基于二次滤波的HHT渡槽模态参数辨识方法

针对大型渡槽安全运行评估问题,提出一种环境激励下基于二次滤波的希尔伯特-黄变换(Hilbert-Huang transform,HHT)模态参数辨识方法。采用小波阈值与经验模态分解(empirical mode decomposition,EMD)结合的二次滤波方法滤除运行环境中强噪声干扰,凸显结构动力特性,有效避免了模态混叠问题;根据奇异熵增量理论确定系统阶数,应用HHT方法辨识降噪后信号,提高辨识准确性和精度。将该方法应用于景泰川二期三泵站输水U型渡槽,辨识正常输水工况下结构前5阶模态参数,建立该渡槽第一跨三维有限元模型,并计算模型同工况下前5阶模态参数,对比模态参数辨识结果与有限元仿真结果。对比结果表明:2种方法计算结果非常接近,最大误差为4.4%。说明基于二次滤波的HHT模态辨识方法能准确高效辨识强噪声背景下结构模态参数,可将该方法推广到大型渡槽结构中,为渡槽安全运行评估和健康在线监测提供新思路,应用前景广阔。

HHT modal parameter identification for aqueduct based on secondary filtering

Abstract: In order to improve the accuracy of safe operation assessment for larger aqueduct structure, a valid Hilbert -Huang Transform (HHT) modal parameter identification method based on secondary filtering under ambient excitation was proposed. For the vibration signal of large aqueduct structure, high-frequency white noise and low-frequency noise are often mixed into structure vibration information, which belongs to non-stationary and nonlinear signal in low signal to noise ratio (SNR). Wavelet threshold method can partially filter out the high-frequency white noise, and Empirical Mode Decomposition (EMD) can be used to obtain a series of intrinsic mode functions from high frequency to low frequency. So the secondary filtering combined wavelet threshold and EMD was used to process vibration signal to achieve a higher precision and a better de-nosing effect. High-frequency white noise and low-frequency water flow noise were filtered out firstly through the secondary filtering, highlighting the useful dominant dynamic characteristics of structure, which reduced the noise interference in a certain extent and avoided the phenomenon of modal frequency mixed effectively during the later modal identification. Then structure system order was determined by theory of singular entropy increment. Finally the modal parameter of large structure was identified through HHT in combination with natural excitation technique (NExT) to process the de-noised signals, which improved accuracy and precision of modal parameter identification. Taking U-shaped aqueduct of Jintai river pumping irrigation as the research object, we collected the vibration response data under normal working condition. We then used this proposed method to identify vibration response data to achieve the modal parameter of aqueduct structure. According to fluid-solid interactions theory, the three-dimensional finite element model of a cross U-shaped aqueduct structure was constructed through finite element model analysis to obtain the structure modal parameter under the same working condition. By comparing the modal parameter results from HHT model parameter identification method with three-dimensional finite element analysis, the result showed that the error between two methods was very small. The study showed that HHT modal parameter identification based on the secondary filtering method can be used to obtain the dominant structure vibration information and to improve the de-nosing precision, which decreased the strong noise disturbance and suppressed the modal frequency mixed phenomenon, and identified effectively structure modal parameter under strong noise background. Thus the method can be extended to lager aqueduct structures to improve the structure safe operation assessment. This proposed method provided a new aspect for safe operation and on-line monitoring of the aqueduct, and can be used effectively to solve the problem of structure modal parameter identification under ambient excitation, especially under the background with strong noise. Thus the method would have a broad prospect in engineering application.