前一篇学习了uORB，用于px4中各个模块的进程间通信，下来学习MAVLink，用于飞控和地面站之间的通信。教程中主要给出了使用MAVLink的发送和接收消息的方法。
完整的MAVLink消息列表见该网页。
uint64_t time_start_usec uint64_t time_stop_usec uint32_t coefficients uint16_t seq_id #TOPICS ca_trajectory
同时在mavlink/include/mavlink/v2.0/custom_message/mavlink_msg_ca_trajectory.h中存在ca_trajectory的自定义mavlink消息。（笔者下载的代码中也没有该部分）
没有的话可以自己添加自定义mavlink消息，教程见官方文档。
先自定义在mavlink/include/mavlink/v2.0/message_definitions/下创建自定义消息custom_message.xml文件，与ca_trajectory中结构一致，内容如下：
<?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>
注意需要添加一个timestamp的成员，因为在编译时为了记录，会增加一个timestamp的成员。然后使用mavlink generator生成c语言源文件。
没有mavlink generator的同学可以用git下载并使用：
git clone https://github.com/mavlink/mavlink mavlink-generator cd mavlink-generator python mavgenerate.py
XML文件定位可以直接定位到mavlink/include/mavlink/v2.0/message_definitions/custom_message.xml，Out目录定位到mavlink/include/mavlink/v2.0/，注意语言选择C，协议选择2.0，如下图所示：
然后点击Generate即可生成c代码源文件。
#include <uORB/topics/ca_trajectory.h> #include <v2.0/custom_messages/mavlink_msg_ca_trajectory.h>
注意添加到已有包含文件的尾部，否则编译时可能会出现类型为定义的错误。
创建自定义消息对应的类
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 uint16_t get_id_static()
{
return MAVLINK_MSG_ID_CA_TRAJECTORY;
}
uint16_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)
{}
bool 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);
return true;
}
return false;
}
};
注意该类的函数成员返回类型和MAVLink v1.0稍有不同，笔者发现下载到的master分支和stable分支的函数类型都有所不同，具体以下载代码为准，可以参考代码中已有的其他消息例子来编写。
在附加流类中添加该自定义项
StreamListItem *streams_list[] = {
...
new StreamListItem(&MavlinkStreamCaTrajectory::new_instance, &MavlinkStreamCaTrajectory::get_name_static, &MavlinkStreamCaTrajectory::get_id_static),
nullptr
};
configure_stream("CA_TRAJECTORY", 100.0f);
#include <uORB/topics/ca_trajectory.h> #include <v2.0/custom_messages/mavlink_msg_ca_trajectory.h>
orb_advert_t _ca_traj_msg_pub;
在类Mavlink_Receiver中增加一个消息处理函数
void handle_message_ca_trajectory_msg(mavlink_message_t *msg);
在mavlink_receiver.cpp中加入如下代码：
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);
}
}
在该函数中实现了对消息的解包及发布。
在MavlinkReceiver::handle_message()中调用消息处理函数
MavlinkReceiver::handle_message(mavlink_message_t *msg)
{
switch (msg->msgid) {
...
case MAVLINK_MSG_ID_CA_TRAJECTORY:
handle_message_ca_trajectory_msg(msg);
break;
...
}
...
}
但是，在nsh里面看不到该消息发布的数据，哪位高手能告诉我哪错了……
原文：http://www.cnblogs.com/spyplus/p/7429751.html