first robotics - frc mechanical presentation
DESCRIPTION
Prepared by Steve MartinTRANSCRIPT
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FIRST Robotics – FRC Mechanical Presentation
Steve MartinMentor FIRST Team 58Co-chair Pine Tree RegionalSouth Portland Package Technology DevelopmentFairchild Semiconductor
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Outline
• Keep it simple• Kitbot frame and how it is assembled• Definition of gears, sprockets and pulleys• Explanation of chains• Explanation of belts and what may be in the KOP this year• Discussion of chains vs belts (based on the Team 234 paper)• Wheel types • Discussion of lifts and manipulators and counter balancing of
arms
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Keep it simple
• A lot of information in this• Decide what you can do with the resources you have• Keep it simple• Keep it light!• A robot that does 1 thing well is better than a robot
that does 2 things poorly• Practice time is key so stop build early
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Kitbot frame pieces
Kitbot frame piecesSix - C-Channel Eight - C-Base Corner ConnectsSix - 3/8-16 x 7" Hex Head BoltsSix - 3/8-16 Nylock NutsThirty Two - individual 1/4-20 x 1.75" Socket Head Cap Screws (Order 50)Thirty Two - individual 1/4-20 Nylock Nuts (Order 100)
C channel
Corner connects
¼-20x1.75 socket cap screws to attach corner connects and c channel
3/8-16x7” Hex head bolts for wheel shafts
AndyMark Kitbot build file
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Kitbot frame assembled
Assembled frame with wheels and motors
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Kitbot frame
• Robot max dimensions are typically 28” x 38” x 60” • Check the rules for 2013 dimensions!!!!!!!• Design your frame to be 1” smaller
• Design to 27”x37”x59”• Frames are not always square• Frames don’t stay square if they started that way• Bolts heads, nuts and other pieces will stick out
beyond the edge of the frame and COUNT as your max dimension
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Ground clearance
• How far above the ground should your frame be?• Depends on the competition field
• Is the field flat?• Do you have to climb over barriers?• Do you have to go up ramps?• Do you need to keep something from going under the
robot?• Are there mechanisms that need certain height for
the frame?• Height for ball to be loaded• Frame height to get arm at given height
2012 ramp and frame height
5.8Ø6.0
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Ground clearance
• Do you adjust wheel diameter or use wheel mounts?• Consider cost and weight
• Larger wheels are more expensive• Only gain 1” height change for 2” diameter change• Supports require good mounting
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Frame blocks
• The first time you run your robot it may not behave as expected!!!
• Before you run your robot for the first time set it on blocks so the wheels are off the ground
• Verify wheel motion is correct before setting the robot on the ground!
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Power transmission
• Power is transmitted from a motor to wheels, arms or other items
• How to you connect a motor to something you want to move?
• Power could be transmitted directly by coupling motor shaft to a wheel but typically need to change• speed and torque or • direction or • act away from the motors
• The change is primarily through the use of gears, sprockets or belts or some combination
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Gears, sprockets, pulleys
• definition of gears (regular and miter), sprockets and pulleys
• how gear ratios work• example with drive• example with arm (this years robot)
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sprockets
• Used with chain• Different pitches
• 25 pitch • Lighter but not as
strong• 35 pitch (3/8
inch)• Stronger but
heavier
• Items can be attached to the chain
25 pitch chain sprockets
35 pitch chain sprockets
Images from AndyMark
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gears
Hex shaft or keyed to connect shaft to gear
Worm gear
Changes direction of power transmission
Miter gear
Spiral gear
Flat gear
Images from AndyMark or Boston Gear
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Pulleys for synchronous belts
• Used with belts
• Different pitches and widths
http://robotics.gatesprograms.com/resources
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Pulleys for non synchronous belts
• Used with belts
• Continuos belts
• Polyurethane belting
http://robotics.gatesprograms.com/resources
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Motor
Center Distance
(center shaft to center shaft)
Take-up/Installation Allowance
Shaft Sizefor driveN
driveR driveNShaft Sizefor driveR
HP, RPM, Efficiency
Backside Idler
Speed/Torque Ratio
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Slow DownMore torque
1:1
Speed UPLess torque
R N
R N
RR N
R = driveR/motorR = driveN/wheel
Speed/Torque Ratio
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Slow Down
R N R NRR N
1:1 Speed up
Diameters 5” 10” 8” 8” 10” 5”
RPM’s 1000 500 1000 1000 1000 2000
Torque (in-lb) 20 40 20 20 20 10
HP 0.3 0.3 0.3 0.3 0.3 0.3
Speed/Torque Ratio Cont.
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Chain vs belts vs gears
• Advantages/disadvantages of chain• Advantages/disadvantages of belts• Comparison of sprockets, gears and pulleys
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Chain
• Advantages• Any length possible• Easy installation• Chains can be put on
without removing wheels
• Little tension required• Disadvantages
• Lubrication required• Messy Environment• Reduced backlash
characteristics• Heavy
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Pulleys and belts
• Advantages• Lightweight• Power dense• Minimal backlash• No lubrication, clean
running• Disadvantages
• Set lengths, can’t break apart
• Have to remove axles to put belts on
• Tension required• Bigger pulleys needed
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Gears
• Advantages• Compact• Power dense• Minimal backlash• Best if used next to
motor and in combination with belts or chains
• Disadvantages• Set distance• Requires lubrication• Heavy if steel gears
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Comparison gears, sprockets, pulleys
Feature Sprockets/chain Belt/Pulleys Gears
Weight Some what heavyLength dependent
Lighter than chain Typically steel so heavy
Distance Driver can be far from driven
Driver can be far from driven
Driver must be near to driven
Rotation direction Driver and driven rotate same direction
Driver and driven typically rotate same direction but belt can be twisted to cause rotation in opposite direction
Each stage rotates in opposite direction
Flexibility Can be any length and can be adjusted
Fixed lengthsCan’t be adjustedChanging pulley size may require different belt
Fixed spacing base on gear sizes
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Kit of Parts Belts
• Expect kits will be similar except for pulley instead of sprockets and belts instead of chain
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Team 234 Belt Drive conclusions
• Based on the results from this testing, the following high level conclusions can be drawn:
• 1. A belt drive system could save approximately 2 pounds on a six wheel drive.
• 2. The Belt drive system is approximately 3 – 4% more efficient. • 3. The belt drive system was approximately 6% faster to a set
distance. • 4. The belt drive system traveled about 8% further for a given
time of power application. • 5. There was no measurable difference in belt or chain stretch
when pulled under load. • 6. There was no noticeable difference in noise level of belt or
chain. • 7. Chain is overall simpler to work with from a design / repair
perspective.
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Wheels
• Wheel types • Wheel chair
• Reasonable traction
• Traction• Good traction • Harder to turn
• Omni• Good traction in rotation direction but can slide ways• Good for steering with center drive wheels• Can be easily pushed sideways
• Mecanum• Rollers at 45 deg to direction of rotation• Allows robot to move sideways or at angle• Need set of 4• Very maneuverable but can be easily pushed• Good where alignment to target is needed• Not good if pushing or resistance to pushing needed• Add torque boost system
• Wheel diameter • Typically 4,
6, 8 or 10inch
• Height will depend on competition and ground clearance
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Manipulators
Articulating ArmsTelescoping LiftsGrippersLatchesTurretsBall Handling SystemsShootersWinches
Information from Andy BakerMech. Engineering Mentor: 45 (1998-present)President and Co-owner: AndyMark, Inc.2003 Championship Woodie Flowers Award
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Four Bar Linkage
• Pin loadings can be very high• Watch for buckling in lower member• Counterbalance if you can• Keep Center of Gravity aft• Limited rotation• Keeps gripper in known location
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Telescoping Lifts
Extension LiftMotion achieved by stacked members sliding on
each otherScissor Lift
Motion achieved by “unfolding” crossed members
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Extension - Rigging
• Continuous • Cascade
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Scissor Lifts
AdvantagesMinimum retracted height - can
go under field barriersDisadvantages
Tends to be heavy to be stable enough
Doesn’t deal well with side loadsMust be built very preciselyStability decreases as height
increasesLoads very high to raise at
beginning of travel
I recommend you stay away from this!
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Arm vs. Lift
Feature Arm LiftReach over object Yes No
Fall over, get up Yes, if strong enough No
Go under barriers Yes, fold down Maybe, limits lift height
Center of gravity (Cg) Not centralized Centralized massSmall space operation No, needs swing room Yes
How high? More articulations, more height (difficult)
More lift sections, more height (easier)
Complexity Moderate High
Powerful lift Moderate High
Combination Insert 1-stage lift at bottom of arm
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Gripper (FIRST definition):Device that grabs a game object
How to gripHow to hang onSpeedControl
Grippers
254 in 2008
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Pneumatic linkage grip1 axis2 axis
Motorized gripRoller gripHoop gripPneumatic grip
How to grip
768 in 2008
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• Pneumatic Cylinder extends & retracts linkage to open and close gripper
• Easy to manufacture• Easy to control• Quick grab• Limited grip force• Requires pneumatic
system
Recommended
Pneumatic linear grip
968 in 2004
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Pneumatic Cylinder, pulling 3 fingers for a 2-axis grip
Recommended
Pneumatic linear grip
60 in 2004
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SlowMore complex
(gearing)HeavierTunable forceNo pneumatics
Motorized Linear Grip
49 in 2001
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Allows for misalignment when grabbing
Won’t let goExtends object as releasingSimple mechanismHave a “full in” sensorSlow
Recommended
Roller Grip45 in2008
148 in 2007
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SlowNeeds alignedCan’t hold on
well
Hoop grip
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Needs vacuum generator
Uses various cups to grab
SlowNot secureNot easy to
controlSimpleProblematic
Not recommended
Pneumatic Grip
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Speed
Quickness covers mistakesQuick to grabDrop & re-grab
FastPneumatic gripper
Not fastRoller, motor gripper, vacuum
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Accumulator: rotational device that collects objectsHorizontal tubes: gathers balls from floor or
platformsVertical tubes: pushes balls between vertical
goal pipesWheels: best for big objects
Ball Systems
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Conveying & Gathering
Conveyor - device for moving multiple objects, typically within your robot
Continuous Belts Best to use 2 running at same speed to avoid
jammingIndividual Rollers
Best for sticky balls that will usually jam on belts and each other
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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)
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Ball System Tips
More control is betterAvoid gravity feeds – these WILL jamTry to reduce “random” movements
Not all Balls are created equalBalls tend to change shape Building adaptive/ flexible systems
Speed vs. VolumeOptimize for the game and strategyThe more capacity, the better (?)
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Roller example: 188
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Intake roller example: 173 & 254
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Ball Shooter Systems
• Secure shooting structure = more accuracy • Feed balls individually, controlling flow• Rotating tube or wheel
• One wheel or two• 2006: 2000-4000 rpm• Protect for safety
• Turret allows for aiming• Sensors detect ball presence
& shot direction
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Winches
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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
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Power = Torque/ TimeOR
Power = Torque x Rotational VelocityPower (FIRST definition) – how fast you can
move something
Power
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Same torque w/ Twice the Power results in Twice the Speed
Power = Torque/ Time
Arm: Power Example
125 Watts, 100 RPM
250 Watts, 200 RPM
10 lbs10 lbs
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SummaryAll motors can lift the same amount (assuming
100% power transfer efficiencies) - they just do it at different rates
No power transfer mechanisms are 100% efficient Inefficiencies (friction losses, binding, etc.)Design in a Safety Factor (2x, 4x)
Power
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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 directionsDisc brakes - like those on your carGear brakes - applied to lowest torque gear in gearbox
Dynamic Breaking in electrical components let go when power is lost
Any gearbox that cannot be back-driven alone is probably very inefficient
Braking: Prevent Back-driving
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Design is an Iterative Process
Final Design
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Keep it simple
• A lot of information in this• Decide what you can do with the resources you have• Keep it simple• A robot that does 1 thing well is better than a robot
that does 2 things poorly• Practice time is key so stop build early
www.fairchildsemi.com57
References
• http://files.andymark.com/ManiupulatorDesign.pdf• http://files.andymark.com/FIRST-Robotics-Drive-Syste
ms.ppt• http://files.andymark.com/am-0952AssemblyInstructi
ons.pdf• http://robotics.gatesprograms.com/first• http://first.gatesprograms.com/assets/first/zips/FIRST.
zip• http://robotics.gatesprograms.com/video