# Macros 功能包的 offboard 模式控制例程
本教程介绍了使用 Gazbo/SITL 中模拟的四轴飞行器 Offboard 控制的基础知识。 在教程的末尾, 你应该看到与下面的视频相同的行为, 即缓慢起飞到2米的高度。
注意
Offboard control is dangerous. If you are operating on a real vehicle be sure to have a way of gaining back manual control in case something goes wrong.
mavros_msgs 功能包中包含操作 MAVROS 包中服务和主题所需的自定义消息文件。 Similar examples in Python can be found here: integrationtests/python_src/px4_it/mavros (opens new window).
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# 代码
Create the offb_node.cpp file in your ROS package (by also adding it to your CMakeList.txt so it is compiled), and paste the following inside it:
/**
* @file offb_node.cpp
* @brief Offboard control example node, written with MAVROS version 0.19.x, PX4 Pro Flight
* Stack and tested in Gazebo SITL
*/
#include <ros/ros.h>
#include <geometry_msgs/PoseStamped.h>
#include <mavros_msgs/CommandBool.h>
#include <mavros_msgs/SetMode.h>
#include <mavros_msgs/State.h>
mavros_msgs::State current_state;
void state_cb(const mavros_msgs::State::ConstPtr& msg){
current_state = *msg;
}
int main(int argc, char **argv)
{
ros::init(argc, argv, "offb_node");
ros::NodeHandle nh;
ros::Subscriber state_sub = nh.subscribe<mavros_msgs::State>
("mavros/state", 10, state_cb);
ros::Publisher local_pos_pub = nh.advertise<geometry_msgs::PoseStamped>
("mavros/setpoint_position/local", 10);
ros::ServiceClient arming_client = nh.serviceClient<mavros_msgs::CommandBool>
("mavros/cmd/arming");
ros::ServiceClient set_mode_client = nh.serviceClient<mavros_msgs::SetMode>
("mavros/set_mode");
//the setpoint publishing rate MUST be faster than 2Hz
ros::Rate rate(20.0);
// wait for FCU connection
while(ros::ok() && !current_state.connected){
ros::spinOnce();
rate.sleep();
}
geometry_msgs::PoseStamped pose;
pose.pose.position.x = 0;
pose.pose.position.y = 0;
pose.pose.position.z = 2;
//send a few setpoints before starting
for(int i = 100; ros::ok() && i > 0; --i){
local_pos_pub.publish(pose);
ros::spinOnce();
rate.sleep();
}
mavros_msgs::SetMode offb_set_mode;
offb_set_mode.request.custom_mode = "OFFBOARD";
mavros_msgs::CommandBool arm_cmd;
arm_cmd.request.value = true;
ros::Time last_request = ros::Time::now();
while(ros::ok()){
if( current_state.mode != "OFFBOARD" &&
(ros::Time::now() - last_request > ros::Duration(5.0))){
if( set_mode_client.call(offb_set_mode) &&
offb_set_mode.response.mode_sent){
ROS_INFO("Offboard enabled");
}
last_request = ros::Time::now();
} else {
if( !current_state.armed &&
(ros::Time::now() - last_request > ros::Duration(5.0))){
if( arming_client.call(arm_cmd) &&
arm_cmd.response.success){
ROS_INFO("Vehicle armed");
}
last_request = ros::Time::now();
}
}
local_pos_pub.publish(pose);
ros::spinOnce();
rate.sleep();
}
return 0;
}
# 代码解释
#include <ros/ros.h>
#include <geometry_msgs/PoseStamped.h>
#include <mavros_msgs/CommandBool.h>
#include <mavros_msgs/SetMode.h>
#include <mavros_msgs/State.h>
我们初始化了一个发布者来发布本地的控制指令，还初始化了请求解锁和更改模式的服务。 请注意，对于您自己的系统，"mavros" 前缀可能不同，因为它将取决于给它的启动文件中的节点指定的名称。
mavros_msgs::State current_state;
void state_cb(const mavros_msgs::State::ConstPtr& msg){
current_state = *msg;
}
PX4 固件在 Offboard 模式下，连续两次控制命令之间有 500ms 的超时。 如果飞控超过，飞控会立即切换回进入 Offboard 模式之前的飞行模式。
ros::Subscriber state_sub = nh.subscribe<mavros_msgs::State>("mavros/state", 10, state_cb);
ros::Publisher local_pos_pub = nh.advertise<geometry_msgs::PoseStamped>("mavros/setpoint_position/local", 10);
ros::ServiceClient arming_client = nh.serviceClient<mavros_msgs::CommandBool>("mavros/cmd/arming");
ros::ServiceClient set_mode_client = nh.serviceClient<mavros_msgs::SetMode>("mavros/set_mode");
在发布对应消息之前，我们需要等待飞控和MAVROS模块的连接。 在收到心跳包之后，代码便会跳出这个循环。
//the setpoint publishing rate MUST be faster than 2Hz
ros::Rate rate(20.0);
尽管PX4飞控在NED坐标系下操控飞机，但MAVROS是在ENU系下进行指令传输的。 这也就是为什么我们设置z为+2。 This is why the publishing rate must be faster than 2 Hz to also account for possible latencies. This is also the same reason why it is recommended to enter Offboard mode from Position mode, this way if the vehicle drops out of Offboard mode it will stop in its tracks and hover.
// wait for FCU connection
while(ros::ok() && !current_state.connected){
ros::spinOnce();
rate.sleep();
}
在切换到offboard模式之前，你必须先发送一些期望点信息到飞控中。 不然飞控会拒绝切换到offboard模式。
geometry_msgs::PoseStamped pose;
pose.pose.position.x = 0;
pose.pose.position.y = 0;
pose.pose.position.z = 2;
我们现在设置要切换的模式为 OFFBOARD。 支持模式 (opens new window) 代码列出供参考：
//send a few setpoints before starting
for(int i = 100; ros::ok() && i > 0; --i){
local_pos_pub.publish(pose);
ros::spinOnce();
rate.sleep();
}
剩下的代码都比较容易自学。 Otherwise the mode switch will be rejected. 我们每隔五秒去检查一下与飞控的连接等是否一切正常。
mavros_msgs::SetMode offb_set_mode;
offb_set_mode.request.custom_mode = "OFFBOARD";
We set the custom mode to OFFBOARD. A list of supported modes (opens new window) is available for reference.
mavros_msgs::CommandBool arm_cmd;
arm_cmd.request.value = true;
ros::Time last_request = ros::Time::now();
while(ros::ok()){
if( current_state.mode != "OFFBOARD" &&
(ros::Time::now() - last_request > ros::Duration(5.0))){
if( set_mode_client.call(offb_set_mode) &&
offb_set_mode.response.mode_sent){
ROS_INFO("Offboard enabled");
}
last_request = ros::Time::now();
} else {
if( !current_state.armed &&
(ros::Time::now() - last_request > ros::Duration(5.0))){
if( arming_client.call(arm_cmd) &&
arm_cmd.response.success){
ROS_INFO("Vehicle armed");
}
last_request = ros::Time::now();
}
}
local_pos_pub.publish(pose);
ros::spinOnce();
rate.sleep();
}
The rest of the code is pretty self explanatory. We attempt to switch to Offboard mode, after which we arm the quad to allow it to fly. We space out the service calls by 5 seconds so to not flood the autopilot with the requests. In the same loop, we continue sending the requested pose at the appropriate rate.
提示
This code has been simplified to the bare minimum for illustration purposes. In larger systems, it is often useful to create a new thread which will be in charge of periodically publishing the setpoints.