1. Building FLL Robots April 22, 2005 Nathan Gray FRC Team #1519 Mechanical Mayhem www.mechanicalmayhem.org

2. April 22, 2005Mechanical Mayhem2 Objective of this talk To teach sound building fundamentals for FIRST LEGO League robots Some discussion may not be applicable to general purpose robots –Assuming the usual FLL parts restrictions –Assumng a vinyl mat on a 4x8’ table with 2x4” walls will be used again next year –And so on…

3. April 22, 2005Mechanical Mayhem3 Contents Robot Components Overview Robot Design Options Common Robot Issues Robot Design Goals Dynamic Environment Some Robust Techniques Examples Resources

4. April 22, 2005Mechanical Mayhem4 Components Overview Technic beams and plates Pins and axles RCX Microcontroller Motors Sensors Gears, Pulleys, Wheels Special pieces

5. April 22, 2005Mechanical Mayhem5 Technic beams and plates STandard Unit of Dimension is a STUD Six studs = five beams, so beams are 6/5 (or 1.2) studs high Three plates = one beam, so plates are 0.4 studs high Hole spacing is the same as stud spacing

6. April 22, 2005Mechanical Mayhem6 Pins and axles Many various kinds Pin, friction pin, and long variants Evil, super friction pin that looks very similar to the normal friction pin Axles, come in various numbers of studs Never bend axles! Axles holding wheels or gears should be closely supported on both sides

7. April 22, 2005Mechanical Mayhem7 RCX Microcontroller 3 outputs for motors or lamps 3 inputs for sensors RCX v1.0 has a power adapter input (which isn’t generally used for FLL) Any of RCX versions 1.0, 1.5, 2.0 are fine for FLL – Use the latest RIS or RoboLab software on all of them

8. April 22, 2005Mechanical Mayhem8 Motors There are several different kinds, but FLL allows three 9 volt geared grey motors Caution! Two different kinds of grey geared 9 volt motors look very similar –The newer version is much lighter, but slightly slower and less powerful For great info on LEGO motors, see –http://www.philohome.com/motors/motorcomp.htm

9. April 22, 2005Mechanical Mayhem9 Sensors FLL allows –Two light sensors that measure 0-100% light – typical FLL table measurements are approximately 30-60 –Two touch sensors which can be used as bumper sensors or limit switches –One rotation sensor Measurement granularity is 1/16 of a rotation Can give bad data if very fast or very slow Rotational speed near motor speed is fine (200-400 rpm)

10. April 22, 2005Mechanical Mayhem10 Sensors (continued) Use all the permitted sensors! Can stack touch sensors on top of light sensor inputs –A closed touch switch reads 100% brightness –Cannot read 100% otherwise, unless pointed at light source Good sensor information at: –http://www.plazaearth.com/usr/gasperi/lego.htm Note: homebrew sensors are not FLL legal

11. April 22, 2005Mechanical Mayhem11 Gears Transfer rotation from one axle to another Even number of gears reverses the direction of rotation The radii determine gear spacing, transferred speed, and power Inverse relationship between power and speed There are lots of gear spacing issues beyond the scope of this talk –Special half-stud beams or diagonal spacing sometimes help –An eight tooth gear has a diameter equal to one stud –8, 24, and 40 tooth gears work well together because their radii are all multiples of 0.5

12. April 22, 2005Mechanical Mayhem12 Gears (continued) Worm gears –Are effectively one tooth gears –Significant efficiency lost to friction –Since they can’t be back driven, they are great for arms that should hold their position Some good gear info at –http://www.owlnet.rice.edu/%7Eelec201/Bo ok/legos.html

13. April 22, 2005Mechanical Mayhem13 Pulleys Work like toothless gears All the same radius principles apply Spacing is more flexible than gears Can be useful when want to allow slip Higher frictional load than gears or chain, and belts can stretch and break Try to use gears instead –Use a clutch gear if necessary

14. April 22, 2005Mechanical Mayhem14 Wheels Like pulleys and gears, the wheel dimension is key! Think of the wheel as the final gear in the drive train –Larger wheels will make the robot move faster, with less power With stability, traction, turning agility, and so on, there are lots of trade-offs in choosing wheels See the LEGO tire traction tests at: –http://www.philohome.com/traction/traction.htm

15. April 22, 2005Mechanical Mayhem15 Special pieces 1x1 beam, and double-hole 1x2 beam, can be used to get half stud spacing Clutch gears: protects motors and LEGO from self-destruction U-joints: can be used when a straight axle just won’t do –Should always be used in pairs –Original and final shaft should be parallel Worm gearbox – the perfect thing to raise and lower an arm Chain – achieves some of the flexibility of pulleys, but more precise, efficient, and reliable than belts

16. April 22, 2005Mechanical Mayhem16 Robot Design Options Drive systems Modular vs. Monolithic Robots

17. April 22, 2005Mechanical Mayhem17 Robot Drive Systems Dual motor / wheel differential drive is a good (and common) choice for FLL –Important to have well-matched motors! Treaded skid steer / tank drive can sometimes be useful for ATV missions –Very stable, but usually slow and sometimes hard to navigate accurately Active steering (e.g., like a car) has some attractive features, but: –Might require a dedicated motor –Cannot spin in place Legs?

18. April 22, 2005Mechanical Mayhem18 Modular versus Monolithic? A “modular” robot is a core robot with attachments that snap on at specific interface points – e.g. might have a different attachment for each FLL mission A “monolithic” (or self-contained) robot has no (or very few) attachments that go on or come off the robot. Either approach can be very successful!

19. April 22, 2005Mechanical Mayhem19 “Monolithic” Considerations Purpose built for each year’s missions Fewer parts to bring to the competition table Fewer attachments create fewer opportunities for operator errors Fewer attachments mean less time spent in base, and more table time available for the robot to actually accomplish the missions Might be easier / better to solve particular missions with dedicated robot features rather than an attachment that uses a standardized interface

20. April 22, 2005Mechanical Mayhem20 “Modular” Considerations Don’t necessarily need to redesign everything every year Can accumulate libraries of reusable code for the core robot base Easy to prototype new ideas Attachments can be used interchangeably on duplicate robot bases Student sub-groups can develop separate attachments in parallel without interfering with other mission solutions

21. April 22, 2005Mechanical Mayhem21 Common Robot “Issues” Robot could fall apart at a bad time It may not drive straight Robot might get “lost” on the table Maybe it is inconsistent and does something slightly different every time

22. April 22, 2005Mechanical Mayhem22 Robot design goals Simple: easy to replicate and less to go wrong! –Ask: Is there an easier solution? Robust: don’t want robots falling apart on the table! Compact –Small enough to turn in tight spaces –Keep the center of gravity between the wheels –Wire routing – tuck wires in so they don’t get pulled loose Predictable and reliable –Behavior should be consistent and repeatable Aesthetics: it’s nice to have a good looking robot!

23. April 22, 2005Mechanical Mayhem23 Some Robust Techniques Shielding light sensors Solid construction Using good batteries Going straight (enough) Reliable Navigation

24. April 22, 2005Mechanical Mayhem24 Dynamic Environment Even with good design, construction and programming, there can still be problems FLL robots and programs are generally designed assuming a well known (static) environment without any interference Unfortunately, things do change: sunlight, spotlights, camera flashes, dust on the tables and wheels, the battery power level, etc There are ways to mitigate some of these

25. April 22, 2005Mechanical Mayhem25 Shielding Light Sensors You cannot control ambient light, but You can control what the sensor sees Build a light box, or other light barrier, around the light sensors

26. April 22, 2005Mechanical Mayhem26 Solid Construction Use cross-bracing and vertical ties Connect enough studs – use significant overlap Use plates to lock the alignment of beams Mount motors and sensors securely Tie down sensor and motor wires

27. April 22, 2005Mechanical Mayhem27 Using Good Batteries Important! Battery levels can affect robot behavior in many ways! Experiment with both strong and weak batteries Know you battery’s discharge behavior We like Sanyo NiMH rechargeables Alkaline have more initial power, but consistency is usually more important

28. April 22, 2005Mechanical Mayhem28 Going Straight Use matched motors (especially if differential drive) Matched frictional drag – minimize drag on both sides Uniform weight distribution Front guide wheels that roll straight (but that will slip sideways when necessary)

29. April 22, 2005Mechanical Mayhem29 Three levels of Navigation Dead reckoning –e.g. aim and shoot for time Odometry / counting rotations –e.g. aim and shoot for wheel spins Feedback orientation –Use walls, mat, field elements so the robot “knows” where it is on the table –Self correcting, no jigs or precise starts

30. April 22, 2005Mechanical Mayhem30 Feedback Orientation Sites Can frequently run a wheel along a wall Turns parallel to a wall can sometimes align on the perpendicular wall Look for reliable “attack points”: –Intersections of linear table features, e.g. wall corners, line intersections, other mat features –Field element corners can also be used for alignment

31. April 22, 2005Mechanical Mayhem31 Sample MAYHEM Robot Core Similar to one used for “City Sights” Missing sensors, weights, and standardized power take off interface, as compared to current robot core base

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48. April 22, 2005Mechanical Mayhem48 Sample MAYHEM attachments Rover retriever Crater transmission

49. April 22, 2005Mechanical Mayhem49 Where to get LEGO? Ebay Look up set parts on: –www.peeron.com PITSCO LEGO Dacta: –www.legoeducationstore.com Bricklink: www.bricklink.com The LEGO Group store: www.lego.com

50. April 22, 2005Mechanical Mayhem50 Other Resources Excellent building guide from Minnesota INSciTE, hightechkids.org -- search for “Building LEGO Robots For FIRST LEGO League by Dean Hystad” Tons of info at LEGO Mindstorms Internals: –www.crynwr.com/lego-robotics/ The Ferrari’s book: Building Robots with LEGO Mindstorms Comprehensive FLL Coach’s Handbook at –http://www.fll-freak.com Mike’s LEGO Cad: –http://www.lm-software.com/mlcad

51. April 22, 2005Mechanical Mayhem51 Summary Probably want to use dual motor / differential drive Probably want to use gears (especially 8, 24, and/or 40 tooth gears) Use vertical ties and cross bracing Have a reliable plan for navigation