A Representation For Robot Plans

A r e p r e s e n t a t i o n f o r r o b o t p lans is p roposed . The r e p r e s e n t a t i o n o f a g i ven p l a n r e f l e c t s the s t r u c t u r e o f the p rocess which produced the p l a n . T h i s i n f o r m a t i o n i s u s e f u l i n b o t h the o r i g i n a l con s t r u c t i o n o f the p l a n and i t s subsequent m o d i f i c a t i o n i f un fo reseen events cause e x e c u t i o n f a i l u r e s . A programmed system t h a t c o n s t r u c t s and execu tes ( i n s i m u l a t i o n ) p lans f o r j ou rneys u s i n g a l a r g e system o f p u b l i c t r a n s p o r t i s d e s c r i b e d t o i l l u s t r a t e the advantages o f the r e p r e s e n t a t i o n . I t is a l s o shown how the r e p r e s e n t a t i o n cou ld be used f o r more t y p i c a l r o b o t p l a n n i n g w o r l d s . 1 . I n t r o d u c t i o n S ince most r o b o t s must f u n c t i o n in a w o r l d whose behav iou r they cannot hope to p r e d i c t e x a c t l y / t hey must be p repared f o r p lans they make to f a i l d u r i n g e x e c u t i o n . (As u s u a l , a p l a n is a l i n e a r sequence o f a c t i o n s o r o p e r a t i o n s i n t e n d e d t o t r a n s f o r m some i n i t i a l s t a t e o f t he r o b o t ' s w o r l d i n t o some goa l s t a t e . ) Such f a i l u r e s may occur because some a c t i o n i n the p l a n f a i l s t o have i t s expec ted e f f e c t o r because o f some unp re d i c t a b l e event o u t s i d e the c o n t r o l o f the r o b o t . In any case , a f t e r such a f a i l u r e the r o b o t must be a b l e t o mod i fy i t s p l a n to cope w i t h the u n expec ted t u r n o f e v e n t s . Rather than throw away the o r i g i n a l p l a n and s t a r t a g a i n f rom s c r a t c h , i t i s c l e a r l y d e s i r a b l e t h a t as much as p o s s i b l e o f the work t h a t went i n t o c o n s t r u c t i n g the o l d p l a n shou ld b e reused i n p r o d u c i n g a m o d i f i e d v e r s i o n . One p r e v i o u s sys tem, S T R I P S 1 ' 2 , t h a t d e a l t w i t h r e p l a n n i n g a f t e r exec u t i o n f a i l u r e s , t r i e d t o save work b y making a r b i t r a r y subsequences o f o p e r a t i o n s f rom the o r i g i n a l p l a n a v a i l a b l e t o the r e p l a n n i n g p rocess as p r i m i t i v e o p e r a t i o n s . No a t t e n t i o n , however, was p a i d to the way in wh ich such subsequences were o r i g i n a l l y i n t e n d e d t o c o n t r i b u t e t o t h e f u l f i l l m e n t o f t h e g o a l o f t h e p l a n , t h u s , a t t i m e s , l e a d i n g to r a t h e r a r b i t r a r y uses o f them. Such a scheme does n o t t r y and use any of the prob lem a n a l y s i s t h a t went i n t o c o n s t r u c t i n g the o r i g i n a l p l a n b u t o n l y i t s r e s u l t s . More h i e r a r c h i c a l l y s t r u c t u r e d systems have proposed t h a t p l a n s shou ld be ske tched o u t a t some l e v e l o f d e t a i l a b s t r a c t e d f rom t h a t o f t he r o b o t ' s p r i m i t i v e a c t i o n s , and each s t e p f u l l y d e t a i l e d o n l y a s i t i s e x e c u t e d . Such a n approach , w h i l e g i v i n g a n i n b u i l t f l e x i b i l i t y w i t h r e g a r d t o d e t a i l s , runs t h e r i s k o f d i s a s t e r i f some o f the u n e l a b o r a t e d s teps t u r n o u t t o be i m p o s s i b l e b e cause o f some c o m p l i c a t i o n a t t he more d e t a i l e d l e v e l s . The scheme o f p l a n r e p r e s e n t a t i o n p resen ted in t h i s paper i s p r i m a r i l y des igned t o f a c i l i t a t e the Hayes Semant ics and C o g n i t i o n , S w i t z e r l a n d r e c o n s t r u c t i o n o f d e t a i l e d p lans a f t e r f a i l u r e i n e x e c u t i o n . I t does t h i s b y e x p l i c i t l y r e c o r d i n g i n the r e p r e s e n t a t i o n o f a p a r t i c u l a r p l a n the s t r u c t u r e o f the p rocess which produced t h a t p l a n . The i n f o r m a t i o n thus reco rded i n c l u d e s the cho i ces made d u r i n g t h e c o n s t r u c t i o n of a p l a n , how they advanced the c o n s t r u c t i o n o f the p l a n i n terms o f s u b g o a l i n g and r e f i n e m e n t o f d e t a i l s , and how they a re l o g i c a l l y r e l a t e d t o each o t h e r . A f t e r f a i l u r e t h i s i n f o r m a t i o n enab les t h a t p a r t o f the development o f a p l a n due to d e c i s i o n s i n v a l i d a t e d by the c i r cumstances o f f a i l u r e to be p r e c i s e l y i d e n t i f i e d and d i s c a r d e d . The r e s u l t i n g d a t a s t r u c t u r e rep resen ts t h a t p a r t o f the o r i g i n a l p rob lem a n a l y s i s which i s s t i l l a p p l i c a b l e a f t e r the f a i l u r e * . Use o f t h i s s t r u c t u r e b y t h e r e p l a n n i n g mechanism w i l l a vo i d t h e c o r r e s p o n d i n g p a r t o f the o r i g i n a l p l a n n i n g e f f o r t b e i n g d u p l i c a t e d d u r i n g r e p l a n n i n g . Whi le the r e p r e s e n t a t i o n makes as few assumpt i o n s a s p o s s i b l e about p l a n n i n g p r o c e s s e s , i t i s b a s i c a l l y o r i e n t e d t o p rocesses based o n h i e r a r c h i c a l l e v e l s o f d e t a i l 3 , 4 , ' 5 . B y making i n f o r ma t i on about a l l s teps o f a p l a n c o n s t a n t l y a v a i l a b l e , t h e r e p r e s e n t a t i o n f a c i l i t a t e s p l a n making f o r w o r l d s i n wh ich the e f f e c t s o f o p e r a t o r s t end t o i n t e r f e r e w i t h each o t h e r t o a s i g n i f i c a n t e x t e n t . I t a l s o p e r m i t s d i f f e r e n t p a r t s o f t he e x e c u t i o n sequence to be deve loped in an o r d e r and t o r e l a t i v e l e v e l s o f d e p t h , dependent o n l y o n t h e c o n s t r a i n t s o f t h e prob lem domain. Such an a b i l i t y i s u s e f u l when t h e r e a re d i f f e r e n t c e r t a i n t i e s as t o whether the d i f f e r e n t s t eps o f an undet a i l e d s k e l e t o n p l a n can be s u c c e s s f u l l y deve loped t o a d e t a i l e d l e v e l . Use o f t h e r e p r e s e n t a t i o n forms the b a s i s o f a w o r k i n g r o b o t p l a n n i n g and ( s imu la ted ) e x e c u t i o n system d e s c r i b e d in s e c t i o n 3. Some aspec ts of t he system implemented a re a t y p i c a l o f r o b o t p l a n n i n g domains in g e n e r a l , and so a d i s c u s s i o n o f how t h e r e p r e s e n t a t i o n would be used w i t h a more u s u a l r o b o t w o r l d f o l l o w s i n s e c t i o n 4 . 2. The Rep resen ta t i on A p l a n i n t h e proposed r e p r e s e n t a t i o n c o n s i s t s o f two i n t e r l i n k e d da ta s t r u c t u r e s : a t r e e wh ich r e p r e s e n t s t h e subgoal s t r u c t u r e o f t he p l a n and a graph wh ich r e p r e s e n t s t h e l o g i c a l r e l a t i o n s h i p s o f the d e c i s i o n s taken i n c o n s t r u c t i n g t h e p l a n . T h i s r e p r e s e n t a t i o n and i t s uses a r e des c r i b e d below i n gene ra l t e rms ; f o r d e t a i l e d examples see s e c t i o n 3. * I n f a c t i t does n o t a lways r e p r e s e n t a l l o f i t , s i n c e t h e r e can b e d e c i s i o n s t h a t a re s t i l l appropr i a t e a f t e r f a i l u r e , b u t are d i s c a r d e d because they were o r i g i n a l l y based on one o f t h e d e c i s i o n s i n v a l i d a t e d b y the f a i l u r e .