PX4 Offboard Control Using MAVROS on ROS

pixhawk4飛控作為一款開源且流行的飛控，在其硬體版本上可以支援APM固件與PX4原生固件。本文針對PX4原生固件進行介紹。PX4中的offboard模式能夠接受來自外部的控制指令，搭配機載或支援MAVROS的協同計算機（如tx1,tx2,樹莓派,dji妙算等等），可在PX4飛控平台上加入視覺處理或人工智能，以實作無人機自動控制功能。這裡詳細介紹一個offboard的例程，外部控制飛機自主飛行2m的高度。

首先需要在Ubuntu的環境下安裝好ros,mavros以及px4原生固件

1.為外部控制例程建立一個工作空間   

mkdir -p px4_offboard_ws/src
           

2.建立一個用于offboard的功能包

cd px4_offboard_ws/src
catkin_create_pkg offboard roscpp std_msgs geometry_msgs mavros_msgs
           

3.建立好ros包過後，建立一個可cpp源檔案用來作為這個功能包的執行檔案

cd offboard/src
gedit offboard_node.cpp
           

4.offboard_node.cpp中的代碼如下

#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;
}

           

5.修改CMakeLists.txt為編譯做準備

cd ..
gedit CMakeLists.txt
           

6.在CMakeLists.txt最末尾添加如下内容後儲存退出

add_executable(${PROJECT_NAME}_node src/offboard_node.cpp)
target_link_libraries(${PROJECT_NAME}_node
  ${catkin_LIBRARIES}
)
           

7.開始編譯

cd ~/px4_offboard_ws
catkin_make
           

8.把這個工作空間添加到.bashrc裡面友善啟動節點

echo "source ~/px4_offboard_ws/devel/setup.bash" >> ~/.bashrc
           

9.通過jmavsim進行軟體在環仿真,打開一個終端啟動仿真

cd px4_source_code/Firmware
make px4_sitl_default jmavsim
           

10.再開一個終端運作mavros

roslaunch mavros px4.launch fcu_url:="udp://:[email protected]:14557"
           

11.打開一個終端運作外部控制的節點

rosrun offboard offboard_node
           

12.打開qgc地面站，可以觀察此時飛機飛行高度為2m