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