020361422] [spawn_entity]: Waiting for service /spawn_entity
[spawn_entity.py-4] [INFO] [1743913847.038242167] [spawn_entity]: Calling service /spawn_entity
[spawn_entity.py-4] [INFO] [1743913847.248558627] [spawn_entity]: Spawn status: SpawnEntity: Successfully spawned entity [simple_robot]
[gzserver-1] [INFO] [1743913847.262162662] [differential_drive]: Wheel pair 1 separation set to [0.340000m]
[gzserver-1] [INFO] [1743913847.262435027] [differential_drive]: Wheel pair 1 diameter set to [0.100000m]
[gzserver-1] [INFO] [1743913847.263072041] [differential_drive]: Subscribed to [/cmd_vel]
[gzserver-1] [INFO] [1743913847.264434814] [differential_drive]: Advertise odometry on [/odom]
[gzserver-1] [INFO] [1743913847.266079557] [differential_drive]: Publishing odom transforms between [odom] and [base_link]
[gzserver-1] [INFO] [1743913847.266098937] [differential_drive]: Publishing wheel transforms between [base_link], [base_left_wheel_joint] and [base_right_wheel_joint]
[INFO] [spawn_entity.py-4]: process has finished cleanly [pid 4640]
[gzclient-2] context mismatch in svga_surface_destroy
[gzclient-2] context mismatch in svga_surface_destroy
^C[WARNING] [launch]: user interrupted with ctrl-c (SIGINT)
[robot_state_publisher-3] [INFO] [1743917858.696625720] [rclcpp]: signal_handler(signum=2)
[gzclient-2] context mismatch in svga_surface_destroy
[gzclient-2] context mismatch in svga_surface_destroy
[INFO] [robot_state_publisher-3]: process has finished cleanly [pid 4638]
[INFO] [gzclient-2]: process has finished cleanly [pid 4636]
[INFO] [gzserver-1]: process has finished cleanly [pid 4634]
root@ros:/data/myrobot/simple_robot_ws# source install/setup.bash && ros2 launch simple_robot robot_gazebo.launch.py
[INFO] [launch]: All log files can be found below /root/.ros/log/2025-04-06-13-37-49-581935-ros-21692
[INFO] [launch]: Default logging verbosity is set to INFO
[INFO] [gzserver-1]: process started with pid [21693]
[INFO] [gzclient-2]: process started with pid [21695]
[INFO] [robot_state_publisher-3]: process started with pid [21697]
[INFO] [spawn_entity.py-4]: process started with pid [21699]
[robot_state_publisher-3] [WARN] [1743917869.961730595] [kdl_parser]: The root link base_link has an inertia specified in the URDF, but KDL does not support a root link with an inertia.  As a workaround, you can add an extra dummy link to your URDF.
[robot_state_publisher-3] [INFO] [1743917869.961810285] [robot_state_publisher]: got segment base_link
[robot_state_publisher-3] [INFO] [1743917869.961840435] [robot_state_publisher]: got segment left_wheel
[robot_state_publisher-3] [INFO] [1743917869.961843738] [robot_state_publisher]: got segment right_wheel
[spawn_entity.py-4] [INFO] [1743917870.115293166] [spawn_entity]: Spawn Entity started
[spawn_entity.py-4] [INFO] [1743917870.115563623] [spawn_entity]: Loading entity published on topic robot_description
[spawn_entity.py-4] [INFO] [1743917870.127828694] [spawn_entity]: Waiting for entity xml on robot_description
[spawn_entity.py-4] [INFO] [1743917870.140340287] [spawn_entity]: Waiting for service /spawn_entity, timeout = 30
[spawn_entity.py-4] [INFO] [1743917870.140550074] [spawn_entity]: Waiting for service /spawn_entity
[spawn_entity.py-4] [INFO] [1743917871.185531000] [spawn_entity]: Calling service /spawn_entity
[spawn_entity.py-4] [INFO] [1743917871.393789340] [spawn_entity]: Spawn status: SpawnEntity: Successfully spawned entity [simple_robot]
[gzserver-1] [INFO] [1743917871.415619114] [differential_drive]: Wheel pair 1 separation set to [0.340000m]
[gzserver-1] [INFO] [1743917871.416094928] [differential_drive]: Wheel pair 1 diameter set to [0.100000m]
[gzserver-1] [INFO] [1743917871.416994931] [differential_drive]: Subscribed to [/cmd_vel]
[gzserver-1] [INFO] [1743917871.418813122] [differential_drive]: Advertise odometry on [/odom]
[gzserver-1] [INFO] [1743917871.421652281] [differential_drive]: Publishing odom transforms between [odom] and [base_link]
[gzserver-1] [INFO] [1743917871.421705231] [differential_drive]: Publishing wheel transforms between [base_link], [base_left_wheel_joint] and [base_right_wheel_joint]
[INFO] [spawn_entity.py-4]: process has finished cleanly [pid 21699]
[gzclient-2] context mismatch in svga_surface_destroy
[gzclient-2] context mismatch in svga_surface_destroy

控制机器人移动

root@ros:/data/myrobot/simple_robot_ws# cd /data/myrobot/simple_robot_ws && source install/setup.bash && ros2 run simple_robot square_movement

代码

https://download.csdn.net/download/haoyujie/90571841
不需要积分。
突然懒病就犯了。不想传到github去了。