Pixhawk原生固件PX4之自定义MAVLink消息

欢迎交流~ 个人 Gitter 交流平台，点击直达：

本着想在PX4基础上加点什么东西的我又开始折腾了，先尝试用串口加传感器通过QGC查看，要是能在原固件上加点内容就棒哉了。先立Flag

自定义uORB消息

ca_trajectory.msg

uint64 time_start_usec
uint64 time_stop_usec
uint32 coefficients
uint16 seq_id

#TOPICS ca_trajectory

自定义MAVLink消息

custom_messages.xml

<?xml version="1.0"?>
<mavlink>
    <include>common.xml</include>
    <!-- NOTE: If the included file already contains a version tag, remove the version tag here, else uncomment to enable. -->
    <!--<version>3</version>-->
    <enums>
    </enums>
    <messages>
        <message id="166" name="CA_TRAJECTORY">
            <description>This message encodes all of the raw rudder sensor data from the USV.</description>
            <field type="uint64_t" name="timestamp">Timestamp in milliseconds since system boot</field>
            <field type="uint64_t" name="time_start_usec">start time, unit usec.</field>
            <field type="uint64_t" name="time_stop_usec">stop time, unit usec.</field>
            <field type="uint32_t" name="coefficients">as it says.</field>
            <field type="uint16_t" name="seq_id">can not cheat any more.</field>
        </message>
    </messages>
</mavlink>

使用python -m mavgenerate打开mavlink消息生成器导入上面的xml文件，生成如下文件：

将生成的custom_messages文件夹拖到Firmware/mavlink/include/mavlink/v1.0目录下

发送自定义MAVLink消息

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);
        }
    }
};

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, &MavlinkStreamCaTrajectory::get_id_static),
nullptr
};

最后在mavlink_main.cpp加入自定义的消息以及期望的更新频率

configure_stream("CA_TRAJECTORY", 100.0f);

接收自定义MAVLink消息

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;
    f.coefficients = traj.coefficients;

    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

# 在mavlink_receiver.cpp文件中添加
MavlinkReceiver::handle_message(mavlink_message_t *msg)
 {
     switch (msg->msgid) {
        ...
    case MAVLINK_MSG_ID_CA_TRAJECTORY:
        handle_message_ca_trajectory_msg(msg);
        break;
        ...
     }

消息查看

将SD卡插入飞控连接电脑，然后会发现串口调试助手不停地吐MAVLink消息，如下所示（选择Hex方式接收，否则看到的都是乱码）：

可以很容易的解析出每一帧MAVLink消息的内容，依次按照下图所示的顺序

可以看到，并不是每一个MAVLink消息都是一直启动的，只有心跳包是时刻不断的发送消息的(1Hz)，其他的自启MAVLink消息规律暂时找不到，至少现在在串口读到的数据中还是找不到我定义的MSGID #166 = 0x6A。

要确认自定义的MAVLink真的存在，目前可以通过NSH查看

ls obj

与预想的直接可以在QGC地面站查看还有一定的差距。

大家有好办法的欢迎交流指导。

参考
1. PX4中文维基之定义MAVlink消息
2. 自定义uORB消息
3. 创建MAVLink消息
4. 生成MAVLink文件

                                          By Fantasy