Question

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MAVLink消息

所有消息的概述可以在这里找到.

创建自定义MAVLink消息

这篇教程是假设你已经在 msg/ca_trajectory.msg 有了一个自定义uORB ca_trajectory
消息，并且在 mavlink/include/mavlink/v1.0/custom_messages/mavlink_msg_ca_trajectory.h 有了一个自定义mavlink
ca_trajectory 消息（点击此处查看如何建立一个自定义mavlink消息以及头文件）。

发送自定义MAVLink消息

这部分介绍如何使用一个自定义uORB消息并且作为mavlink消息发送。
添加mavlink的头文件和uorb消息到mavlink_messages.cpp

#include <uORB/topics/ca_trajectory.h>
#include <v1.0/custom_messages/mavlink_msg_ca_trajectory.h>

在mavlink_messages.cpp中创建一个新的类

class MavlinkStreamCaTrajectory : public MavlinkStream
{
public:
    const char *get_name() const
    {
        return MavlinkStreamCaTrajectory::get_name_static();
    }
    static const char *get_name_static()
    {
        return "CA_TRAJECTORY";
    }
    static uint8_t get_id_static()
    {
        return MAVLINK_MSG_ID_CA_TRAJECTORY;
    }
    uint8_t get_id()
    {
        return get_id_static();
    }
    static MavlinkStream *new_instance(Mavlink *mavlink)
    {
        return new MavlinkStreamCaTrajectory(mavlink);
    }
    unsigned get_size()
    {
        return MAVLINK_MSG_ID_CA_TRAJECTORY_LEN + MAVLINK_NUM_NON_PAYLOAD_BYTES;
    }
private:
    MavlinkOrbSubscription *_sub;
    uint64_t _ca_traj_time;
    /* do not allow top copying this class */
    MavlinkStreamCaTrajectory(MavlinkStreamCaTrajectory &);
    MavlinkStreamCaTrajectory& operator = (const MavlinkStreamCaTrajectory &);
protected:
    explicit MavlinkStreamCaTrajectory(Mavlink *mavlink) : MavlinkStream(mavlink),
        _sub(_mavlink->add_orb_subscription(ORB_ID(ca_trajectory))),  // make sure you enter the name of your uorb topic here
        _ca_traj_time(0)
    {}
    void send(const hrt_abstime t)
    {
        struct ca_trajectory_s _ca_trajectory;    //make sure ca_trajectory_s is the definition of your uorb topic
        if (_sub->update(&_ca_traj_time, &_ca_trajectory)) {
            mavlink_ca_trajectory_t msg;//make sure mavlink_ca_trajectory_t is the definition of your custom mavlink message 
               msg.timestamp = _ca_trajectory.timestamp;
            msg.time_start_usec = _ca_trajectory.time_start_usec;
            msg.time_stop_usec  = _ca_trajectory.time_stop_usec;
            msg.coefficients =_ca_trajectory.coefficients;
            msg.seq_id = _ca_trajectory.seq_id;
            mavlink_msg_ca_trajectory_send_struct(_mavlink->get_channel(), &msg);
        }
    }
};

最后附加流类streams_list的到mavlink_messages.cpp底部

StreamListItem *streams_list[] = {
...
new StreamListItem(&MavlinkStreamCaTrajectory::new_instance, &MavlinkStreamCaTrajectory::get_name_static),
nullptr
};

然后确保启用流，例如通过在启动脚本中添加以下行（-r配置流速率，-u标识UDP端口14556上的mavlink通道）：

mavlink stream -r 50 -s CA_TRAJECTORY -u 14556

接收自定义MAVLink消息

这部分解释如何通过mavlink接收消息并将其发布到uORB。

在mavlink_receiver.h中增加一个用来处理接收信息得函数

#include <uORB/topics/ca_trajectory.h>
#include <v1.0/custom_messages/mavlink_msg_ca_trajectory.h>

在 mavlink_receiver.h中增加一个处理类MavlinkReceiver 中的输入mavlink消息的函数

void handle_message_ca_trajectory_msg(mavlink_message_t *msg);

在 mavlink_receiver.h中加入一个类MavlinkReceiver中的uORB消息发布者

orb_advert_t _ca_traj_msg_pub;

在mavlink_receiver.cpp中实现handle_message_ca_trajectory_msg功能

void
MavlinkReceiver::handle_message_ca_trajectory_msg(mavlink_message_t *msg)
{
    mavlink_ca_trajectory_t traj;
    mavlink_msg_ca_trajectory_decode(msg, &traj);
    struct ca_trajectory_s f;
    memset(&f, 0, sizeof(f));
    f.timestamp = hrt_absolute_time();
    f.seq_id = traj.seq_id;
    f.time_start_usec = traj.time_start_usec;
    f.time_stop_usec = traj.time_stop_usec;
    for(int i=0;i<28;i++)
        f.coefficients[i] = traj.coefficients[i];
    if (_ca_traj_msg_pub == nullptr) {
        _ca_traj_msg_pub = orb_advertise(ORB_ID(ca_trajectory), &f);
    } else {
        orb_publish(ORB_ID(ca_trajectory), _ca_traj_msg_pub, &f);
    }
}

最后确定函数在MavlinkReceiver::handle_message()中被调用

MavlinkReceiver::handle_message(mavlink_message_t *msg)
 {
     switch (msg->msgid) {
        ...
    case MAVLINK_MSG_ID_CA_TRAJECTORY:
        handle_message_ca_trajectory_msg(msg);
        break;
        ...
     }

一般情况

设置流速率

有时，增加单个主题的流速率（例如，为例在QGC中检查）是有用的。这可以通过下面这行代码来实现

mavlink stream -u <port number> -s <mavlink topic name> -r <rate>

你可以通过mavlink status找到端口号，相应地将输出（在其他消息之间）transport protocol: UDP (<port number>)。例如你可能得到

mavlink stream -u 14556 -s OPTICAL_FLOW_RAD -r 300