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FUTURE A I R FORCE AIRCRAFT PROPULSION CONTROL SYSTEMS -
THE EXTENDED SUMMARY PAPER C h a r l e s A. Skira A i r F o r c e A e r o P r o p u l s i o n L a b o r a t o r y F u t u r e m i l i t a r y a i r c r a f t p r o p u l s i o n c o n t r o l s y s t e m s w i l l b e f u l l - a u t h o r i t y ,d i g i t a l - e l e c t r o n i c ,m i c r o p r o c e s s o r - b a s es y s t e m s . By now, t h i s s h o u l d n o t s u r p r i s e a n y o n e , i n f a c t , f o r someone who h a sb e e nc l o s et op r o - p u l s i o nc o n t r o ld e v e l o p m e n t ,t h i ss t a t e m e n t is w i d e l ya c c e p t e d . I f e e l s i l l y j u s t w r i t i n g i t . I f you were l o o k i n gf o r a r e a l g r a b b e ro fa no p e n i n gp a r a - graph, I ' m s o r r y .
The e v i d e n c ei ns u p p o r to fs u c h a b o l dp r e d i c t i o n is overwhelming. Cur- r e n t l ya n df o rt h en e a r - t e r mf u t u r e ,p r o p u l s i o ns y s t e mp e r f o r m a n c ei n c r e a s e s w i l l b e made t h r o u g h t h e e x p l o i t a t i o n of advancedvariablegeometrycomponents.
A s shown i n F i g u r e 1, u n l e s s t h e r e i s a b r e a k t h r o u g hi nc o m p o n e n tt e c h n o l o g y , p e r f o r m a n c ei n c r e a s e s w i l l r e s u l ti na d d i t i o n a le n g i n ec o m p l e x i t y .I no t h e r w o r d s ,t h ec o n t r o ls y s t e m w i l l have t o c o n t r o lm o r ev a r i a b l e s ,m o r ec l o s e l y a n df a s t e rt h a ne v e rb e f o r e .H y d r o m e c h a n i c a lc o n t r o lt e c h n o l o g ys i m p l yc a n - n o tc o m p e t ea g a i n s tt h ep e r f o r m a n c eb e n e f i t so f f e r e db ye l e c t r o n i c s .
TURBINEENGINETECHNOLOGYDEVELOPMENT ADVANCED DEMONSTRATOR TECHNOLOGY ENGINES JTDE, APSI/ATEGG ENGINE COMPLEXITY HIGH THROUGH-FLOW TECHNOLOGY ADVANCED COMPONENT TECHNOLOGY CENTRIFUGAL COMPRESSOR HIGH-TEMPERATURE CERAMICS ~ PERFORMANCE F i g u r e 1 6 3
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Now t h a t t h e f u t u r e h a s b e e n d e f i n e d , w h a t s h o u l d t h e A i r F o r c e ' s r o l e b e inthedevelopmentofthesesystems?Clearly, w e c a n n o te x p e c tt ob e g i nt o develop new andbettermicroprocessorsandassociatedhardware. It i s d i f f i - c u l t j u s t t o k e e p up withtheadvancesincomputertechnology. However, w e c a n b e g i n t o p l a n f o r t h e d a y when microprocessor technology w i l l p e r m i t t h e i n t e g r a t e d c o n t r o l and management of t h e a i r c r a f t f l i g h t c o n t r o l , f i r e c o n t r o l andpropulsioncontrolsystemsandthrow i n maintenanceanddiagnosticinforma- t i o n f o r f r e e .
T h e r e f o r e , i n t h e A i r ForceAeroPropulsionLaboratory, w e haveconcen- t r a t e d on thedevelopmentofcontrollogicalgorithmswitheveryexpectation t h a tt h e y w i l l b ep u ti n t ow o r k a b l es o f t w a r eu l t i m a t e l y . W e are c o n f i d e n t t h a t d i g i t a l e l e c t r o n i c c o n t r o l s s y s t e m s w i l l b e g i n t o r e a l l y p a y o f f when t h e f u l l c a p a b i l i t y and power of themicroprocessor i s u t i l i z e d . A t t h e rate t h a t m i c r o p r o c e s s o r c a p a b i l i t y i s expanding, w e may n e v e r b e a b l e t o u s e i t a l l .
However, o u r u l t i m a t e g o a l i n t h e area of logicdevelopment i s t o b e a b l e t o accomplish real-time, a d a p t i v ec o n t r o lo ft h e a i r c r a f t propulsionsystem.For a p r o p u l s i o ns y s t e m ,t h i s i s a challengingproblemforsure. The p r e s e n tp a t h - way t o w a r da c h i e v i n gt h i sg o a l i s t h es u b j e c t of t h e rest of t h i sp a p e r .
FUTURE PROPULSION CONTROL A I R F R A W INLET OPTIMIZATION
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I 1 Figure 2 6 4 A schematic of the propulsion system control and information management system is shown in Figure 2. Single, closed-loop control of the engine is shown in dark lines. The dark lines indicate which part of the control and information management function can be done real time with current state-of the-art hardware and software technology. In our current activities, we are developing basic control logic algorithms based on linear quadratic synthesi techniques and various schemes based on filter theory for sensor failure detection and accommodation and to a limited extent actuator failure detection and accommodation. At this point in time, actuator failure accommodation con- sists of a reversion to an independent back-up control system. Just how to accommodate an actuator failure without seriously degrading engine performance by way of reconfiguring the control law to account for the loss of controlla- bility is an attractive and needed research area.
Our current planned research activities include an increasing emphasis on the development of real-time system identification techniques. It is obvious to us that this is an extremely important and vital area that will enable us to develop a real-time adaptive control. Research in this area has been ongoing with the initial emphasis on identification of aircraft handling qualities. Research in developing real-time identification methods has begun.
With the knowledge of the engine's current operating characteristics, adaptive control techniques can be implemented. Such a scheme would involve on-line optimization based on continuous observations of engine operating parameters. Adjustments to the control logic would thenbe made.
System identification methods may also be used for engine diagnostics.
The technique would isolate a faulty engine component or sensor based on com parisons of observed engine behavior with nominal engine behavior. The re- sults of such an analysis could be used to trim up or adjust for the loss in of an adjustment to the control logic. A sensor failure, performance by way for example, would result in the reconstruction of that measurement in the signal conditioning logic so it would continue to operate without any per- ceivable change in performance. In any event, the results would be saved and used later for maintenance purposes.
As shown in the figure, the development of a control and diagnostics capability is a logical evolution of such an approach. Unfortunately, the prevailing opinion of Government and industry is that the integration of con- trol and diagnostics is revolutionary, not evolutionary. In an industry where change is both painful and slow, it would appear easier to reduce the nation debt. Despite the internal and political resistance, which is great, tech- nical advancements and a carefully orchistrated effort on the part of the Government agencies who sponsor research in this area may just pull it off.
Such a system, when implemented, would involve several microprocessors working togetherin parallel being monitored by amaster control or super- visory computer. Such a concept of a distributed, microprocessor-based control system is shown in Figure 3 . What looks like a system designer's nightmare will have to be another area of intense research activity. The microprocessor
is breaking down the conventional divisions between software and hardware -
LARGE-SCALE CONTROL DISTRIBUTED SYSTEMS TELETYPE C RT
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SENSORS ACTUATORS Figure 3 the new definition is firmware. Control design engineers will by necessity become electronics engineers.
Fortunately, aircraft propulsion systems will not lead the way, already energy-minded industries involved in process control are utilizing micro- processors to optimize system efficiency and save energy costs. However, the engine control problem is unique and will require more foresight, greater imagination and more coordination on the part of Government and industry Greater emphasis will be placed on concept demonstration and validation. A large ongoing commitment in terms of facilities and test beds within the Government is vital to the successful implementation of the concepts presented in this paper.
In conclusion, we in the Air Force have defined the problem and proposed an outline of an approach to accomplishing a real-time, adaptive control diagnostic information system. Such a task requires further research in several areas. These are listed below in Table I. Some areas have been the focal point of generic development activity and the investigation of how t techniques maybe applied to the propulsion control problem remains to be in- vestigated. In some areas, such as linear quadratic synthesis and multi- 6 6
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variable frequency methods for control logic development, applications to the propulsion control problem have been investigated. In other areas, basic research is needed. In any event, a coordinated research effort on the part of the AirForce, NASA, and the Navy is needed.
TABLE I. - AREAS OF FUTURE RESEARCH
Systems Modeling System Identification Multivariable Control Frequency Domain Time Domain Discrete-Time Control Stochastic Control Distributed Systems Hierarchical Control System ReliabilityIIntegrity Filtering/Estimation Failure Accommodation Fault Detection Fault Isolation Adaptive Control/Optimization Performance Seeking Real-Time Optimization 6 7