# Ros basics in 5 days project

**URL:** <https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160>\
**Category:** ROSDS Support\
**Created:** [March 18, 2023, 8:21pm UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160 "2023-03-18T20:21:02Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![80601](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/80601/32/7019_2.png) [@80601](https://get-help.theconstruct.ai/u/80601)\
**Post date:** [March 18, 2023, 8:21pm UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160/1 "2023-03-18T20:21:03Z")

</div>

why the reading from left and right laser are always very close in value and does not represent the true distance that i see in the gazebo simulation as iam sure they should be very different

```auto

import rospy
import time
from project.srv import FindWall , FindWallResponse
from geometry_msgs.msg import Twist
from sensor_msgs.msg import LaserScan

 
class my_node(object):

    def __init__ (self):
        self.move_2 = Twist()
        self.left = 0 
        self.right = 0  
        self.front = 0 
        self.ready_point = 0 

    def callback_msg(self,msg):
        self.left = msg.ranges[715]
        self.right = msg.ranges[5]
        self.front = msg.ranges[360]
        self.ready_point = msg.ranges[270]
    
    
    def callback(self,request):

        r = rospy.Rate(4)
        rospy.loginfo("left distance"+str(self.left))
        rospy.loginfo("right distance"+str(self.right))
        rospy.loginfo("front distnace"+str(self.front))
        
        while ((self.left == 0) and (self.right == 0) and (self.front == 0)):
            rospy.sleep(1)

        rospy.loginfo('findwall service started...')
    
        if ((self.left < self.right) and (self.left < self.front)):

            rospy.loginfo("first case")
            left_1 = round(self.left , 1)
            front_1 = round(self.front , 1)
            while front_1 != left_1:
                rospy.loginfo('rotating left..')
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.5 #rotate to the left
                pub_s.publish(self.move_2)
                front_1 = round(self.front , 1)
                rospy.loginfo("current left distance"+str(left_1))
                rospy.loginfo("current front distance"+str(front_1))
                r.sleep()
    
                
            while self.front >= 0.3:
                rospy.loginfo("moving forward")
                self.move_2.linear.x = 0.1
                self.move_2.angular.z = 0
                pub_s.publish(self.move_2)
                r.sleep()

            while self.ready_point > 0.3:
                rospy.loginfo("trying to reach the start state (ready_point)")
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.5 #rotate to the left
                pub_s.publish(self.move_2)
                r.sleep()

    
        elif ((self.right < self.left) and (self.right < self.front)):

            rospy.loginfo("second case")
            right_1 = round(self.right , 1)
            front_1 = round(self.front , 1)

            while front_1 != right_1:

                rospy.loginfo("rotating right ..")
                self.move_2.linear.x = 0
                self.move_2.angular.z = -0.5 #rotate to the right
                pub_s.publish(self.move_2)
                front_1 = round(self.front , 1)
                rospy.loginfo("current right distance"+str(right_1))
                rospy.loginfo("current front distance"+str(front_1))
                r.sleep()
            while self.front >= 0.3:

                rospy.loginfo("moving forward")
                self.move_2.linear.x = 0.08
                self.move_2.angular.z = 0
                pub_s.publish(self.move_2)
                r.sleep()
            while self.ready_point > 0.3:
                rospy.loginfo("trying to reach the start state (ready_point)")
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.5 #rotate to the left
                pub_s.publish(self.move_2)
                r.sleep()
    
        else:
            rospy.loginfo("third case")
    
            while self.front >= 0.3:
                rospy.loginfo("moving forward")
                self.move_2.linear.x = 0.1
                self.move_2.angular.z = 0
                pub_s.publish(self.move_2)
                r.sleep()

            while self.ready_point > 0.3:
                rospy.loginfo("trying to reach the start state (ready_point)")
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.5 #rotate to the left
                pub_s.publish(self.move_2)
                r.sleep()
                
        self.move_2.linear.x = 0
        self.move_2.angular.z = 0
        pub_s.publish(self.move_2)
    
        rospy.loginfo('findwall service finished..')
        message = FindWallResponse()
        message.wallfound = True
        return message
    

if __name__ == ' __main__':
    rospy.init_node('service_node')
    my_object = my_node()
    sub_s = rospy.Subscriber("/scan" , LaserScan ,my_object.callback_msg)
    rospy.wait_for_message("/scan" , LaserScan)
    pub_s = rospy.Publisher('/cmd_vel' ,Twist , queue_size= 1 )
    FindWall_service = rospy.Service('/find_wall' , FindWall , my_object.callback)
    rospy.spin()

    

```

---

<div class="post-metadata">

**Author:** ![girishkumar.kannan](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/girishkumar.kannan/32/6994_2.png) [@girishkumar.kannan](https://get-help.theconstruct.ai/u/girishkumar.kannan)\
**Post date:** [March 19, 2023, 5:04am UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160/2 "2023-03-19T05:04:23Z")

</div>

Hi @80601 ,

That is because, unlike in the robot that you used in the course that used 180 degree laser scanner with `0 - right`, `360 - front` and `719 - left`, the turtlebot in rosject uses a 360 degree laser scanner that gives `0/719 - back`, `180 - right`, `360 - front` and `540 - left`.

Refer this post for more info: [Incorrect Robot Scan Values in ROS Basics Course - Wall Follower Robot / TurtleBot3](https://get-help.robotigniteacademy.com/t/incorrect-robot-scan-values-in-ros-basics-course-wall-follower-robot-turtlebot3/18688)

Regards,  
Girish

---

<div class="post-metadata">

**Author:** ![80601](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/80601/32/7019_2.png) [@80601](https://get-help.theconstruct.ai/u/80601)\
**Post date:** [March 20, 2023, 7:04pm UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160/5 "2023-03-20T19:04:17Z")

</div>

actually i do not know why but the front readings at 360 are totally wrong are you sure ?

 ![Screenshot 2023-03-20 210828](https://get-help.robotigniteacademy.com/uploads/default/original/2X/2/2b2f01710269bc530b7a304ac087cbe6f648e92c.png)

---

<div class="post-metadata">

**Author:** ![girishkumar.kannan](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/girishkumar.kannan/32/6994_2.png) [@girishkumar.kannan](https://get-help.theconstruct.ai/u/girishkumar.kannan)\
**Post date:** [March 21, 2023, 4:35am UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160/6 "2023-03-21T04:35:43Z")

</div>

Hi @80601 ,

> [@80601](#):
>
> actually i do not know why but the front readings at 360 are totally wrong are you sure ?

I think I exactly know why you have this problem!

When I worked on this rosject, I found out that the 360 degree laser has 360 readings instead of 720, whereas the real robot TurtleBot has 720 readings for the laser.

You can verify this by doing `ros2 topic echo /scan` and cancel after you get at least one reading.  
You will need to see three fields:

For a 360 degree laser with 0.5 degree angle increment you will have 720 values:

```bash
angle_min: -3.141590118408203
angle_max: 3.141590118408203
angle_increment: 0.008738775737583637 # equals 0.5 degrees

```

For a 360 degree laser with 1.0 degree angle increment you will have 360 values:

```bash
angle_min: -3.141590118408203
angle_max: 3.141590118408203
angle_increment: 0.017477551 # equals 1.0 degrees

```

So now you have two options (choose one, not both):

1. Use a parameter setting in your program (launch file) to use laser scan ranges as 720 values for real robot and 360 values for simulation.
2. Change the value of `samples` to `720` and `resolution` to `1` in the `turtlebot3_burger.gazebo.xacro` file that is located in the `turtlebot3_description` package. This will permanently change simulation robot’s laser scan values from 360 to 720.

I am certain this will solve your problem.

Regards,  
Girish

---

<div class="post-metadata">

**Author:** ![80601](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/80601/32/7019_2.png) [@80601](https://get-help.theconstruct.ai/u/80601)\
**Post date:** [March 21, 2023, 1:39pm UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160/7 "2023-03-21T13:39:47Z")

</div>

i got this the readings seam to be 720 my service code act as expected but i think the problem starts with the wall following function 🤷‍♂️

```auto
header:
  seq: 15
  stamp:
    secs: 13
    nsecs: 233000000
  frame_id: "base_scan"
angle_min: -3.1415998935699463
angle_max: 3.1415998935699463
angle_increment: 0.008738803677260876
time_increment: 0.0
scan_time: 0.0
range_min: 0.11999999731779099
range_max: 3.5

```

code for service

```auto
#! /usr/bin/env python

import rospy
import time
from project.srv import FindWall, FindWallResponse
from geometry_msgs.msg import Twist
from sensor_msgs.msg import LaserScan

class my_node(object):

    def __init__ (self):
        self.move_2 = Twist()
        self.left = 0
        self.right = 0
        self.front = 0
        self.ready_point = 0

    def callback_msg(self, msg):
        self.left = msg.ranges[540]
        self.right = msg.ranges[180]
        self.front = msg.ranges[360]
        self.ready_point = msg.ranges[270]

    def callback(self, request):

        r = rospy.Rate(6)
        rospy.loginfo("left distance"+str(self.left))
        rospy.loginfo("right distance"+str(self.right))
        rospy.loginfo("front distnace"+str(self.front))

        while ((self.left == 0) and (self.right == 0) and (self.front == 0)):
            rospy.sleep(1)

        rospy.loginfo('findwall service started...')

        if ((self.left < self.right) and (self.left < self.front)):

            rospy.loginfo("first case")
            left_1 = round(self.left, 1)
            front_1 = round(self.front, 1)
            while front_1 != left_1:
                rospy.loginfo('rotating left..')
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.45 # rotate to the left
                pub_s.publish(self.move_2)
                front_1 = round(self.front, 1)
                rospy.loginfo("current left distance"+str(left_1))
                rospy.loginfo("current front distance"+str(front_1))
                r.sleep()

            while self.front >= 0.3:
                rospy.loginfo("moving forward")
                self.move_2.linear.x = 0.1
                self.move_2.angular.z = 0
                pub_s.publish(self.move_2)
                r.sleep()

            while self.ready_point > 0.3:
                rospy.loginfo("trying to reach the start state (ready_point)")
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.45 # rotate to the left
                pub_s.publish(self.move_2)
                r.sleep()

        elif ((self.right < self.left) and (self.right < self.front)):

            rospy.loginfo("second case")
            right_1 = round(self.right, 1)
            front_1 = round(self.front, 1)

            while front_1 != right_1:

                rospy.loginfo("rotating right ..")
                self.move_2.linear.x = 0
                self.move_2.angular.z = -0.45 # rotate to the right
                pub_s.publish(self.move_2)
                front_1 = round(self.front, 1)
                rospy.loginfo("current right distance"+str(right_1))
                rospy.loginfo("current front distance"+str(front_1))
                r.sleep()
            while self.front >= 0.3:

                rospy.loginfo("moving forward")
                self.move_2.linear.x = 0.08
                self.move_2.angular.z = 0
                pub_s.publish(self.move_2)
                r.sleep()
            while self.ready_point > 0.3:
                rospy.loginfo("trying to reach the start state (ready_point)")
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.45 # rotate to the left
                pub_s.publish(self.move_2)
                r.sleep()

        else:
            rospy.loginfo("third case")

            while self.front >= 0.3:
                rospy.loginfo("moving forward")
                self.move_2.linear.x = 0.1
                self.move_2.angular.z = 0
                pub_s.publish(self.move_2)
                r.sleep()

            while self.ready_point > 0.3:
                rospy.loginfo("trying to reach the start state (ready_point)")
                self.move_2.linear.x = 0
                self.move_2.angular.z = 0.45 # rotate to the left
                pub_s.publish(self.move_2)
                r.sleep()

        self.move_2.linear.x = 0
        self.move_2.angular.z = 0
        pub_s.publish(self.move_2)

        rospy.loginfo('findwall service finished..')
        message = FindWallResponse()
        message.wallfound = True
        return message

if __name__ == ' __main__':
    rospy.init_node('service_node')
    my_object = my_node()
    sub_s = rospy.Subscriber("/scan", LaserScan, my_object.callback_msg)
    rospy.wait_for_message("/scan", LaserScan)
    pub_s = rospy.Publisher('/cmd_vel', Twist, queue_size=1)
    FindWall_service = rospy.Service(
        '/find_wall', FindWall, my_object.callback)
    rospy.spin()
            rospy.loginfo("action successd")
            self.server.set_succeeded(self._result)

if __name__ == ' __main__':
    rospy.init_node('action_node')
    record_odomClass()
    rospy.spin()

```

main code

```auto
#! /usr/bin/env python

import time
import rospy
from geometry_msgs.msg import Twist
from sensor_msgs.msg import LaserScan
from project.srv import FindWall, FindWallRequest
import actionlib
from project.msg import OdomRecordAction, OdomRecordGoal
from std_msgs.msg import Empty

####service####
rospy.init_node("project_node")
rospy.wait_for_service('/find_wall')
FindWall_service = rospy.ServiceProxy('/find_wall', FindWall)
result = FindWall_service(FindWallRequest())
print(result)

####acttion####
PENDING = 0
ACTIVE = 1
DONE = 2
WARN = 3
ERROR = 4
nImage = 1

def feedback_callback(feedback):
    print("feedback from action server totall distance moved so far\n"+str(feedback))

action_server_name = 'record_odom'
action_client = actionlib.SimpleActionClient(
    action_server_name, OdomRecordAction)
rospy.loginfo('Waiting for action Server '+action_server_name)
action_client.wait_for_server()
rospy.loginfo('Action Server Found...'+action_server_name)
action_client.send_goal(Empty(), feedback_cb=feedback_callback)
state_result = action_client.get_state()

rate = rospy.Rate(1)

rospy.loginfo("state_result: "+str(state_result))

####wall_following####

pub = rospy.Publisher("/cmd_vel", Twist, queue_size=1)
move = Twist()
rospy.loginfo("start wall fllowing")

def callback(msg):
    rospy.loginfo(msg.ranges[360])
    # perform wallfollower programm
    if msg.ranges[180] > 0.3 :
        # approach the wall slowly
        move.linear.x = 0.06
        move.angular.z = -0.09
        pub.publish(move)

    elif msg.ranges[180] < 0.2 :
        # get away from the wall slowly
        move.linear.x = 0.06
        move.angular.z = 0.09
        pub.publish(move)

    elif (2 > msg.ranges[180] > 0.3) and msg.ranges[360] > 0.5 :
        # keep moving forward
        move.linear.x = 0.06
        move.angular.z = 0
        pub.publish(move)

    elif msg.ranges[360] < 0.5:
        # turn left while moving forward
        move.linear.x = 0.06
        move.angular.z = 0.6
        pub.publish(move)
    
    state_result = action_client.get_state()
    if state_result == DONE:
        rospy.loginfo("the action is done succesfully")
        rospy.loginfo(action_client.get_result())

sub = rospy.Subscriber("/scan", LaserScan, callback)
rospy.wait_for_message("/scan", LaserScan)

rospy.spin()

```

---

<div class="post-metadata">

**Author:** ![girishkumar.kannan](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/girishkumar.kannan/32/6994_2.png) [@girishkumar.kannan](https://get-help.theconstruct.ai/u/girishkumar.kannan)\
**Post date:** [March 21, 2023, 2:01pm UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160/8 "2023-03-21T14:01:18Z")

</div>

Hi @80601 ,

I just went through your “Service” code. Long story short, your code will not work. You have some issues in your program, especially in the OOPS concept.  
You must define `Subscriber`, `Publisher` and `Service` inside your `my_node()` object.  
You have two `if __name__ == " __main__":` lines in your program.  
The first one should be just `def main(...)`. The second main function is correct.

Your “ServiceClient” code is fine, but it needs mode “beautification”, that is, rearranging of codes. It would be better if you can implement this as another class.

Regards,  
Girish

---

<div class="post-metadata">

**Author:** ![bayodesegun](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/bayodesegun/32/10684_2.png) [@bayodesegun](https://get-help.theconstruct.ai/u/bayodesegun)\
**Post date:** [March 26, 2023, 2:02pm UTC](https://get-help.theconstruct.ai/t/ros-basics-in-5-days-project/23160/9 "2023-03-26T14:02:19Z")

</div>

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