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Electronic steering and focussing of ultrasonic beams in tube inspection
ULTRASONICS
F O R INDLIb--TR------Y 1 9 8 1 3
Conference Paper
Electronic steering and focussing of ultrasonic beams in tube inspection K. R. Whittington* and B. D. Cox*
I n c r e a s i n g l y s t r i n g e n t r e q u i r e m e n t s for the quality c o n t r o l of s t e e l tubing have led to the d e v e l o p m e n t of c o m p l e x ins p e c t i o n equipment, using l a r g e n u m b e r s of u l t r a s o n i c p r o b e s . Each p r o b e has to be individually a d j u s t e d for any change of tube s i z e . P r o b e s may need to be u n f o c u s s e d , o r f o c u s s e d to a line o r point. Consequently, a s y s t e m is being developed w h e r e b y u l t r a sonic e n e r g y can be a i m e d and f o c u s s e d by purely e l e c t r o nic m e a n s , using an a r r a y of point s o u r c e s of u l t r a s o u n d , r e l a t i v e l y widely s p a c e d , and e n e r g i s e d at calculated t i m e
INTRODUC TION The i n c r e a s i n g l y s t r i n g e n t d e m a n d s on the quality of s t e e l tubing for such a p p l i c a t i o n s a s n u c l e a r fuel cans, b o i l e r tubing for p o w e r s t a t i o n s , and for use in the a i r c r a f t industry, has led to the d e v e l o p m e n t of c o m p l e x u l t r a s o n i c t e s t i n g equipment. If the p r e s e n t t r e n d continues, the m e c h a n i c a l complexity of such equipment, and the t i m e r e q u i r e d f o r initial s e t t i n g up and a d j u s t m e n t f o r change of tube s i z e , is likely to make n e c e s s a r y s o m e fundamental r e - t h i n k i n g of the m e a n s of introducing the u l t r a s o n i c e n e r g y into the tube. Consequently, a l o n g - t e r m r e s e a r c h and d e v e l o p m e n t p r o g r a m m e has been s e t up at Tube I n v e s t m e n t s R e s e a r c h L a b o r a t o r i e s , to i n v e s t i g a t e the f e a s i b i l i t y of s t e e r i n g and f o c u s s i n g u l t r a s o n i c b e a m s by e l e c t r o n i c m e a n s . The f i r s t p a r t of this p a p e r shows how the s p e c i a l r e q u i r e m e n t s of tube t e s t i n g r e l a t e d to t h i s p r o g r a m m e of work; the second p a r t d e s c r i b e s the r e s u l t s a c h i e v e d to date.
ULTRASONIC DETECTION OF DEFECTS IN TUBES Fig 1 shows the m o s t c o m m o n a r r a n g e m e n t for the u l t r a s o n i c d e t e c t i o n of longitudinal d e f e c t s (defects w h o s e long axis is p a r a l l e l to the axis of the tube), using p u l s e t e c h n i q u e s . A p r o b e , P , producing a n a r r o w , c o l l i m a t e d o r f o c u s s e d beam of u l t r a s o u n d is p o s i t i o n e d in such a way that the sound e n e r g y is t r a n s m i t t e d as p r e s s u r e w a v e s through the coupling medium (water) and e n t e r s the tube wall at the angle shown. Mode c o n v e r s i o n o c c u r s at the i n t e r f a c e , and s h e a r w a v e s t r a v e l around the tube wall by s u c c e s s i v e i n t e r n a l r e f l e c tions. A defect in the outside wall of the tube at A, o r in the inside wall at B, will r e f l e c t p a r t of the sound e n e r g y , which will be r e c e i v e d a s an echo p u l s e , e i t h e r by the t r a n s m i t t i n g probe u s e d as a r e c e i v e r , o r by a s e p a r a t e r e c e i v i n g probe. The u l t r a s o u n d f r e q u e n c y u s e d in tube testing is usually 5-10MHz.
i n t e r v a l s so that the i n t e r f e r e n c e p a t t e r n p r o d u c e d r e s u l t s in the r e q u i r e d d i s t r i b u t i o n of e n e r g y . A c o m p u t e r p r o g r a m has been w r i t t e n enabling the effect of v a r i a t i o n s in r e l e v a n t p a r a m e t e r s , (number and spacing of point s o u r c e s , p u l s e length, etc, ) to be studied and e x p e r i mental a r r a y s and digital excitation c i r c u i t r y have been set up to c o n f i r m the t h e o r e t i c a l p r e d i c t i o n s . Beam p a t t e r n s f r o m t h e s e a r r a y s have been plotted, using an X-Y r e c o r d e r or contour p l o t t e r display.
t e s t i n g , in which a ring of 24 p r o b e s is used. With this a r r a n g e m e n t , a rotation of a p p r o x i m a t e l y 10 ° is sufficient f o r c o m p l e t e inspection of the tube c i r c u m f e r e n c e . If it is r e q u i r e d to d i s p e n s e with rotation a l t o g e t h e r , which is the ideal f o r o n - l i n e inspection~ it is found that, for tubes in the 2-3in d i a m e t e r range, a p p r o x i m a t e l y 72 p r o b e s a r e needed. In Fig 1, the w a v e s of u l t r a s o u n d a r e shown t r a v e l l i n g clockw i s e around the tube. Unfortunately, with c l o c k w i s e i n s p e c tion only, d e f e c t s at c e r t a i n angles may not be d e t e c t e d s i n c e the sound e n e r g y r e f l e c t e d f r o m them will be in such a d i r e c t i o n that it can n e v e r be picked up by the r e c e i v i n g p r o b e . Consequently, it is n e c e s s a r y to use a s e c o n d ring of p r o b e s , facing anticlockwise around the tube, in o r d e r to e n s u r e that all longitudinal d e f e c t s may be detected. The situation is f u r t h e r c o m p l i c a t e d by the n e c e s s i t y to d e t e c t t r a n s v e r s e d e f e c t s , in which c a s e two f u r t h e r r i n g s of p r o b e s , facing in e i t h e r d i r e c t i o n along the axis of the tube a r e needed. Thus, c l o s e on 300 p r o b e s may be r e q u i r e d to i n s p e c t tubing on-line for longitudinal and t r a n s v e r s e d e f e c t s , and any a t t e m p t to detect d e f e c t s at all i n t e r m e d i a t e angles can r a i s e this total to o v e r 1000. Even with I00 p r o b e s , the m e c h a n i c a l complexity and the t i m e taken to s e t the p r o b e s up initially, and adjust t h e i r s e t t i n g s (which m u s t be a c c u r a t e within a few m i n u t e s of arc) for changes in tube d i a m e t e r ,
A
It will he a p p a r e n t f r o m the d i a g r a m s that only a s m a l l p o r t i o n of the tube c i r c u m f e r e n c e is i n s p e c t e d by this t e c h nique. Rotation of the tube by only a few d e g r e e s will cause s e v e r e attenuation o r c o m p l e t e l o s s of the defect s i g n a l s f r o m A and B. Consequently, in o r d e r to i n s p e c t the whole of the tube, it is n e c e s s a r y to r o t a t e e i t h e r the p r o b e head a s s e m b l y o r the tube as the l a t t e r is fed p a s t the p r o b e , thus producing a h e l i c a l scan. This n e c e s s i t y for m e c h a n i c a l rotation s e v e r e l y l i m i t s the r a t e at which tube can be i n s p e c t e d . The p r o b l e m can be o v e r c o m e by i n c r e a s i n g the n u m b e r of p r o b e s . Fig 2 shows a p r o b e head a s s e m b l y , at p r e s e n t in use for production * Tube I n v e s t m e n t s R e s e a r c h L a b o r a t o r i e s , Hinxton Hall, Saffron Walden, E s s e x , England 20
ULTRASONICS J a n u a r y 1969
Fig 1 P a t h of u l t r a s o u n d in a s t e e l tube. D i a m e t e r - t o t h i c k n e s s ratio : 4.9999 Angle of incidence : 13. 0000 °
ULTR~ICS
FOR INDLISTR~
1968
Conference Paper
1000 o r m o r e individual t r a n s d u c e r s , so t h a t m u c h of the s o u g h t - a f t e r r e d u c t i o n in complexity is lost. It was decided, t h e r e f o r e , to i n v e s t i g a t e a p o s s i b l e a l t e r n a t i v e s y s t e m b a s e d on a r i n g of r e l a t i v e l y w e l l - s e p a r a t e d point s o u r c e s , f o r m i n g an a r r a y working in the n e a r - f i e l d region, (Fig 4). T h i s i m p l i e s that the t i m e d e l a y s between the f i r i n g of s u c c e s s i v e e l e m e n t s would not, in g e n e r a l , be equal. In a p r a c t i c a l application, a r e l a t i v e l y s m a l l n u m b e r of e l e m e n t s would be p u l s e d in such a way a s to f o r m a focus o r point of m a x i m u m i n t e n s i t y at A in the d i a g r a m ; on the next cycle of o p e r a t i o n a s i m i l a r s e t of e l e m e n t s one p o s i t i o n f u r t h e r on in the a r r a y would be u s e d to f o r m a focus at A', and so on. T h i s s y s t e m e n v i s a g e s the u s e of digitally s y n t h e s i z e d t i m i n g p u l s e s , r a t h e r than analogue d e l a y - l i n e t e c h n i q u e s , for f i r i n g the individual e l e m e n t s of the a r r a y .
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F i g 2 U l t r a s o n i c t e s t h e a d with ring of 24 p r o b e s can be daunting, e s p e c i a l l y s i n c e it is difficult to m a n u f a c t u r e p r o b e s which a r e i d e n t i c a l in t h e i r c h a r a c t e r i s t i c s . It is c l e a r , then, t h a t any s y s t e m of switching o r adjusting the b e a m angle by e l e c t r o n i c m e a n s , and thus r e d u c i n g the n u m b e r of p r o b e s r e q u i r e d , could be of g r e a t value. ELECTRONIC STEERING AND FOCUSSING Most p u b l i s h e d work on t h i s topic h a s dealt with c l o s e s p a c e d a r r a y s of h a l f - w a v e t r a n s d u c e r s , working in the f a r - f i e l d region (Fig 3). The individual e l e m e n t s of the a r r a y a r e fed f r o m a delay line whose p a r a m e t e r s may be v a r i e d so t h a t the t i m e at which e a c h e l e m e n t r e c e i v e s its p u l s e of e l e c t r i c a l e n e r g y m a y be v a r i e d . Consequently, a pulse of u l t r a s o u n d is e m i t t e d f r o m e a c h s u c c e s s i v e e l e m e n t at a slightly l a t e r t i m e and the r e s u l t a n t envelope of the w a v e - f r o n t is at an angle to the a r r a y as shown in the d i a g r a m . T h i s angle, and thus the angle of the u l t r a s o n i c b e a m , may be v a r i e d by v a r y i n g the delayline p a r a m e t e r s . S t e e r a b l e a r r a y s of t h i s n a t u r e a r e f a m i l i a r to w o r k e r s in the fields of both r a d a r 1 and s o n a r 2 ; an application to m e d i c a l u l t r a s o n i c s h a s r e c e n t l y b e e n d e s c r i b e d 3. Such s y s t e m s have s e v e r a l d i s a d v a n t a g e s when applied to the t u b e - t e s t i n g situation. F i r s t l y , the s i z e of h a l f - w a v e t r a n s d u c e r s b e c o m e s e x c e s s i v e l y s m a l l at w a v e l e n g t h s of 5-10MHz, p a r t i c u l a r l y if a two- d i m e n s i o n a l a r r a y is r e quired. M o r e i m p o r t a n t , dividing e a c h p r o b e in a r i n g of 72 into 10 o r 20 e l e m e n t s of t h i s s o r t r e s u l t s once again in
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F i g 4 A r r a y of w i d e l y - s p a c e d point s o u r c e s working in the near-field region
C O M P U T E R SIMULATION In o r d e r to s i m p l i f y the p r o b l e m for i n i t i a l study, c o n s i d e r a tion was f i r s t g i v e n to a s t r a i g h t - l i n e a r r a y of point s o u r c e s (Fig 5). A c o m p u t e r p r o g r a m was w r i t t e n to e n a b l e the f i r i n g t i m e s f o r e a c h e l e m e n t to be c a l c u l a t e d for a focus to be f o r m e d at s o m e c h o s e n point of c o - o r d i n a t e s (x, H). The n u m b e r and spacing of the e l e m e n t s , the o p e r a t i n g f r e q u e n c y and p u l s e length could all be v a r i e d . Using this p r o g r a m it i s p o s s i b l e to p r o d u c e a g r a p h i c a l display showing the d i s t r i bution of i n t e n s i t y of the a c o u s t i c e n e r g y a c r o s s the focal plane. The p r o g r a m was l a t e r modified to allow the e n e r g y d i s t r i b u t i o n along any line p a r a l l e l to the x, H o r z a x e s to be displayed. Fig 6 shows the type of r e s u l t s obtained for the s i m p l e s t c a s e , t h a t of two point s o u r c e s with 5MHz c o n t i n u o u s - w a v e e x c i t a t i o n (infinite pulse length). The s i t u a t i o n is analogous to the optical c a s e of two c o h e r e n t light s o u r c e s , and a s can be s e e n , a s e r i e s of i n t e r f e r e n c e f r i n g e s is f o r m e d , dec r e a s i n g in amplitude away f r o m the a r r a y b e c a u s e of the i n v e r s e s q u a r e law.
F i g 3 C l o s e - s p a c e d a r r a y of h a l f - w a v e t r a n s d u c e r e l e m e n t s working in the f a r - f i e l d r e g i o n
Fig 7 shows a s e r i e s of s i m i l a r p l o t s f o r the c a s e of 10 e l e m e n t s , p l a c e d so a s to p r o d u c e a focus i m m e d i a t e l y below the c e n t r e of the a r r a y at a d i s t a n c e of 10cm. The s o u r c e s a r e s p a c e d 3 m m a p a r t , the p o s i t i o n r e l a t i v e to the X s c a l e being i n d i c a t e d by the i n v e r t e d t r i a n g l e s . In F i g 7a, continuous wave e x c i t a t i o n is again used. It is s e e n t h a t the spacing of the f r i n g e s r e m a i n s the s a m e a s in the twoe l e m e n t c a s e , but the f r i n g e width is r e d u c e d and the a m p l i tude falls off m o r e rapidly away f r o m the focal point. In ULTRASONICS J a n u a r y 1969
21
ULTRABONICS -i
- I-~T
I -
FOR INIDU~nRY -
Conference Paper F i g 7b, the p u l s e l e n g t h i s s h o r t e n e d to 1/xs. At a c e r t a i n d i s t a n c e f r o m the focal point, the p a t h length d i f f e r e n c e b e t w e e n s u c c e s s i v e e l e m e n t s b e c o m e s too g r e a t f o r i n t e r f e r e n c e to take p l a c e , and the m e a n i n t e n s i t y i s t h a t of a s i n g l e e l e m e n t . T h u s , a s the p u l s e length i s s h o r t e n e d , s i d e l o b e s of the p a t t e r n a r e p r o g r e s s i v e l y lost. In F i g 7% the p u l s e l e n g t h h a s b e e n s h o r t e n e d to 200ns (one w a v e l e n g t h ) ; a l l s i d e l o b e s h a v e p r a c t i c a l l y d i s a p p e a r e d and a s i n g l e s h a r p i n t e n s i t y p e a k is left at the f o c u s . F i g 7d s h o w s the s a m e s i t u a t i o n , but with the f i r i n g t i m e s for the e l e m e n t s
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Fig 6 C o m p u t e r plot of i n t e r f e r e n c e p a t t e r n f r o m two e l e m e n t s . The e l e m e n t s p a c i n g i s 0 . 3 0 c m , the focal point i s at x = 0 . 1 5 c m a n d the e l e m e n t s a r e 1 0 . 0 0 c m above the focal p l a n e
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ULTBASONICS J a n u a r y 1969
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F i g 7 C o m p u t e r p l o t s of d i s t r i b u t i o n of u l t r a s o n i c e n e r g y in the focal p l a n e of the a r r a y . The e l e m e n t s p a c i n g is 0 . 3 0 c m and the e l e m e n t s a r e 1 0 . 0 0 c m above the focal p l a n e . a The focal point i s at x 1.35cm and the wave i s c o n t i n u o u s b The focal point i s at x = 1 . 3 5 c m and the p u l s e d u r a t i o n i s 1.00/~ s c The focal point i s at x - 1 . 3 5 c m and the p u l s e d u r a t i o n i s 0.20f~s d The focal point is at x = 4 . 0 0 c m and the p u l s e d u r a t i o n is 0 . 2 0 ~ s e The focal point i s at x = 4 . 0 0 c m and the p u l s e d u r a t i o n is 0.20/~s
Con(vcev~e Paper r e - c a i c u l a t e d to f o r m a focus away f r o m the c e n t r e of the a r r a y . (In e a c h of the f o r e g o i n g c a s e s , the a m p l i t u d e s c a l e is s u c h t h a t unit intensity is defined to be t h a t obtaining at the focal point.) In F i g s 7a-7d, the c o m p u t e r p r o g r a m h a s c a l c u l a t e d the t i m e i n t e r v a l s to s e v e n s i g n i f i c a n t f i g u r e s , that is an a c c u r a c y of l n s . Since it i s c l e a r l y i m p o s s i b l e to a c h i e v e such an a c c u r a c y in p r a c t i c e , the c a s e i l l u s t r a t e d in F i g 7d was r e w o r k e d to an a c c u r a c y of two s i g n i f i c a n t f i g u r e s (the n e a r e s t lOOns). The r e s u l t (Fig 7e) shows, a s m i g h t be expected, a r e d u c t i o n in peak i n t e n s i t y due to s o m e d e s t r u c tive i n t e r f e r e n c e taking p l a c e at the focus; the width of the p e a k a l s o i n c r e a s e s slightly. However, a s can be s e e n , such a r e s u l t would be p e r f e c t l y s a t i s f a c t o r y f r o m a p r a c t i c a l viewpoint. It is i n s t r u c t i v e to note, in view of the p r e s e n t i n t e r e s t in h o l o g r a p h y , that the s y s t e m u n d e r i n v e s t i g a t i o n is, in fact, t r u l y h o l o g r a p h i c , in the s e n s e that e v e r y e l e m e n t c o n t r i butes to the i n t e n s i t y at e a c h point in the b e a m p a t t e r n . Thus, in the e v e n t of f a i l u r e of one o r m o r e e l e m e n t s in a m u l t i e l e m e n t a r r a y , the b e a m p a t t e r n i s not a p p r e c i a b l y changed, only definition being lost, in that the individual i n t e n s i t y p e a k s a r e b r o a d e n e d and t h e i r r e l a t i v e a m p l i t u d e s slightly changed. T h i s gives a v a l u a b l e i n s u r a n c e a g a i n s t f a i l u r e of a c o m p l e x s y s t e m . If a s e r i e s of plots of i n t e n s i t y d i s t r i b u t i o n is m a d e for a s e r i e s of s e c t i o n s t h r o u g h the b e a m , it is p o s s i b l e to p r o d u c e a contour m a p of the b e a m p a t t e r n . Fig 8 shows the r e s u l t s f o r five e l e m e n t s , f i r s t l y , for a 50Ons p u l s e , and secondly f o r a 20Ons pulse. A point of i m p o r t a n c e is that, although the b e a m n a r r o w s rapidly f r o m the plane of the a r r a y to the focal plane, it does not a f t e r w a r d s d i v e r g e again a s m u c h as would be the c a s e with l e n s - f o c u s s e d equivalent. T h i s m a k e s p o s s i b l e the p r o d u c t i o n of a n a r r o w , c o l l i m a t e d b e a m of u l t r a s o u n d well suited for t u b e - t e s t i n g applications. F r o m Fig 8 it can also be s e e n t h a t i n t e r f e r e n c e p a t t e r n s a r e p r o d u c e d along, a s well a s a c r o s s , the axis of the b e a m . T h i s i n d i c a t e s that s o m e c a r e m u s t be t a k e n in choosing the working p o s i t i o n of the u l t r a s o u n d b e a m in o r d e r to avoid dead s p o t s o r unequal r e s p o n s e f r o m s i m i l a r defects.
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Fig 8 C o m p u t e r contour p l o t s of b e a m f r o m an a r r a y of five e l e m e n t s . The e l e m e n t spacing i s 0 . 5 c m and the focal point is at x = 5cm. a The p u l s e d u r a t i o n i s 0 . 5 ~ s b The p u l s e d u r a t i o n i s 0 . 2 ~ s
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E X P E R I M E N T A L PROCEDURE At t h i s stage, it was felt to b e a p p r o p r i a t e to conduct exp e r i m e n t s to c h e c k t h e o r e t i c a l a g a i n s t a c t u a l p e r f o r m a n c e .
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The f i r s t p r o b l e m was to c o n s i d e r what t y p e s of t r a n s d u c e r could be used, which would a p p r o x i m a t e to point (or, initially, line) s o u r c e s . The u s e of flat s t r i p s of p i e z o e l e c t r i c m a t e r i a l of halfw a v e l e n g t h width was not felt to be v e r y p r a c t i c a l b e c a u s e of the s m a l l d i m e n s i o n a s s o c i a t e d with the r e l a t i v e l y high t e s t i n g frequency, although s o m e e x p e r i m e n t s w e r e p e r f o r m e d with flat p l a t e s of h a l f - w a v e t h i c k n e s s , u s e d e d g e - o n . The u s e of w i d e r s t r i p s in a s s o c i a t i o n with d i v e r g i n g l e n s e s was a l s o avoided f o r i n i t i a l e x p e r i m e n t s b e c a u s e of the p o s s i b i l i t y that such s o u r c e s m i g h t have a n o n - u n i f o r m b e a m p a t t e r n , o r m i g h t not a p p e a r c o h e r e n t . The t r a n s d u c e r s finally c h o s e n for the e x p e r i m e n t a l work w e r e r a d i a i l y - p o l a r i s e d l e a d - z i r c o n a t e - t i t a n a t e tubes of 0 . 2 i n d i a m e t e r and 0 . 0 2 i n wail t h i c k n e s s . T h e s e w e r e found to give a u n i f o r m i n t e n s i t y d i s t r i b u t i o n a r o u n d t h e i r whole c i r c u m f e r e n c e . Unfortunately, they p r o v e d difficult to damp effectively, so that m o s t of the e a r l y e x p e r i m e n t s a p p r o x i m a t e d to the c a s e of c o n t i n u o u s - w a v e excitation. L a t e r e x p e r i m e n t s u s e d i n v e r t e d f o c u s s e d bowls of l e a d - z i r c o n a t e t i t a n a t e , which a r e m o r e e a s i l y d a m p e d and have the a d v a n tage of a p p e a r i n g a s point r a t h e r than line, s o u r c e s . In o r d e r to e n a b l e an a r r a y of such t r a n s d u c e r s to be p u l s e d at the c a l c u l a t e d t i m e i n t e r v a l s , a t e n - c h a n n e l pulse g e n e r a t o r was c o n s t r u c t e d . A block d i a g r a m of t h i s i n s t r u m e n t is shown in Fig 9. A 10MHz c r y s t a i - ' c o n t r o U e d o s c i l l a t o r is u s e d as a c l o c k - p u l s e g e n e r a t o r and a s e r i e s of i n t e g r a t e d - c i r c u i t decade c o u n t e r s a r e u s e d to 'count down' f r o m t h i s clock signal. The decoded outputs of the c o u n t e r s in e a c h c h a n n e l a r e s e l e c t e d by two decade s w i t c h e s on the f r o n t panel, thus allowing the f i r i n g t i m e of e a c h t r a n s d u c e r
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to be s e t independently, in 100~s s t e p s up to 10ps f r o m the o c c u r r e n c e of a s y n c h r o n i z i n g pulse. T h i s s y n c h r o n i z i n g pulse, a l s o d e r i v e d by counting down f r o m the clock s i g n a l s , o c c u r s a t a 10Hz r e p e t i t i o n r a t e . F o r full a d v a n t a g e to be t a k e n of t h i s facility, it is n e c e s s a r y that j i t t e r in the output s t a g e s which f i r e the t r a n s d u c e r s should be m i n i m i s e d . Consequently, high s p e e d s w i t c h h l g t r a n s i s t o r s a r e u s e d in p r e f e r e n c e to the conventional t h y r a t r o n or SCR c i r c u i t s . F o r a fully a u t o m a t e d s y s t e m , the decade t i m e i n t e r v a l setting s w i t c h e s could be r e p l a c e d by e l e c t r o n i c s w i t c h e s u n d e r the c o n t r o l of a s m a l l o n - l i n e c o m p u t e r . ULTRASONICS J a n u a r y 1969
23
Conference Paper A plotting tank in which the e x p e r i m e n t a l t r a n s d u c e r s could be mounted, and in which a r e f l e c t i n g t a r g e t o r r e c e i v i n g p r o b e could be s c a n n e d m e c h a n i c a l l y in a suitable r a s t e r , was u s e d in conjunction with the equipment d e s c r i b e d above. F o r the initial e x p e r i m e n t s , a r e c e i v i n g p r o b e , a s opposed to a r e f l e c t i n g t a r g e t , was u s e d e x c l u s i v e l y . T r i a l s using a tubular r e c e i v e r , i d e n t i c a l with the t r a n s m i t t i n g t r a n s d u c e r s , r a n into t r o u b l e on t h r e e counts; f i r s t l y , difficulty in damping the r e c e i v e r ; secondly, the finite d i a m e t e r of the r e c e i v e r which m e a n t that t r a n s m i t t e d p u l s e s which should a r r i v e s e p a r a t e d in t i m e could s t i l l i n t e r f e r e ; and t h i r d l y the finite length of the r e c e i v e r , which m e a n t that s p u r i o u s i n t e r f e r e n c e p a t t e r n s could o c c u r due to c h a n g e s in path length between top and bottom of the t r a n s m i t t e r and r e c e i v e r a s scanning o c c u r r e d , if the two t r a n s d u c e r s w e r e not a c c u r a t e l y aligned. F o r t h e s e r e a s o n s , the type of r e c e i v i n g t r a n s d u c e r e v e n tually s e t t l e d on w a s a s l i v e r of p i e z o e l e c t r i c m a t e r i a l e m b e d d e d , e x c e p t for one c o r n e r , in t u n g s t e n - l o a d e d A r a l d i t e . The e l e c t r i c a l s i g n a l s f r o m this r e c e i v e r w e r e a m p l i f i e d in a w i d e - b a n d a m p l i f i e r and fed to a peak hold c i r c u i t to give a dc envelope which could be fed to the Y a x i s of an X - Y r e c o r d e r . The X axis of the l a t t e r was d r i v e n in s y n c h r o n i s m with the X d r i v e of the m e c h a n i c a l s c a n n e r , thus allowing a plot of a c o u s t i c p r e s s u r e d i s t r i b u t i o n to be p r o d u c e d when s c a n n i n g along any line. As an a l t e r n a t i v e , in o r d e r to p r o duce a t w o - d i m e n s i o n a l d i s p l a y of the beam p a t t e r n , the s i g n a l s f r o m the p e a k - h o l d c i r c u i t could be fed to a H a r w e l l 2 0 0 0 - s e r i e s ' q u a n t i z e r ' w h e r e they could be c o n v e r t e d to d i s c r e t e dc l e v e l s , and fed to a Mufax f a c s i m i l e r e c o r d e r to p r o d u c e a contour plot. The d r i v e of the f a c s i m i l e r e c o r d e r was m e c h a n i c a l l y coupled to the p r o b e scanning m e c h a n i s m . A block d i a g r a m of the c o m p l e t e e x p e r i m e n t a l s y s t e m is shown in Fig 10.
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EXPERIMENTAL RESULTS Fig 11 s h o w s one of the f i r s t plots obtained with the Mufax r e c o r d e r f o r the c a s e of two undamped t u b u l a r t r a n s d u c e r s . The field f r o m e a c h t r a n s d u c e r plotted s e p a r a t e l y a p p e a r s a s the two u n i f o r m g r e y a r e a s at the top of the p i c t u r e ; when both t r a n s d u c e r s a r e e n e r g i s e d s i m u l t a n e o u s l y , the i n t e r f e r e n c e p a t t e r n plotted i m m e d i a t e l y below (T 1 -- T 2 = 0~ s) a p p e a r s . By delaying the p u l s e s into one o r o t h e r t r a n s d u c e r , t h i s p a t t e r n may be shifted, (T 1 -- T 2 = + l ~ s ) , a s shown in the next plot; the e x t e n d e d plot occupying the l o w e r half of the p i c t u r e shows how the i n t e r f e r e n c e p a t t e r n r a d i a t e s o u t w a r d s a s the s e a r c h p r o b e m o v e s away f r o m the t r a n s d u c e r .
[[ements atone at 6cm)
F i g 12 s h o w s the f o r m a t i o n of a focus at d i f f e r e n t points using four t r a n s d u c e r s . The t r a n s d u c e r s u s e d in t h i s c a s e w e r e flat e l e m e n t s , 3 m m s q u a r e , m o d e r a t e l y d a m p e d and fitted with diverging l e n s e s , to give the a p p e a r a n c e of point s o u r c e s . The ability to swing the b e a m o v e r an angle of 171/2 ° can be c l e a r l y s e e n , a s can the axial m a x i m a and m i n i m a r e f e r r e d to in connection with Fig 8, with which it i s i n t e r e s t i n g to c o m p a r e this plot. I n c r e a s e d damping would have the effect of r e m o v i n g side l o b e s and leaving a single s h a r p l y defined, n a r r o w b e a m .
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This p a p e r has outlined the r e s u l t s a c h i e v e d to date, in the e a r l y s t a g e s of a l o n g - t e r m investigation. D e l i b e r a t e l y , no m e n t i o n has been m a d e of the r e c e i v i n g t e c h n i q u e s w h i c h it will be n e c e s s a r y to develop if a p r a c t i c a l s y s t e m i s to be c o n s t r u c t e d . Much work r e m a i n s to be done on p r a c t i c a l and e c o n o m i c m e t h o d s of c o n s t r u c t i n g e l e m e n t a l t r a n s d u c e r s with w i d e - a n g l e d i v e r g e n t b e a m s . The i n v e s t i g a t i o n of i m p r o v e d m e t h o d s of damping i s of the g r e a t e s t i m p o r t a n c e , s i n c e on good damping d e p e n d s the p r o d u c t i o n of n a r r o w b e a m s with a few s i d e lobes. It is n e c e s s a r y to p u r s u e f u r t h e r the question of the behaviour of the u l t r a s o u n d b e a m p a s s i n g through the w a t e r - s t e e l i n t e r f a c e into t h e s p e c i m e n u n d e r t e s t , and to s e e how the s y s t e m behaviour is modified when a c u r v e d a r r a y of e l e m e n t s i s used. A p r o p o s of this l a t t e r point, it is of i n t e r e s t to note that a c u r v e d a r r a y f o r m s a n a t u r a l focus at its c e n t r e of c u r v a t u r e , even when all the e l e m e n t s a r e p u l s e d s i m u l t a n e o u s l y . 24
ULTRASONICS J a n u a r y 1969
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|era Fig 11 F a c s i m i l e r e c o r d i n g of i n t e r f e r e n c e p a t t e r n f r o m two e l e m e n t s displaying r e s u R of t i m e d i s p l a c e m e n t of firing e l e m e n t s . The e l e m e n t s a r e c y l i n d r i c a l , a r e s p a c e d l c m a p a r t and a r e radially p o l a r i z e d . They have outside d i a m e t e r 3 . 2 m m , length 1 2 . 5 m m and h a l f - w a v e t h i c k n e s s at 5MHz. A 10V p u l s e with r i s e t i m e of lOOns i s u s e d
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Fig 12 F a c s i m i l e r e c o r d i n g s of u l t r a s o n i c b e a m swinging with four r e c t a n g u l a r e l e m e n t s . The e l e m e n t spacing is l c m , the t r a n s d u c e r s a r e 0.6cm × 0.3cm with a 0.3cm p e r s p e x convex l e n g t h giving a focal point 10cm f r o m the a r r a y
By delaying the f i r i n g of s u c c e e d i n g e l e m e n t s by equal t i m e i n t e r v a l s , t h i s focus can be s h i f t e d to the r i g h t o r left, and will m o v e in a c i r c u l a r locus a r o u n d the c e n t r e of the a r r a y . Thus, a r e l a t i v e l y s i m p l e m e t h o d m a y be a v a i l a b l e for adapting to differing tube d i a m e t e r s m e r e l y by v a r y i n g the clock r a t e into a s h i f t r e g i s t e r a c t i v a t i n g the e l e m e n t s . F i n a l l y , the b e h a v i o u r of a c y l i n d r i c a l m o s a i c of e l e m e n t s , (Fig 13), c a p a b l e of shifting the focus in t h r e e d i m e n s i o n s , i s to be i n v e s t i g a t e d . In conclusion, we a r e a s yet a long way f r o m being able to build a p r a c t i c a l tube t e s t i n g i n s t a l l a t i o n b a s e d on the p r i n c i p l e s outlined above; h o w e v e r , a l l the i n d i c a t i o n s a r e t h a t the p r o b l e m s involved a r e by no m e a n s i n s o l u b l e , and that a s u c c e s s f u l c o n c l u s i o n to t h i s r e s e a r c h o f f e r s the hope of r e v e r s i n g the p r e s e n t t r e n d t o w a r d s i n c r e a s i n g m e c h a n i c a l complexity which t h r e a t e n s to l i m i t the v e r s a t i l i t y of o u r conventional ultrasonic testing stations. REFERENCES
CyUndricat mosaic of t r a n s d u c e r elements
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Fig 13 C y l i n d r i c a l m o s a i c of t r a n s d u c e r e l e m e n t s e n v i s a g e d for t e s t i n g t u b e s
1
Colley G. J., E d d i e s J . A . , G l o d m a n B . R . , K i l l i c k E . A . 'An e l e c t r o n i c a l l y s t e e r e d r a d a r a e r i a l ' , J o u r n a l of the Royal Naval Scientific Society, Vol 22, No 2
2
T u c k e r D. G., Welsby V. G., Kendell R. ' E l e c t r o n i c s e c t o r s c a n n i n g ' , J o u r n a l of the B r i t i s h I n s t i t u t e of Radio E n g i n e e r s (August 1958)
ACKNOWLEDGEMENTS
3
Somer J.C. 'Electronic sector scanning for ultrasonic d i a g n o s i s ' , U l t r a s o n i c s (July 1968)
The a u t h o r s t h a n k the C h a i r m a n of Tube I n v e s t m e n t s L i m i t e d f o r p e r m i s s i o n to p u b l i s h t h i s p a p e r
ULTRASONICS J a n u a r y 1969
25
F O R INDLIb--TR------Y 1 9 8 1 3
Conference Paper
Electronic steering and focussing of ultrasonic beams in tube inspection K. R. Whittington* and B. D. Cox*
I n c r e a s i n g l y s t r i n g e n t r e q u i r e m e n t s for the quality c o n t r o l of s t e e l tubing have led to the d e v e l o p m e n t of c o m p l e x ins p e c t i o n equipment, using l a r g e n u m b e r s of u l t r a s o n i c p r o b e s . Each p r o b e has to be individually a d j u s t e d for any change of tube s i z e . P r o b e s may need to be u n f o c u s s e d , o r f o c u s s e d to a line o r point. Consequently, a s y s t e m is being developed w h e r e b y u l t r a sonic e n e r g y can be a i m e d and f o c u s s e d by purely e l e c t r o nic m e a n s , using an a r r a y of point s o u r c e s of u l t r a s o u n d , r e l a t i v e l y widely s p a c e d , and e n e r g i s e d at calculated t i m e
INTRODUC TION The i n c r e a s i n g l y s t r i n g e n t d e m a n d s on the quality of s t e e l tubing for such a p p l i c a t i o n s a s n u c l e a r fuel cans, b o i l e r tubing for p o w e r s t a t i o n s , and for use in the a i r c r a f t industry, has led to the d e v e l o p m e n t of c o m p l e x u l t r a s o n i c t e s t i n g equipment. If the p r e s e n t t r e n d continues, the m e c h a n i c a l complexity of such equipment, and the t i m e r e q u i r e d f o r initial s e t t i n g up and a d j u s t m e n t f o r change of tube s i z e , is likely to make n e c e s s a r y s o m e fundamental r e - t h i n k i n g of the m e a n s of introducing the u l t r a s o n i c e n e r g y into the tube. Consequently, a l o n g - t e r m r e s e a r c h and d e v e l o p m e n t p r o g r a m m e has been s e t up at Tube I n v e s t m e n t s R e s e a r c h L a b o r a t o r i e s , to i n v e s t i g a t e the f e a s i b i l i t y of s t e e r i n g and f o c u s s i n g u l t r a s o n i c b e a m s by e l e c t r o n i c m e a n s . The f i r s t p a r t of this p a p e r shows how the s p e c i a l r e q u i r e m e n t s of tube t e s t i n g r e l a t e d to t h i s p r o g r a m m e of work; the second p a r t d e s c r i b e s the r e s u l t s a c h i e v e d to date.
ULTRASONIC DETECTION OF DEFECTS IN TUBES Fig 1 shows the m o s t c o m m o n a r r a n g e m e n t for the u l t r a s o n i c d e t e c t i o n of longitudinal d e f e c t s (defects w h o s e long axis is p a r a l l e l to the axis of the tube), using p u l s e t e c h n i q u e s . A p r o b e , P , producing a n a r r o w , c o l l i m a t e d o r f o c u s s e d beam of u l t r a s o u n d is p o s i t i o n e d in such a way that the sound e n e r g y is t r a n s m i t t e d as p r e s s u r e w a v e s through the coupling medium (water) and e n t e r s the tube wall at the angle shown. Mode c o n v e r s i o n o c c u r s at the i n t e r f a c e , and s h e a r w a v e s t r a v e l around the tube wall by s u c c e s s i v e i n t e r n a l r e f l e c tions. A defect in the outside wall of the tube at A, o r in the inside wall at B, will r e f l e c t p a r t of the sound e n e r g y , which will be r e c e i v e d a s an echo p u l s e , e i t h e r by the t r a n s m i t t i n g probe u s e d as a r e c e i v e r , o r by a s e p a r a t e r e c e i v i n g probe. The u l t r a s o u n d f r e q u e n c y u s e d in tube testing is usually 5-10MHz.
i n t e r v a l s so that the i n t e r f e r e n c e p a t t e r n p r o d u c e d r e s u l t s in the r e q u i r e d d i s t r i b u t i o n of e n e r g y . A c o m p u t e r p r o g r a m has been w r i t t e n enabling the effect of v a r i a t i o n s in r e l e v a n t p a r a m e t e r s , (number and spacing of point s o u r c e s , p u l s e length, etc, ) to be studied and e x p e r i mental a r r a y s and digital excitation c i r c u i t r y have been set up to c o n f i r m the t h e o r e t i c a l p r e d i c t i o n s . Beam p a t t e r n s f r o m t h e s e a r r a y s have been plotted, using an X-Y r e c o r d e r or contour p l o t t e r display.
t e s t i n g , in which a ring of 24 p r o b e s is used. With this a r r a n g e m e n t , a rotation of a p p r o x i m a t e l y 10 ° is sufficient f o r c o m p l e t e inspection of the tube c i r c u m f e r e n c e . If it is r e q u i r e d to d i s p e n s e with rotation a l t o g e t h e r , which is the ideal f o r o n - l i n e inspection~ it is found that, for tubes in the 2-3in d i a m e t e r range, a p p r o x i m a t e l y 72 p r o b e s a r e needed. In Fig 1, the w a v e s of u l t r a s o u n d a r e shown t r a v e l l i n g clockw i s e around the tube. Unfortunately, with c l o c k w i s e i n s p e c tion only, d e f e c t s at c e r t a i n angles may not be d e t e c t e d s i n c e the sound e n e r g y r e f l e c t e d f r o m them will be in such a d i r e c t i o n that it can n e v e r be picked up by the r e c e i v i n g p r o b e . Consequently, it is n e c e s s a r y to use a s e c o n d ring of p r o b e s , facing anticlockwise around the tube, in o r d e r to e n s u r e that all longitudinal d e f e c t s may be detected. The situation is f u r t h e r c o m p l i c a t e d by the n e c e s s i t y to d e t e c t t r a n s v e r s e d e f e c t s , in which c a s e two f u r t h e r r i n g s of p r o b e s , facing in e i t h e r d i r e c t i o n along the axis of the tube a r e needed. Thus, c l o s e on 300 p r o b e s may be r e q u i r e d to i n s p e c t tubing on-line for longitudinal and t r a n s v e r s e d e f e c t s , and any a t t e m p t to detect d e f e c t s at all i n t e r m e d i a t e angles can r a i s e this total to o v e r 1000. Even with I00 p r o b e s , the m e c h a n i c a l complexity and the t i m e taken to s e t the p r o b e s up initially, and adjust t h e i r s e t t i n g s (which m u s t be a c c u r a t e within a few m i n u t e s of arc) for changes in tube d i a m e t e r ,
A
It will he a p p a r e n t f r o m the d i a g r a m s that only a s m a l l p o r t i o n of the tube c i r c u m f e r e n c e is i n s p e c t e d by this t e c h nique. Rotation of the tube by only a few d e g r e e s will cause s e v e r e attenuation o r c o m p l e t e l o s s of the defect s i g n a l s f r o m A and B. Consequently, in o r d e r to i n s p e c t the whole of the tube, it is n e c e s s a r y to r o t a t e e i t h e r the p r o b e head a s s e m b l y o r the tube as the l a t t e r is fed p a s t the p r o b e , thus producing a h e l i c a l scan. This n e c e s s i t y for m e c h a n i c a l rotation s e v e r e l y l i m i t s the r a t e at which tube can be i n s p e c t e d . The p r o b l e m can be o v e r c o m e by i n c r e a s i n g the n u m b e r of p r o b e s . Fig 2 shows a p r o b e head a s s e m b l y , at p r e s e n t in use for production * Tube I n v e s t m e n t s R e s e a r c h L a b o r a t o r i e s , Hinxton Hall, Saffron Walden, E s s e x , England 20
ULTRASONICS J a n u a r y 1969
Fig 1 P a t h of u l t r a s o u n d in a s t e e l tube. D i a m e t e r - t o t h i c k n e s s ratio : 4.9999 Angle of incidence : 13. 0000 °
ULTR~ICS
FOR INDLISTR~
1968
Conference Paper
1000 o r m o r e individual t r a n s d u c e r s , so t h a t m u c h of the s o u g h t - a f t e r r e d u c t i o n in complexity is lost. It was decided, t h e r e f o r e , to i n v e s t i g a t e a p o s s i b l e a l t e r n a t i v e s y s t e m b a s e d on a r i n g of r e l a t i v e l y w e l l - s e p a r a t e d point s o u r c e s , f o r m i n g an a r r a y working in the n e a r - f i e l d region, (Fig 4). T h i s i m p l i e s that the t i m e d e l a y s between the f i r i n g of s u c c e s s i v e e l e m e n t s would not, in g e n e r a l , be equal. In a p r a c t i c a l application, a r e l a t i v e l y s m a l l n u m b e r of e l e m e n t s would be p u l s e d in such a way a s to f o r m a focus o r point of m a x i m u m i n t e n s i t y at A in the d i a g r a m ; on the next cycle of o p e r a t i o n a s i m i l a r s e t of e l e m e n t s one p o s i t i o n f u r t h e r on in the a r r a y would be u s e d to f o r m a focus at A', and so on. T h i s s y s t e m e n v i s a g e s the u s e of digitally s y n t h e s i z e d t i m i n g p u l s e s , r a t h e r than analogue d e l a y - l i n e t e c h n i q u e s , for f i r i n g the individual e l e m e n t s of the a r r a y .
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F i g 2 U l t r a s o n i c t e s t h e a d with ring of 24 p r o b e s can be daunting, e s p e c i a l l y s i n c e it is difficult to m a n u f a c t u r e p r o b e s which a r e i d e n t i c a l in t h e i r c h a r a c t e r i s t i c s . It is c l e a r , then, t h a t any s y s t e m of switching o r adjusting the b e a m angle by e l e c t r o n i c m e a n s , and thus r e d u c i n g the n u m b e r of p r o b e s r e q u i r e d , could be of g r e a t value. ELECTRONIC STEERING AND FOCUSSING Most p u b l i s h e d work on t h i s topic h a s dealt with c l o s e s p a c e d a r r a y s of h a l f - w a v e t r a n s d u c e r s , working in the f a r - f i e l d region (Fig 3). The individual e l e m e n t s of the a r r a y a r e fed f r o m a delay line whose p a r a m e t e r s may be v a r i e d so t h a t the t i m e at which e a c h e l e m e n t r e c e i v e s its p u l s e of e l e c t r i c a l e n e r g y m a y be v a r i e d . Consequently, a pulse of u l t r a s o u n d is e m i t t e d f r o m e a c h s u c c e s s i v e e l e m e n t at a slightly l a t e r t i m e and the r e s u l t a n t envelope of the w a v e - f r o n t is at an angle to the a r r a y as shown in the d i a g r a m . T h i s angle, and thus the angle of the u l t r a s o n i c b e a m , may be v a r i e d by v a r y i n g the delayline p a r a m e t e r s . S t e e r a b l e a r r a y s of t h i s n a t u r e a r e f a m i l i a r to w o r k e r s in the fields of both r a d a r 1 and s o n a r 2 ; an application to m e d i c a l u l t r a s o n i c s h a s r e c e n t l y b e e n d e s c r i b e d 3. Such s y s t e m s have s e v e r a l d i s a d v a n t a g e s when applied to the t u b e - t e s t i n g situation. F i r s t l y , the s i z e of h a l f - w a v e t r a n s d u c e r s b e c o m e s e x c e s s i v e l y s m a l l at w a v e l e n g t h s of 5-10MHz, p a r t i c u l a r l y if a two- d i m e n s i o n a l a r r a y is r e quired. M o r e i m p o r t a n t , dividing e a c h p r o b e in a r i n g of 72 into 10 o r 20 e l e m e n t s of t h i s s o r t r e s u l t s once again in
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F i g 4 A r r a y of w i d e l y - s p a c e d point s o u r c e s working in the near-field region
C O M P U T E R SIMULATION In o r d e r to s i m p l i f y the p r o b l e m for i n i t i a l study, c o n s i d e r a tion was f i r s t g i v e n to a s t r a i g h t - l i n e a r r a y of point s o u r c e s (Fig 5). A c o m p u t e r p r o g r a m was w r i t t e n to e n a b l e the f i r i n g t i m e s f o r e a c h e l e m e n t to be c a l c u l a t e d for a focus to be f o r m e d at s o m e c h o s e n point of c o - o r d i n a t e s (x, H). The n u m b e r and spacing of the e l e m e n t s , the o p e r a t i n g f r e q u e n c y and p u l s e length could all be v a r i e d . Using this p r o g r a m it i s p o s s i b l e to p r o d u c e a g r a p h i c a l display showing the d i s t r i bution of i n t e n s i t y of the a c o u s t i c e n e r g y a c r o s s the focal plane. The p r o g r a m was l a t e r modified to allow the e n e r g y d i s t r i b u t i o n along any line p a r a l l e l to the x, H o r z a x e s to be displayed. Fig 6 shows the type of r e s u l t s obtained for the s i m p l e s t c a s e , t h a t of two point s o u r c e s with 5MHz c o n t i n u o u s - w a v e e x c i t a t i o n (infinite pulse length). The s i t u a t i o n is analogous to the optical c a s e of two c o h e r e n t light s o u r c e s , and a s can be s e e n , a s e r i e s of i n t e r f e r e n c e f r i n g e s is f o r m e d , dec r e a s i n g in amplitude away f r o m the a r r a y b e c a u s e of the i n v e r s e s q u a r e law.
F i g 3 C l o s e - s p a c e d a r r a y of h a l f - w a v e t r a n s d u c e r e l e m e n t s working in the f a r - f i e l d r e g i o n
Fig 7 shows a s e r i e s of s i m i l a r p l o t s f o r the c a s e of 10 e l e m e n t s , p l a c e d so a s to p r o d u c e a focus i m m e d i a t e l y below the c e n t r e of the a r r a y at a d i s t a n c e of 10cm. The s o u r c e s a r e s p a c e d 3 m m a p a r t , the p o s i t i o n r e l a t i v e to the X s c a l e being i n d i c a t e d by the i n v e r t e d t r i a n g l e s . In F i g 7a, continuous wave e x c i t a t i o n is again used. It is s e e n t h a t the spacing of the f r i n g e s r e m a i n s the s a m e a s in the twoe l e m e n t c a s e , but the f r i n g e width is r e d u c e d and the a m p l i tude falls off m o r e rapidly away f r o m the focal point. In ULTRASONICS J a n u a r y 1969
21
ULTRABONICS -i
- I-~T
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Conference Paper F i g 7b, the p u l s e l e n g t h i s s h o r t e n e d to 1/xs. At a c e r t a i n d i s t a n c e f r o m the focal point, the p a t h length d i f f e r e n c e b e t w e e n s u c c e s s i v e e l e m e n t s b e c o m e s too g r e a t f o r i n t e r f e r e n c e to take p l a c e , and the m e a n i n t e n s i t y i s t h a t of a s i n g l e e l e m e n t . T h u s , a s the p u l s e length i s s h o r t e n e d , s i d e l o b e s of the p a t t e r n a r e p r o g r e s s i v e l y lost. In F i g 7% the p u l s e l e n g t h h a s b e e n s h o r t e n e d to 200ns (one w a v e l e n g t h ) ; a l l s i d e l o b e s h a v e p r a c t i c a l l y d i s a p p e a r e d and a s i n g l e s h a r p i n t e n s i t y p e a k is left at the f o c u s . F i g 7d s h o w s the s a m e s i t u a t i o n , but with the f i r i n g t i m e s for the e l e m e n t s
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ULTBASONICS J a n u a r y 1969
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F i g 7 C o m p u t e r p l o t s of d i s t r i b u t i o n of u l t r a s o n i c e n e r g y in the focal p l a n e of the a r r a y . The e l e m e n t s p a c i n g is 0 . 3 0 c m and the e l e m e n t s a r e 1 0 . 0 0 c m above the focal p l a n e . a The focal point i s at x 1.35cm and the wave i s c o n t i n u o u s b The focal point i s at x = 1 . 3 5 c m and the p u l s e d u r a t i o n i s 1.00/~ s c The focal point i s at x - 1 . 3 5 c m and the p u l s e d u r a t i o n i s 0.20f~s d The focal point is at x = 4 . 0 0 c m and the p u l s e d u r a t i o n is 0 . 2 0 ~ s e The focal point i s at x = 4 . 0 0 c m and the p u l s e d u r a t i o n is 0.20/~s
Con(vcev~e Paper r e - c a i c u l a t e d to f o r m a focus away f r o m the c e n t r e of the a r r a y . (In e a c h of the f o r e g o i n g c a s e s , the a m p l i t u d e s c a l e is s u c h t h a t unit intensity is defined to be t h a t obtaining at the focal point.) In F i g s 7a-7d, the c o m p u t e r p r o g r a m h a s c a l c u l a t e d the t i m e i n t e r v a l s to s e v e n s i g n i f i c a n t f i g u r e s , that is an a c c u r a c y of l n s . Since it i s c l e a r l y i m p o s s i b l e to a c h i e v e such an a c c u r a c y in p r a c t i c e , the c a s e i l l u s t r a t e d in F i g 7d was r e w o r k e d to an a c c u r a c y of two s i g n i f i c a n t f i g u r e s (the n e a r e s t lOOns). The r e s u l t (Fig 7e) shows, a s m i g h t be expected, a r e d u c t i o n in peak i n t e n s i t y due to s o m e d e s t r u c tive i n t e r f e r e n c e taking p l a c e at the focus; the width of the p e a k a l s o i n c r e a s e s slightly. However, a s can be s e e n , such a r e s u l t would be p e r f e c t l y s a t i s f a c t o r y f r o m a p r a c t i c a l viewpoint. It is i n s t r u c t i v e to note, in view of the p r e s e n t i n t e r e s t in h o l o g r a p h y , that the s y s t e m u n d e r i n v e s t i g a t i o n is, in fact, t r u l y h o l o g r a p h i c , in the s e n s e that e v e r y e l e m e n t c o n t r i butes to the i n t e n s i t y at e a c h point in the b e a m p a t t e r n . Thus, in the e v e n t of f a i l u r e of one o r m o r e e l e m e n t s in a m u l t i e l e m e n t a r r a y , the b e a m p a t t e r n i s not a p p r e c i a b l y changed, only definition being lost, in that the individual i n t e n s i t y p e a k s a r e b r o a d e n e d and t h e i r r e l a t i v e a m p l i t u d e s slightly changed. T h i s gives a v a l u a b l e i n s u r a n c e a g a i n s t f a i l u r e of a c o m p l e x s y s t e m . If a s e r i e s of plots of i n t e n s i t y d i s t r i b u t i o n is m a d e for a s e r i e s of s e c t i o n s t h r o u g h the b e a m , it is p o s s i b l e to p r o d u c e a contour m a p of the b e a m p a t t e r n . Fig 8 shows the r e s u l t s f o r five e l e m e n t s , f i r s t l y , for a 50Ons p u l s e , and secondly f o r a 20Ons pulse. A point of i m p o r t a n c e is that, although the b e a m n a r r o w s rapidly f r o m the plane of the a r r a y to the focal plane, it does not a f t e r w a r d s d i v e r g e again a s m u c h as would be the c a s e with l e n s - f o c u s s e d equivalent. T h i s m a k e s p o s s i b l e the p r o d u c t i o n of a n a r r o w , c o l l i m a t e d b e a m of u l t r a s o u n d well suited for t u b e - t e s t i n g applications. F r o m Fig 8 it can also be s e e n t h a t i n t e r f e r e n c e p a t t e r n s a r e p r o d u c e d along, a s well a s a c r o s s , the axis of the b e a m . T h i s i n d i c a t e s that s o m e c a r e m u s t be t a k e n in choosing the working p o s i t i o n of the u l t r a s o u n d b e a m in o r d e r to avoid dead s p o t s o r unequal r e s p o n s e f r o m s i m i l a r defects.
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Fig 8 C o m p u t e r contour p l o t s of b e a m f r o m an a r r a y of five e l e m e n t s . The e l e m e n t spacing i s 0 . 5 c m and the focal point is at x = 5cm. a The p u l s e d u r a t i o n i s 0 . 5 ~ s b The p u l s e d u r a t i o n i s 0 . 2 ~ s
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E X P E R I M E N T A L PROCEDURE At t h i s stage, it was felt to b e a p p r o p r i a t e to conduct exp e r i m e n t s to c h e c k t h e o r e t i c a l a g a i n s t a c t u a l p e r f o r m a n c e .
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The f i r s t p r o b l e m was to c o n s i d e r what t y p e s of t r a n s d u c e r could be used, which would a p p r o x i m a t e to point (or, initially, line) s o u r c e s . The u s e of flat s t r i p s of p i e z o e l e c t r i c m a t e r i a l of halfw a v e l e n g t h width was not felt to be v e r y p r a c t i c a l b e c a u s e of the s m a l l d i m e n s i o n a s s o c i a t e d with the r e l a t i v e l y high t e s t i n g frequency, although s o m e e x p e r i m e n t s w e r e p e r f o r m e d with flat p l a t e s of h a l f - w a v e t h i c k n e s s , u s e d e d g e - o n . The u s e of w i d e r s t r i p s in a s s o c i a t i o n with d i v e r g i n g l e n s e s was a l s o avoided f o r i n i t i a l e x p e r i m e n t s b e c a u s e of the p o s s i b i l i t y that such s o u r c e s m i g h t have a n o n - u n i f o r m b e a m p a t t e r n , o r m i g h t not a p p e a r c o h e r e n t . The t r a n s d u c e r s finally c h o s e n for the e x p e r i m e n t a l work w e r e r a d i a i l y - p o l a r i s e d l e a d - z i r c o n a t e - t i t a n a t e tubes of 0 . 2 i n d i a m e t e r and 0 . 0 2 i n wail t h i c k n e s s . T h e s e w e r e found to give a u n i f o r m i n t e n s i t y d i s t r i b u t i o n a r o u n d t h e i r whole c i r c u m f e r e n c e . Unfortunately, they p r o v e d difficult to damp effectively, so that m o s t of the e a r l y e x p e r i m e n t s a p p r o x i m a t e d to the c a s e of c o n t i n u o u s - w a v e excitation. L a t e r e x p e r i m e n t s u s e d i n v e r t e d f o c u s s e d bowls of l e a d - z i r c o n a t e t i t a n a t e , which a r e m o r e e a s i l y d a m p e d and have the a d v a n tage of a p p e a r i n g a s point r a t h e r than line, s o u r c e s . In o r d e r to e n a b l e an a r r a y of such t r a n s d u c e r s to be p u l s e d at the c a l c u l a t e d t i m e i n t e r v a l s , a t e n - c h a n n e l pulse g e n e r a t o r was c o n s t r u c t e d . A block d i a g r a m of t h i s i n s t r u m e n t is shown in Fig 9. A 10MHz c r y s t a i - ' c o n t r o U e d o s c i l l a t o r is u s e d as a c l o c k - p u l s e g e n e r a t o r and a s e r i e s of i n t e g r a t e d - c i r c u i t decade c o u n t e r s a r e u s e d to 'count down' f r o m t h i s clock signal. The decoded outputs of the c o u n t e r s in e a c h c h a n n e l a r e s e l e c t e d by two decade s w i t c h e s on the f r o n t panel, thus allowing the f i r i n g t i m e of e a c h t r a n s d u c e r
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to be s e t independently, in 100~s s t e p s up to 10ps f r o m the o c c u r r e n c e of a s y n c h r o n i z i n g pulse. T h i s s y n c h r o n i z i n g pulse, a l s o d e r i v e d by counting down f r o m the clock s i g n a l s , o c c u r s a t a 10Hz r e p e t i t i o n r a t e . F o r full a d v a n t a g e to be t a k e n of t h i s facility, it is n e c e s s a r y that j i t t e r in the output s t a g e s which f i r e the t r a n s d u c e r s should be m i n i m i s e d . Consequently, high s p e e d s w i t c h h l g t r a n s i s t o r s a r e u s e d in p r e f e r e n c e to the conventional t h y r a t r o n or SCR c i r c u i t s . F o r a fully a u t o m a t e d s y s t e m , the decade t i m e i n t e r v a l setting s w i t c h e s could be r e p l a c e d by e l e c t r o n i c s w i t c h e s u n d e r the c o n t r o l of a s m a l l o n - l i n e c o m p u t e r . ULTRASONICS J a n u a r y 1969
23
Conference Paper A plotting tank in which the e x p e r i m e n t a l t r a n s d u c e r s could be mounted, and in which a r e f l e c t i n g t a r g e t o r r e c e i v i n g p r o b e could be s c a n n e d m e c h a n i c a l l y in a suitable r a s t e r , was u s e d in conjunction with the equipment d e s c r i b e d above. F o r the initial e x p e r i m e n t s , a r e c e i v i n g p r o b e , a s opposed to a r e f l e c t i n g t a r g e t , was u s e d e x c l u s i v e l y . T r i a l s using a tubular r e c e i v e r , i d e n t i c a l with the t r a n s m i t t i n g t r a n s d u c e r s , r a n into t r o u b l e on t h r e e counts; f i r s t l y , difficulty in damping the r e c e i v e r ; secondly, the finite d i a m e t e r of the r e c e i v e r which m e a n t that t r a n s m i t t e d p u l s e s which should a r r i v e s e p a r a t e d in t i m e could s t i l l i n t e r f e r e ; and t h i r d l y the finite length of the r e c e i v e r , which m e a n t that s p u r i o u s i n t e r f e r e n c e p a t t e r n s could o c c u r due to c h a n g e s in path length between top and bottom of the t r a n s m i t t e r and r e c e i v e r a s scanning o c c u r r e d , if the two t r a n s d u c e r s w e r e not a c c u r a t e l y aligned. F o r t h e s e r e a s o n s , the type of r e c e i v i n g t r a n s d u c e r e v e n tually s e t t l e d on w a s a s l i v e r of p i e z o e l e c t r i c m a t e r i a l e m b e d d e d , e x c e p t for one c o r n e r , in t u n g s t e n - l o a d e d A r a l d i t e . The e l e c t r i c a l s i g n a l s f r o m this r e c e i v e r w e r e a m p l i f i e d in a w i d e - b a n d a m p l i f i e r and fed to a peak hold c i r c u i t to give a dc envelope which could be fed to the Y a x i s of an X - Y r e c o r d e r . The X axis of the l a t t e r was d r i v e n in s y n c h r o n i s m with the X d r i v e of the m e c h a n i c a l s c a n n e r , thus allowing a plot of a c o u s t i c p r e s s u r e d i s t r i b u t i o n to be p r o d u c e d when s c a n n i n g along any line. As an a l t e r n a t i v e , in o r d e r to p r o duce a t w o - d i m e n s i o n a l d i s p l a y of the beam p a t t e r n , the s i g n a l s f r o m the p e a k - h o l d c i r c u i t could be fed to a H a r w e l l 2 0 0 0 - s e r i e s ' q u a n t i z e r ' w h e r e they could be c o n v e r t e d to d i s c r e t e dc l e v e l s , and fed to a Mufax f a c s i m i l e r e c o r d e r to p r o d u c e a contour plot. The d r i v e of the f a c s i m i l e r e c o r d e r was m e c h a n i c a l l y coupled to the p r o b e scanning m e c h a n i s m . A block d i a g r a m of the c o m p l e t e e x p e r i m e n t a l s y s t e m is shown in Fig 10.
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EXPERIMENTAL RESULTS Fig 11 s h o w s one of the f i r s t plots obtained with the Mufax r e c o r d e r f o r the c a s e of two undamped t u b u l a r t r a n s d u c e r s . The field f r o m e a c h t r a n s d u c e r plotted s e p a r a t e l y a p p e a r s a s the two u n i f o r m g r e y a r e a s at the top of the p i c t u r e ; when both t r a n s d u c e r s a r e e n e r g i s e d s i m u l t a n e o u s l y , the i n t e r f e r e n c e p a t t e r n plotted i m m e d i a t e l y below (T 1 -- T 2 = 0~ s) a p p e a r s . By delaying the p u l s e s into one o r o t h e r t r a n s d u c e r , t h i s p a t t e r n may be shifted, (T 1 -- T 2 = + l ~ s ) , a s shown in the next plot; the e x t e n d e d plot occupying the l o w e r half of the p i c t u r e shows how the i n t e r f e r e n c e p a t t e r n r a d i a t e s o u t w a r d s a s the s e a r c h p r o b e m o v e s away f r o m the t r a n s d u c e r .
[[ements atone at 6cm)
F i g 12 s h o w s the f o r m a t i o n of a focus at d i f f e r e n t points using four t r a n s d u c e r s . The t r a n s d u c e r s u s e d in t h i s c a s e w e r e flat e l e m e n t s , 3 m m s q u a r e , m o d e r a t e l y d a m p e d and fitted with diverging l e n s e s , to give the a p p e a r a n c e of point s o u r c e s . The ability to swing the b e a m o v e r an angle of 171/2 ° can be c l e a r l y s e e n , a s can the axial m a x i m a and m i n i m a r e f e r r e d to in connection with Fig 8, with which it i s i n t e r e s t i n g to c o m p a r e this plot. I n c r e a s e d damping would have the effect of r e m o v i n g side l o b e s and leaving a single s h a r p l y defined, n a r r o w b e a m .
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This p a p e r has outlined the r e s u l t s a c h i e v e d to date, in the e a r l y s t a g e s of a l o n g - t e r m investigation. D e l i b e r a t e l y , no m e n t i o n has been m a d e of the r e c e i v i n g t e c h n i q u e s w h i c h it will be n e c e s s a r y to develop if a p r a c t i c a l s y s t e m i s to be c o n s t r u c t e d . Much work r e m a i n s to be done on p r a c t i c a l and e c o n o m i c m e t h o d s of c o n s t r u c t i n g e l e m e n t a l t r a n s d u c e r s with w i d e - a n g l e d i v e r g e n t b e a m s . The i n v e s t i g a t i o n of i m p r o v e d m e t h o d s of damping i s of the g r e a t e s t i m p o r t a n c e , s i n c e on good damping d e p e n d s the p r o d u c t i o n of n a r r o w b e a m s with a few s i d e lobes. It is n e c e s s a r y to p u r s u e f u r t h e r the question of the behaviour of the u l t r a s o u n d b e a m p a s s i n g through the w a t e r - s t e e l i n t e r f a c e into t h e s p e c i m e n u n d e r t e s t , and to s e e how the s y s t e m behaviour is modified when a c u r v e d a r r a y of e l e m e n t s i s used. A p r o p o s of this l a t t e r point, it is of i n t e r e s t to note that a c u r v e d a r r a y f o r m s a n a t u r a l focus at its c e n t r e of c u r v a t u r e , even when all the e l e m e n t s a r e p u l s e d s i m u l t a n e o u s l y . 24
ULTRASONICS J a n u a r y 1969
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|era Fig 11 F a c s i m i l e r e c o r d i n g of i n t e r f e r e n c e p a t t e r n f r o m two e l e m e n t s displaying r e s u R of t i m e d i s p l a c e m e n t of firing e l e m e n t s . The e l e m e n t s a r e c y l i n d r i c a l , a r e s p a c e d l c m a p a r t and a r e radially p o l a r i z e d . They have outside d i a m e t e r 3 . 2 m m , length 1 2 . 5 m m and h a l f - w a v e t h i c k n e s s at 5MHz. A 10V p u l s e with r i s e t i m e of lOOns i s u s e d
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Fig 12 F a c s i m i l e r e c o r d i n g s of u l t r a s o n i c b e a m swinging with four r e c t a n g u l a r e l e m e n t s . The e l e m e n t spacing is l c m , the t r a n s d u c e r s a r e 0.6cm × 0.3cm with a 0.3cm p e r s p e x convex l e n g t h giving a focal point 10cm f r o m the a r r a y
By delaying the f i r i n g of s u c c e e d i n g e l e m e n t s by equal t i m e i n t e r v a l s , t h i s focus can be s h i f t e d to the r i g h t o r left, and will m o v e in a c i r c u l a r locus a r o u n d the c e n t r e of the a r r a y . Thus, a r e l a t i v e l y s i m p l e m e t h o d m a y be a v a i l a b l e for adapting to differing tube d i a m e t e r s m e r e l y by v a r y i n g the clock r a t e into a s h i f t r e g i s t e r a c t i v a t i n g the e l e m e n t s . F i n a l l y , the b e h a v i o u r of a c y l i n d r i c a l m o s a i c of e l e m e n t s , (Fig 13), c a p a b l e of shifting the focus in t h r e e d i m e n s i o n s , i s to be i n v e s t i g a t e d . In conclusion, we a r e a s yet a long way f r o m being able to build a p r a c t i c a l tube t e s t i n g i n s t a l l a t i o n b a s e d on the p r i n c i p l e s outlined above; h o w e v e r , a l l the i n d i c a t i o n s a r e t h a t the p r o b l e m s involved a r e by no m e a n s i n s o l u b l e , and that a s u c c e s s f u l c o n c l u s i o n to t h i s r e s e a r c h o f f e r s the hope of r e v e r s i n g the p r e s e n t t r e n d t o w a r d s i n c r e a s i n g m e c h a n i c a l complexity which t h r e a t e n s to l i m i t the v e r s a t i l i t y of o u r conventional ultrasonic testing stations. REFERENCES
CyUndricat mosaic of t r a n s d u c e r elements
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Fig 13 C y l i n d r i c a l m o s a i c of t r a n s d u c e r e l e m e n t s e n v i s a g e d for t e s t i n g t u b e s
1
Colley G. J., E d d i e s J . A . , G l o d m a n B . R . , K i l l i c k E . A . 'An e l e c t r o n i c a l l y s t e e r e d r a d a r a e r i a l ' , J o u r n a l of the Royal Naval Scientific Society, Vol 22, No 2
2
T u c k e r D. G., Welsby V. G., Kendell R. ' E l e c t r o n i c s e c t o r s c a n n i n g ' , J o u r n a l of the B r i t i s h I n s t i t u t e of Radio E n g i n e e r s (August 1958)
ACKNOWLEDGEMENTS
3
Somer J.C. 'Electronic sector scanning for ultrasonic d i a g n o s i s ' , U l t r a s o n i c s (July 1968)
The a u t h o r s t h a n k the C h a i r m a n of Tube I n v e s t m e n t s L i m i t e d f o r p e r m i s s i o n to p u b l i s h t h i s p a p e r
ULTRASONICS J a n u a r y 1969
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