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Mechanical Engineering Crash Course

Mechanical Engineering Crash Course. Steve Evans Team 1294. Acknowledgements. Steve Evans - Team 1294 Brian Gattman - Team 2910 Andy Baker - Team 45. Mechanical Engineering. Very broad discipline Design Modular Design, Mechanical Devices, Drive Base, Design Process Drafting / CAD

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Mechanical Engineering Crash Course

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  1. MechanicalEngineering Crash Course Steve Evans Team 1294

  2. Acknowledgements • Steve Evans - Team 1294 • Brian Gattman - Team 2910 • Andy Baker - Team 45

  3. Mechanical Engineering • Very broad discipline • Design • Modular Design, Mechanical Devices, Drive Base, Design Process • Drafting / CAD • Inventor and ProE • Manufacturing methods • Thermodynamics / Heat Transfer • Fluid Mechanics • Pneumatics • Control and Measurement • LabView, Electrical Design • Dynamics and Vibrations • Organization • Project Management, Team Dynamics, Sources of Parts • We have only 2 hours… MechE related workshops available today

  4. Agenda • Example Problem Statement • FRC Knowledge Depth • Mechanical Engineering: FRC Introduction • Forces & Moments • Free Body Diagrams • Stress & Strain • Mechanics of Materials • Work & Power • FRC Motors • Gear Ratios • Problem Solution • Q&A (and throughout!) • Drivetrains • Arms & Lifts

  5. Quick Poll • Who has taken or understands… (not a contest) • Trigonometry • Physics • Calculus

  6. Example Problem Statement • Lift a ball to the top of a goal • Start: ground • End: 8 feet • 5 lbs • 3 ft diameter • Use primitive design • Has many issues • Not the point Area of Interest

  7. FRC Knowledge Depth • 6 weeks: impossibly short • Know enough to be dangerous • Make informed decisions • Following info is “truth” • Assumptions required • 80/20 Principle • Good for FIRST • Not so good for airplanes

  8. Gravity Holds you on Earth’s surface Turns your “mass” into “weight” Holds planets in orbits, causes tides Magnetism Cousin to electrical current Motors, electromagnets, many more Why your microwave works Non-Contact Forces Tidal Force: Moon’s differential gravity field on earth’s surface

  9. High School Physics Good approximations Works for FRC Industry Accuracy required Contact Forces Classic HS Physics Problem Bird Strike: Simulation on jet engine blades

  10. Moments (AKA Torque) • Forces acting at a distance • M = r x F • Moment = radius (distance) x Force (normal) • Distance • Force • Sine of angle between M r F Bicep Curl: Looks a little like our robot problem…

  11. Fundamentals and Units • F = m * a • Force = mass x acceleration • T = I * α • Torque = Moment of Inertia x Angular Acceleration

  12. Free Body Diagram • BEST way to start a new problem • Pictorial representation which isolates body from world • Shows all loads acting on a body • Problem can be better understood We cut through the robot! Forces needed to represent.

  13. Free Body Diagram • Why this state? • Draw all forces and moments • Solve for reactions 36in 36in Motor & Gearbox attached to robot and arm Arm Gripper Ball

  14. Free Body Diagram Moments Forces FR ? ? ΣMO = I * α = m * (Δω/Δt) = 0 +TM–18in*WA–36in*WG– 54in*WB=0 ΣFY = m * aY = m * (ΔvY/Δt) = 0 +FR – WA – WG – WB = 0 36in 36in FR = WA + WG + WB TM +TM=180inlb+144inlb+270inlb = 594inlb FR = 10lb+4lb + 5lb = 19lb WA WG WB Point O Y Z X

  15. Question Break

  16. Tension & Compression • Tension pulls σ > 0 • Compression squishes σ < 0

  17. Bending smile frown Which portions are in states of tension, compression, or zero stress?

  18. Cross Section Selection • Intuitively, which cross section is preferred for the arm? Section Cut Here Neutral axis A B C D E F G

  19. Stress and Strain • A BRIEF overview • For any individual element • Stress is the average amount of force per unit area σNormal = F / A σBending = M * c / I (Bending Stress = Moment x centroid / Area Moment of Inertia) -centroid is the distance from neutral axis to extreme fiber • Strain is the percent elongation ε = ΔL / L Thanks, Wikipedia!

  20. E Stress and Strain • Young’s Modulus (E) relates them σ = E * ε (like spring theory F = k * x) • Everything is a spring, nothing is truly rigid • Higher modulus denotes a stiffer material (spring) Elastic Deformation σ = E * ε σ

  21. Material Failure Tensile Test Specimen Atomic bonds and dislocations

  22. Material and Gauge Selection • Determine needed size of elements based on criteria • Displacement • Stress • Strain • Buckling, Crack Propagation, Resonance, etc • Can predict failure of elements • Way beyond the scope of this workshop

  23. Material Properties Values are for: Tension condition Polycarbonate, Air Dried Douglas Fir, 6061-T6 Aluminum, 6-4 Titanium, ASTM-A992 Gr 50 Steel

  24. Question Break

  25. Work • Work is a measure of energy added to a system W = F · d W = T * Θ • For the Ball, only gravitational energy is added W = m*g*h W = 5lb*8 feet = 40 ft-lb = 52 Joules Is there other work done?

  26. Power • Power is how fast the work is done P = dW / dt • Lets say we wanted to raise the ball in 2 seconds P = 52J / 2s = 26W • This is the average power delivered, for pure height gain • Arm must trace arc, power required isn’t constant for constant speed • Peak power required is much higher

  27. Motors!

  28. DC Permanent Magnet Motor T

  29. Motor Properties • Important Characteristics • ω (speed, in RPM or rot/s) • T (torque, in N-m) • P (power, in W) • P = T * ω

  30. Motor Properties Ppeak Tstall P T 0 ω ωfree ωfree/2

  31. 2009 FRC Motors Lets pick one From our FBD and Analysis T = 594 inlbs = 67.1 Nm Pmin = 26 W

  32. Globe Motor Properties 55 W 17 Nm T P 8.5Nm 6.8Nm 0 ω 81 rot/s 40.5 – 48.6rot/s

  33. Question Break

  34. Gear Types Spur Bevel Crossed Helical Planetary Rack and Pinion Worm

  35. Gear Torque and Speed Ratio = Ndriven = 30 = 0.5 (:1) Ndriver 60 Tdriven = R*Tdriver = 0.5 Tdriver ωdriven = ωdriver/R = 2 ωdriver

  36. Gear Selection • 6.8 Nm is available from motor at 60% free speed • 67.1 Nm is needed to raise the arm • Ratio = Tdriven = 67.1 Nm = 9.9:1 = 10:1 Tdriver 6.8 Nm ~ 10:1 may need a very large gear for a single stage…

  37. Gear Trains Ratios multiply across gear trains

  38. Solution Globe Motor selected for worst case 10:1 ratio gear train Raise ball in ~2s

  39. Questions

  40. Extra Statics Problem C AB 30x50 rectangular BC 20mm rod Pinned Joints Find the stress in AB and BC d=20mm 600mm 50mm A B 800mm 30kN

  41. Free Body Diagram Cy + ΣMC = 0 = AX(0.6m) – 30kN(0.8m) AX = +40kN Cx + ΣFX = 0 = AX + CX CX = -AX CX = -40kN + ΣFY = 0 = AY + CY – 30kN AY + CY = 30kN 0.6m Ay B Ax 0.8m 30kN

  42. Breakout FBD + ΣMB = 0 = -AY(0.8m) = 0 AY = 0 CY = 30kN Ay B Ax 0.8m 30kN

  43. 2 Force Members Cy CX = -40kN CY = 30kN Cx 0.6m Ay B Ax 0.8m 30kN

  44. FIRST Robotics Drive Systems • Importance • Basics • Drive Types • Resources • Traction • Mobility • Speed • Timing • Importance

  45. Importance The best drive train… • is more important than anything else on the robot • meets your strategy goals • can be built with your resources • rarely needs maintenance • can be fixed within 4 minutes • is more important than anything else on the robot

  46. Basics • Know your resources • Decide after kickoff: • Speed, power, shifting, mobility • Use most powerful motors on drivetrain • Don’t drive ½ of your robot… WEIGH IT DOWN! • Break it early • Give software team TIME to work • Give drivers TIME to drive

  47. Drive Types: 2 wheel drive DrivenWheel Motor(s) Motor(s) • + Easy to design • + Easy to build • + Light weight • + Inexpensive • + Agile • Not much power • Will not do well on ramps • Less able to hold position Caster

  48. Drive Types: 4 wheel drive, 2 gearboxes DrivenWheels Motor(s) Motor(s) Chain or belt • + Easy to design • + Easy to build • + Inexpensive • + Powerful • + Sturdy and stable • Not agile • Turning is difficult • Adjustments needed Resource: Chris Hibner white paper on ChiefDelphi.com Proves that a wide 4wd drive base can turn easily DrivenWheels

  49. Drive Types: 4 wheel drive, 4 gearboxes DrivenWheels Motor(s) Motor(s) • + Easy to design • + Easy to build • + Powerful • + Sturdy and stable • + Many options • Mecanum, traction • Heavy • Costly DrivenWheels Motor(s) Motor(s)

  50. Drive Types: 6 wheel drive, 2 gearboxes + Easy to design + Easy to build + Powerful + Stable + Agile* • *2 ways to be agile • Lower contact point on center wheel • Omni wheels on front or back or both This is the GOLD STANDARD in FRC + simple + easy + fast and powerful + agile Gearbox Gearbox • Heavy ** • Expensive ** • ** - depending on wheel type

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