Pixhawk原生固件PX4之自定义MAVLink消息

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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_usecuint64 time_stop_usecuint32 coefficientsuint16 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

voidMavlinkReceiver::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

字节 内容 数值 0 起始标志位 0~255 1 载荷长度 0~255 2 包序号 0~255 3 SYSID 1~255 4 COMPID 0~255 5 MSGID 0~255 6~(n+6) 载荷 (0~255)字节 (n+7)~(n+8) 冗余校验 XXX

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

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