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How to Build with the Mindstorm Kit
There are many resources available Constructopedias Example Robots
YouTube Etc.
The best way to learn, is to do Remember rule #1: don't be afraid to fail New Rule: don't be afraid to back up and try
something else
Note: some images and content adapted from High Teck Kids, Building LEGO Robots for FIRST LEGO League, Version 1.1
What to Expect
With this presentation we are going to highlight some techniques and theory
Handson!!
You have already built some robots and you've probably played with LEGOs before.
Certainly, you get the basic idea I want to introduce to you some new techniques and
hopefully talk about what all those weird pieces in this kit are actually good for.
Index
LEGO Construction Basics Gears and Pulleys Wheels Robot Drives
1. Lego Construction Basics
1. Bricks
2. Plates
3. Beams
4. Pins and Connectors
5. HalfPins and Axles
6. Frames
1. Cross Bracing
2. Compression/Sheer/Tension
3. Super Frame
Let's Begin at the Beginning
The Brick Dimensions are given in terms of ”studs”
Length x Width x Height Example 2 x 4 x 1
Plates Plates are 1/3 the height of a Brick
Some plates (called technic plates) have holes which allow pins and axles to pass through
Beams Beams are 1x bricks with holes in the side
The holes are placed at 1 stud intervals, between the studs on top
Connector pins allow the beams to be connected together side by side (instead of just one on top of the other, like with regular bricks)
Beams are CRUCIAL for sturdy robot construction.
Pins
The black Technic pin with friction (friction pin) is the most commonly used
The gray Technic pin has a slightly looser fit
Half Pins and Axles The gray pin (really short) can
be used on the side of a beamto mimic studs
Axles come in different lengths measured in studs How can you tell how long an axle is?
Look at them next to a beam Use the stud ruler Note: Even stud length axles are black and odd stud
length axles are gray (most of the time)
Basically, all the other pieces are meant for connecting axles, plates, bricks and beams together in various ways (parallel, perpindicular, at a fixed angle, at a variable angle, rotating freely, or not.)
Note: some connectors pictured don't come in our kit and not all of our connectors are pictured here
Frames Your robot needs a strong backbone, just like you do.
Your robot's backbone is it's frame Often, when designing a robot, you might start with
it's frame It's important to consider the ”forces” that are going
to be trying to tear your robot apart
An EVEN BETTER frame Using ”Cross Bracing”
To cross brace you need to use 1 beam and 2 plates!
Forces on Brick Connections
Try This: Stack two beams on top of one another Apply these three forces using your fingers Which force is most likely to take the two beams
apart? So, why do we need crossbracing then?
Super Frame (1 beam, 2 plates, 1 beam)
You do NOT HAVE to use this frame In fact, many people don't start with the frame,
instead they start with the motors (big and bulky). Regardless, make your robot sturdy and resistant to
the three types of forces (compression, sheer, tension)
2. Gears and Pulleys
1. Gear Basics
2. Spur Gears
3. Gear Ratios, Torque and Gear Trains
4. Crown Gears
5. Bevel Gears
6. Worm Gears
7. LEGO Gear Rack and Pinion
8. Pulleys
1. Gear Basics: Why Use Gears? To transmit torque (rotation) from one axle to
another. To increase or decrease the speed of rotation To reverse the direction of rotation To move rotational motion to a different axis (xaxis
to the yaxis) To change rotary motion to linear motion To keep the rotation of two axles synchronized
(together)
2. Spur Gears Most Common Gears Gears are referenced by their
size (the number of teeth they have) All LEGO Spur Gears have the same size teeth so
they can all mesh properly (no matter the size of the gear, or the number of teeth)
Spur Gear Spacing
Halfstud Spacing
Vertical Gear Spacing
3. Gear Ratio If you have two different sized gears, the ”gear ratio”
is how far the first gear turns vs how far the second gear turns.
This is exactly how your 18 speed (or any speed) bicycle works.
If the first (little) gear turns 1 time then 8 teeth go by
Also, 8 teeth go by for the large gear; BUT the large gear has 24 teeth total so it will only turn 1/3 of a revolution
In other words the first gear must turn 3 TIMES before the second gear turns all the way around
Thus, we say this is a 3:1 gear ratio andthis is called ”gearingdown”
If you switched the gear around you'd get a 1:3 ratio – ”gearing up”
Torque A force that tends to rotate things. For example, you generate a torque any time you
apply a force using a wrench. When you use a wrench, you apply a force to the handle. This force creates a torque on the nut, which tends to turn the nut.
A force applied to the teeth of a large gear will generate more torque than the same force applied to the teeth of a small gear. This also means that for a given torque, a larger gear will transmit less force than a smaller gear
Formula: Torque = Force * Distance
Rule: Gear down to get more torque BUT less speed
Rule: Gear up to get more speed BUT less torque
Choose One:
Strong and Slow
Or
Fast and Weak
Gear Trains: Multiplying Torque
4. Crown Gears The crown gear has teeth that are raised on one side
and roundedoff on the other to give it a crownlike appearance.
Used when the shafts to be turned meet at an angle. It can be meshed to spur gears and worm gears, but it
doesn't mesh well with other crown gears.
5. Bevel Gears The bevel gear has teeth that slope along one surface
of the disc. It is also used when the shafts to be turned meet at an
angle. It has less friction than the crown gear, but can only
mesh with another bevel gear.
LEGO Differential What is a ”differential”?
YouTube Video A differential is a device takes a torque applied to its
housing, and evenly distributes it to two output shafts,
allowing each output to spin at a different speed.
6. Worm Gears A worm gear is a screw which usually turns along a
spur gear. Works with gears on axles that are at right angles. Can create very high gear ratio as each time the shaft
spins one revolution, the spur gear moves one tooth forward.
Worm Gears are SelfLocking You can turn the input shaft to drive the output shaft,
but you cannot turn the output shaft to drive the input shaft.
This is very useful for arms as no torque is required to keep it in place.
7. LEGO Gear Rack and Pinion The gear rack looks like a spur gear laid out flat. It is
usually used in conjunction with a spur gear (which is referred to as the pinion).
Converts rotation into linear motion (left or right). YouTube Video
Note: this is also how the steering in your car works.
8. Pulleys The theory behind pulleys is exactly the same as for
gears, but pulleys don't have to directly touch Instead, they are connected via ”belts” (usually
different sized rubber bands for LEGO) LEGO belts are color coded; small (white), medium
(blue) and large (yellow) It's still possible to ”gear up” and ”gear down”
3. Wheels
Wheels affect your robot's speed, power, accuracy and ability to handle variations in terrain.
How are Wheels like Gears? Big wheels equals more speed
Big wheels equals less torgue (less ability to climb over obstacles, etc)
Can't get more of one without losing some of the other.
Instead of wheels, you can use tracks (like a tank)
Advantages Good traction on rough surfaces Stable Agile (turns in small space)
Disadvantages Slips on smooth surfaces A lot of power loss due to bending of
tread, etc.
Wheel Base and Robot Balance Wheel Base
Balance In order to be balanced you need to make sure the
center of gravity is inside the wheel base The closer the center of gravity is to the wheel base the
better
Finding the ”Center of Gravity”
Why is balance important? For your robot to move in a predictable and
repeatable manner all wheels must be in contact with the ground at all times, and the weight carried by each wheel must be consistent.
It's nearly impossible to program a robot if you can't trust that it will behave the same way on consecutive attempts
Keep in Mind: Momentum and sloping surfaces shift the center of gravity
When jerking a robot really fast in one direction it takes a while for the COG to catch up (can cause the robot to fall over, even)
Also, the further from center the COG is, the more likely a robot is to tip over on an incline
4. Robot Drives
A.K.A. Drive Mechanisms
1. Differential Drive
2. Steering Drive
1. Differential Drive The Tribot (the first robot we built) IS a Differential
Drive Robot All differential drive robots use two motors to drive
two wheels, and then have a third unpowered swivel caster wheel.
These are very common types of robots Things to Consider:
Where should the Center of Gravity be? How do I get the differential drive robot to go
straight? Do I HAVE to have a caster wheel?
Where should the center of gravity be for differential drive robots?
Rule: the less weight that has to be supported by the third wheel the better
That's why the third wheel is usually WAY in the back
How can differential drive robots go straight? First, going exactly straight is REALLY HARD NO
MATTER WHAT!! Think about how hard it can be to make a shopping
cart go exactly straight! It's better to build and program robots so they don't
have to rely very much on being able to go straight Go ”straight enough” Use sensors, etc. to know where you are
Lucky for us, our motors can be synchronized via software; but the problem is when one wheel slips and the other doesn't
There are two mechanical solutions on the next slide
Purple piece is a connecting piece of flexible hose.
You need to figure out a way to stop lock the differential in place (keep it from turning). If it can't turn, then the wheelsMUST turn at the same time (and thus gostraight.It's SO MUCH easier
to not have to worry about going exactly straight!!
Do I HAVE to have a caster wheel? NO, actually! You can use Skid Steer
Remember the tank! It was differential drive too! It's also possible to have multiple wheels on each side
(usually 3 or 4) that are all conrolled by the same motor (one motor per side)
2. Steering Drive Just like your car One motor drives the two back (usually) wheels
Use a differential for letting you get around curves OR – power only ONE back wheel (not as good)
One motor is used to turn the front wheels Use a rack and pinion set up OR – use one motor for EACH front wheel (not as
good – you ONLY HAVE 3 motors)
Turning Comparison
Differential drive robots are more popular because they don't have to move forward to rotate and steering drive robots do
Think parallel parking!
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