FIRST Robotics Drive Trains

Dale Yocum Robotics Program Director

Catlin Gabel School Team 1540, The Flaming Chickens

Coefficient of Friction Material of robot wheels

Soft “sticky” materials have higher COF Hard, smooth, shiny materials have lower COF

Shape of robot wheels Want wheel to interlock with surface for high COF

But not this way!

Always test on playing surface

Traction Basics Terminology

The coefficient of friction for any given contact with the floor, multiplied by the normal force, equals the maximum tractive force can be applied at the contact area.

normal force

tractive force

torque turning the

wheel

maximum tractive force

Normal Force (Weight)

Coefficient of friction = x

weight

Source: Paul Copioli, Ford Motor Company, #217

Traction Fundamentals “Normal Force”

weight front

The normal force is the force that the wheels exert on the floor, and is equal and opposite to the force the floor exerts on the wheels. In the simplest case, this is dependent on the weight of the robot. The normal force is divided among the robot features in contact with the ground.

normal force (rear)

normal force (front)

Source: Paul Copioli, Ford Motor Company, #217

Traction Fundamentals “Weight Distribution” more weight in back

due to battery and motors

front

The weight of the robot is not equally distributed among all the contacts with the floor. Weight distribution is dependent on where the parts are in the robot. This affects the normal force at each wheel.

more normal force

less normal force

less weight in front due to fewer parts in this area EXAMPLE ONLY

Source: Paul Copioli, Ford Motor Company, #217

Weight Distribution is Not

Constant arm position in rear makes the weight shift to the rear

front

arm position in front makes the weight shift to the front

EXAMPLE ONLY

normal force (rear)

normal force (front)

Source: Paul Copioli, Ford Motor Company, #217

Skid Steering

2 vs 4 Wheel

Wheelbase vs track

Long robots go straighter Wide robots turn better

Track > Wheelbase

Track

Wheelbase

Track > Wheelbase

Track W

heelbase

Two Wheels – Casters n  Pros:

n  Simple n  Light n  Turns easily n  Cheap

n  Cons: n  Easily pushed n  Driving less predictable n  Limited traction

n  Some weight will always be over non-drive wheels

n  If robot is lifted or tipped even less drive wheel surface makes contact.

4 Standard Wheels n  Pros:

n  Simpler than 6 wheel n  Lighter than 6 wheels n  Cheaper than 6 wheels n  All weight supported by drive

wheels n  Resistant to being pushed

n  Cons n  Turning! (keep wheel base short) n  Can high center during climbs

n  Bigger wheels = higher COG

4 Wheels With Omni Wheels

n  Pros: n  Same as basic four wheel n  Turns like a dream but not

around the robot center n  Cons:

n  Vulnerable to being pushed on the side

n  Traction may not be as high as 4 standard wheels

n  Can still high center = bigger wheels

6 Wheels n  Pros:

n  Great traction under most circumstances

n  Smaller wheels n  Smaller sprockets = weight savings

n  Turns around robot center n  Can’t be easily high centered n  Resistant to being pushed

n  Cons: n  Weight n  More complex chain paths

n  Chain tensioning can be fun n  More expensive

Note: Center wheel often lowered about 3/16”

8 Wheels Pros: •  Allows for small

wheels and low CG •  Climbs like a tank

Cons: •  Complex chain paths •  Heavy, lots of bearings

and chains

8 Wheels

Team 177

Mecanum n  Pros:

n  Highly maneuverable n  Might reduce complexity elsewhere in robot

n  Simple Chain Paths (or no chain) n  Redundancy n  Turns around robot center

n  Cons: n  Lower traction n  Can high center n  Not great for climbing or pushing n  Software complexity n  Drift dependant on weight distribution n  Shifting transmissions impractical n  Autonomous challenging n  More driver practice necessary n  Expensive See one at

http://www.youtube.com/watch?v=xgTJcm9EVnE

Holonomic Drive

2047’s 2007 Robot

Treads

n  Pros: n  Great traction n  Turns around robot center n  Super at climbing n  Resistant to being pushed n  Looks awesome!

n  Cons n  Not as energy efficient n  High mechanical complexity n  Difficult for student-built teams to

make n  Needs a machine shop or buy them

from Outback Manufacturing

n  Turns can tear the tread off and/or stall motors

Swerve/Crab

Ø  Wheels steer independently or as a set Ø  More traction than Mecanum Ø  Mechanically Complex! Ø  Adds weight

Wild Swerve Based on Wildstang Design

Wild Swerve

8.4 lbs per wheel (less motor and chain)

Chain Wrap

Illustration courtesy Team 488

Chain Wrap

Illustration courtesy Team 488

Chains should wrap at least 120 degrees around sprockets

Chain Tension

How Fast? n  Under 4 ft/s – Slow. Great pushing power if

enough traction. n  No need to go slower than the point that the wheels

loose traction, usually around 6 ft/sec with 4 CIMs

n  6-8 ft/s – Medium speed and good power. Typical of a single speed FRC robot

n  9-12 ft/s – Fast. Low pushing force n  Over 13ft/sec –Hard to control, blazingly fast,

no pushing power. n  CIMs draw 60A+ at stall but our breakers trip

at 40A!

Transmissions

AndyMark Toughbox

12.75:1 Ratio Options for 6:1 and 8.5:1 Long shaft option 2.5 lbs (options for -.85lbs) Encoder option One or two CIMs $88

Toughbox Mini

12.75:1 Ratio Options for 6:1 and 8.5:1 Long shaft option 1.95lbs (options for -.56lbs) Encoder option One or two CIMs $90

Toughbox Nano

12.75:1 Ratio Options for 6:1 and 8.5:1 Long shaft option 1.9 lbs (options for -.28lbs) Encoder option One CIM $78

Nanotube

CIMple Box

4:67:1 One or two CIMs 1.4 lbs Came in last year’s kit

BaneBots

Many gear ratios 3:1- 256:1

Long shaft options

$103

2.5 lbs

Avoid dual CIMs

Planetary not quite as efficient

Order Early!

CIMple Transmissions

Converts Fisher Price or similar into a CIM…around 5:1 ratio.

AndyMark Gen 2 Shifter

11:1 & 4:1 Ratios 3.6 lbs One or two CIMs Servo or pneumatic shifting Two chain paths Encoder included $350

AndyMark SuperShifter

24:1 & 9:1 standard ratios + options Made for direct drive of wheels 4 lbs without pneumatics. (-.6 option) One or two CIMs Servo or pneumatic shifting Direct Drive Shaft Includes encoder $360

WormBox

16:1 Accepts CIM motor $119.00 1.16 lbs

Wheels

Wheels are a Compromise (Like everything else)

n  Coefficient of friction n  You can have too much traction!

n  Weight n  Diameter

n  Bigger equals better climbing and grip but also potentially higher center of gravity, weight, and larger sprockets.

n  Forward vs lateral friction

Wheel Types

n  Conveyer belt covered

n  Solid Plastic

n  Pneumatic

n  Mechanum

n  Omniwheels

AndyMark.biz

Skyway

Tips and Good Practices From Team 488

Ø  Three most important parts of a robot are drive train, drive train and drive train.

Ø  Good practices: Ø  Support shafts in two places. No more, no less. Ø  Avoid long cantilevered loads Ø  Avoid press fits and friction belts Ø  Alignment, alignment, alignment! Ø  Reduce or remove friction everywhere you can Ø  Use lock washers, Nylock nuts or Loctite

EVERYWHERE

Tips and Good Practices: Reparability (also from 488)

Ø  You will fail at achieving 100% reliability Ø  Design failure points into drive train and know where

they are Ø  Accessibility is paramount. You can’t fix what you

can’t touch Ø  Bring spare parts; especially for unique items such as

gears, sprockets, transmissions, mounting hardware, etc.

Ø  Aim for maintenance and repair times of <10 min.

Drive Teams Make the Difference

n  A great drive team can make a average robot great.

n  A weak drive team will make a great robot average (or worse).

n  Drive teams need practice, rest, and freedom from other distractions at the competition.

n  Drive team shouldn’t be the emergency repair crew.

Team 1114 Kitbot on Steroids

http://www.simbotics.org/media/videos/presentations

Questions

So Which is “Best”?

2010 Championship Division Winners and Finalists 2 Four Wheel 5 Six Wheel 10 Eight Wheel 2 Nine Wheel (148, 217 partnership) 1 Mecanum 3 Crab Drive 1 Treads

2011 Championship Division Winners and Finalists