LEGO Theory and Practice

1. LEGO Theory and Practice Mark Green School of Creative Media

2. Introduction • Can do a wide range of things with LEGO • adding motors, sensors and computers gives us even more possibilities • the question is what do we do with this? • Its fun to play with LEGO, but where do we want to take it? • In addition, how do we build fun things?

3. Expressive Robots • One thing is building robots that can express themselves • not just a mechanical thing, but something we can relate to, something with emotions • a good example of this is Feelix, a LEGO robot the expresses feelings: http://www.daimi.au.dk/~chili/feelix/feelix_home.htm

4. Feelix

5. Feelix

6. Feelix • Feelix has been used to study how people recognize emotions • could not recognize as easily as with real humans, but fairly close most of the time • Feelix could react to people through its touch sensors (on feet) • emotion based on frequency and strength of touch

7. PETS • Personal Electronic Teller of Stories • Robots built from LEGO, designed by a team of adults and children • Develop a robot that shows emotions and feelings, can be used to assist with telling stories • Robot acts out part of the story, controlled by computer to give expressions at appropriate times

8. PETS • LEGO used as the robot skeleton and to provide the motion • Skeleton is covered with cloth and other soft things to make a huggable toy • Velcro and glue used to attach “skin” to the robot • Quick way to produce responsive toy without getting into a lot of engineering

9. PETS

10. PETS

11. PETS • Shows how LEGO can be used to prototype intelligent toys • Building out of raw components, plastic and metal, can be difficult and requires special tools and skills • LEGO can be used by most people, doesn’t require anything special • Won’t be the best looking, but quick and easy

12. Building with LEGO • Two general approaches: • Start by deciding what you are going to build, figure out how to build it • Start by putting things together and see what you end up with • Most LEGO projects are a combination of these approaches • Rarely know exactly how to build something before you start

13. Building with LEGO • Good LEGO builders claim that you need three skills: • Mechanics • Electronics • Software • Also need some patience and willingness to try different things • It won’t work right the first few (many) times you try to build it

14. Mechanics • Need to know how to put the blocks together to get the structure you need • Needs to be strong, so it doesn’t fall apart when it moves • Need to understand how to make LEGO move, how to use wheels, gears and axles • Most of this is gained through experience with making things

15. Electronics • Understand how sensors work, how they can be used to control the robot • Understand how motors can be used to move the robot • How to connect the motors and sensors to the LEGO blocks, make the best of the limited resources • Use one motor to produce several motions

16. Software • Write the programs that make the robot work • Read the sensor values, produce the signals required for the motors • Plan how long each motor should run, how it should respond to sensors • Produce the robots behavior, how it will respond to its environment

17. Example • Look at a very simple robot, example of how we build and program them • Based on Tippy from Brian Bagnall’s book “Core LEGO Mindstorms Programming” • Like all good robot projects, this one didn’t go as planned! • Tried to follow instructions from book, but the robot wouldn’t fit together

18. Tippy

19. Tippy

20. Tippy • Tippy is about as simple as it gets • Two wheeled direct drive robot, there are skid plates at the front and back to keep the robot from tipping over • There is a touch sensor at the front to detect collisions • It can only detect collisions at the front, but the robot does go backwards!

21. Tippy

22. Tippy • The touch sensor is quite small, need something bigger to detect collision • The bumper mechanism at the front does this, based on a hinged lift arm • A wide axle is attached to the lift arm, to increase the range of the sensor • When something hits the axle the lift arm hits the touch sensor, signaling the collision

23. Tippy • Two motors are attached to the plate at the bottom, this is not a good design! • Weight of the robot is on the wheels, wheels connected to motors, motors connected to top of plate • Too easy for motors to come off of the plate • Would be better to attach the motors to the bottom of the plate

24. Tippy • Problem: when I tried to attach the understructure of the robot to the RCX I found it was too wide! • Our RCX is narrower, by one row then the one used in the book • Had to design a platform on the bottom of the RCX to mount the structure on • Result: robot is lopsided

25. Lesson • LEGO rarely goes together the way you want it to, must be prepared to improvise • This is the creative part of the project, figuring out how to make the whole thing fit together • Be prepared to rethink your design and build interfaces between the different components of the design

26. Software • We need to make the robot do something • It needs some software for this • What will we make the robot do?? • Its default action is to move forward, both of its motors should spin in the forward direction • When it hits something it should back up and turn so it no longer hits something • Going forward is easy, but how do we turn?

27. Software • Neither wheel turns, there doesn’t appear to be a way to turn the robot • But the two wheels are independent, each have a separate motor • We can make the robot turn by spinning one wheel forward and the other wheel backwards • Only do this for a short period of time

28. Software • Software consists of two parts • First part just drives the robot forward • Turns on the two motors and sets both of them to forward • Second part only runs when there is a collision • It backs up the robot and turns it, then starts it moving forward again

29. Tippy Program

30. Software • The left side turns on the two motors and sets their direction to forward • The right side is connected to the touch sensor • It changes the motor direction and waits for 0.5 second • Set direction so one motor is forward and the other reverse

31. Software • Again wait for 0.5 second • Then set both motors to forward • We don’t measure how far the robot moves or turns, we just wait for 0.5 seconds • Good enough most of the time, but could still get in trouble

32. Summary • We have a robot that basically works • Can be put together in about 20 minutes, most of the effort is finding the right parts • But, neither the software or structure is very robust • It can easily fall apart and it can easily get stuck trying to recover from collisions

33. Summary • Due to the modification I made I didn’t have enough parts to finish the robot • Original design had a plate above the lift arm, but I ran out of plates • Without the plate the arm bounces and causes the robot to turn too much • I later made a plate out of two smaller plates, and it now works better

34. LEGO Theory • If we are going to build things with LEGO we need to understand how it works • Start by looking at the various LEGO parts and then move on to some of the standard structures • Look at some of the standard design and solutions • Get you started on your own designs

35. LEGO Theory • The main structural units are bricks, plates and beams • The size of a LEGO piece is measured in studs, the little round things on the top • Bricks are usually one or two studs wide and from one to eight studs long • Bricks are used to build up structure, they have no other purpose

36. LEGO Theory • Plates are thin bricks, 1/3 the thickness of a brick • Plates can be used to build structure, but they are usually used to connect other units or add strength to a structure • Beams are one stud wide, even number of studs long, with holes running through them

37. LEGO Theory • If a beam is ‘n’ studs long, it has ‘n-1’ holes • Axles and pins can be placed in the holes, so beams are often an important part of a robot’s chassis • Since beams are thin they often need to be reinforced or the structure becomes too weak

38. LEGO Theory • Pins are short and round and fit into the holes in beams • Two types of pins • Free turning pins, can rotate inside the hole • Friction pins, don’t rotate • Pins can be used to attach parts, or to attach wheels and gears to the robot’s chassis

39. LEGO Theory • Wheels, axles and gears are used for movement • A wide range of wheels, the larger the wheel the faster the robot will move • Axles are measured in studs, even though they have no studs • Gear are used to change the speed of motion, or change its direction

40. LEGO Theory • There are a number of other parts used for special purposes and decorations • Lift arms are beams that don’t have studs • They can be connected to other parts using pins and axles • Pulleys can be used to transmit force, but are not as reliable as axles and gears

41. Structures • Mindstorms comes with two motors, and the RCX can only handle three • We can only have a limited number of independent motions, one per motor • In addition, motors rotate, what if we want a linear motion, or one with a limited rotation angle? • Also we cannot control the speed of the motor

42. Structures • The LEGO motor consists of a motor, plus a gear chain • There is no way to control the speed of this motor, we can only control the strength • That is, we can increase the amount of force the motor produces, carry heavier loads, but cannot change speed

43. Structures • This introduces the need for a number of structures to produce different types of motion: • Straight linear motion • Repeated linear motion • Restricted rotations • Faster or slower speed • Axis or rotation

44. Structures • The structures that produce these motions contain combinations of gears and axles • Gears can be used to change speed and axis of rotation • The size of a gear is measure by the number of teeth it has • The standard gear sizes are 8, 12, 16, 24 and 40 teeth

45. Structures • The standard gears mesh together, tooth for tooth • This can be used to control speed of rotation using different gear sizes • Consider a 8 tooth and 24 tooth gear connected together, both with their own axles • Start by turning the axle on the 8 tooth gear

46. Structures

47. Structures • Every complete rotation of the axle will move 8 teeth on the larger gear • This gear has 24 teeth, so we need 3 rotations of the smaller gear for one rotation of the larger gear • Similarly, one rotation of the larger gear will produce 3 rotations of the smaller one • Thus we can go faster or slower

48. Structures • Note: if we speed up there is less force, if we slow down there is more force • Need to consider what you are trying to move • What happens if we want to change the axis of rotation? The motor is facing one way, but we want the rotation in a different direction • Two ways of doing this

49. Structures • One way is to use a worm gear and the other is to use a crown gear • A crown gear meshes with a regular gear at a 90 degree angle • When the crown gear is turned the regular gear with turn, but the axis of rotation has been shifted by 90 degrees

50. Structures – Worm Gear