Building FLL Robots April 22, 2005

1
Building FLL RobotsApril 22, 2005

• Nathan Gray
• FRC Team 1519
• Mechanical Mayhem
• www.mechanicalmayhem.org

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

3
Contents

• Robot Components Overview
• Robot Design Options
• Common Robot Issues
• Robot Design Goals
• Dynamic Environment
• Some Robust Techniques
• Examples
• Resources

4
Components Overview

• Technic beams and plates
• Pins and axles
• RCX Microcontroller
• Motors
• Sensors
• Gears, Pulleys, Wheels
• Special pieces

5
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

6
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

7
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

8
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

9
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)

10
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

11
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

12
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 their position
• Some good gear info at
• http//www.owlnet.rice.edu/7Eelec201/Book/legos.h
  tml

13
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

14
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

15
Special pieces

• 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
  wont do
• Should always be used in pairs
• Original and final shaft should be parallel
• 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

16
Robot Design Options

• Drive systems
• Modular vs. Monolithic Robots

17
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?

18
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!

19
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
  missions with dedicated robot features rather
  than an attachment that uses a standardized
  interface

20
Modular Considerations

• Dont 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

21
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

22
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 routing tuck 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!

23
Some Robust Techniques

• Shielding light sensors
• Solid construction
• Using good batteries
• Going straight (enough)
• Reliable Navigation

24
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

25
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

26
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

27
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

28
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)

29
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

30
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

31
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

32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
(No Transcript)
39
(No Transcript)
40
(No Transcript)
41
(No Transcript)
42
(No Transcript)
43
(No Transcript)
44
(No Transcript)
45
(No Transcript)
46
(No Transcript)
47
(No Transcript)
48
Sample MAYHEM attachments

• Rover retriever
• Crater transmission

49
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

50
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 Ferraris book Building Robots with LEGO
  Mindstorms
• Comprehensive FLL Coachs Handbook at
• http//www.fll-freak.com
• Mikes LEGO Cad
• http//www.lm-software.com/mlcad

51
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