OPTIMIZATION OF FAULT DIAGNOSIS IN HELICOPTER HEALTH AND USA(31)

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in
[21].
There
are
however
no
methods
in
the
literature
which
address
the correction of vibrations signals. This is of interest for long-durations ac-quisitions typical for rotors and epicyclic carriers. In these cases, acquiring a single .nite length signal takes several seconds. This leaves a considerable probability for the environmental context changing throughout the acquisi-tion period. Consequently, the raw signal must be piecewise corrected before any indicators are calculated.
Implementing this method for EuroARMS / MARMS will however require redesign of the airborne segment. This because the method must be applied before synchronous signal averaging. Any modi.cation to the airborne soft-ware is however highly expensive. Consequently, this solution should be kept in mind for the next evolution of the airborne segment.

3.6. M’ARMS AND EUROARMS
Epicyclic Monitoring
Due to high complexity and several parts monitored by a single accelerome-ter, fault detection in epicyclic gearbox stages are inherently di.cult. Apart from the problem of long distance between components and sensors, several components are captured in the same acquisition. This means that the error indicating signature from a faulty component will be buried in the normal state signatures from the healthy state components captured in the acqui-sition. A
method
is
already
proposed
in
[32]
[31],
which
deals
with
this
problem.
Like with contextual signal normalization, this method must be applied before synchronous signal averaging. Further, the method requires an in-dexer on each epicyclic stage, which is currently not available for gearboxes with multiple epicyclic stages. This need for modi.cation of both airborne software and hardware makes this an expensive solution which should be considered for the next generation airborne segment.
New Sensor Technologies
The existing chip detector technology is reliable, but provides warning at a very late stage. For fault types such as gear fretting, the chip detector will provide warning only after severe damage. Obviously, the purpose of condition monitoring is to uncover faults at a much earlier stage. Gear fret-ting is also notoriously di.cult to detect through vibration monitoring, be-cause it produces little low frequency vibration. Oil debris monitoring di.ers from classical chip detection by providing precise quantitative and qualita-tive analysis of oil debris. This technology might prove to be quite e.ective for monitoring of gearboxes, as fretting tends to cause substantial amounts of .ne grained metal debris in the lubrication.
A weak-spot for all HUM System in use today is detection of gear fretting and bearing corrosion. These failure modes typically create metal-to-metal contact, which is a generator of weak signals at high frequency. As the signals are quite weak compared to the low frequency vibration, in addition to being out of the sensitive spectrum of most accelerometers, they are easily lost. Further, the use of synchronous averaging on gears will e.ciently suppress any signal not correlated with the shaft rotation. This includes the metal-to-metal noise created by fretting.
　
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