Mechanical Design Final

8/8/2019 Mechanical Design Final

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Mechanical Design

Anne Bergeron

Mechanical Engineer

SAIC

[email protected]





Anne Bergeron2005 Chesapeake Enrichment Sessions

Work

Work = Force * Distance Example: The arm weighs 10 lbs and moves 3 ft vertically. The

mechanism that contains the balls weighs 5 lbs. The balls weigh 3 lbs.

The mechanism and balls move 6 ft vertically.

(use the center point of object to determine distance)

6 ft

3 ft

lb ft

ft lblb ft lb

d F d F

d F Work

!

!

!

!

78

6)35(310

2211




Power

Power = Work / Time

= (Force * Distance) / Time

= Force * Velocity

= Torque * Angular Velocity

POWER IS LIMITED!!!!!

Example: Desire the motion to be completed in 3 seconds.

watts

lb ft

wattslb ft time

w

ork Pow

er

3.35

min

02259697.0

min

sec60

sec3

78/

!

!

!




Motor Characteristics

Stall Torque

Highest amount of torque a motor cangenerate, the motor will be stalled with this

much load. Stall Current

Amount of current drawn when motor is stalled

Free Speed

Speed of motor under no load, fastest speed Motor Power

How much mechanical power a motor has




Speed-Torque Cur ve

Speed

Stall Torque

(T0)

Free Speed

(Wf)

K (slope)




Current-Torque Cur ve

Torque

Max breaker

current

Max

Design

Torque

Stall current




Power-Torque Cur ve

TorqueMax

design

torque

Max

Power




Combined Motor Cur ves

Torque

Speed-Torque

Current-Torque

Power-Torque

Maxbreaker

current




Motor Equation

Use to get better

estimates from graph

Equation form: Y=mX+b

Calculate the slope, m:

Substitutions:

f f W

T

W

T

Y

X Slope 00

0

0 !

!

(

(!

Speed

T0

Wf

K

0T speed W

T Torque

f

o

!




Motor Exam ples

T0 Wf A0 Af Pmax T40 W40

N-m RPM Amps Amps Watts N-m RPM

Chiaphua 2.45 5,342 114 2.4 342.6 0.80 4,045

Johnson F-P 0.38 15,000 57 1.1 149.2 0.20 7,245

Bosch Drill 0.87 19,670 127 4.5 448.0 0.25 13,970

Motor

Motor Equations:

1. 2002-04 Chiaphua: T = (-2.45/5,342) * W + 2.45

2. 2003 Fisher-Price: T = (-0.38/15,000) * W + 0.38

3. 2003-04 Bosch Drill: T = (-0.87/19,670) * W + 0.87

0T speed W

T Torque

f

o

!




Figuring out gear ratios

Example: A robot is to be designed to have a top

speed of 8 ft/sec. The robot will have 4 wheels that

have a diameter of 8 in and will be using one CIM

motor for each pair of wheels. Find the needed gear ratio.




Ste p one: Gather inf o

Given:

Wheel diameter(d) = 8 in = 0.67 ft

Robot speed (V) = 8 ft/sec

Motor info:

CIM use a 40 amp breaker

Look at graphs or use formula to find

following: T40 = 0.80 N-m = 0.59 ft-lb

W40 = 4045 rpm




Ste p two: Find wheel revolutions

Find distance traveled per revolution of

wheel:

Get wheel revolution needed for desired

speed:

rev

ft d P Perimeter 104.2

2

2)( !! T

min12.228

sec802.3

104.2

sec8

)(

revrev

rev

ft

ft

P

V W Wheelspeed

w

!!

!

!




Ste p 3: Find needed gear ratio

Find ratio to get speed of motor to speedrequired by wheel:

Motor speed (W40) = 4045 rpm

Required wheel speed = 228.12 rpm

This ratio can be achieved using one 17:1ratio or a combination of smaller steps.

Additional steps are multiplied.

17228

404540!!!

rp

rp

W

W ratio

w




Ste p 4: Calculate Force

Find the torque at the wheel:

Find force due to torque per wheel:

lb ft lb ft r atioT T w

!!! 03.101759.040

lb

in

ft in

lb ft R

F w

w 09.30

12

1403.10 !

!!




Ste p 5: Pushing f orce

Get the wheel max friction force (=1) which isequal to the max contact force (weight):

Find torque needed for pushing force:

Greater than torque of motors so not pushingbot.

w pushing

pushing

pushing

pushing

lb ft

lb

in

ft in

F d

u

!

!

!

83.10

5.32

12

1

2

8

2max

lblb

n

weight F

wheels

5.32

4

130)1(max !!! Q




More possi bilities

The same procedure can also be followed using

Torque or Power as the starting points.

Can use the wanted pushing force as starting point

Helps to know the coefficient of friction

Iterations will be needed.

Multiply in efficiencies when # of stages determined

Recalculate with actual ratios This can also be used to calculate ratios for

manipulators as well.




Gear Ty pes and Efficiency

Previous calculations were done under ideal

conditions.

Add efficiency in calculation by multiplying in

with ratio

Spur gears

Efficiency ~ 95% - 98%




Gear Ty pes and Efficiency (cont.)

Chain and Sprockets


Belt and Pulley

Efficiency ~ 85%-98% (timing belt best)




Bevel Gears


Worm Gears

Efficiency ~ 40%-70%

Gear Ty pes and Efficiency (cont.)




Ordering Gears

Remember clearances

Match

Pitch

Pressure angle Good Sources

McMaster Carr

Boston Gear

MSC

Pic-Design

Ord er ear ly!!!




Ordering Wheels

Skyway Wheels (www.skywaywheels.com)

AndyMark (www.andymark.biz)




Ty pical Drive Train

Co

nfigurat

io

ns 2 powered wheels, 2 castors





Co

nfigurat

io

ns (co

nt

.) 4 wheels, several configurations:

4 motors

2 motors

2 gear boxes

2 powered wheels

4 powered wheels





Co

nfigurat

io

ns (co

nt

.) Threads





Co

nfigurat

io

ns (co

nt

.) 2 centered wheels





Co

nfigurat

io

ns (co

nt

.) Swerve




A ppendages

Articulating Arms

Telescoping Lifts

Grippers

Latches

Accumulators




Ar m Design

³ Arm´: device for grabbing & moving objects using

members that rotate about their ends

Thin Walled Tubing (1/16) is your friend

Every Pivot has to be engineered reduce, reuse, recycle ;-)

Pivots on Pivots are confusing to drivers

4 bars linkages help control end of arm

Think about operator interface ± very important




Ar m Advice

Don¶t make it over-complicated

Feedback Control is HUGE

Measure Current Position (potentiometers)

Set Desired Position

Calculate Error

Take Action Based on Error (Search Internet

for P

ID control) Install limits

Design-in sensors from the start of design




Four Bar LinkagePin Loadings can be very high

Watch f or buckling in lower member

Counter balance if you can

Kee p CG af t




Ar m Exam ple: 67 in 2001












Ar m Contrast: 45 in ¶04-¶05








Telesco ping Lif ts

Extension Lift

Scissor Lift




Extension






Extension - Rigging

Continuous Cascade




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

Slider

(Stage3)

Stage2

Stage1

Base




Extension: Continuous Internal Rigging

Even More complex cable

routing

Cleaner and protected

cables

Slider

(Stage3)

Stage2

Stage1

Base




Extension: Cascade Rigging

Up-going and Down-goingCables Have DifferentSpeeds

Different Cable Speeds Canbe Handled with Different

Drum Diameters or MultiplePulleys

Intermediate Sections Don¶tJam

Much More Tension on thelower stage cables

Needs lower gearing todeal with higher forces

I do not prefer this one!

Slider

(Stage3)

Stage2

Stage1

Base




Lif t Exam ple 111 (1997)




Lif t Exam ple 213 (2001)




Scissor Lif t




Scissor Lif t Considerations

Advantages

Minimum retracted height - cango 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 heightincreases

Loads very high to raise atbeginning of travel

I recommend you stay awayfrom this!






Ar m vs. Lif t

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

Much mass, but central

Confided 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-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 is probably very

inefficient




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)




Gri ppers

Gripper = grabbing game object

How to grip

How to hang on

Speed

Control




How to gri p

Pneumatic linkage grip

1 axis

2 axis

Motorized grip Roller grip

Hoop grip

Pneumatic grip




Pneumatic linear gri p

Pneumatic

Cylinder

extends &

retracts

linkage to

open and

close gripper

254 robot:

2004, 1-axis

968 robot:

2004, 1-axis

Recommended




Pneumatic linear gri p

Pneumatic

Cylinder,

pulling 3fingers for a

2-axis grip

60 in 2004

Recommended




Motorized Linear Gri p

Slow

More complex

(gearing)

Heavier Doesn¶t use

pneumatics

49 in 2001

Notrecommended




R oller Gri p

Slow

Allows for

misalignment

when grabbing

Won¶t let go

Extends object as

releasing

Simple

mechanism

45 in 98 and 2004

Recommended




R oller Gri p Exam ple 45 (1998)




R oller Gri p Exam ple 121 (1998)




Hoo p gri p

Slow

Needs

aligned

Can¶t hold onwell

5 in 2000

Not

recommended




Pneumatic Gri p

Vacuum:

generator &cups to grab

Slow

Not secure

Not easy tocontrol

Simple

ProblematicNot

recommended




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

Best grip = roller gripper




Speed

Quickness covers mistakes

Quick to grab

Drop & re-grab

292 example

Fast

Pneumatic gripper

Not fast Roller, motor gripper, vacuum




Gri p contr ol

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




Latches

Spring latches

Hooks / spears

Speed & Control




Latch exam ple: 267

Pneumatic Latch

2001 game

Grabs pipe

No ³smart

mechanism´




Latch exam ple: 469

Spring-loaded

latch

Motorized

release

Smart

Mechanism

2003




Latch exam ple: 118

Spring-loaded

latch

Pneumatic

release

Smartmechanism

2003




Latching advice

Don¶t depend on operator to latch, use asmart 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




Accumulation

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




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 andeach other

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




Conveyors

Why do balls jam on belts?

- Sticky and ru b against each

other as they try to r otate

along the conveyor

Solution #1- Use individual r ollers

- Adds weight and com plexity

Solution #2

- Use pairs of belts

- Increases size and com plexity

Solution #3

- Use a sli ppery material f or the non-

moving surface (Teflon sheet works

great)




R oller exam ple: 111




Accumulator exam ple: 173 & 47



Pneumatics




Pneumatics vs. MotorsSome, but not all im por tant 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 largeforces

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 shockabsorption

Cylinders used with 1-way valves are great for Armageddondevices - stuff happens when power is shut off

This could be good or bad - use wisely




Com ponents

Compressor

Pressure gages

Helps diagnose

problems

Cylinders

custom sizes

Flow controls




Com ponents (cont.)

Tanks

Regulators

60 PSI

Relieving At least one

required after tanks

Inlet labeled

Solenoid Valves




Fittings

Flow Control

Plug Valve



Com pressor

Gage

Regulator

Valve

Tank

Tank Relay

Contr ol

System

Fuse

Box

Piston




Layout of Test Board






Wiring Solenoids

Single Solenoid

Connect the red to M+ of relay

Connect the black to M- of relay

Double Solenoid (as two singles)

Double Solenoid+

-

Single Solenoid

M+

M-

Single Solenoid




Dou ble Solenoid

Double Solenoid

M+ M-

Double Solenoid

M+

M- M-

M+




Pistons

Several sizes available

Bore sizes: ¾´, 1 ½´, 2´

Stroke Lengths: ½´ to 12´

Force= Pressure * Area

Stroke length

Stroke Length

Bore

Size




Forces




Basic Mechanical Ti ps

Know your limitations Machining

Design

KISS

Keep track of weight Spreadsheet

Estimates and actuals

Include materials

Have 5-10 pound buffer

Assign per subsystem

Get a good scale Think about maintenance during design

Access to parts

Determine high maintenance parts




Keep the center of gravity low

Battery/Compressor

Wheel Base

Prototype ideas

Create design drawings

2D or 3D

CAD or paper

Keep at building site

Standardize hardware

Metric or standard

1-2 Sizes (1/4´-20, 10-32)

Lots of lengths

Basic Mechanical Ti ps (cont.)





Avoid set screws

Too much traction can be bad

Be aware of robot systems when drilling or

machining parts on the robot Avoid cantilevered shafts

Avoid stalling your motors

Use the right tool for the job

Key

Pin

Screw

Screw

Shaft





Make spares

Get a base and drive train done quickly

Don¶t forget about pneumatics




Structural Material

Metals

Iron

Steel

Aluminum

Forms

Extruded

Plating

Angle

Tubing Circular

Square

Wood

Fiberglass

Lexan (no Plexiglass)

Carbon fiber




Pr ofiles

Angle: 1´x1´ 1/8´ thick

Square:

1´x1´

1/16´

t

hick Same weight, much more strength and stiffness

Takes more s pace

Extruded: 1´x1´ 1/16´

thick

Heavier, much more strength and stiffness

Takes more s pace

Easy to assemble and connect to




Extruded

Item America www.itemamerica.com

80/20 www.8020.net

Bosh www.boschrextroth.com IPS www.industrialprofile.com



Questions?Thanks to:

Andy Baker (45)

Chris Hussman (330)

Joe Johnson (47)

Raul Olivera (111)www.chiefdelphi.com

www.firstrobotics.net

www.firstrobotics.uwaterloo.ca




Activity

The Task:

Design a robot drive train using AndyMark

gearbox.

Specifications:

Robot Speed= 8 ft/sec

Use 4 Chiaphua motors (Chip) Weight of robot=130 lbs



Ste ps

Decide on a wheel size

Find gear ratio needed from gearbox towheels

Calculate the pushing force of robot

Is it a pushing robot?

Conversion factors: 1 oz-in = 0.0052083 pound foot

Mechanical Design Final

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