Transcript Lego Design - SIUE Robotics

LEGO Design
SIUE
School of Engineering
Fall, 2005
Goals:
• Build better robots
– Minimize mechanical breakdowns
– Build robots that are easy to control
– Encourage good design strategy
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
Structure
• Common pitfall when trying to increase
mechanical robustness:
Structure
• The right way:
Structure
• The right way:
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. We recommend using
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.
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
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.
The structure below demonstrates a very strong design
that will not come apart unless you take it apart
.
Connector pegs
• Black pegs are tight-fitting for locking
bricks together.
• Grey pegs turn smoothly in bricks for
making a pivot
Connector Pegs
Drivetrain
• LEGO Gears
40T
8T
16T
24T
1T Worm
24T
Crown
Bevel
Seesaw Physics
Radius, Torque, and Force on a Gear
torque = r x F
3 to 1 reduction
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.
9 to 1 reduction
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. It’s not that easy!
A three stage gear train with a gear ratio of 27:1
Lego Drive
Trains
Lego Axle
Sample Drive Train
Gear Rack
Worm Gears
• Pull one tooth per revolution
3
1
2
• Result is a 24:1 gearbox
4
Axle Joiner
Toggle Joint
Caster Design
Lego Legs
Grippers
Car Turn
Problem
Lego Differential Gear
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.
Motors
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9V Gear Motor
~ 150 mA
300 RPM (no load)
Polarity
Motors
• 9V Micro Motor
• 20-30 RPM
Mounting Motors
Note Bulge under motor
Mounting Motors
• Add a gear:
Mounting the Motor
Lego Sensors
Light Sensor Mount
This shows an interesting way to mount a photoresistor, as well
as how to sheild it from a dedicated light source.
Touch Sensor Mount
Changing Rotational Axis
Changing Rotational Axis
Spin x-y-z
See more examples at http://constructopedia.media.mit.edu/
Lego RCX Brick
RCX Brick with
sensors & Motors
Lego RCX Brick Display
Build for good control
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Slow vs. fast?
Gear backlash
Stability
Skidding (Tank-tracks vs. wheels)
Differential Steering !!!
Design Strategy
• Incremental
– Test components parts as you build them
• Drivetrain
• Sensors, sensor mounting
• Structure
• Don’t be afraid to redesign
• Internet for design ideas
Design Strategy
• Drive-train driven
• Chassis/structure driven
• Modular?
Testing
• Don’t wait until you have a final robot to
test
– Interaction of systems
– Work division (work concurrently)
• Develop test methods
• Repeatability
Competition Philosophy
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Have fun
Be creative, unique
Strive for cool solutions, that work!
Aesthetics: it’s fun to make beautiful
robots!