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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 an individual
power amplifier and a common con-
troller.
These
arrangements
are
known as multi-exciter systems. The
exciters are controlled such that after
their force amplitudes and phases
have been adjusted, and when sinusoi-
dally excited, the system will produce
only one mode of vibration in the test
structure.
An example of a multi-exciter sys-
tem is shown in Fig. 15.
It
can be ex-
panded to include any number of ex-
citer and accelerometer channels. All
instrumentation below the structure
constitutes the excitation part of
the
system, while that above the
structure
is for data acquisition, processing and
analysis 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. Auto-
matic control can be achieved by using
a digital system-controller, for exam-
ple a desk-top calculator. The ampli-
tude and phase of the force applied to
the test structure must be accurately
controlled by a controller with high
frequency resolution. A suitable con-
trol signal can be supplied by the Vi-
bration Exciter Control Type 1053, via
a power amplifier 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 by
random 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.
~-------.~----~
•
.
.,
.
~~
**
Fig.
16.
Narrow Band Random Noise Unit
Type 5887
6
Cos
Sine
Levell
Phase
5899
870157
Fig. 15. Instrumentation for multi-exciter systems
The Narrow Band Random Noise
Unit Type 5887 converts the Vibra-
tion Exciter Control Type 1053 to nar-
row-band random noise controllers.
The noise bandwidth is selectable
from 3Hz to 1000Hz; selection is by
remote control from the Type 1053.
The narrow-band random noise pro-
duced by the Type 5887 has a Gauss-
ian distribution of the peak-amplitude
probability. Together, the 5887 and
1053 can perform a swept narrow-
band random noise test. This type of
test will simulate the properties of the
vibration experienced in the real envi-
ronment, enabling investigation into
fatigue failure while maintaining the
economic benefit of sinusoidal excita-
tion. The Type 5887 is shown in
Fig. 16.
Multi-point test control
When testing large or complex test-
objects, it is advantageous to use sev-
eral accelerometers to provide the
control signal for the feedback loop.
To simplify this, a signal selector
should be used.
The Control Signal Selector Type
5686 will select the highest, lowest or
mean vibration level from 6 input
channels, for use as the compressor
control signal. The sampling time is
equal to the time period of the vibra-
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