A Fault Detection and Isolation Method Applied to Liquid Oxygen Loading for the Space Shuttle

Process-monitor ing and f a u l t loca t ion techniques have been developed at the Kennedy Space Center in a domain of mixed media con t ro l in NASA's Space Shut t le Launch Processing System. An i n t u i t i v e l y appealing d iagnost ic technique and representat ion of the system's s t ruc tu re and func t ion were formulated in cooperation w i th system engineers. Funct ional re la t ionsh ips that determine the consistency of sensor measurements are represented by symbolic expressions embedded in frames. Funct ional re la t i onsh ips are stored in exact ly one p lace , so they must be inver ted to determine hypothet ica l values for possib ly f a u l t y ob jec ts . Propagating these hypothet ica l s tates to other sensors permits the loca t ion of f a u l t s . Standard symbolic invers ion techniques have been extended to include cond i t i ona l func t ions . A demonstration system is opera t ing , and i t s eva luat ion w i l l soon use l i v e data from the f i r i n g rooms at KSC. The Space Shu t t l e ' s externa l tank is loaded w i th 140,000 gal lons of l i q u i d oxygen (LOX) from a m i l l i o n ga l lon storage tank (Figure 1) about 3.5 miles from c o n t r o l l i n g computers at the Launch Control Center. This c r i t i c a l operat ion begins s ix to e ight hours before launch and must continue to w i t h i n seconds of i g n i t i o n . NASA's automated Launch Processing System cont ro ls t h i s process and monitors sensor readings such as temperature, pressures, and valve pos i t i ons , checking that they are each w i t h i n some spec i f i ed range. If anything goes wrong, a few experts must analyze the problem and determine whether to stop the process, l ea f i ng through 200 pages of schematics. A delay in t roubleshoot ing could abort the Shut t le f l i g h t . This i s the f i r s t app l i ca t i on o f a r t i f i c i a l i n t e l l i gence sponsored by the Kennedy Space Center (KSC). An experimental system ca l led LES (the LOX Expert System) has been constructed to a id in moni tor ing the LOX Loading process, and diagnosing f a u l t s . This domain, i t s h i s t o r y , and fu r the r d e t a i l s of i t s representat ion are described more f u l l y elsewhere (Jamieson, 1984; Delaune, 1985). For more complete desc r ip t ion of diagnosis methods, knowledge representa t ion , and func t iona l invers ion techniques, see (Sca r l , 1985). II DEFINITIONS AND ASSUMPTIONS F i r s t o f a l l , i t i s assumed t h a t : (a) Time between f a i l u r e s is long compared to diagnosis t ime, r u l i n g out simultaneous f a i l u r e s (which F i g u r e 1. LOX TRANSFER AT KSC