Table of Contents
Advertisement
rather large time
constants
of the
compressor
circuit. However, if the
crossing-over
takes place
at
a
very
high or
a very
low frequency
,
the drive
to the
shaker
may
suddenly
decrease and then
slowly
increase
again
until
the normal drive level is regained.
Another
effect
which may be mentioned is the instability
occurring
when
too high
compressor speeds are employed and
the test
specimen contains
high
Q
resonances.
These can
be readily
cured
by
choosing a
lower
com-
pressor
speed
and the
corresponding
lower
sweep
speed.
Notes on Using an Accelerometer as Controlling Element for the
1018 Vibration Exciter Control.
At
low frequencies and low
shaker
displacement levels the
acceleration
level
will be
extremely
low.
(The
acceleration level with a peak-to-peak table
displacement of o.1"
at 5 c/s
is for
example
only o.13 G. G
=
acceleration
of
gravity
=
386
inch/sec.2).
It is therefore necessary to take
special
precautions
when
an
accelerometer is used to
control
low
frequency shaker
displacement
levels within the 1018 level ranges
o.1" and o.01".
(a)
It is recommended to use
accelerometers with
relatively high
charge-
sensitivity,
i.e. the
product
of
accelerometer voltage sensitivity
(mV/G)
and
internal
capacity (,u,uF) should be as
high
as
possible,
preferably
higher than 30 .U.U columb/G.
(b)
In most
cases
it will
be
necessary to mechanically
shield
the
accelero-
meter against the
surrounding air, for even slight
air motion
("wind")
around the pick-up
causes
relatively
great electrical
noise in the
accelero-
meter output
signal.
(c)
The
accelerometer
preamplifier must be
of a very
high quality, prefer-
ably supplied with a sharp cut-off
filters which
attenuates
frequencies
outside
the
operating
range of the 1018.
A very
heavy
stabilization and
smoothing
of the plate
as
well
as
the filament
voltages
are normally
required to avoid low frequency disturbance
signals
from the power
supply, such as
DC
variations,
which upset the proper
functioning
of the
amplifier
(maximum
allowable
disturbance
signal
level is 10
,uV).
In
some cases
it
may,
in
addition
to the
stabilization,
be
advantageous
to
reduce the filament
voltage
by
approximately
10
%.
Great care
has also
been
taken in the design
of
the 1018
Automatic
Vibration
Exciter Control
to diminish the
effects of
low frequency disturbances,
and
a
sharp cut-off
filter has therefore been incorporated in the
vibration
meter
section.
The
cut-off frequency
of the filter is
5
c/
s.
23
Advertisement
Table of Contents
