mindstorms mayhem - 2005con building fll robots

7/31/2019 Mindstorms Mayhem - 2005CON Building FLL Robots

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Building FLL RobotsApril 22, 2005

Nathan Gray

FRC Team #1519

Mechanical Mayhemwww.mechanicalmayhem.org


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April 22, 2005 Mechanical Mayhem 2

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

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

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 ofDimension is a STUD

Six studs = fivebeams, so beams are6/5 (or 1.2) studs high

Three plates = onebeam, so plates are

0.4 studs high

Hole spacing is thesame 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 thenormal friction pin

Axles, come in variousnumbers of studs

Never bend axles! Axlesholding wheels or gearsshould be closely supportedon 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 isnt 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 9volt 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 approximately30-60

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 ofrotation

The radii determine gear spacing, transferred speed,and power

Inverse relationship between power and speed

There are lots of gear spacing issues beyond thescope 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 theirradii 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 cant be back driven, they are

great for arms that should hold theirposition

Some good gear info at http://www.owlnet.rice.edu/%7Eelec201/Bo

ok/legos.html


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Pulleys 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 dimensionis 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 choosingwheels

See the LEGO tire traction tests at: http://www.philohome.com/traction/traction.htm


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Special pieces

1x1 beam, and double-hole 1x2beam, can be used to get halfstud spacing

Clutch gears: protects motorsand LEGO from self-destruction

U-joints: can be used when astraight axle just wont do Should always be used in pairs

Original and final shaft shouldbe parallel

Worm gearbox the perfect

thing to raise and lower an arm Chain achieves some of theflexibility of pulleys, but moreprecise, efficient, and reliablethan 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 (andcommon) choice for FLL Important to have well-matched motors!

Treaded skid steer / tank drive can sometimes beuseful for ATV missions Very stable, but usually slow and sometimes hard to

navigate accurately

Active steering (e.g., like a car) has some attractivefeatures, 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 withattachments that snap on at specificinterface points e.g. might have a

different attachment for each FLLmission

A monolithic (or self-contained) robothas 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 years 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 missionswith dedicated robot features rather than an

attachment that uses a standardized interface


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Modular Considerations

Dont necessarily need to redesign everythingevery year

Can accumulate libraries of reusable code for

the core robot base Easy to prototype new ideas

Attachments can be used interchangeably onduplicate robot bases

Student sub-groups can develop separateattachments in parallel without interfering withother mission solutions


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Common Robot Issues

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: dont want robots falling apart on the table!

Compact Small enough to turn in tight spaces

Keep the center of gravity between the wheels

Wire routingtuck wires in so they dont get pulled loose

Predictable and reliable Behavior should be consistent and repeatable

Aesthetics: its nice to have a good looking robot!


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Some Robust Techniques

Shielding light sensors

Solid construction

Using good batteries

Going straight (enough)

Reliable Navigation


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Dynamic Environment

Even with good design, construction andprogramming, there can still be problems

FLL robots and programs are generallydesigned assuming a well known (static)environment without any interference

Unfortunately, things do change: sunlight,

spotlights, camera flashes, dust on the tablesand 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 batterys 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 dragon both sides

Uniform weight distribution

Front guide wheels that roll straight (butthat 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 robotknows 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 foralignment


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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 RobotsFor FIRST LEGO League by Dean Hystad

Tons of info at LEGO Mindstorms Internals: www.crynwr.com/lego-robotics/

The Ferraris book: Building Robots with LEGOMindstorms

Comprehensive FLL Coachs Handbook at http://www.fll-freak.com

Mikes LEGO Cad: http://www.lm-software.com/mlcad


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April 22 2005 Mechanical Mayhem 51

Summary

Probably want to use dual motor /

differential drive

Probably want to use gears (especially8, 24, and/or 40 tooth gears)

Use vertical ties and cross bracing

Have a reliable plan for navigation

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