Synchronous Averaging

Synchronous averaging, also sometimes redundantly called “Time Synchronous averaging”, was discussed earlier as a method of background noise reduction in spectra of complex signals. Now, we will look at it as a means of greatly increasing the information we can extract from the time-domain vibration waveform.

Synchronous averaging is a fundamentally different process than the usual spectrum averaging that is generally done in FFT analysis. It is used to greatly reduce the effects of unwanted noise in the measurement. The waveform itself is averaged in a time buffer before the FFT is calculated, and the sampling of the signal is initiated by a trigger pulse input to the analyzer. If the trigger pulse is synchronized with the repetition rate of the signal in question, the averaging process will gradually eliminate the random noise because it is not synchronized with the trigger. However, the signal that is synchronous with the trigger will be emphasized, as shown below:

When you do time domain averaging on the vibration signal from a real machine, the averaged time record gradually accumulates those portions of the signal that are synchronized with the trigger, and other parts of the signal, such as noise and any other components such as other rotating parts of the machine, etc., are effectively averaged out. This is the only type of averaging that actually does reduce noise.

An important application of time synchronous averaging is in the waveform analysis of machine vibration, especially in the case of gear drives. In this case, the trigger is derived from a tachometer that provides one pulse per revolution of a gear in a machine. This way, the time samples are synchronized in that they all begin at the same exact point in the angular position of the gear.

Consider a gearbox containing a pinion with 13 teeth and a driven gear with 31 teeth. If a tachometer is connected to the pinion shaft, and its output is used to trigger an analyzer capable of time synchronous averaging, the averaged waveform will gradually exclude vibration components from everything except the events related to the pinion revolution. Any vibration caused by the driven gear will be averaged out, and the resulting waveform will show the vibration caused by each individual tooth on the pinion.

Note that in the figure above, the lower averaged waveform indicates one damaged tooth on the pinion.

Azima DLI provides products and services for Predictive Maintenance including vibration analysis instruments, monitoring and diagnostic software, and consulting for CBM programs.