【无人机开发】通讯协议MavLink详解_mavlink协议原理

1. MAVLink简介

MAVLink（Micro Air Vehicle Link，微型空中飞行器链路通讯协议）是无人飞行器与地面站（Ground Control Station ，GCS）之间通讯，以及无人飞行器之间通讯最常用的协议。它已经在PX4、APM、PIXHAWK和Parrot AR.Drone飞控平台上进行了大量测试。

2.发明者Lorenz Meier简介

MAVLink的最初开发于2009年，由Lorenz Meier完成。Lorenz Meier的LinkedIn主页是：https://www.linkedin.com/in/meierlorenz，个人主页是：https://www.inf.ethz.ch/personal/lomeier/。
根据官网和个人主页，Lorenz Meie的个人经历如下：
 2004年~2008年在德国康斯坦茨大学（Universität Konstanz）就读信息工程专业；
 2008 年~2011年在苏黎世联邦理工学院（德语：Eidgenössische Technische Hochschule Zürich，简称 ETH Zürich或ETHZ）就读视觉计算方向研究生；
 2011~至今在ETHZ攻读博士后，研究方向是：Research on Drones and mobile phones focused on obstacle mapping, path planning and control.
 2011年到现在，Lorenz Meier一直是开源无人机项目Autopilot的建立者和维护者。关于Autopilot，我会另辟章节介绍。
从介绍来看，Lorenz Meier的研究方向包括了无人机避障、基于智能手机或无人机的3D重建、无人机通讯协议等有趣又前言的内容。
这个页面是Lorenz Meier发表的几篇文章：https://www.researchgate.net/profile/Lorenz_Meier3

3. MAVLink相关资料

维基百科：https://en.wikipedia.org/wiki/MAVLink
MavLink官方网站：http://qgroundcontrol.org/mavlink/start
Python写的用于生成C、Java等语言的MavLink生成器软件：https://github.com/mavlink/mavlink

4.协议构成

下面内容引自官网。

• The checksum is the same as used in ITU X.25 and SAE AS-4 standards (CRC-16-CCITT), documented in SAE AS5669A. Please see the MAVLink source code for a documented C-implementation of it. LINK TO CHECKSUM
• The minimum packet length is 8 bytes for acknowledgement packets without payload
• The maximum packet length is 263 bytes for full payload

MavLink的长度是固定的，即 17byte= 6 bytes header + 9 bytes payload + 2 bytes checksum。

5.封包过程

由用户生成的部分包括PlayLoad本身、消息包的STX、COMP、MSG，其他部分自动生成。下图来自于博客http://blog.csdn.net/u013983741/article/details/48053235，侵删。

6.3DR Service实现MavLink协议

3DR Service是Autopilot提供的Android端的app服务，用于做SDK，提供与无人机通讯，以AIDL的方式为上层的App提供服务。基于3DRService，开发者可以不用处理复杂的MavLink通讯，只根据AIDL接口调用服务即可。
这里下载了3DRService用于分析，地址为：https://github.com/ne0fhyk/3DRServices。3DRService将MavLink的协议部分作为单独的包，即项目中的dependencyLibs文件夹。
###6.1. UML图 ###
绘制dependencyLibs的UML图。该包主要提供了MavLink的所有类型的封包类和解析类。

例如，对于MAVLinkPacket类，其核心部分为封包过程。

    /**
    * Encode this packet for transmission.
    *
    * @return Array with bytes to be transmitted
    */
    public byte[] encodePacket() {
        byte[] buffer = new byte[6 + len + 2];

        int i = 0;
        buffer[i++] = (byte) MAVLINK_STX;
        buffer[i++] = (byte) len;
        buffer[i++] = (byte) seq;
        buffer[i++] = (byte) sysid;
        buffer[i++] = (byte) compid;
        buffer[i++] = (byte) msgid;

        final int payloadSize = payload.size();
        for (int j = 0; j < payloadSize; j++) {
            buffer[i++] = payload.payload.get(j);
        }

        generateCRC();
        buffer[i++] = (byte) (crc.getLSB());
        buffer[i++] = (byte) (crc.getMSB());
        return buffer;
    }

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解包的核心部分在Parser类中：

    /**
     * This is a convenience function which handles the complete MAVLink
     * parsing. the function will parse one byte at a time and return the
     * complete packet once it could be successfully decoded. Checksum and other
     * failures will be silently ignored.
     * 
     * @param c
     *            The char to parse
     */
    public MAVLinkPacket mavlink_parse_char(int c) {
        msg_received = false;

        switch (state) {
        case MAVLINK_PARSE_STATE_UNINIT:
        case MAVLINK_PARSE_STATE_IDLE:

            if (c == MAVLinkPacket.MAVLINK_STX) {
                state = MAV_states.MAVLINK_PARSE_STATE_GOT_STX;
            }
            break;

        case MAVLINK_PARSE_STATE_GOT_STX:
            if (msg_received) {
                msg_received = false;
                state = MAV_states.MAVLINK_PARSE_STATE_IDLE;
            } else {
                m = new MAVLinkPacket(c);
                state = MAV_states.MAVLINK_PARSE_STATE_GOT_LENGTH;
            }
            break;

        case MAVLINK_PARSE_STATE_GOT_LENGTH:
            m.seq = c;
            state = MAV_states.MAVLINK_PARSE_STATE_GOT_SEQ;
            break;

        case MAVLINK_PARSE_STATE_GOT_SEQ:
            m.sysid = c;
            state = MAV_states.MAVLINK_PARSE_STATE_GOT_SYSID;
            break;

        case MAVLINK_PARSE_STATE_GOT_SYSID:
            m.compid = c;
            state = MAV_states.MAVLINK_PARSE_STATE_GOT_COMPID;
            break;

        case MAVLINK_PARSE_STATE_GOT_COMPID:
            m.msgid = c;
            if (m.len == 0) {
                state = MAV_states.MAVLINK_PARSE_STATE_GOT_PAYLOAD;
            } else {
                state = MAV_states.MAVLINK_PARSE_STATE_GOT_MSGID;
            }
            break;

        case MAVLINK_PARSE_STATE_GOT_MSGID:
            m.payload.add((byte) c);
            if (m.payloadIsFilled()) {
                state = MAV_states.MAVLINK_PARSE_STATE_GOT_PAYLOAD;
            }
            break;

        case MAVLINK_PARSE_STATE_GOT_PAYLOAD:
            m.generateCRC();
            // Check first checksum byte
            if (c != m.crc.getLSB()) {
                msg_received = false;
                state = MAV_states.MAVLINK_PARSE_STATE_IDLE;
                if (c == MAVLinkPacket.MAVLINK_STX) {
                    state = MAV_states.MAVLINK_PARSE_STATE_GOT_STX;
                    m.crc.start_checksum();
                }
                stats.crcError();
            } else {
                state = MAV_states.MAVLINK_PARSE_STATE_GOT_CRC1;
            }
            break;

        case MAVLINK_PARSE_STATE_GOT_CRC1:
            // Check second checksum byte
            if (c != m.crc.getMSB()) {
                msg_received = false;
                state = MAV_states.MAVLINK_PARSE_STATE_IDLE;
                if (c == MAVLinkPacket.MAVLINK_STX) {
                    state = MAV_states.MAVLINK_PARSE_STATE_GOT_STX;
                    m.crc.start_checksum();
                }
                stats.crcError();
            } else { // Successfully received the message
                stats.newPacket(m);
                msg_received = true;
                state = MAV_states.MAVLINK_PARSE_STATE_IDLE;
            }

            break;

        }
        if (msg_received) {
            return m;
        } else {
            return null;
        }
}

`



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这个类的使用是逐个直接解析，解析完毕后返回完整的包，例如，对字节数组packet，解析过程如下：

for(int i = 0; i < packet.length - 1; i++){
    parser.mavlink_parse_char(packet[i] & 0xFF);//每次解析1位
}
MAVLinkPacket m = parser.mavlink_parse_char(packet[packet.length - 1] & 0xFF);//最后1位即可返回
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###6.2 MavLink包测试###
dependencyLibs提供了测试实例。以msg_altitude为例，判断生成的包和解析的包是否相同，即可判断该类是否正确。

/**
* The current system altitude.
*/
public class msg_altitude_test{

public static final int MAVLINK_MSG_ID_ALTITUDE = 141;
public static final int MAVLINK_MSG_LENGTH = 24;
private static final long serialVersionUID = MAVLINK_MSG_ID_ALTITUDE;

private Parser parser = new Parser();//1位解析类

public CRC generateCRC(byte[] packet){
    CRC crc = new CRC();
    for (int i = 1; i < packet.length - 2; i++) {
        crc.update_checksum(packet[i] & 0xFF);
    }
    crc.finish_checksum(MAVLINK_MSG_ID_ALTITUDE);
    return crc;
}

public byte[] generateTestPacket(){
    ByteBuffer payload = ByteBuffer.allocate(6 + MAVLINK_MSG_LENGTH + 2);
    payload.put((byte)MAVLinkPacket.MAVLINK_STX); //stx
    payload.put((byte)MAVLINK_MSG_LENGTH); //len
    payload.put((byte)0); //seq
    payload.put((byte)255); //sysid
    payload.put((byte)190); //comp id
    payload.put((byte)MAVLINK_MSG_ID_ALTITUDE); //msg id
    payload.putFloat((float)17.0); //altitude_monotonic
    payload.putFloat((float)45.0); //altitude_amsl
    payload.putFloat((float)73.0); //altitude_local
    payload.putFloat((float)101.0); //altitude_relative
    payload.putFloat((float)129.0); //altitude_terrain
    payload.putFloat((float)157.0); //bottom_clearance
    
    CRC crc = generateCRC(payload.array());
    payload.put((byte)crc.getLSB());
    payload.put((byte)crc.getMSB());
    return payload.array();
}

@Test
public void test(){
    byte[] packet = generateTestPacket();
    for(int i = 0; i < packet.length - 1; i++){
        parser.mavlink_parse_char(packet[i] & 0xFF);//每次解析1位
    }
    MAVLinkPacket m = parser.mavlink_parse_char(packet[packet.length - 1] & 0xFF);//最后1位即可返回
    byte[] processedPacket = m.encodePacket();//解析
    assertArrayEquals("msg_altitude", processedPacket, packet);
}
}

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参考

[1]http://mrsxm.mfzgi5lqnfwg65bomnxw2.erenta.ru/wp-content/uploads/sites/6/2015/05/MAVLINK_FOR_DUMMIESPart1_v.1.1.pdf
[2]http://blog.csdn.net/u013983741/article/details/48053235

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