Mechanisms & Manipulators FRC Conference 4/15/04

1. Mechanisms & Manipulators FRC Conference 4/15/04 By Joe Johnson and Raul Olivera

2. Some Basic Physics • Forces, Angles & Torque • Power

3. D Forces, Angles & Torque 10 lbs • Example #1 - Lifting • Same force, different angle, less torque 10 lbs < D

4. Forces, Angles & Torque • Example #2 - Pulling on object • One angle helps secure object • The other does not

5. Forces, Angles & Torque • Example #2 - Pulling on object (cont’d) This one want to rotate clockwise and let go This one want to rotate counter- clockwise and grab even harder

6. Power • Power = Force x Distance / Time OR • Power = Torque x Rotational Velocity Power is all about how fast you can move something

7. 10 lbs Power • Example - Lifting • Same torque, different speed 10 lbs 0.2 HP, 200 RPM Motor w/ 1” sprocket OR 100 RPM w/ 2” sprocket 0.1 HP, 100 RPM Motor w/ 1” sprocket

8. Power • In Summary: • All motors can lift the same amount (assuming 100% power transfer efficiencies) - they just do it at different rates • BUT, no power transfer mechanisms are 100% efficient • If you do not account for these inefficiencies, your performance will not be what you expected

9. Structural Integrity • Materials • Shapes / Weights • Fabrication processes • Environment

10. My Favorite Materials • Spectra Cable • Stronger than steel for the same diameter • Very slippery • Easy to route • Needs special knots to tie • Can only get it from Small Parts and select other suppliers • Pop Rivets • Lighter than screws but slightly weaker - just use more • Steel and Aluminum available • Great for blind assemblies and quick repairs

11. Structural Shapes • Take a look at these two extrusions - both made from same Aluminum alloy: • Which one is stronger? • Which one weighs more? 1.0” 0.8” 1.0” 0.8” Hollow w/ 0.1” walls Solid bar

12. Structural Shapes • The solid bar is 78% stronger in tension • The solid bar weighs 78% more • But, the hollow bar is 44% stronger in bending • And is similarly stronger in torsion

13. Stress Calculations • It all boils down to 3 equations: Bending Tensile Shear Where:  = Bending Stress M = Moment (calculated earlier) I = Moment of Inertia of Section c = distance from Central Axis Where:  = Tensile Stress Ftens = Tensile Force A = Area of Section Where:  = Shear Stress Fshear = Shear Force A = Area of Section

14. Structural Shapes • I am willing to bet that none of our robots are optimized with respect to strength to weight ratios • We all have more material than we need in some areas and less than we need in others. • It would take a thorough finite element analysis of our entire robot with all possible loading to figure it all out • We only get 6 weeks!! • But, this does not mean we cannot improve

15. Structural Shapes • Things to avoid or carefully design: • Sharp inside cuts - leave a radius / fillet • Fastener holes that are too close to an edge • Welding corners without adding a gusset • Brittle materials - bending is easy to repair - cracks are not • Things that might help: • Add thin tension members to stabilize structures • i.e. guy wires, strips of sheetmetal • Use multiple smaller fasteners rather than one big one (did I say I like pop rivets?) • Design in mechanical fuses - a desired place for failure during excessive and unusual forces to avoid catastrophic failure • Crumple zones • Break-away parts - using weaker fasteners that can break (i.e. aluminum pop rivets)

16. Fabrication Processes • Laser cutting causes localized hardening of some metals • Use this to your benefit when laser cutting steel sprockets • Cold forming causes some changes in strength properties • Some materials get significantly weaker • Be aware of Aluminum grades and hardness's • Welding - should not be a problem if an experienced welder does it

17. Environmental Effects • UV exposure - causes some plastics to change their structure and become brittle • ie. Lexan, PVC • Cold temperatures - cause some materials, especially plastics to become brittle • Can cause damage when shipping from cold climates

18. Going Up • Arms • Vertical Lifts • Arms vs. Lifts • Passive Assistance

19. What is an “Arm”? • An “Arm” is a device for grabbing and moving objects using members that rotate about their ends

20. General Arm Advice • Thin Walled Tubing is your friend • 1/16 wall is a good compromise • Known good sources • Mcmaster.com • Onlinemetals.com • Airpartsinc.com

21. General Arm Advice • Every Pivot has to be engineered • reduce, reuse, recycle ;-) • Pivots on Pivots are confusing to drivers • Follow my own advice? • NO… …1996, 1997, 1999, 2000, 2001, 2003 (2/3rds) • “Virtual 4 bars” help, but are still confusing • Drive motors low with chain acting as “4 bar” • Advantage over real 4-bar: • low motor • range of motion • Think about operator interface – very important

22. General Arm Advice Feedback Control is HUGE • Measure Current Position • Set Desired Position • Calculate Error • Take Action Based on Error (Search Internet for PID control)

23. General Arm Advice • Software can only fix so much • Typical: Design for Stall Torque, live with free speed (try to limit in software – extremely hard) • Better: Design for Free Speed, verify that you have “enough” torque (try to limit torque in software – merely difficult) • Best: Do “Better”, but have a mechanical limit to stall torque – friction drive or slip clutch for example

24. General Arm Advice • You can calculate stress! • Pure Compression/Tension: F/A • Beam: Mc/I • Twisting: ?? • Buckling: ?? • Buy Beer & Johnston – Mechanics Text • 6 Degrees of Freedom – Consider them all • Design in “Fuseable Link”

25. Four Bar

26. Four Bar - Design Considerations • Pin Loadings can be very high • Watch for buckling in lower member • Counterbalance if you can • Keep CG aft

27. Vertical Lifts • Extension • Scissors

28. Extension

29. Scissors

30. Scissors vs. Extension • Advantages • Minimum retracted height - can go under field barriers • Disadvantages • Tends to be heavy to be stable enough • Doesn’t deal well with side loads • Must be built very precisely • Stability decreases as height increases • Loads very high to raise at beginning of travel • I recommend you stay away from this!

31. Extension - Design Considerations • Should be powered down as well as up • If not, make sure to add a device to take up the slack if it jams • Segments need to move freely • Need to be able to adjust cable length(s). • Minimize slop / freeplay • Maximize segment overlap • 20% minimum • more for bottom, less for top • Stiffness is as important as strength • Minimize weight, especially at the top

32. Extension - Rigging Cascade Continuous

33. Slider (Stage3) Stage2 Stage1 Base Extension - Rigging - Continuous • Cable Goes Same Speed for Up and Down • Intermediate Sections sometimes Jam • Low Cable Tension • More complex cable routing • The final stage moves up first and down last

34. Slider (Stage3) Stage2 Stage1 Base Extension - Rigging - Continuous- All Internal cabling • Even More complex cable routing • Cleaner and protected cables

35. Slider (Stage3) Stage2 Stage1 Base Extension - Rigging - Cascade • Up-going and Down-going Cables Have Different Speeds • Different Cable Speeds Can be Handled with Different Drum Diameters or Multiple Pulleys • Intermediate Sections Don’t Jam • Much More Tension on the lower stage cables • Needs lower gearing to deal with higher forces • I do not prefer this one!

36. Arms vs. Extension Lifts • Arms can reach over objects; lifts have limited reach • Arms can right a flipped Robot; lifts probably not • Arms can fold down to “limbo” under barriers; lift stay tall • Arms require complex controls and counter-balances; lifts use simple controls • Lifts maintain a better center of gravity over the base; arms do not - can cause tipping • Lifts can operate in confined spaces; arms need space to swing up • Lifts can reach to any height with minimal added complexity; arms need extra articulated joints to reach higher • Combo may be best in some cases

37. Passive Assistance

38. Braking - to Prevent Back-driving • Ratchet Device - completely lock in one direction in discrete increments - such as used in many winches • Clutch Bearing - completely lock in one direction • Brake pads - simple device that squeezes on a rotating device to stop motion - can lock in both directions • Disc brakes - like those on your car • Gear brakes - applied to lowest torque gear in gearbox • Note : any gearbox that cannot be back-driven is probably very inefficient

39. Handling Objects • Accumulators • Conveyors • Grippers • Latches & Grabbers

40. Accumulators • Accumulator = rotational device that pulls objects in • Types: • Horizontal tubes - best for gathering balls from floor or platforms • Vertical tubes - best for sucking or pushing balls between vertical goal pipes • Wheels - best for big objects where alignment is pre-determined • When it comes to gathering balls, there is nothing more efficient • If set up in the proper orientation, will not knock the ball away, just suck it in

41. Conveyors • Conveyor - device for moving multiple objects, typically within your robot • Types: • Continuous Belts • Best to use 2 running at same speed to avoid jamming • Individual Rollers • best for sticky balls that will usually jam on belts and each other • When it comes to gathering balls, there is nothing more efficient • If set up in the proper orientation, will not knock the ball away, just suck it in

42. Conveyors • Why do balls jam on belts? • Sticky and rub against each other as they try to rotate along the conveyor • Solution #1 • Use individual rollers • Adds weight and complexity • Solution #2 • Use pairs of belts • Increases size and complexity • Solution #3 • - Use a slippery material for the non-moving surface (Teflon sheet works great)

43. Grippers

44. General Arm Advice • Rolling balls into and out of gripper can be VERY Effective • Examples Off the top of my head: • Team 222 in 1996 • Team 177 in 1998 • Team 95 in 1998 • Team 45 in 2004 • Team 111 in 2004

45. Latches / Grabbers

46. Other Clever Mechanisms • Wonderful Uses for Spectra cable • Chain turnbuckle • x

47. Wonderful Uses for Spectra Cable • First you must learn to tie a proper knot in this stuff • I use a “triple pretzel knot” (I doubt you will find this name in any scouting book - I made it up) : • Simple lift cables - pretty obvious use, but how do you adjust the slack (steel cables use turnbuckles)? • Use a tourniquet like device - use a dowel pin to twist the cable on the outside of the spool or actuated device, and tie-wrap in place • This works great for adjusting the location of travel also • If slack can occur, add a latex slack tensioner • Remote actuations - this cable is so easy to route within your robot frame efficiently • Linear motions (come see team 111 bumper actuation) • Rotary motions

48. Spectra Cable (cont’d) Remote Rotary Actuations - instead of chain

49. Chain Turnbuckle • Parts Needed: • 1/2” Sq Aluminum bar • 1/4-20 Nut • 1/4-20 Screw • 3/8” dia. CRS rod • 1/16” dia. Steel Dowel pins Dowel Pins 1/4-20 Screw (grind flats) 1/4-20 Nut 1/2 Alum Sq Bar 3/8 Dia. Rod

50. Pneumatics vs. MotorsSome, but not all important differences • Cylinders use up their power source rather quickly • the 2 air tanks we are allowed do not hold much • Motors use up very little of the total capacity of the battery • Cylinders are great for quick actuations that transition to large forces • Motors have to be geared for the largest forces • Our ability to control the position of mechanisms actuated by cylinders is very limited • We are not given dynamic airflow or pressure controls • We are given much more versatile electronic controls for motors • Since air is compressible, cylinders have built-in shock absorption • Cylinders used with 1-way valves are great for Armageddon devices - stuff happens when power is shut off • This could be good or bad - use wisely