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Technical data are subject to change without notice.
ISO 9001 certified. © Copyright SPM 1996-9. 71519.Z
SPM Instrument AB
• Box 4 • S-645 21 Strängnäs • Sweden
Tel +46 152 22500 • Fax +46 152 15075 • [email protected] • www.spminstrument.se
Measure Acceleration - Display Velocity
All three vibration parameters - displacement, accel-
eration, velocity - are mathematically related. One
can, for example, place an accelerometer on a vibrat-
ing surface and convert its signal, via integrating cir-
cuitry in the measuring instrument, into a reading of
vibration velocity or displacement.
The choice of displayed parameter (the instrument
reading) and measured parameter (the transducer type
used) depends on the problem to be solved and on
the cost, the complexity and the reliability of the
measuring equipment.
Experience has shown, that the RMS level of vibration
velocity, measured over a frequency range of 10 to
1000 Hz, is most useful for general assessment of
machine condition. The technical term used is
vibra-
tion severity
, defined as above and displayed in
in/s
RMS
on the instrument. Vibration severity is directly
related to the energy level of machine vibration, and
thus a good indicator of the destructive forces acting
on the machine.
There are transducers which measure velocity directly,
i.e. seismic probes with either moving coils or moving
magnets. These transducers are normally bulky, easily
damaged and expensive to manufacture. They are
therefore gradually being replaced by accelerometers.
An accelerometer is basically a piezo electric crystal (a
crystal that develops an electric charge when it is
compressed or stretched) with a small reference mass
attached. As the transducer is moved back and forth,
the reference mass compresses and stretches the crys-
tal and the transducer gives an output directly related
to acceleration.
Piezo electric accelerometers are small, very robust
and relatively cheap to produce. They can work over a
very large frequency range. They can be mounted on
machines, held by hand against a vibrating surface or
be temporarily attached by wax or magnets. That is
why most practical measuring systems now use an
accelerometer as the transducer and an integrator
within the instrument to give a display in terms of
velocity.
in/s
RMS
Piezo electric
crystal
Housing
Mass
Base
Output
Integrator
Accelerometer
Piezo electric accelerometer
