Frequency analysis

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Since machine vibrations typically consist of a superposition of many single oscillations, it is often necessary to analyze the amplitudes of individual frequencies. This makes it easy to identify and evaluate structural component vibrations.

In the time signal of such a vibration combination, it is difficult to detect individual vibration components. Therefore, a second representation method is used to determine the strength and frequencies of the vibration components, the frequency analysis. This visualization shows the amplitude of the measured oscillations instead of the time over the frequency. This type of analysis is called Fourier transformation or FFT in VibroMatrix.

Practical example on the machine model

In VibroMatrix, the InnoAnalyzer is used for frequency analysis.After opening the instrument, first activate the advanced settings. The operating modes available include FFT with automatic and manual settings.

FFT with automatic settings according to frequency axis

Let's start with the automatic mode first. The user selects the frequency range and the program then automatically calculates the optimum frequency resolution. Changes in the frequency range result in an adjustment of the FFT.

Let's do a practical frequency analysis. All we need to do is start the instrument. We select "velocity" as the measurement parameter. The machine model is also started and now speeds up to the operating speed.

We see lines moving to higher frequencies in the InnoAnalyzer. These are single component vibrations or multiples of them with increasing speed. When the operating speed is reached, the lines stop. We now select the range of 1-100Hz with the right mouse button. The FFT is newly set up and the data progress increases in the lower right-hand corner. At 50 Hz we see a strong line.

This is the operating speed of 3000 rpm. Instead of the display in Hz, you can also change to rpm. The unit of the frequency axis only needs to be changed in the "Display" tab

For basic frequency analysis, the automatic mode offers a quick way to determine frequencies.

FFT with manual settings

Much more possibilities are offered by the manual mode. In manual mode an additional tab "FFT" is inserted. This is where the analysis parameters are defined.

First, a brief explanation of the various setting options.

  1. The time window for 1 spectrum determines the frequency resolution. By means of long time windows, frequencies close to each other can be considered separately.
  2. The different weighting windows influence the amplitude and frequency accuracy. We choose the Hann window. This offers a good compromise between exact amplitude and frequency.
  3. The overlapping offers the possibility to display individual FFTs not only after each time window, but to get updated in every display cycle.
  4. The statistics function can be used to combine several FFTs.

Let us also carry out a practical measurement here.Instead of fine frequency lines we see small boxes in the spectrum. These represent the current frequency resolution of 0.73 Hz. To increase this resolution, only the time window needs to be extended, e. g. to 10s. The frequency curve builds up slowly, the data progress is displayed. The resolution of less than 0.1 Hz produces high resolved frequency lines.

A new FFT appears approximately every 1 second. Frequency analyses are continuously performed by switching on the maximum overlap. This makes it easier to investigate dynamic processes such as start-ups.

The individual frequency bands can be examined easily in the InnoAnalyzer. To do so, switch on the amplitude list. With the help of the marking tolerance, the lines to be displayed can be precisely defined. The list displays the results clearly arranged and can be loaded into the clipboard, for example to be inserted for documentation purposes in external applications.

Now we want to perform a wide-band FFT of 1Hz -10kHz. High-frequency vibrations are falsified due to magnetic coupling, therefore screw or adhesive connections should be used for sensor coupling in analyses above 5 kHz.

We enter the desired analysis range and reduce the time window, as no such high resolution is required. In addition, acceleration is used as a measurement parameter because the vibration velocity underestimates signals at high frequencies. After starting the instrument we see numerous lines up to 3000 Hz. These are excitations from a defective rolling bearing, an unbalance-induced speed component and broadband interference of the installed claw coupling.

In this way, it is possible to identify specific component vibrations or interference effects on the basis of knowledge of the machine speed and the mechanical structure, i. e. ratios, gears, etc..

With the InnoAnalyzer you have a tool with extensive possibilities at your hand.

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