Sampling output levels of accelerometers using largest output or any combination of outputs provide control of shaker for spacecraft vibration qualification tests

Woodbury, R. C.

English

NASA Technical Reports Server

1972	 .	 B72-10091 
NASA TECH BRIEF 
NASA Pasadena Office 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Control of Acceleration in Sine/Random Vibration Tests 
The problem: 
Spacecraft vibration qualification test specifications 
require that accelerations imposed on the test speci-
men not exceed a prescribed limit. Ordinarily, a 
single accelerometer is used as a monitor, and it is 
fastened at the test specimen's point of attachment 
to the shaker. However, when large specimens are 
involved, a test fixture is used and it is attached to 
the shaker at a number of points (usually six), each 
of which is monitored by an accelerometer. Owing 
to the composite structure made up of shaker, fixture, 
and test object, acceleration varies widely with fre-
quency at each test point during a swept-sine test. 
It is thus difficult to select a single accelerometer 
output as representative of the shaker force at any 
instant. 
The solution: 
Sample the output levels of each accelerometer, 
then use the largest output or any combination of 
outputs to control the shaker. 
How it's done: 
The AC signal from each accelerometer is rectified 
and filtered to produce a DC value which is propor-
tional to the acceleration level in its particular channel. 
The outputs of all channels are compared simul-
taneously by a simple diode OR gate arrangement in 
which the largest DC value backbiases the other 
diodes to cut-off. The OR gate output represents the 
largest momentary acceleration; this signal is pro-
cessed further and fed back to the amplitude servo 
for control of shaker force. Whenever the peak ac-
celeration changes from channel to channel, the
various DC voltages passing through the OR gate 
change the DC level of the control signal and hence 
there is no switching or commutation in the conven-
tional sense; the arrangement is inherently free of 
switching transients. 
Peak acceleration control is useful below 200 Hz, 
which is the region in which large displacements may 
damage the spacecraft structure. At higher frequen-
cies, peak control provides an unrealistic under-testing 
of large structures; signal averaging arrangements 
are therefore used. However, the accelerometer sig-
nals are derived from a common source (the shaker); 
this precludes direct summation of accelerometer 
signals because of unacceptable errors caused by 
vectorial addition of the AC accelerometer signals. 
Accordingly, summation is performed after phase-
and frequency-information have been removed by 
converting the accelerometer AC values to DC values 
before averaging. 
Depending upon the test specification, either of 
two types of averaging are utilized. Quadratic-mean 
averaging is employed when it is desired to restrict 
the force input from the shaker to the test object to 
a relatively narrow range and to ensure that the 
force never exceeds the square root of the sum of 
the squares of the accelerometer signals. Thus, should 
there be a loss of signal from one accelerometer in 
a test in which two accelerometers are used, the 
maximum acceleration that can be applied by the 
shaker while under servo-control will never exceed 
1.41 times the level of the servo set-point. In the 
case of a servo-controlled test in which four accelero-
meters are used, the applied acceleration in the event 
of loss of signal from three accelerometers will never 
(continued overleaf) 
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exceed two times the servo set-point level, and in 
the case of six accelerometers, and loss of five, the 
force will not exceed 2.45 times the set-point level. 
These limitations prevent both undertesting and 
overtesting of the structure. For example, consider 
a situation in which there are six points of attach-
ment with six accelerometers, and at each point of 
attachment the measured acceleration is 1 g and the 
servo is set to control at that level. If five accelero-
meters do not produce an output due to structural 
antiresponse at a particular frequency, the servo will 
try to force response by increasing gain of the entire 
system. The use of quadratic mean averaging ensures 
that the maximum output of the system would never 
exceed 2.45 g at any of the other attachment points. 
This effects a conservative test of the spacecraft 
structure. 
Arithmetic mean control is utilized when it is de-
sired to expand the range of test force in a similar 
situation. Assuming 1 g at each accelerometer and 
loss of five channels, the acceleration input will never 
exceed 6 g at the control accelerometer. This effects 
a more rigorous test of the structure. 
Six-channel circuitry for control of peak accelera-
tion has been designed and used for Ranger, Mariner, 
ATS, and Surveyor test programs. Since each channel 
utilizes essentially the same circuitry, it is possible 
to adapt the circuit designs to a larger or smaller 
number of accelerometers. Briefly, the accelerometer 
signal is applied to a follower amplifier through a 
capacitor; this isolates the impedance of the signal 
source from the input impedance of the full-wave 
operational rectifier and filter which consists essen-
tially of two operational amplifiers and associated 
resistors and capacitors. 
For control by arithmetic mean, the DC output of 
each channel is fed to the summing junction of an 
operational amplifier. For peak acceleration control, 
the output DC level of all channels are compared 
by a diode OR gate so that only the largest average 
DC level (corresponding to the channel with the 
maximum acceleration) is further amplified, pro-
cessed, and then used to control the shaker. 
Control by quadratic mean involves circuitry by 
which the filtered DC from the input operational 
rectifier connected to each accelerometer is fed to 
a squaring network fabricated of silicon carbide 
and compensated by suitable resistor networks; the 
squared inputs from all the accelerometers are then 
processed at a summing junction. 
Note: 
Requests for further information may be di-
rected to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP72-10091 
Patent status: 
No patent action is contemplated by NASA. 
Source: Raymond C. Woodbury of

Caltech/JPL
under contract to

NASA Pasadena Office 
(NPO-11482) 
Category 06-02


Control of acceleration in sine/random vibration tests

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