Mechanisms

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Mechanisms Manipulators FRC Conference 4/21/05

• By Raul Olivera

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Some Basic Physics

• Forces, Angles Torque
• Power

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Forces, Angles Torque

• Example 1 - Lifting
• Same force, different angle,
• less torque

10 lbs
10 lbs
lt D
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Forces, Angles Torque

• Example 2 - Pulling on object
• One angle helps secure object
• The other does not

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Forces, Angles Torque

• Example 2 - Pulling on object (contd)

This one want to rotate clockwise and let go
This one want to rotate counter- clockwise and
grab even harder
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Power

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

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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
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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

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Structural Integrity

• Materials
• Shapes / Weights
• Fabrication processes
• Environment

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Materials

• Steel
• High strength
• Many types (alloys) available
• Heavy, rusts,
• Harder to processes with hand tools
• Aluminum
• Easy to work with for hand fabrication processes
• Light weight many shapes available
• Essentially does not rust
• Lower strength

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Materials

• Lexan
• Very tough impact strength
• But, lower tensile strength than aluminum
• Best material to use when you need transparency
• Comes in very limited forms/shapes
• PVC
• Very easy to work with and assemble prefab shapes
• Never rusts, very flexible, bounces back (when
  new)
• Strength is relatively low

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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

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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
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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

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Stress Calculations

• It all boils down to 3 equations

Bending
Tensile
Shear
Where ? Bending Stress M Moment (bending
force) 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
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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

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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)

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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

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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

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Going Up

• Arms
• Vertical Lifts
• Arms vs. Lifts
• Passive Assistance

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What is an Arm?

• An Arm is a device for grabbing and moving
  objects using members that rotate about their
  ends

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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

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General Arm Advice

• Every Pivot has to be engineered
• reduce, reuse, recycle -)
• Virtual 4 bars
• 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

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General Arm Advice

• Feedback Control is HUGE
• Measure Current Position
• Compare to Desired Position
• Calculate Error
• Take Action Based on Error (Search Internet for
  PID control)
• SW/Feedback cannot fix all control problems
  effectively

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Four Bar
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Four Bar - Design Considerations

• Pin Loadings can be very high
• Watch for buckling in lower member
• Counterbalance if you can
• Keep CG aft

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Vertical Lifts

• Extension
• Scissors

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Extension
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Scissors
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Scissors vs. Extension

• Advantages
• Minimum retracted height - can go under field
  barriers
• Disadvantages
• Needs to be heavy to be stable enough
• Doesnt 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!

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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

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Extension - Rigging
Cascade
Continuous
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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

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Extension - Rigging - Continuous- All Internal
cabling

• Even More complex cable routing
• Cleaner and protected cables

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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 Dont Jam
• Much More Tension on the lower stage cables
• Needs lower gearing to deal with higher forces
• I do not prefer this one!

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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

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Passive Assistance

• What is passive assistance?
• SPRINGS or BRAKES!
• Use Elastic Energy to your advantage.
• Surgical Tubing
• Constant Force Springs
• Gas Springs
• Torsion Springs
• Follow FIRST Regulations!

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Braking - to Prevent Back-driving

• Ratchet Device - completely lock in one direction
  in discrete increments ie. 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
• High ratio worm gear (window, van-door motors)
• Note any gearbox that cannot be back-driven is
  probably very inefficient

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Handling Objects (Balls)

• Accumulators
• Conveyors

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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

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Accumulator as Gripper

• 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

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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

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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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Other Clever Mechanisms

• Wonderful Uses for Spectra cable
• Chain turnbuckle

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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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Spectra Cable (contd)

• Remote Rotary Actuations - instead of chain

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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
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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

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Force Values
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Pneumatics

• Warning rated load is only for holding
• Actual power curve is much lower