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Mechanical Engineering Project May 6, 2011
Citation preview
Design Advisor– Sinan Müftü
April 19th, 2011
F.I.R.S.T. ROBOTIC DRIVE BASE
Henry Sick, Rich Phelan, Jose Orozco, Shane LentiniNortheastern University Capstone
Goal Statement:To design and build a universal robotic drive base intended to perform in most standard FIRST competitions.
Why Is This Necessary?• In the past NUtrons has started each year designing a
completely new robot (including drive base)• Historical research has shown that many elements of
the field and regulations are exactly the same from year to year, with few exceptions• Designing a new drive base wastes time and puts the
team at a competitive disadvantage.• With mentors cycling in and out every five years,
knowledge and experience is lost
Problem Statement
• F.I.R.S.T is an organization that hold robotic competitions throughout the country for teams of high school students.
• Northeastern has a team, mentored by college students, called the NUtrons.
• Although there have been other capstone projects with similar problem statements, this is NOT a 2nd stage project.
A Brief History
Design Criteria
Reduced Weight (<30 lbs)
Competitive Speed
(14 fps)
High Pushing Force
(190 lbs)Cost
ServiceabilityManufacturabil
ity
Project Outline
Preliminary
Research• Floor
Material• Field
Elements• Design
Constraints and Rules
• Past Successful Robots
Frame Design
• Frame Material
• Finite Element Analysis
Transmission
• COTS vs Custom
• Ratio Design
• Chain and Sprockets
Wheel Selection
• Traction Test
Drive Method
• Wheel Geometry
• Steering
Drive Method
• 8 Wheel ‘Tank’ Drive• Simple • Lightweight• Easily Maintained• Few points of failure
• 8 Wheel Drive – dropped center wheels• Very stable, prevents rocking of base• Most pressure is on center wheels allowing quick turns• Some turning resistance adds to control – damping effect
• Drawback 8WD• 8 wheels = more weight vs 6WD or 4WD options
Position Weight Distribution per Wheel [lb]
15.5 11.25 11.25
5.167 26.25 26.25
5.167 26.25 26.25
15.5 11.25 11.25
Position 12.5 12.5
Lateral resist torque [ft lb]
rotation torque applicable [ft lb] Ratio
36.168 54.688 1.512
Load Distribution
middle 4 end 4
70.00% 30.00%
*Assumes lateral and medial friction to be equal - ratio is independent – friction values as graphed with µ=1.4
Wheel Geometry & Loading
Wheel geometry to be determined; initially shown here based on footprint maximization
Frame Design
Frame Material: Aluminum 6061 –T6• Lightweight• Low Cost• Easily attainable • Great workability
Final Design• 2”x 1” rectangular tubing• Triangular Cuts for weight
reduction• 1/8” gusset Plate joining side
rails and spanner beams
Overall Dimensions
Beam Cross-Section
Total Frame Weight (lbs)
Side beam
(lbs/ft.)
Spanner Beam
(lbs/ft.)27" X 37" 2"X1" Tube Frame 6.92 0.77 0.5327" X 37" 1"X1" 5.24 0.51 0.51
28" x 37"2" X .0125" Plate
Frame
5.05 (x.1 lbs per additional
standoff) Approx. 6 more on each side so 6.25 lbs
0.45 0.53
23.5" X 33.125"2"X 1" Tube with
Triangular Cutouts Frame
5.17 0.63 0.64
26.5" X 34" 2"X3" Box Frame 7.09 0.94 0.64
Decision Matrix For Frame Type:
• Three different situations analyzed • Straight on front collision (blue
arrows)• Opposite frame rail compression
(shear) (red arrows)• Direct corner collision (combined
loading) (green arrow)
• Forces based on data collected from actual robot collision
• 381 pounds applied per each simulation
• Maximum stress analysis• Nodal solution contour plots• Von Misses stresses
• Minimum Factor of Safety• 2.09
Finite Element Analysis
FEA Results
Max Stress:
17.26 KSI
Max Stress:
11.47 KSI
Max Stress:
15.95 KSI
Side Loading
Front Loading
Corner Loading
Transmission: COTS vs Custom
Gear Box SpeedsWeight
[lb]Volume
[in³]Cost
ESTIMATESCons
AM Gen 1 2 3.5 109.14 $350.00 Large, Heavy, Expensive
AM Gen 2 2 3.5 104.27 $350.00 Large, Heavy, Expensive
AM Super Shifter 2 4 62.5 $360.00 Large, Heavy, Expensive
Dewalt XRP 3 1.5 8.86 $70.00Risky Mod. Only 1 Motor. Some
Labor
Custom Ball Shift 2 2 18.375 $150.00 Labor (Time)
Custom Planetary 2 2 14.13 $350 Labor (Time), Ring Gear Cost
•Why minimizing transmission weight is important:• 30 lb drive base goal• 2.82 lb/CIM = 11.28 lb - 37.6% of total goal (fixed)• 4 lbs / Transmission = 8 lb - 26% of total goal – 42.7% of goal
after motors
Ball Shift is preferred transmission based on above parameters, as well as simplicity and shift on fly performance.
Calculation layout used for determining optimal ratios
•Reduction based around 2.77:1 difference between ratios•Requires 3:1 sprocket reduction (14:42) Transmission-Wheels•Overall reductions, Velocity, Pushing Force (6” wheel)• High- 8:1 14.14 fps 66.67 lbf• Low- 22.2:1 5.09 fps 185.19 lbf
•Custom ball shift vs AM Super Shifter (COTS standard)• 40 in3 vs 62.5 in3 : 2.25 lb vs 4lb : $170 vs $360
Transmission: Ratio Design
Ball Shift Mechanics
Shift on the fly demonstration
• Gear stress & allowable stress were calculated for each gear in both High and Low speeds
• AGMA method of calculating stresses:
• Minimum Safety Factor n=1.59 assuming worst case conditions
Gear Stress Calculations
Factors• Traction• Weight• Cost
Wheel Traction Tests• Load wheels with
appropriate weight• Measure torque
required to slip
Wheel Selection
Traction Tests Results
Albion 6"x2" AndyMark 6" Wedgetop
AndyMark 6" Roughtop
Skyway 6"x1.25"
Shepherd 6"x1.25"
AndyMark 4" Roughtop
AndyMark 4" Wedgetop
Colson 4"0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Final Wheel Traction Test Results(averages of 5 tests)
Torq
ue (ft
*lb)
Our Solution
The figure above illustrates the improvements in both top speed and pushing force compared with previous NUtrons’ robots.
It is clear that the addition of a second gear ratio is very advantageous. • Higher top speed allowable• Higher pushing force
Year FPS Pushing Force Capstone Improvement
2010 12 75.5 17.83% 145.27%
2009 9.5 85.8 48.84% 115.77%
2008 11.97 75.6 18.13% 144.87%
2007 10 83.8 41.40% 121.09%
Capstone 14.14 185.19
•Cost to manufacture:$1300•Total Weight: 38lb•Shift on the fly capabilities•106 individual fabricated components•All parts machined in house by capstone team in 2.5 weeks
QUESTIONS?
Special Thanks To:Brandon HolleyMike ConryBen Van SelousNUtrons Robotics Team
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