FIRSTFare 2012 Manipulators For FIRST FRC Robotics


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Compilation of approaches used for FIRST robotics game piece manipulating systems. Will briefly discuss pro’s, con’s and design trade offs for a number of different manipulator designs.

FIRSTFare 2012 Manipulators For FIRST FRC Robotics

  1. 1. Manipulators for FIRST FRC Robotics FIRST Fare 2012 Bruce Whitefield Mentor, Team 2471
  2. 2. Manipulate What ?Game pieces come in many sizes and shapes
  3. 3. Hanging Manipulate How ?Game objectives change each yearLifting Dumping ThrowingKicking Gathering
  4. 4. Stuff That Don’t WorkKnow the possibilities Things that ManipulateLook on line, at competitions Manipulators thatTalk to mentors, teams work for the gameThis seminarKnow the ObjectivesGame piecesGame & robot rulesYour game strategy Your DesignKnow your capabilitiesTools, Skills, Materials,Manpower , Budget , Time Manipulators you can build Don’t be this team
  5. 5. Many Types of Manipulators Heavy lifting / Long reach Reoccurring o Articulating Arms o Parallel arms themes in FIRST o Telescoping Lifts games Grippers o Rollers o Clamps o Claws FIRST definition for Collect and Deliver a manipulator: o Accumulators & Conveyers o Turrets o Shooters & Kickers o Buckets & Tables Device that Power & Control moves the game o Winches piece from where o Brakes o Latches it is to where it o Pneumatics o Springs and Bungee needs to be o Gears & Sprockets
  6. 6. ArmsShoulderElbowWrist
  7. 7. Torque: Angle and Distance Torque = Force x Distance o Same force, different angle = different torque o Measure from the pivot point o Motor & gearing must overcome torque W= 10 lbs o Maximum torque at 90 degrees Torque = W x D Torque = W x D/2 W=10 lbs 1/2 D D
  8. 8. Power: Torque and Speed Power determines how fast you can move things Power = Torque / Time or Torque x Rotational Velocity Same torque with 2x Power = 2x Speed Or twice the arm length at same speed …. 3 ft Power 10 lbs 20 lbs 10 lbs trade 30 ft-lbs 60 ft-lbs offs 425 100 RPM 50 RPM Power limits arm length. Watts 1.6 rps .83 rps Probably don’t want a 10 ft arm 42.5 10 RPM 5 RPM whipping around at 1000 RPM Watts .16 rps .08 rps anyway…
  9. 9. Arm Design TipsLightweight Materials: tubes, thin wallWatch elbow and wrist weight at ends of armSensors for feedback & control  limit switches, potentiometers, encodersLinkages help control long armsKISS your arm  Less parts… to build or break  Easier to operate  More robustCounterbalance  Spring, weight, pneumatic, bungee…Calculate the forces  Watch out for Center of gravity  Sideways forces when extendedModel the reach & orientation
  10. 10. Telescoping LiftsExtension Lifts  Motion achieved by stacked members sliding on each otherScissor Lift  Motion achieved by “unfolding” crossed members
  11. 11. Extension Lift Rigging Continuous Cascade The final stage moves up first and down last. Previous stage speed plus own speedo Power vs. speed equations apply. Don’t underestimate the power.o Often needs brakes or ratchet mechanism
  12. 12. Cascade Rigging Up-pulling and down-pulling cables have different speeds Different cable speeds can be Slider handled with different drum (Stage3) diameters or multiple Pulleys Intermediate sections don’t jam Stage2 – active return High tension on the lower stage cables Stage1 Base
  13. 13. Continuous Rigging Slider Cable goes same speed for (Stage3) up and down pulling cables Intermediate sections sometimes jam Stage2 Lower cable tension More complex cable routing Stage1 Base
  14. 14. Continuous Rigging Internal Pull-down cable routed back on reverse route as pull-up cable Most complex cable routing Slider All stages have active return (Stage3) Cleaner and protected cables Stage2 Stage1 Base
  15. 15. Extension Lift Design tips Use cables to drive up AND down, or add a cable recoil device Segments must move freely Cable lengths must be adjustable Minimize slop and free-play Maximize segment overlap  20% minimum  More for bottom, less for top Stiffness and strength needed Heavy system, overlapping parts Minimize weight, especially at top
  16. 16. Arms vs. LiftsFeature Arm LiftReach over object Yes NoFall over, get up Yes, if strong enough NoComplexity Moderate HighWeight capacity Moderate HighGo under barriers Yes, fold down Maybe, limits lift heightCenter of gravity () Cantilevered Central massOperating space Large swing space CompactAdding reach Difficult. More Easier. articulations More lift sectionsCombinations Arm with extender Lift with arm on top
  17. 17. Combination Example:  Continuous direct drive chain runs stage 1 up and down  No winch drum needed  Cascade cable lifts slider stage  Gravity return  Got away with this only because slider GOG very well balanced on first stage  Telescoping arm with wrist on slider stage to reach over objects2471 in 2011
  18. 18. Get a GripFIRST definition of a gripper:Device that grabs a game object…and releases it when needed.Design Concerns MethodsGetting object into grip Pneumatic claws /clampsHanging on  1 axis  2 axisSpeed of grip and release Motorized claw or clampPosition control RollersWeight and power source Hoop grips  If at end of arm Suction
  19. 19. Claw or clampPneumaticOne fixed armreduce weight of clawCan make one or two moving sides 768 in 2008
  20. 20. Pneumatic: 2 and 3 point clamps Pneumatic Cylinder extends & retracts linkage to open and close gripper Combined arm and gripper Easy to manufacture Easy to control Quick grab Limited grip force Use 3 fingers for 2-axis grip 968 in 2004 60 in 2004
  21. 21. Motorized clampGenerally slower  May be too slow for frequent grips  Okay if few grabs per game of heavy objectsMore complex (gearing & motors)HeavierTunable forceNo pneumatics 49 in 2001
  22. 22. Suction Grips  Needs vacuum generator  Uses various cups to grab  Slow  Not secure  Not easy to control  Simple  Subject to damage of suction cup or game pieces Not recommended for heavy game pieces Used successfully to hold soccer balls for kickers (Breakaway 2010)
  23. 23. Roller grips Allows for misalignment when grabbing Won’t let go Extends object as releasing Simple mechanism Have a “full in” sensor Many possible variations  Mixed roller & conveyer 45 in 2008  Reverse top and bottom roller direction to rotate object 148 in 2007
  24. 24. Counter Rotating MethodsCrossed round belts  Many ways to achieve counter rotating shafts. Here are few configurations that can run off a single motor or gearbox.  Could also drive each shaft with own motor Counter Stacked pulleys rotating on single drive gearbox shaft
  25. 25. Gripper designHang On!  High friction needed  Rubber, neoprene, silicone, sandpaper … but don’t damage game object  Force: Highest at grip point  Force = 2 to 4 x object weight  Use linkages and toggles for mechanical advantage  Extra axis of grip = More controlThe need for speed  Wide capture window  Quickness covers mistakes  Quick to grab , Drop & re-grab  Fast : Pneumatic gripper. Not so fast: Motor gripper  Make it easy to control  Limit switches , Auto-functions  Intuitive control functions  Don’t make driver push to make the robot pull
  26. 26. Rotating Turrets Tube or post (recommended) Lazy Susan (not for high loads) Know when it is needed o One Goal = good o Nine Goals = not so good o Fixed targets = good o Moving targets = not so good Bearing structure must be solid Rotation can be slow Include sensor feedback o Know which way it is pointing
  27. 27. Accumulator: Rotational device that collects objects  Horizontal rollers: gathers balls from floor or platforms  Vertical rollers: pushes balls up or down  Wheels: best for big objects  Can use to dispense objects out of robot  Pointing the robot is aiming method in this case
  28. 28. Conveyers: For moving multiple objects Point to point within your robot When game involves handling many pieces at onceWhy do balls jam on belts?- Stick and rub against each other as they try to rotate along the conveyorSolution #1- Use individual rollers- Adds weight and complexitySolution #2- Use pairs of belts- Increases size and complexitySolution #3- Use a slippery material for the non-moving surface (Teflon sheet works great)
  29. 29. Conveyer roller ExamplesRollers Belts TowerSolution 1 Solution 2 Solution 3
  30. 30. More control is better  Avoid gravity feeds – these are slow and will jam  Direct the flow: reduce “random” movements.Not all game objects are created equal  Tend to change shape, inflation, etc  Building adaptive/ flexible systems  Test with different sizes, inflation, etc.Speed vs. Volume  Optimize for the game and strategy  The more capacity, the better
  31. 31. Intake Rollers and Accumulators173 2471
  32. 32. Secure shooting structure = more accuracyFeed balls individually, controlling flowRotating tube or wheel  One wheel or two counter rotating 1771 in 2009  High speed & power: 2000-4000 rpm  Brace for vibration  Protect for safetyTurret allows for aimingSensors detect ball presence & shot direction Note Circular Conveyer. One roller on inside cylindrical rolling surface outside
  33. 33. Use for dumping many objectsIntegrate with your accumulator and conveyerHeavy ones may move too slowUsually pneumatic powered but can run with gear, spring or winch 488 in 2009
  34. 34. 2010Many uses  Hanging Robots: 2000, 2004, 2010  Lifting Robots: 2007  Loading kickers 2010Great for high torque applicationsFits into limited spaceEasy to route or rerouteGood Pull. Bad Push 2004
  35. 35. Capture the cableSecure the cable routing Smooth winding & unwinding Leave room on drum for wound up cable Guide the cable Return cable or spring Must have tension on cable to unwind  Can use cable in both directions Guide  Spring or bungee return the Maintain  Gravity return (not recommended) cable Tension Calculate the torque and speed Use braking devices Brake or ratchet
  36. 36. Sudden release of powerUse stored energy:  Springs Bungee, PneumaticDesign & test a good latch mechanism  Secure lock for safety  Fast release1 per game deployments  2011 minibot release
  37. 37.  Hooking and latching devices used to grab goals, bars, and other non- scoring objects Hold stored power in place  Spring or bungee power Several ways: Self latching wheel lock  Hooks  Locking wheels  Rollers Basic Automatic Latch  Pins Bevel for self latching  Springs Removable safety pin Small release force at end of lever Strong pivot in Spring line with large return and force being held stop
  38. 38.  Start design early. Latches tend to be afterthoughts but are often a critical part of the operation Don’t depend on driver to latch, use a smart mechanism  Spring loaded (preferred)  Sensor met and automatic command given  Use operated mechanism to let go, not to latch Have a secure latch  Don’t want release when robots crash Be able to let go quickly  Pneumatic lever  Motorized winch, pulling a string  Cam on a gear motor  Servo (light release force only) Don’t forget a safety pin or latch for when you are working on the robot
  39. 39. Look around see what works and does not workKnow your design objectives and game strategyStay within your capabilitiesDesign before you build  Calculate the forces and speeds  Understand the dimensions using CAD or a modelKeep it simpleMake it well  Poor craftsmanship can ruin the best designAnalyze for failure points.  Use to refine design and decide on spare partsHave fun
  40. 40. Many thanks to teams and companies who made materials for this presentation freely available on web sites to help FIRST students.  Andy Baker : Team 45  Jason Marr : Team 2471  Wildcats: Team 1510  The Flaming Chickens: Team 1540  Society of robots Andy Baker’s original presentation and inspiration for this seminar is available on line There are many examples and resources available. Be sure to use them for your robot designs.
  41. 41. Appendix
  42. 42. Motor Power:Assuming 100% power transfer efficiency:All motors can lift the same amount they just do it at different rates.No power transfer mechanisms are 100% efficient  Inefficiencies due to friction, binding, etc.  Spur gears ~ 90%  Chain sprockets ~ 80% It adds up! 2 spur gears + sprocket =  Worm gears ~ 70% .9 x.9 x.8 = .65  Planetary gears ~80% Losing 35% of power to the drive train  Calculate the known inefficiencies and then design in a safety factor (2x to 4x)Stall current can trip the breakers
  43. 43. Parallel Arms• Pin loading can be very high• Watch for buckling in lower arm• Has limited range rotation• Keeps gripper in fixed orientation
  44. 44. Scissor Lifts Advantages  Minimum retracted height - can go under field barriers Disadvantages  Tends to be heavy when made stable enough  Doesn’t deal well with side loads  Must be built very precisely  Stability decreases as height increases  Stress loads very high at beginning of travel Not recommend without prior experience
  45. 45.  Pneumatic latch, solidly grabs pipe Force and friction only No “smart mechanism”  Spring-loaded latch  Motorized release  Smart Mechanism 469 in 2003
  46. 46. Parallel arm Fixed ArmJointed Arm
  47. 47.  Ratchet - Complete lock in one direction in discrete increments Clutch Bearing - Completely lock in one direction any spot Brake pads - Squeezes on a rotating device to stop motion - can lock in both directions. Simple device  Disc brakes - Like those on your mountain bike  Gear brakes - Apply to lowest torque gear in gearbox  Belt Brake- Strap around a drum or pulley Dynamic Breaking by motors lets go when power is lost.  Use for control, but not for safety or end game  Gearbox that cannot be back-driven is usually an inefficient one.