Deductive Retrieval Mechanisms for State Description Models

T h i s p a p e r p r e s e n t s some p r o g r a m m i n g f a c i l i t i e s f o r m o d e l i n g t h e s e m a n t i c s o f a t a s k doma in and f o r d e s c r i b i n g t h e s i t u a t i o n s t h a t o c c u r i n t h a t doma in a s a t a s k i s b e i n g c a r r i e d o u t . Each such d e s c r i p t i o n mode l s a " s t a t e " o f t he t a s k e n v i r o n m e n t , and any g i v e n s t a t e can b e t r a n s f o r m e d i n t o a new s t a t e b y t h e o c c u r r e n c e o f a n e v e n t t h a t a I t e r s t h e e n v i r o n m e n t . Such m o d e l i n g s y s t e m s a r e v i t a l i n many A l s y s t e m s , p a r t i c u l a r l y t h o s e t h a t d o q u e s t i o n a n s w e r i n g and t h o s e t h a t d o a u t o m a t i c g e n e r a t i o n and e x e c u t i o n m o n i t o r i n g o f p l a n s . The m o d e l i n g mechan isms d e s c r i b e d a r e b a s i c a l l y e x t e n s i o n s and m o d i f i c a t i o n s o f f a c i l i t i e s t y p i c a l l y f o u n d i n A l p r o g r a m m i n g l a n g u a g e s such as PLANNER, CONN1VER, and QA4. In p a r t i c u l a r , we d i s c u s s o u r use o f a 3 v a l u e d l o g i c , g e n e r a t o r f u n c t i o n s t o d e duce answers t o model q u e r i e s , t he s a v i n g and m a i n t a i n i n g o f d e r i ved r e s u l t s , and new f a c i l i t i e s f o r m o d e l i n g stntc changes p r o d u c e d by t h e o c c u r r e n c e o f e v e n t s . I n t r o d u c t i o n T h i s p a p e r d e s c r i b e s some model i n g f a c i l i t i e s t h a t have been d e v e l o p e d as p a r t o f a l a r g e Comp u t e r based C o n s u l t a n t s y s t e m (CBC) b e i n g d e s i g n e d and i m p l e m e n t e d a t t h e SRI A r t i f i c i a l I n t e l l i g e n c e C e n t e r . The CBC i s i n t e n d e d t o s e r v e t h e r o l e o f a n e x p e r t c o n s u l t a n t t o a human a p p r e n t i c e d o i n g m a i n t e n a n c e and r e p a i r o p e r a t i o n s o n e l e c t r o m e c h a n i c a l devices. The a i r c o m p r e s s o r shown in F i g u r e 1 i s an examp le o f such a d e v i c e t h a t has been used d u r i n g t h e ea r l y s t a g e s o f t h e p r o j e c t , A c o m p l e t e o v e r v i e w o f t h e p r o j e c t i s g i v e n e l s e w h e r e i n t h i s vo lume i n R e f e r e n c e 1 . The mode l s t h a t w e a r e c o n c e r n e d a b o u t i n t h i s p a p e r c o u l d b e d e s c r i p t i o n s o f any e n v i r o n m e n t o f i n t e r e s t a t s p e c i f i c i n s t a n c e s i n t i m e . Each such d e s c r i p t i o n i s s a i d t o mode l a " s t a t e " o f t h e e n v i r o n m e n t , and a s t a t e can be t r a n s f o r m e d i n t o a new s t a t e b y t h e o c c u r r e n c e o f a n even t t h a t a l t e r s t h e e n v i r o n m e n t . For t h e CBC, t h e s e mode ls d e s c r i b e t h e s t a t e o f t h e w o r k s t a t i o n i n c l u d i n g t h e d e v i c e , t o o l s , t e s t e q u i p m e n t , e t c . , and t h e e v e n t s a r e p r i m a r i l y m a i n t e n a n c e and r e p a i r o p e r a t i o n s p e r f o rmed b y t h e a p p r e n t i c e . P r o g r a m m i n g f a c i l i t i e s f o r q u e r y i n g s t a t e d e s c r i p t i o n mode l s and f o r u p d a t i n g them t o r e f l e c t t h e o c c u r r e n c e o f a n e v e n t a r e v i t a l i n many A l s y s t e m s , p a r t i c u l a r l y t h o s e t h a t d o q u e s t i o n a n s w e r i n g and t h o s e t h a t d o a u t o m a t i c g e n e r a t i o n and e x e c u t i o n m o n i t o r i n g o f p l a n s . P l a n n e r s , f o r examp l e , use t h e s e m o d e l s t o s i m u l a t e p o t e n t i a l o p e r a t o r ( e v e n t ) sequences and i n v e s t i g a t e t h e i r consequences. One of the goa ls f o r the CBC p r o j e c t is to have the system s y n t h e s i z e p lans f o r t r a n s f o r m i n g a dev i ce from any a r b i t r a r y s t a t e of assembly or d isassembly i n t o any o t h e r such s t a t e . T h i s p l a n n i n g c a p a b i l i t y i s r e q u i r e d f o r component r e mo va 1 or replacement o p e r a t i o n s and, in g e n e r a l , whenever a c o l l e c t i o n of assembly and d i sassemb ly o p e r a t i o n s are needed to he lp accompl ish a t a s k . We w i l l r e f e r to t h i s p l a n n i n g domain f o r examples th roughou t t h i s paper . The mode l ing mechanisms to be d e s c r i b e d here are b a s i c a l l y ex tens ions and m o d i f i c a t i o n s o f f a c i l i t i e s t y p i c a l l y found i n recen t p l a n n i n g p r o grams implemented in languages such as CONNIVEH (Ref . 2 ) , PLANNER (Ref . 3 ) , and QA4 (Ref . 4 ) . The need f o r such a d d i t i o n a l knowledge r e p r e s e n t a t i o n mechanisms is ev iden t as AI p r o j e c t s cont inue to move in the d i r e c t i o n o f c o n s i d e r i n g more complex task domains. The p r e s e n t a t i o n s here a re meant to add an increment to our a b i l i t y to des ign and b u i l d such l a r g e systems. Express ions and S ta tes Our model ing system is implemented as an e x t e n s i o n o f the QLISP programming language (Ref . 5 ) . [No te : QLISP is a d i r e c t descendant of the QA4 language and was des igned to m a i n t a i n QA4 language f e a t u r e s w h i l e e l i m i n a t i n g the i n e f f i c i e n c i e s i n t roduced by the QA4 i n t e r p r e t e r . I t is a se t o f f u n c t i o n s l oadab le i n t o the INTERL1SP (Ref . 6 ) system, and QLISP " s t a t e m e n t s " are b a s i c a l l y c a l l s on these LISP f u n c t i o n s . Hence, the user can f r e e l y i n t e r m i x QLISP and LISP code. The c o n t r o l f e a t u r e s t h a t were l o s t by e l i m i n a t i n g the QA4 i n t e r p r e t e r a re c u r r e n t l y be ing r e s t o r e d by making use of new c o n t r o l f a c i l i t i e s p rov ided by the 1NTERLISP system (Ref . 7 ) . l Each s t a t e d e s c r i p t i o n model can be thought of as a set of QLISP e x p r e s s i o n s , w i t h each exp ress i on hav ing a t r u t h v a l u e a s s o c i a t e d w i t h i t . An exp ress ion t y p i c a l l y i s a s ta tement o f a r e l a t i o n s h i p among e n t i t i e s in the task domain such as o b j e c t s , concep ts , or o t h e r r e l a t i o n s h i p s . For example, the exp ress i on (CONNECTED PUMP PLATFORM) is a s ta tement t h a t the pump is connected to the p l a t f o r m , and the e x p r e s s i o n (FASTENER (CONNECTION PUMP PLATFORM) B0LT:1) is a s ta tement t h a t BOLTrl is a f a s t e n e r of the pumpp l a t f o r m c o n n e c t i o n . A l though each QLISP exp ress i on has a L1SPs t y l e p r o p e r t y l i s t a s s o c i a t e d w i t h i t , w e use