First fare 2011 manipulators
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Transcript of First fare 2011 manipulators
Bruce Whitefield Mentor, Team 2471
Manipulators for FIRST FRC Robotics
� Game pieces come in many sizes and shapes
Game objectives change each year Manipulate How ?
Kicking
Lifting Dumping
Hanging
Throwing Gathering
Know the Objectives Game pieces Game & robot rules Your game strategy
Things that Manipulate
Manipulators that work for the game
Know the possibilities Look on line, at competitions Talk to mentors, teams This seminar
Know your capabilities Tools, Skills, Materials, Manpower , Budget , Time Manipulators you
can build
Your Design
� Heavy lifting / Long reach o Articulating Arms o Parallel arms o Telescoping Lifts
� Grippers o Rollers o Clamps o Claws
� Collect and Deliver o Accumulators & Conveyers o Turrets o Shooters & Kickers o Buckets & Tables
� Power & Control o Winches o Brakes o Latches o Pneumatics o Springs and Bungee o Gears & Sprockets
Many Types of Manipulators Reoccurring
themes in FIRST games
FIRST definition for
a manipulator:
Device that moves the game piece from where
it is to where it needs to be
Shoulder Elbow Wrist
Articulating Arms
� Torque = Force x Distance o Same force, different angle = different torque o Measure from the pivot point o Motor & gearing must overcome torque o Maximum torque at 90 degrees
W=10 lbs
W= 10 lbs
1/2 D
D
Torque = W x D
Torque: Angle and Distance
Torque = W x D/2
� 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 ….
10 lbs Power trade offs
10 lbs 30 ft-‐lbs
20 lbs 60 ft-‐lbs
425 Watts
100 RPM 1.6 rps
50 RPM .83 rps
42.5 Watts
10 RPM .16 rps
5 RPM .08 rps
3 ft
Probably don’t want a 10 ft arm whipping around at 1000 RPM anyway…
Power: Torque and Speed
� 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% � Worm gears ~ 70% � Planetary gears ~80%
� Calculate the known inefficiencies and then design in a safety factor (2x to 4x)
� Stall current can trip the breakers
Motor Power:
It adds up! 2 spur gears + sprocket = .9 x.9 x.8 = .65 Losing 35% of power to the drive train
• Pin loading can be very high • Watch for buckling in lower arm • Has limited range rotation • Keeps gripper in fixed orientation
Parallel Arms
Parallel arm Fixed Arm
Jointed Arm
� 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
Arm Design Tips
� Extension Lifts � Motion achieved by stacked members sliding on each other
� Scissor Lift � Motion achieved by “unfolding” crossed members
Telescoping Lifts
Continuous Cascade
Extension Lift Rigging
� The final stage moves up first and down last. Previous stage speed plus own speed
o Power vs. speed equations apply. Don’t underestimate the power.
o Often needs brakes or ratchet mechanism
� Up-pulling and down-pulling cables have different speeds
� Different cable speeds can be handled with different drum diameters or multiple Pulleys
� Intermediate sections don’t jam – active return
� High tension on the lower stage cables
Slider (Stage3)
Stage2
Stage1
Base
Cascade Rigging
� Cable goes same speed for up and down pulling cables
� Intermediate sections sometimes jam
� Lower cable tension � More complex cable routing
Stage2
Stage1
Base
Continuous Rigging Slider
(Stage3)
� Pull-down cable routed back on reverse route as pull-up cable
� Most complex cable routing � All stages have active return � Cleaner and protected cables
Slider (Stage3)
Stage2
Stage1
Base
Continuous Rigging Internal
� 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 are needed � Heavy system, overlapping parts � Minimize weight, especially at top
Extension Lift Design tips
� 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
Scissor Lifts
Feature Arm Lift Reach over object Yes No
Fall over, get up Yes, if strong enough No
Complexity Moderate High
Weight capacity Moderate High
Go under barriers Yes, fold down Maybe, limits lift height
Center of gravity () Cantilevered Central mass
Operating space Large swing space Compact
Adding reach Difficult. More articulations
Easier. More lift sections
Combinations Arm with extender Lift with arm on top
Arms vs. Lifts
Combinations Can work: � 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 objects
2471 in 2011
FIRST definition of a gripper: Device that grabs a game object …and releases it when needed.
Get a Grip
Methods � Pneumatic claws /clamps
� 1 axis � 2 axis
� Motorized claw or clamp � Rollers � Hoop grips � Suction
Design Concerns � Getting object into grip � Hanging on � Speed of grip and release � Position control � Weight and power source
� If at end of arm
� Pneumatic � One fixed arm � reduce weight of claw � Can make one or two
moving sides
768 in 2008
Claw or clamp
� 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
968 in 2004
Pneumatic linear: 2 point clamp
� Pneumatic Cylinder, pulling 3 fingers for a 2-axis grip
60 in 2004
Pneumatic linear 3 point 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
Motorized clamp
� 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 capture balls for kickers (Breakaway 2010)
Suction 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 � Reverse top and bottom roller
direction to rotate object
45 in 2008
148 in 2007
Roller grips
Hang 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 control
The 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
Gripper design
� 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 � Sensor feedback
o Know which way it is pointing
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
� Typically within your robot � When game involves handling
many pieces at once Why do balls jam on belts? - Stick 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)
Conveyer: For moving multiple objects
Conveyer roller Examples
� 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
Intake Rollers and Accumulators
173 2471
� Secure shooting structure = more accuracy � Feed balls individually, controlling flow � Rotating tube or wheel
� One wheel or two counter rotating � High speed & power: 2000-4000 rpm � Brace for vibration � Protect for safety
� Turret allows for aiming � Sensors detect ball presence & shot direction
1771 in 2009
Note Circular Conveyer. One roller on inside cylindrical rolling surface outside
� 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
� 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
� 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
2010
� Secure the cable routing � Smooth winding & unwinding � Leave room on drum for wound up cable � Guide the cable � Must have tension on cable to unwind
� Can use cable in both directions � Spring or bungee return � Gravity return (not recommended)
� Calculate the torque and speed � Use braking devices
Capture the cable
Maintain Tension
Guide the cable
Brake or ratchet
Retu
rn c
able
or
spring
� 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.
� 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: � Hooks � Locking wheels � Rollers � Pins � Springs
Self latching wheel lock
Basic Automatic Latch
Strong pivot in line with large force being held
Spring return and stop
Small release force at end of lever
Bevel for self latching
� Pneumatic latch, solidly grabs pipe
� Force and friction only � No “smart mechanism”
� Spring-loaded latch � Motorized release � Smart Mechanism
469 in 2003
� 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)
� 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 in CAD or 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
� 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.
http://www.societyofrobots.com/mechanics_gears.shtml