The role of modern control theory in the design of controls for aircraft turbine engines

ONTROL systems for early aircraft turbine engines performed a rather simple task: metering the fuel to the combustor at the proper fuel-to-air ratio for both transient and steady-state operating conditions. More recently, however, things have changed significantly. To achieve higher thrust-to-weight ratios and to improve specific fuel consumption, many additional manipulated engine inputs have been added. Figure 1 shows the trend in complexity in terms of the number of controlled engine variables. Noted are a number of operational engines that have been put into service. Typical of the added engine manipulated inputs are afterburner fuel flow, variable compressor-inlet guide vanes, variable compressor stators, and variable exhaust-nozzle area. The task of designing a control algorithm for an engine having a number of inputs and outputs now becomes a formidable problem. Traditional single-input/single-output techniques can be used, but are often inadequate and require many judgmental interactions to get even close to a suitable engine control law. The designer would really like a direct, straightforward method for handling the multivariable problem. This procedure should be able to eliminate unwanted interactions between the different variables, while bringing into play those interactions that are favorable. Faced with these needs, in the early 1970s the engine control community began to investigate what new design methodologies were available. As a result, a number of researchers began to apply to the engine control design problem tools from the evolving methodology generally termed " modern control theory" (MCT). This paper reviews what has been accomplished in applying MCT to the aircraft turbine engine control design problem over the last 10 or so years and what work yet remains to be done. This review is organized as follows: first is a brief discussion of the evolution of control design methodology, followed by a description of the problems that must be faced in applying MCT to the engine control design task. The past accomplishments in applying MCT to engine control is the subject of the third and most detailed section. The paper concludes with a discussion of the future requirements for advanced engine control design. Evolution of Control Design Methodology In the early 1960s, control theoreticians began to use linear state-space methods to design closed-loop controls for large complex physical systems. At the same time, computers that could easily solve the numerical problems associated with large linear system problems were rapidly evolving and becoming readily accessible. Thus began the era of "modern control theory." This terminology was used to differentiate the new methods from the traditional linear system, single