大纲
在ROS 2（Robot Operating System 2）中，Action是一种强大的通信机制，特别适用于需要长时间运行且需要定期反馈的任务。Action结合了话题（Topic）和服务（Service）的特性，提供了一个更加灵活和强大的方式来控制和管理机器人的复杂行为。
Action概念
组成
Action由三个主要部分组成：
- 目标（Goal）：这是由Action客户端（Client）发送给Action服务器（Server）的请求，指示服务器需要执行的具体任务。例如，在控制一个机器人旋转的任务中，目标可能是指定机器人需要旋转的角度。
- 反馈（Feedback）：在执行目标的过程中，Action服务器会定期向客户端发送反馈，以报告当前的执行状态和进度。这种反馈机制使得客户端能够实时了解任务的执行情况，并在必要时进行干预或调整。
- 结果（Result）：当目标执行完成后，Action服务器会向客户端发送一个结果，表明任务的最终执行状态。这个结果可以是成功完成、失败或者部分完成等状态信息。
特点
- 可抢占性：与服务（Service）不同，Action是可抢占的。这意味着在执行过程中，客户端可以随时取消当前的目标，或者发送一个新的目标来覆盖旧的目标。这种特性使得Action在控制复杂机器人行为时更加灵活和可靠。
- 稳定的反馈：Action提供了持续的反馈机制，这使得客户端能够实时跟踪任务的执行情况。这种稳定的反馈不仅提高了系统的透明度和可控性，还使得客户端能够更好地适应任务执行过程中的变化。
- 长时间的运行：由于Action适用于长时间运行的任务，因此它非常适合用于控制需要较长时间才能完成的复杂机器人行为。例如，在导航、机械臂操作等场景中，Action都能够发挥重要作用。
使用场景
Action在机器人系统中有着广泛的应用场景，包括但不限于：
- 导航：机器人可以使用Action来发送导航指令，并实时接收路径规划的反馈。
- 物流自动驾驶：无人驾驶汽车或物流车可以使用Action来接收路径指令并执行。
- 人机交互：机器人可以通过Action来接收用户的指令和反馈，从而实现更加自然和流畅的交互体验。
- 复杂运动规划：机器人可以使用Action来发送运动指令，并在执行过程中接收运动的反馈，以实现精确和可靠的运动控制。
总之，ROS 2 Action是一种强大而灵活的通信机制，它为机器人系统的控制和管理提供了更加高效和可靠的方式。通过结合话题和服务的特性，Action能够实现对长时间运行任务的精确控制和实时反馈，从而推动机器人技术的进一步发展。
代码
我们通过action_tutorials/action_tutorials_cpp/src/fibonacci_action_server.cpp和action_tutorials/action_tutorials_cpp/src/fibonacci_action_client.cpp两个例子讲解Action服务端和客户端的编码实现。
客户端
客户端代码相对简单，我们可以从它先入手，讲解Action的组成。
构建rclcpp_action::Client
class FibonacciActionClient : public rclcpp::Node
{
public:
using Fibonacci = action_tutorials_interfaces::action::Fibonacci;
using GoalHandleFibonacci = rclcpp_action::ClientGoalHandle<Fibonacci>;
ACTION_TUTORIALS_CPP_PUBLIC
explicit FibonacciActionClient(const rclcpp::NodeOptions & node_options = rclcpp::NodeOptions())
: Node("fibonacci_action_client", node_options)
{
this->client_ptr_ = rclcpp_action::create_client<Fibonacci>(
this->get_node_base_interface(),
this->get_node_graph_interface(),
this->get_node_logging_interface(),
this->get_node_waitables_interface(),
"fibonacci");
this->timer_ = this->create_wall_timer(
std::chrono::milliseconds(500),
[this]() {return this->send_goal();});
}
……
private:
rclcpp_action::Client<Fibonacci>::SharedPtr client_ptr_;
rclcpp::TimerBase::SharedPtr timer_;
}; // class FibonacciActionClient
首先我们需要一个rclcpp_action::Client对象，它的模板类是Fibonacci。
Fibonacci类是通过IDL文件（action_tutorials/install/action_tutorials_interfaces/share/action_tutorials_interfaces/action/Fibonacci.idl）生成的。通过下面这个结构，可以看到我们在“概念”一节中介绍的Goal、Feedback和Result三部分。
// generated from rosidl_adapter/resource/action.idl.em
// with input from action_tutorials_interfaces/action/Fibonacci.action
// generated code does not contain a copyright notice
module action_tutorials_interfaces {
module action {
struct Fibonacci_Goal {
int32 order;
};
struct Fibonacci_Result {
sequence<int32> sequence;
};
struct Fibonacci_Feedback {
sequence<int32> partial_sequence;
};
};
};
而这个文件又是由Fibonacci.action文件（action_tutorials/install/action_tutorials_interfaces/share/action_tutorials_interfaces/action/Fibonacci.action）生成
int32 order
---
int32[] sequence
---
int32[] partial_sequence
准备好基本框架后，我们就要看主要逻辑——定时器具体执行了什么？
进入定时器后，我们先终止该定时器后续再次执行。这是因为后续设置逻辑只要执行一次即可。
ACTION_TUTORIALS_CPP_PUBLIC
void send_goal()
{
using namespace std::placeholders;
this->timer_->cancel();
等待和服务端的连接
if (!this->client_ptr_->wait_for_action_server(std::chrono::seconds(10))) {
RCLCPP_ERROR(this->get_logger(), "Action server not available after waiting");
rclcpp::shutdown();
return;
}
构建目标任务
auto goal_msg = Fibonacci::Goal();
goal_msg.order = 10;
Fibonacci::Goal也是由上述idl文件构建。
构建SendGoalOptions
auto send_goal_options = rclcpp_action::Client<Fibonacci>::SendGoalOptions();
SendGoalOptions是由三个回调函数构成
struct SendGoalOptions
{
SendGoalOptions()
: goal_response_callback(nullptr),
feedback_callback(nullptr),
result_callback(nullptr)
{
}
/// Function called when the goal is accepted or rejected.
/**
* Takes a single argument that is a goal handle shared pointer.
* If the goal is accepted, then the pointer points to a valid goal handle.
* If the goal is rejected, then pointer has the value `nullptr`.
*/
GoalResponseCallback goal_response_callback;
/// Function called whenever feedback is received for the goal.
FeedbackCallback feedback_callback;
/// Function called when the result for the goal is received.
ResultCallback result_callback;
};
我们看下这三个回调的实现
GoalResponseCallback
当Action客户端向服务端发送请求后，服务端会返回“接收”或者“拒绝”该请求，这个时候客户端的这个函数就会被调用。
如果服务端接收了该请求，则goal_handle非空；否则就是nullptr。
send_goal_options.goal_response_callback = [this](
const GoalHandleFibonacci::SharedPtr & goal_handle)
{
if (!goal_handle) {
RCLCPP_ERROR(this->get_logger(), "Goal was rejected by server");
} else {
RCLCPP_INFO(this->get_logger(), "Goal accepted by server, waiting for result");
}
};
FeedbackCallback
在任务处理的过程中，服务端不断向客户端发送Feedback，用于告知任务处理的当前状态。客户端就会通过该回调来接收服务端的响应。
FeedbackCallback有两个参数：
- GoalHandleFibonacci::SharedPtr和GoalResponseCallback回调中的参数一样。
- Fibonacci::Feedback是之前介绍的IDL文件生成的。它用来承载任务中间状态。
本例中它返回了当前服务端计算出来的斐波那契数列的值。
send_goal_options.feedback_callback = [this](
GoalHandleFibonacci::SharedPtr,
const std::shared_ptr<const Fibonacci::Feedback> feedback)
{
std::stringstream ss;
ss << "Next number in sequence received: ";
for (auto number : feedback->partial_sequence) {
ss << number << " ";
}
RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());
};
ResultCallback
当服务端的任务处理完毕，会向客户端发送结果。客户端就是通过这个回调来接收服务端的响应。
send_goal_options.result_callback = [this](
const GoalHandleFibonacci::WrappedResult & result)
{
switch (result.code) {
case rclcpp_action::ResultCode::SUCCEEDED:
break;
case rclcpp_action::ResultCode::ABORTED:
RCLCPP_ERROR(this->get_logger(), "Goal was aborted");
return;
case rclcpp_action::ResultCode::CANCELED:
RCLCPP_ERROR(this->get_logger(), "Goal was canceled");
return;
default:
RCLCPP_ERROR(this->get_logger(), "Unknown result code");
return;
}
std::stringstream ss;
ss << "Result received: ";
for (auto number : result.result->sequence) {
ss << number << " ";
}
RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());
rclcpp::shutdown();
};
ResultCallback的参数WrappedResult 结构如下。它包含了Fibonacci::Result的智能指针，还有代表最终处理结果的ResultCode。
template<typename ActionT>
class ClientGoalHandle
{
public:
RCLCPP_SMART_PTR_DEFINITIONS_NOT_COPYABLE(ClientGoalHandle)
// A wrapper that defines the result of an action
struct WrappedResult
{
/// The unique identifier of the goal
GoalUUID goal_id;
/// A status to indicate if the goal was canceled, aborted, or suceeded
ResultCode code;
/// User defined fields sent back with an action
typename ActionT::Result::SharedPtr result;
};
发送请求
this->client_ptr_->async_send_goal(goal_msg, send_goal_options);
至此，客户端的整个流程走完了。
后续的逻辑主要执行于上述的几个回调函数中。
完整代码
// Copyright 2019 Open Source Robotics Foundation, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include <memory>
#include <string>
#include <sstream>
#include "action_tutorials_interfaces/action/fibonacci.hpp"
#include "rclcpp/rclcpp.hpp"
#include "rclcpp_action/rclcpp_action.hpp"
#include "rclcpp_components/register_node_macro.hpp"
#include "action_tutorials_cpp/visibility_control.h"
namespace action_tutorials_cpp
{
class FibonacciActionClient : public rclcpp::Node
{
public:
using Fibonacci = action_tutorials_interfaces::action::Fibonacci;
using GoalHandleFibonacci = rclcpp_action::ClientGoalHandle<Fibonacci>;
ACTION_TUTORIALS_CPP_PUBLIC
explicit FibonacciActionClient(const rclcpp::NodeOptions & node_options = rclcpp::NodeOptions())
: Node("fibonacci_action_client", node_options)
{
this->client_ptr_ = rclcpp_action::create_client<Fibonacci>(
this->get_node_base_interface(),
this->get_node_graph_interface(),
this->get_node_logging_interface(),
this->get_node_waitables_interface(),
"fibonacci");
this->timer_ = this->create_wall_timer(
std::chrono::milliseconds(500),
[this]() {return this->send_goal();});
}
ACTION_TUTORIALS_CPP_PUBLIC
void send_goal()
{
using namespace std::placeholders;
this->timer_->cancel();
if (!this->client_ptr_->wait_for_action_server(std::chrono::seconds(10))) {
RCLCPP_ERROR(this->get_logger(), "Action server not available after waiting");
rclcpp::shutdown();
return;
}
auto goal_msg = Fibonacci::Goal();
goal_msg.order = 10;
RCLCPP_INFO(this->get_logger(), "Sending goal");
auto send_goal_options = rclcpp_action::Client<Fibonacci>::SendGoalOptions();
send_goal_options.goal_response_callback = [this](
const GoalHandleFibonacci::SharedPtr & goal_handle)
{
if (!goal_handle) {
RCLCPP_ERROR(this->get_logger(), "Goal was rejected by server");
} else {
RCLCPP_INFO(this->get_logger(), "Goal accepted by server, waiting for result");
}
};
send_goal_options.feedback_callback = [this](
GoalHandleFibonacci::SharedPtr,
const std::shared_ptr<const Fibonacci::Feedback> feedback)
{
std::stringstream ss;
ss << "Next number in sequence received: ";
for (auto number : feedback->partial_sequence) {
ss << number << " ";
}
RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());
};
send_goal_options.result_callback = [this](
const GoalHandleFibonacci::WrappedResult & result)
{
switch (result.code) {
case rclcpp_action::ResultCode::SUCCEEDED:
break;
case rclcpp_action::ResultCode::ABORTED:
RCLCPP_ERROR(this->get_logger(), "Goal was aborted");
return;
case rclcpp_action::ResultCode::CANCELED:
RCLCPP_ERROR(this->get_logger(), "Goal was canceled");
return;
default:
RCLCPP_ERROR(this->get_logger(), "Unknown result code");
return;
}
std::stringstream ss;
ss << "Result received: ";
for (auto number : result.result->sequence) {
ss << number << " ";
}
RCLCPP_INFO(this->get_logger(), "%s", ss.str().c_str());
rclcpp::shutdown();
};
this->client_ptr_->async_send_goal(goal_msg, send_goal_options);
}
private:
rclcpp_action::Client<Fibonacci>::SharedPtr client_ptr_;
rclcpp::TimerBase::SharedPtr timer_;
}; // class FibonacciActionClient
} // namespace action_tutorials_cpp
RCLCPP_COMPONENTS_REGISTER_NODE(action_tutorials_cpp::FibonacciActionClient)
扫一扫
744
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
