mavlink-messaging.md

MAVLink Messaging

An overview of all messages can be found here.

Create Custom MAVLink Messages

This tutorial assumes you have a custom uORB ca_trajectory message in msg/ca_trajectory.msg and a custom mavlink ca_trajectory message in mavlink/include/mavlink/v1.0/custom_messages/mavlink_msg_ca_trajectory.h (see here how to create a custom mavlink message and header).

Sending Custom MAVLink Messages

This section explains how to use a custom uORB message and send it as a mavlink message.

Add the headers of the mavlink and uorb messages to mavlink_messages.cpp

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

Create a new class in mavlink_messages.cpp

class MavlinkStreamCaTrajectory : public MavlinkStream
{
public:
    const char *get_name() const
	{
		return MavlinkStreamCaTrajectory::get_name_static();
	}
	static const char *get_name_static()
	{
		return "CA_TRAJECTORY";
	}
	uint8_t get_id()
	{
		return MAVLINK_MSG_ID_CA_TRAJECTORY;
	}
	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_traj_struct_s _ca_trajectory;    //make sure ca_traj_struct_s is the definition of your uorb topic

		if (_sub->update(&_ca_traj_time, &_ca_trajectory)) {
			mavlink_ca_trajectory_t _msg_ca_trajectory;  //make sure mavlink_ca_trajectory_t is the definition of your custom mavlink message

			_msg_ca_trajectory.timestamp = _ca_trajectory.timestamp;
			_msg_ca_trajectory.time_start_usec = _ca_trajectory.time_start_usec;
			_msg_ca_trajectory.time_stop_usec  = _ca_trajectory.time_stop_usec;
			_msg_ca_trajectory.coefficients =_ca_trajectory.coefficients;
			_msg_ca_trajectory.seq_id = _ca_trajectory.seq_id;

			_mavlink->send_message(MAVLINK_MSG_ID_CA_TRAJECTORY, &_msg_ca_trajectory);
		}
	}
};

Finally append the stream class to the streams_list at the bottom of mavlink_messages.cpp

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

Then make sure to enable the stream, for example by adding the following line to the startup script (-r configures the streaming rate, -u identifies the mavlink channel on UDP port 14556):

mavlink stream -r 50 -s CA_TRAJECTORY -u 14556

Receiving Custom MAVLink Messages

This section explains how to receive a message over mavlink and publish it to uORB.

Add a function that handles the incoming mavlink message in mavlink_receiver.h

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

Add a function that handles the incoming mavlink message in the MavlinkReceiver class in mavlink_receiver.h

void handle_message_ca_trajectory_msg(mavlink_message_t *msg);

Add an uORB publisher in the MavlinkReceiver class in mavlink_receiver.h

orb_advert_t _ca_traj_msg_pub;

Implement the handle_message_ca_trajectory_msg function in mavlink_receiver.cpp

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_traj_struct_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);
	}
}

and finally make sure it is called in MavlinkReceiver::handle_message()

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

General

Set streaming rate

Sometimes it is useful to increase the streaming rate of individual topics (e.g. for inspection in QGC). This can be achieved by the following line

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

You can get the port number with mavlink status which will output (amongst others) transport protocol: UDP (<port number>). An example would be

mavlink stream -u 14556 -s OPTICAL_FLOW_RAD -r 300