VM600 MPS hardware manual (standard version) MAMPS-HW/E 7 - 7
Edition 17 - February 2018
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 MPS software.
7.5 Shaft absolute vibration
(1) Description
Absolute vibration is generally measured with seismic transducers (for example,
accelerometers, velocity transducers) mounted on a machine bearing housing or other
mechanical structure. Clearly, this technique cannot be used to measure vibrations on a
rotating shaft. Instead, the measurement is made as follows:
a. The relative shaft vibration (RS) is measured using a non-contacting proximity
transducer mounted on the machine bearing (see Figure 7-4). The transducer and
its signal conditioner output 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.
b. The absolute bearing vibration (AB) is measured using a seismic transducer
mounted on the machine bearing. This will normally be an accelerometer, which
provides a voltage-based signal proportional to the absolute acceleration of the
bearing.
c. The absolute bearing vibration (AB) is processed by the MPS to provide a signal
proportional to the absolute displacement of the bearing.
The RS and AB values must be expressed in the same measuring units. For exam-
ple, if the AB signal is expressed in terms of acceleration, the MPS must perform two
integration operations on the AB signal to convert the relative acceleration into a rel-
ative displacement.
d. The shaft relative displacement found in step (a) and the broad-band absolute
bearing displacement found in step (c) are summed. The resulting signal is
proportional to the absolute shaft displacement (AS).
The following front-end components are most commonly used for measuring the shaft
relative vibration (see Figure 7-5):
• TQxxx proximity transducer + IQSxxx signal conditioner.
The following front-end components are most commonly used for measuring the absolute
bearing vibration (see Figure 7-5):
• CAxxx accelerometer + IPCxxx signal conditioner + GSIxxx galvanic separation
• CVxxx velocity transducer.
NOTE: If different low-pass (LP) cut-off frequencies are defined for the RS and AB
measurements, the lowest frequency will be used for both processes.
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