Abstract

Controls for transient operation of aircraft engine are highly important because the transient performance depends on how the engine responds the throttle command via the control system. The Ndot control is widely used in modern aircraft engine that can produce a consistent acceleration/deceleration despite of engine conditions. But there are some technical challenges in the controller design for closely tracking the fast-changed Ndot command. For a two-spool turbofan engine, the rate of high pressure rotor speed, N2dot as the feedback signal, is not directly measured and calculated using derivative that inevitably amplify the carry-on noise. Although a low pass filter with narrow bandwidth can attenuate the noise, it will also slow down the dynamic response of the closed loop and make the control accuracy completely unacceptable for fast tracking. Given the control authority may be handed over different control loops at any time during engine transient operation, integrator wind-up problem makes the controller design even more sophisticated. The Ndot controller has to be designed in a considerable way to handle with these inherent conflicts and constraints. 

Two controllers designed for Ndot control of a commercial turbofan engine are proposed in this paper. One controller is based on PI controller in tandem with an integrator. And the other is an integral controller with feed-forward structure. The control structure also contains two cascaded 1-order filters in the feedback and a lead compensator on the forward path to improve transient response. A design emphasis is mainly on the anti-windup design when the controller output is not selected by MAX/MIN logic. The corresponding anti-windup logic is  based on condition technique and it can make the switching elegant between rotor speed control at steady state and Ndot control, as well as other limits control at acceleration/deceleration. The controller design procedure and parameter tuning are intended to make straightforward for engineering use purpose.

The controller performance of both design are evaluated and compared comprehensively under engine normal condition and deteriorated conditions via digital simulation. The simulated deterioration of engine components covers the effects on compressor flow and efficiency, combustor efficiency, turbine flow and efficiency and their combinations. The simulation results are satisfactory and show both controllers can meet the design requirements in the presence of sensor noise. The information of sensor noise is obtained from field data analysis and can be representative of the true situation. The control accuracy of the one with feed-forward structure is little better than the other of Pl with integrator for a normal engine. However, the latter design demonstrates better robustness and provides more consistent performance for all engine conditions, below 7% of control error for the simulated worst case.