ROS学习笔记(一):自己动手写一个ROS程序

最近老板安排任务,要把ROS框架在ARM+FPGA平台上实现。但是使用ROS创建程序步骤繁琐,所以这次将官方文档上面的Demo简化写下来,方便以后查看。

ROS版本:Hydro

Linux版本:Ubuntu12.04

在开始第一个ROS(Robot Operating System)程序之前,确保已经按照官方教程(点击打开链接)成功安装了ROS。本文建立的是一个非常简单的发布(Publisher)、订阅(Subscriber)程序。

创建一个工作区(workspace)

工作区可以作为一个独立的项目进行编译,存放ROS程序的源文件、编译文件和执行文件。建立工作区的方法如下:

$ mkdir -p ~/catkin_ws/src
$ cd ~/catkin_ws/src
$ catkin_init_workspace
虽然这时候工作区是空的,但是我们依然可以进行编译:

$ cd ~/catkin_ws/
$ catkin_make

这时候,会在当前文件夹下生成devel,build这两个子文件夹,在devel文件夹下能看到几个setup.*sh文件。

接下来把工作区在bash中注册

$ source devel/setup.bash
要验证是否已经在bash中注册可以使用如下命令:

$ echo $ROS_PACKAGE_PATH
/home/youruser/catkin_ws/src:/opt/ros/indigo/share:/opt/ros/indigo/stacks
如果能看到自己工作区的文件路径就说明已经成功了。

创建一个ROS工程包(Package)

在一个工作区内,可能会包含多个ROS工程包。而最基本ROS工程包中会包括CmakeLists.txt和Package.xml这两个文件,其中Package.xml中主要包含本项目信息和各种依赖(depends),而CmakeLists.txt中包含了如何编译和安装代码的信息。

首先切换到工作区:

$ cd ~/catkin_ws/src
现在可以使用catkin_create_pkg命令去创建一个叫beginner_tutorials的包,这个包依靠std_msgs、roscpp、rospy。

$ catkin_create_pkg beginner_tutorials std_msgs rospy roscpp
接下来在工作区编译这个工程包。

$ cd ~/catkin_ws
$ catkin_make

一个简单的发布(Publisher)、订阅(Subscriber)程序

写一个发布(Publisher节点

节点(node)是连接到ROS网络中可执行的基本单元。我们在这创建一个发布者---“talker”节点,这个节点持续对外发布消息。

首先我们要把目录切换到我们的beginner_tutorials工程包中

$ cd ~/catkin_ws/src/beginner_tutorials
因为我们已经编译过这个工程包了,所以会在beginner_tutorials文件夹下看到CmakeList.txt、package.xml文件和include、src这两个目录。接下来进入src子目录

$ cd src
在src目录中创建一个talker.cpp文件,里面的内容如下:

#include "ros/ros.h"
#include "std_msgs/String.h"

#include <sstream>
int main(int argc, char **argv)
{
  /**
   * The ros::init() function needs to see argc and argv so that it can perform
   * any ROS arguments and name remapping that were provided at the command line. For programmatic
   * remappings you can use a different version of init() which takes remappings
   * directly, but for most command-line programs, passing argc and argv is the easiest
   * way to do it.  The third argument to init() is the name of the node.
   *
   * You must call one of the versions of ros::init() before using any other
   * part of the ROS system.
   */
  ros::init(argc, argv, "talker");

  /**
   * NodeHandle is the main access point to communications with the ROS system.
   * The first NodeHandle constructed will fully initialize this node, and the last
   * NodeHandle destructed will close down the node.
   */
  ros::NodeHandle n;

  /**
   * The advertise() function is how you tell ROS that you want to
   * publish on a given topic name. This invokes a call to the ROS
   * master node, which keeps a registry of who is publishing and who
   * is subscribing. After this advertise() call is made, the master
   * node will notify anyone who is trying to subscribe to this topic name,
   * and they will in turn negotiate a peer-to-peer connection with this
   * node.  advertise() returns a Publisher object which allows you to
   * publish messages on that topic through a call to publish().  Once
   * all copies of the returned Publisher object are destroyed, the topic
   * will be automatically unadvertised.
   *
   * The second parameter to advertise() is the size of the message queue
   * used for publishing messages.  If messages are published more quickly
   * than we can send them, the number here specifies how many messages to
   * buffer up before throwing some away.
   */
  ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);

  ros::Rate loop_rate(10);

  /**
   * A count of how many messages we have sent. This is used to create
   * a unique string for each message.
   */
  int count = 0;
  while (ros::ok())
  {
    /**
     * This is a message object. You stuff it with data, and then publish it.
     */
    std_msgs::String msg;

    std::stringstream ss;
    ss << "hello world " << count;
    msg.data = ss.str();

    ROS_INFO("%s", msg.data.c_str());

    /**
     * The publish() function is how you send messages. The parameter
     * is the message object. The type of this object must agree with the type
     * given as a template parameter to the advertise<>() call, as was done
     * in the constructor above.
     */
    chatter_pub.publish(msg);

    ros::spinOnce();

    loop_rate.sleep();
    ++count;
  }


  return 0;
}


写一个订阅Subscriber节点

还是在src目录下,创建一个listener.cpp文件。内容如下:

#include "ros/ros.h"
#include "std_msgs/String.h"

/**
 * This tutorial demonstrates simple receipt of messages over the ROS system.
 */
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

int main(int argc, char **argv)
{
  /**
   * The ros::init() function needs to see argc and argv so that it can perform
   * any ROS arguments and name remapping that were provided at the command line. For programmatic
   * remappings you can use a different version of init() which takes remappings
   * directly, but for most command-line programs, passing argc and argv is the easiest
   * way to do it.  The third argument to init() is the name of the node.
   *
   * You must call one of the versions of ros::init() before using any other
   * part of the ROS system.
   */
  ros::init(argc, argv, "listener");

  /**
   * NodeHandle is the main access point to communications with the ROS system.
   * The first NodeHandle constructed will fully initialize this node, and the last
   * NodeHandle destructed will close down the node.
   */
  ros::NodeHandle n;

  /**
   * The subscribe() call is how you tell ROS that you want to receive messages
   * on a given topic.  This invokes a call to the ROS
   * master node, which keeps a registry of who is publishing and who
   * is subscribing.  Messages are passed to a callback function, here
   * called chatterCallback.  subscribe() returns a Subscriber object that you
   * must hold on to until you want to unsubscribe.  When all copies of the Subscriber
   * object go out of scope, this callback will automatically be unsubscribed from
   * this topic.
   *
   * The second parameter to the subscribe() function is the size of the message
   * queue.  If messages are arriving faster than they are being processed, this
   * is the number of messages that will be buffered up before beginning to throw
   * away the oldest ones.
   */
  ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

  /**
   * ros::spin() will enter a loop, pumping callbacks.  With this version, all
   * callbacks will be called from within this thread (the main one).  ros::spin()
   * will exit when Ctrl-C is pressed, or the node is shutdown by the master.
   */
  ros::spin();

  return 0;
}

编译创建的节点
在编译我们创建的节点之前,我们还需要编辑Cmakelist.txt文件(注意:是beginner_tutorials项目包下的CMakelist文件),告诉编辑器我们需要编辑什么文件,需要什么依赖。

$ gedit CMakeLists.txt
在文件末尾添加如下语句:

include_directories(include ${catkin_INCLUDE_DIRS})

add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})
add_dependencies(talker beginner_tutorials_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

将目录切换到工作区目录,并执行catkin_make运行命令:

$ cd ~/catkin_ws
$ catkin_make

不出意外的话,会出现如下界面:


至此,程序已经创建完成,而接下来我们要检查一下我们创建的程序是否正确。

测试程序的正确性

首先,我们得要启动ROS核心程序roscore。
$ roscore

在使用我们的程序之前,需要先把程序注册
$ cd ~/catkin_ws
$ source ./devel/setup.bash

运行talker节点:
$ rosrun beginner_tutorials talker 

这时候会看到如下信息:
[INFO] [WallTime: 1314931831.774057] hello world 1314931831.77
[INFO] [WallTime: 1314931832.775497] hello world 1314931832.77
[INFO] [WallTime: 1314931833.778937] hello world 1314931833.78
[INFO] [WallTime: 1314931834.782059] hello world 1314931834.78
[INFO] [WallTime: 1314931835.784853] hello world 1314931835.78
[INFO] [WallTime: 1314931836.788106] hello world 1314931836.79
这就表示发布(Publisher)节点已经正确的运行了。

接下来运行listener节点:
$ rosrun beginner_tutorials listener

这时候会看到如下信息:
[INFO] [WallTime: 1314931969.258941] /listener_17657_1314931968795I heard hello world 1314931969.26
[INFO] [WallTime: 1314931970.262246] /listener_17657_1314931968795I heard hello world 1314931970.26
[INFO] [WallTime: 1314931971.266348] /listener_17657_1314931968795I heard hello world 1314931971.26
[INFO] [WallTime: 1314931972.270429] /listener_17657_1314931968795I heard hello world 1314931972.27
[INFO] [WallTime: 1314931973.274382] /listener_17657_1314931968795I heard hello world 1314931973.27
[INFO] [WallTime: 1314931974.277694] /listener_17657_1314931968795I heard hello world 1314931974.28
[INFO] [WallTime: 1314931975.283708] /listener_17657_1314931968795I heard hello world 1314931975.28
这说明订阅节点(listener)已经成功的接收到了发布节点(talker)发布的信息。至此,整个程序结束!







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