Building competitive manipulators

Building competitive manipulators

Greg Needel

Mentor teams: 131, 1511

• Read the rules

• Outline the game objectives

• Look for the “gimmie” robot design

• Try small simulations

• Whatever you choose STICK WITH IT!

Strategy, Strategy, Strategy!Strategy, Strategy, Strategy!

Types of ManipulatorsTypes of ManipulatorsTypes of ManipulatorsTypes of Manipulators

• Articulating Arms

• Telescoping Lifts

• Grippers

• Latches

• Ball Systems

• Example #1 - Lifting– Same force, different angle,

less torque

10 lbs

10 lbs

< DD

Arm: Forces, Angles & TorqueArm: Forces, Angles & Torque

TorqueTorque

• Torque = Force x Distance

• The farther something is from the pivot the harder it is to lift

• Power is more important then Torque

10 lbs

< D

PowerPower

• Power = Force x Distance / TimeOR

• Power = Torque x Rotational Velocity

Power (FIRST def.) – how fast you can move something

Arm: Power ExampleArm: Power Example

– Same torque, different speed

0.1 HP, 100 RPM Motor w/ 1” sprocket

0.2 HP, 200 RPM Motor w/ 1” sprocket OR 100 RPM w/ 2” sprocket

10 lbs10 lbs

Arm DesignArm 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)

Arm AdviceArm Advice

• K.I.S.S. doesn’t 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 )

Four Bar LinkageFour Bar Linkage

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

4 bar linkage example :229 20054 bar linkage example :229 2005

Arm Example: 234 in 2001Arm Example: 234 in 2001

Arm Example: 330 in 2005Arm Example: 330 in 2005

Arm Example: 1114 in 2004Arm Example: 1114 in 2004

Telescoping LiftsTelescoping Lifts

• Extension Lift

• Scissor Lift

ExtensionExtension

Extension Lift ConsiderationsExtension 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

Extension - RiggingExtension - Rigging

Continuous Cascade

Extension: Continuous RiggingExtension: 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

Slider(Stage3)

Stage2

Stage1

Base

Extension: Continuous Internal RiggingExtension: Continuous Internal Rigging

• Even More complex cable routing

• Cleaner and protected cables

Slider(Stage3)

Stage2

Stage1

Base

Extension: Cascade RiggingExtension: 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 Don’t Jam

• Much More Tension on the lower stage cables– Needs lower gearing to deal

with higher forces

• I do not prefer this one!

Slider(Stage3)

Stage2

Stage1

Base

Team 73 in 2005, Team 340 in 2004Team 73 in 2005, Team 340 in 2004

Scissor LiftScissor Lift

Scissor Lift ConsiderationsScissor Lift Considerations• Advantages

– Minimum retracted height - can go under field barriers

• Disadvantages– Tends to be heavy to be stable

enough– Doesn’t 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!

Team 158 in 2004Team 158 in 2004

Arm vs. LiftArm vs. Lift

Feature Arm LiftReach 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

<-

Braking: Prevent Back-drivingBraking: 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

PowerPower

• 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)

GrippersGrippers

• Gripper (FIRST def) grabbing game object

• How to grip

• How to hang on

• Speed

• Control

How to gripHow to grip

• Pneumatic linkage grip– 1 axis– 2 axis

• Motorized grip• Roller grip• Hoop grip• Pneumatic grip

Pneumatic linear gripPneumatic linear grip

• Pneumatic Cylinder extends & retracts linkage to open and close gripper

• 254 robot: 2004, 1-axis

• 968 robot: 2004, 1-axis

Recommended

Pneumatic linear gripPneumatic linear grip

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

• 60 in 2004

Recommended

Motorized Linear GripMotorized Linear Grip

• Slow• More

complex (gearing)

• Heavier• Doesn’t use

pneumatics• 49 in 2001Not

recommended

Roller GripRoller Grip• Slow• Allows for

misalignment when grabbing

• Won’t let go• Extends object as

releasing• Simple

mechanism• 45 in 98 and 2004

Recommended

Hoop gripHoop grip

• Slow

• Needs aligned

• Can’t hold on well

• 5 in 2000

Not

recommended

Pneumatic GripPneumatic Grip

• Vacuum:– generator &

cups to grab• Slow• Not secure• Not easy to

control• Simple• Problematic

Not recommended

Hang on!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

SpeedSpeed

• Quickness covers mistakes– Quick to grab– Drop & re-grab

• Fast– Pneumatic gripper

• Not fast– Roller, motor gripper, vacuum

Grip controlGrip 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

LatchesLatches

• Spring latches

• Hooks / spears

• Speed & Control

Latch example: 267 Latch example: 267

• Pneumatic Latch

• 2001 game• Grabs pipe• No “smart

mechanism”

Latch example: 469 Latch example: 469 • Spring-

loaded latch

• Motorized release

• Smart Mechanism

• 2003

Latch example: 118Latch example: 118• Spring-

loaded latch• Pneumatic

release• Smart

mechanism• 2002

Latching adviceLatching advice

• Don’t 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

Ball SystemsBall 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

Conveying & GatheringConveying & 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

ConveyorsConveyorsRollers- Use individual rollers- Adds weight and

complexity

Double Belts- Use pairs of belts- Increases size and

complexity

Single Belt- Use a slippery material

for the non-moving surface

Ball System TipsBall System Tips

• More control is better– Avoid Gravity feeds– Try to reduce “random” movements

• Not all Balls are created equal– Balls tend to change shape – Building adaptive/ flexible systems

• Speed vs. Volume– Optimize for the game and strategy

Roller example: 188Roller example: 188

Accumulator example: 173 & 254Accumulator example: 173 & 254

Questions?Questions?Thanks to:

Andy Baker (45)

Raul Olivera (111)

www.chiefdelphi.com

www.robotphotos.org

www.firstrobotics.net

www.firstrobotics.uwaterloo.ca

Extra StuffExtra Stuff

• Pneumatics vs. Motors

• Materials

• Shapes / Weights

• Fabrication processes

• Environment

Pneumatics vs. MotorsSome, but not all important differences

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

MaterialsMaterials• 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

ShapesShapes• Take a look at these two extrusions - both made from

same Aluminum alloy:– Which one is stronger?– Which one weighs more?

1.0”

1.0” 0.8”

0.8”

Hollow w/ 0.1” walls Solid bar

Shapes, cont.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

Stress CalculationsStress Calculations

• It all boils down to 3 equations:

IMc

A

Ftens

tens

A

Fshear

Where: = Bending StressM = Moment (calculated earlier)I = Moment of Inertia of Sectionc = distance from Central Axis

Where: = Tensile StressFtens = Tensile ForceA = Area of Section

Where: = Shear StressFshear = Shear ForceA = Area of Section

Bending Tensile Shear

Structural ShapesStructural 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

Fabrication ProcessesFabrication 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