System features quickly achieved steady state, increased accuracy of spectrum definition, and true Gaussian amplitude distribution of resulting signals. Controlled shock-tests might also be tried with this system

Chapman, C. P.

Sotomayor, B.

English

NASA Technical Reports Server

March 1973	 B78-10138 
•	 NASA TECH BRIEF 
NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Computer-Controlled Vibration Testing 
Conventional closed-loop analog vibration testing 
systems cannot provide real-lime displays of test 
specimen response during test, and their performance 
cannot be corrected during a test because time con-
stants, filter bandwidths, and other control functions 
cannot be readily altered. Moreover, conventional 
systems cannot be fully automated for analog control 
of random noise testing, and they inherently are sub-
ject to drift and must be periodically recalibrated. 
A digital control system which is completely under 
computer command has been constructed to make 
I
possible fully automated control of random-noise 
vibration tests. The system is capable of providing 
real-time analysis with a continuously improving con-
fidence level and, because the computer-controlled 
system can be preprogrammed, it is possible to verify 
spectral test patterns prior to actual testing and to 
avoid operator error in setting up test parameters. 
Preprogrammed test specifications can be either 
tape-fed or introduced by a conversational computer 
language. Test excitation is produced by pseudo-
random noise generated by digital techniques, but 
the excitation, ilthough seemingly random in nature, 
has a predictable format; accordingly, randomness is 
injected in a manner which introduces very little 
variance into the power spectral density (PSD). This 
feature makes it possible to estimate the output PSD 
to a given confidence level much more rapidly than 
is possible with the purely random excitation that is 
obtained from Gaussian noise generated by the usual 
analog techniques. Precisely reproducible test spectra 
can be generated at any time. 
The digital control system includes a Fourier pro-
cessor, which is a special-purpose computer with a
hard-wired, built-in algorithm for effecting a fast 
Fourier transform of time samples of the acceleration 
signal. The Fourier processor is in communication 
with the minicomputer of the digital control system 
via a specially-designed interface, and the transforms 
are used to compute the relative amplitudes of the 
frequency components in the observed test spectrum 
Estimates of the PSD are produced in the minicom-
puter by averaging the square of the moduli of the 
successive transform outputs from the Fourier proces-
sor; the estimates are in terms of bandwidth, resolu-
tion, and confidence level. The average test spectrum 
thus obtained is compared with a reference spectrum 
to produce a resultant in the form of an amplitude 
spectrum. This is multiplied by the pseudo-random 
noise signal and the resultant is subsequently trans-
formed by a novel double sum-and-difference tech-
nique to produce a digital signal which can be con-
verted to analog form for driving the excitation system. 
During the test, the spectral response is displayed 
on a cathode ray tube which is integral with the 
Fourier processor, and a digital plotter is interfaced 
with the computer to plot results upon completion of 
the test. Test results are expressed as a power spec-
trum and in spectral ratios. 
Some of the attractive features of the vibration 
testing system are: the short time required to reach a 
steady state, the increased accuracy of spectrum defini-
tion (there are no gaps as in the conventional noise 
sources), and a true Gaussian amplitude distribution 
of the resulting signals. 
The computer-controlled vibration testing system 
possibly can be used for controlled shock-tests; for 
example, with the described system, it is possible to 
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19730000138 2020-03-17T07:08:20+00:00Z
introduce initially the response characteristics of the 
total test system, and then to devise an input pulse 
which can produce the desired acceleration transient. 
Such a test would be operated as a semi-closed loop 
system. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP 73-10138
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Inquiries concerning non-
exclusive or exclusive license for its commercial de-
velopment should be addressed to: 
NASA Patent Counsel 
Mail Code 1 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Carl P. Chapman and

Bernardo Sotomayor of

Caltech/JPL
under contract to
NASA Pasadena Office 
(NPO-11612)
S 
Ll 
. 
B73-10188	 Category 02


Computer-controlled vibration testing

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