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.

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.

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.