3.2.2 General Contextual Correction
Although the vibration signature from all rotating components is sensitive to rotating speed, some vibration signatures are also sensitive to other fac-tors. Helicopters in normal use experiences a large variation in contextual parameters, such as altitude, speed, oil temperature, torque, etc. Torque is a well known in.uence especially on gears.
Because the environmental context is random in time, variations in envi-ronmental context are manifested as random variations on the recorded vi-bration signals, and consequently the vibrations features. Most commercial HUMS amend this problem by using a contextual window in where acqui-sition is allowed. This involves setting maximum and minimum thresholds for key parameters, such as speed and torque. A drawback of this method is that the contextual variation within the window can be substantial. Reduc-ing windows size might reduce random variation, but risk reducing the data volume collected.
A supplementary method is by using a model representing the in.uence of contextual variations on the di.erent vibration features. Once models are estimated for each feature, they can be used to cancel the e.ect of contextual variations. This method has been successfully deployed using engine torque as
the
only
environmental
context
[21].
3.2.3 Epicyclic Frequency Separation
Frequency separation is a pre-processing technique particular to epicyclic planet gears and bearings. An accelerometer monitoring an epicyclic gear stage must, for practical reasons, be placed outside the gearbox housing. This means that the accelerometer will pick up the vibration signatures of the ring gear, the sun gear and bearing, as well as all planet gears and bearings. The ring, sun and planet vibration signatures can easily be separated using synchronous averaging, as these components rotate at di.erent speeds. This method will however not separate the di.erent planet signatures, as all planet gears and bearing are rotating at the same speed. Consequently, it is not possible to pinpoint any detected planet fault to a speci.c planet gear or bearing. Further, the error-indicating features from one faulty gear or bearing will get buried in the normal state vibration signatures from the other planets, making fault detection di.cult.
A
method
known
as
frequency
separation
[32]
[31]
was
developed
to
amend this problem. Frequency separation method requires an indexer to be placed on the planet carrier, so that it is possible to know when each planet passes the accelerometers. The recorded signal is then split up into equal
