Document reference MAMPS-HW/E HARDWARE MANUAL
Edition 18 - March 2022 VM600 machinery protection system (MPS)
7-7
Shaft absolute vibration
PROCESSING MODES AND APPLICATIONS
The processing outputs one of two values per channel: either rectified displacement or
rectified velocity.
RMS, Mean, True Peak or True Peak-Peak rectification are possible on the AC displacement
or velocity value. The averaging time T can be configured using the VM600 MPSx software.
7.5 Shaft absolute vibration
(1) Description
Absolute vibration is generally measured with seismic transducers such as accelerometers
or velocity sensors mounted on a machine bearing housing or other mechanical structure.
However, this technique cannot be used to measure vibrations on a rotating shaft, so shaft
absolute vibration is measured as follows:
a. Broad-band absolute bearing vibration (BBAB) is measured using a seismic
transducer mounted on the machine bearing (see Figure 7-4). The measurement
chain can use an accelerometer to output a voltage-based signal proportional to the
absolute acceleration of the bearing vibration. Alternatively, it can use a velocity
sensor to output a voltage-based signal proportional to the velocity of the bearing
vibration.
b. The Broad-band absolute bearing vibration (BBAB) is processed by the MPS to
provide a signal proportional to the absolute displacement of the bearing, after
appropriate digital integration of the measurement signal.
To support this processing, one BBAB output value must be in displacement units
(µm, mm or mils). That is, if the BBAB signal is measured in terms of acceleration,
the MPS must perform two digital integration operations on the BBAB signal to
convert the acceleration into a displacement. If the BBAB signal is measured in
terms of velocity, the MPS must perform one digital integration to convert the signal
into a displacement.
c. Relative shaft vibration (RS) is measured using a non-contact proximity transducer
mounted on the machine bearing (see Figure 7-4). The measurement chain outputs
a voltage-based signal proportional to the distance between the transducer tip and
the rotating shaft. This corresponds to the relative displacement between the shaft
and the bearing.
To support this processing, the RS output value must be in displacement units (µm,
mm or mils).
d. The absolute shaft displacement from step (b) and relative shaft displacement from
step (c) are summed. The resulting signal is proportional to the shaft absolute
vibration.
As shown in Figure 7-5, the following front-end components are commonly used for
measuring Broad-band absolute bearing vibration (BBAB):
• CAxxx accelerometer + IPCxxx signal conditioner + GSIxxx galvanic separation unit.
• CExxx accelerometer.
• CVxxx or VExxx velocity sensor.
As shown in Figure 7-5, the following front-end components are most commonly used for
measuring Relative shaft vibration (RS):
• TQxxx proximity transducer + IQSxxx signal conditioner.
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