超维空间S2无人机使用说明书——52、使用PID算法进行基于yolo的目标跟踪

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  0.40

using 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。