The Light Sensor
Tracking a line
Richard McMullin
fllcoach.broadacres@gmail.com
Oakville Robotics Symposium
22-Oct-2011
Introduction – Richard McMullin
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Leader in IT
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Entrepreneur
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4th year coaching FLL
Tournaments: Toronto West, Provincial Championships
FRC Volunteer
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Started several technology companies
Finance, Medicine
FLL Coach
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Engineer, University of Waterloo – Systems Design
Technical Leader at National Bank of Canada
Changing hearts and minds to improve effectiveness of over 300 people
Tournament setup
Field Referee
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Agenda
1.
Attaching the light sensor(s)
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b.
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Testing the light sensor
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3.
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Left right method
Program
Tracking a line
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b.
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Programming blocks that use the light sensor
Stopping on a line – Program
Walking a line
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b.
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Built-in program on the robot
Controlling the light sensor
a.
b.
4.
Placement on the Robot
Attaching to the ports (default port 3)
Line tracking method
Program
Questions
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Attaching the light sensor
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Light Sensor Placement on the Robot
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Attaching to the Ports
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Facing down towards the mat
Better if forward of the wheels (for steering)
Allow a gap for the light to be reflected (not touching the mat)
Place somewhere stable (consistent readings)
Place somewhere protected (avoids contact and external light sources)
Default port is port 3.
Any port can be used, but always check that the program is set for the correct port.
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Testing the light sensor
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Built in testing
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Programs exist on the Robot to test the attached light sensor
But they just make sounds
Here is a program to show the light sensor value
White value = 60
Black value = 34
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Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Controlling the light sensor
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Programming blocks that use the light sensor
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Read the light sensor
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Wait for a light sensor value
Oakville Robotics Symposium – The Light Sensor
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Branch on a light sensor
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Loop until a light sensor value
22-Oct-2011
Program Example – stopping on a line
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Steps in the program
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Go forward forever
Wait until the line is detected
Stop
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Program Example – Walk a line
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Steps in the program
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If the sensor senses white, steer to the right
If the sensor senses black, steer to the left
Bounded unstable program
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Advanced – Track a line
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General idea
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Math
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Assume the following values:
Black: 36, White 60.
Ideal ‘grey’ value is (60+36)/2 = 48
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In order to find out the amount to steer, take the current sensor value and subtract it from the
ideal ‘grey’ value.
Steering value = 48 – sensor value
Max white steering = 48 – 60 = - 12 (left turn)
Max black steering = 48 – 36 = + 12 (right turn)
Grey steering
= 48 – 48 = 0 (go straight)
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Track along the ‘grey’ value between black and white.
Use the difference between the current value and the ideal ‘grey’ value to tell the robot how
hard to turn.
The more ‘white’ the value is, the harder you steer to the left
The more ‘black’ the value is, the harder you steer to the right.
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Advanced – Track a line
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The simple program
Read the
light sensor
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Subtract
from 48
Use the
result to set
the steering
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Program Example – Track a Line
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Use additional math blocks to set the sensitivity of the turning.
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Whole number math only available with multiplication and division
In order to multiply by .75, multiply by 3 then divide the result by 4.
Always do the multiplication first to keep the highest accuracy
Use a second light sensor to stop the walk along the line
Oakville Robotics Symposium – The Light Sensor
22-Oct-2011
Questions
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Contact Info:
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Richard McMullin
fllcoach.broadacres@gmail.com
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Oakville Robotics Symposium – The Light Sensor
22-Oct-2011