Transcript Document

Building FLL Robots
April 22, 2005
Nathan Gray
FRC Team #1519
Mechanical Mayhem
www.mechanicalmayhem.org
Objective of this talk
• To teach sound building fundamentals for
FIRST LEGO League robots
• Some discussion may not be applicable
to general purpose robots
– Assuming the usual FLL parts restrictions
– Assumng a vinyl mat on a 4x8’ table with
2x4” walls will be used again next year
– And so on…
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Contents
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Robot Components Overview
Robot Design Options
Common Robot Issues
Robot Design Goals
Dynamic Environment
Some Robust Techniques
Examples
Resources
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Components Overview
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Technic beams and plates
Pins and axles
RCX Microcontroller
Motors
Sensors
Gears, Pulleys, Wheels
Special pieces
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Technic beams and plates
• STandard Unit of
Dimension is a STUD
• Six studs = five
beams, so beams are
6/5 (or 1.2) studs high
• Three plates = one
beam, so plates are
0.4 studs high
• Hole spacing is the
same as stud spacing
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Pins and axles
• Many various kinds
• Pin, friction pin, and long
variants
• Evil, super friction pin that
looks very similar to the
normal friction pin
• Axles, come in various
numbers of studs
• Never bend axles! Axles
holding wheels or gears
should be closely supported
on both sides
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RCX Microcontroller
• 3 outputs for motors or lamps
• 3 inputs for sensors
• RCX v1.0 has a power adapter input
(which isn’t generally used for FLL)
• Any of RCX versions 1.0, 1.5, 2.0 are
fine for FLL – Use the latest RIS or
RoboLab software on all of them
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Motors
• There are several different kinds, but
• FLL allows three 9 volt geared grey motors
• Caution! Two different kinds of grey geared 9
volt motors look very similar
– The newer version is much lighter, but slightly
slower and less powerful
• For great info on LEGO motors, see
– http://www.philohome.com/motors/motorcomp.htm
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Sensors
• FLL allows
– Two light sensors that measure 0-100% light –
typical FLL table measurements are approximately
30-60
– Two touch sensors which can be used as bumper
sensors or limit switches
– One rotation sensor
• Measurement granularity is 1/16 of a rotation
• Can give bad data if very fast or very slow
• Rotational speed near motor speed is fine (200-400 rpm)
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Sensors (continued)
• Use all the permitted sensors!
• Can stack touch sensors on top of light
sensor inputs
– A closed touch switch reads 100% brightness
– Cannot read 100% otherwise, unless pointed at
light source
• Good sensor information at:
– http://www.plazaearth.com/usr/gasperi/lego.htm
• Note: homebrew sensors are not FLL legal
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Gears
• Transfer rotation from one axle to another
• Even number of gears reverses the direction of
rotation
• The radii determine gear spacing, transferred speed,
and power
• Inverse relationship between power and speed
• There are lots of gear spacing issues beyond the
scope of this talk
– Special half-stud beams or diagonal spacing sometimes help
– An eight tooth gear has a diameter equal to one stud
– 8, 24, and 40 tooth gears work well together because their
radii are all multiples of 0.5
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Gears (continued)
• Worm gears
– Are effectively one tooth gears
– Significant efficiency lost to friction
– Since they can’t be back driven, they are
great for arms that should hold their
position
• Some good gear info at
– http://www.owlnet.rice.edu/%7Eelec201/Bo
ok/legos.html
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Pulleys
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Work like toothless gears
All the same radius principles apply
Spacing is more flexible than gears
Can be useful when want to allow slip
Higher frictional load than gears or
chain, and belts can stretch and break
• Try to use gears instead
– Use a clutch gear if necessary
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Wheels
• Like pulleys and gears, the wheel dimension
is key!
• Think of the wheel as the final gear in the
drive train
– Larger wheels will make the robot move faster,
with less power
• With stability, traction, turning agility, and so
on, there are lots of trade-offs in choosing
wheels
• See the LEGO tire traction tests at:
– http://www.philohome.com/traction/traction.htm
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Special pieces
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1x1 beam, and double-hole 1x2
beam, can be used to get half
stud spacing
Clutch gears: protects motors
and LEGO from self-destruction
U-joints: can be used when a
straight axle just won’t do
– Should always be used in pairs
– Original and final shaft should
be parallel
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Worm gearbox – the perfect
thing to raise and lower an arm
Chain – achieves some of the
flexibility of pulleys, but more
precise, efficient, and reliable
than belts
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Robot Design Options
• Drive systems
• Modular vs. Monolithic Robots
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Robot Drive Systems
• Dual motor / wheel differential drive is a good (and
common) choice for FLL
– Important to have well-matched motors!
• Treaded skid steer / tank drive can sometimes be
useful for ATV missions
– Very stable, but usually slow and sometimes hard to
navigate accurately
• Active steering (e.g., like a car) has some attractive
features, but:
– Might require a dedicated motor
– Cannot spin in place
• Legs?
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Modular versus Monolithic?
• A “modular” robot is a core robot with
attachments that snap on at specific
interface points – e.g. might have a
different attachment for each FLL
mission
• A “monolithic” (or self-contained) robot
has no (or very few) attachments that
go on or come off the robot.
• Either approach can be very successful!
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“Monolithic” Considerations
• Purpose built for each year’s missions
• Fewer parts to bring to the competition table
• Fewer attachments create fewer opportunities for
operator errors
• Fewer attachments mean less time spent in base,
and more table time available for the robot to actually
accomplish the missions
• Might be easier / better to solve particular missions
with dedicated robot features rather than an
attachment that uses a standardized interface
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“Modular” Considerations
• Don’t necessarily need to redesign everything
every year
• Can accumulate libraries of reusable code for
the core robot base
• Easy to prototype new ideas
• Attachments can be used interchangeably on
duplicate robot bases
• Student sub-groups can develop separate
attachments in parallel without interfering with
other mission solutions
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Common Robot “Issues”
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Robot could fall apart at a bad time
It may not drive straight
Robot might get “lost” on the table
Maybe it is inconsistent and does
something slightly different every time
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Robot design goals
• Simple: easy to replicate and less to go wrong!
– Ask: Is there an easier solution?
• Robust: don’t want robots falling apart on the table!
• Compact
– Small enough to turn in tight spaces
– Keep the center of gravity between the wheels
– Wire routing – tuck wires in so they don’t get pulled loose
• Predictable and reliable
– Behavior should be consistent and repeatable
• Aesthetics: it’s nice to have a good looking robot!
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Some Robust Techniques
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Shielding light sensors
Solid construction
Using good batteries
Going straight (enough)
Reliable Navigation
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Dynamic Environment
• Even with good design, construction and
programming, there can still be problems
• FLL robots and programs are generally
designed assuming a well known (static)
environment without any interference
• Unfortunately, things do change: sunlight,
spotlights, camera flashes, dust on the tables
and wheels, the battery power level, etc
• There are ways to mitigate some of these
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Shielding Light Sensors
• You cannot control ambient light, but
• You can control what the sensor sees
• Build a light box, or other light barrier,
around the light sensors
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Solid Construction
• Use cross-bracing and vertical ties
• Connect enough studs – use significant
overlap
• Use plates to lock the alignment of
beams
• Mount motors and sensors securely
• Tie down sensor and motor wires
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Using Good Batteries
• Important! Battery levels can affect
robot behavior in many ways!
• Experiment with both strong and weak
batteries
• Know you battery’s discharge behavior
• We like Sanyo NiMH rechargeables
• Alkaline have more initial power, but
consistency is usually more important
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Going Straight
• Use matched motors (especially if
differential drive)
• Matched frictional drag – minimize drag
on both sides
• Uniform weight distribution
• Front guide wheels that roll straight (but
that will slip sideways when necessary)
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Three levels of Navigation
• Dead reckoning
– e.g. aim and shoot for time
• Odometry / counting rotations
– e.g. aim and shoot for wheel spins
• Feedback orientation
– Use walls, mat, field elements so the robot
“knows” where it is on the table
– Self correcting, no jigs or precise starts
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Feedback Orientation Sites
• Can frequently run a wheel along a wall
• Turns parallel to a wall can sometimes align
on the perpendicular wall
• Look for reliable “attack points”:
– Intersections of linear table features, e.g. wall
corners, line intersections, other mat features
– Field element corners can also be used for
alignment
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Sample MAYHEM Robot Core
• Similar to one used for “City Sights”
• Missing sensors, weights, and
standardized power take off interface,
as compared to current robot core base
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Sample MAYHEM attachments
• Rover retriever
• Crater transmission
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Where to get LEGO?
• Ebay
• Look up set parts on:
– www.peeron.com
• PITSCO LEGO Dacta:
– www.legoeducationstore.com
• Bricklink: www.bricklink.com
• The LEGO Group store: www.lego.com
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Other Resources
• Excellent building guide from Minnesota INSciTE,
hightechkids.org -- search for “Building LEGO Robots
For FIRST LEGO League by Dean Hystad”
• Tons of info at LEGO Mindstorms Internals:
– www.crynwr.com/lego-robotics/
• The Ferrari’s book: Building Robots with LEGO
Mindstorms
• Comprehensive FLL Coach’s Handbook at
– http://www.fll-freak.com
• Mike’s LEGO Cad:
– http://www.lm-software.com/mlcad
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Summary
• Probably want to use dual motor /
differential drive
• Probably want to use gears (especially
8, 24, and/or 40 tooth gears)
• Use vertical ties and cross bracing
• Have a reliable plan for navigation
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