4.不同类型的常见激励技术 – 随机,猝发,扫频,正弦等等,其优点是什么?

模态激励的普遍问题

The main excitation technique used in modal testing today is burst random, used more often than others such as random, sine chirp and digital stepped sine. These are discussed with some brief comments. (Other techniques such as pseudo-random, periodic random, burst chirp and others are variations on these signals and are not discussed at length here.)

今天,用在模态试验中的主要激励技术是猝发随机,比其他诸如随机,正弦扫频和数字步进正弦更常用。讨论这些技术,给些简短的评论。(其他技术,例如伪随机,周期随机,猝发扫频还有其他,是这些信号的变种,在此不做详细讨论。)

Random excitation was one of the first excitation techniques used because it was simple to create. The problem with random excitation is that the signal is never periodic in the sample interval of the FFT measurement and requires a window (commonly a Hanning window) to mediate the effects of leakage. Unfortunately, even with a window applied, the frequency response measurement suffers from leakage especially at the resonant peaks of the measurement.

随机激励是首选激励技术之一,因为它很容易生成。随机激励的问题是,在FFT测量的采样时间段内信号永远不是周期的,需要一个窗函数(通常是汉宁窗)来减轻泄漏的影响。遗憾的是,即使加了窗函数,频响测量结果仍然受到泄漏之害,特别是在测量结果的共振峰上。

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图9 – 随机激励的典型测量步骤

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图10 – 输入,输出随机时域信号,以及FRF和相干

Sine chirp is an excellent technique for testing systems that are fairly linear. This signal is a very fast swept sine where the frequency is swept from low frequency to high frequency within the time of one sample of the FFT analyzer. As a result, the signal is periodic in the sample interval once steady state response is achieved. This signal does not require any windows and does not suffer from leakage.

对测试相当线性的系统,正弦扫频是一种优秀的技术。此信号是一种非常快速的扫频正弦信号,其中在FFT分析仪的一次样本时间内,频率从低频扫到高频。因此,只要达到稳态,信号在采样时间段内是周期的。这种信号不需要任何窗函数,不受泄漏影响。

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图11 – 正弦扫频激励的典型测量步骤

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图 12 – 输入和输出正弦扫频时域信号,以及FRF和相干

Burst random excitation was developed in the early 80s and has remained as one of the more commonly used excitation techniques for experimental modal testing. The vast majority of all shaker modal tests today employ burst random technique.

在1980年代早期开发出了猝发随机激励,并且作为试验模态测试最常用的激励技术之一,一直保留下来。今天,绝大多数激振器模态试验利用猝发随机技术。

Burst random is formed as follows: A random excitation is generated but is only applied for a portion of the data block. In this way, the excitation signal is totally observable within one sample interval of the FFT analyzer and there is no need for the use of windows since there is no leakage associated with the captured signal. (Note: Providing that the response measured on the structure is also totally observable within one sample interval of the FFT analyzer then there is no need for the use of windows since there is no leakage associated with the captured signal.) However, once the excitation is turned off, the structural response will die exponentially depending on the damping associated with the structure. If the response of the structure does not die out within one sample interval, then the burst should be shortened such that the response does end before the end of the sample interval. The burst can be controlled by specifying the percentage of the block over which the excitation is to be applied. Generally, this can be accomplished with most structures.

猝发随机按下述方式构成:生成一个随机激励,但只在一部分数据块时间内施加。用这种方法,在FFT分析仪的一次样本时间段内激励信号是完全观察得到的,无需加窗,因为采集的信号没有泄漏。(备注:在FFT分析仪的一次样本时间段内,只要结构上所测的响应也是完全观察得到的,那么就无需加窗,因为采集的信号没有泄漏。)但是,一旦关掉了激励,结构响应将按照指数方式衰减,取决于结构的阻尼。如果在一次采样时间段内结构响应没有衰减完毕,那么应该缩短猝发时间,这样在采样时间段的尾部响应确实结束了。通过规定块的百分比,在这个范围内施加激励,来控制猝发。一般地,对大多数的结构,可以达到这点。

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图13 – 猝发随机激励的典型测量步骤

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图14 – 输入和输出猝发随机时域信号,以及频响和相干