Control Of A PCB Drilling Machine By Visual Feedback

A v i s u a l i n p u t i s l i k e l y t o p l a y a n i m p o r t a n t r o l e i n many f u t u r e m e c h a n i c a l h a n d l i n g and i n s p e c t i o n systems. T h i s paper w i l l d e s c r i b e one e x p e r i m e n t a l i m p l e m e n t a t i o n o f a machine o f t h i s t y p e ; a n a u t o m a t i c p r i n t e d c i r c u i t b o a r d d r i l l i n g machine i n w h i c h a T V camera i s used t o v i e w t h e b o a r d and a s m a l l computer i s used t o p r o c e s s t h e TV image. The computer c o n t r o l s t h e machine u s i n g o n l y t h e i n f o r m a t i o n e x t r a c t e d f r o m t h e p i c t u r e . I n t r o d u c t i o n I t i s becoming i n c r e a s i n g l y d i f f i c u l t f o r i n d u s t r y t o f i n d p e o p l e who a r e w i l l i n g t o d o d a n g e r o u s or monotonous j o b s . Those who a r e p e r f o r m i n g b o r i n g o r r e p e t i t i v e t a s k s a r e l i a b l e t o become f r u s t r a t e d and e r r o r p r o n e . Problems o f t h i s t y p e t o g e t h e r w i t h t h e i n c r e a s i n g c o s t o f manpower p r o v i d e s t r o n g i n c e n t i v e s f o r i n d u s t r y t o automate t h e i r p r o d u e t i o n l i n e s . W i t h t h e advent o f c o m p u t e r s , t r a d i t i o n a l f i x e d a u t o m a t i o n i s b e i n g s e l e c t i v e l y r e p l a c e d b y programmable n u m e r i c a l l y cont r o l l e d m a c h i n e s . These machines n e v e r t h e l e s s f o l l o w a f i x e d s e t o f i n s t r u c t i o n s , and i n g e n e r a l t h e y c a n n o t cope w i t h a change i n t h e i r e n v i r o n m e n t . A s c o m p u t e r s have become cheaper and more p o w e r f u l i t has become p o s s i b l e t o i n t r o d u c e i n t o f a c t o r i e s more i n t e l l i g e nt machines ( 1 ) w h i c h a r e aware o f t h e i r e n v i ronment and w h i c h a r e c a p a b l e o f r e a c t i n g t o changes i n t h a t e n v i r o n m e n t . Examples o f t h i s t y p e o f machine i n c l u d e H i t a c h i ' s v i s u a l l y c o n t r o l l e d b o l t t i g h t e n e r , and G e n e r a l M o t o r ' s system f o r p l a c i n g w h e e l s o n hubs ( 2 , 3 ) . I n t h i s paper w e a r e c o n c e r n e d w i t h m a c h i n e s w h i c h examine t h e i r e n v i r o n m e n t w i t h v i s u a l s e n s o r s and w h i c h we c a l l v i s u a l l y c o n t r o l l e d m a c h i n e s . Much o f t h e d i s c u s s i o n however w o u l d a p p l y t o machines e q u i p p e d w i t h o t h e r t y p e s o f s e n s o r . J u s t i f i c a t i o n f o r V i s u a l l y C o n t r o l l e d A u t o m a t i o n I t can b e a r g u e d t h a t t h e r e s h o u l d b e n o need f o r v i s u a l l y c o n t r o l l e d m a c h i n e s i n t h e i d e a l a u t o m a t i c f a c t o r y . C o n s i d e r f o r example, t h e p r o b l e m o f a u t o m a t i c a s s e m b l y . I f t h e o r i e n t a t i o n and p o s i t i o n o f a l l components were p r e s e r v e d f r o m t h e p o i n t o f i n i t i a l f a b r i c a t i o n , where t h e y are w e l l known, t o t h e f i n a l assembly i n t o t h e c o m p l e t e d p r o d u c t , t h e n ' b l i n d a u t o m a t i o n ' w o u l d b e q u i t e s a t i s f a c t o r y . I n p r a c t i c e however t h e r e a r e many r e a s o n s why t h i s d e s i r a b l e g o a l c a n n o t be a c h i e v e d . I t may o f t e n b e n e c e s s a r y t o M e t go' o f p a r t s , ( f o r example when s m a l l s h e e t m e t a l p r e s s i n g s a r e p l a t e d o r d e b u r r e d ) , and t o s t o r e t hem between m a n u f a c t u r e and a s s e m b l y . If t h e components a r e s t o r e d i n a ' l o o s e ' s t a t e , t h e n e i t h e r a p e r s o n o r a machine i s r e q u i r e d t o f e e d t h e component t o t h e a u t o m a t i c assembly machine. I n many c a s e s , t h i s p r o b l e m i s s o l v e d by i n g e n i o u s m e c h a n i c a l d e s i g n s such as bowl f e e d e r s . Some components however p r o v i d e h a r d e r p r o b l e m s f o r t h e m e c h a n i c a l d e s i g n e r and i t i s i n t h e s e c i r c u m s t a n c e s t h a t a v i s u a l l y c o n t r o l l e d machine i s u s e f u l . Thus v i s u a l l y c o n t r o l l e d machines may b e c o s t e f f e r t i v e when i t i s uneconomic t o keep components i n j i g s , when f r e q u e n t p r o d u c t changes make c o m p l e t e programming e x p e n s i v e , o r when i t i s i m p o r t a n t t o d e t e c t d r i f t s i n t h e p r o d u c t o r machine ( 2 5 ) . The A p p l i c a t i o n o f V i s u a l C o n t r o l t o D r i l l i n g P r i n t e d C i r c u i t Boards The d r i l l i n g o f P r i n t e d C i r c u i t Boards (PCBs) under v i s u a l c o n t r o l i s a p r o b l e m i n w h i c h b o t h t h e m e c h a n i c s and t h e scene a n a l y s i s a r e r e l a t i v e l y s i m p l e . The s o l u t i o n t o t h e p r o b l e m c o u l d however have g e n u i n e p r a c t i c a l a p p l i c a t i o n . I n r e s e a r c h l a b o r a t o r i e s and o t h e r e s t a b l i s h ments i n w h i c h PCBs a r e made i n v e r y s h o r t r u n s o f s m a l l numbers o f b o a r d s , t h e b o a r d s a r e o f t e n d r i l l e d by hand u s i n g a s i n g l e s p i n d l e d r i l l i n g machine such a s t h a t i l l u s t r a t e d d i a g r a m m a t i c a l l y i n F i g . l a . 1 . The Manual D r i l l i n g o f P r i n t e d C i r c u i t Boards The o p e r a t o r v i e w s t h e b o a r d t h r o u g h t h e e y e p i e c e , w h i c h c o n t a i n s a c r o s s w i r e i n d i c a t i n g t h e p o s i t i o n o f t h e d r i l l ( F i g . l b ) . T o d r i l l a h o l e , he moves t h e b o a r d u n t i l t h e p o i n t t o be d r i l l e d c o i n c i d e s w i t h t h e c r o s s w i r e , and t h e n a c t u a t e s t h e d r i l l . The e x p e r i m e n t a l equipment b u i I t a t MRL t o aut o m a t e t h i s p r o c e s s and r e p l a c e t h e human o p e r a t o r b y a computer v i s i o n s y s t e m , i s i l l u s t r a t e d d i a g r a m m a t i c a l l y i n F i g . 2 and b y t h e p h o t o g r a p h i n F i g . 3 . The b o a r d i s v i e w e d , v i a a h a l f s i l v e r e d m i r r o r , b y a T V camera w h i c h i s i n t e r f a c e d to a H o n e y w e l l 516 c o m p u t e r . The TV v i d e o s i g n a l can b e sampled w i t h a maximum r e s o l u t i o n o f 300 x 400 p i c t u r e e l e m e n t s ( p i x e l s ) o v e r t h e f i e l d o f v i e w o f t h e camera, and each p i x e l can b e d i g i t i s e d t o 5 b i t s .