How to use RPI3 as a companion device between pixhawk(PX4,APM) and ground station,with Dronee Pilot.

Below topics cover how to use RPI3 as repeater, as companion device and both.

First thing in RPI3 is to enable serial port for the the transmitter. Pixhawk is shown in /dev/ttyACM0 serial port, and the transmitter is in /dev/ttyS0.

Most of the linux systems favor buffering data to optimize throughput as same as in Raspbian,but in this case we need low latency transmission/handling. First of all we need to tell to the linux to threat with both pixhawk and transmitter as low latency devices.

setserial /dev/ttyACM0 low_latency
setserial /dev/S0 low_latency
 

PX4 connects with 115200 of baud rate by default,make sure that you set the transmitter's baudrate 115200 or above.

Using RPI3 as repeater.

The easiest way is to use socat,socat is upgraded version of netcat which comes default in all linux with many functionality and options. To install socat:

sudo apt-get install socat
 

socat /dev/ttyACM0,nonblock,raw,echo=0,waitlock=/var/run/ttyLock,min=0,b115200 /dev/ttyS0,nonblock,raw,echo=0,waitlock=/var/run/tty,min=0,b115200
  

  static SerialPort sp1(port1,baudRate1),sp2(port2,baudRate2);
  sp1.open_serial();
  sp2.open_serial();
  static char buffer1[BUFFER_LENGTH],buffer2[BUFFER_LENGTH];
  thread([](){
  	while(true){
    	int l = sp2._read_port(buffer2,BUFFER_LENGTH);
        if(l>0){
	   sp1._write_port(buffer2, l);
        }
           usleep(5*1000);
          }
   }).detach();
        
        
  while(true){
  	int l = sp1._read_port(buffer1,BUFFER_LENGTH);
    if(l>0){
    	sp2._write_port(buffer1, l);
   	 }
     usleep(5*1000);         
 }
 

git clone https://github.com/maksimpiriyev/serial_port_repeater
 

./compile
 

./repeater /dev/ttyACM0 115200 /dev/ttyS0 115200
 

Using RPI3 as companion device.

If you just want to write code that will work inside the drone. You just open the /dev/ttyACM0 serial port and parse the bytes with mavlink parser, here is the below code and github repository

 static char buffer1[BUFFER_LENGTH];
 int l = sp1._read_port(buffer1,BUFFER_LENGTH);
 if(l>0){
    static mavlink_message_t msg;
    static mavlink_status_t status;
    for (int i = 0; i < l; ++i)
    {
    	if (mavlink_parse_char(MAVLINK_COMM_0, buf[i], &msg, &status)){
           // Packet received
        	printf("\nReceived packet: SYS: %d, COMP: %d, LEN: %d, MSG ID: %d\n", msg.sysid, msg.compid, msg.len, msg.msgid);
         }
     }
  }
 

./compile_companion
 

./companion /dev/ttyACM0
 

Using RPI3 as repeater and companion device at the same time by handling some functionality in the RPI3 drone itself.

This is is the same as above repeater but handling mavlink is also integrated. To compile:

./compile_companion_repeater
 

./repcomp /dev/ttyACM0 /dev/ttyS0

Handling mavlink messages without interfering repeating.

There are sometimes cases handlink mavlink packets can take some long amount of time,especially when you do vision processing.In order to prevent latency problems in retransmission mavlink messages should be handled in another thread.

Below is the easy readable thread safe queue gets data and push it to another thread.

template
class thread_safe_queue{
    queue q;
    mutex m;
    condition_variable cv;
public:
    K pop(){
        unique_lock l(m);
        if(!q.size()){
            cv.wait(l);
        }
        auto m = q.front();
        q.pop();
        return m;
    }
    void push(K k){
        lock_guard l(m);
        q.push(k);
        cv.notify_one();
    }
};
 

Here is the handling code.Note that extracting mavlink messages have deterministic reasonable time,so they can be handled in the retransmission thread.

 static thread_safe_queue q;
 thread([]{
         while(true){
             auto m = v.pop();
             if(m.msgid == MAVLINK_MSG_ID_HEARTBEAT){
			 	// handling heartbeat
			 }else{
			 	// some vision code
			 }
         }
     }).detach();
	 
 
 static char buffer1[BUFFER_LENGTH];
 int l = sp1._read_port(buffer1,BUFFER_LENGTH);
 if(l>0){
    static mavlink_message_t msg;
    static mavlink_status_t status;
    for (int i = 0; i < l; ++i)
    {
    	if (mavlink_parse_char(MAVLINK_COMM_0, buf[i], &msg, &status)){
           q.push(msg);
         }
     }
  }
 

Restarting program in possible crash.

To make the program restart itself we will use supervisor. Supervisor deosn't come built i in Raspbian. To install.

sudo apt-get install supervisor
sudo service supervisor restart
 

  [program:repeater]
  command=/home/pi/repeater /dev/ttyACM0 115200 /dev/ttyS0 115200
  autostart=true
  autorestart=true
  stderr_logfile=/var/log/repeater.err.log
  stdout_logfile=/var/log/repeater.out.log
 

  supervisorctl reread
  

 supervisorctl update
  

Any time you make a change to any program configuration file, running the two previous commands will bring the changes into effect.

Now, you are all set!