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引言：在实际工程项目中，为了提高系统的响应速度和稳定性，往往需要采用一定的控制算法进行目标跟踪。这里抛砖引玉，仅采用简单的PID算法进行目标的跟随控制，目标的识别依然采用yolo。对系统要求更高的，可以对算法进行改进，也欢迎读者与我们联系，合作开发。

步骤一：打开摄像头

注意：为了获取目标物的三维位置信息，我们采用了D435深度摄像头，仅供参考，可根据需要自行选择即可

roslaunch realsense2_camera rs_camera.launch

查看话题，需要/camera/color/image_raw和/camera/depth/image_rect_raw

步骤二：打开yolo识别节点，具体yolo版本可以根据需要选择

 roslaunch darknet_ros darknet_ros.launch 

没有报错的情况下，会弹出识别效果图，如下：

## 注：我这里训练的是自己打印的H型地标，具体可以根据需要选择合适的目标物

步骤三：打开三维坐标转换节点

该节点可以直接一话题的形式输出目标物的名称和真实的位置信息

roslaunch darknet_real_position darknet_real_position.launch

launch文件解析

此处的launch文件，以参数的方式指定了识别目标。比如landing，因此这个节点只会把指定的landing地标位置信息打印出来，其他的目标通通忽略

查看话题数据/object_position

从上述图片可以看出，系统非常准确的给出了目标物的名称和真实的位置信息，单位是米。需要指出的是，这里的位置是相对于D435摄像头的位置信息，X表示横向位置，Y表示纵向位置，Z表示实际的距离信息

步骤四：启动PID跟随节点。注意，可以先不要启动mavros，仅仅测试PID控制器发布出的速度是否正确。在确认了没问题后在启动mavros节点，无人机就可以进行正常的跟随运动了

roslaunch follow_pid follow_pid.launch 

launch文件解析

这里仅仅进行偏航角度和距离的控制，如果需要对高度方向控制。可以直接复制代码进行简单的修改即可。参数linear_x_p和linear_x_d是距离的PID控制，同理yaw_rate_p和yaw_rate_d是角度的控制。参数target_x_angle是期望保持的角度，通常设置为0即可。最后参数target_distance是期望保持的距离，单位是毫米

代码如下：

#include <ros/ros.h>
#include <std_msgs/Bool.h>
#include <geometry_msgs/PoseStamped.h>
#include <geometry_msgs/TwistStamped.h>
#include <mavros_msgs/CommandBool.h>
#include <mavros_msgs/SetMode.h>
#include <mavros_msgs/State.h>
#include <mavros_msgs/PositionTarget.h>
#include <cmath>
#include <tf/transform_listener.h>
#include <nav_msgs/Odometry.h>
#include <mavros_msgs/CommandLong.h>   
#include <string>#define MAX_ERROR 0.20
#define VEL_SET   0.10
#define ALTITUDE  0.40using namespace std;float target_x_angle = 0;
float target_distance = 2000;
float linear_x_p = 0.5;
float linear_x_d = 0.33;
float yaw_rate_p = 4.0;
float yaw_rate_d = 15;geometry_msgs::PointStamped object_pos; 
nav_msgs::Odometry local_pos;
mavros_msgs::State current_state;  
mavros_msgs::PositionTarget setpoint_raw;
//检测到的物体坐标值
string current_frame_id   = "no_object";
double current_position_x = 0;
double current_position_y = 0;
double current_distance   = 0;//1、订阅无人机状态话题
ros::Subscriber state_sub;//2、订阅无人机实时位置信息
ros::Subscriber local_pos_sub;//3、订阅实时位置信息
ros::Subscriber object_pos_sub;//4、发布无人机多维控制话题
ros::Publisher  mavros_setpoint_pos_pub;//5、请求无人机解锁服务        
ros::ServiceClient arming_client;//6、请求无人机设置飞行模式，本代码请求进入offboard
ros::ServiceClient set_mode_client;void pid_control()
{static float last_error_x_angle = 0;static float last_error_distance = 0;				float x_angle;float distance;if(current_position_x == 0 && current_position_y == 0 && current_distance == 0){x_angle  = target_x_angle;distance = target_distance;}else{x_angle = current_position_x / current_distance;distance = current_distance;}float error_x_angle = x_angle - target_x_angle;float error_distance = distance - target_distance;if(error_x_angle > -0.01 && error_x_angle < 0.01)  {error_x_angle = 0;}if(error_distance > -80 && error_distance < 80) {error_distance = 0;}setpoint_raw.velocity.x = error_distance*linear_x_p/1000 + (error_distance - last_error_distance)*linear_x_d/1000;if(setpoint_raw.velocity.x < -0.3)  {setpoint_raw.velocity.x = -0.3;}else if(setpoint_raw.velocity.x > 0.3) {setpoint_raw.velocity.x = 0.3;	}setpoint_raw.yaw_rate = error_x_angle*yaw_rate_p + (error_x_angle - last_error_x_angle)*yaw_rate_d;if(setpoint_raw.yaw_rate < -0.5)  {setpoint_raw.yaw_rate = -0.5;}else if(setpoint_raw.yaw_rate > 0.5) {setpoint_raw.yaw_rate = 0.5;}mavros_setpoint_pos_pub.publish(setpoint_raw);last_error_x_angle  = error_x_angle;last_error_distance = error_distance;
}void state_cb(const mavros_msgs::State::ConstPtr& msg)
{current_state = *msg;
}void local_pos_cb(const nav_msgs::Odometry::ConstPtr& msg)
{local_pos = *msg;
}void object_pos_cb(const geometry_msgs::PointStamped::ConstPtr& msg)
{object_pos = *msg;current_position_x = object_pos.point.x*(-1000);current_position_y = object_pos.point.y*(-1000);//此处将距离由单位米改称毫米，方便提高控制精度current_distance   = object_pos.point.z*1000;current_frame_id   = object_pos.header.frame_id; pid_control();	 //ROS_INFO("current_position_x = %f",current_position_x);//ROS_INFO("current_position_y = %f",current_position_y);//ROS_INFO("current_distance = %f"  ,current_distance);
}int main(int argc, char *argv[])
{ros::init(argc, argv, "follow_pid");ros::NodeHandle nh;state_sub     = nh.subscribe<mavros_msgs::State>("mavros/state", 100, state_cb);local_pos_sub = nh.subscribe<nav_msgs::Odometry>("/mavros/local_position/odom", 100, local_pos_cb);object_pos_sub = nh.subscribe<geometry_msgs::PointStamped>("object_position", 100, object_pos_cb);mavros_setpoint_pos_pub = nh.advertise<mavros_msgs::PositionTarget>("/mavros/setpoint_raw/local", 100);   arming_client = nh.serviceClient<mavros_msgs::CommandBool>("mavros/cmd/arming");set_mode_client = nh.serviceClient<mavros_msgs::SetMode>("mavros/set_mode");ros::Rate rate(20.0); ros::param::get("linear_x_p",linear_x_p);ros::param::get("linear_x_d",linear_x_d);ros::param::get("yaw_rate_p",yaw_rate_p);ros::param::get("yaw_rate_d",yaw_rate_d);ros::param::get("target_x_angle", target_x_angle);ros::param::get("target_distance",target_distance);//等待连接到PX4无人机/* while(ros::ok() && current_state.connected){ros::spinOnce();rate.sleep();}*/setpoint_raw.type_mask = /*1 + 2 + 4 + 8 + 16 + 32*/ + 64 + 128 + 256 + 512 /*+ 1024 + 2048*/;setpoint_raw.coordinate_frame = 1;setpoint_raw.position.x = 0;setpoint_raw.position.y = 0;setpoint_raw.position.z = 0 + ALTITUDE;mavros_setpoint_pos_pub.publish(setpoint_raw);for(int i = 100; ros::ok() && i > 0; --i){mavros_setpoint_pos_pub.publish(setpoint_raw);ros::spinOnce();rate.sleep();}//请求offboard模式变量mavros_msgs::SetMode offb_set_mode;offb_set_mode.request.custom_mode = "OFFBOARD";//请求解锁变量mavros_msgs::CommandBool arm_cmd;arm_cmd.request.value = true;ros::Time last_request = ros::Time::now();//请求进入offboard模式并且解锁无人机，15秒后退出，防止重复请求       /*while(ros::ok()){//请求进入OFFBOARD模式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 {//请求解锁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();}}if(ros::Time::now() - last_request > ros::Duration(15.0))break;mavros_setpoint_pos_pub.publish(setpoint_raw);ros::spinOnce();rate.sleep();}*/   while(ros::ok()){//ROS_INFO("11111");ros::spinOnce();rate.sleep();}}

步骤五：在上述基础上再打开mavros，即可开始跟随控制。代码后续会在B站进行讲解。同时会提供相应的实机演示。链接会在后续给出。