Bruce Whitefield

Mentor, Team 2471

Manipulators for FIRST

FRC Robotics

FIRST Fare 2012

Game pieces come in many sizes and shapes

Manipulate What ?

Game objectives change each year

Manipulate How ?

Kicking

Lifting Dumping

Hanging

Throwing

Gathering

Know the Objectives

Game pieces

Game & robot rules

Your game strategy

Things that Manipulate

Manipulators that

work for the game

Know the possibilities

Look on line, at competitions

Talk to mentors, teams

This seminar

Know your capabilities

Tools, Skills, Materials,

Manpower , Budget , Time Manipulators you

can build

Your Design

Stuff That Don’t Work

Don’t be this team

Heavy lifting / Long reacho Articulating Armso Parallel armso Telescoping Lifts

Gripperso Rollerso Clampso Claws

Collect and Delivero Accumulators & Conveyerso Turretso Shooters & Kickerso Buckets & Tables

Power & Controlo Wincheso Brakeso Latcheso Pneumaticso Springs and Bungeeo Gears & Sprockets

Many Types of Manipulators

Reoccurring themes in FIRST

games

FIRST definition for a manipulator:

Device that moves the game piece from where

it is to where it needs to be

ShoulderElbowWrist

Arms

Torque = Force x Distanceo Same force, different angle = different torque

o Measure from the pivot point

o Motor & gearing must overcome torque

o Maximum torque at 90 degrees

W=10 lbs

W= 10 lbs

1/2 D

D

Torque = W x D

Torque: Angle and Distance

Torque = W x D/2

Power determines how fast you can move things

Power = Torque / Time or Torque x Rotational Velocity

Same torque with 2x Power = 2x Speed

Or twice the arm length at same speed ….

10 lbsPower trade offs

10 lbs30 ft-lbs

20 lbs60 ft-lbs

425 Watts

100 RPM1.6 rps

50 RPM.83 rps

42.5 Watts

10 RPM.16 rps

5 RPM.08 rps

3 ft

Power limits arm length.

Probably don’t want a 10 ft arm

whipping around at 1000 RPM

anyway…

Power: Torque and Speed

Lightweight Materials: tubes, thin wall Watch elbow and wrist weight at ends of arm Sensors for feedback & control

limit switches, potentiometers, encoders

Linkages help control long arms KISS your arm

Less parts… to build or break Easier to operate More robust

Counterbalance Spring, weight, pneumatic, bungee…

Calculate the forces Watch out for Center of gravity Sideways forces when extended

Model the reach & orientation

Arm Design Tips

Extension Lifts Motion achieved by stacked members sliding on each other

Scissor Lift Motion achieved by “unfolding” crossed members

Telescoping Lifts

Continuous Cascade

Extension Lift Rigging

The final stage moves up

first and down last.

Previous stage speed plus

own speed

o Power vs. speed

equations apply. Don’t

underestimate the power.

o Often needs brakes or

ratchet mechanism

Up-pulling and down-pulling cables have different speeds

Different cable speeds can be handled with different drum diameters or multiple Pulleys

Intermediate sections don’t jam – active return

High tension on the lower stage cables

Slider

(Stage3)

Stage2

Stage1

Base

Cascade Rigging

Cable goes same speed for

up and down pulling cables

Intermediate sections

sometimes jam

Lower cable tension

More complex cable routing

Stage2

Stage1

Base

Continuous Rigging

Slider

(Stage3)

Pull-down cable routed back on

reverse route as pull-up cable

Most complex cable routing

All stages have active return

Cleaner and protected cables

Slider

(Stage3)

Stage2

Stage1

Base

Continuous Rigging Internal

Use cables to drive up ANDdown, or add a cable recoil device

Segments must move freely

Cable lengths must be adjustable

Minimize slop and free-play

Maximize segment overlap 20% minimum More for bottom, less for top

Stiffness and strength needed

Heavy system, overlapping parts

Minimize weight, especially at top

Extension Lift Design tips

Feature Arm Lift

Reach over object Yes No

Fall over, get up Yes, if strong enough No

Complexity Moderate High

Weight capacity Moderate High

Go under barriers Yes, fold down Maybe, limits lift height

Center of gravity () Cantilevered Central mass

Operating space Large swing space Compact

Adding reach Difficult. More

articulations

Easier.

More lift sections

Combinations Arm with extender Lift with arm on top

Arms vs. Lifts

Combination Example: Continuous direct drive chain

runs stage 1 up and down

No winch drum needed

Cascade cable lifts slider stage

Gravity return

Got away with this only because slider GOG very well balanced on first stage

Telescoping arm with wrist on slider stage to reach over objects

2471 in 2011

FIRST definition of a gripper:

Device that grabs a game object

…and releases it when needed.

Get a Grip

Methods

Pneumatic claws /clamps 1 axis

2 axis

Motorized claw or clamp

Rollers

Hoop grips

Suction

Design Concerns

Getting object into grip

Hanging on

Speed of grip and release

Position control

Weight and power source

If at end of arm

Pneumatic

One fixed arm

reduce weight of claw

Can make one or two moving sides

768 in 2008

Claw or clamp

Pneumatic Cylinder

extends & retracts linkage

to open and close gripper

Combined arm and gripper

Easy to manufacture

Easy to control

Quick grab

Limited grip force

Use 3 fingers for 2-axis grip968 in 2004

Pneumatic: 2 and 3 point clamps

60 in 2004

Generally slower May be too slow for

frequent grips

Okay if few grabs per

game of heavy objects

More complex (gearing

& motors)

Heavier

Tunable force

No pneumatics

49 in 2001

Motorized clamp

Needs vacuum generator Uses various cups to grab Slow Not secure Not easy to control Simple Subject to damage of

suction cup or game pieces

Not recommended for heavy game pieces

Used successfully to hold soccer balls for kickers (Breakaway 2010)

Suction Grips

Allows for misalignment when

grabbing

Won’t let go

Extends object as releasing

Simple mechanism

Have a “full in” sensor

Many possible variations Mixed roller & conveyer

Reverse top and bottom roller

direction to rotate object

45 in 2008

148 in 2007

Roller grips

Counter Rotating Methods

Many ways to achieve counter

rotating shafts. Here are few

configurations that can run off a

single motor or gearbox.

Could also drive each shaft with

own motor

Crossed round belts

Counter

rotating

gearbox

Stacked pulleys

on single drive

shaft

Hang On! High friction needed

Rubber, neoprene, silicone, sandpaper … but don’t damage game object

Force: Highest at grip point

Force = 2 to 4 x object weight

Use linkages and toggles for mechanical advantage

Extra axis of grip = More control

The need for speed Wide capture window

Quickness covers mistakes

Quick to grab , Drop & re-grab

Fast : Pneumatic gripper. Not so fast: Motor gripper

Make it easy to control Limit switches , Auto-functions

Intuitive control functions

Don’t make driver push to make the robot pull

Gripper design

Tube or post (recommended)

Lazy Susan (not for high loads)

Know when it is needed

o One Goal = good

o Nine Goals = not so good

o Fixed targets = good

o Moving targets = not so good

Bearing structure must be solid

Rotation can be slow

Include sensor feedback

o Know which way it is pointing

Rotating Turrets

Accumulator: Rotational device that collects objects

Horizontal rollers: gathers balls from floor or platforms

Vertical rollers: pushes balls up or down

Wheels: best for big objects

Can use to dispense objects out of robot

Pointing the robot is aiming method in this case

Point to point within your robot

When game involves handling

many pieces at once

Why do balls jam on belts?- Stick 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)

Conveyers: For moving multiple objects

Conveyer roller Examples

Belts TowerRollers

Solution 1 Solution 3Solution 2

More control is better

Avoid gravity feeds – these are slow and will jam

Direct the flow: reduce “random” movements.

Not all game objects are created equal

Tend to change shape, inflation, etc

Building adaptive/ flexible systems

Test with different sizes, inflation, etc.

Speed vs. Volume

Optimize for the game and strategy

The more capacity, the better

Intake Rollers and Accumulators

173 2471

Secure shooting structure = more accuracy

Feed balls individually, controlling flow

Rotating tube or wheel One wheel or two counter rotating

High speed & power: 2000-4000 rpm

Brace for vibration

Protect for safety

Turret allows for aiming

Sensors detect ball presence

& shot direction

1771 in 2009

Note Circular Conveyer. One roller on inside cylindrical rolling surface outside

Use for dumping many objects

Integrate with your accumulator

and conveyer

Heavy ones may move too slow

Usually pneumatic powered but

can run with gear, spring or winch

488 in 2009

Many uses Hanging Robots: 2000, 2004, 2010

Lifting Robots: 2007

Loading kickers 2010

Great for high torque applications

Fits into limited space

Easy to route or reroute

Good Pull. Bad Push2004

2010

Secure the cable routing

Smooth winding & unwinding

Leave room on drum for wound up cable

Guide the cable

Must have tension on cable to unwind

Can use cable in both directions

Spring or bungee return

Gravity return (not recommended)

Calculate the torque and speed

Use braking devices

Capture the cable

Maintain Tension

Guide the cable

Brake or ratchet

Retu

rn c

able

or

spri

ng

Sudden release of power

Use stored energy:

Springs Bungee, Pneumatic

Design & test a good latch

mechanism

Secure lock for safety

Fast release

1 per game deployments

2011 minibot release

Hooking and latching devices used to

grab goals, bars, and other non-

scoring objects

Hold stored power in place

Spring or bungee power

Several ways:

Hooks

Locking wheels

Rollers

Pins

Springs

Self latching wheel lock

Basic Automatic Latch

Strong pivot in line with large force being held

Spring return and stop

Small release force at end of lever

Bevel for self latching

Removable safety pin

Start design early. Latches tend to be afterthoughts but are often a critical part of the operation

Don’t depend on driver to latch, use a smart mechanism Spring loaded (preferred)

Sensor met and automatic command given

Use operated mechanism to let go, not to latch

Have a secure latch Don’t want release when robots crash

Be able to let go quickly Pneumatic lever

Motorized winch, pulling a string

Cam on a gear motor

Servo (light release force only)

Don’t forget a safety pin or latch for when you are working on the robot

Look around see what works and does not work

Know your design objectives and game strategy

Stay within your capabilities

Design before you build Calculate the forces and speeds

Understand the dimensions using CAD or a model

Keep it simple

Make it well Poor craftsmanship can ruin the best design

Analyze for failure points. Use to refine design and decide on spare parts

Have fun

Many thanks to teams and companies who made

materials for this presentation freely available on

web sites to help FIRST students.

Andy Baker : Team 45

Jason Marr : Team 2471

Wildcats: Team 1510

The Flaming Chickens: Team 1540

Society of robots

Andy Baker’s original presentation and inspiration for this

seminar is available on line

There are many examples and resources available. Be

sure to use them for your robot designs.

http://www.societyofrobots.com/mechanics_gears.shtml

Appendix

Assuming 100% power transfer efficiency:

All motors can lift the same amount they just do it at

different rates.

No power transfer mechanisms are 100% efficient

Inefficiencies due to friction, binding, etc. Spur gears ~ 90%

Chain sprockets ~ 80%

Worm gears ~ 70%

Planetary gears ~80%

Calculate the known inefficiencies and then design in a

safety factor (2x to 4x)

Stall current can trip the breakers

Motor Power:

It adds up! 2 spur gears + sprocket =

.9 x.9 x.8 = .65

Losing 35% of power to the drive train

• Pin loading can be very high

• Watch for buckling in lower arm

• Has limited range rotation

• Keeps gripper in fixed orientation

Parallel Arms

Advantages Minimum retracted height - can go

under field barriers

Disadvantages Tends to be heavy when made

stable enough

Doesn’t deal well with side loads

Must be built very precisely

Stability decreases as height increases

Stress loads very high at beginning of travel

Not recommend without prior experience

Scissor Lifts

Pneumatic latch, solidly

grabs pipe

Force and friction only

No “smart mechanism”

Spring-loaded latch

Motorized release

Smart Mechanism

469 in 2003

Parallel arm Fixed Arm

Jointed Arm

Ratchet - Complete lock in one direction in discrete increments

Clutch Bearing - Completely lock in one direction any spot

Brake pads - Squeezes on a rotating device to stop motion - can lock in

both directions. Simple device

Disc brakes - Like those on your mountain bike

Gear brakes - Apply to lowest torque gear in gearbox

Belt Brake- Strap around a drum or pulley

Dynamic Breaking by motors lets go when power is lost.

Use for control, but not for safety or end game

Gearbox that cannot be back-driven is usually an inefficient one.