abc ABC-MEMS, User Manual, Page: 11 / 43

User’s Manual 10
Figure 2 Signal Processing
Figure 2 shows the signal processing flow for one axis of the accelerometer.
The raw acceleration signal is sampled at an adjustable rate. The sampling rate can go up to 4 kHz.
The signal is then optionally high-pass filtered to remove the DC and low-frequency components. This
is usually required to capture RMS vibration levels. The cutoff of the high-pass filter is adjustable.
Alternately, the acceleration signal is digitally integrated to yield a velocity signal.
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In the Raw-Signal branch the signal (acceleration or velocity) is recorded at the sampling
frequency.
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In the Signal Statistics branch Min-Max-Avg processing captures the min, max and average of
the acceleration or velocity signal over an adjustable recording (log) interval. For each new
interval the statistics are cleared, then the statistics are updated during the course of the
interval, then at the end of the interval the statistics are recorded, and a new interval is started.
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In the RMS-Level branch the signal is squared to calculate the instantaneous power. The
instantaneous power is low-pass-filtered with an adjustable time constant to produce an
average. A short time constant provides an average that is capable of tracking fast transients,
while a longer time constant provides a smoother and less noisy average. A typical fast time
constant is around 100ms. While a typical long time constant is around 1s. Finally the square-
root of the power is taken to present RMS vibration or velocity levels. The RMS vibration levels
can be displayed in linear or dB scale.
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In the Level-Statistics branch Min-Max-Avg processing captures the min, max and average
vibration levels over an adjustable recording interval. For each new interval the statistics are
cleared, then the statistics are updated during the course of the interval, then at the end of the
interval the statistics are recorded, and a new interval is started. The RMS vibration levels can
be displayed in linear or dB scale.
Irrespective of the setting, the instrument always observes 100% of the acceleration samples.
6.5.2 Upper Frequency Limit
Figure 3 shows the response of the accelerometer structure and its acquisition chain, along the X and
Y axes, at 4 kHz sampling rate.