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First fare 2011 drive trains
1. FIRST Robotics
Drive Trains
Dale Yocum
Robotics Program Director
Catlin Gabel School
Team 1540, The Flaming Chickens
2. 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
Always test on playing surface
But not this way!
3. Traction Basics
Terminology
maximum Coefficient Normal Force
torque
turning the tractive = of friction x (Weight)
wheel force
weight
tractive normal
force force
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.
Source: Paul Copioli, Ford Motor Company, #217
4. Traction Fundamentals
“Normal Force”
weight
front
normal normal
force force
(rear) (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.
Source: Paul Copioli, Ford Motor Company, #217
5. Traction Fundamentals
“Weight Distribution”
more weight in back
due to battery and less weight in front
motors due to fewer parts
EXAM in this area
PL
ONLY E
front
more less
normal normal
force force
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.
Source: Paul Copioli, Ford Motor Company, #217
6. Weight Distribution is Not
Constant
arm position in
rear makes the weight
shift to the rear arm position in front
makes the weight
shift to the front
EXAM
PL
ONLY E
front
normal normal
force force (front)
(rear)
Source: Paul Copioli, Ford Motor Company, #217
12. 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.
13. 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
14. 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
15. 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”
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
18. 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
21. 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
22. Swerve/Crab
Ø Wheels steer independently or as a set
Ø More traction than Mecanum
Ø Mechanically Complex!
Ø Adds weight
33. 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!
42. 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
43. 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
44. WormBox
16:1
Accepts CIM motor
$119.00
1.16 lbs
46. 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
50. 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
51. 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.
52. 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.
53. Team 1114
Kitbot on Steroids
http://www.simbotics.org/media/videos/presentations
55. 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
