BRUEL & KJAER 4320 User guide

Triaxial
Accelerometer Set Type 4320
The Tr i axial Accelerometer is virtually three
accelerometers mounted in one housing. It
indicates the vibration in three mutually per-
pendicular directions . Principal application
areas are, balancing of machinery , vibration
testing , multidirectional shock testing and
others where the direction of vibration is not
known or where measurements in more than
one
direction are of interest.
BRiJEL &KJJER
Na!rum, Denmark . eye 80 05 00 . ,;;:' BRUKJA, Copenhagen . Telex: 5316
BB 4320

Accelerometer Set
Type 4320
April1967

Contents
0. Introduction 3
1. Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Factory Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3. Mounting Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4. Measuring Systems ................................. 28
5. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
7. Accessories ........................................ 46
8. Conversion Charts, etc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
9. Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
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f,
0. Introduction
The accelerometer is an electromechanical transducer which produces at its
output terminals an e.m.f., proportional to the acceleration to which the trans-
ducer is subjected. The output signal can be electronically processed and
read on a
meter or some other suitable indicating device.
The Triaxial
Accelerometer is virtually three accelerometers mounted in one
housing. It
indicates the vibration in three mutually perpendicular directions
Principal application areas are, balancing of machinery, vibration testing, multi-
directional shock testing and others where ·the direction of vibration is not
known
or where measurements in more than one direction are of interest.
The
accelerometer is small, light and of rugged construction, suitable for
most vibration measurements, both in the laboratory and in field environments.
Versatility in use has been a prime goal and special
care has been taken to
ensure a
wide frequency range, high sensitivity and good temperature charac-
teristics. Furthermore, the accelerometer has low transverse sensitivity,
negligible mounting error and low sensitivity to severe environmental, con-
ditions, such as humidity, high temperature,
corrosive atmospheres and
magnetic fields. The acoustical sensitivity and influence of cable whip are
also minimized.
For absolute measurements it is necessary to know the sensitivity of the
vibration transducer employed. Each accelerometer is supplied with individual
calibration data and frequency response curves, all taken as part of the
production test procedure. The aim has been to supply the maximum amount
of information on the calibration sheet to ensure a completely predictable
performance of the accelerometer.
The Triaxial Accelerometer has type number 4340 when on its own in the
production and calibration stages. When it is sold it is accompanied by
certain
accessories such as connecting cable, mounting studs etc., and is
available only as
Accelerometer Set Type 4320. See Specifications at the end
of this book.
3

1. Description
Construction.
The Accelerometer Type 4340 is of the piezoelectric compression type with
a construction indicated schematically in Fig. 1.1 .
Mass
Base
Output
terminals
Fig. 1.1. Schematic drawing of a piezoelectric accelerometer.
Each transducing element consists
of two piezoelectric*) discs on which is
resting a heavy mass. The mass is preloaded by a
stiff spring and the whole
assembly is mounted in a metal housing with a thick base. When the accelero-
meter is subjected to vibration, the mass will exert a variable force on the
piezoefectric discs . This force is exactly proportional to the acceleration of
the mass. Due to the piezoelectric effect a variable potential will be developed
across
the two discs , which is proportional to the force and therefore to the
acceleration of the mass. For frequencies much lower than the resonance fre-
quency
of the mass and the stiffness of the whole accelerometer system the
acceleration
of the mass will be virtually the same as the acceleration of the
whole
transducer , and the potential produced will therefore be proportional to
the acceleration to which the transducer is subjected. This potential can be
picked' up from the
output terminals of the accelerometer and used for
determination of the vibration amplitude, waveform and frequency. ,
Although the accelerometer has been designed especially with multidirectipnal
measurement in mind , it is also a versatile general purpose accelerometer
which can successfully meet the requirements of most vibration laboratories.
*) Lead Zirconium Titanate.
4

Fig. 1.2. Physical dimensions of the triaxial accelerometer.
Physical Dimensions.
It is generally true that an accelerometer should be as light as possible in
order not to influence the vibration of the specimen on which it is mounted.
For measurements on heavy machinery etc. this causes no problem but
for
lighter structures, such as for example a thin metal plate, the transducer
weight is important.
The resonance frequency of a vibrating single degree of freedom system is
fo= _1_
1/ k -
2n V mo
where mo is the effective lumped
mass and k is the lumped stiffness restrain-
ing the mass.
It is easily seen that adding another mass, e.g. an accelerometer, will result
in a change in resonance frequency according to the formula
V
---
~- -- ~
fo - ml + mo
where f1is the new resonance frequency and m1is the added mass.
Thus any mass added
to a vibrating structure will influence the vibration to
some extent, but provided the added mass is small the influence is often
negligible . A 10% increase in mass will reduce the resonance frequency by
about
5%.
The sensitivity of a piezoelectric accelerometer is directly proportional to
the mass acting on the piezoelectric disc. This limits the extent to which the
accelerometer weight may be reduced without loosing too much of the
sensitivity. For many
applications it is also important that any strain developed
in the material on which the accelerometer is fixed is not transmitted to the
piezoelectric element as this would show up in the output signal, indicating
an acceleration which actually is not present. It is therefore necess:ary to
make the accelerometer base thick , in order to isolate the piezoelectric disc
from such strain. The requirements to high sensitivity, low strain sensitivity

and good high frequency response are conflicting with requirements to small
physical dimensions and weight, so that
for general purpQse accelerometers
some useful compromise must be found.
The Triaxial Accelerometer Type
4340 has a titanium base and a total weight
of
35 grammes (1 . 23 oz). Fig. 1.2 gives its physical dimensions.
Environmental Sensitivity.
Accelerometers are often used to measure vibration in the field or on
specimens subjected to severe environmental tests. It is therefore important
that
their sensitivity to environmental changes is as small as possible. The
factors that may influence accelerometer performance are primarily tem-
perature, humidity and rapidly varying ambient pressure (Sound). The
temperature effect is to reduce the voltage sensitivity
of the accelerometer
at higher temperature, but if the accelerometer has undergone a suitable
temperature cycling process in the production stages, the sensitivity will
revert to its normal value w~en the temperature is brought back to normal
again. Beyond a certain temperature (the Curie point) the piezoelectric element
is permanently damaged.
-148'F
-100
dB
6
-3
-4
-5
-6
Voltage Sensitivity
571'F
300 •c
Temperature
1'+622
Fig. 1.3. Typical performance characteristics for the accelerometers in the
temperature range -100 to +260°C.
The triaxial accelerometer Type. 4340 is designed to be used for temperatures
up to 260°C (500°F) without cooling. The sensitivity will be slightly reduced
at the
higher temperatures, but' the necessary heat cycling process has been
carried out, so that no permanent change will take place.
6

It should1 be noted that great care has been taken to use materials which will
withstand high temperatures. The thermal coefficient of expansion of in-
sulating ', material and metal parts are carefully matched in order to maintain
humidity sealing.
It is, however, very
difficult to guarantee a perfect seal for immersion in
water when the accelerometers have been exposed
to the high temperature
limit. Therefore accel.erometers which are to be used in liquids should not
be exposed to more than 150°C for long periods of time.
The accelerometers a're
brought up to 250°C and left to cool down to about
50°C several times during production and calibration until performance is
stable.
Tests have also been conducted in order to find the influence of low
temperatures on accelerometer performance. The voltage sensitivity increases
steadily down to some -100°C and then levels out, while the capa<J:ity under-
goes a gradual decrease with decreasing temperature. The charge sensitivity
is
practically constant.
Typical performance characteristics are given in Fig. 1.3 for the temperature
range -100
to +260°C (- 150 to +500° F).
Cooling.
The Triaxial
Accelerometer has provision for water cooling , so that it may be
used
for measurements on surfaces with temperatures up to 1000°C (1800° F)
without seriously affecting performance.
Threaded connection tubes are supplied with the accelerometer
for easy
connection to a water supply. See Fig. 1.4.
2 67031
Fig. 1.4. Arr~ngement for water cooling of the accelerometer.
The water should preferably be supplied via
silicon rubber tubes, ~uch as
the B & K Type AF 502, 3.8 mm (0.15") internal diameter. The amount of
water necessary may be in the order of 1 to 20 litres per hour (1 /4 to 5
gallons per hour}.
7

It is also possible to reduce the temperature of the accelerometer base by
inserting a
cooling plate between the base and the measuring surface.
When a
mica washer surch as the one included in the Accelerometer S' et
4320 is used between the mounting surface and the cooling plate, experience
has shown
that temperatures up to 350-400°C may be measured on the
mounting surface with less than 250°C in the acca.lerometer base.
A stream
of air passing over the cooling plate consi. derably aids the cooling
process.
For high temperature measurements one should always employ Teflon cable,
B & K
part No. AO 0038. The cable delivered with the accelerometer , B & K
No. AO 0037 is based on Polyethylene and PVC which
will not withstand more
than 100°C. Also for low temperature measurements, from - 40 °C downwards ,
the
T~flon cable should be employed.
267 0 30
Fig. 1.5. Sealing of cable entry.
Vacuum Test.
The
accelerometer is sealed and tested under water in a evacuated jar for
leaks. This eliminates the risk
of inferior performance in moist atmospheres or
environmental test chambers where heavy condensation may take place .
When the accelerometers are used for measurements in liquids or in very
moist environments it is necessary to seal the cable entry, as shown in
Fig. 1.5.
A good se?lant
for the cable entry is for example Dow Corning Silastic
RTV 731 (room temperature vulcanizing silicon rubber) or General Electric
equivalent. These sealants show excellent performance for a wide tempera-
ture range, (- 100 to +.500° F).
Acoustical Sensitivity.
Effects due
to acoustical excitation are unavoidable with piezoelectric
accelerometers, but may pe reduced by careful design. The B & K
accelero -
meters are
of a rigid, mechanically isolated construction and pressure vari-
ations in the air will have little effect on the force exerted on the piezo-
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