BRUEL & KJAER 2010 Instruction Manual page 16

to get the curve enlarged in the X-di-
rection, and the range potentiome-
ter of the recorder was changed to
50 dB in
or~er
to have the curve en-
larged in the Y-direction. The fre-
quency marker function was incor-
porated as use was in this case
made of uncalibrated paper. The
marker circuit which is connected
with the nixie display circuitry was
set to give an impulse for every
10Hz. As can be seen from the re-
cording in Fig. 1 0 the marker on the
recorder was activated by the
marker circuit and a frequency cali-
bration of the recording paper was
thus obtained. The marker circuit
gives a long pulse for multiples of
10Hz.
As shown in the figure a very ac-
curate frequency determination of
the enlarged curve is possible.
Tracking Analysis
Tuned from a Tracking Frequency
Multiplier Type 1901 the 2010 per-
forms as an automatic tracking anal-
yzer, see Fig. 11. Typical applica-
tions are synchronous acoustic and
vibration analysis of rotating ma-
chinery. Also complex harmonic an-
alysis of loudspeakers, for example,
can be made in an extremely simple
manner.
The 1 901 can tune the Hetero-
dyne Analyzer to multiples of the
fundamental frequency. Multiplica-
tion factors between 1 and 999 can
be selected in steps of 1 . The 1 901
accepts periodic triggering signals
between 30 mV and 300V RMS in
the frequency range 20Hz to
200kHz. The signal waveform is
not critical, and e. g. a pulse, sine,
square or triangular waveform is ap-
plicable .
Measurement of Mechanical
Impedance
Fig . 1 2 shows a set-up for the
measurement of the mechanical im-
pedance of a beam in a structure.
The mechanical impedance of a
structure is defined as the ratio of
the exciting force F to the velocity V
obtained when applying the force,
i.e .:
Z
=~(Newtons
per metre/s)
. v
As can be seen from the sketch
the excitation and measurement are
carried out at the same point result-
ing in the measurement of the
10
Magnetic Transducer MM 0002
·~ ~
-
~-
--.;---
Control of paper
movement, for
example propor-
tional to RPM
Level Recorder 2307
or X-Y Recorder
-,
I
I
Tracking Frequency
L.- _ _ _ _ _ - - - - - - - - - - - - - - - - - - - -
...J
Multiplier 1901 DC
a:
input frequency or N x input frequency
173257
Fig.11. Set-up for performing tracking analysis
ducer Accelerometer 8306
8200
Force Signal
Preamplifier
Mechanical
or electrical
sweep drive
------,
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-
.
.
.
.
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Power Amplifier 2707
Level Recorder 2307
173258/1
Fig.12. Set-up for measurement of mechanical impedance
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50 25
Potonti-
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Rectifier.___!!M,§_ L - LiM.
Fr.q.: ~z
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Speed:~
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Speed:~
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Concrete beam
~
15
6~
10
·~
10 dB
5~10
m/s per N
2 5
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Sign.: (
0 0
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QP112-4 10
20 Hz 50
100 200 500 1000 2000 5090 10000 20000 .0000 D A
8 C Un.
Multiply Frequency Scale
br 1
Z...OL.e..l: 1612/2112
A
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Fig.13. Recording of mobility measured with set-up in Fig.12
"point impedance" i.e. the struc-
ture's ability to withstand or absorb
vibrations.
The force is applied by means of
an exciter 4801 with mode study
head Type 481 4 driven from the
BFO output on the 201 0 through a
power amplifier Type 2707 and ap-
plied to the structure through the
force transducer Type 8200. The
8200 is connected to the compres-
sor input of the 2010 via the condi-
tioning amplifier Type 2626 in or-
der to secure constant force at the
excitation point throughout the en-
tire frequency sweep. The resulting
acceleration is measured by the low
g accelerometer Type 8306 and the
signal is integrated in the 2635 in
order to obtain the velocity signal
for calculating the mechanical im-
pedance. The velocity signal is fre-
quency analyzed in the 2010 and
recorded on the Recorder 2307. As
the applied force is kept constant
the recording (as can be seen from
the formula) will represent the mo-
bility of the beam at the measuring
point . The mobility being the recipro-
cal of the mechanical impedance.
The recording is shown in Fig. 13.