Drive System Engineering Presented by Ken Zaballos FIRST

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

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!

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

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

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

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 • • •

First, Understand Motor Characteristics

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

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

A FIRST Motor Curve

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

Power-Torque Power (Watts) Speed (RPM) Torque (Oz-In) CONVERT TO SI UNITS FOR CALCULATIONS!

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

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

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

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

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

MOTOR PRIORITIES Rank motors by power @ 30 amps Make Name Vexpro 775 Pro Max Power (w) Power @ 30 A Torque @ 30 A Speed @ 30 A Stall Torque (oz-in) Stall Torque Free Speed Free Current Stall Current (N-m) (rpm) (A) 346. 3 237. 5 0. 16 14590 100. 2 0. 71 18700 0. 7 134 Bane. Bots RS-775 Motor 266. 6 236. 6 0. 26 8682 110. 9 0. 78 13000 1. 8 86. 7 Andy. Mark CIM 337. 1 223. 3 0. 51 4197 343. 4 2. 42 5310 2. 7 133 Bane. Bots RS-550 Motor 245. 7 221. 2 0. 17 12697 68. 9 0. 49 19300 1. 4 85 VEX 227. 2 203. 2 0. 47 4109 198. 3 1. 40 6200 1. 5 86 Andy. Mark 9015 Motor 179. 4 178. 2 0. 20 8639 60. 6 0. 43 16000 1. 2 63. 8 VEX 146. 6 118. 4 0. 29 3929 56. 6 0. 40 14000 1. 8 41 Andy. Mark Snow Blower Motor 29. 6 -49. 2 14. 87 -32 1600. 0 11. 30 100 5 24 Denso Window Motor 23. 3 -64. 2 15. 57 -39 1501. 1 10. 60 84 1. 8 21 Andy. Mark RS 775 -5 Motor 36. 9 -75. 8 0. 34 -2131 35. 0 0. 25 5700 0. 6 22 Andy. Mark RS 775 -125 Motor 36. 9 -75. 8 0. 34 -2131 35. 0 0. 25 5700 0. 6 22 Denso Window Motor 23. 3 -106. 2 17. 79 -57 1501. 1 10. 60 84 1. 8 18. 6 Denso Throttle Motor 18. 0 -1336. 1 0. 63 -20317 18. 4 0. 13 5300 1 7 Mini CIM Motor BAG Motor Eliminate weak motors

Back to the Lift Design

How much power do we need? Hint: Convert to SI units! ? ? ? KPPs: 40 lbs = 177. 9 N 44 in = 1. 118 m 1 seconds 44 in We need a motor that can deliver AT LEAST 226 watts. 40 lbs

We Choose … Make Name Bane. Bots RS-775 Motor Max Stall Free Stall Power Torque Speed Current (w) @ 36 A (oz-in) (N-m) (rpm) (A) 266. 8 256. 5 110. 9 0. 783 13000 1. 8 86. 7

Max Motor Torque (Design) • The Algebra is left to the student as an exercise!

Calculate Max Motor Torque (Design)

Calculate Torque on the Output Shaft 0. 26 Nm ? 2. 88 Nm ? Sprocket: #25, 16 T P. D. 44 in 16 T, #25 Chain (Design Choice) 40 lbs www. vexpro. com

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

Ratios in gear trains F t 1 , w 1 r 2 F t 2 , w 2 t 1 = r 1 * F t 2 = r 2 * F v = r 1 * w 1 v = r 2 * w 2 t 2 = (r 2 / r 1) * t 1 Increases Torque w 2 = (r 1 / r 2) * w 1 Reduces Speed

Find Required Gearbox Reduction 0. 26 Nm 2. 88 Nm ? Gearbox Reduction? 40 lbs THIS WOULD BE A BIG MISTAKE!

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% Involute gear form • Depends on the number of planets, lubrication, quality – Worm gear = 60% *-YOUR ASSUMPTIONS MAY VARY

Find Required Gearbox Reduction (considering mechanical efficiency) 0. 32 Nm 2. 88 Nm ? Stage 1 Stage 2 Sprocket Carriage Gearbox Reduction? Vexpro planetary gearbox Pick the nearest BIGGER reduction you can make. 2 stages 3: 1 and 9: 1 = 27: 1 4: 1 and 7: 1 = 28: 1(more durable) 40 lbs http: //www. vexrobotics. com/vexpro/motion/gearboxes/versaplanetary. ht

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!

Important things to consider • Motors are rated @ 12 VDC – Batteries are only 12 VDC 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!

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

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

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