Larson Davis HVM100, Manual

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1-6 HVM100 User Manual 4/11/03
low level applications a high sensitivity accelerometer
should be used.
NOTE: Transverse sensitivity is also
something to be considered. Transverse
sensitivity is defined as: The unwanted
output signal of a motion sensor when
subjected to motion perpendicular to the
sensitive axis-usually expressed as a per-
cent of the normal axis sensitivity. For
example if you are using a tri-axial
accelerometer and place an input signal
on the Z axis, your X axis could also be
reading a level even though there is
really no x-axis signal present.
The resonant frequency of the transducer should also be con-
sidered. The resonant frequency is defined as the frequency
at which the structure will readily vibrate. For accelerome-
ters, there is one frequency where the accelerometer will
vibrate much easier that at other frequencies. At this point,
the reading will be very high, and could overload the input
of the HVM100. However, for most of the recommended
accelerometers, the resonant frequency is well above the
upper limit of the HVM100, and will subsequently be
masked out by the low pass filter on the HVM100. If the res-
onant frequency becomes an issue, it is recommended that
an external mechanical filter be used.
Frequency range is the next thing to take into consideration.
Most recommended accelerometers will have a frequency
range sufficient for use with the HVM100. See the fre-
quency response tables (appendix A) and graphs (appendix
D) for more information on the HVM100’s frequency
response.
The last thing to consider is temperature, humidity, and other
physical agents, such as mounting surface, mass and envi-
ronment. The mass of the accelerometer should ideally be no
more that 1/50 of the mass of the object being measured. A
ratio of as little as 1/10 is acceptable in extreme circum-
stances. The affect of these elements should always be con-
sidered when choosing a transducer. Please also be aware of
the different methods of mounting an accelerometer and the
affects of each of those methods on the measured data.
The following is a selection from the PCB catalog listing the
characteristics of a transducer. Please note the items that we
have discussed and note where they appear in a transducer
specification.