St Albans Robots

Wall sensors

1 How they work

The robot's position in the maze is read by reading 3 wall sensors
It needs to detect the presence of left, right and front walls.
It needs to steer straight down a corridor, and to detect gaps in the walls as they appear.
It can detect the end of a left turn, by seeing the re-appearance of the left wall.

Set up the inputs

from machine import Pin, ADC
    
Lsensor = ADC(27)  # Left wall sensor
Msensor = ADC(26)  # Front
Rsensor = ADC(28)  # Right

tst = Pin(14, Pin.OUT) #Controls illuminating LEDs

Read the three sensors with no illumination

tst.value(0)       # switch off LED illumination
time.sleep(0.001)  # wait 1 millisecond
LMin = Lsensor.read_u16()  # read the dark values (Left)
MMin = Msensor.read_u16()  # Front
RMin = Rsensor.read_u16()  # Right

Read the sensors with illumination

tst.value(1)       # switch on LED illumination
time.sleep(0.001)  # wait 1 millisecond
LMax = Lsensor.read_u16()  # read the light values (Left)
MMax = Msensor.read_u16()  # Front
RMax = Rsensor.read_u16()  # Right

Calculate the brightness increases

Left  = LMax-LMin
Front = MMax-MMin
Right = RMax-RMin

Take an average
The code below uses the new reading to pull the average towards it
The factor controls how much it pulls the average. 1 = full 0= not at all.
The average has to be initialised at the start of the run:

Factor = 0.5
Laverage = 0
Faverage = 0
Raverage = 0

It is then updated with each reading of the sensors

Factor = 0.5
Laverage = Laverage(1-*Factor) + Left *Factor
Faverage = Faverage(1-*Factor) + Front*Factor
Raverage = Raverage(1-*Factor) + Right*Factor

Saving the calibration

Writing data
New file to write data

f = open("demofile.txt", "r")print("Saving")
f = open("calib.py", "w")
f.write("leftThresh  =" + str(leftThresh)+ "\n")
f.write("frontThresh  =" + str(frontThresh)+ "\n")
f.write("rightThresh  =" + str(rightThresh)+ "\n")
f.close()
print("Saved")

Read existing file:

import calib as Ca
return( (Ca.leftThresh,Ca.frontThresh,Ca.rightThresh) )

Writing data to a file Writing string (no end of line)

f.write("Start of data")

Writing string with end of line:

f.write("Start of data\r\n")

Writing integer value with end of line:

f.write("dfactor="+str(value)+"\r\n" )

4. Reading data

4.1 Read Only Parts of the File
You can also specify how much you want to read.
Example - the 5 first characters of the file:

f = open("demofile.txt", "r")
print(f.read(5))

Read one line of the file:

f = open("demofile.txt", "r")
print(f.readline())

Loop through the file line by line:

f = open("demofile.txt", "r")
for x in f:
  print(x)

5 Closing the file

Close file

f.close()

Wall sensors

To delete a file, you must import the OS module, and run its os.remove() function: Example - Remove the file "demofile.txt":

import os
os.remove("demofile.txt")

To avoid getting an error, you might want to check if the file exists before you try to delete it:

import os
if os.path.exists("demofile.txt"):
  os.remove("demofile.txt")
else:
  print("The file does not exist")

Conclusion

Files can be used to store data permanently, or to record the results of a run.
They are however a bit complicated to code.
They take a long time to read and write, and should not be used inside a tight control loop.