Building competitive manipulators

1
Building competitive manipulators

• Greg Needel
• DEKA RD, Rochester Institute of technology
• Owner, www.midnightinvention.com
• Mentor teams 131, 1511

2
Strategy, Strategy, Strategy!

• Read the rules
• Outline the game objectives
• Look for the gimmie robot design
• Try small simulators
• Whatever you choose STICK WITH IT!

3
Types of Manipulators

• Articulating Arms
• Telescoping Lifts
• Grippers
• Latches
• Ball Systems

4
Arm Forces, Angles Torque

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

10 lbs
10 lbs
lt D
5
Power

• Power Force x Distance / Time
• OR
• Power Torque x Rotational Velocity
• Power (FIRST def.) how fast you can move
  something

6
Arm Power Example

• Same torque, different speed

10 lbs
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
7
Arm Design

• Arm device for grabbing moving objects using
  members that rotate about their ends
• Think of your materials (thin wall is good)
• Every Pivot has to be engineered (less is more)
• Linkages help control long arms.
• Use mechanical advantage (it is your friend)
• Think of the drivers (pivots on pivots are hard)
• Operator Interface (keep this in mind)

8
Arm Advice

• K.I.S.S. doesnt mean bad
• Feedback Control is HUGE
• Potentiometers, encoders, limits
• Automatically Take Action Based on Error
• Design-in sensors from the start of design
• Think outside the box.
• Off the shelf components are good (andymark.biz,
  banebots.com )

9
Four Bar Linkage

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

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4 bar linkage example 229 2005
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Arm Example 234 in 2001
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Arm Example 330 in 2005
13
Arm Example 1114 in 2004
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Telescoping Lifts

• Extension Lift
• Scissor Lift

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Extension
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Extension Lift Considerations

• Should be powered down AND 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 / free-play
• 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 Continuous Rigging

• 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 Continuous Internal Rigging

• Even More complex cable routing
• Cleaner and protected cables

20
Extension Cascade Rigging

• 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!

21
Team 73 in 2005 elevator
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Scissor Lift
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Scissor Lift Considerations

• Advantages
• Minimum retracted height - can go under field
  barriers
• Disadvantages
• Tends 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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Team 158 in 2004
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Arm vs. Lift
Feature Arm Lift
Reach over object Yes No
Fall over, get back up Yes, if strong enough No
Go under barriers Yes, fold down No, limits lift potential
Center of gravity (Cg) Can move it out from over robot Centralized mass over robot
small space operation No, needs swing room Yes
How high? More articulations, more height (difficult) More lift sections, more height (easier)
Complexity Moderate High
Accumulation 1 or 2 at a time Many objects
Combination Insert 1-stage lift at bottom of arm lt-
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Braking Prevent Back-driving

• 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
  directions
• Disc brakes - like those on your car
• Gear brakes - applied to lowest torque gear in
  gearbox
• Note any gearbox that cannot be back-driven
  alone is probably very inefficient

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Power

• 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
• Inefficiencies (friction losses, binding, etc.)
• Design in a Safety Factor (2x, 4x)

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Grippers

• Gripper (FIRST def) grabbing game object
• How to grip
• How to hang on
• Speed
• Control

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How to grip

• Pneumatic linkage grip
• 1 axis
• 2 axis
• Motorized grip
• Roller grip
• Hoop grip
• Pneumatic grip

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Pneumatic linear grip

• Pneumatic Cylinder extends retracts linkage to
  open and close gripper
• 254 robot 2004, 1-axis
• 968 robot 2004, 1-axis
• Recommended

31
Pneumatic linear grip

• Pneumatic Cylinder, pulling 3 fingers for a
  2-axis grip
• 60 in 2004
• Recommended

32
Motorized Linear Grip

• Slow
• More complex (gearing)
• Heavier
• Doesnt use pneumatics
• 49 in 2001
• Not
• recommended

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Roller Grip

• Slow
• Allows for misalignment when grabbing
• Wont let go
• Extends object as releasing
• Simple mechanism
• 45 in 98 and 2004
• Recommended

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Hoop grip

• Slow
• Needs aligned
• Cant hold on well
• 5 in 2000
• Not
• recommended

35
Pneumatic Grip

• Vacuum
• generator cups to grab
• Slow
• Not secure
• Not easy to control
• Simple
• Problematic
• Not recommended

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Hang on!

• Friction High is needed (over 1.0 mu)
• Rubber, neoprene, silicone, sandpaper
• Force Highest at grip point
• Force multiple x object weight (2-4x)
• Linkage, toggle mechanical advantage
• Extra axis of grip More control

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Speed

• Quickness covers mistakes
• Quick to grab
• Drop re-grab
• Fast
• Pneumatic gripper
• Not fast
• Roller, motor gripper, vacuum

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Grip control

• Holy grail of gripping
• Get object fast
• Hang on
• Let go quickly
• This must be done under excellent control
• Limit switches
• Auto-functions
• Ease of operation

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Latches

• Spring latches
• Hooks / spears
• Speed Control

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Latch example 267

• Pneumatic Latch
• 2001 game
• Grabs pipe
• No smart mechanism

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Latch example 469

• Spring-loaded latch
• Motorized release
• Smart Mechanism
• 2003

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Latch example 118

• Spring-loaded latch
• Pneumatic release
• Smart mechanism
• 2002

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Latching advice

• Dont depend on operator to latch, use a smart
  mechanism
• Spring loaded (preferred)
• Sensor met and automatic command given
• Have a secure latch
• Use an operated mechanism to let go
• Be able to let go quickly
• Pneumatic lever
• Motorized winch, pulling a string

44
Ball Systems

• 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

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Conveying Gathering

• 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

46
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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Roller example 188
48
Accumulator example 173 254
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Questions?

• Thanks to
• Andy Baker (45)
• www.chiefdelphi.com
• www.robotphotos.org
• www.firstrobotics.net
• www.firstrobotics.uwaterloo.ca

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Extra Stuff

• Pneumatics vs. Motors
• Materials
• Shapes / Weights
• Fabrication processes
• Environment

51
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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Materials

• Aluminum, thin-wall tubing
• Polycarbonate sheet, PVC tubing
• Fiberglass (used rarely, but advantages)
• 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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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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Shapes, cont.

• 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 (calculated
earlier) 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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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