A Survey of Intelligent Control and Health Management Technologies for Aircraft Propulsion Systems <s> Life Extending Control <s> The consumption of engine life characterized by low EGT margin, expended life-limited parts, and slow engine accelerations is the principal cause of aircraft engine removal. Life extending control results from a conscious effort on the part of control system designers to extend the life of an engine by modifying the control logic or control hardware to influence one or more of these life-consuming factors. General Electric Aircraft Engines and NASA Glenn Research Center are currently engaged in a collaborative research programme to investigate control technologies applicable to extending on-wing life of aircraft engines. A trade study of potential schemes that may have a positive impact on engine life has been performed, and the results of this study are used to narrow the focus of further research under this programme. <s> BIB001 </s> A Survey of Intelligent Control and Health Management Technologies for Aircraft Propulsion Systems <s> Life Extending Control <s> Aircraft engine controllers are designed and operated to provide desired performance and stability margins. The purpose of life -extending -control (LEC) is to study the relationship between control action and engin e component life usage, and to design an intelligent control algorithm to provide proper trade -offs between performance and engine life usage. The benefit of this approach is that it is expected to maintain safety while minimizing the overall operating co sts. With the advances of computer technology, engine operation models, and damage physics, it is necessary to reevaluate the control strategy for overall operating cost consideration. This paper uses the thermo -mechanical fatigue (TMF) of a critical com ponent to demonstrate how an intelligent engine control algorithm can drastically reduce the engine life usage with minimum sacrifice in performance. A Monte Carlo simulation is also performed to evaluate the likely engine damage accumulation under variou s operating conditions. The simulation results show that an optimized acceleration schedule can provide a significant life saving in selected engine components. Nomenclature <s> BIB002 </s> A Survey of Intelligent Control and Health Management Technologies for Aircraft Propulsion Systems <s> Life Extending Control <s> This paper addresses the requirements of a control system for active turbine tip clearance control in a generic commercial turbofan engine through design and analysis. The control objective is to articulate the shroud in the high pressure turbine section in order to maintain a certain clearance set point given several possible engine transient events. The system must also exhibit reasonable robustness to modeling uncertainties and reasonable noise rejection properties. Two actuators were chosen to fulfill such a requirement, both of which possess different levels of technological readiness: electrohydraulic servovalves and piezoelectric stacks. Identification of design constraints, desired actuator parameters, and actuator limitations are addressed in depth; all of which are intimately tied with the hardware and controller design process. Analytical demonstrations of the performance and robustness characteristics of the two axisymmetric LQG clearance control systems are presented. Takeoff simulation results show that both actuators are capable of maintaining the clearance within acceptable bounds and demonstrate robustness to parameter uncertainty. The present model-based control strategy was employed to demonstrate the tradeoff between performance, control effort, and robustness and to implement optimal state estimation in a noisy engine environment with intent to eliminate ad hoc methods for designing reliable control systems. <s> BIB003 </s> A Survey of Intelligent Control and Health Management Technologies for Aircraft Propulsion Systems <s> Life Extending Control <s> ** * * The objective of this work is to extend the on-wing life of aircraft engines using advanced control techniques. Previous studies demonstrated that, out of various schemes that have a positive impact on engine life, improved active clearance control (ACC) is the one with the best potential. Modern commercial turbofan engines are already implementing various forms of ACC schemes to maintain turbine tip clearances within appropriate ranges for safety and efficiency reasons. However, one limitation of these schemes is that they are designed for a "nominal" engine, and as the engine deteriorates over time the ACC performance degrades. The present paper proposes to improve the high-pressure turbine ACC in a high bypass commercial engine to account for deterioration. A key component of this new ACC scheme is an estimator of the clearance variation due to deterioration. This estimator processes the difference between some of the actual engine outputs and outputs from a model of the nominal engine running in real-time. The new ACC scheme was implemented on a high-fidelity model of the commercial engine. Extensive simulations show that exhaust gas temperatures are clearly lowered by the new ACC, which translate into more cycles between engine overhauls. <s> BIB004
Engine controllers are designed to meet certain operability and performance constraints. Some are engine-related, such as that the engine should not stall on acceleration; some are externally imposed, such as the Federal Aviation Administration's (FAA) rise time requirement for thrust in commercial engines. The rise time requirement results in the development of an acceleration schedule (the maximum rotor speed rate-of-change loop as a function of rotor speed, sometimes called the Ndot control mode) that is designed simply to accelerate the fan and core to achieve desired thrust within the required time, while not exceeding other constraints. Durability is one of the key VAATE goals, so it is reasonable that it should be taken into consideration in the design process of future engine control algorithms. The idea of Life Extending Control is to design a control system which provides acceptable engine response while minimizing component damage. The concept of Life Extending Control has demonstrated that by using smart acceleration logic for engine control, the thermomechanical fatigue damage accumulated during a typical engine acceleration transient from idle to full power can be significantly decreased without any noticeable loss in engine performance. By slightly reducing the peak temperature during a transient, significant part life can be saved. A typical baseline acceleration schedule for a commercial turbofan engine, as well as schedules optimized to achieve acceptable take-off transients while minimizing life consumption for a specific hot section engine component, is shown in figure 4 . The thrust transients that correspond to these schedules are shown in figure 5 . Simulation results demonstrated that the optimized acceleration schedule decreased component life consumption during takeoff significantly for the same rise time, and extending the rise time slightly reduced the life consumption even more. BIB002 A recent study BIB001 looked at several potential component life extending technologies, and it showed that smart acceleration/deceleration logic was the highest ranked software-only technology while active clearance control was the highest ranked software/hardware technology. BIB003 BIB004 One potential architecture that enables adjustment of the schedules in response to accumulated engine damage is shown in figure 6 (ref. 17). 