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Appendix
Use of Type 1018 as Vibration Meter only.
The Automatic Vibration Exciter Control can be u sed as a vibration meter
when the "Compressor
Function"
switch is set to "Off".
However,
to obtain
a correct reading on the instrument meter the "Displacement-Velocity
Level"
and "Acceleration
Level"
potentiometers should then be set to
"10"
.
If it is desired to monitor the vibration
level,
this can be done by connecting
an AC level recorder to the terminal marked "Vibration" on the front panel.
Due to the relatively high output impedance the load impedance should not
be less than 100 kohms.
8ome Notes
·
R~garding
Cycling Systems Operation.
If a cycling system is used together with a shaker (and power amplifier
)
which contairis no overdriving protection circuits, damage J?ight under
certain circumstances occur to the shaker. Two examples of damage-
producing operations are mentioned in the
following:-
'When specimens with very high Q resonances are tested outside the normal
operating range of the
shaker,
the compressor arrangement may not be able
to maintain a constant vibration level on the shaker table due to overdriving
in the power amplifier. This again will result in overdriving of one or more
of the oscillator
and·
amplifier stages. The situation can be observed on the
vibration meter, the indication of which will decrease instead of keeping a
fixed value. If now the "Meter
Switch"
on the 1018 is switched from its
position "Vibration Level" to "Oscillator Voltage", i.e. if the operator wants
to check the oscillator voltage, the feedback in the oscillator output circuit
will suddenly
change,
correspondingly changing the overdrive level. The
change introduces a DC component in the shaker drive voltage, causing the
shaker to
" jump",
which will eventually produce
damage.
A similar situation is created if the "Output
Voltage"
potentiometer of the
Vibration Exciter Control is turned quickly down during the above described
operating conditions.
However, not all types of
"unusual"
effects will produce damage. For
example, during automatic cross-over operation a sudden increase in the
deflection of the vibration meter may be observed at the moment when the
crossing-over takes place. This may mislead the operator to assume that the
vibration level of the shaker has suddenly increased. This effect is due to an
electrical transient in the first stages of the regulating amplifier and does
not normally influence the actual vibration level of the shaker
,
due to the
22
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