US3695607A

US3695607A - Fluid driven pneumatic displays

Info

Publication number: US3695607A
Application number: US37382A
Authority: US
Inventors: Ronald Stouffer
Original assignee: Bowles Fluidics Corp
Current assignee: Bowles Fluidics Corp
Priority date: 1970-05-07
Filing date: 1970-05-07
Publication date: 1972-10-03
Anticipated expiration: 1989-10-03

Abstract

The apparatus comprises a display device comprising a hollow body having an air bubble trapped therein. The body is located in a tank substantially filled with liquid. Apparatus is provided for selectively pressurizing the tank to force water into the body through a flap valve to increase its weight and cause it to sink. Release of pressure permits the pressurized water in the body to bleed out through a rear vent pipe permitting the body to rise. By fluctuating the pressure, water exiting through the vent pipe propels the body at a speed determined by the rate of fluctuations. The level of the body is determined by the average pressure maintained in the tank and steering is effected by the duty cycle of the pressure fluctuations, a rudder tending to produce rotation of the body in one direction and water being expelled through the vent pipe tending to produce rotation of the body in the opposite direction.

Description

United States Patent 1151 3,695,607 Stouffer [45] Oct. 3, 1972 [54] FLUID DRIVEN PNEUMATIC 131,912 10/1920 Great Britain ..46/91 DISPLAYS 291,614 12/1931 Italy ..46/95 [72] Inventor: Ronald Stouffer, Silver Spring, Md. 454015 1950 Italy [73] Assignee: Bovvles Fluidics Corporation, Silver P i r Examiner--Anton O, oechsle p g Assistant Examiner-Arnold W. Kramer 2 Filed: May 7 1970 Attorney--Rose & Edell [21] Appl. No.1 37,382 57 ABSTRACT Related US. Application Data The apparatus comprises a display device comprising a hollow body having an air bubble trapped therein. [63] gg g sglggg 716378 March The body is located in a tank substantially filled with liquid. Apparatus is provided for selectively pressuriz- 52 us. c1 ..272/8 D, 46/94 46/95 hlg the tank fOrce water the through 40710625 flap valve to increase its weight and cause it to sink. 51 1111.01. ..A63h 23/06 Release of Pressure permits the vpressurized Water in [58] Field of Search ..272/8, 8 N, s D; 46/91, 94, the y to bleed out through rear vent p 4 95; 40 mitting the body to rise. By fluctuating the pressure, water exiting through the vent pipe propels the body [56] References Cited at a speed determined by the rate of fluctuations. The

level of the body is determined by the average pres- UNITED STATES PATENTS sure maintained in the tank and steering is effected by 2,509,112 5/1950 Seaman ..272/8 N the duty eyele of the pressure fluctuations, a rudder 2,525,232 10/1950 McGaughy ..272/8 R ux tending to Produce rotation of the y in one 1,596,934 8/1926 McI-Iugh et al. ..46/ direction and water being expelled through the vent 2,120,151 6/1938 Miller ..46/94 pip en ing o produce rotation of the body in the 3,071,375 1/1963 Moore ..272/8 pp s t direction. 3,382,606 5/1968 Johnson ..46/91 FOREIGN PATENTS OR APPLICATIONS Austria ,46/95 I I l l I I I I I I I l a I Inna/[m 6 Claims, 4 Drawing Figures P'AIENTEDncIs m2 IjNVENTOR RONALD STOUFFER ATTORNEYS FLUID DRIVEN PNEUMATIC DISPLAYS This application is a continuation of copending application Ser. No. 716,378, filed Mar. 27, 1968, and now abandoned.

The present invention relates to an apparatus which may be employed as an animated display simulation device or a toy and, more particularly, to a display device employing a hollow body which can be made to selectively dive or rise in a tank and may be selectively propelled either directly forward or with rotation to the right or left as desired. The device employs a movable body which may take the form of almost any freely movable body such as various types of vehicles or animals such as submarines, airplanes, space vehicles, fish, birds, etc.

In accordance with the present invention, a small hollow body or craf is provided with a flap or one-way valve. The body is located in a closed tank of liquid such as water. The valve permits water to enter the interior of the body but does not permit water to escape through the valve. When the craft is initially placed in the water, it has air therein and the weight of the craft causes water to enter the craft. The weight of the apparatus may be designed such that with the tank at ambient pressure an equilibrium condition is established with the craft floating on the surface of the water with a part of the hull submerged as with normal surface vessels. A jet pipe extends out of the back of the craft so that if and when water within the craft is pressurized, it may escape through the pipe.

In operation, by increasing the pressure in the tank, the air in the hollow body is compressed by water entering through the one-way valve until the pressure of the compressed air equalizes the pressure of the water. The water added to the craft is sufficient to cause the craft to seek a new level and, if the pressure developed interiorly of the vessel is sufficient, the craft sinks to the bottom. Propulsion of the craft can be effected by causing the pressure to fluctuate. Fluctuating pressure causes the craft to act as a pump since, when the tank is pressurized, water enters the craft and, when the pressure in the tank is reduced, water escapes through the tail pipe of the craft. By relatively rapidly fluctuating the pressure, the craft can be made to travel at a relatively interesting velocity at a level determined by the average pressure in the tank.

In one form of the invention, the tail pipe or jet pipe is angled outwardly from the back of the body in one direction and a rudder is angled in the other direction. If the duty cycle of the pressure fluctuation is large, then water is expelled from the body over only a short period of time and the effect of the rudder prevails. If the duty cycle is small, water is expelled from the craft over a long period of time (over half each pumping interval) and the direction control exerted by the jet pipe prevails.

The air bubble in the craft is analogous to a spring in its elasticity characteristics so that the craft may be made selective as to the frequency of the pressure fluctuations to which it will respond by choosing the size of the air bubble. The sensitivity of the system to control flow to and from the tank is determined by the surface area of the liquid in the tank exposed to variation in air pressure. As an example, if a piston is used to introduce and withdraw air from the system, a surface area ratio of 100 in the tank to one in the piston provides a system that is quite sensitive and easily controlled.

It is an object of the present invention to provide a display device or toy which can be made to dive or rise in a tank at will.

It is another object of the present invention to provide a display device or toy which can be made to dive or rise and/or can be propelled through a tank in which it is situated.

In accordance with still another object of the present invention, there is provided an animated display which can be steered by increasing and decreasing the fluid pressure in a tank in which the craft is situated, the direction of steering being determined by the rate of fluctuation of pressure.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is an illustration in elevation of an embodiment of apparatus of the invention; I

FIG. 2 is a bottom view of the vessel illustrated in FIG. 1;

FIG. 3 is a side view of a portion of the wall of the tank illustrated in FIG. 1 with a trigger mechanism for releasing a tank pressurizing apparatus; and

FIG. 4 is a diagrammatic illustration of a fluidic system for controlling operation of the vessel illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring specifically to FIG. 1 of the accompanying drawings, there is illustrated a tank or chamber 1 substantially filled with water to a level generally as indicated by the reference numeral v2. The interior of the chamber 1 is connected via a pipe or other means of connection 3 to a variable source of pressure which is illustrated for purposes of example only as a piston 4. Located within the tank 1 is a body 6 which may be initially selected to have one of a wide variety of forms such as an aircraft, a fish, a bird, a diver, a space vehicle, a heavenly body or any other appropriate form. In the example selected for discussion purposes only, the body is chosen as a submarine having a flap valve 7 located in the bottom thereof and having a hollow interior chamber generally designated by the reference numeral 8 which communicates through the valve 7 with the water in the surrounding tank. The interior 8 of the body 6 further communicates with the interior of the tank through a jet pipe 9 located at the back of the body. The body is provided with a rudder 11 although this is not essential. It will be noted that the tank 1 is provided with a removable cover 12 schematically illustrated as a screw-on type cover. The pump and connection assembly to the interior of the tank is illustrated as being connected through the top 12 but, of course, it may just as well be connected through any of the walls of the tank.

In operation, the top 12 of the tank 1 is removed; the device is substantially filled with water; and the body or craft 6 is placed therein. The craft may be made of a material such as rigid plastic or very lightweight metal such that it will normally float on the surface of the water in the tank. Of course, some water will enter through the flap valve 7 into the interior of the vessel but the amount will be minimal until the tank is placed under pressure. Continuing, the cover is placed on the tank 1 and the pressure is increased, moving the piston 4 toward the tank forcing air into the vessel. The air in the tank is compressed and increases the pressure in the water. The pressure of the water rises above the pressure of the air in the interior 8 of the body 6, water enters the vessel through the flap valve 7. The air bubble in the vessel is compressed to, for instance, a level indicated by the water level line 13. The addition of water to the interior of the vessel 6 increases the weight of the vessel and when the weight of the water and the weight of the vessel is greater than the weight of the water displaced thereby at the surface, the craft sinks to a level where the weight of displaced water is equal to the combined weight of water interiorally of the craft 6 and the material from which the craft is manufactured.

When the pressure in the tank is reduced by withdrawing the piston 4, for instance, the water in the vessel cannot escape through the flap valve 7 and therefore it must escape through the jet pipe 9. If the pressure is released gradually the vessel will simply rise; if the pressure is decreased rapidly the relatively rapid flow of water through the pipe 9 causes the vessel to travel in the direction opposite to the flow water through the pipe or as illustrated in FIG. 1, causes the vessel to travel from right to left. If the piston 4 is oscillated so that the pressure in the tank 1 is pulsed, rapid increases and decreases in pressure are produced and a relatively large flow of water passes through the jet pipe 9 so that the vessel moves at a relatively uniform rate through the water. The rate at which the vessel travels is determined by the quantity of water moving through the jet pipe per unit of time. Both the amplitude of the pressure fluctuations and the frequency of pressure variations affect water flow per unit of time. It is expected that usually the amplitude of fluctuations will be fixed and thus frequency will be used to control speed of the vessels.

From the above it is apparent that average pressure controls depth of the craft, and frequency controls its speed. However, design of the craft is also important. Vessels having different weights, for instance, seek different levels in the tank. In this respect the size of the cavity 8 is also of importance since the quantity of water which enters the vessel, and thus the weight change, is determined by the size of the cavity. Taking into account both weight and cavity size, a light craft with a small cavity requires a large pressure change to change levels whereas a heavy craft with a large cavity is very sensitive to pressure changes.

The size of the cavity also determines the frequency of pressure fluctuations to which the craft is particularly sensitive. An air bubble has an elastic characteristic and thus responds in a resonant manner to specific frequencies. The water flow in and out is much greater at the resonant frequency of the bubble than at any other frequency and thus craft may be selectively controlled by use of appropriate frequencies. Thus, if two vessels are in the tank, one with a small and one with a large air bubble, use of an appropriate high and an appropriate low frequency respectively will produce selective control of the vessels.

In this respect, one vessel may have two cavities of different sizes so that the two cavities respond to different frequencies. This concept is illustrated in FIG. 1 as applied to the faint-line position of the vessel 6 designated in this position by the reference numeral 6'. A solid partition 10 divides the interior of the vessel 6' into two cavities, a larger cavity 8 and a smaller cavity 8". Each cavity 8' and 8" communicates with the exterior of the vessel through distinct flap valves 7 and 7" and tubes 9 and 9 respectively.

It is apparent from the drawing that the cavity 8" is considerably smaller than the cavity 8'. Thus, the cavity 8" resonates at a higher frequency than the cavity 8' and the craft is propelled left to right in FIG. 1 if the cavity 8" is excited and is propelled from right to left if the cavity 8 is excited.

The tubes 9 and 9' may both extend from one end of the vessel but at different angles so as to effect steering by frequency selection rather than by duty cycle. Also, the passages may be positioned at right angles relative to one another so that one propels axially and the other rotates the vessel.

Referring now specifically to FIG. 2 of the accompanying drawings, if the rudder ll of the craft is set so that as the craft moves the rudder tends to produce steering in one direction and the jet pipe 9 is directed in the opposite direction from the rudder 11, control of direction of movement of the craft may be achieved. Specifically, if the reaction produced by water flowing through the pipe 9 is relatively small, then the steering action produced by the rudder 1 1 predominates and in the illustration of FIG. 2 the craft tends to rotate counterclockwise. However, if reaction to flow through the pipe 9 predominates the craft rotates or steers clockwise as illustrated in FIG. 2.

Whether the effect of the rudder 11 or jet pipe 9 predominates is determined by the average length of time liquid is expelled from 9. If the time interval of flow from the pipe 9 is sufficient, the effect of the pipe will predominate. The time interval of flow from the vessel is determined by several factors but only the duty cycle of the pressure fluctuations may be controlled once the system is put into operation. By the duty cycle of the pressure fluctuations is meant the ratio of the interval during which pressure is increased to the interval during which it is decreased. High duty cycle is when pressure increases for more than half the time and low duty cycle is the opposite. Therefore, in a high duty cycle system, the rudder effects will usually predominate since it is effective over half the time. In a low duty cycle system, water is expelled from pipe 9 for over half the time and the jet pipe effect tends to predominate.

In summary, the average pressure controls depth of the vessels, frequency controls speed, and may be used to select one craft relative to another and duty cycle controls direction of turning of a moving vessel.

It is apparent that if steering is not desired, the jet pipe 9 may be directed straight into the back of the craft and the rudder may be dispensed with or if desired for stabilization may lie along the main axis of the craft.

The apparatus has been described as employing a water-air system. Actually any combination of an incompressible and a compressible fluid may be employed so long as they are relatively immiscible or some other means; such as, a thin elastic membrane, is employed to maintain separation of the fluids. Further, the apparatus is described in terms of pressurizing the system. Actually, the system may be initially pressurized so that control is effected by reducing pressures. Thus, in FIG. 1, the piston 4 may be moved away from tank 1 to reduce pressure therein to cause vessels which normally lie on the bottom to rise to prescribed heights.

An application of the latter approach to operation of the system is illustrated in FIG. 3 of the accompanying drawings. The figure actually illustrates a partial horizontal section through the tank 1 to which is applied a device consisting of a piston 16 located in a passage 17 in communication with the interior of a tank 1. The piston 16 may be withdrawn; that is, moved out wardly away from the interior of the vessel 1 by means of a handle 18 operating against a compression spring 19. A latch arrangement 21 is provided for holding the piston 16 in its retracted position.

The operator manipulates the piston 4 to cause the vessel to rise to the height of the latch mechanism 21 and operates the piston at a duty cycle such as to steer the vessel to the latch 21 from the direction indicated by the arrow 22. If the vessel is made to contact the portion of the latch 21 extending into the interior of the vessel, then the latch 21 is withdrawn from in front of the piston 16 and the piston 16 moves toward the interior of the vessel 1 under the force supplied by the compression spring 19. This action increases the pressure in the tank and causes the vessel to fall to the bottom indicating a hit. The interior pressure of the vessel is initially such that when the piston 16 is in its forwardmost or leftmost position, the interior of the vessel 1 is pressurized sufficiently to cause the vessel to be at the bottom and when the piston 16 is withdrawn to the position illustrated in FIG. 3, the interior pressure of the vessel is such that the piston 4 may readily select the height at which the vessel is to operate.

Thus, the apparatus of FIG. 3, if incorporated in the vessel 1, provides a child playing with the toy an opportunity to practice and assess his skill at manipulating the vessel and also if two are playing the game may per mit an enemy to capture a vessel and cause it to be sunk by tripping the latch 21. Two latches may be provided each increasing pressure a different amount. If two vessels are of different bouyancies then releasing one latch may cause only one vessel to sink while releasing both latches may cause both to sink.

As an additional feature, the latch 21 may be employed to release a depth charge which could be a magnet. If the magnet contacts the vessel which in this case would have to contain a high permeability material, the magnet adheres to the vessel and causes it to sink.

The piston 4 may be replaced by a bellows and either the piston or the bellows may be driven by a variable speed, variable stroke drive. Relatively complex linkages would be necessary to provide variable duty cycle and, although such are known and could be provided, the better approach would appear to be to employ fluidic controls throughout thus permitting ready control of the various factors involved in the system.

Referring now specifically to FIG. 4 of the accompanying drawing, there is illustrated a fluidic system which may be employed to control the apparatus of FIG. 1. Specifically, the apparatus comprises a variable frequency oscillator 23 which may be of the type illustrated in Warren US. Pat. No. 3, l 58,166. The variable frequency oscillator 23 is connected to supply periodic pulses to a pulse width modulator generally designated by the reference numeral 24. The pulse width modulator may be of the type illustrated in Warren et al. U.S. Pat. No. 3,228,410 particularly as illustrated in FIG. 1. The oscillator of FIG. 1 of the Warren et al. patent is replaced with the variable frequency oscillator of the aforesaid Warren patent. Obviously, any of the variable frequency oscillators of the Warren patent or other known fluidic oscillators may be employed with any of the various pulse width modulation schemes of the Warren et al. patent.

The pulse width modulator does not change appreciably the basic frequency supplied by the oscillator 23 but does vary the duty cycle of the pulses supplied to

output passages

26 and 27 of the modulator. The

passages

26 and 27 are connected to supply signals to the control nozzles 28 and 29 of a pure fluid analog amplifier 31. An output passage 32 of the amplifier 31 is connected to the passage 3 supplying air to the tank 1 of FIG. 1. Bias control signals for the pulse width modulator are applied through passages 33 and 34 and bias signals are applied to control nozzles 28 and 29 via passages 36 and 37 respectively.

In operation, the frequency of the oscillator 23 may be varied to determine the speed of the craft. The bias signals supplied via passages 33 and 34 to the modulator 24 determine the duty cycle of the fluid supplied to the tank 1 and thus determine the turning or direction of movement of the craft. By controlling the fluid supplied to the passages 36 and 37 the quiescent position of the power stream of the amplifier 31 is adjusted to determine the average dc level of fluid supplied to the passage 3. Thus, with the various sets of adjustment an operator may control speed, direction of movement and level of the craft in the tank. Where frequency selective type vessels are used, the variable frequency oscillator may have its frequency varied to determine the particular craft to be manipulated. Actual control of the oscillator is by control of a pair of valves 38 and 39 which control flow of fluid from the feedback path of the fluid amplifier and thus,as explained in the Warren patent, control its frequency of oscillation.

As indicated in the introduction of the application, the apparatus of the present invention may be employed as a toy or may be employed for display purposes. If it is desired to independently and concurrently control two or any number of vessels in a tank any number of apparatus as illustrated in FIG. 4 may be employed although it is to be pointed out that the interaction between the devices would tend to produce an average internal pressure and also disrupt operation except where frequency selective vessels are employed.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variation of the details of construction which are specifically illustrated and described may be resorted to without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. A pneumatic device comprising:

a closed tank containing an incompressible liquid;

a body adapted to be immersed in said tank and having a predetermined axis, an interior cavity in said body containing a compressible fluid, a one-way valve for admitting said liquid into said cavity when the pressure of said liquid in the vicinity of said body is greater than the pressure in said cavity, a fluid passage communicating between said cavity and the exterior of said body, said fluid passage having an impedance to flow therethrough which is large relative to the impedance of said one-way valve to inflow of said liquid when the pressure of said liquid in the vicinity of said body exceeds the pressure in said cavity, whereby substantially all liquid inflow to said cavity is via said one-way valve and substantially all liquid outflow from said cavity is via said fluid passage, said fluid passage being oriented such that liquid outflow from said cavity has flow components parallel and perpendicular to said predetermined axis, and a rudder member extending from said body for producing a turning force to turn said body relative to said predetermined axis whenever said body has a component of momentum along said axis and no liquid is flowing out of said cavity via said fluid passage;

said valve and said fluid passage being located below the top of said cavity;

control means for cyclically varying the pressure of said liquid in said tank at a frequency which is sufficiently great to propel said body in a direction opposite the direction of outflow through said fluid passage;

wherein said perpendicular flow component from said fluid passage is oriented to provide a turning force on said body which is directed oppositely to the turning force produced by said rudder member; and

additional means for selectively varying the duty cycle of the cyclically varying liquid pressure variations provided by said control means;

wherein said control means comprises a fluidic oscillator for providing a train of fluid pulses and additional means for selectively varying the frequency of said fluid pulses, and wherein said additional means comprises a fluidic pulse width modulator responsive to said fluid pulses from said fluidic oscillator for selectively varying the duration of said pulses and applying said pulses to said tank to cyclically vary the pressure of said liquid.

2. A pneumatic display device comprising:

a tank adapted to be closed and to contain an incompressible liquid;

a body adapted to be immersed in said tank and having an interior cavity;

means for dividing said interior cavity into first and second fluid-isolated sub-cavities each adapted to contain a compressible fluid;

first and second fluid passages extending into said tank from said first and second sub-cavities, respectively;

said first sub-cavity being substantially larger than said second sub-cavity, whereby said first sub-cavity has a first resonant frequency different from a second resonant frequency possessed by said second sub-cavity,

said fluid passages being located below the top of said sub-cavities; and

control means for cyclically varying the pressure in said tank over a range of frequencies including the resonant frequencies of said sub-cavities, wherein said control means comprises a fluidic oscillator of the type wherein a stream of pressurized fluid is cyclically deflected relative to at least one outlet passage by at least one relatively low pressure fluid stream, said outlet passage being arranged to issue stream into said tank.

3. The device according to claim 2 wherein said first and second fluid passages comprise first and second tubes, respectively, extending from said first and second sub-cavities, respectively, and wherein said control means further includes: means for at will varying the duty cycle of said cyclically varying pressure.

4.. A pneumatic display device, comprising:

a tank adapted to be closed and to contain an incompressible liquid;

a body adapted to be immersed in said tank, said body having first and second interior cavities each adapted to contain a compressible fluid, means for permitting entry of said liquid into said cavities in response to increasing pressure in said tank, and for expelling said liquid from said cavities in a predetermined direction in response to decreasing pressure in said tank; and

control means for translating said body through said liquid in a direction generally opposite said predetermined direction by cyclically varying the pressure in said tank,

said control means including: a fluid passage in pressure communication with said tank; means for issuing a stream of pressurized fluid generally toward said fluid passage; and means for cyclically deflecting said stream relative to said fluid passage.

5. The device according to claim 4 wherein said cavities have different volumes and different resonant frequencies, and wherein said control means includes means for at will varying the frequency of said cyclically varying pressure, said frequency being variable over a continuous frequency range which includes the resonant frequencies of said cavities.

6. A pneumatic device comprising:

a tank adapted to be closed for containing an incompressible liquid;

control means for cyclically varying the pressure of said liquid in said tank; and

a body adapted for insertion into said tank and having an internal cavity adapted to contain a compressible fluid and means for admitting said liquid into said cavity when the liquid surrounding said body is at a pressure in excess of the pressure in said cavity and expelling said liquid from said cavity when the pressure of said liquid in the vicinity of said body exceeds the pressure in said cavity,

wherein said control means comprises a fluidic oscillator including: a fluid passage; means for issuing a fluid stream generally toward said fluid passage; means for cyclically deflecting said fluid stream relative to said fluid passage; and means for pressurizing the liquid in said tank as a function of the pressure in said fluid passage.

Claims

1. A pneumatic device comprising: a closed tank containing an incompressible liquid; a body adapted to be immersed in said tank and having a predetermined axis, an interior cavity in said body containing a compressible fluid, a one-way valve for admitting said liquid into said cavity when the pressure of said liquid in the vicinity of said body is greater than the pressure in said cavity, a fluid passage communicating between said cavity and the exterior of said body, said fluid passage having an impedance to flow therethrough which is large relative to the impedance of said one-way valve to inflow of said liquid when the pressure of said liquid in the vicinity of said body exceeds the pressure in said cavity, whereby substantially all liquid inflow to said cavity is via said one-way valve and substantially all liquid outflow from said cavity is via said fluid passage, said fluid passage being oriented such that liquid outflow from said cavity has flow components parallel and perpendicular to said predetermined axis, and a rudder member extending from said body for producing a turning force to turn said body relative to said predetermined axis whenever said body has a component of momentum along said axis and no liquid is flowing out of said cavity via said fluid passage; said valve and said fluid passage being located below the top of said cavity; control means for cyclically varying the pressure of said liquid in said tank at a frequency which is sufficiently great to propel said body in a direction opposite the direction of outflow through said fluid passage; wherein said perpendicular flow component from said fluid passage is oriented to provide a turning force on said body which is directed oppositely to the turning force produced by said rudder member; and additional means for selectively varying the duty cycle of the cyclically varying liquid pressure variations provided by said control means; wherein said control means comprises a fluidic oscillator for providing a train of fluid pulses and additional means for selectively varying the frequency of said fluid pulses, and wherein said additional means comprises a fluidic pulse width modulator responsive to said fluid pulses from said fluidic oscillator for selectively varying the duration of said pulses and applying said pulses to said tank to cyclically vary the pressure of said liquid.

2. A pneumatic display device comprising: a tank adapted to be closed and to contain an incompressible liquid; a body adapted to be immersed in said tank and having an interior cavity; means for dividing said interior cavity into first and second fluid-isolated sub-cavities each adapted to contain a compressible fluid; first and second fluid passages extending into said tank from said first and second sub-cavities, respectively; said first sub-cavity being substantially larger than said second sub-cavity, whereby said first sub-cavity has a first resonant frequency different from a second resonant frequency possessed by said second sub-cavity, said fluid passages being located below the top of said sub-cavities; and control means for cyclically varying the pressure in said tank over a range of frequencies including the resonant frequencies of said sub-cavities, wherein said control means comprises a fluidic oscillator of the type wherein a stream of pressurized fluid is cyclically deflected relative to at least one outlet passage by at least one relatively low pressure fluid stream, said outlet passage being arranged to issue stream into said tank.

4. A pneumatic display device, comprising: a tank adapted to be closed and to contain an incompressible liquid; a body adapted to be immersed in said tank, said body having first and second interior cavities each adapted to contain a compressible fluid, means for permitting entry of said liquid into said cavities in response to increasing pressure in said tank, and for expelling said liquid from said cavities in a predetermined direction in response to decreasing pressure in said tank; and control means for translating said body through said liquid in a direction generally opposite said predetermined direction by cyclically varying the pressure in said tank, said control means including: a fluid passage in pressure communication with said tank; means for issuing a stream of pressurized fluid generally toward said fluid passage; and means for cyclically deflecting said stream relative to said fluid passage.

6. A pneumatic device comprising: a tank adapted to be closed for containing an incompressible liquid; control means for cyclically varying the pressure of said liquid in said tank; and a body adapted for insertion into said tank and having an internal cavity adapted to contain a compressible fluid and means for admitting said liquid into said cavity when the liquid surrounding said body is at a pressure in excess of the pressure in said cavity and expelling said liquid from said cavity when the pressure of said liquid in the vicinity of said body exceeds the pressure in said cavity, wherein said controL means comprises a fluidic oscillator including: a fluid passage; means for issuing a fluid stream generally toward said fluid passage; means for cyclically deflecting said fluid stream relative to said fluid passage; and means for pressurizing the liquid in said tank as a function of the pressure in said fluid passage.