FIRST Robotics - FRC Mechanical Presentation

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FIRST Robotics – FRC Mechanical Presentation

Steve MartinMentor FIRST Team 58Co-chair Pine Tree RegionalSouth Portland Package Technology DevelopmentFairchild Semiconductor


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


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


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

http://www.andymark.com/product-p/am-0202.htm









Kitbot frame assembled

Assembled frame with wheels and motors


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


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


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


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!


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


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)


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


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


Pulleys for synchronous belts

• Used with belts

• Different pitches and widths

http://robotics.gatesprograms.com/resources


Pulleys for non synchronous belts

• Used with belts

• Continuos belts

• Polyurethane belting

http://robotics.gatesprograms.com/resources


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


Slow DownMore torque

1:1

Speed UPLess torque

R N

R N

RR N

R = driveR/motorR = driveN/wheel

Speed/Torque Ratio


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.


Chain vs belts vs gears

• Advantages/disadvantages of chain• Advantages/disadvantages of belts• Comparison of sprockets, gears and pulleys


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


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


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


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


Kit of Parts Belts

• Expect kits will be similar except for pulley instead of sprockets and belts instead of chain


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.


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


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


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


Telescoping Lifts

Extension LiftMotion achieved by stacked members sliding on

each otherScissor Lift

Motion achieved by “unfolding” crossed members


Extension - Rigging

• Continuous • Cascade


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!


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


Gripper (FIRST definition):Device that grabs a game object

How to gripHow to hang onSpeedControl

Grippers

254 in 2008


Pneumatic linkage grip1 axis2 axis

Motorized gripRoller gripHoop gripPneumatic grip

How to grip

768 in 2008


• 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


Pneumatic Cylinder, pulling 3 fingers for a 2-axis grip

Recommended

Pneumatic linear grip

60 in 2004


SlowMore complex

(gearing)HeavierTunable forceNo pneumatics

Motorized Linear Grip

49 in 2001


Allows for misalignment when grabbing

Won’t let goExtends object as releasingSimple mechanismHave a “full in” sensorSlow

Recommended

Roller Grip45 in2008

148 in 2007


SlowNeeds alignedCan’t hold on

well

Hoop grip


Needs vacuum generator

Uses various cups to grab

SlowNot secureNot easy to

controlSimpleProblematic

Not recommended

Pneumatic Grip


Speed

Quickness covers mistakesQuick to grabDrop & re-grab

FastPneumatic gripper

Not fastRoller, motor gripper, vacuum


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


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


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)


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 (?)


Roller example: 188


Intake roller example: 173 & 254


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


Winches


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


Power = Torque/ TimeOR

Power = Torque x Rotational VelocityPower (FIRST definition) – how fast you can

move something

Power


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


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


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


Design is an Iterative Process

Final Design


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


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

http://files.andymark.com/ManiupulatorDesign.pdf

http://files.andymark.com/FIRST-Robotics-Drive-Systems.ppt

http://files.andymark.com/FIRST-Robotics-Drive-Systems.ppt

http://files.andymark.com/am-0952AssemblyInstructions.pdf

http://files.andymark.com/am-0952AssemblyInstructions.pdf

http://robotics.gatesprograms.com/first

http://first.gatesprograms.com/assets/first/zips/FIRST.zip

http://first.gatesprograms.com/assets/first/zips/FIRST.zip

FIRST Robotics - FRC Mechanical Presentation

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