This section outlines the procedures required to perform the random
vibration test using the tape deck unit. During the random vibration test the
true RMS voltmeter shall be utilized to accurately measure the overall RMS
amplitude of the vibration spectrum (see Figure 5-1 )
5-3.2.1 The nominal Grms for the required test spectrum
was established by basic test specification. This value may have to be
modified to correct for charge amplifier bandpass.
5-18.104.22.168 The correction for the charge amplifier bandpass
may be necessary to compensate for control accelerometer and charge amplifier
sensitivity to the vibration energy above the nominal g Hz upper limit of the
test spectrum. This energy is associated with mechanical noise and harmonics
of the test spectrum. An increase of 5% in the nominal Grms level is
recommended to compensate for this energy.
If the charge amplifier or RMS voltmeter have a 2000 Hz low
pass filter, no correction factor is necessary.
5-3.2.2 Prior to the test verify that
the shaker system is in an operate mode and the amplitude or master gain
control is in the zero or off position.
the tape using the fast forward control until the tape deck counter reads two
to three counts beyond the start of the Random Noise Spectrum (see Tape Deck
Sheet Chapter 3, Figure 3-3
5-3.2.4 Activate the playback control
of the tape deck. Observe the left channel VU meter for a positive reading
(above-20dB) indicating the start of the random signal. At this point slowly
increase the amplitude or master gain until a reading of the required Grms
times the charge amplifier sensitivity is realized on the True RMS voltmeter
(Example: a 6.0 Grms test with a charge amplifier sensitivity of 10 mv/g will
read 60 mvrms on the voltmeter).
5-3.2.5 At the conclusion of the
required random test time, the amplitude or master gain control be reduced to
zero and the tape deck de-energized.
5-3.2.6 The maximum duration of the
taped random signal is 17.5 minutes. The end of the signal is indicated by a
gradual decrease in noise level over 15 seconds followed by 45 seconds of no
signal. Test time duration greater than 17 minutes will require rewinding the
tape to the random signal start location on the tape deck counter and then
repeating steps 5-3.2.4 and 5-3.2.5.
It should not be necessary to readjust the amplitude
or master gain control to maintain the overall Grms level. I. If any change
is evident during the test period it may indicate a problem involving the
test article or test equipment. If the change is greater than 10% of the
test level, terminate the test by turning the amplitude or master gain to
zero (off) and refer to Chapter 6, Troubleshooting,