The analog output of an accelerometer, mounted on a machine, includes normal vibration signals and stress wave energy over the entire response bandwidth of the sensor system. The normal vibration portion of the signal consists of lower frequencies and the stress wave portion consists of high frequencies.
For normal vibration measurements, the normal component is separated from the stress wave activity by routing the analog signal through a high order, low-pass filter followed by the conversion to the digital domain. The sampling rate is 2.56 x Fmax.
For PeakVue measurements, the stress wave component of the signal is sepa-rated form the normal vibration by routing the signal through a high order high-pass analog filter. Prior to routine digitization of the resultant signal for further analysis, the high frequency signal is further processed.
The important parameters to capture from stress wave activity are:
• Amplitude of each event
• Approximate time required for the detected event to occur
• Rate (periodic or non-periodic) at which events are occurring with emphasis on event rate versus specific fault frequencies which are dependent on both the specific component and on machine rotational speed.
The method developed by CSI that captures peak values of the analog signal from the sensor post-passing through the high-pass filter, called PeakVue, pro-vides the three key parameters specified above. The appropriate time resolution is accomplished by the selection of the maximum frequency, Fmax, to obtain adequate resolutions of possible fault frequencies, e.g., an Fmax of 3 or 4 times the inner race fault frequency when monitoring bearings. Once the Fmax is spec-ified, peak values will be collected at a rate of 2.56*Fmax. The inverse of the sampling rate defines the time increment over which the peak value is captured.
PeakVue Processing
These peak values are captured sequentially until the total desired block length is accumulated. The total time in the PeakVue waveform depends on the number of shaft revolutions desired by the analyst and the block of data consists of sequential constant time intervals of peak values (the PeakVue spectrum is computed from the time block data by an FFT algorithm as are vibration spectra). For bearing fault analysis, the block time should be sufficient to pro-vide adequate resolution on the lowest fault frequency (cage fault). This sug-gests a minimum of 15 revs (preferably 20) be included in the captured peak value data block.
Dynamic Range
Dynamic range is defined as the ability of the analyzer to distinguish between the highest and lowest amplitude signals. It is controlled by the Analog to Dig-ital (A/D) processor.
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The 2120 has a greater than 90 dB dynamic range. If two vibration frequencies have amplitudes greater than 90 dB apart, the lower amplitude signal will not be visible in the spectrum. It will be "lost in the noise". Put another way, the lower amplitude signal will be lower than the noise floor of the analyzer.
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Low amplitude stress wave energy is particularly difficult to resolve when the signal is dominated by unbalance, misalignment and other low frequency vibra-tions. Filtering out the non stress wave energy assures stress wave signals are measured with good signal to noise ratio.
PeakVue Processing
Auto-ranging
The AUTORANGE function of the 2120 analyzer selects a signal input range based on the incoming voltage signal. The Autorange feature optimizes the dynamic range of the 2120 analyzer. The autorange function is typically always enabled when measuring periodic signals. When the Enter button is pressed on the 2120 analyzer, AUTO-RANGING is the first thing seen on the screen.
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The F.S. Range function can be disabled on the 2120 analyzer itself if acquiring data in the ANALYZE or OFFROUTE modes or through a route point config-uration defined in MasterTrend or RBMware. A F.S. Range value of ZERO (0) instructs the analyzer to autorange. Any number, other than zero, in the F.S.
range field forces the analyzer's input buffer to be fixed to a specific vibration level. The number entered into the F.S. range is always in waveform units.
PeakVue Filter Types
PeakVue uses of two types of filters: Band Pass and High Pass.
The purpose of filtering the signal is to remove non-stress wave energy that typ-ically constitutes much of the signal's amplitude. By removing the non stress wave signals, the 2120's entire 90 db of dynamic range is focused on resolving the stress wave energy.
Band-Pass Filter
The bandpass filter removes all data above and below the filter corner values.
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High-Pass Filter
The high-pass filter removes low-frequency vibrations. All data below the filter value are removed from the signal. Selection of the high pass filter frequency filter is the most important consideration when using PeakVue. The goal of the filtering process is to remove the rotational vibration frequencies such as turning speed harmonics, bearing frequencies, multiples of gear mesh fre-quency, etc. The high pass filter should be selected to remove these rotational frequencies. Select a filter above the highest operational or defect frequency present in the signal. Generally, the 1000-Hz high pass filter is a good choice.
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PeakVue Processing
Rectified Signal
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Only the top half of the waveform is shown in the final PeakVue waveform.