Nonlinear System Identification Of A Folding Rudder With Freeplays

With the continuous expansion of the aerospace industry,folding rudders are used in many aircrafts in order to maximize space saving during storage and transportation.While freeplay nonlinearity is the most common centralized structural nonlinearity in folding rudders,and its existence will inevitably change the stiffness characteristics of a folding rudder and greatly affect the aeroelastic characteristics of the aircraft.At present,the research focus of the folding rudder with freeplays confines to the positive research area.However,it is difficult to simulate the freeplay nonlinearity in the structure so that the kinetic model established by mechanistic analysis is hard to describe the system characteristics accurately.Therefore,it is necessary to obtain the dynamic modeling of the folding rudder with freeplays based on system identification.This paper proceeded from the perspective of identification,researched on the identification of the folding rudder with freeplays using direct parameter estimation method base on time domain identification.The main work includes the following aspects.(1)The research background and research significance of the folding rudder with freeplays are introduced.By reviewing the history and investigating state-of-the-art of nonlinear system identification,the problem and necessity of identification of the folding rudder with freeplays are presented.(2)Two time domain identification methods for nonlinear system identification are introduced:restoring force surface method and direct parameter estimation method.The expressions of direct parameter estimation under partial and basic excitation are given respectively.Meanwhile,the significance factor and mean square error function are introduced and the steady state diagram is used to modify the model and evaluate the identification results.(3)The direct parameter estimation method is applied to the nonlinear system identification of a 3-DOF lump mass numerical model with a freeplay under base excitation and a finite element model of a folding rudder.The steady state diagram and the comparison between estimated and actual responses show that the obtained system model provides better dynamics of the reaction system which verified the theoretical feasibility of direct parameter estimation method.(4)An experimental system of an overhanging beam with adjustable freeplays is built.The restoring force surface method and direct parameter estimation method is applied to the system identification of the experimental system.The identification results based on different excited forms shows that base excitation is better for system with freeplays requiring specific bandwidth excitation.(5)The direct parameter estimation is applied to the nonlinear system identification of a folding rudder with freeplays under base excitation.The influence of different location,number and freeplay on identification results is discussed.It is found that as the model order increases.The position of the physical poles tends to be stable,which is in good agreement with the peak position of the system response FFT curve.