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Fig. 12. Resonance Dwell Unit Type 5885
filtering of the control and response
signals.
Response
Signal
Accelerometer
e.g.
4383
-----~·
AO 0127
Measuring
Amplifier
2610
X-Y Recorder
2308
AO 0087
Vibration Exciter 4801
+Head
Power Amplifier
2707
870421
Band-pass filter bandwidths provid-
ed are 3, 10, and 30%; selection is by
remote control from the Type 1053.
The Slave Filter Type 5888 is shown
in Fig. 11.
Fig. 14.
Instrumentation for
mobility
measurement
Resonance dwell testing
One principal application of sine ex-
citation is fatigue
testing,
which in-
volves continuous vibration testing at
constant, discrete frequencies. Howev-
er, the repeated stressing of a test
structure changes the resonance fre-
quencies during the test, due to
changes in the structure's material.
Resonance-dwell
testing
enables
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 reso-
nance frequency, there will be a large
change in the phase
angle.
Conse-
quently, monitoring the change in the
phase angle is a more accurate method
of tracking the resonance frequency
than monitoring the maximum ampli-
tude of the response.
The Resonance Dwell Unit Type
5885 will activate the frequency sweep
of the controllers, such that it in-
creases or decreases the generator fre-
quency, to maintain a constant phase
angle between the force and accelera-
tion.
The
Resonance Dwell Unit
tracks the resonance frequency by
means of a phase-locked loop. The in-
strument is shown in Fig. 12.
An instrumentation set-up for reso-
nance-dwell
testing
is
shown
in
Fig. 13. A Force Transducer Type
8200 measures the input force and
supplies the input force signal to the
Response
Response Signal
Accelerometer
e.g.
4384
AO 0038
Input
Force
Signal
Resonance Dwell
Unit 5885
AO 0034
Channel 2
I
Channel 1
Input
A
Input
'
I
Remote
WL 0658
I
Control
Vibration Exciter Control
1053
AO 0087
Charge
Amplifier
2635
AO 0231
Input
Force
Signal
Vibration Exciter
4805
Power Amplifier
2707
870422
Fig.
13.
Instrumentation for resonance-dwell testing
controller charge input. This enables
compressor regulation of the excita-
tion force by means of a feedback
loop. The regulated force signal is sup-
plied to the channel 2 input of the
Resonance Dwell Unit.
An Accelerometer, for example 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 for accurate
phase measurement by the Resonance
Dwell Unit.
The Type 5885 remotely controls
the frequency sweep up and down
functions of the 1053. The resonance
dwell system searches for a resonance,
using a downward sweeping frequency
signal. On resonance detection, the
Type 5885 will lock the 1053 generator
frequency to the detected resonance
frequency 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
Type 4383 for supplying 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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