5.为什么要不怕麻烦地用MIMO试验?难道就不能多做几次SISO试验吗?

模态激励的普遍问题

Multiple Input Multiple Output (MIMO) testing requires the use of multiple shakers to excite the structure with uncorrelated signals supplied to each of the shakers. This requires that all the shakers be simultaneously attached to the structures for the modal test, as well as separate independent uncorrelated output sources from the data acquisition system.

多输入多输出(MIMO)试验需要使用多个激振器来激励结构,给每个激振器施加以不相关的信号。这需要所有的激振器同时连接到结构上进行模态试验,同时需要从数据采集系统中分开独立的不相关的输出源。

While it seems possible to test the structure with one shaker and then move that shaker to all the different shaker locations, conducting the test in this manner typically results in an inconsistent set of frequency response functions. This can be due to a variety of reasons such as an inconsistent mass load distribution from roving transducers or environmental changes altering the structures mass and stiffness properties. When the different sets of data are combined, the resulting frequency responses are not as consistent as when all the data is collected simultaneously. The best measurement results have been achieved when all of the test data is acquired in a single snapshot, eliminating any issues related to time invariance or structure stationarity.

尽管看上去有可能用一个激振器测试结构,然后移动这个激振器到所有不同的位置,按这种方式进行试验通常得到不一致的一组频响函数。造成这个的原因有很多种,例如从逐点移动传感器造成的不一致的质量载荷分布或者环境变化改变了结构的质量和刚度特性。当不同组的数据合在一起时,得到的频响不像数据同时采集时的那样一致。所有的测试数据在“一次抓拍”中采集时,可以获得最好的测量结果,去掉了与时不变或结构稳态相关的任何问题。

To illustrate this point, reciprocal frequency response measurements were taken with a single shaker moved between two different locations using a SISO approach. Each measurement was taken twice, first using a random and then a burst random signal to illustrate the differences. The results are shown below. Notice that the random signal has more variance and suffers from leakage even though a Hanning window was used. Clearly there is a difference in the two measurements shown; these two measurements should be exactly the same.

为了说明这点,用SISO方法,利用单个激振器在两个不同的位置移动,进行互易的频响测量。每个测量结果采集两次,先用随机,后用猝发随机信号来说明差别。结果如图15所示。注意,即使加了汉宁窗,随机信号也有更大的误差,受泄漏之害。显然,在所示的两次测量结果中有差别;这两次测量结果应该完全相同。

This measurement was repeated with a MIMO approach. Again a random excitation with a Hanning window and a burst random excitation were used. The variance using the random excitation can still be seen in the measurement even using the MIMO approach. Notice that the burst random MIMO approach provides the best measurement overall with a good frequency response where reciprocity is observed in the measurement.

用MIMO方法重复测量。同样使用随机激励加汉宁窗,以及使用猝发随机激励。即使是采用MIMO方法,仍然能看出用随机激励的误差。注意,猝发随机MIMO方法提供了最优的整体测量结果,有良好的频响,在测量结果中可以观察到互易性。

ShakerFAQ19-01

图15 – SISO互易性FRF,随机激励(左侧) – 猝发随机(右侧)

ShakerFAQ22-01

图16 – MIMO互易性FRF,随机(左侧) – 猝发随机(右侧)