MAVLink message structure

minhhieutran0407 · May 13, 2020, 7:30am

Hello. I want to know the structure of the MAVLink message when I send a request to Drone, example takeoff and land. Where I can find document about it? Thank you!

JulianOes · May 13, 2020, 8:02am

What exactly are you trying to achieve? I’m not entirely sure how to answer you, there would be a lot to explain.

Have you browsed through https://mavlink.io/en/ ?

minhhieutran0407 · May 13, 2020, 10:25am

Sorry, my English skills are not good. I want to see MAVLink messages when I send a command to Drone, such as takeoff and landing. I am also looking at https://mavlink.io/en/, but I would like to learn more documents for a more specific look.

JulianOes · May 13, 2020, 11:19am

The command sent to takeoff is: https://github.com/mavlink/mavlink/blob/ddc09ce654f0923f3780a7422e14aa0d643f8a80/message_definitions/v1.0/common.xml#L1350-L1359

It’s a MAVLink COMMAND_INT or COMMAND_LONG: https://github.com/mavlink/mavlink/blob/ddc09ce654f0923f3780a7422e14aa0d643f8a80/message_definitions/v1.0/common.xml#L4993-L5022

And as an example how the bytes are put on the buffer, check:

github.com

mavlink/c_library_v2/blob/de33a4b53592a0530a5ddadec841210b62670929/common/mavlink_msg_command_int.h#L95-L136


static inline uint16_t mavlink_msg_command_int_pack(uint8_t system_id, uint8_t component_id, mavlink_message_t* msg,
                               uint8_t target_system, uint8_t target_component, uint8_t frame, uint16_t command, uint8_t current, uint8_t autocontinue, float param1, float param2, float param3, float param4, int32_t x, int32_t y, float z)
{
#if MAVLINK_NEED_BYTE_SWAP || !MAVLINK_ALIGNED_FIELDS
    char buf[MAVLINK_MSG_ID_COMMAND_INT_LEN];
    _mav_put_float(buf, 0, param1);
    _mav_put_float(buf, 4, param2);
    _mav_put_float(buf, 8, param3);
    _mav_put_float(buf, 12, param4);
    _mav_put_int32_t(buf, 16, x);
    _mav_put_int32_t(buf, 20, y);
    _mav_put_float(buf, 24, z);
    _mav_put_uint16_t(buf, 28, command);
    _mav_put_uint8_t(buf, 30, target_system);
    _mav_put_uint8_t(buf, 31, target_component);
    _mav_put_uint8_t(buf, 32, frame);
    _mav_put_uint8_t(buf, 33, current);
    _mav_put_uint8_t(buf, 34, autocontinue);

        memcpy(_MAV_PAYLOAD_NON_CONST(msg), buf, MAVLINK_MSG_ID_COMMAND_INT_LEN);

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And an example how this is used, e.g. in MAVSDK:

github.com

mavlink/MAVSDK/blob/62fa8925d1d7e98b0dd4f167b9a319e84d16c152/src/plugins/action/action_impl.cpp#L269-L279


void ActionImpl::takeoff_async(const Action::ResultCallback& callback) const
{
    MAVLinkCommands::CommandLong command{};

    command.command = MAV_CMD_NAV_TAKEOFF;
    command.target_component_id = _parent->get_autopilot_id();

    _parent->send_command_async(command, [this, callback](MAVLinkCommands::Result result, float) {
        command_result_callback(result, callback);
    });
}

minhhieutran0407 · May 13, 2020, 12:17pm

Thank you very much! It will help me a lot in the process of learning about MAVLink protocol.