日志
[翻译]4模态试验移动力锤还是移动加速度计有差别吗？Pete Avitabile著 westrongmc译
热度 20 ||
MODAL SPACE - IN OUR OWN LITTLE WORLD
模态空间–在我们自己的小世界中
Pete Avitabile著 KINGSCI INSTRUMENTS-KSI科尚仪器 组织 westrongmc译
Is there a difference between a roving hammer and roving accelerometer modal test?
Well ... it depends
Let's explain what the differences could be.
模态试验移动力锤还是移动加速度计有差别吗？
嗯… 那要看情况了
让我们解释会有什么差别？
Basically, there is no difference between a roving hammer and roving accelerometer modal test. This is true providing the same measurements are collected. Let me explain by discussing this seemingly simple but tricky fine point about a modal test.
从根本上讲，模态试验中移动力锤和移动加速度计之间没有什么差别。假若采集到了相同的测量结果，那么这个结论是正确的。让我通过讨论，来解释这个关于模态试验的看似简单、实则非常复杂的问题。
Back when we performed a modal test with a 2 channel analyzer, it was fairly straightforward to perform an impact test. Usually, the hammer roved around the structure with a stationary accelerometer. Typically, we impacted the structure at every point in the x, y, and z directions to obtain FRFs relative to the reference location of the stationary accelerometer. But when we started using multichannel analyzers to perform the same test, there are some slight differences that need to be addressed. Let's consider an impact test for the 9 points shown on the structure. Let's also assume that I have an impact hammer and a tri-axial accelerometer with a 4 channel FFT analyzer or acquisition system.
先前用一个2通道的信号分析仪进行模态试验时，进行锤击试验是相当简单的。通常加速度计固定不动，力锤在结构上逐点移动。一般情况下，我们在每个测点位置，沿着x、y、和z向对结构进行脉冲激励，以得到频响函数，频响函数是以固定不动的加速度计为参考点的。但是，当我们开始用多通道的信号分析仪进行相同的试验时，尚有些许差别需要注明。让我们考虑一个锤击试验，9个测点示于结构之上。并假定我有一把冲击力锤，一个三轴加速度计，和一套4通道的FFT分析仪或采集系统。
One way to run the test is to place the tri-axial accelerometer at a fixed location and impact, in one direction, at all 9 points. We would then obtain 27 FRFs for the structure. Another way to run the test is to impact at one point, in one direction, and have the tri-axial accelerometer rove to all 9 points. Again we would collect 27 FRFs. So in both cases, we measure 27 FRFs by impacting in only one direction.
一种试验的方法是，将三轴加速度计固定于一个确定的位置，然后在所有9个测点位置，沿某个方向进行锤击。那么对这个结构，我们可以得到27个频响函数。另一种试验方法是，在某一个测点位置，沿某一个方向进行锤击，然后将三轴加速度逐步移动到所有9个测点位置。我们再次得到27个频响函数。那么这两种情况下，我们仅仅是沿着某一个方向进行锤击，测得了27个频响函数。
But are the two tests the same? At first glance, you would think that both test setups should produce the same results. In order to confirm whether this is true or not, let's step through the measurement process and list out what measurements are actually being made for each test setup.
但是这两种试验一样吗？乍看之下，你或许认为这两种试验方案应该得到相同的结果。为了证实这是对的还是错误的，让我们来一步一步深入这个测量过程，并且对每种试验方案，列出实际上得到了什么样的测试结果。
Test Setup #1
试验方案#1
Let's say that we want to run a modal test shown in setup #1. In this test, the tri-axial accelerometer is stationary at point 9 and measures x, y, and z outputs. The input hammer force is applied in the z direction only and roves to each of the 9 points shown.
假设我们想要进行方案#1所示的模态试验。这个试验中，三轴加速度计固定于测点9位置不动，然后测量x、y、和z向的输出。输入力锤激振力仅是沿着z向施加，并且逐点移动到所显示的9个测点位置。
Now let's list each of the FRFs that will be collected from this test setup. When we impact point 1 in the z direction, the response is measured at 9x, 9y, and 9z. So the FRFs measured are 9x/1z, 9y/1z, 9z/1z for the first measurement made. Next we impact point 2 in the z direction and the response is measured at 9x, 9y, and 9z. This set of FRFs are 9x/2z, 9y/2z, 9z/2z. We can continue on here but I think you get the hang of it. But what did we actually measure? Let's arrange all of these measurements in the FRF matrix to see what we have.
现在，根据这个试验方案，让我们列出将要采集的每个频响函数。沿z向锤击测点1时，在9x、9y、和9z位置测量响应。则对于第一次测量，测得的频响函数为9x/1z、9y/1z、和9z/1z。接下来，沿z向锤击测点2，并在9x、9y、和9z位置测量响应。这组频响函数为9x/2z、9y/2z、和9z/2z。这里我们可以一直继续下去，但我认为你应该能找到窍门了。可是，我们实际上测得了什么呢？让我们对频响函数矩阵中的所有这些测量结果进行整理，看看得到了什么。
When we take a close look at the FRF matrix, we notice that we have measured only parts of three different rows of this matrix. So we only have three partial descriptions of the characteristic of the system. But in each of the partial descriptions, we can only see the characteristic information in the z direction. This would be fine if there was only motion in the z direction. But what if there was significant motion in the z direction when the structure is excited in the x direction? We have only measured response due to excitation in the z direction!
仔细观察这个频响函数矩阵时，我们注意到，仅仅测得了这个矩阵的三个不同行的部分元素。因此，只能得到系统特性的三个局部描述。另一方面，在每个局部描述中，我们仅能观察到沿z向的特征信息。如果只是在z向有运动，这没有问题。但是，当沿x向激励结构时，倘若沿z向有明显的运动，又会怎样呢？我们仅仅测得了沿z向激励引起的响应！
Test Setup #2
试验方案#2
Now let's say that we also want to run the modal test shown in setup #2. In this test, the hammer impacts only in the z direction at point 9. The tri-axial accelerometer roves to each of the 9 points shown for this test, measuring the x, y, and z directions.
现在，假设我们也要进行方案#2所示的模态试验。这个试验中，力锤仅是沿着z向在测点9位置进行锤击。三轴加速度计逐点移动到本试验所示的9个测点位置，测量x、y、和z向。
Let's list each of the FRFs that will be collected from this test setup. When we impact point 9 in the z direction, the response is measured at 1x, 1y, and 1z. So the FRFs measured are 1x/9z, 1y/9z, 1z/9z for the first measurement made. Next we move the accelerometer to point 2 and the response is measured at 2x, 2y, and 2z. This set of FRFs are 2x/9z, 2y/9z, 2z/9z. So what did we actually measure? Again, let's arrange all of these measurements in the FRF matrix to see what we have.
现在，根据这个试验方案，让我们列出将要采集的每个频响函数。沿z向锤击测点9时，在1x、1y、和1z位置测量响应。则对于第一次测量，测得的频响函数为1x/9z、1y/9z、和1z/9z。接下来，移动加速度计到测点2，并在2x、2y、和2z位置测量响应。这组频响函数为2x/9z、2y/9z、和2z/9z。那么，我们实际上测得了什么呢？让我们再次对频响函数矩阵中的所有这些测量结果进行整理，看看得到了什么。
Now we notice that we have measured one complete column of the FRF matrix. Now we can describe the response of the system in a more complete sense. We have now measured enough FRFs that we can describe the response of the system for all points. Of course, I'm assuming that the reference location at point 9 in the z direction is not the node of a mode!
现在我们注意到，已经测得了频响函数矩阵的完整的一列元素。那么我们可以在一个更为完备的意义上去描述系统响应。我们已经测量了足够的频响函数，这样可以描述所有测点的系统响应。当然，我假定了沿z向的测点9处的参考点位置不是某一阶模态的节点！
So what should I do?
那我应该怎么做呢？
So while it appeared on the surface that both tests were the same, there actually is a difference!!! So how could I change these test setups so that the same data is measured. Well, there are two ways. First, Setup #1 could be changed as follows. Instead of using a tri-axial accelerometer, we could use a single uniaxial accelerometer to acquire data at 9z, for instance. But the difference would be that the impact excitation needs to be applied in the x, y and z directions. Then the data collected would be a row of the FRF matrix with 9z as the reference. This is exactly the same data as collected in Setup#2 provided that reciprocity holds true.
所以尽管在这个平面上，两次试验好像是相同的，但实际上它们有差别！！！那么我该如何改变这些试验方案，以便测得相同的数据。嗯，有两种方法。首先，方案#1按如下方式进行改变。例如，不用三轴加速度计，相反我们用一个单轴向加速度计来采集9z位置的数据。但不同的是，需要沿x、y、和z向来施加锤击激励。这样，采集到的数据将是频响函数矩阵的一行，并以9z为参考点。假设互易性成立，则这个数据与方案#2中采集的数据完全相同。
The other way to make sure that the same data is collected is as follows. In Setup #1, the impact hammer needs to be used to excite the x, y, and z direction. So the roving hammer needs to impact in all three directions. In Setup #2, the stationary impact at point 9 would need to be used to excite the structure in all three directions. Both tests would then produce 3 complete rows or columns of the FRF matrix.
另一种保证采集到相同数据的方法如下。在方案#1中，需要用力锤来激励x、y、和z向。于是移动的力锤就需要沿着所有三个方向进行锤击。在方案#2中，锤击于测点9位置固定不动，沿着所有三个方向对结构进行激励。那么这两次试验会生成频响矩阵的3个完整的行或列元素。
Now you still may be a little confused by this. I know it's not easy to comprehend the first time you hear it. The best way to convince yourself is to write out all the FRF measurements that you intend to collect to assure that at least one complete row or one complete column of the FRF matrix is acquired.
这会儿你对此或许仍然有点摸不着头脑。你听到这个问题，第一次就能理解，我知道这不容易。让自己明白的最好方法是写出所有你打算采集的频响函数测量结果，要保证至少可以采集到频响矩阵的完整的一行或一列元素。
I hope this simple explanation helps to clear up your question. You need to carefully think about the measurements you are going to make. Remember what I always say: "Thinking is not optional!" If you have any more questions about modal analysis, just ask me.
我希望这个简单的解释有助于澄清你的问题。需要仔细思考你要进行的测量。记住我总说的话：“思考不是可有可无的，而是必须的！”。如果你有关于模态分析的其他任何问题，尽管问我好了。
