STA – Synchronous Time Averaging – How to isolate one machinery from the workshop vibration?

Time-to-frequency transformation using FFT buries the spectral result with a large standard deviation which can be reduced by averaging. This averaging can be done at four different steps of the FFT/SOA process:

• The most common, Frequency/Order-domain Averaging (FA, OA), averages the amplitude spectra. The entire amplitude frequency distribution is displayed. The average count is 3 to 5 to remove the standard deviation.

• Synchronous Time-domain Averaging (STA) starts time domain block acquisition synchronously with the unit-under-test cycles. Typically, this is one pulse/rev for rotating machinery or an impact threshold for bump-tests. Since asynchronous waves are out of phase, repetitive averaging cancels out their positive and negative amplitudes. The synchronous waves (same phases) are maintained and smoothed to provide a clean, unique time block. As a result, FFT provides a standard deviation-free, complex spectrum/FRF of the unit-under-test signature. The average count is 30 to 50 times.

• The Synchronous Angular-domain Averaging (SAA) method is derived from STA. It applies in the angular domain using Synchronous Order Analysis (SOA) resampling for variable-speed machinery. It provides order spectra/profiles with better elimination of unwanted signals. STA and SAA benefit from the high-accuracy Synch inputs to synchronize the samples down to 1/64th of the (re-)sampling.

• Frequency Domain Synchronous Averaging (FDSA) uses the phase of one frequency from the measurement to synchronize. The averaging applies in the complex frequency domain with a phase shift. FDSA features synchronizing from a vibration or noise instead of the time domain edge.

Synchronous averaging is the best solution to clean and isolate the targeted measurements with various applications:

• Health check of rotating machinery in a workshop: using STA to isolate the vibration of one steady-state machine eliminates the unwanted vibrations from the other units in the workshop.

• Extraction of the background spectrum on operating machinery with a bump test: using STA synchronized with the impact edge of a bump test provides structural dynamics spectra without the operating vibrations of the machinery.

• In-vehicle sub-systems: the noise and vibration signature of cars, trains, ships and aircraft. These units operate at variable speed; the SAA method manages these variations to prevent them affecting results and to reject all asynchronous signals from the unit-under-test environments.

• • High-accuracy order tracking for turbine (engine, gas, aero) balancing: SAA is perfect for improving order analysis stability with jitter-free order extraction thanks to the accuracy of the oversampled triggers.

• Industrial fan diagnostics, without access to the fan cycles: FDSA can be used on the fan first order with a simple microphone to isolate the vibrations from the fan.

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