3.2 Data Validation and Correction
It is of course possible to test a mechanical assembly in a test-rig using a static environmental context, i.e. constant torque, constant rotation speed, constant temperature, and so on. A helicopter must however sustain sub-stantial variations in operating conditions. The vibrations signature of all components is to some extent sensitive to variations in environmental con-text. Consequently, such variations must be compensated for before data is passed on to the classi.er.
3.2. DATA VALIDATION AND CORRECTION
Obviously, any change in rotating speed for a mechanical assembly will change its vibratory signature. Even though the rotating speed of a helicopter drive-train is relatively constant, any variations which might occur must be compensated for. Further, the vibratory signature for some components is also susceptible to other contextual factors, such as torque. It is indeed of interest to compensate for such factors as well, so that the information passed on to the classi.er is as consistent as possible.
3.2.1 Correction of Speed Variations
The vibration signature of a component is a function of its rotating speed. A gear will generate a tone, known as the meshing tone, at the frequency corre-sponding to the tooth pass frequency. The frequency of this tone, measured in Hertz, is obviously dependent on rotating speed. To uncouple rotating speed and vibration signature, the signal is re-sampled using synchronous sampling. Synchronous sampling means that the sampling interval is synchronous with the shaft rotation rather than time. Consequently, the resulting output has a .xed number of samples per shaft rotation rather than per second.
Synchronous
averaging
[43]
refers
to
the
process
of
recording
a
given
num-
ber of rotations of a component, re-sample the signal to synchronize it with the shaft rotating speed, and adding together each segment representing one complete rotation. This will amplify any signal being periodic with the shaft rotating speed, and attenuate everything else. Synchronous averaging is a convenient tool for removing background noise. This is especially e.ective for gearboxes, where a single accelerometer will capture the vibration signa-tures of several components. By creating a re-sampled and averaged signal for each component, each resulting signal contains the vibration signature from only a single component. A few cases do however exist, where a signal captures the signals from several components. These are the cases where two similar components, like two gears or two bearings, are located in close proximity rotate at the same speed. In these cases, only the applicable selec-tion of vibration features can separate the characteristics of each component. For some components, like the epicyclical planet gears, also the same vibra-tion features are applicable for each component in the acquisition. Thus, no unambiguous error localization can be made.
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