Larson Davis HVM100, Manual

Page: 13 / 174

4/11/03 Introduction 1-5
Selecting the Proper Transducer
There are several important considerations when selecting a
transducer to perform your measurements. The first decision
that must be made is what type of transducer to use.
There are two general categories of accelerometers, ICP®
and Charge.
• ICP® accelerometers are also called “Voltage Mode” or
“Low impedance” and may be known by various other
vendor trade names. ICP® is PCB’s registered trademark
which stands for “Integrated Circuit Piezoelectric” and
identifies PCB sensors which incorporate built-in, signal
conditioning electronics. The built-in electronics serve to
convert the high impedance charge signal that is
generated by the piezoelectric sensing element to a
usable low impedance voltage signal which can be
readily transmitted over ordinary 2 wire or coaxial cables
to any voltage readout or recording device. The low
impedance signal can be transmitted over long cable
distances and used in dirty field or factory environments
with little signal degradation.
NOTE: The HVM100 has built-in ICP
®
power supplies and charge amplifiers.
This will allow the HVM100 to interface
directly with ICP
®
or charge transduc-
ers, and eliminates the need for external
signal conditioning.
• Charge mode sensors output a high impedance, electrical
charge signal that is generated by the piezoelectric
sensing element. This signal is extremely sensitive to
corruption from environmental influences. To conduct
accurate measurements, it is necessary to condition this
signal to a a low impedance voltage before it can be input
to a readout or recording device. A charge amplifier or
in-line charge converter is generally used for this
purpose.
The final way to use the input of the HVM100 is through the
direct input. This would require the use of some kind of
external signal conditioning unit. An external source of
ICP® power, or an external charge amplifier.
The next item to consider is the sensitivity of the transducer.
The sensitivity of the accelerometer will depend on the
application being performed. If the application is a high
level application, a low sensitivity should be selected. For