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Drive System Engineering

Drive System Engineering. Presented by: Ken Zaballos FIRST Team 1983 – Skunk Works Robotics Mechanical Engineer Boeing Research & Technology 777X Wing Assembly Technology Development ken.zaballos@gmail.com. Today’s Session. Drive System Design Process Lift System Case

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Drive System Engineering

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  1. Drive System Engineering Presented by: Ken Zaballos FIRST Team 1983 – Skunk Works Robotics Mechanical Engineer Boeing Research & Technology 777X Wing Assembly Technology Development ken.zaballos@gmail.com

  2. Today’s Session Drive System Design Process Lift System Case Lift a load within a given time Motor Characteristics Motor Selection Gearbox Selection This IS the math!

  3. Today’s Case: Lift Systems ARMS Arms, Elevators, Drivetrain The process is the same. ELEVATORS

  4. ELEVATOR LIFT SYSTEM Motor Drivetrain Reduction Load Chain and sprocket lift system

  5. Key Performance Parameters Drive Sprocket ? ? ? Motor Drivetrain Reduction Figure these out when you decide how you want to play the game ? in KPPs: 40 lbs 44 in 1 second ? lbs

  6. Some Terms and Things • Force = a push or pull upon an object • Torque = a twisting force that tends to cause rotation. • Voltage = an electromotive force or potential difference • Current = a flow of electrical charge • Friction = the resistance one object encounters when moving over another • Efficiency = the effectiveness of transforming power input to an output force and movement • PMDC = Permanent Magnet Brushed DC Motor

  7. First, UnderstandMotor Characteristics

  8. Key Motor Values • Stall Torque • Torque at the output shaft when it can’t rotate • Stall Current • Current draw when the output shaft can’t rotate • Free Current • Current draw when the output shaft is free • Free Speed • Speed of the output shaft when it is free

  9. Motor Data from FIRST (2015) All values @ 12VDC http://www.usfirst.org/sites/default/files/2015%20Motor%20Information.pdf

  10. A FIRST Motor Curve

  11. Speed-Torque Speed (RPM) Torque (Oz-In) CONVERT TO SI UNITS FOR CALCULATIONS!

  12. Power-Torque P=Torque*Speed P = (, ) Power (Watts) Speed (RPM) Torque (Oz-In) CONVERT TO SI UNITS FOR CALCULATIONS!

  13. Power-Torque Max Power Power (Watts) Speed (RPM) Torque CONVERT TO SI UNITS FOR CALCULATIONS!

  14. Current-Torque Power (Watts) Speed (RPM) Current (Amps) Torque Torque CONVERT TO SI UNITS FOR CALCULATIONS!

  15. Efficiency Mechanical Power = Electrical Power = Efficiency = Mechanical Power / Electrical Power e = Power (Watts) Speed (RPM) Current (Amps) Efficiency (%) Torque Torque CONVERT TO SI UNITS FOR CALCULATIONS!

  16. Efficiency e = Power (Watts) Speed (RPM) Current (Amps) Efficiency (%) Torque Torque CONVERT TO SI UNITS FOR CALCULATIONS!

  17. Efficiency NEVER DESIGN TO THE RIGHT OF PEAK POWER! Peak Power Peak Efficiency e = Power (Watts) Speed (RPM) Current (Amps) Efficiency (%) Smoke! Torque Torque CONVERT TO SI UNITS FOR CALCULATIONS!

  18. Motor Data from FIRST (2015) Not necessarily at a usable current http://www.usfirst.org/sites/default/files/2015%20Motor%20Information.pdf

  19. MOTOR PRIORITIES Rank motors by power @ 30 amps Eliminateweakmotors

  20. Back to the Lift Design

  21. How much power do we need? Hint: Convert to SI units! ? ? ? KPPs: 40 lbs = 177.9N 44 in = 1.118m 1 seconds 44in 40 lbs We need a motor that can deliver AT LEAST 226 watts.

  22. We Choose …

  23. = Torque at max allowed current = Maximum allowed current draw = Stall torque = Motor free current = Motor stall current Max Motor Torque (Design) The Algebra is left to the student as an exercise!

  24. Calculate Max Motor Torque (Design) • = 30 amps (a design choice) • = 0.78 Nm (from motor data) • = 1.8 amps (from motor data) • = 86.7 amps (from motor data)

  25. Calculate Torque on the Output Shaft Sprocket: #25, 16T 0.26 Nm 2.88 Nm ? ? P.D. 44in 16T, #25 Chain (Design Choice) 40 lbs www.vexpro.com

  26. What is a Gear Reduction? 40T 12T A gearbox REDUCES speed A gearbox INCREASES torque Motor Drives Small Gear

  27. Ratios in gear trains F t1 ,w1 r1 r2 t2 ,w2 F v = r1 * w1 t1 = r1 * F v = r2 * w2 t2 = r2 * F w2 = (r1 / r2) * w1 t2 = (r2 / r1) * t1 Reduces Speed Increases Torque

  28. Find Required Gearbox Reduction 0.26 Nm 2.88Nm ? Gearbox Reduction? 40 lbs THIS WOULD BE A BIG MISTAKE!

  29. Mechanical Efficiency • Friction (primary source of inefficiency) • Gear meshing (center-center distance) • Gear form accuracy • Lubrication • Cleanliness • Bearings • Rules of thumb* • Sprocket (chain) = 95% • Spur gear stage = 95% • Planetary Stage = 75% • Depends on the number of planets, lubrication, quality • Worm gear = 60% *-YOUR ASSUMPTIONS MAY VARY Involute gear form

  30. Find Required Gearbox Reduction(considering mechanical efficiency) 0.32 Nm 2.88Nm ? Gearbox Reduction? Stage 1 Stage 2 Sprocket Carriage Vexpro planetary gearbox 40 lbs Pick the nearest BIGGER reduction you can make. 2 stages 3:1and 9:1 = 27:1 2 stages 4:1and 7:1 = 28:1 (more durable) http://www.vexrobotics.com/vexpro/motion/gearboxes/versaplanetary.html

  31. Is this good enough? • No? • Decrease Gearbox Load (can be difficult) • Increase Gearbox Power (use bigger motor or add a motor) • Live with the low speed… • Risk failure • Design is all about tradeoffs • Finally, • Design for one motor (torque and speed) • Then, add another motor! • Factor of Safety = 2!

  32. Important things to consider • Motors are rated @ 12VDC • Batteries are only 12VDC once during a match! • Circuits are limited to 20, 30 and 40 amps. • Motor Controller (PWM) • Pulse Wave Modulation – understand how it works • PID control • Proportional-Integral-Differential – understand how it works and practice tuning them. • Mechanical efficiency of the things you build • Accuracy and quality play huge roles • Typically, make your system go FAST!

  33. Important Equations Speed at a given torque Current at a given torque Power output Motor efficiency e = Torque at Design current Gearbox reduction Output Shaft Speed Time to lift

  34. Useful Websites • www.andymark.com(robot marketplace) • www.vexpro.com(robot marketplace) • www.banebots.com(motors and gearboxes) • www.digikey(electronics) • www.chiefdelphi.com(technical and social) • www.onlinemetals.com(raw material) • www.mcmaster.com(everything else)

  35. THE END GOOD LUCK! WE’LL SEE AT THE COMPETION, or sooner if you want some help!

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