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10
begins to rattle turn the AMPLITUDE control
ant.
i clockwise until the
rattling ceases and readjust the RESONANCE control until the meter
indication again peaks at maximum value. Since the RESONANCE
control is mechanically coupled to the table's spring suspension, it is
best whilst monitoring the meter indication to remove one's hand
from the control after making each adjustment.
6. Turn the AMPLITUDE control clockwise until the ball in the table
just commences to rattle. At this point the acceleration level of the
table is 1 g peak (i.e. 9.81 m/sec
2
at sea level).
The condition where the ball just commences to rattle can rapidly be
determined by listening. However, for a more accurate determination
of this condition, or for self education on the first calibration
attempt, one of the following methods may be used:
a) Monitor the output signal from the accelerometer using a pair of
headphones or a loudspeaker. These should be connected to the out-
put of the indicating instrument using a power amplifier. When the
ball begins to rattle the level of background noise heard will sharply
increase.
b) If the indicating instrument is provided with frequency weighting
networks, select A-weighting and turn the AMPLITUDE control
clockwise. At first the meter needle of the instrument will rise slowly,
as the network will only pass the harmonics of the vibration table's
resonance and not the fundamental. However, as the AMPLITUDE
control is turned further, the meter needle will reach a point on the
meter scale where the rate at which it rises will sharply increase, due
to the increase in harmonics produced when the ball begins to rattle.
Therefore the meter needle should be set to this point on the scale.
After this has been achieved reset the instrument for operation using
its linear frequency mode.
c) If a 2107 is used as the indicating instrument then a similar proce-
dure to that described in item b) may be followed, but with the 2107
set to its rejection mode and its centre
freq~ency
tuned to 79.6 Hz.
d) By far the most accurate method of determining when the ball
begins to rattle is to monitor the output signal from the accelerometer
using an oscilloscope connected to the output of the indicating instru-
ment. The moment the ball begins to rattle a small amount of distor-
tion wi II occur on the negative slope of the monitored signal as shown
in Fig.3.2.
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