Problem with usb serial RPI -> Arduino

Question

I'm working on a robot, more precisely on his head which have to track an human face.

I'm using a RPI which will detect the face and then send to the Arduino the angles for both servomotors.

I wrote a python script which will take 2 parameters and send them to the Arduino, this script will open the serial connection and send the command.

python script:

#!/usr/bin/env python

import serial
import time
import sys

ser = serial.Serial("/dev/ttyACM0", 9600)
time.sleep(2)

coords = [str(sys.argv[1]), str(sys.argv[2])]

commande = 'X' + coords[0] + 'Y' + coords[1] + 'V'

ser.write(commande)

ser.close()

The control of servos is working well on the Arduino but the problem is that each time I execute my script, the opening of the serial connection firstly reset both servos and once the Arduino receive the command both servos will go to the correct angles.

Arduino sketch:

//Bibliothèque :
#include <Servo.h> //Ajoute la librairie Servo

Servo servo1; //servo 180°
Servo servo2;    //servo 360°
int pos1 = 0; //Position initiale du servo1
int pos2 = 0; //Position initiale du servo2
int maxHaut = 90;
int maxBas = -35;
int maxGauche = 90;
int maxDroite = -85;

/**
 * Déclarations variables commandes
 */
 String commande = "";
 int coords[] = {0,0};
 int newCoords[] = {0,0};
 int indexX = 0;
 int indexY = 0;
 int indexFinCommande = 0;
 int result = 0;

void setup() {
  servo1.attach(9); //servo 180° sur pin 9
  servo1.write(35);
  servo2.attach(10); //servo 180° sur pin 10
  servo2.write(85);
  Serial.begin(9600);
}

void loop() {
  //pos1 = servo1.read();
  //pos2 = servo2.read();
  commande = "";
  if(Serial.available())
  {
    //Serial.println("Traitement de la commande..");
    commande = Serial.readString();
    //Serial.println(commande);
    traitementCommande(commande);
  }
}

void traitementCommande(String uneCommande)
{
  Serial.println(uneCommande);

  indexFinCommande = uneCommande.indexOf('V');
  indexX = uneCommande.indexOf('X');
  indexY = uneCommande.indexOf('Y');

  if((indexFinCommande==-1) || (indexX==-1) || (indexY==-1))
  {
    Serial.println("Commande non reconnue..");
    goto commandNotRecognized;
  }

  newCoords[0] = uneCommande.substring(indexX+1,indexY).toInt();
  newCoords[1] = uneCommande.substring(indexY+1, indexFinCommande).toInt();

  if((newCoords[0] < maxBas) || (newCoords[0] > maxHaut) || (newCoords[1] < maxDroite) || (newCoords[1] > maxGauche))
  {
    Serial.println("Coordonees invalides..");
    goto commandNotRecognized;
  }

  coords[0] = newCoords[0];
  coords[1] = newCoords[1];
  pos1 = coords[0] + 35;
  pos2 = coords[1] + 85;

  servo1.write(pos1);
  servo2.write(pos2);

  Serial.println("OK");
  commandNotRecognized:
  Serial.print(coords[0]);
  Serial.print(" ");
  Serial.println(coords[1]);
}

I want to know if there is an alternative to keep the serial connection open or if you guys had a solution for my problem.

I hope you will understand my request. Sorry for the fews french word used in my code, if something seems strange to you just ask me to explain it ;) And I'm also sorry for my english that should not be perfect..

Thank you for your time and your help.

Answer 1

Keep the script alive and use a infinite loop to keep it running without closing it:

while True:

    coords = [str(sys.argv[1]), str(sys.argv[2])]

    commande = 'X' + coords[0] + 'Y' + coords[1] + 'V'

    ser.write(commande)

Answer 2

I found something and it's actually working as I expected !

I used SimpleXMLRPCServer (https://docs.python.org/2/library/simplexmlrpcserver.html#simplexmlrpcserver-example) which permits me to store my serial instance !

Here is my first python script which will start my serial connection and store it into the server:

file: openSerialToArduino.py

#!/usr/bin/env python

import serial
import time
from SimpleXMLRPCServer import SimpleXMLRPCServer
from SimpleXMLRPCServer import SimpleXMLRPCRequestHandler

class RequestHandler(SimpleXMLRPCRequestHandler):
        rpc_paths = ('/RPC2',)

server = SimpleXMLRPCServer(("localhost", 8000), requestHandler=RequestHandler)

server.register_introspection_functions()

ser = serial.Serial("/dev/ttyACM0", 9600)
time.sleep(2)

server.register_instance(ser)

server.serve_forever()

Then I modify this file to be executable:

chmod +x openSerialToArduino.py

So I can execute this one in background to let me access to my serial instance:

nohup /home/pi/Documents/Projects/SerialPiArduinoServo/openSerialToArduino.py &

And finally I can execute my main script and get the instance of serial without re-open it:

file: serialControlServoAbsPi.py

#!/usr/bin/env python

import serial
import sys
import xmlrpclib

s = xmlrpclib.ServerProxy('http://localhost:8000')

coords = [str(sys.argv[1]), str(sys.argv[2])]

command = 'X' + coords[0] + 'Y' + coords[1] + 'V'

s.write(command)

And now both servos are going directly to the desired angles !

Thank you Luis Diaz for leaded me on this way ! Creating the serial instance before executing my script was the main part of the solution ! (BTW your answer didn't seems me too rude, you was totally correct ;) )

Hope this will help some Googlers !

Edit:

I get an error when I restarted RPI, don't know why and I was not able to fix this one so I used a fresh Raspbian and another version of SimpleXMLRPCServer for python 2 and it's working again even after reboot :)