Automatica, Vol. 16, pp. 595-613
Pergamon Press Ltd. 1980. Printed in Great Britain
International Federation of Automatic Control
0005- 098/80/
0 -0595 502.00/0
Assembly Research*
J. L. N E V I N S t and D. E. W H I T N E Y t
Assembly is a process performed by people and is poorly understood. To automate
assembly requires an information-control approach which accurately describes the
process, the workpieces and the assembly equipment so that reproducible systems
can be designed.
Key Words--Computer controlled assembly; robot assembly; manufacturing automation.
Abstract--Assembly research comprises the definitive
description of how parts interact during assembly (called Part
Mating Science), the collection of part mating processes into
systems that assemble products, and the integration of this
process into the factory as a whole. Products can be
assembled by four techniques: manual labor, special purpose
machines, programmable systems, or hybrids of the above.
Each system includes material handling, parts feeding,
assembly, and inspection. Issues of design include technology,
part mating and product system design tools, and economics.
lie
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INTRODUCTION
C O N T I N U I N G inflation, competition from other
countries, and record deficits in international
trade have created a widespread awareness in
various countries of the need to increase
productivity in manufacturing, which usually
means decreasing the man-hours, materials,
energy or capital required to produce industrial
goods of all kinds. An additional stimulus for
increasing productivity arises from the desire to
improve the quality of life, including the life of
workers now engaged in stultifying, repetitive and
sometimes hazardous tasks. For most of the past
century growth in manufacturing productivity
has been maintained by the substitution of
power-driven machines and new technological
methods for labor. Today there are pressures to
use power, materials and capital more efficiently.
The conventional remedies need to be reexamined and new solutions need to be sought.
To illustrate, Fig. 1 gives the distribution of
direct labor in four major U.S. durable-goods
industries and demonstrates the large role that
manual-assembly labor still plays, even in the
ifN
O00nnnnn~n
.....
.......
[1
Fl(;. 1. Distribution of direct labour.
highly mechanized motor-vehicle industry. On
first impulse automation would be best applied
by attacking the functions with the highest labor
percentages. However, a better perspective is
obtained if these percentages are scaled by the
value of shipped goods, as Figs 2 and 3 illustrate.
These figures are averaged over the indicated
specific industries.
Thus, for the farm machinery and equipment
industries 30.2 per cent of direct labor is involved
in assembly, but when this is scaled by the value
of shipments it represents only 7 per cent of
costs. Similarly, 33.4 per cent of direct labor is
assembly in the automobile and parts industry,
but again this is only 4.7 per cent of the value of
the shipped goods.
If we were to completely automate the
assembly functions in these two industries we
would only change our total costs by 7 per cent
for farm machinery and 4.7 per cent for the auto
industry. This, of course, assumes we know how
to do so. ]~igure 4 illustrates this perhaps more
*Received October 17 1979; revised May 9 1980. The
original version of this paper was presented at the 2nd
IFAC/IFIP Symposium on Information Control Problems in
Manufacturing Technology which was held in Stuttgart,
Federal Republic of Germany during October, 1979. The
published Proceedings of this IFAC Meeting may be ordered
from: Pergamon Press Limited, Headington Hill Hall, Oxford
OX3 0BW, England. This paper was recommended for
publication in revised form by associate editor A. Longmuir.
tCharles Stark Draper Laboratory, Inc., 555 Technology
Square, Cambridge, MA 02139, U.S.A.
595
596
J.L. NEVINS and D. E. WHITNEY
SIC 3522
FARM MACHINERY & EQUIPMENT
BREAKDOWN OF
VALUE OF SHIPMENTS
% TOTAL LABOR
% VALUE OF
SHIPMENTS
Function Co,If
100.0
23.4
Nonproductlon
42.7
10.0
Pro¢lucti=~
560
~3 1
~%i TOTAL
l
LABOR
%SHIPMENTS
VALUE OF
4
1426
59A
84
47
16.6
2.4
Asmmbly
~0.2
71
33.5
47
1.3
9 0
1.3
5.0
4.6
0.3
M=terlal Colt|
54.0
Oull~ty Cc~.
15
Othl~ CO¢~, Profit, etc.
.
.
.
.
.
21 1
.
.
.
SIC 3541
- - MACHINE TOOLS.
METAL CUTTING TYPES
SIC 3601
RADIO AND TV RECEIVING SETS
BREAKDOWN OF
VALUE OF SHIPMENTS
% TOTAL
TOTAl LABOR
% VALUE OF
SHIPMENTS
% TOTAL LABOR
% VALUE OF
SHIPMENTS
F~tkon
100,0
37.5
100.0
15.g
Cost~
Nonproduction
32.3
12.1
52.7
8.4
Production
665
24.9
45.g
72
Plwts Fal~i~tion
50.1
10.8
84
1.3
~bly
11.0
41
239
3.8
2.0
5.4
35.4
IBll~r ial Coltl
i
Quality Co~t~
.
13.3
17
0E
1.2
MilcJdlar4ou$
I
s AND P A R T ~
202
Inspectio.
.
~ic: 37,7-
' V
IWO
lETO
HRI C L E
Parts F#;)ri~ t i ~
Mi*cllllmmlou s
.
F-
~ts, Profit,~t¢.
.
F'l(is 2 a n d
.
3.
2.1
0.3
615
15
1.5
25.6
21.0
.
]
.
Labor costs as a percentage of value of
shipments.
ILLUSTRATIVEEXPERIMENTS
F=rm M ¢ h i ~
MI~i~T~I=' f
and ECmipcnent
C~I Elects
ble~tCu~ing Types
IS*C a5221
tSIC 3541>
Radi°al~V
ReceivmgSets
Motm Vehicle~
~d parts
ISIC3651}
(SIC 37171
+.
*5O
la~t
Stiff
E~p~V~ R,lat,om
Finan~
Ml.k,t,ng
Co~p~t,, Op~aUO,,
En~i~,,ng
+20 0
i
+5 0
100
Mamri=l Movem~t
m0
A~mblv
mo
Inl~tlon
900
5.0
100
500
20
50
In~lory
+0 i
~han~= m P,oduct Cost
Ch~np mA ~
Re(IUir~ n t i
NOTE All v a l ~ are e x p ~
100
100
~0
38
as a ~
-25
07
O0
2~
c h a ~ In t ~ ratio of c~t5 Io val~ of thipm~ts
FI(i. 4.
hnpact calculation.
dramatically. Here, not only are production
functions changed, but also non-production
functions "white
collar
workers"-- like
engineering. Again dramatic changes in the
specific function cause only minor a few per
cent -modification in the final sum.
Clearly, no one single device or technique can
be used to accomplish such huge changes.
Neither will a haphazard application of singular
solutions accomplish the goal. Rather, what is
needed is a systematic multi-discipline approach
if one is going to achieve the kind of changes
needed to cause a significant effect on industrial
productivity.
Although there are many ways of increasing
manufacturing productivity financial, fiscal, and
social we shall focus here on the possibilily o f
raising productivity through the application of
science and advanced technology to an old field:
assembly. Technology has brought about radical
changes in many areas power generation,
transportation,
chemical
manufacturing,
communications, and data processing but it has
had only a minor effect on the way the broad
spectrum of consumer goods, from electric
toasters to automobiles, are actually assembled.
The rest of this paper will be devoted to
describing assembly research in this context. That
is, we will generate a n overall s c h e m e o r
system required for automatic assembly a n d
then identify the research directions currently m
process and how they appear to fit into this
overall system. In particular, with this kind of
approach, systematic tools that illuminate these
relationships or help define the performance
requirements of the supporting subsystems
appear at the moment to be the items most
needed. Later, of course, with good agreement on
the outlines of the system defined, the emphasis
will shift to the refinement of the supporting
systems needed. By then, the priority of this
supporting work will also be fairly clear.
In the simplest view, assembly can be
categorized into two principal activities, namely:
(a) part mating or what occurs when two or
more parts interact during the assembly process,
and (b) the aggregation of this process activity
into the assembly of an entire product. If one is
to consider the "assembly system" then
additional things must be considered. Specifically,
assembly cannot be considered without the
integration of both the feeding of the parts and
the inspection steps necessary during the process.
Finally, the integration of the above activities
into the context of the total factory produces the
desired assembly system.
At the present time research has been carried
out on part mating and product assembly. F o r
"'assembly systems" only unrelated items a r e
being pursued. Integration of these systems into
the factory arc restricted to the present tools of
industrial
engineers
or
simple
heuristic
techniques.
However,
the
application
of
operational research techniques or knowledge
based systems to these last two problems offer
more interesting solutions which will be
illustrated later.
PART MATING
In order to define assembly
systematic way must be used to
functions, group them in some
categorize the statistics of their
processes, some
identify tasks or
generic way and
occurrence a s a
function of various classes of products. An early
Assembly Research
study by Kondoleon (Kondoleon, 1976) of ten
products with various part counts and sizes (Fig.
5) yielded the range of tasks shown by Fig. 6.
The statistics for the assembly functions for those
products are illustrated by Fig. 7. Additionally,
Fig. 7 illustrates the number of degrees-offreedom required in order to move the parts from
some location to the vicinity where assembly
takes place. These are pertinent data necessary to
specify the number of articulations required if
linkage type machines (industrial robots) are used
in product assembly.
ELECTRIC TIMER
COVER SUBASSEMBLY*
7
45-
ELECTRICTtM~R CASE
AND FINAL ASSEMBLY*
18
45"
REFRIGERATORCOMPRESSOR
FAMILY
26
10"
15
6 to 8
6"
10'
29
7'
BICYCLE COASTER BRAKE
TRANSFORMER ELECTRIC
BUSHING FAMILY
END CAP SUBASSEMBLIES FOR
SMALL INDUCTION MOTORS
INDUCTION ivtOTOR MAIN BODY SUBASSEMBLY AND FINAL ASSEMBLY
ELECTRIC JIGSAW
TOASTER OVEN
21
15-
58
41
12"
15-
AUTOMOBILE ALTERNATORFAMILY*"
17
8'
*CURRENTLY ASSEMBLED AUTOMATICALLY
**CURRENTLY ASSEMBLED BY A MIXTURE OF MANUAL AND AUTOMATIC
WORKSTATIONS
FIG. 5. Ten products analysed.
597
Additional statistics are also needed for
weights, part tolerances, and force levels
occurring at assembly, etc. Using these
techniques, General Motors found that 90 per
cent of the pieces used in the assembly of an
automobile weigh less than 2kg. John Deere and
Company, who manufacture large agricultural
machines like combines and tractors as well as
large earth moving equipment, in a similar study
found that 80 per cent of their pieces weigh less
than 4 kg.
Round peg-hole insertion tasks with chamfered
corners are by far the most frequent in the
assembly of metal products with machined or
cast parts (Fig. 7). This task has been extensively
analyzed, and a complete statement of the
requirements on relative errors is now available.
The important design variables have been
identified and for many parts it has been
determined
that
conditions
for successful
assembly can be met more easily than the
clearances between the parts would make it at
first appear. These analyses have been amply
verified experimentally and it has been shown
that the conditions can be obtained from the blue
prints for the parts (Nevins and co-workers,
1976). The analysis and experiments also form a
valuable pattern which can be followed in the
analysis of other tasks. In particular, the
extension of this work to compliant parts has
proved to be particularly useful in the design of
parts used in the electronics industry.
¢~.MF
ON~ miNI
ACT
tWO POq~r
¢aNWAe~
D~LAYEe Te
r
ANGULARERRORIGRE~N)
FiG. 6. Assemblytasks.
DURINGtb*K)POINTC O N T A C t
THE
T~
OF THE pE~ MUST ~ [ U A I ~ W I 1 H I N TNE
IHE F ~ N E L
IS
REO FUNNEt
SMALLE. IF T N t R A ~ I O OF CLt~
~NCETOPE¢ mAMEVERqS ~ * t t e ~
IgG. 8. Basic geometry.
............
.,,.~,,,, . . . . . . . .
D I m E { ~ I O ~ OF" A T T A C H ~ I L ~ [
OF" IPAR T~*~ u .R~l~.*~l m the
FIG.
7.
Task
' ':,~-~,~ . ~ " ~ , ~', ~,,~ .,,~
" ~ n ~r
statistics.
~
~
I~
,
Cases where the peg touches one or both sides
of hole have been considered (Fig. 8). Two types
of "jamming" during assembly have been
identified (Fig. 9), and the conditions for
preventing them specified in terms of geometry,
friction coefficient (Fig. 10), and-arrangement of
the applied forces (Fig. 11). The geometry of
598
J.L. NEVINS and D. E. WHITNEY
. d ~ PEG
Fz
DIFFERENCE BETWEEN WEDGING AND JAMM/~G
was chlrtlJed din'in8 the d e v e l o p
mlml ~ cemtldJl~t-lp, Jppe~ moelbma/m~. ~
foe' m p l e ,
• INIreml dllwe~ be¢omm wedled
(IeJ~), N b Ute8111~ IIo~..keclL A j a r ~
spt~lkmffea eG force wUI d d o r m the dJrawe~ mr die b u r m u e r b o 4 ~ 1~ee¢7 8bows t k t t w e d s ~ arbes w b e a *ke drltwe¢ Js imes-ted at such ~ ~ ~ e
c4--t the r u t l o eli L / D b lees t l m t t k e c o d ~ J e a ( o~r IrrklJoa (;,) wltea I w o - p o i n t coutact I r n t
occurs. 1['be e a ~ r e m e d y b to p u U 1be d r a w e r o u t m d ~
8 p b L If, however, t k e retio L / D
b larse~ t k m p at d i e time o( ~
(wo-pebst eoutact (nEht), wedJrln8 c l n m o t result, mlt b o u l b furtbe~ movement ~ m be impeded by j J m m l n 8. l r b e remedy Is to break the t w o - p o i n t
c o n l l c t by p u ~
I t A, Ihereb¥ c l ~
Ibe d h ' e ~
o( be4tk the a p p l i e d force t u d the applied enomemL -
r~ Z
JAMMING REG(ON
Fl(i. 9. Wedging and jamming.
1
QX
rj
JAMMING
R
Wt4EN THER| iS k~ FRiCTiONCON.
TACT r~f~c| (IILUEI IS FELT|QQALL¥
AND A rUnWAYS RE~TION (RED)
LATERAL FORC~CAN ~IE t.~EO AJ A
CUE TO THE OEI~RED CORRECTIVE
MOTION(REO)
4
~REGION OF NO
JAMMING
~A
WHEN THERE IS MO FRiCTiON COil.
TACTFO~qcESI|LUE)CAU~EAI4OM~N
T
/~DUT THE TIP IRE(}I. WP*41CHOlVI[I A
CUE/UStO T.E O~R~O CO~lCTWE
MOT~C~InED)
F'IG. l l. Force moment relationships to avoid ammmg.
....L .
CO'TACTFaeCESe~TW~E~PARTSCANU SaNStO~VO~ 0 TO
auloE C O 4 ~ C T W E N O T ~
FR~TION ca~¢ CO~TRmUTE
FORCIES ~
M4J~ TNE FC~CE ff4FOgtMAT~N CONTACT
FORCES ARE FIEFIPENOCCUt.AR TO TNE CONTACTING ILmFACES
FRK:T~D4~ FOp,CE8 RItE PARALLEL TO C~ITACTING St,reFACES
MAQI4f~
OF I')'NE FRK:TION P ~ C E i i ~q~qOXIMATELY p ~ l
FO~ITIOklAL TO TNIE NJU3N4TUOIE Of THIE C~WTACT F,I~IC| THE
CIQWSTANTOF P ~ O F O I N T I ~ I T Y I I CALLED TI,I~ECOIFFICI|NT
OF FRICTION ITJVALUERAIIIH]iE$FNONO;' FORIWE| L PART|TO
I o ~ LARGER F ~ AL~ 1 ~
p~TI
•
FI(;. 10. Eorce friction relationships.
angular errors in screw thread mating has also
been analyzed and shown to have looser error
tolerances than the typical peg-hole problem.
To test these theories a number of carefully
constructed experiments have been carried out.
One of the first experiments (Fig. 12)involved
the use of a pair of Unimate 5000 manipulators
to test the capability of these kinds of
manipulators to perform the assembly of simple,
rigid, machined pieces without adaptability. What
was shown was that assembly was possible, but
the probability
of success
depended
on
unspecified compliances unique to each machine
that were at least time and temperature
dependent.
A second group of experiments was carried out
to test friction and jamming theories (Fig. 13).
These early experimenrs were conducted on a
second generation pedestal force sensor, but the
sensitivity of the unit to moment information was
inadequate.
This
same
sensor
was
used
FIG. 12. Programmability experiment. Two Unimatc 5000
Industrial Manipulators were used to assemble pieces of a
small gasoline engine where nominal clearances of parts was
0.001 0.002 in. (0.025 0.05mm). Positioning error of the
manipulators was 0.020--0.028 in. (0.525 0.72mm) and about
0.5' in angle (neglecting backlash). The experiment showed
that this kind of system would work for pieces with slightly
lubricated mating surfaces, chamfers, bevels and rotationally
compliant grippers, but the probability of successful assembly
could not be stated.
599
Assembly Research
FIG. 13. First friction and j a m m i n g test. Experimental system
shown was used to verify coefficient of friction for various
materials and to test j a m m i n g theories. Although system used
second
generation
force
sensor
system
(pedestal
configuration), its m o m e n t sensitivity was almost two orders
of magnitude less sensitive than force readings.
successfully to perform the first experiments for
very large errors (Error<radius of hole) where
estimation techniques might be useful. To
compensate for the lack of sensitivity in the
sensor, the test pieces were made ten times
oversized (Fig. 14) (Simunovic, 1979).
The first definitive part mating experiments
required the development of a more sensitive
force sensor array and a wrist with only one
carefully specified compliance. This device was
attached to a milling machine base and specific
relative errors were imposed on insertion tasks
using special test pegs and holes. All data were
gathered and processed on-line by a computer.
The results verified the conditions for one of the
two predicted types of lamming and showed that
unambiguous force and moment data could be
obtained (Fig. 15). A newer more sensitive
instrument
for
studying
small
electrical
components with a minimum threshold of 4g in
the x and y direction and 10g in the axial
direction has recently been developed to support
these activities (Fig. 16).
{WLL~E~t
*27
~ l t ~ E ~
6
G~
oF
~
Aer
a
To
iim~mN
~lm~
~
}
Flcl. 15. First definitive part mating cxpcrimcm.
FIG. 14. Tilt strategy estimator. Use of estimator for part
mating errors approximately equal to radius of hole. Pedestal
force sensor measured tip-over moment from which estimator
calculated the azimuth of tilt, and from this, the direction to
the center of the hole from the center of the peg. The peg was
then driven sideways until it struck the opposite side of the
hole. The tilt was then removed and the peg inserted into the
hole. The sequence of moves constitutes an information
strategy without search.
The inaccuracies in the U-5000 manipulator and the lack of
moment sensitivity in the force sensor required that the pieces
be ten times over size.
FIG. 16. 4 G r a m 6 Axis force sensor array.
600
J . L . NEVINS and D. E. WHITNt ',
The combination of geometric and friction
analyses, plus the stimulus of the above data gave
rise to the invention of a totally new device for
accomplishing assembly--the Remote Center
Compliance (RCC) (Watson, 1976) (Figs 17 24).
This device accomplishes mechanically what
active force feedback does with sensors and
servos. It allows chamfered peg-hole insertions
with 0.0003 clearance ratio* with 1 mm (0.04in.)
or larger initial position errors with a few degrees
of angular misalignment. The error tolerance of
/
MOMENT
,',,,,
'*<IMF~,I
C F N T E N :l~ C O M ~ L I A N C I
~LATERALrOOC
E
nOT
CfNTER
~ORC~
AV
OF COMPLIANCF
I
RCC APPLICATIONS
Categories and examples:
Clearance fits Bearings in housings
Shafts in bearings
Splines
Castings or forgings onto locating pins
Noncircular pegs into broached holes
FIG. 19. RCC clearance fit apolication~
-,q
FORCE ~,~
CENTER ~ CO~PL I a n c ~
t J(;. 17. Requircment for remote center,
A
II
-7
L1
I. IG. 20. Model 4A R C C The Mod 4A has been designcd as a
rugged relatively foolproof unit with overall protection in all
directions - done to provide a unit for evaluation by industr,,
m the industrial environment. Several models of the 4A ha~e
been placed in industry. Its nominal specifications are: Mass
(weight), 1.36kg (31b); focal length from base, 20cm (Sin):
lateral stiffness, lOONcm ~ (551bin 1); torsional stiffness.
0.1NM mrad ~ (0.9in lbs/mrad).
_A- /
CONNEC/
LINK NGC~T
i
/
J
I I
INTERFERENCE FITS
-?
t-it;. 18. Concept RCC. The vertical
compliance center effectively at infinity.
produce only lateral motion. The other
compliance center at the end of the peg
angular rotation in response to a moment
Locating pins
Calibrated orifices
links (B) give a
So lateral forces
links (C) give a
which allows only
applied at the tip.
Bearings in bores
Shafts in bearings
Pins in bores
Noncircular geometries
lq~. 21. RCC interference fit application.
*Clearance Ratio
dia. of Hole - dia. of Peg
Diameter of Hole
Assembly Research
601
MANUFACTUR ING
Reduction of tooling forces
Reduction of tooling breakage
Tool-guide bushing mating
Tool mating to bushing
I" IG. 22. RCC applic~ltion~ ill inzlnul'zlctiuit/g.
l-t(i. 23. Designed to provide a lighter weight (1 lb or 0.45 kg
massl unit capable of easier repair and lower cost. The focal
length and elastic parameters are nominally the same as the
model 4A. Fifteen units are currently being evaluated by .
industry.
this passive compliance device is much larger
than was originally thought possible for passive
devices. Further, the device works for both
negative as well as positive clearances.
To illustrate the usefulness of the RCC device,
a comparison can be made of a number of
techniques for absorbing the error occurring between industrial robots and the pieces they might
be required to perform tasks on (Fig. 25).
Industrial devices tend to be quite imprecise
compared to a rigid machine tool.
Therefore, some kind of device is necessary to
absorb the error that arises due to the lack of
repeatability of these machines when they are
required to interact with piece parts. Figure 25
shows two classes of systems, namely passive and
active devices, that could be used for a variety of
tasks that include material handling, assembly,
and manufacturing. Figure 26 compares the estimated probability-of-success of accomplishing
tasks for three passive techniques and one active
technique vs the initial position error. The active
system shown--the Hitachi Hi-T-Hand (Fig. 27)
(Goto, Inoyama and Takeysau, 1974}--is also
capable of solving the chamferless insertion problem. Figure 28 further compares the active
FIG. 24. Fixture for aiding rapid, reproducible manual
instrument bearing assembly. Instrument ball bearings are a
light interference fit at operating temperatures and are
assembled into a heated housing to provide assembly
clearance. The assembly must be done quickly and at light
force levels. It is c o m m o n in unaided assembly that a ball
bearing will j a m and have to be driven home. A manual
assembly stand assisted by the RCC to avoid jamming, and a
six degree-of-freedom force sensor to document the assembly
force history, is expected to yield significant improvements in
the process. A brake is included in the design; activated by
the force sensor, it can retard the drive and limit the seating
forces exerted on the bearing. Any one of the six forces and
moments measured may be selected for analog display (on the
meter to the left of the assembly stand). Each or all of the
measured quantities may be stored on a computer file or
recorded for permanent record.
ERROR ASSONBER
PASSIVE SYSTEM
No Complilmce
Undocurmmed C o m p ~
Enwn.erld Compl~nce
RCC
x - Y T.bM
TASKS
1. MATERIAL HANDLING
L old/Unlold Machines
Stick/P~:k
Tnzr,s~r Pzrtz
Hold Puts
2. ASSEMBLY
ACTIVE SYSTEM
F~c~ Fa~Jb~ck
Hi-T-Hand
ViRon
Clear eeoc Fits
Inlerf~en~ Fits
Other Tasks
3. MANUFACTURING
Drilling
R~Im9
I-'l(:. 25. Error absorbers for industrial robots.
•
I
.:/' "L~
o.ooi
"
\
!
i I
o01
1
dl
ERROR (ram)
ERROR ABSORBERS PROBABILITYOF SUCCESSFULASSEMBLY
~Y TECHNIQUEvmu$ INITIAL POSITRON ERROR
FIG. 26.
1.0 ' - - ' - LOCATION
OEPENOENT
ON CHAMFER
SIZE
602
J . L . NEVINS and D. E. WHIINEY
POST
IO
I NN
I Gi
PROGRAMMABLE
> ~
MECHANISM I
I ~'Fz
~,~,,
1> I
MEOHAN[SM
I
1
I "+:->
Fx
0.,c
Two dimensional representation
of inserting operation.
FIG. 27. Hi-T-Hand.
•//////
.
.
~
/
/
/
/
L
-
Z
~
I RCC
0,',
d,
,%
4
tRROR (~1
ERROR A B ~ R B E ~ $
~LUTION
TIME ~,ut
INITIAL ~SITIONAL
ERROR
0
Fk~. 28.
system (Hi-T-Hand) and the passive system
(RCC) by solution time and the initial position
error. For the class of errors it can handle, the
totally passive system is quite simple, cheap, and
very fast. The research challenge thus is to create
a new device capable of solving the chamferless
ins&tion problem, but is simpler and not quite as
slow as the Hi-T-Hand.
Compared to the above systems, the use of
vision (Agin, 1977) is limited by cost, and the
x y passive table (Rosen and co-workers, is
limited to certain applications. The above devices
are only the start. Since the general requirements
for solutions have been determined (Simunovic,
1979; Nevins and co-workers, 1978a, b; Drake,
1977) it will only be a matter of time before we
are flooded with a whole group of new devices.
SYSTEMS
To identify and quantify the kinds of assembly
tasks required to assemble products, the study
shown earlier was made of ten products and
subassemblies to answer two questions: what
assembly tasks occur, and what gross directions
relative to the assembly do the parts approach
from? Eight of these products were of cast or
machined metal, one of molded plastic, and one
of a variety of plastic, sheet metal stampings, and
wires. The latter product was the exception to the
findings from the others: 70 per cent of the parts
arrive from one direction, and 35 per cent of all
tasks are single peg-hole insertions. Adding a
second direction antiparallel to the first picks tip
another 20 per cent of the parts. Screw insertions
represent 25 per cent of all tasks, and ten other
tasks make up the rest. This study justified the
choice of tasks to analyze, although more extensive surveys could alter the results. It can be
concluded that the first nine products form a
group with quantifiable characteristics. In particular, because most parts arrive from one direction, it should be possible to assemble them with
devices which have much fewer than six degreesof-freedom. This has important economic and
system configuration ramifications.
A second survey, Fig. 29, examined industrial
design practice with respect to sizes and clearances between parts. Here it was found that
particular types of parts, made by certain manufacturing techniques, reliably fall into predictable
clearance ratio ranges from 0.001 to 0.01. This
confirms the choice of assembly task difficulty to
analyze. It categorizes parts by their geometric
properties and allows the analytical tools developed to be applied to general manufactured
parts.
One of the principal problems facing programmable assembly researchers is the fact that
there are many technological options to consider
for these very complex systems. For our early
work, we chose a fairly straightforward, but very
powerful, economic modeling technique (Lynch,
1976) to help systematize these issues. Economic
models have been made of programmable assembly machines and systems of such machines.
Simple assumptions have been made concerning
machine component costs and cycle times.
Comparison models of special purpose assembly
machines and manual assembly have been developed from similar assumptions so relative
costs can be obtained (Figs 30-33). The economic
model for programmable assembly systems identified a price time product relationship as an
important tool for the selection or design of
assembly robots to be used in programmable assembly systems. The price time product is defined
Assembly Research
603
INDUSTRIAL PRACTICE-- DIAMETER VS CLEARANCE RATIO
I0
HALL BEARING FIT (OUTER DIA. TO HOUSING)
~ - r - ~ . -~
-~---~z--~l
ID
I ~ ~
\
AUTO VALVES 1974
AUTO CAMSHAFTS 1974
--GENERAL MACHINE BEARING PRACTICE
. . . . ELECTRIC MOTOR PRACTICE~
--'--PRECISION LAPPED BEARING
•
BRONZEBUSHING BEARINGS
\
\
\
~ WASHERS •
\,
\
\
\
I
[
BALL BEARING Fir (SHAFT TO RING)
0.1
I
I
I
I
I~
1 I I1~1--
I
1
I
I 1111
I
I
O.Ol
0.001
0.0001
I
" CLEARANCE RATIO
I
I I II
I
0 1
D
FIG. 29. Industrial practice.
where
Manual
MCPU =
MCPU MATP -
assemb1~ cost:
MATP * LABCST * NPART
manual assembly cost per u n i t
manual assembly tlme per p a r t , sec
ASSUMPTIONS
LABCST = cost rat e of l a b o r , S/see
NPART - number of part s in the product
a .to mat lnn I t r a n ; f |
Ft~d
TCPU -
where
r machtne~ assembly c o s t :
TSCST
PAYPER * VOL
TCPU - t r a n s f e r machine assembly cost per u n l t
TSCST - t r a n s f e r machine t o t a l c o s t
PAVPER * payback p e r i o d tn years
Next,
TSCST - NRART * TMCPP
where
TMCPP " t r a n s f e r machine cost per p a r t
Therefore
NPART * TMCPP
TCPU = PAYPER * VOL
T h i s model i s
based on the payback p e r i o d method r a t h e r
c o u n t e d cash f l o w f o r
simplicity.
worked o u t u s i n g the more a c c u r a t e
Any p a r t i c u l a r
than d i s -
10
PROGRAMMABLE STATION PRICE
$30,000
TOOLING PER PART
$7500
TRANSFER MACHINE
COST PER PART
$30.000
PART STATION
TIME
3 s
PAYBACK PERIOD
2 years to 4 years
LABOR COST
$ 7 . 5 0 / h r to
$10.OO/hr
MANUAL ASSEMBLY
STATION TIME
7 s
NUMBER OF SECONDS
PER YEAR
1.152 x TO7
case can be
method.
FIG. 30. Economic models for manual assembly and fixed
automation.
Programmable s~stem cost:
PSCST - NSTA * STAP + NPART * TOLPP
where
PSCST - programmable system cost
NSTA = number of assembly stations
STAP - s i n g l e s t a t i o n price
• TOLPR = t o o l i n g p r i c e per p a r t
The number of s ta tio n s required is
NHTA = VO6 * NPART * PARTTIME
NHPY
where
NUMBER OF PARTS
PARTTIME- assembly tlme per p a r t , sec
RSPY - 1.152 X I07 sec/yr f or an uptime f r a c t i o n of
O.E, a 250 day year and a 2 s h i f t , i6 hour day.
The assembly system cost Is then
PSCST = STAP * VOL * NPART * PARTTIME + NPART * TOLPP
NSPY
and, using the same payback period model as in e q ( I I - 2 7 ) , the assembly cost per u n i t is
TOLRP ] . _ _ _ _
PCPU = NPART [ STAP * PARTTIME
PAYPER
NSPY
+ VOL
*Includes basic feeding mechanism (bowl feeders, hoppers, magazines.
e t c ) , feed tracks and chutes, and placement or escapement devices
or conveying mechanisms t h a t l i n k the parts together.
FIG. 31. Economic model for programmable assembly.
FIG. 32. Cost assumptions for comparing manual, fixed
automation and programmable assembly systems.
as the cost of the assembler (including installation in the system) multiplied by the average
single part assembly time for that assembler. Thus,
the price-time product measures the effects of the
cost of the assembler and its speed. Assemblers
capable of comparable motions which have equal
price-time products will, when combined into
programmable assembly systems, result in the
same cost of assembly for that product. For
example, if assembler B costs $100,000 and assembler A costs $50,000, but assembler B can
assemble one part, on the average, in 2s while
assembler A takes 4s, then assemblers A or B,
when used in the correct quantity in programmable assembly systems will result in the same
cost of assembly for that product since both have
price-time products of 200,000 $-s.
604
J . L . NEVINS and D. E. WHITNEY
PRODUCTION VOLUME REGIONS WHERE
PROGRAMMABLE ASSEMBLY MIGHT BE ECONOMIC
COST TO ASSEMBLE
ONE UNIT OF
PRODUCT WITH
TEN PARTS, S
FACTORS
•
lO
PRICE-TIME PRODUCTS
•
L A B O R COST
•
R E Q U I R E D R A T E OF R E T U R N
SlOlh
MANUAL LABOR = = i a ~
@ S7 SO/h
iiI
mi
•
I|
0.01
01i
S
•
MANUAL
ASSY i
1 0
D AUTO.'~
=~III~/XE
%,,
MANUAL me # l l l l l ~ { n ~ PROGRAMMARLEAssYm~
m~
ANNUAL PRODUCTION
VOLUME. MILLIONS
FIXED AUTO ~ . . , D I ~
[Q
~l(i. 33. l-;conomic comparison of manual, fi×ed automaliom and progranlmable
assembly systems.
Also, the economic model can be used to
obtain an upper bound on the price--time product for which an assembler cannot be economically competitive with either manual assembly or
fixed automation assembly. For an assembler to
be significantly economic, with a relatively wide
range of production volumes, its price-time product value should be well below this bound. For
assumptions based on typical 1976 values of the
cost of system components, labor rate, and cost
of capital used, the upper bound was estimated at
293,000 $-s.
The structure of this model has been examined,
and conclusions have been drawn concerning the
sensitivity of unit assembly costs to various factors. The sensitivity is equally important for
identifying research issues or directions that need
to be examined. Without an analytical model,
these vital sensitivity analyses cannot be
performed.
To study adequately the area of programmable
systems, it was found necessary to construct a
programmable system test bed. This test bed
consists of a single electric arm originally constructed by the Bendix A & M Division (Dayton,
Ohio) as a prototype arm for industrial assembly.
Supporting the arm are sensors, tooling, parts
feeding, minicomputer systems and a specifically
developed software system. To test the knowledge
developed to date an automobile alternator with
17 parts was assembled on the system (Fig. 34).
I~[(;. 34. I)rogrammable system test bed.
At present it can assemble the alternator m
2min and 42s* using six tools and eight tool
changes. In principle it can assemble products
about one foot cube in size requiring insertion
motions vertically down, or (with appropriate
tools) horizontally, or vertically down with some
spin about the vertical axis. Electric motors,
some types of gear boxes, electric entrance boxes
and terminal board arrays are examples. This test
has yielded much knowledge of a technical and
*Applying routine industrial engineering this number can
be reduced to about 70s tNevins and co-workers, 1978a).
Assembly Research
e c o n o m i c n a t u r e c o n c e r n i n g p r o g r a m m a b l e assembly. S o m e of the results are s h o w n in Figs
35-37 a n d are s u m m a r i z e d in T a b l e 1.
P~RTA~EMeLYCYCLE
~'~
GRO~ MOTIONTIME
(FEEOEATOWORKAREA)
~ G~OSS
E MrmON TDME. i
,N~ERFACE MORIONT*M~feR
APPRaACH & WITHDRAWAL
A
B
-
~
DOWNWABDMOTqoN
UPWARD~OTION
B - PART"~L~ASE
C U~WARDMO~tO~
- O~WAROMOTION
U~AflDMOTION
, FINEMOTIONTIMEFORPARTPbCKUP
B
¢
~ A ~ pickup
U~ARO MOTION
FIG, 35. Robot arm trajectory definitions.
TABLE
I.
605
RESULTS
FROM
EXPERIMENTAL
PROGRAMMABLE
TEST
BED
(ll Adaptable, programmable assembly of a real industrial
product was accomplished:
adaptability was demonstrated by the system's ability to
absorb position error of 1.25ram (0.05in.) or more due
to part variations and uncertainty in the location of part
feeders;
programmability was demonstrated by ability to program
different assembly sequences using substitute tools in place
of broken tools or new "tricks" to improve system
performance;
approximately 75 alternators were assembled to test the
system.
(21 Performance of the system was thoroughly documented
(Figs. 35-37).
t3) Concept of engineered compliance was shown to be
sufficient to perform a variety of difficult single-direction
tasks that could either not have been performed at all, or
at least with the necessary reliability.
(4) The software and teaching technique appear adequate for
this class of assembly, For this problem planning was
done off-line and explicit locations of tools and feeders
was taught on-line, i.e. on the shop floor.
ASSEMBLY CYCLE TIME BREAKDOWN
ESTIMATED
TIME (158.6)
[~
MEASURED
TIME (16~ II
1
60
7-50
=,
40
30
i
I \
0
GROSS
MOTION
TIMEFOR
ASSY
INTERFACE
AND FINE
MOTIONTIME
FOR ASSY
GROSS
MOTION
TIMEFOR
TOOL
INTERFACE
AND FINE
MOTIONTIME
FOR TOOL
CHANGE
CHANGE
CYCLE
TIME
FOR
ASSY
CYCLE
TIME
FOR
TOOL
CHANGE
FIG. 36. Assembly cycle time analysis from programmable
system experiment.
A l t h o u g h the previously described e x p e r i m e n t
was the first p r o g r a m m a b l e assembly e x p e r i m e n t
to generate any real d a t a on the p e r f o r m a n c e of
these classes of systems, it w o u l d only be considered m a r g i n a l l y e c o n o m i c by industry. Clearly
m o r e definitive tools are necessary to define
system p e r f o r m a n c e in a m o r e specific way. T h e
early e c o n o m i c m o d e l i n g suffers from simplifying
a s s u m p t i o n s and the assembly stations are treated as if they were all the same kind. F u r t h e r ,
other technologies like parts feeding, m a t e r i a l
handling, a n d inspection must be i n c o r p o r a t e d if
the tool is to be of any general use.
T o this end, a new tool, based on mixed linear
and i n t e g e r - p r o g r a m m i n g , has been d e v e l o p e d
(Whitney and Graves, 1979) which on initial tests
holds p r o m i s e in this area. To illustrate, using the
assembly of the a l t e r n a t o r as a reference case, a
n u m b e r of e x a m p l e systems were analyzed for the
range of a n n u a l volume of 60,000 units to
480,000 units in eight steps (Figs 38 42). At each
step the p r o g r a m selected the system most suitable from a cost a n d p r o d u c t i o n constraint.
ANALYTICAL
DESIGN TOOL
• Mixed linear/integer programming formulation
8
I0
TIMEILf
FIG. 37. Trajectory data for the up down axis for six
assembly tasks.
• INPUT:
Robot capabilities, task requirements
• OUTPUT:
Selected robots, task assignment fractions,
task assignments when one or another
machine is broken, utilizations, costs
FIG. 38. Analytical design tool for manufacturing assembly
systems.
606
J . L . NEVINS and D. E. WHITNEY
'ij I$)
TIME/OPERATION
I
PAX.
3
~A,
$~0.000+
~.000
2 PAX
slz,ooo
RESOURCEi
4
2 PUMA
5
6
PROGRAM
TASK I
NUT
LOCKWASNER
PULLEY
FAN
SPACER
FRONTHOUSING
BEARING
RETAINEF~
3 SCREWS
USES
•
System preliminary
•
Technology
•
System
•
Transport
topology
•
Parts flow
management
•
Phased introduction
needs
design
assessment
I
breakdown
resistance
3
S~ACER
1~
)2
13
14
ROTOR
TIGHTENNUT
REARhOUSING
3 SCREWS
I
8
100
100
8
:(,. 39. Input example using alternator as an cxamplc.
~Ax
a
4
s
X
X
........
....!!!!I!!
e
7
8
X
X
X
x = UNIT IS FULLY
x = U N I T I~ A L ~ T
LOADE0
FULLY
1-tG. 42. List of p r o g r a m uses.
Many simplifying assumptions and approximate
data were used in these runs. The results should
therefore be taken as an indication of the
program's capabilities, but not used to generalize
on the suitability of any particular robot.
Although robot systems have been shown for
illustration, the program will work equally well
for complex systems involving manufacturing,
material handling, parts feeding, and inspection
assuming that the necessary data base on these
systems has been established. The necessary data
bases (Fig. 43) include the equipment data base
(available technology) as well as the operations
data base for the manufacturing and assembly
constraints.
LOAOEU
PUMA
2 PUMA
5pSCR DR
AUTOPL I
. . . . . . . . .I
AUTOPL 2
........
~NUAL
i
X
......
b
.
.
.
plan
.
ON~SHIF~UNIT
*f ~. ~AVBACK
F-I(~. 40. Typical results.
L I M I T I N G V A L U E S OF C O S T / U N I T vs
V O L U M E FOR S A T U R A T E D C O N F I G U R A T I O N S ONE Y E A R P A Y B A C K
$2.00
$1.00
I-
8
w
<
$0.10
10000
I
J
i
I
I
I I I I
l
100,000
UNITS/YEAR
FIG. 41. E c o n o m i c
comparisons.
I
I
I
I
I J I t
1,000.000
Assembly Research
607
A DESIGN SYSTEM (FUTURE)
(AVAILABLE TECHNOLOGY)
(MANUFACTURING & ENGINEERING)
PROGRAM TO
CALCULATE
OPERATION TIMES
ANO EQUIPMENT
COSTS
.EGR,STIC
INPUTS
l
I ,N'EGER
L.NEAR1
PROGRAMS
~
J
f REL,AB,L,TY,
_l
-L
8REAKOOWN
s uoY
MACHINE SELECTIONS
AND TASK ASSIGNMENTS
FIG. 43. Completedesign showing required data bases (economicand technical) and
method of inputinginnovativedesignideas.
ASSEMBLY SYSTEMS
The previous discussions dealt with tools and
techniques of importance to the assembly of a
product in a very limited way. That is, only
assembly functions were considered and the supporting systems like part feeding, inspection and
material handling were ignored. However, the
latest design tool based on mixed linear and
integer-programming does offer the potential for
handling these systems as well as the ability for
examining specific technology items like vision
system. So if the problem can be reduced to one
of annual volume and costs, then this technique
will work.
Let us consider more complex problems. For
example, consider a product where assembly
steps and manufacturing steps are intertwined
and may actually share the same machines with
different setupts. Here it may be necessary to
cluster the operations into separate subsystems or
to consider possible redesign of the product. The
choice of clusters depends not only on the technological capabilities of machines, but also the
need to support different manufacturing strategies. Depending on the market for the product,
one may have to build to order, or build subassemblies to stock and final assemblies to order.
Or one may, for large enough production volume
and order quantity, build entirely to stock.
Economic studies show that increased volume
will be best served by less programmable but
more efficient technologies. At present there are
no design tools for systematically attacking such
problems.
Pieces of the necessary tools have been
developed principally by the management science
groups for solving the above class of problems.
However, these tools must be developed and
integrated. The artificial intelligence people have
also been examining related issues and have
created deductive planning and path finding
techniques for relational data bases (Kellog and
co-workers, 1978). These systems are quite big
and expensive to operate. Further, they, and for
that matter all the tools mentioned so far, need
extensive development of data bases, both
technological and economical, if they are to be at
all useful.
We are just now beginning to apply these tools
and beginning to appreciate the complex set of
questions that arise when one seeks to apply
them in industry. But, only through this kind of
process is there any hope of creating the
understanding necessary for designing the newer
systems of the future. As the result of the PSP
study indicated (Figs 2-4), only a wide spectrum,
multidisciplined approach can achieve the kind of
productivity gains industries and nations are
seeking.
ASSEMBLY TECHNOLOGY
Assembly technology can be classified into two
broad groupings, namely: (1) system issues, and
(2) component issues.
System issues
Product design. Although the importance of
this work has not really been highlighted, it is
quite obvious that maximum system efficiency
will be realized only when system and product
design are truly integrated. It does not matter
what the system is. It can be a manual or special
machine, or a programmable system. The only
thing that matters is that the limitations of the
608
J . L . NEVINS and D. E. WHITNEY
particular assembly techniques are compensated
for in the product design. It has often been found
in special machine design that once a product
has been redesigned for that assembly technique,
it is often significantly cheaper to assemble
manually.
The part mating studies at the present are our
best technique for indicating system design
requirements. But this work is not complete, so a
complete programmable system product design
specification cannot be written.
Analytical tools and simulation. Three types of
analytical tools are needed in this field: (1) part
mating analysis techniques, especially to treat
problems of non-rigid parts; (2) assembly system
design and operational tools, to specify system
needs in the face of uncertain demands as
described earlier; and (3) assembly system
financial analysis and justification methods which
adequately take account of system flexibility.
Parts fi, eding. The parts to be assembled must
be presented to the assembly system somehow.
There are two crucial issues, as noted below.
(1) Should the parts be fed oriented in the
proper way for the robot, as they are for other
automated processes, or should they be fed (more
cheaply) unoriented, with the robot having the
responsibility for orienting them'? The former
makes for a simple robot system and pushes the
responsibility off onto other parts of the factory.
The latter requires a complex system, but one
that can more easily be introduced into present
factories one work site at a time.
12) How to keep bad parts from entering the
assembly system and how to carry out in process
and final inspection'? People are very good at
both of these when their performance is up to
par. Machines usually detect bad parts by
jamming, taking time and requiring a person to
free them. The required inspections can be quite
subtle and often have not been quantified ("click
means it's good, clunk means it's bad"). So far
this problem has been addressed only by
proposing presorting of the parts by people or by
supposing better and better computer TV vision
systems. Table 2 lists some alternative feeding
methods and comments on them.
Inspection systems. Currently there are no
systematic ways of looking at inspection only
singular solutions and devices. Some description
of this current work is described below in this
section on sensors under component issues.
Sol?ware and hardware control systems. This
section discusses design of computer controls for
robot arms, while the next section discusses
philosophies for teaching new assembly tasks to a
robot.
T A B L E 2. A L I E R A N I F
Method
Conventional feeder
tracks, vibratory
bowls, etc.
I>ARF I I [{DIN(i MI I l l ( ) l ) S
Comments
Examples
The least programmable.
Paris arrive oriented
but often a person
docs the orienting.
('an use simple robot
lfewer than 6 axes
could be sufficientl
[)raper PAX
U-6000
SIGMA
('onvcntional
assembly
machines
Parts dumped
randomly onto a
vision station
Paris arrive disoriented.
Requires TV/Computer.
Requires 6 axis arm and
time to reorient
Bendix PAX
U. of
Rhode Island
SRI
Palletized kits of
identical parts
Parts arrive oriented.
A person orients them
(could be oriented}.
Simple robot
Weslinghouse
design
Palletized kits of
parts needed for one
product unit
Parts arrive oriented.
A person orients them
(could be automated).
Simple robot
?
The same kit plus
all the necessary
tools
Better for model mix
assembly.
Simple robot
?
A computer control system for a robot[s) must
consist of one or more computers plus programs
to run the arms, read sensors, input new
assembly instructions, execute these instructions,
and interact with the operator. Data must be
inputed, consisting of points in space, times and
speeds, and values for forces. The arms may be
controlled in a variety of endpoint coordinate
systems. Separate computers may be used for
sensor processing, operator interaction and so on.
Three approaches have been taken so far, as
described below.
(1) Complex systems consisting of compilerbased languages, IBM 360 or similar host
computer, IBM System 3 or System 1 real time
controller, and
perhaps
additional
sensor
processors. Examples include work at tile
Stanford
University
Artificial
Intelligence
Laboratory (Finkel and co-workers, 1974) and
IBM (Lieberman and Wesley, 1975).
(2) True minicomputer based systems (PDP
11-34 or Data General NOVA) stand alone,
usually with interpreter based languages which
are simpler than the above compilers (examples
include Bendix PAX, DL DIAL, SIGMA)
(D'Auria and Salmon, 1975).
(3) The simplest: hardwired logic set up to
execute a sequence of taught spatial points one at
a time. A computer may be used to input these
points. Such systems have quite limited
adaptability. The most they can do is stop if
conditions are observed to change. The Unimate
class of machines are an example.
Assembly Research
Naturally, the simplest is the cheapest and
most reliable. The major issue again is how much
complexity is necessary. It is nice to contemplate
an adaptable system, but one must be able to
show exactly what events the system is expected
to be able to recover from. The hard fact is that
many simple events (dropped parts is a severe
one) which a person could handle easily would
require a dazzling robot system.
A second major issue is how to obtain the
desired arm motions. In our work we have
identified three motion regimes: gross motions
(high speed, low accuracy transfers), fine motions
(when parts are touching during assembly), and
interface motions (between gross and fine when
the arm is lining up the parts) (Fig. 35). At
present the robot performs all of these except if a
Hi-T-Hand or Remote Center Compliance (RCC)
are in use. Exceptions occur where binary force
sensing (detecting contact) (D'Auria and Salmon,
1975) or visual servoing (Agin, 1977) (using wrist
TV to locate a hole) are used, but even here the
main control technique is for the computer to
drive the arm to a position or through a
sequence of positions. Forces, velocities, even
frequencies, could be used as control references,
but so far have not. The required computer
complexity for such methods has not been
evaluated. Table 3 compares a number of current
systems.
609
Teaching methods. Teaching a robot a new
assembly task involves determining the assembly
actions required, their nominal sequence, and
their detailed breakdown into robot motions,
tests, loops, tool actions, contingency actions, and
so on. This breakdown
must then be
communicated to the robot controller.
At present all robot programming is geometric.
That is, points and orientations in space are the
fundamental data used in programming. Force
data is used mostly as a check or to conform to
load requirements of the assembly itself ("torque
nut to 10N-m'). Intellectually this means that
all programming is physical, giving detailed "how
to" guidance to the robot. There is, as yet, no
functional programming ("this is what I want,
you figure out how"), although various
approaches have been suggested in research
laboratories (Whitney, 1969).
The major current research issue in robot
teaching involves what is termed explicit versus
implicit programming. Explicit programming
consists of expressing the desired robot actions in
terms of explicit points in space and a sequence
of logical actions. This technique is common
among the simpler computer architectures. The
implicit technique requires much more computer
power and assumes that the robot and work
comprise a surveyed environment which has been
stored in the computer as a "world model".
TABLE 3. COMPUTER CONTROL SYSTEM OF VARIOUS ROBOTS
Bendix PAX
Draper PAX
U-6000
U-500
Shuttle
SRI
Hardware
architecture
One mini
plus disk
One mini
plus disk
External
servo hard-wired
control
6 micros
plus diskette
One
computer
PDP-10
host plus
2 minis
Control
complexity
Several coordinate
systems,
Stored points,
Control of
2 arms.
Program
seq. with
conditionals
Two coordinate
systems for
teaching.
Stored points.
Force
feedback
teaching aid
Extra computer
for teaching,
Stored points,
Stored points
Two arms
Seve.ral
coordinate
systems,
Stored points.
Several
coordinate
systems,
Stored points,
Manual control
Program
sequence with
conditionals
visual servo
No
Multiple arm
control
Trajectory
control
Vibration
suppression?
Use of
sensors
Yes
No
Yes
Nominal
velocity
between taught
points
Nominal
velocity
between points.
Position error
No
No
No
Force, vision
Force
monitoring
None
No
No
Velocity
ref.
Nominal
velocity
No
No
No
None yet
None
Contact,
force vision
NOTE: Draper PAX is an assembly station built around the prototype model of the Bendix PAX arm, but having a computer
system and tooling developed by Draper Laboratory.
610
J . L . NEVINS and D. E. WHITNEY
TABLE 4.
Bendix PAX
COMPARISON
O I~ R O B O I ( O N I R O L
I.ANGUA(iES
Draper PAX
SIGMA
U-500
Stanford A.I. Lab
SIGLA
VAL
AL
M ix
Implicit
Language
Name
?
FSYS
Explicit/Implicit?*
Implicit
Explicit
Compiler/Interpreter ?
Interp.
lnterp,
lnterp,
lnterp.
Compiler
Features
Conditionals.
Multi-arm
interlocks.
Link to vision
station.
Macros.
Link to force
monitoring.
Conditionals.
Loops.
Interlocks.
Link to force
monitoring,
Conditionals.
Interlocks.
Immediate
execution of
single commands.
World model
coord, frames.
Block structure.
Conditionals.
Two arm
coordination.
*Judgment here grossly oversimplified.
Locations and orientations can be referred to by
name, and displacements caused by the robot's
actions and the buildup of parts are kept track of
automatically. If this is combined with trajectory
planning, then something closer to functional
programming results, because the computer will
calculate all the missing data. Figure 37
illustrates the complicated trajectory data that
would have to be described and stored.
Explicit programming can be compared to NC
programming in APT, whereas implicit is more
like projected CAD/CAM techniques in which
the computer's internal model of the part is
converted automatically into the data needed to
support the NC programmer's description of the
sequence of cuts. There is general agreement that
implicit programming makes the programmer's
job easier, but only if he is a programmer by
training. Explicit programming requires less of
the programmer and the computer, but it will
probably limit the degree of adaptability which
can be programmed in and incorporated into the
program.
Table 4 compares several current systems.
errors, very close clearances, and no chamfers or
lead-ins.
(1) Active (motorized) wrist with sensing this
device holds the workpiece, the robot carries it to
the work site and the device maneuvers the
pieces together. The best known of these is the
Hi-T-Hand (Goto, Inoyama and Takesau, 1974).
It can assemble parts that have no chamfers, and
do so with very light touch. Assembly usually
takes 3 to 5s, which is relatively long, and the
device is rather fragile and in need of continual
adjustment (Fig. 27).
(2) Active wrist with estimation theory this is,
as yet, a theoretical technique with little
laboratory verification. Modern control theory
and mathematical models of the assembly
operation are combined to yield a closed loop
technique for assembling parts (Simunovic, 1979:
Nevins and Whitney, 1973).
In all of these approaches, the major issues
involve how much complexity is enough to
handle common assembly tasks and how can
these tasks be modeled adequately so that
effective devices can be designed. If one assumes
the existence of chamfers and enough repeatability
Component issues
Articulating devices such as robot arms. There
are many concerns in this area. They range from
kinematic design, number of degrees-of-freedom
(in the arm or in the workplace), size, actuators,
actuator location (shoulder or at respective
joints) and accuracy. A paper by Whitney
(Whitney, 1979) goes into great detail in this with
Table 5 summarizing the issues and Table 6
listing the various assembly robots presently
available.
Adaptive devices. We describe here a number of
other devices used in assembly. With the
exception of the conventional fixed mechanical
workhead, all these are recent developments.
They are mostly wrist or hand devices and their
goal is to help a robot perform assembly under
difficult conditions, such as large part-to-part
TABLE 5. RESEAR( H ISSUESFOR ROBO] ARMS
il) Kinematic design and number of axes. Included in thi~
discussion is the question of, should you have an elbo,a
or slide axes like machine tools'?
I21 Size two issues here. Namely, how big a reach should an
arm have. and why are they so inefficient? Typically it
takes a 500 to 1000 kg system consuming 40kW to carry
2kg loads a distance of one meter in three to five
seconds.
13) Actuators
should they be hydraulic or electric'?
(4} Actuator location
at tbe shoulder'?
should they be located at tile joints or
(5) Accuracy- this issue is irrelevant if the arm is used only
in a playback or incremental mode. In this case only
repeatability and resolution are important. An arm, when
coupled with an RCC, requires only enough accuracy to
get to the chamfer (1 to 2mm) which is much cruder than
present systems, and therefore possibly cheaper.
Assembly Research
611
TABLE 6. COMPARISON OF ASSEMBLY ROBOTS
Bendix PAX
Unimate 6000
Unimate 500
SIGMA
Shuttle
manipulator
Kinematics
Cylindrical plus
wrist
Elbow
Elbow
XYZ
Elbow
No. of axes
Six
Six
Five
Three-five
Six
Size
0.5m × 0.5m × 200 °
~ l . 3 m reach
~lm
~0.5m 3
~18m
Actuators
Electric/gears,
ball screws
Hydraulic/gears,
ball screws
Electric/gears
Electric/
ball screws
Electric, gears
Repeatability
~0.2m
~0.2 m m
~0.1 m m
~0.1 m m
5cm
Weight of
moving parts
200 kg ?
900 kg ?
60 kg
50 kg ?
500 kg
Payload
2 kg
10 kg?
3 kg
?
32 tons
NOTES:
(1)
(2)
(3)
(4)
(5)
(6)
PAX was a research effort by Bendix, now abandoned.
Unimate 6000 is a cooperative effort between Unimation, Inc., and Ford Motor Co.
Unimate 500 is being built to General Motors Co. specifications.
S I G M A is marketed by Olivetti, s.P.a.
The Shuttle Manipulator is being built by SPAR, Ltd., Toronto, Canada.
All data in this table are estimates by the author.
in robots, parts, grippers, jigs and feeders so that
the chamfers will meet during assembly, then the
simple Remote Center Compliance will assemble
the parts.
When parts have 0.01 mm clearances and no
chamfers, the Hi-T-Hand suffices, but often so
does simple vibration (Dunne, 1977) so the issue
is still open. What does seem clear is that visual
imaging is less useful in the 0.01 to 1.0mm range
than contact-based techniques (Fig. 44). Further
description of this problem can be found under
sensing in the next section.
0.01
am
0.1
I
am
I sm
lO am
10 cm
I
I
|
THE ROBOT OPERATES IR THIS REGION
,41
IF THE
I m
I
L
ARE CHAMFERS, THE REMOTE
CENTER O~qpLIANCE OPERATES 1N THIS REGION
THE H I - T I - H A N D OPERATES IN
THIS REGION EVEN NITH NO CHAMFERS
COMPUTER-T¥ CAN AID
L
":ASSEMBLY IN THIS 'REGION
PROXIMITY SENSORS
"OPERATE IN THIS ~EGIOR
NEGATIVE
CLEARANCE
reach
EAR&RCE BETWEEN PARTS OR
POSITIONING ERROR OF ROBOT,
~K
PANTS, Off JIBS
~
I
I
RANGE OF MOTION REQUIRED
I
I
FIG. 44. Hierarchy of sensors and error absorbers vs linear
distance or error.
Sensing systems. It is generally agreed that
sensing, especially monitoring, is essential to
much of automation, and assembly is no
exception. A conclusion of Draper research is
that quite a lot of assembly can be done without
direct sensing of any kind, except monitoring. In
other areas of sensing, especially TV vision, there
has not yet emerged an adequate balance
between cost and effectiveness. An idea of the
relative applica.tion areas of passive wrists, active
wrists, robot arm inherent accuracy and vision is
emerging, however, and this is expressed in Fig.
44.
A list of some current sensing techniques, with
comments, are listed in Table 7. For the time
being, it would appear that force sensing is
confined to a monitoring role while vision can be
used for gross inspection and as an aid in part
feeding, if one does not ask too much (a variety
of parts, all jumbled together, is too much for
people sometimes).
Tools and grippers. An assembly robot will
need at least one tool or gripper to do its job.
How many more than one is the question. There
are several options in use and others have been
proposed (see Table 8). The issue is whether each
part or operation should have its own tool, the
problem being cost and time for tool changing.
While both of these can be attacked, the nagging
philosophical issue is whether one has a
reprogrammable system and is there a large fixed
investment in specialized tools? On the other
hand, it is doubtful that a truly universal tool can
be achieved. Even the human hand needs aid
from other tools.
612
J . L . NEVINS and D. E. WHITNEY
TABLE 7. SENSING OPTIONS
Option
Comments
Examples
Active force
sensing to guide
assembly
Remote center compliance,
if applicable, will
be much faster.
Can handle large
errors
Hi-Ti-Hand
Draper demo,
1972, 1975
Stanford A.I.
Lab
IBM
Force sensing to
monitor the progress
of the assembly
Record history
Detect drift
Prevent damage
Verify operations
Abort failed operation
Draper bench
unit
SIGMA
Proximity sensor
Collision avoidance
Jet propulsion
Lab
TV "vision"
Part Orientation
Gross inspection
Product integrity
Feeder/tool operation
monitor.
Visual servo for
endpoint guidance
of robot
Bendix PAX
U. of
Rhode Island
SRI
Stanford A.I.
Lab
N o v r : The Draper bench unit is a manually operated inserter
equipped with a load cell and a remote center compliance.
Electronic logic will halt insertion if the forces rise above a set
level. This unit was built to aid close clearance assembly of
delicate gyroscope components.
TAt~t,F. 8. TOOL AND GRIPPER r)PTIONS
Options currently
in use
Comments
Examples
Universal socket on
robot plus tool
changing and external
tool storage
Takes time to change
tools.
Really universal
within range of tools
themselves
Draper PAX
U-6000
Multiple arms or
arm at each
work station, each
a n n with one
"'permanent" tool
Limited range of tasks
suited to intermittent
changeover
U-6000
Bendix PAX
Westinghouse
design
SIGMA
Small tool turret on
robot wrist
Limited range of tasks.
Could be heavy
U-6000
degree
of
specialization.
Hopefully,
this
specialization will occur within a carefully
designed general environment (see below l.
Software
Tools
General
apparatus
Compiler/Editor
Debugger
The Robot
Interface
Real time operator
system
Robol wrist
with tool socket
Specific items
Programs for specific
assembly operations, and
specific products
Specific tools
"'Libraries'" of useful programs and tool
concepts can be expected to emerge. For
example, four types of tools (six tools in all) were
needed by Draper Laboratory to assemble the
alternator: (1) expanding gripper for small object
I.D.; (2) contracting gripper for large object
O.D.; (3) contracting gripper for small O.D.: and
(4) screwdriver.
Part feeder
and
inspection
devices.
There
are
many devices for part feeding and inspection.
Some of these were described in previous sections
and tables. Singular solutions and devices will be
the order of the day until systematic ways of
looking at lhe problems and defining the
requirements (such as those being worked on by
Birk (Kelley, Birk and Wilson, 1977) and
Boothroyd (Abraham et al., 1977) are fully
developed.
CONCLUSIONS
We have tried to list here (but make no claim
for completeness), the reserve issues in this area.
The intent was to show that this is truly a virgin
research area. Our small successes, while
significant, have mainly helped us to get visibility
into the larger issues, and to help focus our
research.
Other possibilities
Parts fed with tool
Extends universality of
wrist socket, but takes
time
Truly universal tool
Possible'?
Product redesign to
reduce number and
variety of tools
needed
Not done as often as it
could be
A similar problem arises in software system
design. Should one expect future robots to come
preprogrammed for any assembly operation, or is
it more reasonable to expect software facilities
which can be used to compose programs for
specific operations? In both tools and software it
appears more reasonable to accept a certain
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