M.Sc Thesis


M.Sc StudentPavel Gold
SubjectModelling and Control of Nonlinear Aeroservoelasticity
Due to Actuator Free Play
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Idan Moshe
Professor Emeritus Karpel Mordechay


Abstract

This work deals with aeroservoelastic (ASE) modeling where the control-surface actuators are grossly nonlinear due to free play. Efficient ASE models are constructed and control techniques applied for dealing with actuator nonlinearities. The actuator free play leads to a discontinuity in its stiffness coefficient, which might lead to excessive vibrations around the static position for which the aerodynamic hinge-moment nullifies.

The linear state-space ASE formulation is expanded to address the nonlinear actuator stiffness characteristics, represented by a nonlinear feedback loop. The fictitious masses method is used to account for the actuator stiffness changes. The stiffness actuation mechanics are represented using two modeling approaches. Analyzing performances and characteristics of the two approaches has lead to a preferred method, and it is used to obtain a credible model of the nonlinear actuator behavior.

An aeroservoelastic trim analysis algorithm that allows the presence of nonlinear structural elements was developed to determine the initial aeroelastic states of the aircraft in a steady trimmed flight. Simulations of complete aircraft structural dynamic response to maneuvers commands exhibit limit-cycle oscillations (LCO), when one of the actuator forces changes sign.

A comprehensive LCO cancellation algorithm was developed for the nonlinear ASE model derived in this work. The algorithm includes an LCO detection strategy and aileron command compensation to terminate the LCO. The performance of the proposed LCO cancellation algorithm was evaluated via a closed loop simulation study. The numerical results presented show a good closed loop performance and full LCO cancellation.