U.S. patent number 3,848,115 [Application Number 05/408,181] was granted by the patent office on 1974-11-12 for vibration control system.
This patent grant is currently assigned to Time/Date Corporation. Invention is credited to Charles L. Heizman, Edwin A. Sloane.
United States Patent |
3,848,115 |
Sloane , et al. |
November 12, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
VIBRATION CONTROL SYSTEM
Abstract
A vibration control system which compares the actual spectral
density of a specimen with a predetermined spectral density and
modifies a random signal based on the results of the comparison to
generate a driving signal is disclosed. A single frame of the
driving signal is randomized to permit the single frame of
information to be used a number of times for controlling the
system. A technique referred to as "windowing" is also utilized on
successive frames of information.
Inventors: |
Sloane; Edwin A. (Los Altos,
CA), Heizman; Charles L. (Los Altos, CA) |
Assignee: |
Time/Date Corporation (Palo
Alto, CA)
|
Family
ID: |
23615185 |
Appl.
No.: |
05/408,181 |
Filed: |
October 19, 1973 |
Current U.S.
Class: |
700/280; 73/664;
340/683 |
Current CPC
Class: |
G01M
7/022 (20130101) |
Current International
Class: |
G01M
7/00 (20060101); G01M 7/02 (20060101); G06F
17/00 (20060101); G01n 029/00 (); G06f
015/34 () |
Field of
Search: |
;235/150.5,151,151.3,156
;73/67.2,71.5,71.6 ;340/261 ;328/129 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gruber; Felix D.
Assistant Examiner: Smith; Jerry
Attorney, Agent or Firm: Spensley, Horn & Lubitz
Claims
We claim
1. A vibration control system for generating a control signal
comprising:
sensing means for sensing the movement of a specimen;
a first converting means for converting at least one frame of
information from said sensing means to the frequency domain;
comparator means for comparing said converted frame of information
with a predetermined spectrum and for providing a comparison frame
of data representative of said comparison, coupled to said first
converting means;
a generator for generating a random signal;
combining means for combining said random signal with said frame of
data coupled to said generator and said comparator means;
a second converting means for converting the output of said
combining means to the time domain and for providing a control
frame of information, coupled to said combining means; and
randomization means coupled to said second converting means for
randomizing said control frame such that said control frame may be
repeatedly used as said control signal for said vibration control
signal.
2. The system defined in claim 1 wherein said control frame
comprises a plurality of digital signals and said randomization
means randomly arranges said digital signals to provide a plurality
of control frames.
3. The system defined in claim 2 wherein said first and second
converter means are a common means.
4. The system defined in claim 3 wherein Fourier transforms are
utilized by said converter means.
5. A digital system for generating a plurality of control frames of
digital data for controlling a vibration system where said
plurality of control frames are bases on a first frame of digital
data comprising:
computer means for generating said first frame of digital data as a
function of a comparison between a measured spectrum and a
predetermined spectrum; and,
randomizing means for randomly arranging said first frame of
digital data into said plurality of control frames;
whereby said control frames may be converted to an analog signal
for controlling said vibration system.
6. A vibration control system comprising:
sensing means for sensing the movement of a specimen;
a generator for generating a random signal;
computer means for converting a predetermined duration of said
sensed movement to the frequency domain, for comparing said
converted predetermined duration of sensed movement with a
reference spectrum, for multiplying the results of said comparison
with said random signal and for reconverting the results of said
multiplication to a time domain function to provide a control
signal of finite duration; and
randomizing means for successively randomizing said control signal
of finite duration to provide a continuous control signal for
controlling said vibration control system.
7. In a vibration control system which generates a digital control
signal comprising a plurality of digital words based on a reference
spectrum and a measured spectrum, an improvement for utilizing a
predetermined segment of said digital control signal for providing
a continuous control signal comprising:
a memory for storing said segment of said digital control
signal;
random generator means; and
control means coupled to said memory and said random generator
means for causing said digital control signal to be randomized such
that said signal is read from said memory with a different one of
said words being first, said different one of said words being
selected by said random generator means;
whereby a continuous control signal may be generated.
8. The vibration control system defined in claim 7 wherein the
contents of said memory are periodically updated with another
digital control signal comprising a plurality of digital words.
9. In a vibration control system which compares the actual spectrum
of a specimen being subjected to vibrations with a reference
spectrum and modifies a random signal based on the comparison to
generate a control signal, an improvement for allowing a
predetermined duration of said control signal to be utilized for a
period of time greater than the time represented by said
predetermined duration for controlling a vibration system
comprising:
a recirculating memory for storing said predetermined duration of
said control signal and for recirculating said signal within said
memory; and
random generation means for causing said control signal to be read
from said memory such that said control signal is randomized;
whereby said control signal of predetermined duration may be used
as a continuous random driving signal.
10. A method for controlling the movement of a vibration producing
apparatus with a random signal to produce vibrations having a
predetermined power spectral density comprising:
comparing the power spectral density representation of the movement
of the apparatus with the predetermined power spectral density and
generating a digital signal representative of said comparison;
generating a random signal;
converting the results of said comparison and said random signal
into a time domain random driving signal of predetermined duration
by utilizing an inverse Fourier transform;
randomizing said driving signal of predetermined duration into a
continuous random driving signal; and
controlling the movement of the apparatus with said continuous
random driving signal.
11. A method for controlling the movement of a vibration producing
apparatus to produce vibrations having predetermined frequency
contents comprising the following steps:
sensing the movement of said apparatus and producing a digital
signal representative of the sensed movement;
converting a predetermined duration of the digital signal to a
frequency domain representation;
comparing the converted digital signal to the predetermined
frequency contents and producing an output signal representative of
said comparison;
generating a random signal;
combining said random signal with said output signal representative
of said comparison;
converting said combined signal into a time domain driving
signal;
randomizing said time domain driving signal to form a continuous
driving signal;
converting said continuous driving signal into an analog driving
signal; and
driving the vibration producing apparatus with said analog driving
signal.
12. The method defined in claim 11 wherein said converting of said
predetermined duration of the digital signal occurs periodically so
as to allow a different time domain driving signal to be
randomized.
13. The method defined in claim 12 wherein the frequency domain
representation comprises a power spectral density
representation.
14. The method defined in claim 13 wherein said random signal
comprises the sine and cosine of a random digital signal
representative of an angle and said combining means comprises means
for multiplying said sine and cosine of said angle by said output
signal representative of said comparison.
15. A digital system for generating a plurality of control frames
of digital data for controlling a vibration system where said
plurality of control frames are based on a first frame of digital
data comprising:
computer means for generating said first frame of digital data as a
function of a comparison between a measured spectrum and a
predetermined spectrum; and
windowing means for multiplying at least the beginning and end of
said first frame of digital data by some predetermined function and
for adding a plurality of said frames such that at least one end of
one frame overlaps said beginning of a subsequent frame;
whereby said conrol frames may be converted to an analog signal for
controlling said vibration system.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of vibration control
systems.
2. Prior Art
The closest prior art known to the applicant in U.S. Pat. No.
3,710,082 wherein a vibration control system is disclosed. In this
system the vibrations to which a specimen is subjected on a
vibration apparatus is sensed and changed to a digital signal. The
digital signal is converted to a frequency domain representation,
such as with the use of the Fourier transform. Further, a spectrum
of the frequency domain representation, such as the power spectral
density (PSD) is computed. This actual PSD to which the specimen is
subjected is then compared with a desired or reference PSD and the
results of the comparison are combined such as with multiplication
means with a random signal. The results of this combination are
then converted to the time domain function such as with the use of
an inverse Fourier transform and the results of this computation,
after being converted to analog form, are used to provide a control
or driving signal for the vibration apparatus.
In some applications it may be desirable to utilize only a single
frame of sensed information from the specimen to produce a driving
signal having a time duration greater in time than the sensed
signal. This is particularly desirable where slower and less
expensive computational means are to be utilized for performing the
Fourier transforms and for performing the comparison. As will be
seen in the present invention the results of the frequency
domain-to-time domain conversion is randomized before being
converted to analog form, thus permitting a single frame of sensed
information to be utilized to produce a plurality of frames of
driving signal without requiring a continuous flow of sensed
information through the entire system. Also windowing is utilized
to minimize discontinuities between frames of the signal.
SUMMARY OF THE INVENTION
A system for digitally controlling a vibration testing environment
or apparatus is described. The system includes means for sensing
the vibrations to which a specimen on a vibration apparatus is
subjected and an analog-to-digital converter for converting the
sensed signal to digital form. The digital form of the sensed
signal is converted to a frequency domain representation such as
through the use of Fourier transform in the presently preferred
embodiment. From this a spectral representation of the signal is
derived such as a power spectral density (PSD). The actual PSD to
which the specimen is subjected is then compared with a
predetermined or reference spectrum and the results of the
comparison are then combined with a random or pseudo-random signal
to produce a driving signal. The driving signal is then converted
into the time domain through the use of an inverse transform such
as the inverse Fourier transform and this time domain
representation is randomized in a randomization means to provide a
continuous driving signal. In the presently preferred embodiment
the randomization means includes a recirculating memory which
continuously recirculates a frame of the driving signal, such frame
including a plurality of digital words. A random number generator
within the randomization means causes the recirculating driving
signal to be read from the memory such that a randomly selected
word of the signal is utilized as the first word of a driving
frame. A plurality of driving frames of information are derived
from this recirculating memory and are converted to analog form to
provide a continuous driving signal for the vibration apparatus.
The contents of the recirculating memory are periodically updated
bases on new frames of information derived from the above described
comparison of the actual PSD with the reference PSD. The system
permits a single frame of the sensed information to be utilized a
number of times and, hence, the computations involved in the time
domain to frequency domain and frequency domain-to-time domain
conversions may be at a slower rate than the rate at which the
control frames are applied to the vibration apparatus. The
randominzation means provides a multiframe random signal to the
vibration apparatus even when the driving signal is based on a
single frame of computed information.
Also a windowing technique is employed. A single frame of driving
signal is miltiplied by some known function and then added to other
such frames which are delayed in time to form a continuous driving
signal. Windowing maybe employed in conjunction with the
randomization means above described.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a general block diagram illustrating the entire vibration
control system.
FIG. 2 is a detail block diagram illustrating the randomization
unit shown in FIG. 1.
FIG. 3 is an embodiment of the random number generator illustrated
in FIG. 2.
FIG. 4 is a block diagram of the windowing means utilized in
conjunction with the system of FIG. 1.
FIG. 5 is a series of waveforms which are used to explain the
operation of the windowing means.
FIG. 6 is a block diagram of an alternate embodiment of the random
number generator.
DETAILED DESCRIPTION OF THE INVENTION
A system and method for controlling a vibration testing environment
or apparatus, such as a shaker table, is described. The system
allows a specimen in the environment or on the apparatus to be
subjected to vibrations having a predetermined spectrum, such as a
predetermined power spectral density (PSD) or auto spectral
density. In this specification the system is described with
reference to an apparatus such as a shaker table, it will be
obvious that the system may be used to control a vibration
environment such as is used for high intensity sound testing and
other accoustical testing. For purposes of the specification, the
words "shaker apparatus" or "shaker system" shall mean any of
numerous vibration testing apparatuses or systems commonly known
and used for subjecting specimens to acceleration forces or
vibrations. Throughout the specification reference will be made to
the term "random signal". Included within the meaning of this term,
unless otherwise indicated, are pseudo-random signals such as those
that are digitally generated.
The system illustrated in FIG. 1, aside from the randomization unit
28 and windowing means 29 is similar to the systems described in
U.S. No. 3,710,082. The system of FIG. 1 includes a vibration
apparatus 10 used to subject a specimen 11 to vibrations. A sensing
means 12 is coupled to the specimen 11 in order that the vibrations
to which the specimen 11 is subjected may be sensed. Commonly known
accelerometers or other sensing means may be utilized for the
sensing means 12 as is known in the art. The sensed vibrations are
coupled to an analog-to-digital converter 14 where they are
converted to digital form. The results of the conversion to digital
form are then communicated to the PSD processor 16. The PSD
processor 16 converts the digital signal from converter 14 to a
frequency domain representation, such as through the use of a
Fourier transform, and further, computes, in the presently
preferred embodiment, the PSD to which the specimen is
subjected.
Numerous known computers may be utilized for obtaining the PSD or
other frequency domain representation. One such computer is
illustrated in U.S. Pat. No. 3,638,004. This computer also may be
utilized for performing the inverse Fourier transform performed by
converter 26. Other algorithms which may be used for performing the
time domain-to-frequency domain conversion and the conversion from
frequency domain-to-time domain are described in "An Algorithm for
Machine Computation of Complex Fourier Series," Math of
Computation, Volume 19, pages 297-301, April 1965, by Cooley and
Tukey, and in the Franklin Institute Journal, Vol. 233, April 1942
in an article by Danielson and Tanczos. Also, in U.S. Pat. No.
3,638,004 an analog-to-digital converter is illustrated which may
be utilized for the analog-to-digital converter 14. Other
references cited in this patent disclose other algorithms for
performing Fourier transforms.
Typically, in computational systems such as the one illustrated in
FIG. 1, a finite sample of information referred to hereinafter as a
frame is utilized in the various computational means. For example,
the analog-to-digital converter 14 may sense the vibrations to
which the specimen 12 is subjected for a predetermined period of
time and convert that sensed information into a plurality of
digital words which comprise the frame. In the presently preferred
embodiment a frame of approximately 500 digital words is utilized
wherein each digital word comprises a plurality of bits, for
example, 16.
The actual PSD is compared in the PSD comparator 18 with a
reference PSD or desired PSD stored within the reference PSD means
20. The reference PSD means 20 may be an ordinary memory for
storing digital information while the PSD comparator may be a
digital computer programmed with known techniques to perform
comparisons. Numerous comparisons are described in U.S. Pat. No.
3,710,082, see column 4, and another comparison is described in
column 14. Commercially available computer means may be utilized
for the PSD comparator 18 and the multiplier 22 of FIG. 1, such as
Model PDP-11 computer manufactured by Digital Equipment
Corporation, or "Super-Nova" manufactured by Data General
Corporation, or the Model 620/S computer manufactured by Varian
Corporation.
The random phase angle generator 24 may be any device for
generating a random number or pseudo-random number, in the
preferred embodiment, in digital form. Numerous techniques and
circuits may be utilized to generate a pseudo-random signal. One
such circuit which is utilized in the randomization unit 28 is
illustrated in FIG. 3. Techniques for generating a random and
pseudo-random signal are discussed in "Random-Process Simulation in
Measurements," by Granino A. Korn, a McGraw-Hill Company
publication.
The angle generated by the random phase angle generator 24 is
combined with the results of the comparison generated by the PSD
comparator 18, in the presently preferred embodiment, by
multiplication within multiplier 22. Multiplier 22 may be a
separate digital multiplier fabricated from prior art techniques or
may be a part of the PSD comparator 18. In the presently preferred
embodiment of the disclosed system, a random phase angle having
only four possible values is utilized. These values are arbitrarily
chosen as +/45% and +/135% and the digital signal representative of
this angle are sequentially applied to the multiplier 22. Note the
control signals, particularly the timing signals utilized for the
application of these signals to the multiplier 22 and for the
control of the remainder of the system are not shown in order to
simplify the explanation. For a more thorough analysis of the
mathematics involved in generating the pseudo-random signal
utilizing four phase angles and their application to a system such
as the one disclosed, see "A Digital Control Vibration or Acoustic
Testing System," by Charles L. Heizman, Proceedings of the
Institute of Environmental Sciences, Meeting on Environmental
Testing, Anaheim, Calif., Apr. 20-23, 1969, pages 387-409.
The results of the multiplication from multiplier 22 are converted
into a time domain function by the frequency domain-to-time domain
converter 26 by use of the inverse Fourier transform within
converter 26. The output from converter 26 which are frames of data
representing a control signal for the vibration apparatus 10 is
communicated to the randomization unit 28 or to the windowing means
29 on lead 54. In the prior art the output from the converter 26
was applied directly to the digital-to-analog converter 30 and
converted into analog form for driving the vibration apparatus
10.
The randomization unit 28 provides a means of randomizing the
output from converter 26 such that a single frame of information
from converter 26 may be utilized to produce a plurality of frames
of driving signal which after communication to the
digital-to-analog converter 30, via lead 55, may be converted into
analog form and used to drive the vibration apparatus. As will be
explained the windowing means 29 may be used to smooth the
transition between frames. The present invention permits the
processor 16, comparator 18, multiplier 22 and converter 26 to
operate at a slower rate and hence utilize less expensive
components and still permits the vibration apparatus 10 to be
controlled by a continuous random signal. The presently described
technique is particularly useful where the specimen 11 is
reasonably stable or invariant under test and where its transfer
function and the transfer function of the vibration apparatus
remains substantially constant or are slowly changing with respect
to time.
In order to understand the function of the randomization unit, a
brief examination of some mathematical concepts will readily
explain the results obtained from the unit. First assume that
X.sub.t (w) is the Fourier transform of a single frame of control
data such as will be obtained in the time domain from the converter
26. If a plurality of frames are to be formed from the data
contained in the single frame, the lth frame in the concatenated
series will be
X.sub. tl (.omega.) = X.sub. t
(.omega.)C.sup.-.sup.j.sup..omega..sup..tau.
C.sup.-.sup.j.sup..omega.lt (1)
where lT is the delay associated with the lth frame and .tau..sub.l
is the amount by which the lth frame is rotated. If the data in the
frame is subjected to a circular shift which is uniform from frame
to frame, then .tau..sub.l will equal l.alpha. T where .alpha.T is
incremental shift each frame. It can be shown that the resultant
spectrum S (.omega.) where ##SPC1##
will be ##SPC2##
The quantity in Equation (2) which includes the ratio of two sine
functions will cause the spectrum to be modulated or scalloped with
periodic peaks at frequency intervals of 2.pi./(.alpha.+1)T . Thus,
if the data in the frame is rotated or shifted by a constant
amount, the series of frames will result in a scalloped spectrum,
this being undesirable since a continuous smooth function is
required. On the other hand, if .tau..sub.l is random and
independent of l, the resultant spectrum will be defined by the
following equation: S(.omega.) = (1/T).vertline.X.sub.t
(.omega.).vertline..sup.2 (3)
From this equation it may be readily seen that by repeatedly
randomly shifting the data contained in a single frame in each
successive frame a plurality of frames may be formed based on the
information contained in the single frame which results in the
desired output. This is the function performed by the randomization
means illustrated in FIG. 3.
Referring to FIG. 3, the randomization means includes a
recirculating memory 32 which receives an input frame of data on
lead 54. The recirculating memory continually recirculates the
information contained within the frame, word by word, via lead 52
on command from clock pulses supplied by line 50. Any one of
numerous memories may be utilized for this function including
dynamic storage devices. The output from the memory 32 as it is
recirculated on line 52 is also connected to one input terminal of
the AND gate 36. The other input terminal of AND gate 36 is coupled
to the output from comparator 40 and one input terminal of the AND
gate 34. The other terminal of AND gate 34 is coupled to the clock
line 50. The output of AND gate 34 is coupled to an N-1 counter 44.
The N-1 counter 44 is an ordinary digital counter adaptable for
counting to N-1. N is assumed to be the number of words in the
frame of information received by the recirculating memory 32 on the
input line 54. The contents of the N-1 counter 44 are sensed by
decoder 46, this decoder upon sensing the number N-1 contained
within the N-1 counter 44 at the next clock pulse prOvides a reset
signal on line 48 for resetting counter 38 and for causing the
random number generator 42 to present a number to comparator 40.
Comparator 40 compares the contents of counter 38 with the number
presented by the random number generator 42 and upon sensing a
match between the two numbers provides an output signal which is
communicated to the AND gates 34 and 36. The output from AND gate
36 is coupled to a digital buffer 35 and the output from the
digital buffer 35 is the output line 55. The random number
generator 42 may be any means for generating a random or
pseudo-random output number such as the circuit illustrated in FIG.
3. The numbers generated by generator 42 are preferably between
zero and N.
The control circuit of FIG. 2 such as the AND gates 34 and 36, the
counters 38 and 44, the decoder 46, comparator 40, and the buffer
35 may be ordinary digital circuits.
In operation when a single input frame of information is
communicated into the recirculating memory 32, via lead 54, it is
continually recirculated through the memory as controlled by the
clocking pulses on line 50. The contents of the memory are
continually presented to the AND gate 36 but are not communicated
into the buffer 35 unless and until a signal is received from
comparator 40. At the beginning of the operation assume that a
reset pulse has been received by counter 38 and by the random
number generator 42. When this occurs the counter 38 will begin
counting and the random number generator will present a random
number to the comparator 40. When the number in the counter 38
matches the number presented to the comparator a signal is
presented on line 40 causing the information in the recirculating
memory to be shifted through the AND gate 36 into the buffer 35.
Also when this occurs, the conditions for the AND gate 34 are met
and the N-1 counter 44 begins counting. When the N-1 counter 44 has
counted through N-1, on the next clock pulse the counter 38 is
reset to zero and another random number is presented to the
comparator 40 by the generator 42. The operation repeats itself and
when the number in counter 38 again is the same as the numbers
presented by the random number generator 42 the words comprising
the frame of information stored within the memory 32 are
communicated to the buffer 35 through the AND gate 36. It is
readily apparent that a different one of the words comprising the
frame will be the first word communicated to the buffer 35 each
time a new number is generated by generator 42. Thus, each frame of
information based on the original frame communicated into the
recirculating memory will be randomized thereby providing a control
signal having the desired spectrum at the output line 55.
The recirculating memory is periodically updated with new
information furnished to line 54. Thus, the processor 16,
comparator 18, multiplier 22 and converter 26 of FIG. 1 may operate
at a slower rate than with some prior art systems and still allow a
driving signal to be furnished to the vibration apparatus 10 having
the required spectral contents.
In the circuit of FIG. 2 the timing signals applied to line 50 may
operate at a faster rate than the timing signals used throughout
the remainder of the system to assure a relatively continuous flow
of data to the buffer 35.
Referring to FIG. 3, the pseudo-random number generator which may
be utilized for generator 42 of FIG. 2, includes a 25 bit shift
register 58. The information in the shift register is shifted at a
clock rate provided by line 66 which in the presently preferred
embodiment is different than the clock rate provided to line 50.
The contents of the 25th stage of the register are applied to the
input terminal of inverter 60 and one input terminal of OR gate 63.
The contents of the third stage of the register are applied to one
terminal of OR gate 64 and the input terminal of inverter 61. The
other terminal of OR gate 63 is coupled to the output of inverter
61, while the other input terminal of OR gate 64 is coupled to the
output of inverter 60. The outputs from OR gates 63 and 64 are
coupled to the input terminals of OR gate 62 and the output from OR
gate 62 is coupled, via line 68, to the first stage of the shift
register. With the circuit of FIG. 3, the shift register 58 will
contain a pseudo-random number as the information in the shift
register is shifted and as information is returned to the first
stage of the register via lead 68. Other known circuits may be
utilized for generating the random number utilized in the circuit
of FIG. 2. The positive and negative value of a single number maybe
utilized where the sign determination is made randomly, i.e., by
the "flip of a coin." A single number and zero maybe utilized where
the choice between these two is randomly made.
Referring to FIG. 6, an alternate embodiment for the random number
generator is illustrated which includes an analog noise generator
90. This generator may be any known device for generating noise.
The noise generated by generator 90 is coupled to the
analog-to-digital converter 91 where it is periodically sampled on
command from clock pulses applied to the converter via lead 92 or
where it is sampled on command from the reset signal (line 48, FIG.
2). The digital signal from the converter 92 is communicated to an
output register 93 where it is stored. The output from register 93
may be communicated, via lead 94, to the comparator 40 of FIG. 2
for use in the comparison with the number generated by counter
38.
Referring to FIG. 1 the windowing means 29 performs the function of
taking a single frame of driving information and forming it into a
series of frames for driving the shaker table 10. The windowing
means receives input information on line 54 from the converter 26
and then applies an output signal to the converter 30. Windowing
means 29 may also utilize input information from the randomization
unit 28 and operate upon this information, in such event the output
from the randomization unit is not applied to the converter 30 but
rather to the windowing means 29 on line 55 with the output from
the windowing means 29 being converted to analog form by the
converter 30. In the presently preferred embodiment, the
information from the randomization unit 28 is applied to the
windowing means 29 with the output from the windowing means being
converted to analog form for driving the shaker table 10.
Referring first to FIG. 5, there is shown a graph which readily
explains the operation of the windowing means 29. The horizontal
lines 84, 85, 86, 87 and 88 represent time, with line 89
representing time, t = 0. Note that the information shown in FIG. 5
is in analog form while in reality the windowing means 29 operates
on digital information. Line 88 contains the driving signal which
is formed from either a single frame or driving signal from
converter 26 (FIG. 1) or from the series of frames received from
randomization unit 28. It is the information on line 88, when
converted to analog form by the converter 30, which is used to
drive the table 10. First, assume that the input to the windowing
means 29 consists of a single frame of information from converter
26. This single frame of information is shown repeated on lines 84,
85, 86 and 87. The amplitude of the frame of information had been
multiplied by some known function such as a sine function in order
to define the envelopes 82 of the frames. The series of frames are
added together as is shown in FIG. 5 such that the information on
lines 84, 85, 86 . . . 87 when summed form the information shown on
line 88. By way of example, the frame shown on line 85 would be
added to the information on line 84 beginning at approximately the
mid-point (in terms of time) of the frame shown on line 84.
Likewise, the frame shown on line 86 is added to the frame shown on
line 85 beginning at approximately the midpoint of the frame shown
on line 85. For the case where a series of frames are provided to
the windowing means 29 by unit 28, the summing and multiplication
by a known function to form the envelopes 82 would occur in the
same manner. In this case though the information shown on lines 84,
85, 86. . . 87 would have been randomized by unit 28, prior to
being "windowed."
The apparatus for performing this function is illustrated in FIG. 4
includes a pair of recirculating memories 71 and 72. Memory 71
circulates information on line 74 while memory 72 recirculates
information on line 73. The output from recirculating memory 71 is
applied to an amplitude control means 77, while the output from
recirculating memory 72 is applied to an amplitude control means
76. The amplitude of the signals contained within the amplitude
control means 76 and 77 are controlled by the function generator
80. The output from the amplitude control means 76 and 77 are
applied to a summing means 79 where they are summed. Timing signals
are applied to the various components in the circuit of FIG. 4 as
is illustrated by line 69. Ordinary electrical circuitry may be
utilized for the devices of FIG. 4.
The function generator 80 generates a function (in digital form)
such as that defined by the envelopes 82 of FIG. 5. In the
presently preferred embodiment this is a sine function. It will
appreciated that function generator 80 must generate a signal of a
first and a second phase of a function. In the case of the sine
function one signal is 90 degrees out of phase with the second
signal. One of said signals is applied to the amplitude control
means 77 and the other to the amplitude control means 76. The
amplitude control means 76 and 77 may be multipliers which multiply
the signal from the function generator by the signal from the
recirculating memory.
It will be appreciatd that the windowing may also be accomplished
by tapering only the beginning and the end of a repeated frame,
and, such that only the tapered portions of the repeated frame
overlap to provide a continuous signal. In the preferred
embodiment, the sums of the squares of the overlapping segments are
equal to unity. Thus, the beginning of the frame may utilize a sine
taper and the ends of the frame may utilize a cosine taper. Then
when the end of a frame is summed with the beginning of another
frame or repeated frame the sums of the square, that is, the sum of
sin.sup.2 .theta. and cos.sup.2 .theta. will equal unity. This
assures that the "power" applied to the shaker system does not have
peaks or valleys at the frame periods.
Assume for the sake of discussion that windowing means 29 of FIG. 4
is utilized for that shown as 29 in FIG. 1, without the
randomization unit 28. In that case a single frame of data from
converter 26 would be applied to both recirculating memories 71 and
72. At time t = 0 the information received by recirculating memory
71 is applied to the amplitude control means 77, and
simultaneously, the function generator 80 begins applying the
predetermined function, such as a sine function, to the amplitude
control means 77. The output from amplitude control 77 is applied
to the summing means 79. This output will correspond to the
waveform shown on line 84 of FIG. 5, but in digital form. After
approximately half the frame stored within memory 71 has been read
from memory 71, the frame stored within recirculating memory 72 is
read from that memory to amplitude control means 76. The function
generator 80 again furnishes a predetermined function which is used
to define the envelope 82 of FIG. 5. The output from amplitude
control means 76 which by way of example is shown on line 85, is
summed with the output from amplitude control means 77 within the
summing means 79. As the information is read from each of the
recirculating memories 71 and 72 it is recirculated on lines 74 and
73 respectively, thus, permitting the information in the memories
to be subsequently read from the memories. The output from the
summing means 79 is shown on line 88 of FIG. 5 and this signal when
converted into analog form controls the shaker table 10.
In the presently preferred embodiment the information from the
randomization unit 28 is applied to the windowing means 29 and not
to the digital-to-analog converter 30. When the windowing means is
utilized in conjunction with the randomization unit 28 the series
of frames from randomization unit 28 are alternately coupled to the
memories 71 and 72. For this mode of operation the memories 71 and
72 are not operated as recirculating memories but as ordinary
memories and the information which once read from them is not
recirculated. The series of frames alternately supplied to memory
71 and 72 are read from these memories out of phase as illustrated
in FIG. 5 and again the amplitudes of these signals are combined
with a known function from function generator 80 in order to define
the envelopes 82 of FIG. 5.
Referring again to FIG. 1 in operation the converter 14 converts
the signal from the sensor into a digital signal which is
communciated to the processor 16. In the presently preferred
embodiment a single frame of information from the converter 14 is
utilized to compute the PSD to which the specimen is subjected.
This PSD is compared in comparator 18 to the reference or desired
PSD and the results of the comparison are multiplied within
multiplier 22 with a random phase angle generated by generator 24.
The results of the multiplication are then converted to a time
domain function by the converter 26 and it is this information
which is applied to the randomization unit 28 and its randomized
before being communicated to windowing means 29. A plurality of
frames of driving signal are provided by the randomization unit 28
for providing a continuous driving signal to the vibration
apparatus 10. Periodically, converter 26 applies an updated frame
of information which replaces the information previously stored in
the randomization unit 28 and allows an additional series of
control frames to be generated by the randomization unit 28.
Thus, a vibration control system has been disclosed which allows a
single frame of control information to be reused a number of times
to provide a continuous control signal which contains the desired
spectral contents and exhibits a high degree of randomness.
* * * * *