전달경로분석을 적용한 함정용탑재장비의 고체음 예측에 관한 연구

Abstract

In general, structure borne noise(SBN) causes various vibration problems in machinery and structures. SBN is vibration or noise generated at the receiving point due to source induced vibration or noise transmitted through transfer path. In case of naval shipboard equipment, it must meet higher standards than equipment manufactured by general production process. This is to ensure stability and reliability of the ship and the equipment, and also to ensure airtightness of underwater radiation noise. Thus, the double mounting systems are often used and in this case, according to MIL-STD--740-2, the measuring position of SBN is at the upper surface of the base frame, and must meet the acceptance criteria. In general, for double mounting system the mounting equipment and the base frame are modeled as rigid bodies, and the type of mount, the installation location, and the size and weight of the base frame are designed as rigid mode of the low frequency band. The required frequency range of SBN of MIL-STD-740-2 is up to 10kHz, and SBN is highly affected in this range by flexible mode of the base frame. Therefore, SBN prediction should done in different point of view. In order to predict the SBN of the base frame using forced vibration analysis, it is necessary to analytically calculate or measure the frequency response function(FRF) and the excitation force of the base frame. First, for directly measuring the FRF method, it is difficult to excite up to 10kHz, which is the standard of MIL-STD-740-2. Therefore, finite element analysis(FEA) is used to obtain FRF up to 10 kHz. In order to obtain a high-precision FRF by FEA, detailed data of the entire system, such as the dynamic characteristics of the on-board equipment, base frame, and upper and lower mounts, and accurate excitation force must be provided. However, detailed data on ship payload equipment is difficult to obtain due to the military secrets. Therefore, there is a need for a practical method of calculating FEA without detailed data of the upper elements of the base frame. Because the SBN of the base frame is up to high frequency, it is expected that the flexible mode of the base frame will have much influence. It is also expected that the dynamic characteristics of the upper elements of upper mounts to have little effect on the base frame. This study proposes a method to obtain the FRF only for the base frame, ignoring the upper elements of base frame. Also, this study confirms the validity of analysis method by comparing measured FRF of the base frame with and without the upper elements. As a result, the FRF of the base frame with or without upper elements marginally differs in the low-frequency region of the rigid body mode, and the difference in high-frequency region of the flexible body mode is negligible. Therefore, for SBN analysis, even if the upper elements of the base frame are ignored and only the base frame is analyzed, it is judged that SBN can be predicted without having significant differences compared to the analysis results that consider upper element. Second, it is a method for calculating the excitation force acting from the upper mount to the base frame. The transfer path analysis(TPA) is a method of calculating the excitation force by expressing the relationship between the force of the source and the vibration of the receiver. This is especially useful when the actual vibration mechanism is too complex to be modeled or measured directly. Using the measured SBN according to MIL-STD-740-2 and the calculated FRF of base frame by FEA, the excitation force transmitted from the upper mount to the base frame is calculated using matrix inversion method of TPA. Then the SBN is calculated by applying the excitation force obtained by TPA with the FRF of base frame calculated by FEA. By comparing and reviewing the measured SBN and calculated SBN, the validity of the method of calculating the excitation force by the TPA is confirmed. Also, it is confirmed that the predicted SBN satisfy the MIL-STD-740-2 standard only by changing the shape of the base frame without changing the excitation force or the mount dynamic characteristics. In the future, to improve the accuracy of calculation through reliable modeling and excitation force calculation for various models, it is necessary to measure FRF and predict SBN for various types of naval shipboard equipment. Based on this principle, continuous research on optimal design of base frame for SBN is required.