编写tf2监听程序 (c ++) [待校准@8673]

Goal目标: Learn了解如何使用tf2访问帧转换。 [待校准@8674]

Tutorial教程级别: Intermediate中级 [待校准@6713]

时间: 10分钟 [Alyssa@7452]

背景

在之前的教程中，我们创建了一个tf2广播器，将海龟的姿势发布到tf2。 [待校准@8675]

在本教程中，我们将创建一个tf2监听程序以开始使用tf2。 [待校准@8676]

先决条件

本教程假设您已经完成了 tf2 broadcaster tutorial (C++) 。在之前的教程中，我们创建了一个 learning_tf2_cpp 包，在这里我们将继续工作。 [待校准@8677]

任务

1写入监听节点 [待校准@8678]

让我们先创建源文件。转到我们在上一教程中创建的 learning_tf2_cpp 包。在 src 目录中，通过输入以下命令下载示例侦听器代码: [待校准@8679]

LinuxmacOSWindows

wget https://raw.githubusercontent.com/ros/geometry_tutorials/ros2/turtle_tf2_cpp/src/turtle_tf2_listener.cpp

使用您喜欢的文本编辑器打开文件。 [待校准@8591]

#include <geometry_msgs/msg/transform_stamped.hpp>
#include <geometry_msgs/msg/twist.hpp>

#include <rclcpp/rclcpp.hpp>
#include <tf2/exceptions.h>
#include <tf2_ros/transform_listener.h>
#include <tf2_ros/buffer.h>
#include <turtlesim/srv/spawn.hpp>

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

using std::placeholders::_1;
using namespace std::chrono_literals;

class FrameListener : public rclcpp::Node
{
public:
  FrameListener()
  : Node("turtle_tf2_frame_listener"),
    turtle_spawning_service_ready_(false),
    turtle_spawned_(false)
  {
    // Declare and acquire `target_frame` parameter
    this->declare_parameter<std::string>("target_frame", "turtle1");
    this->get_parameter("target_frame", target_frame_);

    tf_buffer_ =
      std::make_unique<tf2_ros::Buffer>(this->get_clock());
    transform_listener_ =
      std::make_shared<tf2_ros::TransformListener>(*tf_buffer_);

    // Create a client to spawn a turtle
    spawner_ =
      this->create_client<turtlesim::srv::Spawn>("spawn");

    // Create turtle2 velocity publisher
    publisher_ =
      this->create_publisher<geometry_msgs::msg::Twist>("turtle2/cmd_vel", 1);

    // Call on_timer function every second
    timer_ = this->create_wall_timer(
      1s, std::bind(&FrameListener::on_timer, this));
  }

private:
  void on_timer()
  {
    // Store frame names in variables that will be used to
    // compute transformations
    std::string fromFrameRel = target_frame_.c_str();
    std::string toFrameRel = "turtle2";

    if (turtle_spawning_service_ready_) {
      if (turtle_spawned_) {
        geometry_msgs::msg::TransformStamped transformStamped;

        // Look up for the transformation between target_frame and turtle2 frames
        // and send velocity commands for turtle2 to reach target_frame
        try {
          transformStamped = tf_buffer_->lookupTransform(
            toFrameRel, fromFrameRel,
            tf2::TimePointZero);
        } catch (tf2::TransformException & ex) {
          RCLCPP_INFO(
            this->get_logger(), "Could not transform %s to %s: %s",
            toFrameRel.c_str(), fromFrameRel.c_str(), ex.what());
          return;
        }

        geometry_msgs::msg::Twist msg;

        static const double scaleRotationRate = 1.0;
        msg.angular.z = scaleRotationRate * atan2(
          transformStamped.transform.translation.y,
          transformStamped.transform.translation.x);

        static const double scaleForwardSpeed = 0.5;
        msg.linear.x = scaleForwardSpeed * sqrt(
          pow(transformStamped.transform.translation.x, 2) +
          pow(transformStamped.transform.translation.y, 2));

        publisher_->publish(msg);
      } else {
        RCLCPP_INFO(this->get_logger(), "Successfully spawned");
        turtle_spawned_ = true;
      }
    } else {
      // Check if the service is ready
      if (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
        auto request = std::make_shared<turtlesim::srv::Spawn::Request>();
        request->x = 4.0;
        request->y = 2.0;
        request->theta = 0.0;
        request->name = "turtle2";

        // Call request
        using ServiceResponseFuture =
          rclcpp::Client<turtlesim::srv::Spawn>::SharedFuture;
        auto response_received_callback = [this](ServiceResponseFuture future) {
            auto result = future.get();
            if (strcmp(result->name.c_str(), "turtle2") == 0) {
              turtle_spawning_service_ready_ = true;
            } else {
              RCLCPP_ERROR(this->get_logger(), "Service callback result mismatch");
            }
          };
        auto result = spawner_->async_send_request(request, response_received_callback);
      } else {
        RCLCPP_INFO(this->get_logger(), "Service is not ready");
      }
    }
  }
  // Boolean values to store the information
  // if the service for spawning turtle is available
  bool turtle_spawning_service_ready_;
  // if the turtle was successfully spawned
  bool turtle_spawned_;
  rclcpp::Client<turtlesim::srv::Spawn>::SharedPtr spawner_{nullptr};
  rclcpp::TimerBase::SharedPtr timer_{nullptr};
  rclcpp::Publisher<geometry_msgs::msg::Twist>::SharedPtr publisher_{nullptr};
  std::shared_ptr<tf2_ros::TransformListener> transform_listener_{nullptr};
  std::unique_ptr<tf2_ros::Buffer> tf_buffer_;
  std::string target_frame_;
};

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

1.1检查代码 [待校准@8367]

要了解产卵海龟背后的服务是如何工作的，请参考 writing a simple service and client (C++) 教程。 [待校准@8680]

现在，让我们看一下与访问帧转换相关的代码。 tf2_ros 包含一个 TransformListener 头文件实现，使接收转换的任务更容易。 [待校准@8681]

#include <tf2_ros/transform_listener.h>

在这里，我们创建一个 TransformListener 对象。创建侦听器后，它开始通过导线接收tf2转换，并将其缓冲长达10秒。 [待校准@8682]

transform_listener_ =
  std::make_shared<tf2_ros::TransformListener>(*tf_buffer_);

最后，我们向侦听器查询特定转换。我们使用以下参数调用 lookup_transform 方法: [待校准@8683]

1. 目标帧 [待校准@8559]

2. 源帧 [待校准@8561]

3. 我们想要转换的时间 [待校准@8684]

提供 “tf2::TimePointZero()” 将为我们提供最新的可用转换。所有这些都包装在一个try-catch块中，以处理可能的异常。 [待校准@8685]

transformStamped = tf_buffer_->lookupTransform(
  toFrameRel, fromFrameRel,
  tf2::TimePointZero);

2构建和运行 [待校准@7729]

使用文本编辑器，打开调用 turtle_tf2_demo.launch.py 的launch文件，并在第一个 turtle1 广播器节点后添加以下行。此外，在文件开头包括 DeclareLaunchArgument 和 LaunchConfiguration 的进口: [待校准@8686]

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration

from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        ...,
        DeclareLaunchArgument(
            'target_frame', default_value='turtle1',
            description='Target frame name.'
        ),
        Node(
            package='learning_tf2_cpp',
            executable='turtle_tf2_broadcaster',
            name='broadcaster2',
            parameters=[
                {'turtlename': 'turtle2'}
            ]
        ),
        Node(
            package='learning_tf2_cpp',
            executable='turtle_tf2_listener',
            name='listener',
            parameters=[
                {'target_frame': LaunchConfiguration('target_frame')}
            ]
        ),
    ])

这将宣布一个 target_frame launch的论点，为第二只乌龟启动一个广播器，我们将产卵，听众将订阅这些转变。现在，您可以开始完整的turtle例程了: [待校准@8687]

ros2 launch learning_tf2_cpp turtle_tf2_demo.launch.py

你应该看看有两只海龟的海龟sim。在第二个终端窗口中，键入以下命令: [待校准@8688]

ros2 run turtlesim turtle_teleop_key

3检查结果 [待校准@8476]

要查看是否正常工作，只需使用箭头键绕开第一只乌龟 (确保终端窗口处于活动状态，而不是模拟器窗口)，你会看到第二只乌龟跟着第一只! [待校准@8689]

总结

在本教程中，您学习了如何使用tf2访问帧转换。你也已经完成了你自己的turtlesim例程，这是你第一次在 Introduction to tf2 教程中尝试。 [待校准@8690]