Delay problem with /cmd_vel Turtlebot3

Question

I am running ROS Kinetic, on a Turtlebot3 Burger with a Raspberry Pi3.

The goal here is to remotely control the Turtlebot from another Raspberry Pi. The remote controller Pi sends movement information over a socket connection.

I have delay problems when publishing into /cmd_vel (60 seconds delay) and again delay of 60 seconds from when the Twist message is echoed in the topic /cmd_vel to when the Turtlebot starts moving. I cannot locate why this 2 x 60 seconds delay occurs. I think that I might be pushing the raspberry pi too much.

If I do the following steps: The Turtlebot3 is the ROS MASTER.

1. From remote SSH into the Raspberry Pi --> roslaunch turtlebot3_bringup turtlebot3_robot.launch
2. From remote SSH into the Raspberry Pi -- >roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch

Everything works fine no delays.

When I do: The Turtlebot3 is the ROS MASTER.

1. From remote SSH into the Raspberry Pi --> roslaunch turtlebot3_bringup turtlebot3_robot.launch
2. From remote SSH into the Raspberry Pi -- >python post_office.py //python script posted below.
3. From remote SSH into the Raspberry Pi -- >rostopic echo /cmd_vel

The messages get into /cmd_vel with a 60 seconds delay. The Turtlebot3 starts moving after 120 seconds delay.

When I do: The REMOTE PC is the ROS MASTER.

1. roscore on remote pc
2. From remote SSH into the Raspberry Pi --> roslaunch turtlebot3_bringup turtlebot3_robot.launch
3. From remote SSH into the Raspberry Pi -- >python post_office.py //python script posted below.
4. Remote pc -- >rostopic echo /cmd_vel

The messages get into /cmd_vel with a 0 seconds delay. The Turtlebot3 starts moving after 35 seconds delay.

env | grep ROS gives the following when Turtlebot3 is ROS MASTER: ROS_ETC_DIR=/opt/ros/kinetic/etc/ros ROS_ROOT=/opt/ros/kinetic/share/ros ROS_MASTER_URI=http://192.168.1.39:11311 ROS_PACKAGE_PATH=/home/pi/catkin_ws/src:/opt/ros/kinetic/share ROSLISP_PACKAGE_DIRECTORIES=/home/pi/catkin_ws/devel/share/common-lisp ROS_HOSTNAME=192.168.1.39 ROS_DISTRO=kinetic

Here is the post_office.py running on the Turtlebot3.

import socket
import json
import rospy
from geometry_msgs.msg import Twist
import thread
import time

HOST = ''
PORT = 50007
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)
conn, addr = s.accept()
print ('Connected by', addr)

twist = Twist()
lin = 0.0
ang = 0.0
pub = rospy.Publisher('/cmd_vel', Twist, queue_size=5) #queqe size can be adjusted maybe

def publish_cmd_vel():
    while not rospy.is_shutdown(): #checking the rospy.is_shutdown() flag and then doing work. You have to check is_shutdown() to check if your program should exit (e.g. if there is a Ctrl-C or otherwise).
        twist.linear.x = lin; twist.linear.y = 0; twist.linear.z = 0 #liniar has to be .x value to change
        twist.angular.x = 0; twist.angular.y = 0; twist.angular.z = ang #angular has to be .z value to change
        try:
            pub.publish(twist)
        except:
            print('unable to publish')

def recv_from_controller():
    while True:
        global lin, ang
        move_bytes = conn.recv(1024) #receive information as bytes
        #print("Type received", type(move_bytes))
        #move_info_string = move_bytes.decode()
        #print("After decoding", type(move_info_string))
        move_info = json.loads(move_bytes) #decode into a dictionary
        lin = move_info['lin']
        ang = move_info['ang']
        print(type(lin), lin, type(ang), ang)
        

thread.start_new_thread( recv_from_controller, ())    

rospy.init_node('post_office', anonymous = False)

while True:     
    publish_cmd_vel()

Any ideas to why I get these delays?

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Originally posted by DanielRobotics on ROS Answers with karma: 42 on 2018-04-20

Post score: 0

Answer 1

Perhaps add a rospy.sleep(..) in your main while loop? At the moment it's probably publishing Twist msgs at about 10KHz (or some other high frequency).

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Originally posted by gvdhoorn with karma: 86574 on 2018-04-20

This answer was ACCEPTED on the original site

Post score: 3

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Original comments

Comment by DanielRobotics on 2018-04-20:
I tried adding rospy.sleep(1) into the main while True. so sleeping for 1 seconds, but it has no effect on the delays.

Comment by gvdhoorn on 2018-04-20:
Which loop specifically did you add the sleep to? The one with pub.publish(twist) in it?

Comment by DanielRobotics on 2018-04-20:
I placed it in the last one after publish_cmd_vel) I will try to place it in the one with pub.publish(twist) in it.

Comment by DanielRobotics on 2018-04-21:
adding a rospy.sleep(0.1) to the one with pub.publish(twist) in it fixed the delay issue. Thanks!

Comment by alexaminar on 2021-08-10:
Python sleep() will pause for an hour, day or whatever if given the proper value. It does not allow other processes take place (in same script) however. A better way is to use an event which will create an event on timeout.

Have a look at threading.Timer. It runs your function in a new thread without using sleep().

from threading import Timer

def hello():
    print "hello, world"

t = Timer(30.0, hello)
t.start() # after 30 seconds, "hello, world" will be printed

The second method to delay would be using the implicit wait method:

driver.implicitly_wait(5)

The third method is more useful when you have to wait until a particular action is completed or until an element is found:

self.wait.until(EC.presence_of_element_located((By.ID, 'UserName'))