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5
Primary device functions (continued)
Vibration Acceleration Statistics
The BCM can calculate three statistical values:
– Crest factor
– Kurtosis
– Skewness
Crest factor
The
Crest factor
describes the ratio of the peak value of
the vibration to its RMS value. It is unitless and always
lager than 1. It is used for bearing fault detection, because
worn out bearings create very short and high peaks in the
vibration signal that contain little energy, resulting in a high
Peak-to-RMS-ratio that is represented here as
Crest
factor
.
The
Crest factor
is calculated axis depending or with the
overall vibration acceleration as
Crest factor
magnitude.
Kurtosis
Kurtosis is a statistical parameter that describes the
“peakedness” of a vibration signal. The BCM calculates
non-excess kurtosis (Pearson's kurtosis). It is calculated by
dividing the fourth statistical moment by the square of the
second statistical moment. In vibration analysis, it is used
to see how many peaks in the time domain signal occur
compared to the Gaussian distribution. A high
Kurtosis
value can be caused by fatigued components or potential
damage in the machine.
Kurtosis
of the acceleration signal is calculated depending
on the axis.
Skewness
Skewness is a statistical value and the ratio of the average
deviation from the mean vibration cubed, divided by the
standard deviation of the vibration cubed. It describes an
asymmetrical shift of the Gaussian distribution to the left or
to the right. For vibration signals, a non-Gaussian
distribution can come from the bearings’ defect region or
uneven load distribution, that causes the time domain
signal to have peaks with positive or negative trend.
Skewness of the acceleration signal is calculated
depending on the axis.
Vibration Velocity
The internal MEMS measures acceleration and is able to
calculate the velocity. The bandwidth of the velocity signal
ranges from 2 Hz to 6000 Hz. All occurring acceleration
signals within this range are used to calculate the velocity
signal that is forwarded to the sensors output. The BCM
uses a user-selectable velocity time window (for possible
values see Tab. 5-9) for the calculation of the output data.
Vibration Velocity Configuration
The relevant parameters can be selected in the vibration
acceleration configuration menu (ISDU 0x2160). The time
window is selectable from predefined values (see Tab. 5-9).
Since there is a strong dependency between the time
window and the lower bandwidth limit of the detectable
signal, the minimum values for the lower bandwidth limit
for each time window are shown in Tab. 5-10. The lower
and upper bandwidth limit can be chosen in whole
numbers between 2 Hz and 6000 Hz. The upper value
must always be higher than the lower value.
Value
Used time window length
0
20 ms
1
100 ms
2
250 ms
3
500 ms
4
1000 ms
Tab. 5-9: Vibration velocity time window
a-Time Window
Minimum a-Lower Bandwidth
Limit
20 ms
100 Hz
100 ms
20 Hz
250 ms
8 Hz
500 ms
4 Hz
1000 ms
2 Hz
Tab. 5-10: Vibration velocity time window vs. lower bandwidth limit
BCM R16E-004-CI0 _ - _ _ , _-S4
Condition Monitoring Sensor