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I
·-
Fig. 12. Resonance Dwell Unit T ype 5885
gether with the controller ensures that
the feedback signal is free from noise
and is controlled using the excitation
frequency alone.
Resonance Dwell Testing
Accelerometer
e.g.
4383
Response
Signal
.--....
-;ret
.t--__,.t--~-,1
Measuring
Amplifier
2610
X-Y Recorder
2308
Slave Filter
5888
Vibration Exciter
Control 1 050
Force
Transduce
8200
Vibration Exciter 4801
+
Head
Power Amplifier
2707
8602521
1
One principle application of sine ex-
citation is fatigue testing, which in-
volves continuous vibration testing at
constant, discrete frequencies. Howev-
er, the repeated stressing of the test
structure causes changes in the reso-
nance frequencies during the test due
to damage to the structure material.
Fig.
14. Instrumentation for m
obility
measuremen
t
Resonance dwell testing provides
continuous testing of an object at a
particular resonance. At resonance the
phase angle between the input force
and the response acceleration is 90°.
For a small change in the resonance
frequency there will be a large change
in the phase angle. Consequently,
monitoring the change in the phase
angle is a more accurate method of
tracking the resonance frequency than
by monitoring the maximum magni-
tude of the response.
The Resonance Dwell Unit Type
5885 will activate the frequency sweep
of the controller so that it increases or
decreases the generator frequency to
maintain a constant phase angle be-
tween the excitation force and re-
sponse acceleration. The Resonance
Dwell Unit tracks the resonance fre-
quency by means of a phase-locked
loop. The instrument is shown in
Fig. 12.
An instrumentation set-up for reso-
nance dwell testing is shown in Fig. 13.
A Force Transducer Type 8200 mea-
sures the input force and supplies the
input force signal to the controller
charge input. This enables compressor
Response Signal
Response
Accelerometer e.g. 4384
.--------~ Force Transducer
AO
0038
8200
Input
Force
Signal
Resonance Dwell
Unit 5885
AQ
0034
Channel 2
1
Channel 1
Input
A
Input
+
·
I
AO
0087
Charge
Amplifier
2635
Input
Force
Signal
Vibration Exciter
4805
~ : -:~
t - - - - - - . - - - - - - - i
!
@
i
'
-~- .
·
-@
-
~
-
;
~----~~~--r
Vibration Exciter
Control 1050
Fig. 13.
Instrumentation for
resonance
dwell
testing
Power Amplifier
2707
85202411
regulation of the excitation force by
means of a feedback loop. The regu-
lated force signal is supplied to the
channel 2 input of the Resonance
Dwell Unit.
An Accelerometer, e.g. the Type
4384, measures the frequency re-
sponse of the test structure. The re-
sponse signal is conditioned by the
Charge Amplifier Type 2635 and is
supplied to the channel 1 input of the
Type 5885. This channel uses an inter-
nal compressor circuit to enable accu-
rate phase measurement by the Reso-
nance Dwell Unit.
The Type 5885 remotely controls
the frequency sweep up and down
functions of the 1050. The two instru-
ments can be used to search for reso-
nances using either an upward or
downward sweeping frequency signal.
On resonance detection, the Type
5885 will lock the 1050 generator fre-
quency to the detected resonance fre-
quency to enable continuous excita-
tion of the test structure resonance.
Mobility Measurement
An instrumentation set-up for mo-
bility
measurement
is
shown
in
Fig. 14. The input force to the test
object is controlled by using the Force
Transducer Type 8200 to provide the
feedback signal to the compressor.
The acceleration of the test object is
measured
using
an Accelerometer
fype 4383 which supplies a signal to
the X-Y Recorder Type 2308 for hard
copy of the frequency response. Be-
cause the input force is controlled, the
frequency response directly represents
the system's accelerance.
5
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