function abrir_ventana(url,nombre,tam_x,tam_y) {
    var xMax = screen.width, yMax = screen.height;
    var xOffset = (xMax)/6, yOffset = (yMax)/6;

    window.open(url,nombre,'scrollbars=yes, resizable=NO, width='+ tam_x +',height='+ tam_y +',screenX='+xOffset+',screenY='+yOffset+',top='+yOffset+',left='+xOffset+'');
  }