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BRUEL & KJAER System V - Page 54

BRUEL & KJAER System V
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sponse
of
the
Head
itself
is
eliminated
as a
source
of
error.
The
next
consideration
is
the
response
of
the
Standard
Accelerometer.
Below
5 kHz
this
response
is
independent
of
frequency.
Above
5 kHz
there
is a
slight
upward
swing
to a
maximum
of
+
0,5
dB
(6%) at
10kHz.
If
a ca
libration
is
to
be
run
at
these
high
frequencies,
it
is a
simple
matter
to
adjust
the
sensitivity
setting
on
the
preamplifier
to
compensate
for
the
increased
output,
by
reference
to
the
accelerometer
calibration
chart.
The
influence
of
test
levelon
sensi-
tivity
is
so
small
as to be
not
measurable
over
the
dynamic
range
of
the
4815.
The
final
consideration
is
the
cross-motion
of
the
Head.
If,
for
example,
this
is
taken
as
2%,
the
contribution
to
the
measured
acceleration
level
is
dependent
on
the
difference
in
transverse
sensitivity
between
the
Standard
Accelerometer
and
the
test
accelerometer.
For
example,
if
the
Exciter
is
running
at
100
g
at
1 kHz,
there
is a
transverse
acceleration
of
about
2 g. If
the
Standard
Accelerometer
has
a
transverse
sensitivity
of
1,7%
it
will
be
reading
2 x
0,017
g
high,
i. e. 1
00,034g.
If
the
test
accelerometer
has
a
transverse
sen-
sitivity
of
3%,
it
will
be
reading
2 x
0,03
high,
or
100,060
g.
Since
the
ca
libration
of
the
test
accelerometer
is
based,
not
on
what
the
absolute
acceleration
level
is,
but
on
what
the
Standard
Accelerometer
measures,
there
will
be
an
error
of
+
0,026
g
introduced.
If
both
accelerometers
had
the
same
maximum
transverse
sensitivity
and
were
aligned
in
the
direction
of
greatest
sensitivity,
there
would
be
no
error
introduced
by
cross-motion.
53

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