IME 106 LEGO Design

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IME 106LEGO Design

• SIUE
• School of Engineering
• Fall 2006

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Robot

• Any software controlled mechanical device
• Actuators and Effectors
• Sensors
• Controller

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Industrial Robots
Check out the Mitsubishi Robotic Arms in IME CIM
lab (EB 1022) !
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Mobile Robots Remote control, autonomous, or a
mixture of two
Go To http//roboti.cs.siue.edu/ to control
Elmer, Taz, or Marvin
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Goals

• Build better robots
• Minimize mechanical breakdowns
• Build robots that are easy to control
• Encourage good design strategy

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Who Builds Robots?

• ECE - designs the brain, sensors, actuators
  wiring.
• ME - designs body, gearing, actuators
• IME - designs and integrates controls.
• CS designs robot software
• All disciplines listed above work together to
  design/build robots.

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Robotics made easy?

• Design Problem -
• Design and build a robot to vacuum your house.

What are some of the challenges?
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Design Challenges for Mobile Robots

• Position -
• How does robot know where it is (or has been)?
• Navigation -
• How does it navigate around obstacles?
• Object Recognition -
• How does it recognize money, toys, even cats?

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Design Approaches

• "Top-down" design
• the process of starting with the goal of the
  project and then developing a solution.
• "Bottom-up" design
• the process of first learning about the available
  materials and then determining what can be done
  with them.

Add Project Planning and Testing phases to
robot design process.
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Lego RCX Brick
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RCX Brick withsensors Motors
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Lego RCX Brick Display
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Design Strategy

• Incremental
• Test components parts as you build them
• Drivetrain
• Sensors, sensor mounting
• Structure
• Dont be afraid to redesign
• Internet for design ideas

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Design Strategy

• Drive-train driven
• Chassis/structure driven
• Modular?

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Geometry

• Three plates 1 brick in height

• 1-stud brick dimensions exactly 5/16 x 5/16 x
  3/8 (excluding stud height 1/16),
• This is the base geometry for all LEGO
  components

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Structure

• Common pitfall when trying to increase mechanical
  robustness

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Structure

• The right way

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Structure

• The right way

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A good robot starts with a good foundation. A
robot whose body is not structurally sound will
be fraught with problems for the designers. The
first and most important is that the friction
between stacked bricks should not be relied upon
for structural strength. Use connector pegs to
help create a "skeleton" like the one below. A
design like this is both light and strong but
usually requires a number of rebuilds to get
perfect.
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Structural supports like the ones shown below can
be placed on almost any chassis design. Use this
to your advantage. You can get by with fewer
legos and have a stronger chassis this way
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The picture below demonstrates a very
structurally sound way of constructing a frame
with Legos. The 3 wide connector peg can be used
for one of the 3 join points, or an additional
4x1 brick can be used.
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The structure below demonstrates a very strong
design that will not come apart unless you take
it apart.
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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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Connector pegs

• Black pegs are tight-fitting for locking bricks
  together.
• Grey pegs turn smoothly in bricks for making a
  pivot

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Connector Pegs
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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 Lego design

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

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Worm Gears

• Pull one tooth per revolution

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1
2
Result is a 241 gearbox
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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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Drivetrain

• LEGO Gears

40T
8T
16T
Bevel
1T Worm
24T
24T Crown
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Robot Basics - Gears

• Speed
• Torque (climb over obstacles)
• Turns
• Tips -
• Try different size gear combinations, different
  types of gears (worm), and different motor
  placement (rear wheel drive or 4 wheel drive).

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Seesaw Physics
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Radius, Torque, and Force on a Gear
torque r x F
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(No Transcript)
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3 to 1 reduction
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Since the forces between the teeth of the two
gears are equal in magnitude but act opposite in
directions, the torque exerted on the right axle
is three times the torque exerted on the left
axle (since the radii of thee gears differ by a
factor of three). Thus this gear system as acts
as a torque converter, increasing the torque at
the expense of decreasing the rate at which the
axle turns.
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9 to 1 reduction
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The torque at the output shaft is 9 times
the torque provided on the left(input) axle.
The output shaft will of course spin 9 times
slower than the input shaft, but it will be much
harder to stall. Have someone grab the output
shaft and try to stall your fingers as you spin
the input axle. Its not that easy!
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A three stage gear train with a gear ratio of
271
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Sample Drive Train
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Lego Axle
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Gear Rack
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Axle Joiner
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Toggle Joint
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Caster Design
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Lego Legs
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Grippers
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Changing Rotational Axis
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Changing Rotational Axis
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Spin x-y-z
See more examples at http//constructopedia.medi
a.mit.edu/
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Car Turn Problem
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Lego Differential Gear
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Differential Drive
The differential gear is used to help cars turn
corners. The differential gear (placed midway
between the two wheels) allows one wheel to turn
at a greater speed than the other. Even though
the wheels may be turning at different speeds,
the action of the differential means that the
torque generated by the motor is distributed
equally between the half-axles upon which the
wheels are mounted. Assuming the robot's weight
is sufficient and distributed properly, the robot
should be able to turn with its drive motors at
full power without causing either wheel to slip.
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Motors

• 9V Gear Motor
• 150 mA
• 300 RPM (no load)
• Polarity

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Motors

• 9V Micro Motor
• 20-30 RPM

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Mounting Motors
Note Bulge under motor
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Mounting Motors

• Add a gear

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Mounting the Motor
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Lego Sensors
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Light Sensor Mount
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This shows an interesting way to mount a
photoresistor, as well as how to sheild it from a
dedicated light source.
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Touch Sensor Mount
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Sensor Issues

• Two light sensors that measure 0-100 light
  typical 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

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Build for good control

• Slow vs. fast?
• Gear backlash
• Stability
• Skidding (Tank-tracks vs. wheels)
• Differential Steering !!!

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Design Strategy

• Incremental
• Test components parts as you build them
• Drivetrain
• Sensors, sensor mounting
• Structure
• Dont be afraid to redesign
• Internet for design ideas

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Design Strategy

• Drive-train driven
• Chassis/structure driven
• Modular?

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Testing

• Dont wait until you have a final robot to test
• Interaction of systems
• Work division (work concurrently)
• Develop test methods
• Repeatability

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Competition Philosophy

• Have fun
• Be creative, unique
• Strive for cool solutions, that work!
• Aesthetics its fun to make beautiful robots!

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Be aware of the common problems!

• Wheels stick, slip, or slide depending on surface
• Rotations are not always accurate or consistent
• Different motor strengths
• Touch sensor activation
• 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 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!

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Some Robust Techniques

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

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Control Structures

• No matter what language you use, there are 3
  basic control structures for organizing the
  programming commands
• Selection
• Repetition (Loops)
• Conditional

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RCX Program Code

• Commands
• Sensor Watchers
• Stack Controllers
• My Commands

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How To Write Programs
Click on Program RCX. Stack puzzle
pieces. Move unused pieces to the trash. Download
program to the RCX.
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RCX Programming
Commands Tell robot what to do (e.g. stop, go,
turn, etc.).
Sensor Watchers Test conditions (e.g. light,
touch, count) and determine actions based on
conditions.
Stack Controllers Allows robot to repeat
commands or wait until condition is true.
My Commands Makes several actions a
subroutine which can be used as a single
command.
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Demo Robot
Robot backs up for 1 s. Both motors stop in
preparation of power change. Power increases to
overcome wheel friction when turning. Wheel A
changes to forward,so robot turns to the right
for 1s. Power decreases to protect sensors when
robot bumps objects. Wheel C changes to forward,
so robot moves forward.
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Selection

• Selection statements are defined as a list of
  commands that are executed in order.
• For example
• Set Forward Direction
• Go forward for 3 s
• Stop

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Repetition

• Repetition statements allow for a series of
  commands to be repeated for a set number times.
• For example
• Repeat 3 times
• Set forward direction
• Move forward for 3 s
• Stop
• End Repeat

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Conditional

• Conditional statements allow for two (or more)
  different sets of commands to be executed
  depending on a condition.
• For example,
• If certain conditions are true - one set of
  commands will be execute.
• Else if any (or all) are false - another set of
  commands will be executed.

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Example of Conditional Statements

• For example -
• If the light is
• Set Direction Forward
• Move Forward for 3 s
• Stop
• Else If light is 50
• Stop
• End

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How To Download Programs
Select button to download
Select program number (1-5)
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How To Save Programs
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Features of RCX Software

• Multi-threaded language
• Different parts of the program execute at the
  same time.
• Can cause unexpected results!
• Loops in main program interfere with subprogram.
• Variables limited to 1 or 0
• Use counter as variable.
• Not quite C (NQC) language allows for more
  variables.

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NQC (Not Quite C) Programming
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Useful Links

• http//www.crynwr.com/lego-robotics/
• http//www.plazaearth.com/usr/gasperi/lego.htmbac
  kground
• http//www.oreilly.com/catalog/lmstorms/resources/
  index.html
• http//member.nifty.ne.jp/mindstorms/Gallery
• http//www.robotbooks.com/

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Interesting Lego related websites

• (many links) http//www.oreilly.com/catalog/lmstor
  ms/resources/index.html
• (interesting sites for ideas) http//member.nifty.
  ne.jp/mindstorms/Gallery
• http//www.mi-ra-i.com/JinSato/MindStorms/index-e.
  html
• http//staticip.cx/benw/lego/
• http//www.verinet.com/dlc/botlinks.htm
• http//www.medialab.nl/Company/Crew/daan/legodiff.
  htm
• http//www.mindspring.com/clagett/bill/lego/geome
  try/index.html
• http//www.robotbooks.com/
• (good introduction to gear and beam construction)
  http//ldaps.ivv.nasa.gov/Curriculum/legoengineeri
  ng.html
• http//www.fischermellbin.com/Marcus/Lego/Gear_Mth
  /gear_math.html
• http//phred.org/alex/lego/
• (ideas for sensors)
• http//www.plazaearth.com/usr/gasperi/lego.htmbac
  kground
• http//www.umbra.demon.co.uk/legopages.html
• http//www.primenet.com/johnkit/Projects.html
• http//www.mnsinc.com/wesmat/TouchSensor.html
• http//www.daimi.au.dk/mic/speciale/RCX
• http//www.crynwr.com/lego/lego-robotics/extreme-r
  otation-sensor.htm
• http//www.csepainball.com/chris/radarbot.html