Patterns ofbuildinglegos

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And now for something completely different. This is something I wrote back in 2007 for my FLL team when teaching them basic concepts for robot design.

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Patterns ofbuildinglegos

  1. 1. Patterns of Building Legos Kyle Brown
  2. 2. Acknowledgements Fred G. Martin, The Art of Lego Design, The Robotics Practitioner: The Journal for Robot Builders, v1, #2, Spring 1995 Dean Hystad, Building Lego Robots for FIRST Lego League, v1.3, available from www.hightechkids.org
  3. 3. Glossary Rotary motion – motion in a circle; like a wheel spinning or an axle turning Linear motion – motion in a straight line; like your hand pushing forward Reciprocal motion – linear motion that switches direction, like a car piston Torque – the amount of force on a rotating axle (different from the speed of rotation)
  4. 4. Angles 90° angle (also known as a right angle) 90°
  5. 5. What’s a pattern? A solution to a commonly recurring problem If you come across the problem again, you can apply the solution again!
  6. 6. Frame Building
  7. 7. 2 hole plate frame Problem: How do you build a sturdy rectangular frame with holes for axles? Solution: Connect technic beams with 2- hole plates on the end. For even stronger frames use 2-hole plates on top and bottom
  8. 8. Cross-bracing Problem: How do you join technic beams together so they won’t slip apart? Solution: Use cross-bracing
  9. 9. EXERCISE Each team build two rectangular bases that: • Are at least two technic beams high • Won’t slip when the corners are pushed to the right or left • Are at least 1 ½ times longer than an NXT motor and at least 3 times as wide
  10. 10. Basic Rotary Motion
  11. 11. Stop Bushing Problem: How do you keep gears from sliding around on an axle or keep axles from sliding out of a frame? Solution: Use stop bushings
  12. 12. When do you need more torque? If you need to move a heavy load or move something with a long swing distance, you need more torque Technically it’s defined as force (weight) times a distance (a moment arm). Point of rotatio force n distance
  13. 13. Gear Ratios Problem: How do you make an axle move either faster (with less torque) or slower (with more torque) than a motor? Solution: Use gear ratios to gear up or *the gear ratio is the number of gear down teeth on each gear expressed as a fully reduced fraction
  14. 14. Ganging Problem: How do you create gear ratios that go beyond the ratio of a single pair of gears? Solution: Gang the gears together by placing large and small gears together on the same axle See: gear ratios
  15. 15. Gear Train Problem: How do you change the torque or speed of rotation between two axles? Solution: Mesh two or more gears together into a gear train See: frame building patterns
  16. 16. Idler Gear Problem: How do you make two axles separated over a small distance turn in the same direction? Solution: Use an idler gear Note: Idler Gears do NOT change the gear ratios between Input and output axles!
  17. 17. Exercise Using the rectangular bases from the last exercise: • Attach two wheels (one on each side) to an axle so that the wheels turn at 3/5 the speed of the drive axle For every five turns of this drive axle The wheel should turn three times
  18. 18. Robot Basics
  19. 19. Drive Base Problem: How do you allow your robot to have a stable base that allows both navigation and manipulation of objects? Solution: Use a separate drive base that moves the robot and attachments that manipulate objects.
  20. 20. Example Drive Bases Drive bases can have 3 wheels, 4 wheels, tank treads, skids, or any combination of the above! A drive base consists of a sturdy frame with motors, controller brick , skids, casters or wheels attached to the frame
  21. 21. Drive base combinations Differential Drive Tank treads Two Wheel Drive Synchro drive
  22. 22. Differential Drive  Has two powered wheels plus casters or skidsSkids or casters (front, back or both) Independently powered wheels
  23. 23. Building a simple caster The key is that both the wheel axle and the center axle need to swivel freely
  24. 24. Differential Drive Advantages • Simple • Turns on the spot Disadvantages • May not drive straight accurately • Friction may lead to varying accuracy in turns
  25. 25. Tank Treads Instead of wheels and casters, use tank treads Advantages • Good grip • Low slippage Disadvantages • Unpredictable rotation
  26. 26. Two-wheel Drive Front Wheel steering with powered back wheels Just like on a car
  27. 27. Two Wheel Drive When you turn all the wheels move at different speeds • Use a differential on the back wheels You need a mechanism to turn the front wheels • Usually a rack and pinion
  28. 28. Two Wheel Drive Advantages • Can carry very heavy payloads Disadvantages • Very, very complicated to build • Comparatively large turning radius
  29. 29. Synchro Drive Have all the wheels simultaneously powered and turned • One motor powers the wheels • One motor turns the wheels • Use a Lego turntable to independently turn the wheels Advantage: EXTREMELY accurate Disadvantage: VERY complicated
  30. 30. EXERCISE Build a robot with the following attributes: • It has a stable rectangular base that does NOT use the NXT brick as a structural member • It uses differential drive with a front caster or skid • The drive wheels turn at only 3/5 of the speed of the drive wheel motors
  31. 31. Advanced Rotary Motion
  32. 32. Bevel Gears Problem: How do you convert rotary motion into rotary motion at a 90° angle with a 1:1 gear ratio? Solution: Use two bevel gears
  33. 33. Crown gear How do you convert rotary motion to rotary motion at a 90° angle with a differing gear ratio? Use a regular gear and a crown gear
  34. 34. Worm Gears Problem: How do you convert rotary motion to rotary motion at a 90° angle that is self locking? Solution: Use a worm gear with a crown gear*self locking means that the follower axle can’t move the drive axle
  35. 35. Clutch Gear Problem: How do you limit the torque in a gear train? Solution: Use a clutch gear *you often want to limit torque to prevent lego pieces from breaking under strain.
  36. 36. Ratchet Problem: How do you limit rotary motion to a single direction only? Solution: Use a ratchet
  37. 37. Pulleys and Belts Problem: How do you connect two widely separated axles turning in the same direction? Solution: Use pulleys and belts *pulleys and belts can also be used to limit torque since the belt will slip when the torque is too high
  38. 38. Belts at an Angle Problem: How do you connect two widely separated axles that are at an odd angle? Solution: Use pulleys and belts
  39. 39. Linear Motion
  40. 40. Rack and Pinion Problem: How do you convert rotary motion to linear motion 90° away from the rotating axle over a short distance? Solution: Use a Rack and Pinion *notice that the rack has to be able to slide on a smooth track.
  41. 41. Piston Rod Problem: How do you convert rotary motion to reciprocal linear motion 90° away from the rotating axle over a very short distance? Solution: Use a piston rod Note: a Piston like this has a bit of side- to-side motion to it…
  42. 42. Lead Screw Problem: How do you convert rotary motion into continuous linear motion in the same direction as the rotating axle over a short distance? Solution: Use a Lead Screw
  43. 43. Scissor Arm Problem: How do you convert a small linear motion into a larger linear motion at a 90° angle? Solution: Use a scissor arm
  44. 44. Exercise Build a crank-powered Lego construction to: • (1) Move a lego minifigure 1 ½ inches forward • or • (2) Move a lego minifigure 6 inches forward
  45. 45. Object manipulation
  46. 46. Pusher Piston Problem: How do you move an object a short linear distance? Solution: Use a pusher piston Picture shows a piston connected to a crankshaft
  47. 47. Dumper How do you release one or more objects all at once? Solution: Use a gravity-fed dumper You can either rotate the dumper into position or lift it up on one end with a piston or lead screw
  48. 48. Water wheel Problem: How do you release several objects from a hopper over a period of time? Solution: Use a gravity-fed water wheel
  49. 49. Pincer Problem: How do you grasp an object when you have clearance on two sides? Solution: Use a pincer
  50. 50. Pincer types Parallel Gripper Pinch Gripper
  51. 51. Simple fork tines Problem: How do you grab an object at a fixed height and deposit it in base? e.g. how do you pick a loop up? Solution: Use a simple fork tine attachment, either powered or unpowered The simplest one is a fork directly attached to a forward-facing motor Multiple tines allow for variation in accuracy in getting to the loop. Single tines give more control but require precise navigation
  52. 52. Forklifts Problem: How do you grab an object at a variable height and deposit it later at a different height? e.g. how do you pick a loop up and put it back down? Solution: Use a forklift attachment Can use either belt drive (simple) or gear drive (more robust)
  53. 53. Scoops Problem: How do you relocate an object freely sliding on the board? Solution: Use a scoop or plow attachment
  54. 54. Exercise Each team build either a pinch gripper or a parallel gripper • Capable of grabbing a minifigure How would you power these from an NXT motor?
  55. 55. Wedge Problem: How do you separate two objects? Solution: Use a wedge to force the two apart
  56. 56. Navigation patterns
  57. 57. Odometry Problem: How do you navigate simple turns and short straight distances Use odometry; measure your distances and turn radius and program the robot to move exactly that much Odometric methods are prone to wheel slippage and center of gravity variations
  58. 58. Line Following Problem: How do you more precisely navigate to specific obstacles when a line is available leading to that obstacle. Solution: Use a line following algorithm and a light sensor. A simple switch one is available in the NXT education instructions. More advanced (PID) algorithms can be found here: http://nxt- progs.blogspot.com/2011/02/line- following-pid-controller.html Others are available online
  59. 59. Wall hugging Problem: How do you navigate precisely to an obstacle that is adjacent to a wall? Solution: Use a wall- hugging approach. Have the robot turn a bit into the wall as it moves. This usually requires a wall- following attachment and is compatible with odometry. The design of the attachment should account for variable distances between the mat and the wall.

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