LEGO Design

SIUESchool of Engineering

Fall, 2005

Goals:

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

Geometry

• 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

• Three plates = 1 brick in height

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

8T

16T

24T

40T

24TCrown

1T Worm 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

1

2

3

4• Result is a 24:1 gearbox

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

• 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 withsensors & Motors

Lego RCX Brick Display

Build for good control

• 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

• Have fun

• Be creative, unique

• Strive for cool solutions, that work!

• Aesthetics: it’s fun to make beautiful robots!