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Acquisition and Interpolation
Figures (10) shows the initial planetary orientation for the OH-58C. The relative positions of accelerometers and the OPR sensor are also provided. For both systems, the output shaft is splined to the carrier so that one carrier cycle is the same as the output shaft cycle.
During testing, data is collected representing many car-rier cycles. Figure 11 shows an example of a raw data set for a given accelerometer
Fig. 11. Aquisition and Interpolation: Partitioning the Raw
Data
The output shaft’s tachometer provides the OPR pulse used to parse these signals into the individual carrier cycles. Since the .rst cycle and last cycles are incomplete, these two cycles are rejected.
Since data is needed from each carrier cycle, time syn-chronous averaging, which would average all cycles into one, is not performed prior to extraction but instead rele-gated to the point just before assembly. Because of this, non-synchronous dynamics associated with other subsys-tems are included in every extraction. Frequency .ltering can be used to get rid of unwanted signals known a-priori as shown in Figure 12 for the pinion’s .rst mesh frequency and its harmonics. The bene.t of using the .ltered signal is that a high correlation between waveforms is expected from the onset making it possible that faults become appar-ent earlier in the processing.
Equally Spaced Tooth Mesh Periods
For every carrier cycle, there are Nr planet/ring tooth mesh periods, (TMPs). Thus, partitioning the carrier cycles at intervals representing a single TMP, should produce the same number of points in each partition. However, due to
Fig. 9. Vibration Separation FlowChart
Fig. 12. Effect of Filtering Before Interpolation
slight variation in shaft speeds, this is not always the case in practice. Interpolation is used to ensure that all cycles have measurement points at the same angular position which is necessary when doing time-synchronous averaging and that all cycles contain the same number of points. Interpola-tion makes the assumption that the rotation speed is either constant in between carrier cycles or that factors causing a non-uniform rotation are repeated every cycle. The number of interpolation points, N, is chosen to satisfy two require-
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