Time-varying transfer function extraction of an unstable launch vehicle via closed-loop identification

The aerospace launch vehicle, developed today by manufacturers, is characterized by high nonlinearity, open loop instability and time-varying behaviors. The transfer functions of the vehicle can be extracted using well-known linearization methodology. This paper presents an alternative to obtain the transfer functions via closed-loop identification using six degrees of freedom nonlinear simulation software. The pitch program is taken into account as the external excitation. Control and stability of the process are performed using robust PID controller. The model structure with some unknown parameters is obtained after mathematical modeling, thus the case of our problem is a parameter identification one. Time-variant parameters are estimated by Kalman filter approach with the aid of ARX model structure.