Spectral Analysis

A time history record is converted into the frequency domain by use of a Fourier transform. The Fourier transform is a block data processing technique. The common Fourier analysis options Power and Auto Spectrums.

Spectral analysis techniques enable the frequency components of a sensor signal to be analysed separately if required. The analysis software can be used to look at Kurtosis values within signal bands as well as for values for the full bandwidth of the input signal.

Fourier techniques also enable vibration signals to be integrated very efficiently. For example an acceleration time history can be integrated to determine the relative displacement for the movement of a machine without loss of spectral detail that often occurs when data is processed similar time based techniques and reduces the amount of data to be analysed.

The Keynes Controls analysis software allows up to 512 point spectrums with signal enhancement features to be processed and displayed.

The data analysis options within the software enable the user to set the number of spectra to be plotted. The spectra are calculated based on processing the data from a 4000 series instruments. For the run up tests the vibration time histories are acquired using a synchronised input system.

Waterfall diagrams clearly show peaks of interest such as resonances as a machine speeds up and slows down in a very convenient manner. It becomes relatively easy to make out the operating characteristics of a machine.

Information From Periodic Waveforms

Condition monitoring often involves the interpretation of waveforms from measurements by transducers. Most waveforms from rotating machinery will be periodic or precisely approximately periodic in nature. Condition monitoring of machinery normally use a measuring device that produces a signal that can be processed to represent a physical variable. Many of these signals are periodic or approximately periodic, so a considerable amount of information can be gleamed from Fourier Analysis.

Crest Factor Measurements For Bearing Damage Analysis

When small defects occur on the elements or when races of rolling element bearings they issue high frequency vibration pulses which can be measured. Bearing damage can occur when defects are not repaired. Due to high frequency of the vibrations the effect of the vibrations on the RMS statistic is minimal as this parameter is affected greater with low frequency components than high one even though the peak levels will increase significantly.

The Peak/RMS ratio is called the crest factor and by measuring this ratio, rolling element bearing faults can be detected at an early stage and their development monitored. The Peak/RMS ratio is the maximum peak value divided by the RMS value of a specified block of data. The Keynes Controls software can calculate this ratio and plot trends .

Frequency Ranges

BS 4675 part 2 (ISO 2945) gives the standard requirements for vibration monitoring instruments covering a frequency range from 0 Hz to 1KHz. This adequately covers fundamental speeds and first harmonics from 600 rpm to 3000 rpm.

The standard 24 bit 2 KHz interface cards for the NetPod 4000 series instrument fullfill the correct specifications for these operating ranges. For higher harmonic analysis the 70 KHz interface card can be used for spectral analysis operations.

For high speed synchronised continuous operations such as those required for the analysis of turbo drives then the NDACS 5000 series instruments should be used. The 5000 series instruments offer analysis rates to 100KHz/channel distributed on a network.

The NDACS 5000 series instruments also features synchronised operations between channels for each instrument on that network essentially making one large synchronised data acquisition system, no matter how the instruments are deployed.

Hazardous & Underwater Environments

Instruments can be supplied for use in hazardous and underwater environments. The NDACS 5000 series instruments can be supplied within a titanium enclosure making them impervious to most chemically hazardous environments short of immersing the systems within chemical tanks and waterproof to a depth of > 1 Km enabling
seabed located machinery to be protected.

See Part Numbers: NDACS5002, NDACS5004, NDACS5008