After detecting an obstacle and turning 90 degrees left, the robot immediately turns left again instead of moving forward. It stays stuck in a loop of turning

AntonioMayorga · February 18, 2025, 2:05pm

Problem

After detecting an obstacle and turning 90 degrees left, the robot immediately turns left again instead of moving forward. It stays stuck in a loop of turning.

Possible Cause

It seems that the laser scan data doesn’t update fast enough after turning. As a result:

The old obstacle data is still being used.
The robot thinks there’s still an obstacle in front and turns again.
This only happens after turning, meaning the sensor might need extra time to refresh.

Tried Solutions

Added a short pause (sleep) after turning to wait for updated LaserScan data.
Implemented a “WAIT_FOR_UPDATE” state to force a sensor refresh.
Printed real-time sensor values (front, left, right) to check if they update correctly.

Results

The problem still occurs; the front distance does not update immediately after a turn.
This was not an issue in an earlier simpler version of the code, suggesting the bug is related to how data is being processed after a turn.

Question for the Forum

Why is the LaserScan data not updating immediately after a turn?
How can I ensure the robot correctly detects the new front distance after turning?

#!/usr/bin/env python

import rospy
from sensor_msgs.msg import LaserScan
from geometry_msgs.msg import Twist
import time

class LaserAvoidance:
def init(self):
rospy.init_node(‘topics_quiz_node’, anonymous=True)

    # Publisher to move the robot
    self.cmd_vel_pub = rospy.Publisher('/cmd_vel', Twist, queue_size=10)

    # Subscriber to laser scan data
    self.laser_sub = rospy.Subscriber('/kobuki/laser/scan', LaserScan, self.laser_callback)

    self.twist = Twist()
    self.safe_distance = 1.5  # Distance threshold in meters
    self.turn_speed = 1.0  # Speed for 90-degree turn
    self.turn_duration = 1.3  # Time to complete a 90-degree turn
    self.state = "MOVE_FORWARD"  # Initial state

    self.front_distance = 10.0  # Store updated front distance

def turn_90_degrees(self, direction="left"):
    """
    Rotates the robot by 90 degrees using a time-based approach.
    """
    rospy.loginfo(f"Turning 90 degrees {direction}...")

    twist = Twist()
    twist.linear.x = 0.0
    twist.angular.z = self.turn_speed if direction == "left" else -self.turn_speed

    start_time = time.time()

    while time.time() - start_time < self.turn_duration:
        self.cmd_vel_pub.publish(twist)
        rospy.sleep(0.1)

    # Stop turning after 90 degrees
    twist.angular.z = 0.0
    self.cmd_vel_pub.publish(twist)

    # Transition to waiting for new sensor data
    self.state = "WAIT_FOR_UPDATE"

def laser_callback(self, msg):
    ranges = msg.ranges
    num_ranges = len(ranges)

    # Ensure valid data is received
    if num_ranges == 0:
        rospy.logwarn("Empty LaserScan data received!")
        return

    # Identify key laser readings
    front_index = num_ranges // 2
    left_index = int(num_ranges * 3 / 4)  # 75% index (left side)
    right_index = int(num_ranges * 1 / 4)  # 25% index (right side)

    self.front_distance = ranges[front_index] if ranges[front_index] != float('inf') else 10.0  # Replace inf with 10m
    left = ranges[left_index] if ranges[left_index] != float('inf') else 10.0
    right = ranges[right_index] if ranges[right_index] != float('inf') else 10.0

    rospy.loginfo(f"State: {self.state} | Front: {self.front_distance:.2f}, Left: {left:.2f}, Right: {right:.2f}")

    # State-based decision making
    if self.state == "MOVE_FORWARD":
        if self.front_distance > self.safe_distance:
            # Continue moving forward
            self.twist.linear.x = 0.2
            self.twist.angular.z = 0.0
        else:
            # Obstacle detected, stop and transition to turning
            self.twist.linear.x = 0.0
            self.cmd_vel_pub.publish(self.twist)
            rospy.sleep(2)
            self.state = "TURN_LEFT"

    elif self.state == "TURN_LEFT":
        self.turn_90_degrees("left")  # Turn 90 degrees left
        self.state = "WAIT_FOR_UPDATE"

    elif self.state == "WAIT_FOR_UPDATE":
        rospy.sleep(2)  # Allow time for sensor data to refresh

        if self.front_distance < self.safe_distance:
            # If front still blocked, turn again
            self.state = "TURN_LEFT"
        else:
            # If path is clear, move forward
            self.state = "MOVE_FORWARD"

    # Publish movement command
    self.cmd_vel_pub.publish(self.twist)

def run(self):
    rospy.spin()

if name == ‘main’:
try:
robot = LaserAvoidance()
robot.run()
except rospy.ROSInterruptException:
pass

[INFO] [1739887115.211071, 12.431000]: State: MOVE_FORWARD | Front: 1.55, Left: 2.17, Right: 10.00
[INFO] [1739887115.240988, 12.458000]: State: MOVE_FORWARD | Front: 1.55, Left: 2.15, Right: 10.00
[INFO] [1739887115.265539, 12.480000]: State: MOVE_FORWARD | Front: 1.55, Left: 2.17, Right: 10.00
[INFO] [1739887115.302420, 12.519000]: State: MOVE_FORWARD | Front: 1.55, Left: 2.14, Right: 10.00
[INFO] [1739887115.315217, 12.533000]: State: MOVE_FORWARD | Front: 1.56, Left: 2.12, Right: 10.00
[INFO] [1739887115.336326, 12.555000]: State: MOVE_FORWARD | Front: 1.54, Left: 2.12, Right: 10.00
[INFO] [1739887115.361659, 12.580000]: State: MOVE_FORWARD | Front: 1.53, Left: 2.11, Right: 10.00
[INFO] [1739887115.387551, 12.607000]: State: MOVE_FORWARD | Front: 1.53, Left: 2.13, Right: 10.00
[INFO] [1739887115.421732, 12.636000]: State: MOVE_FORWARD | Front: 1.53, Left: 2.11, Right: 10.00
[INFO] [1739887115.449797, 12.659000]: State: MOVE_FORWARD | Front: 1.50, Left: 2.10, Right: 10.00
[INFO] [1739887115.470958, 12.681000]: State: MOVE_FORWARD | Front: 1.52, Left: 2.09, Right: 10.00
[INFO] [1739887115.501245, 12.706000]: State: MOVE_FORWARD | Front: 1.51, Left: 2.08, Right: 10.00
[INFO] [1739887115.525626, 12.732000]: State: MOVE_FORWARD | Front: 1.51, Left: 2.08, Right: 10.00
[INFO] [1739887115.554157, 12.756000]: State: MOVE_FORWARD | Front: 1.49, Left: 2.07, Right: 10.00
[INFO] [1739887117.584153, 14.759000]: State: TURN_LEFT | Front: 1.48, Left: 2.06, Right: 10.00
[INFO] [1739887117.586531, 14.759000]: Turning 90 degrees left…
[INFO] [1739887118.908810, 16.062000]: State: WAIT_FOR_UPDATE | Front: 1.49, Left: 2.07, Right: 10.00
[INFO] [1739887120.913808, 18.063000]: State: TURN_LEFT | Front: 1.49, Left: 2.03, Right: 10.00
[INFO] [1739887120.916002, 18.064000]: Turning 90 degrees left…
[INFO] [1739887122.247642, 19.365000]: State: WAIT_FOR_UPDATE | Front: 1.47, Left: 2.05, Right: 10.00
[INFO] [1739887124.288336, 21.370000]: State: TURN_LEFT | Front: 1.50, Left: 2.03, Right: 10.00
[INFO] [1739887124.290680, 21.372000]: Turning 90 degrees left…
[INFO] [1739887125.611361, 22.672000]: State: WAIT_FOR_UPDATE | Front: 1.47, Left: 2.06, Right: 10.00
[INFO] [1739887127.667812, 24.673000]: State: TURN_LEFT | Front: 1.48, Left: 2.05, Right: 10.00
[INFO] [1739887127.672869, 24.677000]: Turning 90 degrees left…
[INFO] [1739887129.002014, 25.979000]: State: WAIT_FOR_UPDATE | Front: 1.48, Left: 2.05, Right: 10.00
[INFO] [1739887131.026794, 27.981000]: State: TURN_LEFT | Front: 1.49, Left: 2.06, Right: 10.00
[INFO] [1739887131.029142, 27.983000]: Turning 90 degrees left…
[INFO] [1739887132.348461, 29.287000]: State: WAIT_FOR_UPDATE | Front: 1.49, Left: 2.06, Right: 10.00
[INFO] [1739887134.393447, 31.289000]: State: TURN_LEFT | Front: 1.50, Left: 2.06, Right: 10.00
[INFO] [1739887134.395219, 31.290000]: Turning 90 degrees left…
[INFO] [1739887135.716043, 32.594000]: State: WAIT_FOR_UPDATE | Front: 1.49, Left: 2.06, Right: 10.00
[INFO] [1739887137.756216, 34.600000]: State: TURN_LEFT | Front: 1.50, Left: 2.04, Right: 10.00
[INFO] [1739887137.758357, 34.601000]: Turning 90 degrees left…
[INFO] [1739887139.078719, 35.903000]: State: WAIT_FOR_UPDATE | Front: 1.50, Left: 2.06, Right: 10.00
[INFO] [1739887141.114462, 37.903000]: State: MOVE_FORWARD | Front: 1.49, Left: 2.06, Right: 10.00
[INFO] [1739887143.144590, 39.905000]: State: TURN_LEFT | Front: 1.50, Left: 2.07, Right: 10.00
[INFO] [1739887143.146632, 39.907000]: Turning 90 degrees left…
[INFO] [1739887144.487925, 41.208000]: State: WAIT_FOR_UPDATE | Front: 1.49, Left: 2.05, Right: 10.00
[INFO] [1739887146.504573, 43.212000]: State: TURN_LEFT | Front: 1.49, Left: 2.05, Right: 10.00
[INFO] [1739887146.506871, 43.214000]: Turning 90 degrees left…
[INFO] [1739887147.825577, 44.514000]: State: WAIT_FOR_UPDATE | Front: 1.48, Left: 2.06, Right: 10.00
^C[topics_quiz_node-1] killing on exit
shutting down processing monitor…
… shutting down processing monitor complete

bayodesegun · February 19, 2025, 11:22am

For better context, which specific exercise are you trying to solve?

In any case, your code looks too complicated! To keep things simple,

only use the laser reading in the front center of the robot (“the sensor”)
No need to do a 90-degree turn. Just do a small turn when the sensor detects a close obstacle.
Don’t time the turn. Let the feedback from the laser data handle that.
Resume the forward movement when the sensor can no longer see a close obstacle.

The laser scan data is fast enough to achieve this “turn then move straight” without crashing or turning forever.

AntonioMayorga · February 19, 2025, 9:43pm

thanks for your reply, yes with simpler code it works, but I was just playing with the 90 degrees method, it seems that the program just don’t support it at the end.

bayodesegun · February 21, 2025, 11:36am

I don’t see the need for the 90-degree turn - that’s a big angle! The robot should only turn as much as necessary and continue on its journey, as a real vehicle would.

system · February 26, 2025, 11:36am

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