Hi,

I heard sombody talk about a cepstrum.
Can anyone explain what a cepstrum is? Is somebody using that to analyse vibration measurements?

It´s a way to treat signal that at first I saw from B&K. Basically you make a new FFT and you use a normal FFT as input. What to put on the axis and what to call it is a problem so some new names was invented like cepstrum being spectrum turned around etc. In practice it may be a nice tool for gearbox analysis. If you take away one data point in the cepstrum and reverse the calc back to the spectrum a complete family of frequencies goes away so you can find out what peaks are connected. Today it´s rare so far to find this tool in software. Olov

Thanks OLI,

Your explanation helpt me a lot to understand cepstrum.
You say it's rare so why does nobody use it anymore. As I understand is it a powerfull tool to analyse gearboxes.

Greetings from Tim

Tim,

It´s not that often anybody pay me as a consultant to do deeper investigations in gearboxes either. Normally the decisions are kept as simple as with bearings, is it good fine, is it bad fix it. The interest in looking at what is bad does not exist. Example, gearbox for feedpump. high gearmesh detected after 15-20 years of operation, only gearmesh, no multiples, sidebands or anomaly otherwise but gearmesh is relatively high 5-8mm/s. They have run it at 30-40% overload, all the time and it´s still happilly running all hours of the operating season here. It has consumed 1 coupling set and is working on the next as levels are higher axially towards the pump. So if you run a thing way outside specs and it still have acceptable maint cost, why bother? I think this may change with modern design of gearboxes.... Olov

Another way to look at cepstrum is that it gives you peaks at the fundamental frequency of a harmonic string. For example, if you have strong tones at 1x, 2x, 3x, 4x, 5x, etc, the cepstrum will show a peak at 1x.

DLI Engineering uses cepstrum to look for bearing defects. Suppose a defect frequency generates a string of harmonics at 4.2 x rotation. This string of harmonics may not be easily recognizable in the spectrum, but cepstrum analysis will indicate a peak at 4.2x. If you look at the cepstrum and find non-integer peaks, it's a strong indication of rolling element bearing defects.

Jon
Spintelligent Labs

Oli,

You've got a point.

thanks.

Jon,

What software do you use to convert a spectrum into a cepstrum?
I use the Odessey software but I think a cannot perform such a convertion.

Greetings from Tim

Tim,

Cepstrum is a feature of DLI's Expert ALERT software. I don't believe there is any option to actually show the cepstrum signature. It is used as an input to the expert system, where non-integer peaks are used as a flag to indicate bearing deterioration.

Jon

From my B&K years i recall this technique referred to as: " ..gisnal alanysis of torating chamines" i.e. everything turned around. Spectrum->cepstrum, frequency->quefrency, harmonics->rahmonics. It was sometimes useful to make a messy looking spectrum with lots of harmonic families more readable. IMO enveloping/demodulation can be used for the same.

Yeah; that's the way we always refered to it - a spectrum turned inside out. I'm wanting to say B&K put it on thier box in 82 and Hilbert Transform in 83.

Tim,

The ceptrum is a sort of "spectrum of a spectrum"Hance, the name CEPStrum from SPECtrum. However, this is not really its distrtinctive feature. The distrtinctive feature of the spectrum is the logarithmic conversion of the spectrum. You can find a good book of Ceptrum. Neue Ergebnisse der akustischen Überwachung von Betriebsdampfturbinen" juni 1979.And from Lyon, R.H. and Ordubadi, A"Use of Ceptra in Acoustical Signal Analysis
Rgs Hans

From the Vibration Institute:
The cepstrum is the forward Fourier transform of a spectrum. It is thus the spectrum of a spectrum, and has certain properties that make it useful in many types of signal analysis. One of its more powerful attributes is the fact that any periodicities, or repeated patterns, in a spectrum will be sensed as one or two specific components in the cepstrum. If a spectrum contains several sets of sidebands or harmonic series, they can be confusing because of overlap. But in the cepstrum, they will be separated in a way similar to the way the spectrum separates repetitive time patterns in the waveform. Gearboxes and rolling element bearing vibrations lend themselves especially well to cepstrum analysis. The cepstrum is closely related to the auto correlation function.

Hans EPP,

Thanks fot the titles. I'll look into it.
Greetz

Dave G.,

Thank You for the explanation.
I'm now a little smarter. Every day I learn a little more.

Thanks

http://www.dliengineering.com/section.asp?nID=6

You can find 2 short papers on Cepstrum analysis by following the link above. You can now display a Cepstrum graphically in DLI's ExpertALERT 3.0 software.

For high frequency events systems (roller element bearings and gears)each company try to have its own point of view and to find the way how to analize, for example IRD Mechanalysis has the Gs, CSI has the peakvue and Bruel&Kjaer has the cepstrum wich simplest is defined like spectrum of the spectrum.

Out of the vibrations world, mathematics new events can be builted from one original and real, each one of they has is own wave form at its charasteristic frequency.

Cepstrum is no new way how to analize high frequency event no broadly used, I dont know why. Can any body explain advantages of Gs, Peakvue and Cepstrun?

Hi Tim and my collegues,

From my principal, they're will be explain you about cepstrum.

Download attachment for more detail explaination.

Let me know by my private email, if you have any questions.

Regards,
Sketska Naratama
sketska at klaras dot co dot id

8-3_-_Cepstrum_-_V2.pdf (118 Kb, 47 downloads) Cepstrum from 01dB-Metravib Articles

Alan,

The link, I look in to it.

Greez Tim

quote:

Originally posted by Alan F:
http://www.dliengineering.com/section.asp?nID=6

You can find 2 short papers on Cepstrum analysis by following the link above. You can now display a Cepstrum graphically in DLI's ExpertALERT 3.0 software.

Miguel,

It's comparable with the HFd measurement of SKF.
Isn't it?

Greetings from Tim

quote:

Originally posted by Miguel Kovac:
For high frequency events systems (roller element bearings and gears)each company try to have its own point of view and to find the way how to analize, for example IRD Mechanalysis has the Gs, CSI has the peakvue and Bruel&Kjaer has the cepstrum wich simplest is defined like spectrum of the spectrum.

Out of the vibrations world, mathematics new events can be builted from one original and real, each one of they has is own wave form at its charasteristic frequency.

Cepstrum is no new way how to analize high frequency event no broadly used, I dont know why. Can any body explain advantages of Gs, Peakvue and Cepstrun?

Nrtama,

Thank you for the Pdf file.
I will ake e look.

Tim

quote:

Originally posted by Naratama:
Hi Tim and my collegues,

From my principal, they're will be explain you about cepstrum.

Download attachment for more detail explaination.

Let me know by my private email, if you have any questions.

Regards,
Sketska Naratama
sketska at klaras dot co dot id