Active Control Of Gearbox Induced Broadband Noise In Helicopter Interior

For many years,helicopter cabin noise has always been a plaguing problem.During the flight,the whole drive train system radiates and transmits noises to the helicopter interior.Intrusive and irritating noises are not only harmful to the health of passengers,but also affect the working efficiency,and moreover,structural vibration and acoustic fatigue aroused by them may affect the helicopter safety seriously.Therefore,reducing helicopter cabin noise level has been one of the most crucial problems in the development of helicopters.To realize this target,using Active Structural Acoustic Control(ASAC)technology is an effective means of attenuating the interior noise introduced by the gearbox,while intelligent materials and the development of control algorithm gradually make it possible to control the broadband noise.With the background of the active control of the gear-meshing noise in a helicopter cabin and a perspective of vibration control aiming to attenuate the noise level,a series of topics related to the application of active struts in an ASAC system has been studied in this dissertation.The main research work was listed below:(1)For the problem of helicopter cabin noise caused by the main gearbox,the specific reasons why the active feedforward algorithm cannot fulfill the broadband control are analyzed.Based on the analysis,an ASAC hybrid control law is proposed: the feedforward controller attenuates the primary vibration that is correlated with the reference signal,while the feedback controller cancels the remaining components that are not observed by the reference sensor.(2)Based on the state space and the input-output model of the helicopter vibro-acoustic system,a discrete predictive sliding mode feedback controller is newly presented.With the robustness and response speed considered primarily,an equivalent sliding mode controller is employed.Then with the assistance of model predictive control,the chattering phenomenon is eliminated and the system states are steered to reach the sliding surface precisely in an optimal manner.Also,the stability,robustness,and state tracking error bound of the feedback controller are analyzed.As conventional sliding mode control cannot be directly used to manage non-minimum phase systems,two improvements are offered for the feedback controller to manage the systems.(3)According to the hybrid control algorithm and the control target,an overall sketch of ASAC system is proposed.A scaling model of the Z-11 helicopter is designed in detail as the simulation experimental platform,with corresponding gearbox and intelligent supporting strut systems.Then the dynamic model of the gearbox strut installed with piezoelectric actuators and the vibro-acoustic model of the model helicopter are built.By combining the structural finite element method and the acoustic finite element method,the cabin sound field distribution is achieved,as well as the transfer functions from each excitation point to the target points.This work provides support for simulation and experiment research for the ASAC algorithm.(4)Relatively comprehensive simulation work is carried out to verify the effectiveness and stability of the proposed hybrid control law.First,the simulation of broadband control with vibration feedback is conducted.The control characteristics under the circumstance are analyzed,and the influence of main control parameters is reviewed.Further,the simulation of broadband control with noise feedback is conducted to verify the effectiveness of the algorithm.The characteristics of the algorithm are verified under various cases,with important conclusions drawn,which lay a foundation for the ASAC experimental research.(5)According to the overall scheme of the active control,the software and hardware of the real-time active control system are newly developed.Based on the system,the algorithm's control abilities are sufficiently checked by single-input single-output and multiple-input multiple-output experiments via different feedback strategies progressively.Results confirm that the active control system is practical for cancelling broadband helicopter interior noise.For many cases,the attenuations of the measured noise exceed the level of 15 d B,with maximum reduction reaching 31 d B.In addition,reduction of 8 d B broadband noise attenuation can be obtained.Different feedback strategies are compared to provide instructions for deeply researching the coupling characteristics of the vibration and noise of the helicopter.(6)In response to the urgent requirements of helicopter vibration suppression,the proposed hybrid control law is introduced into the active control of structural response system.Performance enhancement of the hybrid control law is verified by simulations based on a simplified helicopter finite element model.By comparison with the parallel Fx-LMS algorithm under various cases,the results clearly demonstrate that the algorithm deals with the rotor main passing frequency component and its harmonics simultaneously with faster convergence and better stability.Considering the practical necessity,experiments on the free-free elastic beam structure have been conducted under various cases such as excitation change and path change.The experimental results show that the proposed algorithm deals with the multifrequency vibration with faster convergence rate and remarkable self-adaptability.The research results of this paper will provide important theoretical analysis basis and experimental basis for the development of active noise reduction systems in helicopter cabins.