© Daniel E Whitney

Assembly Workstation Design Issues

• Goals of this class– understand workstation elements– look at part feeding and presentation alternatives– design a process and a single station system for it

© Daniel E Whitney

Assembly = Reduction in DoF Uncertainty

0.001

0.01

0.1

1.0

10.0

100 or more STORAGE

BOWLFEEDER

FIXTURE

AMOUNT OFUNCERTAINTYIN PART LOCATION, INCHES

CHAMFERVISIONSENSOR

SPECIALIZEDGRIPPER

FINALASSEMBLYPOINT

PASSIVECOMPLIANCE

PASSIVE/ACTIVE FINEMOTIONDEVICE

PERSON,SHAKER,OR BIN-PICKER PERSON

PERSON

UNSPECIALIZEDGRIPPER

SPECIALIZEDGRIPPER

OEMPARTSCARRIER

SPECIALIZEDGRIPPER

SPECIALIZEDGRIPPER

ORIGINAL MFR

UNSPECIALIZEDGRIPPER

PartLevel

FeatureLevel

ChamferLevel

ClearanceLevel

GeographyLevel

PALLET

BOWLTRACKEXIT

© Daniel E Whitney

What Happens in a Workstation• An incomplete assembly arrives (or several at once)• Parts to be assembled arrive

– as single parts– as a subassembly

• Parts may have to be separated, oriented, given a final check

• Parts are joined to the assembly• Assembly correctness is checked• Documentation may have to be filled out• The assembly is passed on to the next station

© Daniel E Whitney

Major Issues

• Get done within the allowed cycle, which is usually short

• Avoid the three common errors– wrong part– correct part installed wrong, damaged, or causing

damage to the rest of the assembly– bad part used anyway

• Error-proofing or poka-yoke• Handle a lot of distractions

© Daniel E Whitney

Cycle Time

• Varies from milliseconds for cigarettes to days for aircraft

• Components– work in/out– move to get tool– move to get part– move to insertion point– insert– move to get new tool

• Each move includes accel-steady speed-decel-creep

timeRAMPUP TOFULLSPEED

RUN ATFULL SPEED

RAMPDOWN

CREEP

SPEEDDISTANCE

22 40 38

14 68 18

60% OF THE TIME IS SPENT COVERING ONLY 32% OF THE DISTANCE

% TIME

% DISTANCECOVERED

© Daniel E Whitney

http://www.silma.com

Simulation Softw

areCourtesy of Applied Computing & Engineering (c) - http://www.acel.co.uk

© Daniel E Whitney

Coordinate Transfer and Part Control

• Machine assembly requires transfer of coordinates– from where part is palletized to where it is gripped– from where it is gripped to where it mates– grip points may or may not be functional features

• These coordinates are usually on different locations on a part

• From each of these coordinates runs a chain of coordinate frames back around until they meet at the point of assembly

© Daniel E Whitney

Assem

blability

AB

C

D

E

F & A

Σ

G

SOME OF THE MANY SOURCES OF ERROR THAT MUST BECONTROLLED IN ORDER TOACHIEVE SUCCESSFUL ASSEMBLY

A. PART CONSTRUCTIONB. PART JIGGINGC. JIG LOCATIOND. ROBOT ACCURACY AND CALIBRATIONE. TOOL SOCKETF. PART GRIPG. OFFLINE MODEL

2 = A + B + C + D + E + F + G2 222222

Σ

© Daniel E Whitney

Some Important Decisions

• Choice of assembly “resource”– cost, reach, speed, multi-task capability, load cap,

dexterity, etc: people, robots, dedicated/fixed

• Part presentation at the station or elsewhere– accuracy of palletizing or carrier strips almost the same

as that of assembly– economics of palletizing: how/who; pipeline of WIP

• Serial vs parallel parts presentation – vibratory bowl or parts strip vs pallet

• Tool change vs multi-purpose tools– similar issues apply to manual and robotic

© Daniel E Whitney

Other Important Decisions

• Allocation of degrees of freedom– all in one place– shared between two, as in 4 DOF robot and 2 DOF

workholder

• Combinations of fab and part arrangement with assembly– creates parts or subassemblies on the spot– examples: pre-assembly of valve keepers, spring

winding, lubrication, sorting

© Daniel E Whitney

Valve Keepers

SelectedSolidLifter

Cam

Cylinder Head

Valve

© Daniel E Whitney

Workstation Layout• Part presentation

– Automatic feeders– Chutes loaded from opposite side– Bulk parts vs kits

• Station layout to provide– Parts and tools within easy reach– Things laid out in process sequence– Instructions - paper, video for each version– Instructions - what version is this– Documentation - tests performed, parts installed

• Line layout to provide– Space for materials at lineside– Space for transporters

© Daniel E Whitney

Sony APOS

• Offline shakers fill pallets (~ 10” x 12”)• Part jams, if any, occur off line and do not stop the

assembly system• Rather complex parts can be presented automatically• Pallets occupy considerable space at the workstation• The robot spends a lot of time slewing over to the

pallet to get a part• So you trade time for space: do you win?

© Daniel E Whitney

Sony APOS - Palletizer

TRACK FORPALLETS TOARRIVE AND LEAVE

PALLET ONSHAKER

PARTS HOPPERS

MECHANISM FOR RETURNINGEXCESS PARTS TO HOPPERS

© Daniel E Whitney

Sony APOS - Assembly Station

NEXTDONE WORKINGPALLET

1

2

3

4

5

ROBOT WORKENVELOPE

CONVEYOR

...........

...........

......

TO PALLETFILLINGEQUIPMENT

PARTSPALLETS

© Daniel E Whitney

Other Architectures• Escort parts and tools (early Sony FX-1)• Flexibility based on station lockout

– one simple station per part or version thereof– assembly passes through unneeded stations– lots of floor space

• Roving robot (Hitachi, 1980)– carries assembly in its “lap” – visits stations that feed parts and hold special tools

• Roving robot teams (Denso, 2000)– Robots carry tools, assemblies ride conveyor, parts delivered at

stations– Robots can be added or removed from system to adjust capacity– Robots can share work at highly loaded stations

• Parts made in or fastened to carrier strips - separates part prep from part feeding for higher feeding reliability

© Daniel E Whitney

Pallet Arrangement for Large Parts

ASSEMBLYPALLET

UNIQUEPARTS FORONE ASSEMBLY

BULK PARTSFOR SEVERALASSEMBLIES

© Daniel E Whitney

Spot

Line

Area

Cube

Network

FMS-0 FMS-1 FMS-2 FMS-3

Improvement of Denso Production System

Mechanical Spot Automation

TransferLine

RobotCell

MobileRobot

(Slide from Denso)

See “Nippondenso: a case study of strategic product design”. Courtesy of Denso Wave, Inc., Japan.Used with permission.

© Daniel E Whitney

Cartesian Robot

Workpiece Circulation System

Stocker

End Effector

High Rigidity Robot

Conveyor

Small Volume

4 times circulation

twice circulation

Medium Volume

No Circulation

Large Volume

FMS-3(Fluctuation Adaptive System)

-Change the number of assembly cellsto adapt to the fluctuation of the production volume

Assembly Cell

(Slide from Denso)

Courtesy of Denso Wave, Inc., Japan.Used with permission.

© Daniel E Whitney

Starter Assembly Automation Line

End Frame

Armature

Magnet Switch

Clutch

Yoke

Housing

Starter(Slide from Denso)

Courtesy of Denso Wave, Inc., Japan. Used with permission.

© Daniel E Whitney

Increasing and Decreasing Mobile Robots

Higher Production Volume : 3 mobile robots

- Increase/Decrease the number of RobotsAccording to Production Volume

LowerProduction Volume : 2 mobile robots

(Slide from Denso)

Courtesy of Denso Wave, Inc., Japan.Used with permission.

© Daniel E Whitney

Making Stacks - Method 1

12

aa a

a aa

b bb

b b b

a/bstacks

3

need astack every 25 sec

individual a'sand b's arriving

camera neededto aid gripping

ALTERNATIVE 1: ROBOT GRIPS 1 a AND 1 b

PROGRAM IS 1, 2, 3, 1...EACH SUBMOVE TAKES T SEC:1 TO 2, 2 TO 3, 3 TO 1, 5 TIMES.TIME FOR A STACK OF 5 a's, 5 b's = 15*T SECSO T MUST BE 25/15 = 1.67 SEC.

ALTERNATIVE 2: ROBOT CAN GRIP UP TO 5 a's AND 5 b's

PROGRAM IS (3 TO 2), {(2 TO 1) 5 TIMES, (1 TO 2) 4 TIMES INTERLEAVED}PLUS (1 TO 3).TIME FOR A STACK OF 10 = 11*T SEC.SO T MUST BE 25/11 = 2.27 SEC.

SINCE T FOR A ROBOT OF THIS SIZE IS TYPICALLY 5 SEC,ALT 1 WOULD REQUIRE 4 SUCH STATIONS, WHILE ALT 2WOULD REQUIRE 3.

© Daniel E Whitney

Making Stacks - Method 2

12

aa

aa a

a

bb

bb

b b

SPINDLE

3

need astack every 25 sec

ROBOT PICKS UP SPINDLE IN TIME T.ROBOT'S TOOL SWINGS SPINDLE FROM 1 TO 2 IN TIME 0.7*T.ROBOT DEPOSITS STACK WITH SPINDLE IN TIME T.TOTAL TIME = (9*0.7+2)*T = 8.3*T, SO T MUST BE 3.01 SEC.

ON THIS BASIS, TWO SUCH STATIONS WILL SUFFICE.

© Daniel E Whitney

Making Stacks - Method 3

3

need astack every 25 sec

12

aa

aa a

bb

bb

b

ab

FLIPPER

SLIPPERS

FLIPPER FLIPS EVERY 2 SEC. SLIPPERS SLIDEA DISK ONTO EACH SPINDLE ON THE FLIPPER.TWO COMPLETE STACKS ARE DONE IN 10 SEC.ROBOT TRANSFERS THEM TO CONVEYOR IN 5 SEC.NET TIME PER STACK = 7.5 SEC

IF THERE ARE N FLIPPER-SLIPPERS, THE ROBOT SERVESEACH ONE EVERY 5N SEC. SO EACH ONE PRODUCES ASTACK EVERY 2.5N SEC. IF EACH MAKES A STYLE THAT ISNEEDED ONCE EACH 25 SEC, THEN A ROBOT CAN SERVE 10FLIPPER-SLIPPERS.

© Daniel E Whitney

AMP Ignitor

ALUMINUM TOPWITH HOLE

PLASTICWINDOW

FUSE WIRE

PLASTICHOUSING

STEEL CAP

WASHER

STEEL CAP

FIBERWASHER

STEEL BODY

POWDER

(A)

ACCESS HOLEFOR FIRINGIGNITOR

STEEL CAP

WASHER

(B)

(C)

STEEL CAP

WASHER

© Daniel E Whitney

Manual A

ssembly Sequence

STEEL CAP

WASHER

STEEL CAP

WASHER

PLACEWINDOW

INSERTAND BENDWIRE

PRESS HOUSINGAND WIRE INTOTOP

POUR INPOWDER

PRESS ONCAP

BEND WIRE PLACE WASHER

PRESS SUBASSEMBLYINTO BODYMANUAL ASSEMBLY SEQUENCE

© Daniel E Whitney

Alternate Process

STEEL CAP

WASHER

STEEL CAP

WASHER

PLACE WASHER

PRESS SUBASSEMBLYINTO BODY

INSERTAND BENDWIRE

PRESS HOUSINGAND WIRE INTOCAP

POUR INPOWDER

PLACE WINDOW(A PRESS FIT)

PRESS ONTOP WITHWINDOW

© Daniel E Whitney

Alternate Process O

ne-handed

X

HOUSINGGRIPPER

WIREPINCHERS

PLACE HOUSINGOVER PINCHERS

FEED WIRETHROUGH

PINCH WIRE ROTATE CLOCKWISE90 DEGREES

PRESS IN CAP CUT WIRE ROTATE COUNTER-CLOCKWISE 90 DEGREES

CARRY TO POWDERFILL STATION

CARRY TOTOP PRESS

STATION

CARRY TO BODYPRESS STATION

© Daniel E Whitney

Single Station System

HousingFeeder

Wire Feed and CutStation

Cap Feedand PressStation

WindowFeeder

TopFeeder

Press windowinto topPress topinto housing

PowderFill Station

Body FeederWasher Feedand Drop intoBody

Press sub-assy intobody

TestOK

NotOK