US3691516A

US3691516A - Acoustic pulse generator utilizing a mechanism for changing the natural frequency of oscillation

Info

Publication number: US3691516A
Application number: US878776A
Authority: US
Inventors: Walton Graham; Irving E Melnick; Tullio De Filippis
Original assignee: Control Data Corp
Current assignee: Control Data Corp
Priority date: 1969-11-21
Filing date: 1969-11-21
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12

Abstract

An acoustic pulse generator for generating acoustic pulses of varying frequency in water is disclosed including a mechanically resonant structure, a drive mechanism for causing the mechanically resonant structure to oscillate, a mechanism for changing the natural frequency of oscillation of the mechanically resonant structure, and one or more transducers for transmitting the oscillations of the mechanically resonant structure to the water in which the acoustic pulse is to be generated.

Description

United States Patent Graham et al.

ACOUSTIC PULSE GENERATOR UTILIZING A MECHANISM FOR CHANGING THE NATURAL FREQUENCY OF OSCILLATION Inventors: Walton Graham, Roslyn; Irving E. Melnlck, Syosset; Tullio De Filippis, Garden City, all of NY.

Control Data Corporation, neapolis, Minn.

Filed: Nov. 21, 1969 Appl. No.: 878,776

Assignee: Min- U.S. Cl. ..340/8 R, 181/.5 H Int. Cl. ..G0lv 1/02 Field of Search ..340/8, 12; 181/.5 H

References Cited UNITED STATES PATENTS Lyons et al. ..l8l/.5 l-l 51 Sept. 12, 1972 7/1968 Dickie et al ..340/8 X Primary Examiner-Carl D. Quarforth Assistant Examiner-J. M. Potenza Attorney-Darby & Darby [57] ABSTRACT An acoustic pulse generator for generating acoustic pulses of varying frequency in water is disclosed including a mechanically resonant structure, a drive mechanism for causing the mechanically resonant structure to oscillate, a mechanism for changing the natural frequency of oscillation of the mechanically resonant structure, and one or more transducers for transmitting the oscillations of the mechanically resonant structure to the water in which the acoustic pulse is to be generated.

29 Claims, 11 Drawing Figures PATENTEDSEP 12 1972 SHEET 1 [IF 5 FIG.

FIG. 2

POWER (WATTS) I,OO0,000-

PATENTEUSEP 12 m2 SHEET 2 OF 5 JOmFZOU moEEO PATENTEDSEPIZIHIZ 3.691.516

sum 3 0F 5 INVENTORS WALTON GRAHAM IRVING E. MELNICK TULLIO De FlLLlPlS AT TORNEYS PATENTEU 3,691,516

SHEEI '4 OF 5 ATTORNEYS ACOUSTIC PULSE GENERATOR UTILIZING A MECHANISM FOR CHANGING TI-IE NATURAL FREQUENCY OF OSCILLATION This invention relates generally to apparatus for generating acoustic pulses such as are used in geological exploration, and particularly in offshore oil exploration. The travel times of signals reflected by internal layers of the earth are used to calculate the position of such layers. In early exploration work, explosives were commonly used to generate acoustic pulses. Explosives generate strong, sharp acoustic pulses having a fairly wide frequency range. However, the use of explosives is not permitted in certain areas if, for example, damage to structures or wild life is likely. For this reason, attempts have been made to provide a safe, controlled acoustic pulse generator which is capable of spreading the requisite acoustic energy over a period of time.

Devices which are capable of transmitting a lowpower acoustic signal for a period of several seconds are generally well-known to those skilled in the art. However, in order to obtain accurate geological information, it is necessary to be able to accurately measure the time between the transmission of the acoustic signal and the reception of the signal reflected back from the various internal layers of the earth. Because the acoustic signal is transmitted over a period of several seconds, it is necessary to be able to associate with each instant of the period of transmission some unique property of the transmitted signal which can be identified in the reflected signal. Accordingly, such acoustic pulse generators used in geological exploration are capable of generating an acoustic signal having a frequency which varies with time. For example, a typical acoustic signal for use in underwater geological exploration might have a duration of several seconds beginning at a frequency of hz and sweeping linearly up to 100 hz, or vice versa. The frequencies of the reflected signals can readily be identified and correlated with the frequency of the transmitted signal with an accuracy equal to a small part of the total duration of the transmitted signal.

The useful acoustic power output of an underwater acoustic pulse generator is limited by the fact that cavitation will occur during the reduced-pressure portions of the acoustic wave if the strength of the acoustic signal is too great. However, the strength of the acoustic signal should be maximized to assure a strong reflection from the various layers of earth. Since the strength of the transmitted acoustic pulse is limited by the cavitation phenomenon, the acoustic pulse generator should be lowered well below the water surface in order to take advantage of the higher threshold of cavitation at those depths. In addition, the reflection from the water surface of the transmitted signal (in the 10 hz to 100 hz frequency range) can be made favorable by placing the source at approximately 40 feet beneath the surface of the water.

In addition, if the transducer is a simple piston in contact with the water, the amplitude of the motion of the piston must be changed with the frequency of the acoustic signal so as to keep the acoustic signal strength at its maximum value just below the cavitation threshold.

Certain mechanisms for producing mechanical oscillations of variable frequency and variable amplitude are known to those skilled in the art. For example, the

desired mechanical oscillations can be produced by a hydraulic servo system including a hydraulic power supply, a hydraulic servo valve and an actuating piston. The hydraulic servo valve converts the static hydraulic pressure into the desired flow to the actuating piston. The hydraulic servo valve is actuated by an electric solenoid, and control of the mechanism is effected by impressing the required electric signal on the solenoid.

While hydraulic servo systems provide good efficiency when driving dissipative loads, their overall efficiency drops to a low value when driving a reactive load, such as water, particularly when the wavelength of the acoustic signal is large in relation to the diameter of the transducer piston.

For example, in a hydraulic system for generating acoustic waves in water, the transducer might be an acoustic piston rigidly attached to the hydraulic actuating piston. The acoustic piston would be of sufficient diameter and rigidity to produce the maximum acoustic signal strength throughout the range of frequencies of interest. The loading on the acoustic piston is predominately reactive rather than dissipative. That is to say that the water acts as a large mass attached to the acoustic piston. In many oscillatory or reciprocating mechanical systems, a reactive load poses no great problem because the reactive energy oscillates between the power source and the load with only a small internal dissipative loss. On the other hand, hydraulic servo systems are unable to transfer the energy from the load back through to the power source. This deficiency arises from the nature of the servo valve which passes hydraulic fluid to the actuating piston to move the load, but only converts the reactive energy from the load into heat on the return stroke.

It is therefore an object of this invention to provide improved apparatus for generating controlled acoustic pulses in water.

It is also an object of this invention to provide a compact self-contained acoustic pulse generator which may be contained within a submarine capsule of moderate size.

It is a further object of this invention to provide a self-contained acoustic pulse generator having an electric power input only.

It is another object of this invention to provide an acoustic pulse generator which is capable of generating acoustic pulses in water with high efficiency.

According to the above and other objects, the present invention provides an acoustic pulse generator including a mechanically resonant structure, a drive mechanism for causing the mechanically resonant structure to oscillate, a transducer connected to the resonant structure for converting the oscillations of the mechanically resonant structure into acoustic waves in a medium such as water, and a mechanism for changing the natural frequency of oscillation of the mechanically resonant structure and the stroke of the transducer in order to provide a long stroke at low frequencies and a short stroke at high frequencies. In addition, the preferred form of the present invention includes a cocking device for storing energy in the resonant structure to permit the acoustic output pulse to be initiated at a high power level. Sensing devices and feedback control mechanisms are provided to control the system so that acoustic pressures generated by the piston do not exceed the cavitational limit.

The above and other objects and advantages of the present invention will be apparent to those skilled in the art from the following detailed description and accompanying drawings which set forth the principles of the invention and, by way of illustration, the preferred embodiment for carrying out those principles and several modifications thereof.

In the drawings:

FIG. 1 is an overall view of a geological survey ship with a submarine capsule containing the acoustical pulse generator of the present invention.

FIG. 2 is a graph of the factors limiting the power output of the acoustic pulse generator of the present invention over the range of frequencies of interest.

FIG. 3 is a side elevational view in cross-section taken along line 3-3 of FIG. 6 of a submarine capsule containing a preferred form of acoustic pulse generator according to the present invention.

FIG. 4 is a cross-sectional view of the submarine capsule and acoustic pulse generator taken along the line 4-4 of FIG. 3.

FIG. 5 is a block diagram of the preferred form of feedback control system for the drive mechanism of the subject acoustic pulse generator.

FIG. 6 is a plan view with parts broken away of the submarine capsule and acoustic pulse generator.

FIG. 7 is a cross-sectional view of the submarine capsule and the acoustic pulse generator taken along the line 77 of FIG. 6.

FIG. 8 is a side elevational view in cross-section of a submarine capsule containing a modified form of acoustic pulse generator according to the present in vention.

FIG. 9 is a crosssectional view of the submarine capsule and modified acoustic pulse generator taken along the line 99 of FIG. 8.

FIG. 10 is a perspective view, partly broken away, of a second modified form of acoustic pulse generator according to the present invention.

FIG. 11 is a perspective view, partly broken away, of a third modified form of acoustic pulse generator according to the present invention.

Referring now to FIG. 1 of the drawings, there is shown a general overall view of an offshore seismic prospecting operation using the acoustic pulse generator of the present invention. The survey ship 1 moves over a prescribed course which is provided by various navigational devices not shown. A submarine capsule 2 is towed behind the ship 1 preferably at a depth of 40 feet beneath the surface of the water. Electrical power is supplied via cable 4 to the acoustic pulse generator contained within the submarine capsule 2. At regular intervals, under the control of the crew of the survey ship 1, the acoustic pulse generator is triggered and an acoustic pulse is transmitted to the

water b3acoustical pistons

5 and 6. The acoustic waves pass through the water and on into the earth beneath, whence they are reflected from various subsurface layers. The reflected waves, or echoes, are received by the hydrophones 7 which are placed at internals along the streamer 8 which is towed behind the survey ship 1. The signals from the hydrophone are recorded, and later analyzed to determine the depths and locations of the various subsurface layers to provide a picture of the subsurface structure of the earth beneath the water. The details of the subsurface structure can then be analyzed to determine if the characteristics of petroleum-bearing strata or other valuable mineral deposits are present.

Referring now to FIG. 2 of the drawings, there is shown a graph of the factors limiting the power output of an acoustic pulse generator of the type shown in FIG. 1 over the range of frequencies of interest. There are three limits on the acoustic power output. There is a cavitation limit (P, which is due to the properties of the medium (water) through which the acoustic waves are to be transmitted. Attempts to exceed the cavitation limit will result in the production of cavitation bubbles rather than increased acoustic power output. At a depth of about 43 feet, the power limitation imposed by the cavitation phenomenon is:

P (watts) 40,000[b(meters) In addition, if for mechanical engineering reasons the velocity of the acoustic pistons should not exceed a certain maximum value, v there will be a resulting limit, P,, on output power. P,, is given by:

P (watts) 41 [f(hz)] [v (meters/sec.)] [12 (meters) Similarly, if for mechanical reasons, the displacement of the acoustic pistons should not exceed a certain maximum value d,,,,,, there will be a resulting limit, P on output power. P is given by:

P (watts) 1630 [f(hz)] [d (meters)] [d(meters) 14 These three limiting factors on the acoustic output power are graphically shown in FIG. 2 for a typical acoustic pulse generator according to the present invention wherein the radius of the piston, b 0.5 m, v 2 m/sec., d,,,,, 0.15 m. According to equations 1, 2 and 3 above, P 10,000 watts, P, 10f and P 2.3 f. In the frequency range between l0hz and l00hz, P or P, are the limiting factors on the acoustic power output. Referring now to FIG. 3 of the drawings, there is shown a side elevation view in cross-section of a submarine capsule containing the preferred form of acoustic pulse generator according to the present invention. The submarine capsule, generally designated 20, is of oblong shape so that it may be relatively easily towed under water by a survey ship as shown in FIG. 1. The outer shell of submarine capsule 20 may be made of steel or other suitable material and should be watertight so as to prevent the entry of seawater into the interior where the acoustic pulse generating apparatus is located.

As shown in FIG. 3, a pair of

acoustic pistons

22 and 23 are located at opposite ends of the submarine capsule 20. Piston 23 slides within a cylinder 25. A circumferential sealing ring 26 surrounds piston 23 and contacts the walls of cylinder 25 to prevent water from entering the cylinder 25 behind piston 23. Piston 22 slides within cylinder 28 and is provided with a similar circumferential sealing ring.

Rigidly mounted at the center of piston 23 is a connecting rod or shaft 29 which is connected, at its other end, to the beam member 31 which forms a part of the variable frequency drive mechanism which will be explained in greater detail hereinafter. A similar connecting rod or shaft 32 connects piston 22 to beam member 34. Connecting shaft 29 slides longitudinally in bearings and 36. Similar bearings are provided for connecting shaft 32. Connecting

shafts

29 and 32 are preferably mounted in colinear relation so that the forces generated by

pistons

22 and 23 are balanced and there is no net force or torque acting upon submarine capsule 20.

Projecting inwardly from piston 23 is an annular structure or cylindrical wall 37, the inner surface of which engages the outer surface of fixed cylindrical projection 38. Annular structure 37, the central portion of the inner face of piston 23 and the end of fixed cylindrical projection 38 form a variable volume chamber which is connected via conduit 41 and guide release valve 41a, to an air surge tank 42. Variable volume chamber 40 is also connected via conduit 24 and air pressure regulator and valve 27 to the main air reservoir 77. Piston 22 is provided with a similar chamber, not shown, which is connected via conduit 43 and quick release valve 43a to air surge tank 44 and, via conduit 30 and air pressure regulator and valve 33 to the main air reservoir 77.

In operation the variable volume air chambers, such as chamber 40, perform two functions. They are used to cock the resonant structure prior to the generating of an output pulse so as to provide a high output power level at the start of the pulse, and they are used to balance the static pressure of the water on the outer surfaces of

pistons

22 and 23.

For example, referring to variable volume chamber 40, the cocking of the resonant structure is accomplished by admitting air from reservoir 77 via conduit 24 to chamber 40 under control of air pressure regulator valve 27. The air pressure which is used to cock the resonant structure is preferably sufficient to move the piston 23 to the outwardmost limit of its stroke acting against both the static pressure of the water on the outer surface of piston 23 and against the restoring force of the pneumatic spring system which will be described in greater detail hereinafter. Normally, the air pressure which is required to cock the resonant structure is about twice that which is required to balance the static pressure of the water when the

pistons

22 and 23 are centered at their neutral positions.

When the

pistons

22 and 23 have been moved to their outwardmost positions, the air pressure regulator valves 27 and 33 are closed to place the apparatus in condition to begin the generating of an acoustic output pulse. The output pulse is started by opening quick release valves 41a and 43a to permit air to pass from the variable volume chambers thru the

conduits

41 and 43 to

air surge tanks

42 and 44 thus sharply reducing the air pressure in the variable volume chambers to a level which just balances the static force of the water on the outer surfaces of

pistons

22 and 23. The sharp pressure reduction in the variable volume chambers, such as chamber 40, causes the

pistons

22 and 23 to move inward thus initiating the acoustic output pulse.

The

air surge tanks

42 and 44 have a volume which is sufficient to produce the required air pressure reduction. The necessary volume depends upon the pressure within the

air surge tanks

42 and 44 before the start of the acoustic pulse. If the

tanks

42 and 44 are evacuated prior to the start of the pulse, each tank will have a volume which is approximately equal to the mean volume of its associated variable volume chamber. lf, prior to the start of an acoustic pulse, the

air surge tanks

42 and 44 contain air at a certain pressure, such as the ambient pressure within submarine capsule 20, each

tank

42 and 44 must have a somewhat larger volume in order to produce the required pressure reduction in their associated variable volume chambers.

During the generating of an acoustic output pulse, the air surge tank 42 provides additional volume so that the pressure within chamber 40 will remain more nearly constant as the size of chamber 40 changes with the motions of piston 23. Air surge tank 44 performs a similar function in connection with the variable volume chamber associated with piston 22.

After an acoustic output pulse has been generated, quick release valves 41a and 43a are closed and exhaust valves 42a and 44a are opened to permit

air surge tanks

42 and 44 to be evacuated by the vacuum pump 47 or, alternatively, to vent the air in

tanks

42 and 44 into the interior of submarine capsule 20. Air pressure regulator valves 27 and 33 may be opened when it is next desired to cock the system.

Surrounding annular structure 37 and fixed cylindrical projection 38 is an annular vacuum chamber 45. A similar vacuum chamber 46 is associated with piston 22. The

vacuum chambers

45 and 46 are evacuated by a vacuum pump 47 via conduits 48 and 49 respectively.

Chambers

45 and 46 are evacuated so as to avoid significant pressure changes arising from the motions of

pistons

22 and 23 and to prevent leakage of air into water when pressure on piston is minimum. This permits use of a relatively loose seal around

pistons

22 and 23 thus reducing friction loss.

It will be appreciated by those skilled in the art that the entire space behind each of the

pistons

22 and 23 might be devoted to pressure balancing chambers such as chamber 40. In that case, however, the

air surge tanks

42 and 44 would have to be considerably larger in order to maintain a substantially constant pressure in the pressure balancing chambers throughout the range of motion of the

pistons

22 and 23.

The vacuum pump removes both air and seawater which may have leaked into

annular chambers

45 and 46. The air and seawater are separated, and the sea water is pumped overboard via conduit 51 while the air is discharged inside the hull of submarine capsule 20.

As described above, the connecting rod or shaft 29 of piston 23 is connected to a beam member 31. One end of beam 31 is connected to a pneumatic spring system which receives the reactive energy transmitted from the reactive load (seawater, in this case) back to the piston 23 connecting shaft 29 and beam member 31. The other end of the member 31 is connected to the drive mechanism which provides the motive force for the piston 23 and which provides the desired variation in frequency and stroke.

More particularly, one end of beam member 31 is connected by a rod 53 to a piston 54 which slides within an air cylinder 55. The air cylinder 55 is pivotally mounted on the frame of the submarine capsule 20 by a pivot pin 56, and the connecting rod 53 is pivotally mounted on beam member 31 by a pivot pin 57 in order to permit free movement of these parts during the operation of the acoustic pulse generating apparatus of the present invention.

Similarly, beam member 34 is connected by a rod 63 to a piston 64 which slides within an air cylinder 65. Air cylinder 65 is pivotally mounted on the frame of submarine capsule by a pivot pin 66, and connecting rod 63 is pivotally mounted on the beam member 34 by a pivot pin 67.

The chamber 58, which is the portion of air cylinder 55 to the right of piston 54 as shown in FIG. 3, is connected by a relatively large diameter flexible pneumatic hose 59 to the air surge tank 70. Similarly, the chamber 68, which is the portion of air cylinder 65 to the left of piston 64 as shown in FIG. 3, is also connected by a relatively large diameter flexible pneumatic hose 69 to the air surge tank 70. The chamber 72, which is the portion of air cylinder 55 to the left of piston 54 as shown in FIG. 3, is connected by the large diameter flexible pneumatic hose 73 to a second air surge tank 71 which is not shown in FIG. 3, but is shown in FIG. 5. The chamber 74, which is the portion of air cylinder 65 to the right of piston 64 as shown in FIG. 3, is connected by the large diameter flexible pneumatic hose 75 to the air surge tank 71 as shown in FIG. 5.

The two

air cylinders

55 and 65 and the two air surge tanks 70 and 71 act as a pneumatic spring system for the acoustic pulse generating apparatus of the present invention. When the

acoustic pistons

22 and 23 are in their neutral positions as shown in FIG. 3, the

pistons

54 and 64 are approximately centered in their respective air cylinders 55 and 65and the air pressures in the two air surge tanks 70 and 71 are equal so that the forces on

pistons

54 and 64 are balanced. When the

acoustic pistons

22 and 23 move outward, the volumes of chambers 58 and 68 are reduced and the pressure in chambers 58 and 68 and air surge tank 70 is increased. At the same time, the volumes of chambers 72 and 74 are increased and the pressure in chambers 72 and 74 and air surge tank 71 is increased. As a result of these different pressures, there is a restoring force on

pistons

54 and 64 which tends to return them to their neutral positions shown in FIG. 3. Conversely, when the

acoustic pistons

22 and 23 move inward, the pressure in air surge tank 70 is reduced while the pressure in air surge tank 71 is increased. Again, the difference in pressures provides a restoring force which tends to restore

pistons

54 and 64 to their neutral positions shown in FIG. 3. Hence, the

air cylinders

55 and 65 and the air surge tanks 70 and 71 act as a pneumatic spring system. The volumes of

air cylinders

55 and 65 and air surge tanks 70 and 71 are preferably related to the diameter and stroke of

pistons

54 and 64 so that the restoring force arising from the above-mentioned pressure differences is proportional to the displacement of the

pistons

54 and 64 from their neutral position shown in FIG. 3. There is a general limitation that fractional change in volume produced by the motions of

pistons

54 and 64 should be small enough so that the nonlinearities will be acceptable. The actual dimensions and pressures are selected to produce the required spring constant.

The air in air surge tank 70 and 71 is supplied from an air reservoir 77 through suitable pressure regulators which are not shown in FIG. 3 and which may be of a type well-known to those skilled in the art. Air is supplied to the air reservoir 77 by a conduit'78 from an air compressor 79 through an air supply regulation and pressure sensing device 80. Another conduit 81 from air supply regulation and pressure sensing device supplies

high pressure tanks

82 and 83 shown in FIG. 4.

The drive mechanism includes a hydraulic actuator having

output shafts

91 and 92. The output shaft 91 is pivotally connected to the cross-slide 93 which engages ball screw 94. The ball screw 94 is rotated by a motor 95 to adjust the point at which beam member 31 is driven by hydraulic actuator 90. The forces generated by hydraulic actuator 90 are preferably transmitted directly through cross-slides 93 to beam member 31 rather than through the ball screw 94. If desired, roller bearings may be provided to facilitate the motion of cross-slide 93 along beam member 31 in response to the ball screw 94.

Similarly, the output shaft 92 of hydraulic actuator 90 is pivotally connected to a cross-slide 96 which engages a ball screw (not shown) which is driven by a motor 97. In operation, the two

output shafts

91 and 92 of hydraulic actuator 90 move in unison, which is to say that both shafts move outward at the same time and both move inward at the same time.

The hydraulic actuator 90 is controlled by a servo valve 100 which is connected to a hydraulic reservoir 101 and a hydraulic pump 102 which is driven by a motor 103. The servo valve 100 is controlled by the feedback control system shown in block diagram form in FIG. 5. Feedback signals are provided to the electronic control system by

acceleration sensing devices

105 and 106 mounted on

piston shafts

29 and 32 respectively as shown in FIG. 3. The

acceleration sensing devices

105 and 106 may be accelerometers of the type well-known to those skilled in the art.

The acoustic pressure generated by

pistons

22 and 23 is directly proportional to their acceleration. Hence, the maximum acceleration signal produced by

acceleration sensing devices

105 and 106 gives a measure of the maximum acoustic pressure generated by

pistons

22 and 23 during their outward stroke, and the minimum pressure which occurs during the return stroke. Because cavitation will occur if the pressure at the faces of

pistons

22 and 23 drops below a certain minimum value on the return stroke, it is desirable to control the acceleration of the

pistons

22 and 23 in order to avoid cavitation.

In the preferred form of the acoustic pulse generator of the present invention, control of the maximum acceleration of the

acoustic pistons

22 and 23 is accomplished by the feedback control system of FIG. 5. The time-varying signals from the acceleration sensing device shown in FIG. 5 are fed into peak-following circuit 151 which provides an output signal which corresponds to the peak value of the time-varying input signal. The output signal from peak-following circuit 151 is compared, in comparator 152, with a reference signal corresponding to the acceleration at which cavitation would occur at the particular depth at which the acoustic pulse generator is being operated. The output signal from comparator 152 corresponds to difference between the maximum acceleration signal provided by acceleration sensing device 150 and peak-following circuit 151 and the reference signal and is fed to the servo valve orifice control 153 to control the size of the orifice of the servo valve 100. If the difierence is large, the orifice will be opened wide so as to provide more force to the

output shafts

91 and 92 of hydraulic actuator 90 thus increasing the maximum acceleration of

acoustic pistons

22 and 23. On the other hand, if the difference between the reference signal and the maximum acceleration signals from

acceleration sensing devices

105 and 106 approaches zero, the orifice of servo valve 100 will be restricted so as to limit the force applied by

output shafts

91 and 92 of hydraulic actuator 90, thus limiting the acceleration of the

acoustic pistons

22 and 23 to a value just below that which would produce cavitation. In operation, the effect of the above-described feedback control system is to operate the acoustic pulse generator of the present invention so as to produce the maximum possible acoustic output power as limited by the threshold at which cavitation effects occur.

It will be appreciated that the reference signal is preferably adjustable to correspond to the operating depth of the acoustic pulse generator of the present invention. The greater the depth, the greater the maximum acceleration which can be tolerated without producing cavitation effects.

In the preferred form of acoustic pulse generator shown in FIG. 3, the frequency of operation is changed by changing the effective lengths of

beam members

31 and 34. This is accomplished by operation of the screw jack, such as screw jack 94 of beam member 31, to

A change the positions of

cross-slides

93 and 96. When the

cross-slides

93 and 96 are located at their maximum distances from

piston shafts

29 and 32 respectively, the acoustic waves produced by the motions of

pistons

22 and 23 will be at the low end of the frequency scale, nominally hz as suggested hereinabove. The reason for this is that the pneumatic spring system including

air cylinders

55 and 65 and air surge tanks 70 and 71 sees a large load relative to its fixed spring constant. During the production of each acoustic pulse, the

motors

95 and 97 drive the ball screws to move crossslides 93 and 96 towards the centers of

beam members

31 and 34 respectively. As the

cross-slides

93 and 96 approach the

piston shafts

29 and 32 respectively, the pneumatic spring system acquires a greater mechanical advantage and, hence, sees a smaller load relative to its fixed spring constant. Therefore, the natural frequency of oscillation of the apparatus increases as the

cross-slides

93 and 96 approach the points of attachment of

piston shafts

29 and 32 at the center of

beam members

31 and 34 respectively.

From the foregoing it will be appreciated that each

beam member

31 and 34 acts as a second-class lever wherein the cross-slide is the fulcrum, the piston shaft is the load and the pneumatic spring system is the applied force. It will be appreciated that the fulcrum is essentially stationary because the motions of the hydraulic actuator are extremely small in relation to the motions of the

acoustic pistons

22 and 23 and the

pistons

54 and 64 of the pneumatic spring system. For example, the acoustic pulse generator of the present invention would typically have a Q on the order of 100 at lOhz. Therefore, the stroke of the

output shafts

91 and 92 of hydraulic actuator 90 would be only one-fiftieth of the stroke of the

acoustic pistons

22 and 23. Because of the short stroke of the hydraulic actuator 90, losses in the hydraulic system are minimized so that, for a given acoustic output power level, the input power requirements of the system are relatively low.

In the preferred form of acoustic pulse generator according to the present invention, the frequency of operation of the hydraulic actuator is changed to match the natural resonant frequency which changes with the operation of the ball screws as described above. The frequency of operation of hydraulic actuator 90 is controlled by servo valve which is in turn controlled by the feedback control system which is shown in FIG. 5. The signal which controls the frequency of operation of servo control valve 100 is derived from the time-varying output signals of acceleration sensing device 150 which corresponds to either of the acceleration sensing devices and 106 shown in FIG. 3. The output signals from

acceleration sensing devices

105 and 106 are oscillating signals having the same frequency as the motions of the

acoustic pistons

22 and 23 which correspond to the natural resonant frequency of the system. Hence, the output signals from acceleration sensing device may be used, with appropriate phase modification by phase delay circuit 155, to control the servo valve 100 so that the hydraulic actuator will continue to operate at the natural resonant frequency of the system as that natural frequency changes with the operation of the ball screws.

FIGS. 8 and 9 are cross-sectional views of a submarine capsule 200 containing a modified form of acoustic pulse generator according to the present invention. The outer shell of the submarine capsule 200 is preferably made of steel or other suitable material and has a streamlined oblong shape so that it may be relatively easily towed under water by a survey ship as shown in FIG. 1. In addition to the acoustic pulse generating apparatus which will be described in greater detail hereinafter, the submarine capsule 200 may contain various types of auxiliary equipment such as, ballast tanks, pumps and valves, bilge pumps, etc.

As shown in FIG. 8, a pair of

acoustic pistons

201 and 202 are located at opposite ends of the submarine capsule 200.

Acoustic pistons

201 and 202 are located within suitable

cylindrical openings

203 and 204 respectively in submarine capsule 200. Suitable circumferential sealing rings are provided in order to prevent seawater from leaking in around the edges of

pistons

201 and 202 into the interior of submarine capsule 200.

The acoustic piston 201 is connected to the drive mechanism by a connecting rod 205 which slides longitudinally within a bearing 207 which is supported by the structure of submarine capsule 200. Similarly, the acoustic piston 202 is connected to the drive mechanism by a connecting rod 206 which slides longitudinally within a bearing 208 which is supported by the structure of the submarine capsule 200. The axes of travel of the two

acoustic pistons

201 and 202 are colinear so that the forces generated by the motions of the pistons are balanced and there is no net force or torque on the submarine capsule 200 as a result of the operation of the acoustic pulse generating apparatus.

The acoustic pulse generating apparatus of FIGS. 8 and 9 include an elastic beam 210 which is preferably made of steel or other material capable of efficiently storing substantial amounts of mechanical energy when elastically distorted. The elastic beam 210 is supported by two

movable support structures

211 and 212. Each of the

movable support structures

211 and 212 includes roller bearings 213 contacting the upper and lower surfaces of the elastic beam 210. The lower portions of

movable support structures

211 and 212 are in the form of

cross-slides

215 and 216 which ride within

ways

217 and 218 and engage

ball screws

219 and 220 respectively. The ball screws 219 and 220 are rotated by a motor 221 which operates to move both

support structures

211 and 212 simultaneously inward towards the center of beam 210, or to move both

support structures

211 and 212 simultaneously outward toward the ends of the beam 210. Changing the positions of

movable support structures

211 and 212 will change the natural resonant frequency of the elastic beam 210. When the two

movable support structures

211 and 212 are near the center of the elastic beam 210, the natural resonant frequency will be relatively lower, and the excursions of the ends of the beam 210 will be relatively longer. On the other hand, when the

movable support structures

211 and 212 are near the outer ends of the elastic beam 210, the natural resonant frequency will be relatively higher and the excursions of the ends of the beam 210 will be relatively shorter. Hence, the characteristics of the elastic beam 210 are well matched to the requirements for operating the

acoustic pistons

201 and 202 to provide the maximum acoustic power output over the prescribed frequency range. As explained previously, the stroke of the acoustic pistons should be longer at lower frequencies and shorter at higher frequencies in order to produce the maximum output power over the prescribed frequency range without exceeding the cavitation limit.

The elastic beam 210 is connected to acoustic piston 201 by a link 223 which is connected to a bell crank 225 which is connected to a link 227 which is connected to the connecting rod 205 of acoustic piston 201. Similarly, elastic beam 210 is connected to acoustic piston 202 by a link 222 which is connected to bell crank 224 which is, in turn, connected to link 226.

The length and cross-sectional dimensions of the elastic beam 210 and the proportions of the linkages connecting the elastic beam 210 to the

acoustic pistons

201 and 202 are interrelated and depend upon the forces and displacements to be applied to the

acoustic pistons

201 and 202 in order to achieve the prescribed acoustic output power over the prescribed frequency range.

The elastic beam 210 is driven by a pair of

actuators

229 and 230 which form portions of the movable supporting

structures

211 and 212 respectively. The

actuators

229 and 230 are preferably hydraulic actuators which are controlled by a feedback control system similar to that described in connection with the preferred embodiment shown in FIGS. 3-7. Briefly, the feedback control system includes acceleration sensing devices mounted, for example, on connecting rods S and 206. The time-varying signals from the acceleration sensing devices are used, with appropriate phase delays, to control the servo-valve which controls the

actuators

229 and 230. The magnitude of the signals from the acceleration sensing devices is compared with a reference signal, and the difference is used to control the force applied by the

actuators

229 and 230 in order to keep the acoustic output power of the apparatus at a value just below the cavitation limit over the prescribed frequency range.

Although hydraulic actuators are preferred, it will be appreciated by those skilled in the art that other types of actuators may be employed within the spirit and scope of the present invention, such, as, for example, stacks of piezoelectric crystals. In any event, it will be appreciated that because of the large 0 of the system and because of the displacement multiplying effect of the linkages between the elastic beam 210 and the

acoustic pistons

201 and 202, the displacements of the

actuators

229 and 230 need only be on the order of fractions of an inch.

Referring now to FIG. 10 of the drawings, there is shown a perspective view of another modified form of the acoustic pulse generating apparatus of the present invention. The apparatus of FIG. 10 includes an elastic beam 300 which is preferably made of steel or other material capable of efficiently storing mechanical energy when elastically distorted. In the acoustic pulse generating apparatus of FIG. 10, the elastic beam 300 is adapted to be twisted or torsionally distorted by the action of the actuator 301. One end of the elastic beam 300 is pivotally supported by a stationary supporting structure 302 which may be, for example, a portion of the hull or supporting structure of a submarine capsule of the type shown in FIG. 1 and FIGS.38. The other end of the elastic beam 300 is rigidly connnected to a shaft 303 having a crank 304 which is operated by the actuator 301. If desired, the shaft 303 may be journalled in a portion of the stationary supporting structure 305.

The natural resonant frequency of the elastic beam 300 is controlled by the position of the movable supporting structure 310 which includes sets of roller bearings 311 which contact the upper and lower surfaces of the elastic beam 300. The lower portion 312 of the movable supporting structure 310 slides within a way 313 which is mounted on the stationary supporting structure indicated by fragmentary portion 314. The lower portion 312 of movable supporting structure 310 engages the ball screw 315 which is operated by the motor 316. The lower portion 312 of movable supporting structure 310 preferably fits within the way 313 so that forces on the movable supporting structure 310 are transmitted directly to the way 313 and not to the ball screw 315.

It will be appreciated by those skilled in the art that the natural resonant frequency of the elastic beam 300 will be lowest when the movable supporting structure 310 is farthest from the end of the elastic beam 300 which is driven by the actuator 301. Conversely, the resonant frequency will be highest when the movable supporting structure 310 is closest to the end driven by the actuator 301. Moreover, as in the case of the elastic beam 210 of FIGS. 8 and 9 which was subjected to longitudinal bending, the rotational motions of the driven end of the elastic beam 300 of FIG. 9 are greatest when the frequency of oscillation is low, and are shortest when the frequency of oscillation is high.

The motions of the driven end of elastic beam 300 are transmitted to the acoustic piston 320 by the crank arm 321 which is connected to link 322 which is in turn connected to the connecting rod 323 of piston 320.

The connecting rod 323 slides longitudinally in an appropriate bearing 324 in the supporting structure 325. Similarly, a crank arm 331 which is mounted on the driven end of elastic beam 300 is connected to a link 332 which is in turn connected to the connecting rod 333 of acoustic piston 330. The connecting rod 333 slides longitudinally in a suitable bearing 334 in supporting structure 335.

As in the case of the acoustic pulse generating apparatus of FIGS. 8 and 9, the length and cross-sectional dimensions of the elastic beam 300 and the proportions of the connecting linkages of the acoustic pulse generating apparatus of FIG. 10 are determined by the forces and displacements which are to be applied to the

acoustic pistons

320 and 330 over the intended frequency range.

The actuator 301 is preferably a hydraulic actuator controlled by a feedback control system of the type described in connection with the previous embodiments shown in FIGS. 3-9. It will be appreciated, however, that other types of actuators and control systems may be used within the spirit and scope of the present invention.

FIG. 11 shows a perspective view of yet another modified form of the acoustic pulse generator of the present invention. The elastic beam 400 is preferably made of steel or other material capable of efficiently storing mechanical energy when elastically distorted. The natural frequency of oscillation of the elastic beam 400 is controlled by the movable supporting

structures

401 and 402 which include sets of roller bearings 403 which engage the upper and lower surfaces of the elastic beam 400. The

lower portions

405 and 406 of the movable supporting

structures

401 and 402 respectively slide within a way 407 which is mounted on stationary supporting structures 408 which may be portions of the hull or internal structure of a submarine capsule of the type shown in FIG. 1 and FIGS. 3-9. The

lower portions

405 and 406 of the movable supporting

structures

401 and 402 engage

ball screws

409 and 409a which are driven by

motors

410 and 410a, respectively. The ball screws 409 and 409a operate to move the two movable supporting

structures

401 and 402 simultaneously inward toward the center of the elastic beam 400, or simultaneously outward toward the ends of the elastic beam 400. The natural resonant frequency is the lowest, and the excursions of the ends of the elastic beam 400 are greatest when the movable supporting

structures

401 and 402 are positioned near the center of the beam. Conversely, the resonant frequency is highest, and the excursions of the ends of the elastic beam 400 are shortest when the movable supporting

structures

401 and 402 are positioned near the ends of the beam 400.

The motions of the end s of elastic beam 400 are transmitted to the

acoustic pistons

411 and 412 by a connecting

linkage including links

413 and 414 which connect the ends of the elastic beam 400 to the crank

arms

415 and 416 which project from the shaft 417 which is preferably journalled at both ends in the stationary supporting structure as represented by

fragments

418 and 419. The motions of the ends of elastic beam 400 thus cause the shaft 417 to oscillate, in a rotational sense, about its longitudinal axis. The crank arm 421, which is mounted on shaft 417, transmits the motions of shafts 417 to link 422 which is connected to lever 423 which is, in turn, connected to link 424 which is connected to the connecting rod 425 of acoustic piston 411. The connecting rod 425 slides longitudinal ly within a bearing 426 which is mounted in the supporting structure 427. Similarly, the crank 421 transmits the motions of shaft 417 to link 432 which is connected to the leverarm 433 which is, in turn, connected to the link 434 which is connected to the connecting rod 435 of acoustic piston 412. The connecting rod 435 slides longitudinally within the bearing 436 which is mounted in supporting structure 437.

The elastic beam 400 is preferably driven at its center by a hydraulic actuator 440 which is controlled by a feedback control system of the type described in connection with the embodiments shown in FIGS. 2-10. It will be appreciated, however, that other types of actuators may be used within the spirit and scope of the present invention. It will further be appreciated that the elastic beam 400 might be actuated alternatively by two actuators forming parts of the movable supporting

structures

401 and 402 as described above in connection with the embodiment of FIGS. 8 and 9.

It will further be apparent to those skilled in the art that other modifications and adaptations of the present acoustic pulse generating apparatus may be made without departing from the spirit and scope of the invention as set forth with particularity in the appended claims.

What is claimed is:

1. An acoustic pulse generator for generating acoustic pulses in a medium which acts as a reactive load, each acoustic pulse comprising an acoustic wave having a frequency which varies over a predetermined range during the period of said pulse, said acoustic pulse generator comprising:

a mechanically resonant structure capable of storing mechanical energy in the form of oscillatory movements; transducer connected to said mechanically resonant structure, said transducer being in contact with said medium to convert the oscillatory movements of said mechanically resonant structure into acoustic waves in said medium;

drive means for imparting sufficient energy to said mechanically resonant structure to produce an acoustic pulse; and

means for changing the natural of oscillatory of said mechanically resonant structure over a predetermined range during the period of an acoustic pulse.

2. The acoustic pulse generator of claim 1 wherein said drive means comprises an actuator for imparting oscillatory motions to said mechanically resonant structure.

3. The acoustic pulse generator of claim 2 wherein said transducer comprises a piston connected to said mechanically resonant structure, the face of said piston being in contact with said medium to convert the oscillatory movements of said mechanically resonant structure into acoustic waves in said medium.

4. The acoustic pulse generator of claim 3 further comprising means responsive to the motions of said system for controlling the frequency of operation of said actuator.

5. The acoustic pulse generator of claim 1 wherein said mechanically resonant structure comprises a pneumatic spring system including an air cylinder, a piston disposed within said air cylinder so that when said piston is displaced from a neutral position, a differential pressure is created which tends to return said piston to said neutral position.

6. The acoustic pulse generator of claim 5 further comprising a beam pivotally connected to said piston of said pneumatic spring system so that the action of said piston is substantially transverse to said beam, said transducer and said drive means being connected to said beam at different points along the length thereof.

7. The acoustic pulse generator of claim 6 wherein said means for changing the natural frequency of oscillatory of said mechanically resonant structure comprises means for changing the ratio of the distance between the point of connection of said pneumatic spring system to said beam and the point of connection of said drive means to said beam, and the distance between the point of connection of said transducer and said beam and the point of connection of said drive means and said beam.

8. The acoustic pulse generator of claim 7 wherein said means for changing the natural frequency of oscillatory of said mechanically resonant structure comprises a ball screw mounted on said beam along a portion of the length thereof, a motor for driving said ball screw, and a cross-slide engaging said ball screw, said drive means being connected to said ball screw so that, upon operation of said motor, said cross-slide is caused to move along the length of said beam, thereby changing the point of application of said drive means to said beam.

9. The acoustic pulse generator of claim 1 wherein said mechanically resonant structure comprises an elastic beam capable of storing mechanical energy when elastically deformed and releasing said energy in the form of oscillatory movements.

10. The acoustic pulse generator of claim 9 wherein said transducer is connected by a mechanical linkage to a free end of said elastic beam.

11. The acoustic pulse generator of claim 10 wherein said drive means comprises an actuator for imparting oscillatory motions to said elastic beam.

12. The acoustic pulse generator of claim 11 wherein said means for changing the frequency of oscillatory of said elastic beam comprises a support member engaging said elastic beam, said support member being stationary in a plane perpendicular to the longitudinal axis of said elastic beam but movable along the length of said elastic beam to change the natural frequency of oscillatory thereof.

13. The acoustic pulse generator of claim 11 wherein said actuator engages said elastic beam at approximately right angles to the longitudinal axis thereof so as to impart bending oscillations thereto.

14. The acoustic pulse generator of claim 11 wherein said actuator engages a crank arm extending at right angles to the longitudinal axis of said elastic beam so as to impart torsional oscillatory to said elastic beam.

15. Acoustic pulse generating apparatus for generating acoustic pulses in water, each acoustic pulse comprising an acoustic wave having a frequency which varies over a predetermined range during the period of said pulse, said acoustic pulse generating apparatus comprising:

a mechanically resonant structure capable of storing mechanical energy in the form of oscillatory movements;

a pair of acoustic pistons connected to said mechanically resonant structure, the faces of said acoustic pistons being in contact with the water to convert the oscillatory movements of said mechanically resonant structure into acoustic waves in the water;

means for changing the natural frequency of oscillatory of said mechanically resonant structure over a predetermined range during the period of an acoustic pulse.

16. The acoustic pulse generating apparatus of claim 15 wherein said acoustic pistons are connected to travel in opposite directions, the axes of travel of said acoustic pistons being colinear so that there is no net force or torque on the apparatus as a result of the operation of said acoustic pistons.

17. The acoustic pulse generating apparatus of claim 16 further comprising a watertight capsule enclosing said mechanically resonant structure, said drive means, and said means for changing the natural frequency of oscillatory of said mechanically resonant structure so that said acoustic pulse generating apparatus may be operated at a depth below the surface of the water.

18. The acoustic pulse generating apparatus of claim 17 further comprising means for compensating the effect of static water pressure on said acoustic pistons during the generating of an acoustic pulse.

19. The acoustic pulse generating apparatus of claim 18 wherein said pressure compensating means comprises an air chamber associated with each of said acoustic pistons, the inner surface of each of said acoustic pistons comprising one wall of its associated air chamber, each of said air chambers containing air under pressure sufficient to balance the static pressure of the water on the outer faces of said acoustic pistons during the generating of an acoustic pulse.

20. The acoustic pulse generating apparatus of claim 19 wherein said means for storing energy in said mechanically resonant structure comprises a source of high pressure air and means for introducing said high pressure air into each of said chambers before the start of an acoustic pulse to move said acoustic pistons outwardly of their neutral positions.

21. The acoustic pulse generating apparatus of claim 20 wherein the pressure of the air provided by said source is sufficient to move said pistons to the outwardmost limits of their strokes.

22. The acoustic pulse generating apparatus of claim 20 wherein said means for releasing the energy stored in said mechanically resonant structure comprises means associated with each of said air chambers for quickly reducing the air pressure in said air chambers to a level which substantially balances the static pressure of the water on the outer faces of said pistons.

23. The acoustic pulse generating apparatus of claim 22 wherein said means for quickly reducing the air pressure in said air chambers comprises a quick release valve connected to each of said air chambers and an air tank connected to each of said quick release valves to receive air through said quick release valves from said air chambers.

24. The acoustic pulse generating apparatus of claim wherein said drive means comprises an hydraulic actuator for imparting oscillatory motions to said mechanically resonant structure.

25. The acoustic pulse generating apparatus of claim 24 further comprising control means responsive to the oscillations of said acoustic pistons for controlling said hydraulic actuator.

26. The acoustic pulse generating apparatus of claim 25 wherein said control means comprises motion sensing means for producing a signal in response to the motions of said acoustic pistons, delay means for delaying said signals from said motion sensing means, and a servo control valve for controlling said hydraulic actuator in response to signals from said delay means.

27. The acoustic pulse generating apparatus of claim 26 wherein said motion sensing means comprises acceleration sensing means mounted on said acoustic pistons for producing output signals corresponding to the acceleration of said acoustic pistons.

28. The acoustic pulse generating apparatus of claim 27 further comprising means responsive to the difference between the magnitude of said signal from said acceleration sensing means and a reference signal for controlling said servo valve to cause said actuator to apply greater force to said mechanically resonant structure when said signal from said acceleration sensing means is less than said reference signal, and to apply less force to said mechanically resonant structure when the magnitude of said signal from said acceleration sensing means is greater than said reference signal.

29. The acoustic pulse generator of claim 1, further comprising means for storing energy in said mechanically resonant structure prior to the start of an acoustic pulse, and means for releasing the energy stored in said mechanically resonant structure at the start of an acoustic pulse so as to cause said acoustic pulse to start at a high output power level.

Claims

1. An acoustic pulse generator for generating acoustic pulses in a medium which acts as a reactive load, each acoustic pulse comprising an acoustic wave having a frequency which varies over a predetermined range during the period of said pulse, said acoustic pulse generator comprising: a mechanically resonant structure capable of storing mechanical energy in the form of oscillatory movements; a transducer connected to said mechanically resonant structure, said transducer being in contact with said medium to convert the oscillatory movements of said mechanically resonant structure into acoustic waves in said medium; drive means for imparting sufficient energy to said mechanically resonant structure to produce an acoustic pulse; and means for changing the natural of oscillatory of said mechanically resonant structure over a predetermined range during the period of an acoustic pulse.

15. Acoustic pulse generating apparatus for generating acoustic pulses in water, each acoustic pulse comprising an acoustic wave having a frequency which varies over a predetermined range during the period of said pulse, said acoustic pulse generating apparatus comprising: a mechanically resonant structure capable of storing mechanical energy in the form of oscillatory movements; a pair of acoustic pistons connected to said mechanically resonant structure, the faces of said acoustic pistons being in contact with the water to convert the oscillatory movements of said mechanically resonant structure into acoustic waves in the water; drive means for imparting sufficient energy to said mechanically resonant structure to produce an acoustic pulse; and means for changing the natural frequency of oscillatory of said mechanically resonant structure over a predetermined range during the period of an acoustic pulse.

24. The acoustic pulse generating apparatus of claim 15 wherein said drive means comprises an hydraulic actuator for imparting oscillatory motions to said mechanically resonant structure.