Identification Of Longitudinal Load Of Propeller-shaft-boat System Based On Regularization Technology And Subspace Method

In the research of propeller-shaft-boat system,the vibration and noise of hull structure have a great impact on the safety,applicability and stealth of propeller-shaft-boat.Studying the force distribution and size of propeller under fluid excitation will provide an important theoretical basis for reducing vibration and noise of hull.At present,there are few literatures on the identification of the excitation force of propeller-axle-boat system in time domain,but there is a relatively complete theoretical system in the field of aviation.In this paper,from the point of view of actual hull,by studying the regularized load identification method and subspace system identification theory,the time domain load identification of hull structure under simplified load is studied.In this paper,the load identification of the propeller-shaftboat system can be divided into two parts: one is the identification of the transfer function matrix of the propeller-shaft-boat system,the other is the identification of the dynamic load on the blade.Firstly,in practical engineering structures,the condition number of transfer function matrix measured in time domain is usually large.When the response data of structures are mixed with noise,the least square method can not be used to obtain the solution that is in line with engineering practice.This is an ill-posed problem in mathematics,and the regularization theory can solve the problem of structural load identification under ill-posed conditions,including Tikhonov method,TSVD method,L-curve and GCV method.In this paper,the regularization errors under different sampling frequency intervals,different regularization load identification theories,different sampling points and different structural excitation forms are verified by two examples and various working conditions.It is concluded that better load identification results can be obtained when TSVD+GCV regularization method is selected with smaller sampling frequency and more measuring points and lower noise figure.Secondly,due to the complexity of the actual structure and the influence of structural noise,the subspace recognition theory is introduced into the solution of the system structure matrix.The theory divides the noise subspace and the system subspace and filters them to obtain the required subspace system.The subspace identification accuracy of the original structure under different subspace orders,different noise coefficients,different loads and locations,and different subspace inputs is studied by two examples under six working conditions.The results show that the method has good filtering characteristics for noise signals.Finally,combined with two methods,the system identification and excitation force identification of Suboff submarine and KP-505 propeller shaft system are carried out.Firstly,the transfer matrix on the propeller-shaft ship is identified in subspace,and then the load on the blade under axial loading is identified mechanically.The results show that when the natural frequency of propeller differs greatly from the excitation frequency,the subspace method and TSVD+GCV regularization theory can make the mechanical identification of propeller-shaftboat system achieve higher accuracy requirements.However,attention should be paid to the selection of parameters when solving the problem,such as the location of excitation point and measurement point,the selection of sampling time,the selection of subspace order.