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04-11-2017 12:35 PM

I'm attempting to calculate accurate displacements from measured accelerometer data. I've tried using both the 'Second Overall Level' converted to diplacement in Online Processing, and using Throughput Processing. I've also tried to use the double integration option in the Time Signal Calculator, feeding my Throughput Accel time signals back into the calculator.

I'm getting wildly high numbers for displacement either way, especially while using the Calculator, and the numbers aren't even close to matching.

Is there some trick to double-integration that I'm missing? Perhaps this isn't feasible while using ICP accels? Has anyone used this feature in Test.Lab with any degree of accuracy/confidence in the data?

Thanks.

Shelly

Solved! Go to Solution.

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04-11-2017 02:34 PM

Hello Shelly.

In order to integrate time histories correctly a little bit of care is needed. Any drift in the original signal can cause integration problems so we use a DETREND_AC command to remove this drift and any low frequency components. Because some integration methods introduce high frequency components (a sawtooth effect on the result) we normally suggest upsampling before the integration and then downsampling later. We then use a HP Filter to remove the constants of integration. So in order to get meaningful results we suggest DETREND_AC, RESAMPLING up by a factor of 4, INTEGRATE or DOUBLEINTEGRATE, RESAMPLING down by a factor of 4, HP_FILTER.

An FAQ is attached. It is also available on the Global Technical Assistance Center (GTAC) website under the solution center.

With respect to the Overall Level question, the Overall level includes frequencies from 0 Hz to Bandwidth. Those low frequencies show up as really large numbers because their integration in the frequency domain is 1 / wj and when frequency is close to 0, that integration is infinity. There are two workarounds I can suggest:

- On the waterfall calculation, there is an option "First bins to clear". If you set this to 2, it will clear two bins setting 0 Hz and the first spectral line to 0 amplitude. That will then improve the integration of the Overall Level. In Time Data Processing that option is under Channel processing for the Acoustic, Vibration and Other channel groups.
- Rather than calculating Overall Level (which includes 0 Hz), calculate a Frequency section starting at 1 or 2 Hz and ending at your Bandwidth Frequency. You can then integrate that. As an example with a Bandwidth of 6400 Hz, I would calculate a frequency section from 1 Hz to 6400 Hz as shown here.

Re: Online (or Signature Throughput) Processing - Second Overall Level - Vibration

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04-11-2017 03:00 PM

Excellent! Thank you for the quick response, and detailed explanations.

I will run my data through using both methods (Time Data Calculator with extra signal conditioning, and Overall Level via a Frequency Section in the Time Data Processing), and report back once I have a chance to analyze the data.

Re: Online (or Signature Throughput) Processing - Second Overall Level - Vibration

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04-11-2017 05:31 PM

I haven't tried the Frequency Section method yet.

Thanks again!!

Re: Online (or Signature Throughput) Processing - Second Overall Level - Vibration

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01-25-2019 11:33 AM

Hi Kevin,

Thanks for this explanation.

If we wanted to use a DC accel (say attached to the nose of the vehicle to calculate the vertical distance travelled due to pitch) and integrate that, what procedure can we follow to get us a good displacement answer? In this case, I believe that we are basically interested in zero Hz and low frequency part of the accel.

We can generate this data in a CAE tool and can try out the procedure in LMS if there is a recommendation on how this can be done.

Thanks, much appreciated.

regards,

Makarand

Re: Online (or Signature Throughput) Processing - Second Overall Level - Vibration

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01-25-2019 12:08 PM

Hello MakD,

If you use the Search Documentation tool in Testlab rev 17 or rev 18 or look on the GTAC Support site in the solution center for "Integrate" you should find FAQ which is titled "How to 'correctly' integrate time data within Time Domain Integration? " I believe it is also attached further up on this forum article.

Unfortunately integrating at 0 Hz will be very difficult due to the effects of the integration in the time domain. We normally apply a HP filter after the integration to remove those effects.

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