Measuring device, Randomdec, provides continuous on-line signatures representative of system free vibration curve. Selected points on curve are used in control and failure detection systems. Apparatus applied to both linear and nonlinear systems under nonstationary vibratory states

Cole, H. A., Jr.

English

NASA Technical Reports Server

August 1971	 Brief 71-10284 
.
	
NASA TECH BRIEF 
Ames Research Center 
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. 
On-Line Analysis of Random Vibrations 
A measuring apparatus, Randomdec, can be used 
to analyze the vibrations resulting from random 
excitations of systems operating in natural en-
vironments. The apparatus provides continuous
In the new device (see Fig. 1.), the random 
signal from a vibration transducer attached to the 
structure is connected to the input. A voltage 
reference level (1) and a clock rate (2) are set, 
Extern 
Start 
0---
External 
Stop
Timing Output I 
Figure 1. 
on-line signatures representative of a system free-
vibration curve, from which meaningful data such 
as damping decrement and period may be ascer-
tained. Selected points on the curve may then be used 
in control and failure detection systems. 
The device is applicable to both linear and 
nonlinear systems under nonstationary vibratory 
conditions. Prior art spectral analyzers and cor-
relation computers do not, in general, give satis-
factory results under these conditions.
and the external start is activated. Flip-flop (3) 
activates switch (4) which sends the input signal 
to comparators (5) and (10). When the input signal 
reaches the voltage reference with a positive slope 
(Al in Figure 2), comparator (5) activates switch 
(7) and clock (2) via flip-flop (6), sending the 
input to the sample and hold block (8). An n-stage 
sequential switch (9) sequentially feeds a sample with 
each clock pulse into output lines 1, 2, . . . n. 
Similarly, comparator (10) activates blocks (11), 
(continued overleaf) 
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(12), (13), and (14) when the input signal crosses
	 through an analog or digital averaging circuit. The 
the
.
	 with a negative slope (BI in Figure
	 real time curve can be compared by computer with 
2). '
 The outputs of (14) are fed into the corre-
	 a stored standard curve. The computer can be con-
sponding outputs of (9). When the nth sample is
	 nected to a servo that modifies the status of the 
stStaeA 1st Stage B
2nd Stage A I 	 I	 I
2nd Stage B I, 
Vref
.
	 - - 
fl__ -	 - B2	 131 
-
Time 
1st Stage 
Timing Output 2nd Stage Timing Output
Figural. 
taken, feedback signals from (9) and (14) turn off 
flip-flops (6) and (11), respectively. Also, when 
switch (12) is activated, a timing signal is fed back 
to flip-flop (3) so that the system will not operate 
until another start signal is received. A timing signal 
is also put out by block (14) shortly after BI is 
reached. This signal may be fed back to the external 
start for single-stage operation, or fed to the second-
stage Randomdec (Figure 2) for two-stage opera-
tion. 
In the two-stage operation, the timing output 
from the first stage activates a second stage, which 
operates on the next peak at points A2 and B2. The 
timing output of the second stage feeds back to 
the first stage, which operates on the next peak. In 
this manner, all of the peaks are included in the 
calculations. Ordinarily, only two stages are needed 
to obtain the random damping decrement because 
the stage 'operation time needed is only slightly 
longer than one period. (In certain laboratory ap-
plications, it may be desirable to examine more 
than one period of the decrement. In these instances, 
any number of stages may be added to ensure that 
all peaks are included.) 
The outputs. of the sequential switches may 
be read in any of several ways. For example, they 
can be displayed on a CRT after being passed
vibrating system when a predetermined variance is 
noted. If such a system were to be installed in an 
aircraft, for example, the servo might control the 
thrust of one or more of the engines. In an aircraft, 
multiple transducers could be placed about the air-
craft and a selector switch could enable the 
flight engineer or operator to sequentially monitor 
each transducer location. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B71-10284 
Patent status: 
Inquiries about obtaining domestic and foreign 
licenses of this invention may be made to: 
Patent Counsel 
Mail Code 200-1 LA 
Ames Research Center, 
Moffett Field, California 94035 
Source: H. A. Cole, Jr.

Ames Research Center

(ARC-10154)
S 
Brief 71-102 . 84 Category 09 


On-line analysis of random vibrations

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