ORTOP
NXT Robotics Techniques
Workshop 2012
“Opening doors to the worlds of science and
technology for Oregon’s youth”
6/22/12
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Goals
• Audience: Rookie and experienced coaches
and mentors
• Build confidence using more advanced
navigation techniques
• Gain experience with 3 motor robot and
missions
• You should have already attended NXT
workshops I & II and/or coached a team in
2011 tournament
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Instructor Contacts
• Ken Cone
• kencone@gmail.com
• 503-415-1465
• Jim Ryan
• james.r.ryan@intel.com
• 971-215-6087
• Terry Hamm
• Terry.hamm@gmail.com
• (503) 720-5157
• Roger Swanson
• swanson@hevanet.com
• 503-297-1824
• Dale Jordan
• Dale_A_Jordan@msn.com
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Agenda
• Robot Design
– Building strong structures
– Gears
– Motors
• Navigation
– Going straight
– Turning accurately
• Sensors
– Light sensor and calibration
– Distance sensor
• Mission Planning & The Buoy Mission
• Next Steps
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Robot Design
• What engineering problems are we trying to solve?
– Understanding each mission
– Building a robot to move objects around on a playing field
– Other ideas?
• Observe workshop robot noting:
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Complexity
Sensors – location with respect to wheels
Control panel
Beam & pin structural design
What differences exist between this robot design and
others you have seen?
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Engineering Design
Process
Define the
Problem
Research the
Problem
Redesign
Develop
Possible
Solutions
Communicate
Test and
Evaluate
Choose the
Best Solution
Create a
Prototype
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Building Strong
Structures
– Note: parts are all pinned together using Technic pins &
beams
– Here are three cross bracing examples
• Note: 3-4-5 Pythagorean theorem
– Ref: Minnesota High Tech Kids
• http://www.hightechkids.org/for-teams/coacheslibrary
– 2 inch drop test
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Gears
• Gears
– Spur Gear (show example)
• instructor holds beam
• class person turns axles and
feels torque on other axle
• tactile feedback = learning
about gears and torque
– Gear Ratio and Speed
• turn the large gear one complete turn
• How may revolutions does the small gear turn?
• Is gear ratio the same as the ratio of gear teeth?
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Gears
• Gears & Pulleys
– Worm Gears (show example)
• one way transfer of power
– Crown & Bevel Gears
• turn 90 degrees
– Spur Gears
• change direction & change torque
– Note gears used on workshop robot fork lift
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Robot Design
• Three motor base
– Level, square
– Screen location
• Attachments
– Soda can pusher
– Fork
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Robot Design
• Mission Attachments
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–
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Measure all mission items
Grabbers – e.g. to catch food factor truck & trailer
Pushers/pullers – e.g. food factor fish
Caribiners /hooks - locks onto item
Carts – for collecting or delivering mission items
Use of 3rd motor – for a grabber or fork lift
Can we group missions together using one attachment?
• Mission Planning
– Use a chart/drawings to show attachments used on each
mission, more on this topic in the next segment.
– Any questions on robot design?
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ORTOP FLL Workshop 3
Navigation
Sometimes the robot does not go
where you want it to. Has this ever
happened to you?
In this segment, let’s explore basic
robot move and turn accuracy.
Here is an example mission…
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Mission:
Insert Stent into Artery
Moves:
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Start box to ball
Turn left 40 deg
Ball to black line
Turn left 70 deg
Follow black line
for 5 sec
Can this mission
run successfully 5
times??
What factors
influence move
and turn accuracy?
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Navigation
• Let’s do a simple “Going Straight” lab
test to understand how the robot moves:
– Tape down 2 pieces of paper on the table,
2 feet apart
– Draw a starting line on one paper
– Create a simple program to move 2 feet
then stop
– Run speeds at 20, 50, 90
– Use a “soda can pusher” or similar
attachment on the front of the robot
– Run test 5-10 times
– Mark a large dot where front
“can pusher” arm stops
– Begin test now
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Navigation
• Going Straight test:
– Run test 5-10 times, record results
– Draw box around landing points
– What does the box tell us?
• Goal: To understand how the robot
moves – its accuracies and
inaccuracies
• We can then compensate for
inaccuracies with other techniques
– Examples: use a wide fork for the
buoy mission, or a wide soda can
pusher
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Navigation
• Theory - The S curve inaccuracy:
– A Move block rotates both wheels to
the programmed position using built-in
rotation sensors as feedback
– If one wheel slows down, the other
wheel slows down, causing the robot to
move in a lazy S curve
• Did you observe this in your going
straight lab tests?
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Navigation
• Compensation for navigational
errors – “reset to zero”
• Use mechanical means
– Angled corners (back into a corner)
– Wall follower (wheel)
– Back against a wall
• Use sensors
– Light & touch to “reset to zero” and
re-establish the exact robot location
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Navigation
• Variables that affect going straight:
– Starting box position
– Speed
– Battery charge
– Tire size
– Motor friction, gear backlash (Google these terms)
– NXT software tries to keep both wheels moving at
same rate
– What is distinct about the last two variables?
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Navigation
• Turning Accuracy
<<<Move block slider >>>
– Lets review two types of turns
• Spin Turn
– Position Move block slider all
the way to the LEFT or RIGHT
– A spin turn uses 2 wheels, one
forward, one backward - robot
“spins” around center point
between the wheels
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Navigation
• One Wheel Turn
– Use Move block, turn one motor off
– One wheel moves, one is stationary –
robot turns around the stationary
wheel
One wheel
turned off
• Which turn is more accurate?
• Where could we use the spin turn?
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Navigation
• Extra Credit
– Program a move-turn sequence
– Use “spin” turns and “one-wheel” turns
– Place a pencil mark where robot stops
– Run program 5-10 times and collect data
• Variables
– Left-right error
– Turn angle error
– Any more variables?
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Navigation
• Review:
– Use navigation and “reset to zero” to know where
robot is at all times
– Understand variables and how they influence robot
movement. We learned left-right moves are less
accurate than front-back moves.
– Long dead reckoning moves are ok, if
robot/attachments can compensate for move
inaccuracies, e.g.
• Width of can pusher, width of buoy pickup fork
• Backing into wall or corner to re-align robot
• Go for simple & sufficient solution
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Sensors
• Now that we know how to move and turn with some
precision, lets take a look at sensors
• Sensors we can use in FLL:
– touch, light, rotation, distance
• Teams sometimes give up on sensors because they
seem complex and don’t seem to work in a predictable
manner
• But, we all use the built-in rotation sensors in the
motors to tell us the number of rotations/degrees
• Let’s explore the light sensor to help us navigate on the
playing field
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Sensors
• Let’s take a detailed look at the light sensor
(LS)
– Connect LS to NXT brick – use port 3
– Create a program to turn on LS and read values,
use yellow LS icon, add a 5 sec wait to keep the LS
on for a few seconds while you read values
– Read LS values: click LS icon, read value in LS
control panel
– Mount the LS about 3/8 inch from the mat and
observe Black, Green, and White values
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Sensors
• Let’s look at LS-to-mat distance & record some data:
– Mount LS on the robot about 1/8” from the mat
– Record Black, Green, & White values
– Move the LS to 3/8” from the mat, record values
– How does the distance affect LS values?
– As the robot moves, it bounces. Which distance do you
think would work best, and why?
LS # _____
Black
Green
White
Calculated
trigger point
1/8” from mat
3/8” from mat
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Sensors
• Now let’s record some more data:
– Use your hand to cover the LS to simulate a dark room
– Record Black, Green, & White values
– Trigger point is halfway between white & black, we will
come back to this calculation
– What does this tell us about LS performance in various
room lighting conditions?
Black
Green
White
Calculated
trigger point
LS-1
uncovered
LS-1
covered
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Sensors
• Theory: Light sensor returns values of reflected light
– E.g. white = 63
– Trigger point (programmed) =
• (( white – black) / 2) + black
– black = 34
– Trigger point example: ((63-34) / 2) + 34 =
– Take a minute and calculate trigger points on previous
slides
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Sensors
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•
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Now lets use what we learned in a program..
Theory: sensor “wait until” program action (orange hour glass icons)
Program example:
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Runs the Move block,
– Waits on the Light Sensor block, Until sensor trigger point is crossed,
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Continues to the next program step
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•
What does this program do?
What is the purpose of
the third block?
•
Note:
– Don’t forget to select
less than< or greater than>
on the control panel
W63
Trigger point
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B33
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Sensors
• Data from 7 light sensors - uncalibrated, room light on
• What trigger point would we use to stop on black? On green?
80
Uncal room light on
70
Uncal value
60
50
Black
40
Green
30
White
20
10
0
0
1
2
3
4
5
6
7
8
Sensor number
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Sensors
• Calibration: A process to compensate for varying lighting
conditions
120
• Data from two light sensors – uncalibrated, calibrated
100
LS1Cal
LS2Cal
80
BLACK
LS2Uncal
60
GREEN
LS1Uncal
LS1Cal
40
WHITE
LS2Uncal
LS1Uncal
LS2Cal
LS2Uncal
LS1Uncal
20
0
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LS1Cal
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LS2Cal
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Sensors
• Example: Light Sensor Calibration Blocks
– Program Steps:
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Turn on light sensor
Say “white”
Wait 3 seconds to position sensor over white area
Sample and store the “white” value
Say “Black”
Wait 3 seconds to position sensor over black area
Sample and store the “black” value
Wait 1 second and end the program
– Why is this important?
• Calibrating the light sensor provides widest range of values between white and black,
making it easier to use the sensor in a variety of lighting conditions
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Sensors
• Using the internal calibration program
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Connect NXT brick to your computer, and make sure MINDSTORMS is running.
Verify the NXT is properly connected, click “NXT Window” in lower right corner of
screen, click “Scan” and “Connect” and “Close”
On the upper left corner, click “Tools” and “Calibrate Sensors”
To Un-calibrate the NXT:
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Click “Port3”, “Default”, and “Refresh”.
Important: If you calibrate one of the workshop robots, please “un-calibrate” it before
leaving class.
To Calibrate, do the following:
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Select the light sensor and the port the sensor is connected to. Then click the Calibrate button. This will
download a small program to the NXT and run it automatically.
On the NXT’s screen you will see text that reads “Min Value:”. Point the light sensor towards a material or
spot that represents what the light sensor should measure as dark. Press the orange Enter button on the
NXT.
Next you will see text that reads “Max Value:”. Point the light sensor towards a material or spot that
represents the brightest location the sensor will encounter during the program. Press the orange Enter
button again. Calibration is complete.
Caution: After Calibration, read light sensor values from the Light Sensor program block to determine
trigger points. Do not use NXT View function, View gives incorrect values for calibrated light sensors.
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Sensors
• Extra Credit – The Color Light Sensor
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See Color Sensor Block “?” for programming details
Detects color(s) within a defined range
Also works as black & white light sensor
Our tests show it works better for detecting black lines
Color sensor does not require calibration when detecting colors
Review how to set “stop on black”
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Example:
Turn on B motor
“Wait until” color red
Stop B motor
• Use Color Lamp Block – for turning on/off RGB lamps
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Sensors
• Mounting LS on the robot:
– Front center – detect line or follow line to target
– If LS is too close to turning center, then robot wobbles
– Distance to the mat, shoot for about 3/8” to avoid false
triggers with small bumps in mat
– room light influence, LS is buried inside robot or out in
open
• Extra Credit:
– What changes to the LS mounting would you recommend?
– When could we use two sensors?
– Review LS help sections in NXT-G
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Sensors
• Homework assignment:
– Add a “Wait for Distance” block
– Program the robot to stop at 4 inches
– Shape and texture of target object is important
• This completes the four sensors:
1. Rotation
2. Touch
3. Light
4. Distance
• Questions on Sensors?
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ORTOP FLL Workshop 3
The Buoy Mission
In this segment:
• Use a Mission Planning Sheet
• Plan moves, turns, & attachments
• Program the robot
• Win the points!
s
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The Buoy Mission
•
Engineering Problem:
– Pick up and carry the buoy from its current position,
– Place it in the box at the end of the mission board,
– Park the robot on the blue finish line
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•
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Note the box has a fence in front, so to score
points you must lift the buoy over the fence and
into the box. Extra credit points for detecting
the fence using the distance sensor.
First, write a mission plan (moves, turns &
attachments), then write a program to execute
your plan
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Mission Planning Sheet
Mission(s):______________________________________ Maximum Score: _________
Program Name: __________________Memory size: ____ Time to run: _____________
Plan Overview: ___________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
Starting Position:__________________________________________________________
_______________________________________________________________________
_
Sensors Used (Port): __________(___), __________(___), __________(___)
__________(___), __________(___), __________(___)
Navigation Details:________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
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Buoy Mission
End here
s
Place
buoy
here
Start here
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Next Steps
• Register your team with FLL
• Share with your team
– NXT Robotics Techniques slides
– Mindstorms NXT Tutorial (Dale Yocum)
• www.ortop.org/NXT_Tutorial
– Build and test 2-3 robot designs, then have your team select a hybrid of
best features
– Project resource video: Google FLL project dvd, should find:
www.youtube.com/watch?v=LFBBnyz8syM
• Challenge the kids!
– Make up your own exercises – stress basics and repeatability
• Fill out workshops survey, let us know highlights, lowlights,
changes, keeps www.ortop.org/workshopsurvey/
• Good Luck and thanks for being a coach/mentor!
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