7. Obstacle Avoidance Robot – Bit:Bot Tutorials

Requires Ultra Sonic sensor

In this tutorial we will create a obstacle avoidance robot using the HC-SR04 Ultra sonic sensor. This is a cheap sensor that uses sound waves to calculate distances to objects. The function sends out a 10 microsecond pulse, starts a timer then stops the timer when an echo of the pulse is received, Thge round trip time is then used to calculate the distance to the object.

Using Ultrasound to Calculate Distance

def sonar():  # This function will get distance using the sonar module

    global SONAR

    start_time = 0
    end_time = 0

    SONAR.write_digital(1) # Send 10us Ping pulse
    sleep_us(10)
    SONAR.write_digital(0)
    SONAR.set_pull(SONAR.NO_PULL)
    while SONAR.read_digital() == 0: # ensure Ping pulse has cleared
        pass
    start = ticks_us() # define starting time
    while SONAR.read_digital() == 1: # wait for Echo pulse to return
        pass
    end = ticks_us() # define ending time
    echo = end-start
    distance = int(0.01715 * echo) # Calculate cm distance
    return distance

Now we have a function to detect objects we need to create a simple path finding algorithm so the robot knows how to behave when an object is detected. This will be a simple path finding algorithm that will tell the robot to turn when an object is within 50cm of the sensor. The code below is the comlete code listing where the code under the while loop is the path finding algorithm.

Full Code Listing for a Simple Path Finding Robot

from microbit import *  # generic microbit functionality
import neopixel  # used to illuminate the LEDs
import radio  # used to communicate with other microbits
from utime import ticks_us, sleep_us, ticks_ms  # used to calc distance

robotType = ""
SONAR = pin15

minDist = 50  # Distance in CM when the robot will turn

def detectModel():  # Detects which model were using XL or classic
    global robotType
    try:
        value = i2c.read(28, 1, repeat=False)  # Read i2c bus
        robotType = "XL"  # If we can read it then it must be XL
        display.show("X")
    except:
        robotType = "classic"  # If we can't read it it must be classic
        display.show("C")      # or Micro:bit is unplugged
    sleep(1000)  # Do this so the user can see if the correct model is found

def sonar():  # This function will get distance using the sonar module

    global SONAR

    start_time = 0
    end_time = 0

    SONAR.write_digital(1) # Send 10us Ping pulse
    sleep_us(10)
    SONAR.write_digital(0)
    SONAR.set_pull(SONAR.NO_PULL)
    while SONAR.read_digital() == 0: # ensure Ping pulse has cleared
        pass
    start = ticks_us() # define starting time
    while SONAR.read_digital() == 1: # wait for Echo pulse to return
        pass
    end = ticks_us() # define ending time
    echo = end-start
    distance = int(0.01715 * echo) # Calculate cm distance
    return distance

def Drive(_leftSpeed, _rightSpeed, _leftDirection, _rightDirection):
    # speed values between 1 - 1023
    # smaller values == faster speed moving backwards
    # Smaller values == lower speeds when moving forwards
    # direction 0 == forwards, 1 == backwards
    leftSpeed.write_analog(_leftSpeed)  # Set the speed of left motor
    rightSpeed.write_analog(_rightSpeed)  # Set the speed of right motor
    if (_leftDirection != 2):
        leftDirection.write_digital(_leftDirection)  # left motor
        rightDirection.write_digital(_rightDirection)  # right motor
    
detectModel()

# Motor pins; these tell the motor to go
# forward, backwards or turn
if robotType == "classic":
    leftSpeed = pin0
    leftDirection = pin8
    rightSpeed = pin1
    rightDirection = pin12

else:  # Bit:Bot XL
    leftSpeed = pin16
    leftDirection = pin8
    rightSpeed = pin14
    rightDirection = pin12
    
while True:
    # path finding alogirthm
    distance = sonar()  # get a distance measurement
    if distance < minDist:
        sleep(200)
        Drive(1, 1023, 1,1)
    else:
        Drive(1023,1023,0,0)

Code Files

pathFinderAUTO.zip