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
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)
Angles 90° angle (also known as a right angle) 90°
What’s a pattern? A solution to a commonly recurring problem If you come across the problem again, you can apply the solution again!
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
Cross-bracing Problem: How do you join technic beams together so they won’t slip apart? Solution: Use cross-bracing
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
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
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
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
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
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
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!
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
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.
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
Drive base combinations Differential Drive Tank treads Two Wheel Drive Synchro drive
Differential Drive Has two powered wheels plus casters or skidsSkids or casters (front, back or both) Independently powered wheels
Building a simple caster The key is that both the wheel axle and the center axle need to swivel freely
Differential Drive Advantages • Simple • Turns on the spot Disadvantages • May not drive straight accurately • Friction may lead to varying accuracy in turns
Tank Treads Instead of wheels and casters, use tank treads Advantages • Good grip • Low slippage Disadvantages • Unpredictable rotation
Two-wheel Drive Front Wheel steering with powered back wheels Just like on a car
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
Two Wheel Drive Advantages • Can carry very heavy payloads Disadvantages • Very, very complicated to build • Comparatively large turning radius
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
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
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
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
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
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.
Ratchet Problem: How do you limit rotary motion to a single direction only? Solution: Use a ratchet
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
Belts at an Angle Problem: How do you connect two widely separated axles that are at an odd angle? Solution: Use pulleys and belts
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.
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…
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
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
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
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
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)
Scoops Problem: How do you relocate an object freely sliding on the board? Solution: Use a scoop or plow attachment
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?
Wedge Problem: How do you separate two objects? Solution: Use a wedge to force the two apart
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
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
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.