Once the process of synthesizing the random
equalization is accomplished using the sine sweep data, the next step is to
determine if system stability permits the use of the same random equalization
factors over and over again in day to day testing. (An automatic random
equalization system continually samples the spectral density levels at the
shaker head and adjusts the equalization every few seconds as required).
Since the sine equalization factors are being used to predict the random
equalization, the factors that affect the sine equalization can be examined to
gauge their effect on the random spectrum. It was pointed out in paragraph 1-3.1 that electrical and
mechanical resonances of the shaker system are the main causes of variations
in the input voltage required for constant shaker head motion. Since the
resonant frequencies vary with the mass and stiffness of the system
(mechanical) as well as the capacitance and inductance of the circuitry
(electrical) variations of these parameters must be minimized by imposing the
following restrictions on the use of a particular equalization curve:
a. No changes in the shaker and power amplifier.
b. No changes in the test article.
c. No changes in the test conditions, i.e., (test axis, alignment,
hardware, torque, etc.).
If these potential variables are stabilized the single equalization
curve should be usable indefinitely, without problems.
Another possible source of instability not
directly related to the equalization curve is the medium used to reproduce the
equalization voltage, namely the magnetic tape system. The tape system used
must be capable of accurate reproduction of the equalization voltage through
repeated playbacks in day to day test operations. The detailed procedures used
in evaluating the fidelity and stability of the tape system are presented in
Chapters 2 and 4.