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();
}
}
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