TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN Madison Krass - and - Rahul Bhardwaj This is insane. Don’t do this.

TEAM XBOT FRC488 – SEATTLE, WA Madison  2013 Season Will Be 15 th  Mentor since 2000  With FRC488 since 2005  Mechanical Designer  Auto crash test systems  Retail display systems  Marketing events  Hydropower Rahul  2013 Season Will Be 4th  Junior at International School  FRC488 Team Captain and student lead of mechanical team  Intern at DANTERRY, Inc.  FTC team leader since 2011  FRC drive team since 2011 DRIVE DESIGN INTRODUCTION Non-Destructive Test Sled FTC Robot

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN EXPECTATIONS  You won’t know everything you need to know to do this right when you leave here today. Hopefully, you’ll know where to start, though.  This presentation tries to favor graphics and common sense explanations over pages and pages of equations. When you want to begin building high-performance drives, you’ll need to understand the math.  We are competitive team on the field and off. We are trying to be World Champions and so that goal informs all of our decisions and opinions.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN REAL TALK  Nothing else your robot does matters if you get the drive wrong. A bad drive will ruin your season on the field.  We know our stuff. Heed our advice and you’ll be better off for it.  Every team has the ability to make a reliable, functional drive. Most have the ability to make an excellent drive if they can reign in the crazy.  Do NOT mimic what you see on the internet if you can’t provide a strong explanation for why things were done that way in the first place.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN CONCEPTS AND WORDS  Friction – a mercenary force that works for and against us. Generally opposes movement.  Coefficient of Friction – a unit-less, experimentally derived value that describes how ‘sticky’ two surfaces are when sliding against one another. A higher value means more ‘stickiness’.  Torque – occurs when a force acts at some distance from an object’s center of mass. Torque is the force that makes robots move and is applied by friction.  Wheelbase – the distance between points of contact with the floor, both along the length and width of your robot.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN CONCEPTS AND WORDS  Center of Mass – a.k.a. ‘center-of-gravity’, a.k.a. ‘the reason your robot falls over’, a.k.a. ‘the reason your robot hops while turning’. Keeping this near to the ground is important. Locating it in different places along the ground plane will produce different turning characteristics.  Power – the availability of power determines how much stuff your robot can do in a given span of time. Trade-off between doing a lot of stuff slowly or less stuff more quickly. The difference between you and world champions is how you use power.  Gearing – the mechanical components that help us to make the trade-off described above. Fractions make your wheels go ‘round.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN CENTER OF MASS

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN FRICTION  Friction is everywhere, but we only care about it in a few places.  Maximizing friction between wheels and floor maximizes robot’s ability to push things around the field.  Minimize friction everywhere else to avoid giving energy away to heat, noise, vibration. Save it for robot hugs.  Force of Friction = Supported Weight x coefficient of friction  To maximize pushing ability, maximize weight on powered wheels.  Never have unpowered wheels.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN TURNING TORQUE  Decisions about 2WD vs. 4WD vs. 6+WD are foremost about managing turning torque.  Making a robot turn easily and reliably is the most under-estimated aspect of drive design and is important to quickly position a robot on the field.  High friction wheels that maximize a robot’s pushing advantage also will impede its turning performance.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN TURNING TORQUE d F Torque = F x d d d1 Torque = (F x d) + (-F1 x d1) -F1 +F

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN TURNING TORQUE This gap is very, very important. Wheel Base

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN TURNING TORQUE

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN WHEEL CONFIGURATION  2WD – One or more unpowered points of contact makes this a last resort. It is outclassed on the field in nearly every respect.  4WD – Easy to implement and maximum pushing ability is available. Tricky to get turning right while maintaining that pushing ability.  6WD – Solves 4WD turning challenges by shortening wheelbase; solves problems associated with short wheelbases – e.g. tipping – by creating two effective wheelbases. Consider extra two wheels as powered wheelie bars.  8+WD – This is often dumb, but can sometimes be helpful for getting on or over obstacles.  Increasing the number of wheels has no appreciable affect on pushing force

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN OMNIDIRECTIONAL DRIVE Driving the wheels together makes the robot behave like a typical 4WD Driving the wheels individually allows for omnidirectional motion

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN OMNIDIRECTIONAL DRIVE Swerve drive maximizes pushing ability while maintaining the maneuverability of mecanum drive systems. VERY complex or very expensive; also difficult to program.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN MOTOR SELECTION Breaker (A) Motor Name Max Power (W) Peak Power (W) Free Speed (RPM) Stall Torque (N*m) Stall Current (Amp) Free Current (Amp) Spec Voltage Gearbo x Ratio Motors 40Atwood CIM Motor Nippon-Denso Window Motor RH ( ) Nippon-Denso Window Motor LH ( ) FP 2011 ( ) RS-775 (18V 18V) RS-775 (18V 12V) RS RS RS (Max power is the power output at the breaker or peak power, whichever is greater) RPM Torque & Current This is the power (W) curve of a motor. We aim to have it run at max. power so it will do the most work for our robot or, put another way, move the most stuff in the shortest possible time. Courtesy of Jesse K. on chiefdelphi.com 40A

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN GEARING  Gearing is comprised of everything between the motor shaft and the wheels.  Often a mixture of gears, sprockets, pulleys, chain and belt.  Gears are more compact and require less maintenance than sprockets and chain or pulleys and belts. It requires accurate manufacturing processes to make custom gearboxes if something commercially available doesn’t meet your needs.  Sprockets and chain / Pulleys and belt are great for transmitting power over ‘long’ distances. Gears are heavy, a single sprocket is not.  Use toothed belts where possible. Tensioning V- or flat-belts is a nightmare.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN GEARING / 4 = / 2 = / 2 =.063 Gearing stages are multiplicative. Real world gearing is not 100% efficient, so each stage loses some energy to heat, noise, and vibration. Speed Calculation

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN GEARING vs. Advantage:  All motor power is always available, regardless of how many wheels are on the ground. Disadvantage:  All wheels share same gearing regardless of weight distribution. 2 CIM Motors There’s that gap again. Advantage:  With a well known weight distribution, you can select different gearing for each wheel. Disadvantage:  As soon as a wheel comes off the floor, 25% of your power is gone.  Advantage above is impractical.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN MORE REAL TALK  Drives are the mechanism most often prototyped in the off-season because the need for mobility around the field is a safe bet in FRC.  Consequently, teams often want to show off what they’ve prototyped by using it during the season, even if it’s a terrible drive for the game. We were the first FRC team to build a drive using AndyMark’s mecanum wheels in the Fall of We didn’t put a mecanum drive on the field until five years later and, even then, it was the absurd ‘Octocanum’ that was designed explicitly for the 2011 game.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN MORE REAL TALK  A lot of work has been done on your behalf because a working, reliable drive is the most important part of your robot and, arguably, the most important part of a successful year and enthusiasm for the next.  As a result, many newer teams have NO IDEA what they’re talking about. When we did this stuff for the first time, there was no kitbot or AndyMark.  This makes the leap from a kitbot to a custom-designed drive of any sort a big one for a lot of teams. Don’t attempt it until you’ve built a reliable, successful scoring mechanism in a prior season.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN ANATOMY OF A DRIVE Traction Wheel Omniwheel Wheelbase Sprockets Chain CIM Motor Gearbox

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN ITERATION 2008 Season Drive -Cantilevered, ‘live’ axles - Custom-made 4” omniwheels - Modified Dewalt drill transmissions with servo shifting 2008 Fall Prototype - Dead axles on 80/20 rails - COTS wheels everywhere - Improved chain wrap on gearbox output

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN ITERATION 2009 required special wheels that minimized friction to mimic playing on ice. Simple 4WD was all that was needed with extra time spent developing traction control software to maximize acceleration and improve turning performance. In 2010, we built an 8WD robot with the center two wheels lowered to shorten its wheelbase and improve turning performance. We chose 8WD to allow it to climb over these midfield obstacles.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN ITERATION For 2011, our game strategy prioritized maneuverability because of the narrow entrances into the loading zone, but recognized that defense of the scoring zone would be a large obstacle. Octocanum provides omnidirection motion via mecanum wheels, but can pneumatically lower high- traction wheels to maximize pushing ability. This summer, we returned to the simple, powerful 6WD while experimenting with new manufacturing resources.

TEAM XBOT FRC488 – SEATTLE, WA DRIVE DESIGN QUESTIONS DISCUSS.