将标记数据类型与“tf2_ros::MessageFilter”结合使用

目标： 学习如何使用“tf2_ros::MessageFilter”来处理带标记的数据类型。

教程级别： 中级

时间： 10 分钟

背景

本教程介绍如何将传感器数据与 tf2 结合使用。

一些现实世界中的传感器数据示例：

• 相机，单声道和立体声

• 激光扫描

假设创建了一个名为“turtle3”的新海龟，并且它没有良好的里程计，但有一个高架相机跟踪其位置并将其发布为与“世界”框架相关的“PointStamped”消息。

“turtle1”想知道“turtle3”与自身相比的位置。

为此，“turtle1”必须监听发布“turtle3”姿势的主题，等到准备好转换为所需框架，然后执行其操作。 为了使这更容易，“tf2_ros::MessageFilter”非常有用。 tf2_ros::MessageFilter 将订阅任何带有标头的 ROS 2 消息并将其缓存，直到可以将其转换为目标框架为止。

任务

1 编写 PointStamped 消息的广播器节点

在本教程中，我们将设置一个演示应用程序，该应用程序有一个节点（使用 Python）来广播 turtle3 的 PointStamped 位置消息。

首先，让我们创建源文件。

转到我们在上一个教程中创建的 learning_tf2_py package。 在“src/learning_tf2_py/learning_tf2_py”目录中输入以下命令下载示例传感器消息广播器代码：

LinuxmacOSWindows

wget https://raw.githubusercontent.com/ros/geometry_tutorials/ros2/turtle_tf2_py/turtle_tf2_py/turtle_tf2_message_broadcaster.py

使用您喜欢的文本编辑器打开该文件。

from geometry_msgs.msg import PointStamped
from geometry_msgs.msg import Twist

import rclpy
from rclpy.node import Node

from turtlesim.msg import Pose
from turtlesim.srv import Spawn


class PointPublisher(Node):

    def __init__(self):
        super().__init__('turtle_tf2_message_broadcaster')

        # Create a client to spawn a turtle
        self.spawner = self.create_client(Spawn, 'spawn')
        # Boolean values to store the information
        # if the service for spawning turtle is available
        self.turtle_spawning_service_ready = False
        # if the turtle was successfully spawned
        self.turtle_spawned = False
        # if the topics of turtle3 can be subscribed
        self.turtle_pose_cansubscribe = False

        self.timer = self.create_timer(1.0, self.on_timer)

    def on_timer(self):
        if self.turtle_spawning_service_ready:
            if self.turtle_spawned:
                self.turtle_pose_cansubscribe = True
            else:
                if self.result.done():
                    self.get_logger().info(
                        f'Successfully spawned {self.result.result().name}')
                    self.turtle_spawned = True
                else:
                    self.get_logger().info('Spawn is not finished')
        else:
            if self.spawner.service_is_ready():
                # Initialize request with turtle name and coordinates
                # Note that x, y and theta are defined as floats in turtlesim/srv/Spawn
                request = Spawn.Request()
                request.name = 'turtle3'
                request.x = 4.0
                request.y = 2.0
                request.theta = 0.0
                # Call request
                self.result = self.spawner.call_async(request)
                self.turtle_spawning_service_ready = True
            else:
                # Check if the service is ready
                self.get_logger().info('Service is not ready')

        if self.turtle_pose_cansubscribe:
            self.vel_pub = self.create_publisher(Twist, 'turtle3/cmd_vel', 10)
            self.sub = self.create_subscription(Pose, 'turtle3/pose', self.handle_turtle_pose, 10)
            self.pub = self.create_publisher(PointStamped, 'turtle3/turtle_point_stamped', 10)

    def handle_turtle_pose(self, msg):
        vel_msg = Twist()
        vel_msg.linear.x = 1.0
        vel_msg.angular.z = 1.0
        self.vel_pub.publish(vel_msg)

        ps = PointStamped()
        ps.header.stamp = self.get_clock().now().to_msg()
        ps.header.frame_id = 'world'
        ps.point.x = msg.x
        ps.point.y = msg.y
        ps.point.z = 0.0
        self.pub.publish(ps)


def main():
    rclpy.init()
    node = PointPublisher()
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass

    rclpy.shutdown()

1.1 检查代码

现在让我们看一下代码。 首先，在 on_timer 回调函数中，我们通过异步调用 turtlesim 的 Spawn 服务来生成 turtle3，并在海龟生成服务准备就绪时将其位置初始化为 (4, 2, 0)。

# Initialize request with turtle name and coordinates
# Note that x, y and theta are defined as floats in turtlesim/srv/Spawn
request = Spawn.Request()
request.name = 'turtle3'
request.x = 4.0
request.y = 2.0
request.theta = 0.0
# Call request
self.result = self.spawner.call_async(request)

随后，节点发布主题“turtle3/cmd_vel”、主题“turtle3/turtle_point_stamped”，并订阅主题“turtle3/pose”，并对每条传入消息运行回调函数“handle_turtle_pose”。

self.vel_pub = self.create_publisher(Twist, '/turtle3/cmd_vel', 10)
self.sub = self.create_subscription(Pose, '/turtle3/pose', self.handle_turtle_pose, 10)
self.pub = self.create_publisher(PointStamped, '/turtle3/turtle_point_stamped', 10)

最后，在回调函数“handle_turtle_pose”中，我们初始化“turtle3”的“Twist”消息并发布它们，这将使“turtle3”沿着圆圈移动。然后我们用传入的“Pose”消息填充“turtle3”的“PointStamped”消息并发布它们。

vel_msg = Twist()
vel_msg.linear.x = 1.0
vel_msg.angular.z = 1.0
self.vel_pub.publish(vel_msg)

ps = PointStamped()
ps.header.stamp = self.get_clock().now().to_msg()
ps.header.frame_id = 'world'
ps.point.x = msg.x
ps.point.y = msg.y
ps.point.z = 0.0
self.pub.publish(ps)

1.2 编写启动文件

为了运行此演示，我们需要在包“learning_tf2_py”的“launch”子目录中创建一个启动文件“turtle_tf2_sensor_message_launch.py​​”：

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch_ros.actions import Node


def generate_launch_description():
    return LaunchDescription([
        DeclareLaunchArgument(
            'target_frame', default_value='turtle1',
            description='Target frame name.'
        ),
        Node(
            package='turtlesim',
            executable='turtlesim_node',
            name='sim',
            output='screen'
        ),
        Node(
            package='turtle_tf2_py',
            executable='turtle_tf2_broadcaster',
            name='broadcaster1',
            parameters=[
                {'turtlename': 'turtle1'}
            ]
        ),
        Node(
            package='turtle_tf2_py',
            executable='turtle_tf2_broadcaster',
            name='broadcaster2',
            parameters=[
                {'turtlename': 'turtle3'}
            ]
        ),
        Node(
            package='turtle_tf2_py',
            executable='turtle_tf2_message_broadcaster',
            name='message_broadcaster',
        ),
    ])

1.3 添加入口点

要允许“ros2 run”命令运行您的节点，您必须将入口点添加到“setup.py”（位于“src/learning_tf2_py”目录中）。

在“’console_scripts’:”括号之间添加以下行：

'turtle_tf2_message_broadcaster = learning_tf2_py.turtle_tf2_message_broadcaster:main',

1.4 构建

在工作区根目录中运行“rosdep”以检查是否缺少依赖项。

LinuxmacOSWindows

rosdep install -i --from-path src --rosdistro rolling -y

然后我们就可以构建包：

LinuxmacOSWindows

colcon build --packages-select learning_tf2_py

2 编写消息过滤器/侦听器节点

现在，为了可靠地获取“turtle1”框架中“turtle3”的流式“PointStamped”数据，我们将创建消息过滤器/侦听器节点的源文件。

转到我们在上一个教程中创建的“learning_tf2_cpp”：doc：package。 在“src/learning_tf2_cpp/src”目录中，输入以下命令下载文件“turtle_tf2_message_filter.cpp”：

LinuxmacOSWindows

wget https://raw.githubusercontent.com/ros/geometry_tutorials/ros2/turtle_tf2_cpp/src/turtle_tf2_message_filter.cpp

使用您喜欢的文本编辑器打开该文件。

#include <chrono>
#include <memory>
#include <string>

#include "geometry_msgs/msg/point_stamped.hpp"
#include "message_filters/subscriber.h"
#include "rclcpp/rclcpp.hpp"
#include "tf2_ros/buffer.h"
#include "tf2_ros/create_timer_ros.h"
#include "tf2_ros/message_filter.h"
#include "tf2_ros/transform_listener.h"
#ifdef TF2_CPP_HEADERS
  #include "tf2_geometry_msgs/tf2_geometry_msgs.hpp"
#else
  #include "tf2_geometry_msgs/tf2_geometry_msgs.h"
#endif

using namespace std::chrono_literals;

class PoseDrawer : public rclcpp::Node
{
public:
  PoseDrawer()
  : Node("turtle_tf2_pose_drawer")
  {
    // Declare and acquire `target_frame` parameter
    target_frame_ = this->declare_parameter<std::string>("target_frame", "turtle1");

    std::chrono::duration<int> buffer_timeout(1);

    tf2_buffer_ = std::make_shared<tf2_ros::Buffer>(this->get_clock());
    // Create the timer interface before call to waitForTransform,
    // to avoid a tf2_ros::CreateTimerInterfaceException exception
    auto timer_interface = std::make_shared<tf2_ros::CreateTimerROS>(
      this->get_node_base_interface(),
      this->get_node_timers_interface());
    tf2_buffer_->setCreateTimerInterface(timer_interface);
    tf2_listener_ =
      std::make_shared<tf2_ros::TransformListener>(*tf2_buffer_);

    point_sub_.subscribe(this, "/turtle3/turtle_point_stamped");
    tf2_filter_ = std::make_shared<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>>(
      point_sub_, *tf2_buffer_, target_frame_, 100, this->get_node_logging_interface(),
      this->get_node_clock_interface(), buffer_timeout);
    // Register a callback with tf2_ros::MessageFilter to be called when transforms are available
    tf2_filter_->registerCallback(&PoseDrawer::msgCallback, this);
  }

private:
  void msgCallback(const geometry_msgs::msg::PointStamped::SharedPtr point_ptr)
  {
    geometry_msgs::msg::PointStamped point_out;
    try {
      tf2_buffer_->transform(*point_ptr, point_out, target_frame_);
      RCLCPP_INFO(
        this->get_logger(), "Point of turtle3 in frame of turtle1: x:%f y:%f z:%f\n",
        point_out.point.x,
        point_out.point.y,
        point_out.point.z);
    } catch (const tf2::TransformException & ex) {
      RCLCPP_WARN(
        // Print exception which was caught
        this->get_logger(), "Failure %s\n", ex.what());
    }
  }

  std::string target_frame_;
  std::shared_ptr<tf2_ros::Buffer> tf2_buffer_;
  std::shared_ptr<tf2_ros::TransformListener> tf2_listener_;
  message_filters::Subscriber<geometry_msgs::msg::PointStamped> point_sub_;
  std::shared_ptr<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>> tf2_filter_;
};

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<PoseDrawer>());
  rclcpp::shutdown();
  return 0;
}

2.1 检查代码

首先，您必须包含来自“tf2_ros”包的“tf2_ros::MessageFilter”标头，以及之前使用的“tf2”和“ros2”相关标头。

#include "geometry_msgs/msg/point_stamped.hpp"
#include "message_filters/subscriber.h"
#include "rclcpp/rclcpp.hpp"
#include "tf2_ros/buffer.h"
#include "tf2_ros/create_timer_ros.h"
#include "tf2_ros/message_filter.h"
#include "tf2_ros/transform_listener.h"
#ifdef TF2_CPP_HEADERS
  #include "tf2_geometry_msgs/tf2_geometry_msgs.hpp"
#else
  #include "tf2_geometry_msgs/tf2_geometry_msgs.h"
#endif

其次，需要有“tf2_ros::Buffer”、“tf2_ros::TransformListener”和“tf2_ros::MessageFilter”的持久实例。

std::string target_frame_;
std::shared_ptr<tf2_ros::Buffer> tf2_buffer_;
std::shared_ptr<tf2_ros::TransformListener> tf2_listener_;
message_filters::Subscriber<geometry_msgs::msg::PointStamped> point_sub_;
std::shared_ptr<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>> tf2_filter_;

第三，必须使用主题初始化 ROS 2 message_filters::Subscriber。 并且必须使用该 Subscriber 对象初始化 tf2_ros::MessageFilter。 MessageFilter 构造函数中值得注意的其他参数是 target_frame 和回调函数。 目标框架是确保 canTransform 成功进入的框架。 回调函数是数据准备就绪时将调用的函数。

PoseDrawer()
: Node("turtle_tf2_pose_drawer")
{
  // Declare and acquire `target_frame` parameter
  target_frame_ = this->declare_parameter<std::string>("target_frame", "turtle1");

  std::chrono::duration<int> buffer_timeout(1);

  tf2_buffer_ = std::make_shared<tf2_ros::Buffer>(this->get_clock());
  // Create the timer interface before call to waitForTransform,
  // to avoid a tf2_ros::CreateTimerInterfaceException exception
  auto timer_interface = std::make_shared<tf2_ros::CreateTimerROS>(
    this->get_node_base_interface(),
    this->get_node_timers_interface());
  tf2_buffer_->setCreateTimerInterface(timer_interface);
  tf2_listener_ =
    std::make_shared<tf2_ros::TransformListener>(*tf2_buffer_);

  point_sub_.subscribe(this, "/turtle3/turtle_point_stamped");
  tf2_filter_ = std::make_shared<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>>(
    point_sub_, *tf2_buffer_, target_frame_, 100, this->get_node_logging_interface(),
    this->get_node_clock_interface(), buffer_timeout);
  // Register a callback with tf2_ros::MessageFilter to be called when transforms are available
  tf2_filter_->registerCallback(&PoseDrawer::msgCallback, this);
}

最后，当数据准备就绪时，回调方法将调用“tf2_buffer_->transform”并将输出打印到控制台。

private:
  void msgCallback(const geometry_msgs::msg::PointStamped::SharedPtr point_ptr)
  {
    geometry_msgs::msg::PointStamped point_out;
    try {
      tf2_buffer_->transform(*point_ptr, point_out, target_frame_);
      RCLCPP_INFO(
        this->get_logger(), "Point of turtle3 in frame of turtle1: x:%f y:%f z:%f\n",
        point_out.point.x,
        point_out.point.y,
        point_out.point.z);
    } catch (const tf2::TransformException & ex) {
      RCLCPP_WARN(
        // Print exception which was caught
        this->get_logger(), "Failure %s\n", ex.what());
    }
  }

2.2 添加依赖项

在构建包“learning_tf2_cpp”之前，请在此包的“package.xml”文件中添加另外两个依赖项：

<depend>message_filters</depend>
<depend>tf2_geometry_msgs</depend>

2.3 CMakeLists.txt

并在“CMakeLists.txt”文件中，在现有的依赖项下方添加两行：

find_package(message_filters REQUIRED)
find_package(tf2_geometry_msgs REQUIRED)

下面几行将处理 ROS 发行版之间的差异：

if(TARGET tf2_geometry_msgs::tf2_geometry_msgs)
  get_target_property(_include_dirs tf2_geometry_msgs::tf2_geometry_msgs INTERFACE_INCLUDE_DIRECTORIES)
else()
  set(_include_dirs ${tf2_geometry_msgs_INCLUDE_DIRS})
endif()

find_file(TF2_CPP_HEADERS
  NAMES tf2_geometry_msgs.hpp
  PATHS ${_include_dirs}
  NO_CACHE
  PATH_SUFFIXES tf2_geometry_msgs
)

之后，添加可执行文件并将其命名为“turtle_tf2_message_filter”，稍后您将在“ros2 run”中使用它。

add_executable(turtle_tf2_message_filter src/turtle_tf2_message_filter.cpp)
ament_target_dependencies(
  turtle_tf2_message_filter
  geometry_msgs
  message_filters
  rclcpp
  tf2
  tf2_geometry_msgs
  tf2_ros
)

if(EXISTS ${TF2_CPP_HEADERS})
  target_compile_definitions(turtle_tf2_message_filter PUBLIC -DTF2_CPP_HEADERS)
endif()

最后，添加“install（TARGETS…）”部分（在其他现有节点下方），以便“ros2 run”可以找到您的可执行文件：

install(TARGETS
  turtle_tf2_message_filter
  DESTINATION lib/${PROJECT_NAME})

2.4 构建

在工作区根目录中运行“rosdep”以检查是否缺少依赖项。

LinuxmacOSWindows

rosdep install -i --from-path src --rosdistro rolling -y

现在打开一个新终端，导航到工作区的根目录，然后使用命令重建包：

LinuxmacOSWindows

colcon build --packages-select learning_tf2_cpp

打开一个新终端，导航到工作区的根目录，然后获取安装文件：

LinuxmacOSWindows

. install/setup.bash

3 运行

首先我们需要通过启动启动文件“turtle_tf2_sensor_message_launch.py​​”来运行几个节点（包括PointStamped消息的广播节点）：

ros2 launch learning_tf2_py turtle_tf2_sensor_message_launch.py

这将打开“turtlesim”窗口，其中有两个海龟，其中“turtle3”沿圆圈移动，而“turtle1”最初不动。 但你可以在另一个终端运行“turtle_teleop_key”节点来驱动“turtle1”移动：

ros2 run turtlesim turtle_teleop_key

现在如果你回应主题“turtle3/turtle_point_stamped”：

ros2 topic echo /turtle3/turtle_point_stamped

然后会有这样的输出：

header:
  stamp:
    sec: 1629877510
    nanosec: 902607040
  frame_id: world
point:
  x: 4.989276885986328
  y: 3.073937177658081
  z: 0.0
---
header:
  stamp:
    sec: 1629877510
    nanosec: 918389395
  frame_id: world
point:
  x: 4.987966060638428
  y: 3.089883327484131
  z: 0.0
---
header:
  stamp:
    sec: 1629877510
    nanosec: 934186680
  frame_id: world
point:
  x: 4.986400127410889
  y: 3.105806589126587
  z: 0.0
---

当演示运行时，打开另一个终端并运行消息过滤器/监听器节点：

ros2 run learning_tf2_cpp turtle_tf2_message_filter

如果运行正确，您应该会看到如下流数据：

[INFO] [1630016162.006173900] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.493231 y:-2.961614 z:0.000000

[INFO] [1630016162.006291983] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.472169 y:-3.004742 z:0.000000

[INFO] [1630016162.006326234] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.479420 y:-2.990479 z:0.000000

[INFO] [1630016162.006355644] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.486441 y:-2.976102 z:0.000000

摘要

在本教程中，您学习了如何在 tf2 中使用传感器数据/消息。 具体来说，您学习了如何在主题上发布“PointStamped”消息，以及如何监听主题并使用“tf2_ros::MessageFilter”转换“PointStamped”消息的框架。