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Multi-exciter Testing Systems
The modal parameters of large
scale
objects can be found by using several
vibration exciters operating in paral-
lel, each being driven by individual
power amplifiers and a common
con-
troller.
These
arrangements
are
known as multi-exciter systems. The
exciters are controlled to have their
force amplitudes and phases adjusted
so that, when sinusoidally excited, the
system will produce
one
mode of vi-
bration only in the test
structure.
An
example of a multi-exciter system is
shown
in Fig. 15. It can be expanded
to include any number of exciter and
accelerometer channels. All instru-
mentation below the structure consti-
tutes the excitation part of the system,
while
that
above the structure is for
data aquisition, processing and analy-
sis
of results.
Due to the large number of instru-
ments used in the set-up, automatic
system control is desirable although
manual operation is possible. This can
be achieved by using a digital system
controller, for example a desk-top cal-
culator. The amplitude and phase of
the force applied to the test structure
must be accurately controlled by a
controller with a high frequency reso-
lution. A suitable control signal can be
supplied by the Vibration Exciter
Control Type 1050 via a power ampli-
fier and a level/phase controller.
Narrow Band Random Noise
Testing
Narrow band random noise testing
can be used to simulate real environ-
ments in which the excitation is ran-
dom vibration confined to a narrow
frequency band. Such environments
can be caused by mechanical filtering
and resonance amplification of broad-
band random noise resulting in nar-
row band random noise.
r-------,
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Fig.
16.
Narrow Band Random Noise
Unit
Type 5887
6
Sin
Vibration Exciter
Control 1 050
HP Desk-Top
Calculator
Series 200/300
l:-====:::~~~
851844
Fig.
15. Instrumentation for multi-exciter
systems
The Narrow Band Random Noise
Unit Type 5887 converts the Vibra-
tion Exciter Control Type 1050 for use
as a narrow band random noise con-
troller. Noise bandwidths are selec-
table in the range 3Hz to 1000Hz and
selection is made by remote control
from the Type 1050.
The Narrow Band Random Noise
Unit Type 5887 produces narrow band
random noise having a Gaussian dis-
tribution of the peak amplitude prob-
ability. By using this instrument to-
gether with the controller, a swept
narrow band random noise test can be
performed. This type of test will simu-
late the properties of the vibration ex-
perienced in the real environment and
enable investigation into fatigue fail-
ure while maintaining the economic
benefit of sinusoidal excitation. The
Type 5887 is shown in Fig. 16.
Multi-point Test Control
When testing large or complex test
objects it is advantageous to use sever-
al accelerometers to provide the con-
trol signal for the feedback loop. To
facilitate this a signal selector should
be employed.
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