Understanding your vibrations

Taking a closer look at different analysis approaches

Back to the basics - what does 'understanding your vibrations' really mean?

Understanding your vibrations and being able to act upon them is, of course, crucial. So to start things off, you need to have some vibration recordings from a real-life example that has been recorded with a vibration sensor in a real-life example.

And the entire analysis starts off by looking at the time domain of vibration, i.e. the amplitude of the vibration plotted against a time axis, and analyse changes in the same domain through a few different parameters. The parameters that we want to take a look at are amplitude, peak-to-peak, and RMS.

The reason that we want to do this is to at first understand how powerful the vibration is and if it is a stable vibration over time or if the vibrations are a bit more random. But, let us first explains the terms a bit closer:

 

sinus vibration wave

 Image illustrating a sinus wave

  • Amplitude is a measure for the height of the wave but as a simple expression of amplitude doesn’t take into account the length of the wave, it does not give us any information on how much energy actually is stored in the vibration.
  • Peak-to-peak amplitude is a different way of measuring the change between the two peaks: the highest peak and the lowest peak in a wave. Like to amplitude, the peak-to-peak amplitude doesn’t provide us with any information around the amount of energy inside the vibration.
  • RMS (or Root mean square amplitude) on the other hand gives us a lot more information! As the RMS value takes into account the changes in the curve over time, it gives us information on how much energy is stored in the vibration (expressed in g’s or m/s^2) which in turn gives us information on how powerful the vibration is.

After having looked at the amplitude and the RMS we now know how powerful vibration is. But, in order to get a better picture of the situation, it is also necessary to understand how intense the vibration is. And this is something that we can find out by examining the frequency (expressed in hertz/Hz) of the vibration, i.e. how many times the object moves per second. To do this, we have to complete a spectrum analysis.

Different types of analysis to uncover the frequency of a vibration

There are 3 different analysis that we would recommend to understand your vibration a bit closer

“FFT” – An FFT analysis calculates the discrete Fourier transform of the signal. This provides information about the average spectral content in the signal. The resolution of the FFT is directly proportional to the length of the measurement and the sample rate used. 

FFT analysis of vibrations
FFT analysis indicating the peak acceleration vs the frequency

“PSD” – The PSD is based on the FFT algorithm but normalises the values to frequency bin width. Practically, this reduces the influence of sampling duration on the spectrum. If a random signal is sampled for 10 seconds, or 100 seconds, the spectral content will appear to be different if using FFT, although the signal is similar throughout the measurement. With the PSD however, they will be similar, despite the difference in signal length.

PSD analysis of vibration signal
Power Spectral Density from an industrial asset where vibrations are centered below 250 Hz

“Spectrogram” – A spectrogram is computed by calculating several FFTs, each for an individual segment of the time-domain signal. The segments together span the entire signal and may be overlapping. This gives insight into how the frequency content changes over time in the signal. It may be useful for e.g. tracking how the operating speed of a motor changes throughout the measurement

Spectrogram of vibrations
Spectrogram from sine sweep from 100 Hz to 6000 Hz

A real life example - analysing vibrations from a tram

As usual, nothing is better than actually showing how we record and analyse vibrations rather than only writing about it. So, let us take a look at a situation where we helped with analysing vibrations on trams in the city of Stockholm to understand if they were so severe that it could lead to physical damage for the staff working on the trams. 

Setup and preparation

To the measurement site, we brought the following in terms of equipment:

  • One ReLog S
  • Mounting magnets
  • Cleaning kit to ensure solid mounting of the ReLog

After discussing the situations and understanding your vibrations, it was decided that we should focus on vibrations on the beams that made up the bulk of the tram. The next step was to properly clean the beams where we intended to mount the ReLog to ensure that the quality of the measurements would be as good as possible. We choose magnets as we only wanted to look at fairly low vibrations (up to 200 Hz), and it made our job easier in terms of mounting the ReLog. 

Setting the sample rate and measuring

As we only wanted to look at vibrations up to 200 Hz we set the sample rate to 500 Hz to ensure the quality of the measurement would be good enough. After having mounted the ReLog, we started the measurement by powering on the ReLog and simply pressing the start button.

The entire measurement took about 80 minutes, which corresponded to the tram going from one end-stop to the other one. Below, you can see an image visualising the entire signal where our first piece of vital information is seen in the top left corner: the RMS value for the entire signal.

vibration signal acceleration

The RMS value for the entire signal was found to be 0.37 g’s. But, what we immediately can see is that the vibrations stop completely on a regular basis. This coincided with when the tram was standing still at the station and letting people on and off. And, the large spikes that can be seen corresponded well with the times when the tram accelerated and applied the brakes to either speed up or slow down. While running at a constant speed, the vibrations seemed to be slightly lower. 

Analysis - understanding your vibrations

The next step for us was to take a closer look at the vibrations and perform an FFT as well as a PSD analysis to truly understand which vibration levels that could be found on frequencies between 0 – 100 Hz. These levels predominately are the vibrations that the staff would experience as irritating and potentially dangerous which takes us a lot closer to understanding your vibrations and their impact. 

The analysis was performed in VibInspect and handed over to the client for closer inspection. 

What type of data logger is required to get high quality analysis?

The choice of vibration data logger has a big part in getting high-quality data. For us, we mainly look at three things when using or recommending a data logger:

  • Accelerometer specifications, and more specifically the following specs:
    • Resolution in g (the lower the more precise data)
    • Bandwidth (the higher it is the better possibility of measuring high-frequency vibrations)
    • Sample rate (should be customisable so you can adapt per the necessary measurements)
  • Mounting possibilities, to ensure that the data logger can be properly mounted to guarantee the correct transfer of the vibrations to the data logger
  • Battery size & memory size, to ensure that measurements can take place for a long time without risking that the data logger runs out of memory or battery.

If you are interested in a more detailed analysis of which data logger that could be a good choice for your purposes, please have a look at our article “Which is the best vibration data logger?