Closed-Loop Identification of Flight Control System Using Two-Stage Method

Abstract

In this paper, a two-stage method is developed for the closed-loop identification of the pitch angle and speed plant transfer functions of a commercial Boeing 747 transport aircraft. A PID controller is used for the pitch angle control while a PD controller is used for the speed control. To generate identification data, the continuous-time transfer functions and the continuous-time controllers are first converted to discrete-time systems. Independent zero-mean unit variance Gaussian white noise reference signal and noise disturbance signal are used to simulate the closed-loop discrete time system. In the first step of the two-stage method, a causal high-order FIR (Finite impulse response) model is used to identify the sensitivity function of the closed loop system. The estimated sensitivity function is then used to reconstruct a noise-free uncorrelated plant input signal. In the second step the open-loop plant transfer functions are identified using the uncorrelated input and output signals. The estimated plant transfer functions are compared with the actual ones in the form of Bode plots. The results obtained from the two-stage method are also compared with those obtained using the indirect identification method.