- Email: [email protected]
Inspection of Rolls in Roller Bearings
Theoretical and Applied Aspects of Eye Movement Research A.G. Gale and F. Johnson (Editors) 0 Elsevier Science Publishers B.V. (North-Holland), 1984
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INSPECTION OF ROLLS I N ROLLER BEARINGS
M. A. S i n c l a i r Department o f Human Sciences Loughborough U n i v e r s i t y o f Technology Loughborough, Leics. L E l l 3TU
The problem w i t h eye movement studies i s t h a t though the eye movements are r e l a t i v e l y easy t o measure and the data are p r o l i x , eye movements do n o t provide much i n s i g h t i n t o r e a l tasks w i t h o u t the use o f major assumptions, u s u a l l y on t h e nature o f i n f o r m a t i o n processing associated w i t h each f i x a t i o n . These assumptions are u s u a l l y j u s t i f i e d by c u r v e - f i t t i n g , which does n o t necessarily v a l i d a t e the model. An example (Drury and S i n c l a i r , 1983) t o i l l u s t r a t e how eye movements do n o t necessarily help i s discussed. I n addition, the r e l a t i v e f a i l u r e of a simple automatic inspection device intended t o replace the human inspector i s discussed. Over t h e past 15 years, as i n many other f i e l d s t h e r e has been an explosive growth i n the study o f v i s i o n i n human v i s u a l inspection and d e t e c t i o n tasks. There have been several studies i n v o l v i n g eye movements, b u t i n t e r e s t i n these has wained f a i r l y r a p i d l y . This has n o t been occasioned by any indeed, they are f a i l u r e s i n the methods f o r measuring eye movements almost t o o successful i n producing data b u t because there has been no what do we good t h e o r e t i c a l answer t o the question, 'Now we have the data do w i t h i t ? ' Certainly, one nay p i c k up one's s t a t i s t i c a l c l u b and beat the data t o death t o produce p r o b a b i l i t y d i s t r i b u t i o n s and other d e s c r i p t i v e s t a t i s t i c s , b u t t h i s i s merely surface analysis.
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There are some basic assumptions made about which there i s general agreement. These are:-
1)
The purpose o f v i s i o n i n i n d u s t r i a l inspection tasks i s t o gather information relevant t o decisions about q u a l i t y .
2)
The purpose o f eye movements i s f i r s t l y t o ensure t h a t the v i s u a l f i e l d i s scanned, and secondly t o b r i n g foveal v i s i o n t o bear on p a r t s o f t h e v i s u a l f i e l d t h a t are information-rich.
3)
The purpose of information-gathering i s t o make decisions about q u a l i t y based upon a s e t o f i n t e r n a l i s e d standards.
Unfortunately, w h i l e these assumptions are c l e a r l y i n t e r - r e l a t e d , they do n o t form a closed set. This would r e q u i r e a f o u r t h assumption, about which t h e r e i s general disagreement:-
4) A l l other variables are immaterial; i n other words, the human being acts as a robot.
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From i n t r o s p e c t i o n , if n o t f r o m o b s e r v a t i o n , t h i s i s c l e a r l y u n t e n a b l e . T h i s t h e n i m p l i e s t h a t eye movements cannot be regarded as i n d i c a t o r s o f mental a c t i v i t y s i n c e t h e e x c l u s i v e l i n k between t h e two has been broken. The o n l y r e m a i n i n g way whereby such a l i n k may be e s t a b l i s h e d i s by p o s t u l a t i n g a model w h i c h s t a t e s such a l i n k and t h e n d e m o n s t r a t i n g t h e l i n k b y some e x p e r i m e n t a l means. An example o f t h i s i s t h e scanpath t h e o r y , f i r s t p u t forward b y Noton and S t a r k (1971) and c o n s i d e r e d most r e c e n t l y b y Groner e t a1 (1983) a t t h i s conference. However t h e t h e o r y S n o t w i d e l y accepted, and i n t h e o p i n i o n o f t h i s a u t h o r r e q u i r e s more s u p p o r t f r o m experiments. I f one accepts t h e i d e a s o f Schneider and S h i f f r i n (1977) and v i c e v e r s a (1977), i n c l u d i n g t h e e x t e n s i o n i n t o v i g i l a n c e ( F i s k and Schneider, 1981), t h e n i t seems t o t h i s a u t h o r t h a t he o n i y c o n d i t i o n s under w h i c h such an.approach c o u l d b e t e n a b l e would be i n s i t u a t i o n s where t h e whole process was ' a u t o m a t i c ' , i n t h e i r sense. T h i s o f course r e q u i r e s t h a t t h e c r i t i c i s m s d i s c u s s e d by Broadbent (1982) should be d i s r e g a r d e d , o r o t h e r w i s e answered. Such a long-term i n v e s t i g a t i o n has y e t t o be c a r r i e d o u t . Another example where an e x p l i c i t l i n k i s b u i l t i n i s t h e v i s u a l sampling model (Senders, 1966) though t h e model was n o t envisaged f o r use i n t a r g e t d e t e c t i o n . O t h e r models i n t h i s g e n e r a l f i e l d o f t a r g e t d e t e c t i o n and r e c o g n i t i o n do n o t make t h i s e x p l i c i t l i n k , though as s t a t e d b e f o r e t h i s i s due i n many cases t o t h e e x c l u s i o n of p s y c h o l o g i c a l and p h y s i o l o g i c a l v a r i a b l e s e n t i r e l y f r o m t h e models. A r e v i e w a r t i c l e b y Greening (1975) g i v e s some e x c e l l e n t examples o f t h i s , i n e a r l y work on models o f m i l i t a r y t a r g e t a c q u i s i t i o n . The same t r e n d can be seen i n more r e c e n t models p u b l i s h e d i n t h e l i t e r a t u r e (e.g. S i l b e r n a g e l , 1982, Akerman and K i n z l y , 1979, and K r a i s s and Kuaeuper, 1982). O f course t h e r e a r e reasons f o r t h i s i n t h a t t h e r e l a t i o n s h i p s between search v a r i a b l e s and p s y c h o l o g i c a l v a r i a b l e s a r e i l l - u n d e r s t o o d , and a r e n o t w e l l q u a n t i f i e d a t a l l . What has n o t a s s i s t e d i n c o r r e c t i n g t h i s s i t u a t i o n i n t h e o p i n i o n o f t h e a u t h o r i s t h e tendency t o v a l i d a t e models by means o f c u r v e - f i t t i n g ; f o r example, by e x a m i n a t i o n o f t h e c u m u l a t i v e % of d e t e c t i o n s ( w i t h r e s p e c t t o passage o f t i m e ) . By c o n f l a t i n g temporally-dependant d a t a i n t o a g l o b a l v a r i a b l e such as t h i s most o f t h e f i n e d e t a i l i s e l i m i n a t e d and t h e d a t a t h e n appears t o be v e r y well-behaved, and a l l o w s of exemplary mathematical t r e a t m e n t . An example o f t h i s i s t h e e x c e l l e n t match o b t a i n e d by f i t t i n g exponential curves t o cumulative p r o b a b i l i t y o f d e t e c t i o n data. T h i s i s u s u a l l y performed on t h e assumption t h a t random search has o c c u r r e d , and t h e f i t o b t a i n e d i s u s u a l l y v e r y good even i n t h o s e s i t u a t i o n s where i t i s p a t e n t l y obvious t h a t search i s non-random. An example t h a t perhaps i l l u s t r a t e s t h e l a c k o f d i r e c t a s s o c i a t i o n ( i n t h e s t a t i s t i c a l sense) between search and d e c i s i o n s i s g i v e n i n what f o l l o w s , w h i c h i s a b r i e f r e p o r t o f work t o b e p u b l i s h e d elsewhere (Drury and S i n c l a i r , 1983). The work a l s o i l l u s t r a t e s p a r t i c u l a r advantages t o be o b t a i n e d f r o m human v i s u a l i n s p e c t i o n as opposed t o a f a i r l y s i m p l e m i croprocessor-based o p t i c a l d e v i c e i n t e n d e d t o accomplish t h e same function. The p r o j e c t t o o k p l a c e i n an i n d u s t r i a l s e t t i n g , and was an i n v e s t i g a t i o n o f t h e e f f i c i e n c y o f i n s p e c t o r s o f r o l l s f o r r o l l e r - b e a r i n g s t o be used i n t h e manufacture o f j e t engines. A l l r o l l s p a s s i n g t h r o u g h t h e i n s p e c t i o n process were i n s p e c t e d t w i c e ; once f o r dimensional t o l e r a n c e and once f o r s u r f a c e d e f e c t s . I t i s t h e l a t t e r o p e r a t i o n which was examined. The r o l l s
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vary i n size; i n t h i s e x e r c i s e o n l y those from 7 t o l l m m i n length, and 7 t o l l m i n diameter were considered. F a u l t s c o u l d occur i n any o f 3 regions - the b a r r e l , t h e ends, o r t h e curved edges between t h e ends and the b a r r e l . The l a t t e r areas are c a l l e d t h e corners. Four classes o f f a u l t c o u l d occur; p i t s (pock marks, t y p i c a l l y caused by r u s t i n g ) , scratches, n i c k s and dents, and t o o l marks ( u s u a l l y a c i r c u m f e r e n t i a l gouge on a corner, o r p a r a l l e l c u t s on an end). Only t h e l a s t 3 classes are considered h e r e a f t e r . R e j e c t thresholds f o r these f a u l t s v a r i e d depending upon t h e n a t u r e o f the f a u l t ; f o r t h e purposes o f t h i s d i s c u s s i o n any f a u l t o f a s i z e g r e a t e r than 25/10000inch i n diameter was r e j e c t a b l e , (It should be remembered t h a t even t h e hardest o f metals appear p l a s t i c if the u n i t o f measurement i s small enough). To measure i n s p e c t i o n performance, a t e s t batch o f r o l l s was created. T h i s comprised a s e t o f 110 f a u l t y r o l l s , w i t h 182 f a u l t s upon them, p l u s a f u r t h e r 52 r o l l s deemed acceptable. A l l t h e r o l l s were numbered and mapped p h o t o g r a p h i c a l l y . During the experiment t h e r o l l s were kept i n o i l - f i l l e d tubes t o minimise handling damage; even so, t h e r o l l s had t o be resubmitted t o t h e f i n i s h i n g (SWECO) process t o r e s t o r e the surface f i n i s h . The r o l l s were checked t o make sure t h e maps were s t i l l r e l e v a n t . Twelve i n s p e c t o r s were used i n t h e experiment. A l l had had a t l e a s t 2 years experience, and had undergone a t r a i n i n g programme. T h e i r ages ranged from 20 t o 50 and, by company p o l i c y , had had t h e i r eyesight checked and c o r r e c t e d by an o p t i c i a n w i t h i n the p a s t 3 years. One i n s p e c t o r was t e s t e d each day a t t h e beginning o f h i s o r her s h i f t (7.00am o r 3.00pm). T y p i c a l l y , t h e t e s t l a s t e d f o r 4 hours, and took place a t a normal works t a t i o n i n an adjacent room. The w o r k s t a t i o n consisted o f a desk-mounted l i g h t f i x t u r e w i t h two f l u o r e s c e n t tubes p o s i t i o n e d above the i n s p e c t o r s ' hands when h a n d l i n g t h e r o l l s . A t r o l l l e v e l , t h e i l l u m i n a t i o n averaged 887 l u x , w i t h a background i l l u m i n a t i o n o f 95 l u x . Given the m a t e r i a l o f t h e c y l i n d e r s , t h e r e was a g l a r e problem from the surface. Viewing d i s t a n c e averaged 25 cms, which f o r a f a u l t t h r e s h o l d value of25/10,000 i n c h gives an angular subtense a t t h e eye o f 50 seconds. To a s s i s t i n t h e task, i n s p e c t o r s were given a r e c t a n g u l a r g l a s s x4 m a g n i f i e r . Most i n s p e c t o r s a f f i x e d t h i s t o t h e i r l i g h t f i x t u r e w i t h rubber bands; one i n s p e c t o r refused t o make any use o f i t . The t e s t batch o f r o l l s was presented on trays, as was normal. Each i n s p e c t o r would then i n s p e c t t h e r o l l s s i n g l y , r o t a t i n g the r o l l s i n the hands, and u s i n g e i t h e r gloves, a c l o t h , o r barrier-cream-protected f i n g e r s , t o r u b o f f d u s t p a r t i c l e s . This was a necessary process, as i t was f r e q u e n t l y d i f f i c u l t t o d i s t i n g u i s h between dust and a f l a w . As each f l a w was found t h e i n s p e c t o r would use a s t y l u s f o r f a c t u a l confirmation, and then t h e type, p o s i t i o n , and f a u l t c l a s s i f i c a t i o n was recorded. Normally, as soon as any f a u l t i s found, t h a t r o l l i s r e j e c t e d . I n t h i s experiment, because t h e r e were occurences o f m u l t i p l e f a u l t s on t h e r o l l , i n s p e c t o r s were asked t o keep on i n s p e c t i n g u n t i l they thought they had found a l l f a u l t s ( i f any). No time l i m i t was placed on t h i s . Watching t h e i n s p e c t o r s a t work revealed immediately t h a t t h e r e was a f a i r l y standard method o f search involved. A l l i n s p e c t o r s organised t h e i r search i n t o 3 phases - b a r r e l , corner and end though t h g order v a r i e d between i n s p e c t o r s . The r o l l was always h e l d a t about 45 t o t h e v e r t i c a l i n the s a g g i t a l plane f o r i n s p e c t i o n o f t h e b a r r e l . I t was then r o t a t e d s l o w l y about t h i s a x i s w h i l e t h e eyes performed v e r t i c a l saccades up and down t h e b a r r e l . This produced a zig-zag p a t t e r n o f f i x a t i o n s around the b a r r e l ; t y p i c a l l y 3 f i x a t i o n s were used on each z i g ( s t a r t , middle, and
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f i n i s h ) . The c o r n e r was i n s p e c t e d by h o l d i n g t h e r o l l p o i n t i n g towards t h e body i n t h e s a g g i t a l plane, and t h e n p e r f o r m i n g a s m a l l , 2 - a x i s r o t a t i o n o f t h e r o l l t o a l l o w a r e f l e c t e d b a r o f l i g h t t o p l a y a l l round t h e c o r n e r . A s i m i l a r , more o s c i l l a t o r y movement was performed t o i n s p e c t t h e end, In t h e l a t t e r case t h e f i x a t i o n p a t t e r n was r o u g h l y c i r c u l a r , whereas f o r t h e edge t h e eyes f o l l o w e d t h e b a r o f l i g h t , s i n c e changes i n t h i s r e v e a l e d t h e presence o f a f l a w . These comments a r e based upon o b s e r v a t i o n o f t h e eye movements u s i n g a s m a l l m i r r o r , and a r e t h e r e f o r e q u a l i t a t i v e . The d a t a o b t a i n e d was compared w i t h t h e maps, and was s o r t e d i n t o t h e f a u l t c l a s s i f i c a t i o n s . W i t h i n each c l a s s i f i c a t i o n i t was f u r t h e r s o r t e d i n t o H i t s , Misses, C o r r e c t Acceptances, and F a l s e Alarms. T h i s was performed f o r t h e d e f e c t i v e r o l l s and t h e good r o l l s s e p a r a t e l y ( t h o u g h i n t h e l a t t e r case, t h e r e were no h i t s , by d e f i n i t i o n ) . The h i t s and t h e f a l s e a l a r m c a t e g o r i e s f o r each i n s p e c t o r were then p l o t t e d g r a p h i c a l l y on R e c e i v e r O p e r a t i n g C h a r a c t e r i s t i c s diagrams ( F i g s . 1 - 3 ) .
0
20
F i g . 1.
40
60
% False Alarm Rate R.O.C.
80
f o r N i c k s and Dents
100
Rolls in Roller Bearings
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I
s
% False Alarm Rate Fig. 2.
R.O.C.
for toolmarks
% False Alarm Rate Fig. 3.
R.O.C.
for scratches
311
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I n p a r a l l e l w i t h t h i s experiment u s i n g human i n s p e c t o r s , an e v a l u a t i o n o f an o p t i c a l , microprocessor-based devise was undertaken, which was i n a breadboard stage o f development. I t was intended t h a t t h i s device should r e p l a c e t h e i n s p e c t o r s . The sensing element was an a r r a y o f photodiodes, behind a focussing system. The d e t e c t i o n p r i n c i p l e was t o compare t h e o u t p u t o f each diode t o i t s 8 nearest neighbours, and t o s i g n a l any d i f f e r e n c e s t h a t were g r e a t e r than a s e t t h r e s h o l d . I n machine u n i t s , t h i s t h r e s h o l d c o u l d be s e t t o any value between 0 and 255; i n p r a c t i c e , o n l y t h e range between 100 and 225 was o f any importance. 100 would r e j e c t almost every r o l l ; 225 r e j e c t e d v e r y l i t t l e . Because a range o f thresholds t h i s i s shown i n could be used, i t was p o s s i b l e t o o b t a i n an R.O.C.; F i g s . 1 - 3. These t h r e e f i g u r e s i n d i c a t e t h a t f i r s t l y t h e s c a t t e r o f t h e p o i n t s f o r t h e i n s p e c t o r s r e v e a l s very d i f f e r e n t performance among t h e i n s p e c t o r s d e s p i t e t h e very s i m i l a r search behaviour used, thereby i l l u s t r a t i n g t h e p o i n t s made e a r l i e r . The wide s c a t t e r i s probably due t o poor i n i t i a l t r a i n i n g and no subsequent r e c a l i b r a t i o n o f t h e i n s p e c t o r s , a l l o w i n g them t o s l i p t o an i n a p p r o p r i a t e s e t o f perceived t h r e s h o l d s f o r r e j e c t i o n . Secondly, t h e r e i s t h e comparison between the i n s p e c t o r s and t h e microprocessor-based device. I f one makes use o f t h e usual i n t e r p r e t a t i o n o f R.O.C. diagrams on t h e b a s i s o f s i g n a l d e t e c t i o n theory, then those i n s p e c t o r s p l o t t e d i n t h e area above and t o t h e l e f t o f t h e machine's R.O.C. have performed b e t t e r . Since t h i s l o o s e l y accords w i t h d e s i r a b l e p r a c t i c e i n production, t h i s c r i t e r i o n i s used here. I t w i l l be seen t h a t i n general t h e human i n s p e c t o r s perform b e t t e r than t h e machine. On scratches and n i c k s and dents, humans a r e s i g n i f i c a n t l y b e t t e r (p<0.05).
A f u r t h e r a n a l y s i s was undertaken t o i l l u s t r a t e the d i f f e r e n c e between search performance and d e c i s i o n performance by human i n s p e c t o r s . Successf u l search corresponds t o t h e d e t e c t i o n o f any f a u l t o r f l a w i n t h e mapped p o s i t i o n on a r o l l . Successful decision-making corresponds t o making t h e c o r r e c t d e c i s i o n once a v e r i f i a b l e f l a w has been detected. The data allowed t h i s a n a l y s i s t o be performed, and t h e r e s u l t s a r e shown i n Table I ( i t should be noted t h a t by m u l t i p l y i n g t h e search performance by t h e d e c i s i o n making performance f o r each i n s p e c t o r one o b t a i n s t h e p o i n t s 3). T h i s t a b l e r e v e a l s the considerable d i f f i c u l t y i n p l o t t e d i n Figs. 1 f i n d i n g f l a w s below t h e t h r e s h o l d value, and i n d i c a t e s t h a t t h i s t a s k i s near t h e l i m i t s of human performance under normal c o n d i t i o n s . Secondly, i t a l s o shows t h e importance o f psychological v a r i a b l e s i n t h a t t h e d e c i s i o n performance shows considerable a b e r r a t i o n .
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Search Function
Decision-Making Function
Type o f F a u l t s
Success on faults
Success on flaws
HR
FAR'
Nicks and Dents
56.8%
29.6%
75.2%
60.9%
Tool Marks
75.1%
43.3%
92.4%
46.1%
Scratches
80.9%
37.5%
79.6%
38.2%
Table I .
Mean search and decision-making performance of i n s p e c t o r s
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This study i s reported much more f u l l y i n Drury and S i n c l a i r (1983). Even w i t h the b r i e f d e s c r i p t i o n herein, i t i s hoped t h a t t h e dangers o f i n f e r r i n g from eye-movement data alone, without considering other more global aspects o f the system, have been indicated. Furthermore, i t i s hoped t h a t t h e power o f the human v i s u a l system has i n some small way been demonstrated compared t o a purpose-built device, and t h a t automatic devices are n o t i n v a r i a b l y t h e best answer t o i n d u s t r y ' s problems (though they f r e q u e n t l y are). REFERENCES Akerman, A. 111, and Kinzly, R.E., P r e d i c t i n g a i r c r a f t d e t e c t a b i l i t y , Human Factors 21 (3), (1979), 277-291. Broadbent, D.E., Task combination and s e l e c t i v e i n t a k e o f information, Acta Psychologica 50 (1982) 253-290. Drury, C.G. and S i n c l a i r , M.A., Human and machine performance i n an inspection task, Human Factors (1983) I n press. Fisk, A.D. and Schneider, W., Control and automatic processing during tasks r e q u i r i n g sustained a t t e n t i o n : a new approach t o Vigilance, Human Factors 23 (6), (1981), 737-750. Greening, C.P., Mathematical modelling o f air-to-ground t a r g e t a c q u i s i t i o n , Human Factors 18 (2), (1976), 111-148. Groner, R., Menz, C. and Walder, F., Testing the scanpath hypothesis on a l o c a l and global l e v e l , i n : Gale, A.G. and Johnson, C.W., (eds):2nd European Conference on Eye Movements, Nottingham, England. Kraiss, K.-F. and Knaeuper, A.-M., Using v i s u a l lobe area measurements t o p r e d i c t v i s u a l search performance, Human Factors 24 ( 6 ) , (1982), 673-682. Noton, D . and Stark, L., Scanpaths i n eye movements during p a t t e r n perception, Science, 171, (1971), 308-311. Schneider, W. and S h i f f r i n , R.M., C o n t r o l l e d and automatic human i n f o r m a t i o n processing : I , detection, search and a t t e n t i o n , Psychol. Rev. 84 ( l ) , (1977), 1-66. Senders, J.W. , A re-analysis o f the p i l o t eye-movement data, I.E.E.E. Trans. Human Factors 7, (1966), 103-106. (11) S h i f f r i n , R.M. and Schneider, W., C o n t r o l l e d and automatic human i n f o r m a t i o n processing : I 1 perceptual learning, automatic attending and a general theory, Psychol. Rev. 84 (1), (1977), 127-190. (12) Silbernagel, 6. , Using r e a l i s t i c t a r g e t , sensor, and .scene characteri s t i c s t o develop a t a r g e t a c q u i s i t i o n model, Human Factors 24 ( 3 ) , (1982) , 321-328.
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M. A. S i n c l a i r Department o f Human Sciences Loughborough U n i v e r s i t y o f Technology Loughborough, Leics. L E l l 3TU
The problem w i t h eye movement studies i s t h a t though the eye movements are r e l a t i v e l y easy t o measure and the data are p r o l i x , eye movements do n o t provide much i n s i g h t i n t o r e a l tasks w i t h o u t the use o f major assumptions, u s u a l l y on t h e nature o f i n f o r m a t i o n processing associated w i t h each f i x a t i o n . These assumptions are u s u a l l y j u s t i f i e d by c u r v e - f i t t i n g , which does n o t necessarily v a l i d a t e the model. An example (Drury and S i n c l a i r , 1983) t o i l l u s t r a t e how eye movements do n o t necessarily help i s discussed. I n addition, the r e l a t i v e f a i l u r e of a simple automatic inspection device intended t o replace the human inspector i s discussed. Over t h e past 15 years, as i n many other f i e l d s t h e r e has been an explosive growth i n the study o f v i s i o n i n human v i s u a l inspection and d e t e c t i o n tasks. There have been several studies i n v o l v i n g eye movements, b u t i n t e r e s t i n these has wained f a i r l y r a p i d l y . This has n o t been occasioned by any indeed, they are f a i l u r e s i n the methods f o r measuring eye movements almost t o o successful i n producing data b u t because there has been no what do we good t h e o r e t i c a l answer t o the question, 'Now we have the data do w i t h i t ? ' Certainly, one nay p i c k up one's s t a t i s t i c a l c l u b and beat the data t o death t o produce p r o b a b i l i t y d i s t r i b u t i o n s and other d e s c r i p t i v e s t a t i s t i c s , b u t t h i s i s merely surface analysis.
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There are some basic assumptions made about which there i s general agreement. These are:-
1)
The purpose o f v i s i o n i n i n d u s t r i a l inspection tasks i s t o gather information relevant t o decisions about q u a l i t y .
2)
The purpose o f eye movements i s f i r s t l y t o ensure t h a t the v i s u a l f i e l d i s scanned, and secondly t o b r i n g foveal v i s i o n t o bear on p a r t s o f t h e v i s u a l f i e l d t h a t are information-rich.
3)
The purpose of information-gathering i s t o make decisions about q u a l i t y based upon a s e t o f i n t e r n a l i s e d standards.
Unfortunately, w h i l e these assumptions are c l e a r l y i n t e r - r e l a t e d , they do n o t form a closed set. This would r e q u i r e a f o u r t h assumption, about which t h e r e i s general disagreement:-
4) A l l other variables are immaterial; i n other words, the human being acts as a robot.
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From i n t r o s p e c t i o n , if n o t f r o m o b s e r v a t i o n , t h i s i s c l e a r l y u n t e n a b l e . T h i s t h e n i m p l i e s t h a t eye movements cannot be regarded as i n d i c a t o r s o f mental a c t i v i t y s i n c e t h e e x c l u s i v e l i n k between t h e two has been broken. The o n l y r e m a i n i n g way whereby such a l i n k may be e s t a b l i s h e d i s by p o s t u l a t i n g a model w h i c h s t a t e s such a l i n k and t h e n d e m o n s t r a t i n g t h e l i n k b y some e x p e r i m e n t a l means. An example o f t h i s i s t h e scanpath t h e o r y , f i r s t p u t forward b y Noton and S t a r k (1971) and c o n s i d e r e d most r e c e n t l y b y Groner e t a1 (1983) a t t h i s conference. However t h e t h e o r y S n o t w i d e l y accepted, and i n t h e o p i n i o n o f t h i s a u t h o r r e q u i r e s more s u p p o r t f r o m experiments. I f one accepts t h e i d e a s o f Schneider and S h i f f r i n (1977) and v i c e v e r s a (1977), i n c l u d i n g t h e e x t e n s i o n i n t o v i g i l a n c e ( F i s k and Schneider, 1981), t h e n i t seems t o t h i s a u t h o r t h a t he o n i y c o n d i t i o n s under w h i c h such an.approach c o u l d b e t e n a b l e would be i n s i t u a t i o n s where t h e whole process was ' a u t o m a t i c ' , i n t h e i r sense. T h i s o f course r e q u i r e s t h a t t h e c r i t i c i s m s d i s c u s s e d by Broadbent (1982) should be d i s r e g a r d e d , o r o t h e r w i s e answered. Such a long-term i n v e s t i g a t i o n has y e t t o be c a r r i e d o u t . Another example where an e x p l i c i t l i n k i s b u i l t i n i s t h e v i s u a l sampling model (Senders, 1966) though t h e model was n o t envisaged f o r use i n t a r g e t d e t e c t i o n . O t h e r models i n t h i s g e n e r a l f i e l d o f t a r g e t d e t e c t i o n and r e c o g n i t i o n do n o t make t h i s e x p l i c i t l i n k , though as s t a t e d b e f o r e t h i s i s due i n many cases t o t h e e x c l u s i o n of p s y c h o l o g i c a l and p h y s i o l o g i c a l v a r i a b l e s e n t i r e l y f r o m t h e models. A r e v i e w a r t i c l e b y Greening (1975) g i v e s some e x c e l l e n t examples o f t h i s , i n e a r l y work on models o f m i l i t a r y t a r g e t a c q u i s i t i o n . The same t r e n d can be seen i n more r e c e n t models p u b l i s h e d i n t h e l i t e r a t u r e (e.g. S i l b e r n a g e l , 1982, Akerman and K i n z l y , 1979, and K r a i s s and Kuaeuper, 1982). O f course t h e r e a r e reasons f o r t h i s i n t h a t t h e r e l a t i o n s h i p s between search v a r i a b l e s and p s y c h o l o g i c a l v a r i a b l e s a r e i l l - u n d e r s t o o d , and a r e n o t w e l l q u a n t i f i e d a t a l l . What has n o t a s s i s t e d i n c o r r e c t i n g t h i s s i t u a t i o n i n t h e o p i n i o n o f t h e a u t h o r i s t h e tendency t o v a l i d a t e models by means o f c u r v e - f i t t i n g ; f o r example, by e x a m i n a t i o n o f t h e c u m u l a t i v e % of d e t e c t i o n s ( w i t h r e s p e c t t o passage o f t i m e ) . By c o n f l a t i n g temporally-dependant d a t a i n t o a g l o b a l v a r i a b l e such as t h i s most o f t h e f i n e d e t a i l i s e l i m i n a t e d and t h e d a t a t h e n appears t o be v e r y well-behaved, and a l l o w s of exemplary mathematical t r e a t m e n t . An example o f t h i s i s t h e e x c e l l e n t match o b t a i n e d by f i t t i n g exponential curves t o cumulative p r o b a b i l i t y o f d e t e c t i o n data. T h i s i s u s u a l l y performed on t h e assumption t h a t random search has o c c u r r e d , and t h e f i t o b t a i n e d i s u s u a l l y v e r y good even i n t h o s e s i t u a t i o n s where i t i s p a t e n t l y obvious t h a t search i s non-random. An example t h a t perhaps i l l u s t r a t e s t h e l a c k o f d i r e c t a s s o c i a t i o n ( i n t h e s t a t i s t i c a l sense) between search and d e c i s i o n s i s g i v e n i n what f o l l o w s , w h i c h i s a b r i e f r e p o r t o f work t o b e p u b l i s h e d elsewhere (Drury and S i n c l a i r , 1983). The work a l s o i l l u s t r a t e s p a r t i c u l a r advantages t o be o b t a i n e d f r o m human v i s u a l i n s p e c t i o n as opposed t o a f a i r l y s i m p l e m i croprocessor-based o p t i c a l d e v i c e i n t e n d e d t o accomplish t h e same function. The p r o j e c t t o o k p l a c e i n an i n d u s t r i a l s e t t i n g , and was an i n v e s t i g a t i o n o f t h e e f f i c i e n c y o f i n s p e c t o r s o f r o l l s f o r r o l l e r - b e a r i n g s t o be used i n t h e manufacture o f j e t engines. A l l r o l l s p a s s i n g t h r o u g h t h e i n s p e c t i o n process were i n s p e c t e d t w i c e ; once f o r dimensional t o l e r a n c e and once f o r s u r f a c e d e f e c t s . I t i s t h e l a t t e r o p e r a t i o n which was examined. The r o l l s
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vary i n size; i n t h i s e x e r c i s e o n l y those from 7 t o l l m m i n length, and 7 t o l l m i n diameter were considered. F a u l t s c o u l d occur i n any o f 3 regions - the b a r r e l , t h e ends, o r t h e curved edges between t h e ends and the b a r r e l . The l a t t e r areas are c a l l e d t h e corners. Four classes o f f a u l t c o u l d occur; p i t s (pock marks, t y p i c a l l y caused by r u s t i n g ) , scratches, n i c k s and dents, and t o o l marks ( u s u a l l y a c i r c u m f e r e n t i a l gouge on a corner, o r p a r a l l e l c u t s on an end). Only t h e l a s t 3 classes are considered h e r e a f t e r . R e j e c t thresholds f o r these f a u l t s v a r i e d depending upon t h e n a t u r e o f the f a u l t ; f o r t h e purposes o f t h i s d i s c u s s i o n any f a u l t o f a s i z e g r e a t e r than 25/10000inch i n diameter was r e j e c t a b l e , (It should be remembered t h a t even t h e hardest o f metals appear p l a s t i c if the u n i t o f measurement i s small enough). To measure i n s p e c t i o n performance, a t e s t batch o f r o l l s was created. T h i s comprised a s e t o f 110 f a u l t y r o l l s , w i t h 182 f a u l t s upon them, p l u s a f u r t h e r 52 r o l l s deemed acceptable. A l l t h e r o l l s were numbered and mapped p h o t o g r a p h i c a l l y . During the experiment t h e r o l l s were kept i n o i l - f i l l e d tubes t o minimise handling damage; even so, t h e r o l l s had t o be resubmitted t o t h e f i n i s h i n g (SWECO) process t o r e s t o r e the surface f i n i s h . The r o l l s were checked t o make sure t h e maps were s t i l l r e l e v a n t . Twelve i n s p e c t o r s were used i n t h e experiment. A l l had had a t l e a s t 2 years experience, and had undergone a t r a i n i n g programme. T h e i r ages ranged from 20 t o 50 and, by company p o l i c y , had had t h e i r eyesight checked and c o r r e c t e d by an o p t i c i a n w i t h i n the p a s t 3 years. One i n s p e c t o r was t e s t e d each day a t t h e beginning o f h i s o r her s h i f t (7.00am o r 3.00pm). T y p i c a l l y , t h e t e s t l a s t e d f o r 4 hours, and took place a t a normal works t a t i o n i n an adjacent room. The w o r k s t a t i o n consisted o f a desk-mounted l i g h t f i x t u r e w i t h two f l u o r e s c e n t tubes p o s i t i o n e d above the i n s p e c t o r s ' hands when h a n d l i n g t h e r o l l s . A t r o l l l e v e l , t h e i l l u m i n a t i o n averaged 887 l u x , w i t h a background i l l u m i n a t i o n o f 95 l u x . Given the m a t e r i a l o f t h e c y l i n d e r s , t h e r e was a g l a r e problem from the surface. Viewing d i s t a n c e averaged 25 cms, which f o r a f a u l t t h r e s h o l d value of25/10,000 i n c h gives an angular subtense a t t h e eye o f 50 seconds. To a s s i s t i n t h e task, i n s p e c t o r s were given a r e c t a n g u l a r g l a s s x4 m a g n i f i e r . Most i n s p e c t o r s a f f i x e d t h i s t o t h e i r l i g h t f i x t u r e w i t h rubber bands; one i n s p e c t o r refused t o make any use o f i t . The t e s t batch o f r o l l s was presented on trays, as was normal. Each i n s p e c t o r would then i n s p e c t t h e r o l l s s i n g l y , r o t a t i n g the r o l l s i n the hands, and u s i n g e i t h e r gloves, a c l o t h , o r barrier-cream-protected f i n g e r s , t o r u b o f f d u s t p a r t i c l e s . This was a necessary process, as i t was f r e q u e n t l y d i f f i c u l t t o d i s t i n g u i s h between dust and a f l a w . As each f l a w was found t h e i n s p e c t o r would use a s t y l u s f o r f a c t u a l confirmation, and then t h e type, p o s i t i o n , and f a u l t c l a s s i f i c a t i o n was recorded. Normally, as soon as any f a u l t i s found, t h a t r o l l i s r e j e c t e d . I n t h i s experiment, because t h e r e were occurences o f m u l t i p l e f a u l t s on t h e r o l l , i n s p e c t o r s were asked t o keep on i n s p e c t i n g u n t i l they thought they had found a l l f a u l t s ( i f any). No time l i m i t was placed on t h i s . Watching t h e i n s p e c t o r s a t work revealed immediately t h a t t h e r e was a f a i r l y standard method o f search involved. A l l i n s p e c t o r s organised t h e i r search i n t o 3 phases - b a r r e l , corner and end though t h g order v a r i e d between i n s p e c t o r s . The r o l l was always h e l d a t about 45 t o t h e v e r t i c a l i n the s a g g i t a l plane f o r i n s p e c t i o n o f t h e b a r r e l . I t was then r o t a t e d s l o w l y about t h i s a x i s w h i l e t h e eyes performed v e r t i c a l saccades up and down t h e b a r r e l . This produced a zig-zag p a t t e r n o f f i x a t i o n s around the b a r r e l ; t y p i c a l l y 3 f i x a t i o n s were used on each z i g ( s t a r t , middle, and
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f i n i s h ) . The c o r n e r was i n s p e c t e d by h o l d i n g t h e r o l l p o i n t i n g towards t h e body i n t h e s a g g i t a l plane, and t h e n p e r f o r m i n g a s m a l l , 2 - a x i s r o t a t i o n o f t h e r o l l t o a l l o w a r e f l e c t e d b a r o f l i g h t t o p l a y a l l round t h e c o r n e r . A s i m i l a r , more o s c i l l a t o r y movement was performed t o i n s p e c t t h e end, In t h e l a t t e r case t h e f i x a t i o n p a t t e r n was r o u g h l y c i r c u l a r , whereas f o r t h e edge t h e eyes f o l l o w e d t h e b a r o f l i g h t , s i n c e changes i n t h i s r e v e a l e d t h e presence o f a f l a w . These comments a r e based upon o b s e r v a t i o n o f t h e eye movements u s i n g a s m a l l m i r r o r , and a r e t h e r e f o r e q u a l i t a t i v e . The d a t a o b t a i n e d was compared w i t h t h e maps, and was s o r t e d i n t o t h e f a u l t c l a s s i f i c a t i o n s . W i t h i n each c l a s s i f i c a t i o n i t was f u r t h e r s o r t e d i n t o H i t s , Misses, C o r r e c t Acceptances, and F a l s e Alarms. T h i s was performed f o r t h e d e f e c t i v e r o l l s and t h e good r o l l s s e p a r a t e l y ( t h o u g h i n t h e l a t t e r case, t h e r e were no h i t s , by d e f i n i t i o n ) . The h i t s and t h e f a l s e a l a r m c a t e g o r i e s f o r each i n s p e c t o r were then p l o t t e d g r a p h i c a l l y on R e c e i v e r O p e r a t i n g C h a r a c t e r i s t i c s diagrams ( F i g s . 1 - 3 ) .
0
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F i g . 1.
40
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% False Alarm Rate R.O.C.
80
f o r N i c k s and Dents
100
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I
s
% False Alarm Rate Fig. 2.
R.O.C.
for toolmarks
% False Alarm Rate Fig. 3.
R.O.C.
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378
I n p a r a l l e l w i t h t h i s experiment u s i n g human i n s p e c t o r s , an e v a l u a t i o n o f an o p t i c a l , microprocessor-based devise was undertaken, which was i n a breadboard stage o f development. I t was intended t h a t t h i s device should r e p l a c e t h e i n s p e c t o r s . The sensing element was an a r r a y o f photodiodes, behind a focussing system. The d e t e c t i o n p r i n c i p l e was t o compare t h e o u t p u t o f each diode t o i t s 8 nearest neighbours, and t o s i g n a l any d i f f e r e n c e s t h a t were g r e a t e r than a s e t t h r e s h o l d . I n machine u n i t s , t h i s t h r e s h o l d c o u l d be s e t t o any value between 0 and 255; i n p r a c t i c e , o n l y t h e range between 100 and 225 was o f any importance. 100 would r e j e c t almost every r o l l ; 225 r e j e c t e d v e r y l i t t l e . Because a range o f thresholds t h i s i s shown i n could be used, i t was p o s s i b l e t o o b t a i n an R.O.C.; F i g s . 1 - 3. These t h r e e f i g u r e s i n d i c a t e t h a t f i r s t l y t h e s c a t t e r o f t h e p o i n t s f o r t h e i n s p e c t o r s r e v e a l s very d i f f e r e n t performance among t h e i n s p e c t o r s d e s p i t e t h e very s i m i l a r search behaviour used, thereby i l l u s t r a t i n g t h e p o i n t s made e a r l i e r . The wide s c a t t e r i s probably due t o poor i n i t i a l t r a i n i n g and no subsequent r e c a l i b r a t i o n o f t h e i n s p e c t o r s , a l l o w i n g them t o s l i p t o an i n a p p r o p r i a t e s e t o f perceived t h r e s h o l d s f o r r e j e c t i o n . Secondly, t h e r e i s t h e comparison between the i n s p e c t o r s and t h e microprocessor-based device. I f one makes use o f t h e usual i n t e r p r e t a t i o n o f R.O.C. diagrams on t h e b a s i s o f s i g n a l d e t e c t i o n theory, then those i n s p e c t o r s p l o t t e d i n t h e area above and t o t h e l e f t o f t h e machine's R.O.C. have performed b e t t e r . Since t h i s l o o s e l y accords w i t h d e s i r a b l e p r a c t i c e i n production, t h i s c r i t e r i o n i s used here. I t w i l l be seen t h a t i n general t h e human i n s p e c t o r s perform b e t t e r than t h e machine. On scratches and n i c k s and dents, humans a r e s i g n i f i c a n t l y b e t t e r (p<0.05).
A f u r t h e r a n a l y s i s was undertaken t o i l l u s t r a t e the d i f f e r e n c e between search performance and d e c i s i o n performance by human i n s p e c t o r s . Successf u l search corresponds t o t h e d e t e c t i o n o f any f a u l t o r f l a w i n t h e mapped p o s i t i o n on a r o l l . Successful decision-making corresponds t o making t h e c o r r e c t d e c i s i o n once a v e r i f i a b l e f l a w has been detected. The data allowed t h i s a n a l y s i s t o be performed, and t h e r e s u l t s a r e shown i n Table I ( i t should be noted t h a t by m u l t i p l y i n g t h e search performance by t h e d e c i s i o n making performance f o r each i n s p e c t o r one o b t a i n s t h e p o i n t s 3). T h i s t a b l e r e v e a l s the considerable d i f f i c u l t y i n p l o t t e d i n Figs. 1 f i n d i n g f l a w s below t h e t h r e s h o l d value, and i n d i c a t e s t h a t t h i s t a s k i s near t h e l i m i t s of human performance under normal c o n d i t i o n s . Secondly, i t a l s o shows t h e importance o f psychological v a r i a b l e s i n t h a t t h e d e c i s i o n performance shows considerable a b e r r a t i o n .
-
Search Function
Decision-Making Function
Type o f F a u l t s
Success on faults
Success on flaws
HR
FAR'
Nicks and Dents
56.8%
29.6%
75.2%
60.9%
Tool Marks
75.1%
43.3%
92.4%
46.1%
Scratches
80.9%
37.5%
79.6%
38.2%
Table I .
Mean search and decision-making performance of i n s p e c t o r s
Rolls in Roller Bearings
379
This study i s reported much more f u l l y i n Drury and S i n c l a i r (1983). Even w i t h the b r i e f d e s c r i p t i o n herein, i t i s hoped t h a t t h e dangers o f i n f e r r i n g from eye-movement data alone, without considering other more global aspects o f the system, have been indicated. Furthermore, i t i s hoped t h a t t h e power o f the human v i s u a l system has i n some small way been demonstrated compared t o a purpose-built device, and t h a t automatic devices are n o t i n v a r i a b l y t h e best answer t o i n d u s t r y ' s problems (though they f r e q u e n t l y are). REFERENCES Akerman, A. 111, and Kinzly, R.E., P r e d i c t i n g a i r c r a f t d e t e c t a b i l i t y , Human Factors 21 (3), (1979), 277-291. Broadbent, D.E., Task combination and s e l e c t i v e i n t a k e o f information, Acta Psychologica 50 (1982) 253-290. Drury, C.G. and S i n c l a i r , M.A., Human and machine performance i n an inspection task, Human Factors (1983) I n press. Fisk, A.D. and Schneider, W., Control and automatic processing during tasks r e q u i r i n g sustained a t t e n t i o n : a new approach t o Vigilance, Human Factors 23 (6), (1981), 737-750. Greening, C.P., Mathematical modelling o f air-to-ground t a r g e t a c q u i s i t i o n , Human Factors 18 (2), (1976), 111-148. Groner, R., Menz, C. and Walder, F., Testing the scanpath hypothesis on a l o c a l and global l e v e l , i n : Gale, A.G. and Johnson, C.W., (eds):2nd European Conference on Eye Movements, Nottingham, England. Kraiss, K.-F. and Knaeuper, A.-M., Using v i s u a l lobe area measurements t o p r e d i c t v i s u a l search performance, Human Factors 24 ( 6 ) , (1982), 673-682. Noton, D . and Stark, L., Scanpaths i n eye movements during p a t t e r n perception, Science, 171, (1971), 308-311. Schneider, W. and S h i f f r i n , R.M., C o n t r o l l e d and automatic human i n f o r m a t i o n processing : I , detection, search and a t t e n t i o n , Psychol. Rev. 84 ( l ) , (1977), 1-66. Senders, J.W. , A re-analysis o f the p i l o t eye-movement data, I.E.E.E. Trans. Human Factors 7, (1966), 103-106. (11) S h i f f r i n , R.M. and Schneider, W., C o n t r o l l e d and automatic human i n f o r m a t i o n processing : I 1 perceptual learning, automatic attending and a general theory, Psychol. Rev. 84 (1), (1977), 127-190. (12) Silbernagel, 6. , Using r e a l i s t i c t a r g e t , sensor, and .scene characteri s t i c s t o develop a t a r g e t a c q u i s i t i o n model, Human Factors 24 ( 3 ) , (1982) , 321-328.