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Robotic Arms vs. Lifts. What is an Arm?. A device for grabbing & moving objects using members that rotate about their ends. What is a Lift?. A device for grabbing and moving objects in a predominately vertical direction. Relative Advantages of Arms Over Lifts. Very flexible

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what is an arm
What is an Arm?

A device for grabbing & moving objects using members that rotate about their ends

what is a lift
What is a Lift?

A device for grabbing and moving objects in a predominately vertical direction

relative advantages of arms over lifts
Relative Advantages ofArms Over Lifts
  • Very flexible
  • Can right a flipped robot
  • Can place object in an infinite number of positions within reach
  • Minimal height - Great for going under things
relative advantages of lifts over arms
Relative Advantages ofLifts Over Arms
  • Typically simple to construct
  • Easy to control (don’t even need limit switches)
  • Maintain CG in a fixed XY location
  • Don’t require complex gear trains
articulating arm
Articulating Arm
  • Shoulder
  • Elbow
  • Wrist
arm forces angles torque

D

Arm: Forces, Angles, & Torque
  • Example: Lifting at different angles
  • Torque = Force x Distance
  • Same force, different angle, less torque

10 lbs

10 lbs

< D

arm power
Arm: Power
  • Power = Torque / Time
    • OR –
  • Power = Torque x Rotational Velocity
  • Power (FIRST definition): How fast you can move something
arm power9
Arm: Power
  • Example: Lifting with different power output
  • Same torque with twice the power results in twice the speed
  • Power = Torque / Time

10 lbs

10 lbs

125 Watts,

100 RPM

250 Watts,

200 RPM

arm design considerations
Arm: Design Considerations
  • Lightweight Materials: tubes, thin wall sheet
  • Design-in sensors for feedback & control
    • limit switches and potentiometers
  • Linkages help control long arms
  • KISS
    • Less parts… to build or break
    • Easier to operate
    • More robust
  • Use off-the-shelf items
  • Counterbalance
    • Spring, weight, pneumatic, etc.
types of lifts
Types of Lifts
  • Elevator
  • Forklift
  • Four Bar (can also be considered an Arm)
  • Scissors
elevator advantages disadvantages
Elevator: Advantages & Disadvantages
  • Advantages
    • Simplest structure
    • On/Off control
    • VERY rigid
    • Can be actuated via screw, cable, or pneumatics
  • Disadvantages
    • Single-stage lift
    • Lift distance limited to maximum robot height
    • Cannot go under obstacles lower than max lift
elevator design considerations
Elevator: Design Considerations
  • Should be powered down as well as up
  • Slider needs to move freely
  • Need to be able to adjust cable length--a turnbuckle works great
  • Cable can be a loop
  • Drum needs 3-5 turns of excess cable
  • Keep cables or other actuators well protected
elevator calculations
Elevator: Calculations
  • Fobject = Weight of Object + Weight of Slider
  • Dobject = Distance of Object CG
  • Tcable= Fobject
  • Mslider = Fobject• Dobject
  • Fslider1 = - Fslider2 = Mslider / 2Dslider
  • Fpulley = 2 Tcable
  • Fhit = (Weight of Object + Weight of Slider) • G value [I use .5]
  • Mhit = Fhit • Hslider
  • Mbase = Mslider + Mhit

Fpulley

Mslider

Fobject

Fslider1

Fhit

Dobject

Dslider

Fslider2

Tcable

Hslider

Mbase

forklift advantages disadvantages
Forklift: Advantages & Disadvantages
  • Advantages
    • Can reach higher than you want to go
    • On/Off control
    • Can be rigid if designed correctly
    • Can be actuated via screw, cable, or pneumatics, though all involve some cabling
  • Disadvantages
    • Stability issues at extreme heights
    • Cannot go under obstacles lower than retracted lift
forklift design considerations
Forklift: Design Considerations
  • Should be powered down as well as up
  • Segments need to move freely
  • Need to be able to adjust cable length(s).
  • Two different ways to rig (see later slide)
  • MINIMIZE SLOP
  • Maximize segment overlap
  • Stiffness is as important as strength
  • Minimize weight, especially at the top
forklift calculations

Mslider

Forklift: Calculations

Fobject

Fslider1

Fhit

Dobject

Dslider

Fslider2

Hupper

Fupper1

  • Fobject = Weight of Object + Weight of Slider
  • Dobject = Distance of Object CG
  • Mslider = Fobject• Dobject
  • Fslider1= - Fslider2 = Mslider / 2Dslider
  • Fhit = G value [I use .5] • (Weight of Object + Weight of Slider)
  • Mhitlower = Fhit•Hlower + [(Weight of Upper + Weight of Lower) • (Hlower / 2)]
  • Flower1= - Flower2 = [Mslider + Mhitlower]/ 2Dslider
  • Mhit = Fhit • Hslider + [(Weight of Lift • G value • Hslider ) / 2]
  • Mbase = Mslider + Mhit

Mupper

Dupper

Hlower

Dupper/2

Fupper2

Hslider

Flower1

Mlower

Dlower

Dlower/2

Flower2

Mbase

forklift rigging
Forklift: Rigging

Cascade

Continuous

forklift rigging continuous
Forklift: Rigging (Continuous)
  • Cable goes same speed for up and down
  • Intermediate sections often jam
  • Low cable tension
  • More complex cable routing
  • Final stage moves up first and down last
  • Tcable = Weight of Object + Weight of Lift Components Supported by Cable
forklift rigging cascade
Forklift: Rigging (Cascade)

Tcable3

Slider

(Stage3)

  • Up-going and down-going cables have different speeds
  • Different cable speeds can be handled with different drum diameters or multiple pulleys
  • Intermediate sections don’t jam
  • Very fast
  • Tcable3 = Weight of Object + Weight of Slider
  • Tcable2 = 2Tcable3 + Weight of Stage2
  • Tcable1 = 2Tcable2 + Weight of Stage1
  • Much more tension on the lower stage cables
    • Needs lower gearing to deal with higher forces

Tcable2

Stage2

Stage1

Tcable1

Base

four bar advantages disadvantages
Four Bar: Advantages & Disadvantages
  • Advantages
    • Great for fixed heights
    • On/off control
    • Lift can be counter-balanced or spring-loaded to reduce the load on actuator
    • Good candidate for pneumatic or screw actuation
  • Disadvantages
    • Need clearance in front during lift
    • Can’t go under obstacles lower than retracted lift
    • Have to watch CG
    • If pneumatic, only two positions (up & down)
four bar design considerations
Four Bar: Design Considerations
  • Pin Loadings can be very high
  • Watch for buckling in lower member
  • Counterbalance if you can
  • Keep CG back
  • Limit rotation
  • Keep gripper on known location
four bar calculations
Four Bar: Calculations

Mgripper

Fobject

Fhit

Dobject

Dgripper

Fgripper1

  • Under Construction Check Back Later

Llink

Fgripper2

Flink1

Dlink

Flink2

Mlink

Hgripper

Dlower/2

Mbase

scissors advantages disadvantages
Scissors: Advantages & Disadvantages
  • Advantages
    • Minimum retracted height
  • Disadvantages
    • Tends to be heavy
    • High CG
    • Doesn’t deal well with side loads
    • Must be built precisely
    • Loads very high on pins at beginning of travel
scissors design considerations
Scissors: Design Considerations
  • Members must be good in both bending and torsion
  • Joints must move in only one direction
  • The greater the separation between pivot and actuator line of action, the lower the initial load on actuator
  • Best if it is directly under load
  • Do you really want to do this?
scissors calculations
Scissors: Calculations
  • I don’t want to go there

THIS IS NOT RECOMMENDED

stress calculations
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

stress calculations cont

bo

do

bi

ho

di

hi

c

Stress Calculations (cont.)
  • A, c and I for Rectangular and Circular Sections
stress calculations cont38

Y

cy

cx1

h1

b1

h2

cx2

b2

Stress Calculations (cont.)
  • A, c and I for T-Sections

X

stress calculations cont39
Stress Calculations (cont.)
  • A, c and I for C-Sections (Assumes Equal Legs)

Y

cy

cx1

h1

b1

X

h2

cx2

b2

stress calculations cont40
Stress Calculations (cont.)
  • A, c and I for L-Angles

Y

cy2

cy1

cx1

h1

b1

X

h2

cx2

b2

allowable stresses
Allowable Stresses
  • allowable = yeild /Safety Factor
  • For the FIRST competition, try to use a Static Safety Factor of 4.
  • While on the high side it allows for unknowns and dynamic loads
  • Haven’t had anything break yet!
allowable stresses42
Allowable Stresses

Here are some properties for typical robot materials:

Material Desig Temper Yield Tensile Shear Modulus

(ksi) (ksi) (ksi) (msi)

Alum 6061 O 8 18 12 10

Alum 6061 T6 40 45 30 10

Brass C36000 18-45 49-68 30-38 14

Copper C17000 135-165? 165-200? 19

Mild Steel 1015-22 HR 48 65 30

PVC Rigid 6-8 0.3-1