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BRUEL & KJAER System V - Characteristics; Square Table Heads; Frequency Response and Resonance

BRUEL & KJAER System V
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43
7.
CHARACTERISTICS
7.1.
SQUARE
TABLE
HEADS
The
three
Square
Table Heads, as
might
be
expected,
have
similar
frequency
response
and
loading
characteristics
.
They
are
grouped
together
in
this
section
for
comparison.
Since
frequency
response
and
resonance
are
characteristics
of
the
Heads
alone,
the
curves
shown
in
Figs.7.1,
7.2, 7.3,
7.7
and
7.9
apply
to
bot
h
4801
and
4805
Bodies
.
7.
1.1
.
Frequency
Response
and
Resonance
In Fig. 7.1 is
shown
a
plot
of
the
frequency
response
of
an
Exciter
Head Type
4811
. The
acceleration
in g is
plotted
with
respect
to
frequency
for
a
constant
voltage.
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8"
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en
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30
15
3,16g
.I
y.
~
l
oaded
wit
h an
Accelerometer
I.
YP~21
10
1-
, g
--
-
-K'
0,3169
R.<.,,""--
';.:::
.
'~J~-7'
O 0
1=
0,1 9
OP
112,4
JO
20
HI 50 100
200
Muhlp/y Fr..que
l\C)"
Segle
b.,
•
00
."
2 "
O O
500
1000
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SOOO
10000
20000.&0000
D A B C Uno
leroL.e-..l
:
16
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A B C lin. 27/053
Fig. 7. 1.
Frequency
response
of
a
High
g
Head
4811
One
may
note
a
levelling
off
at
slightly
above
300
Hz,
the
n a
small
step
upwards
be-
tween
500
and
700
Hz.
At
this
point
the
mass
of
the
flexures
is
de-coupled
from
the
mass
of
the
skeleton
and
table.
This
reduces
the
dynamic
mass,
and
hence
there
is a
slight
increase
in
acceleration
for
the
same
voltage.
Af
ter
700
Hz
the
acceleration
remains
at
the
same
level
until
about
3 kHz
at
which
point
it
begins
to
rise
to
the
peak
at
8 kHz.
This
peak
is
the
first
major
resonance
peak
of
the
moving
element
itself
.
Fig .7 .2
shows
the
frequency
response
of
the
Type
4812
Head.
Here
there
is a
smaller
decoupling
step
in
the
curve,
since
the
mass
of
the
flexures
is
smaller
with
respect
to
the
mass
of
the
moving
element.
The
acceleration
remains
level
between
400
Hz
and
3 kHz
and
rises
to
a
resonance
peak
at
about
7,2
kHz.

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