US2602327A - Transducing system - Google Patents

Description

y 3, 1952 'w.- L. BOND TRANSDUCING SYSTEM Filed July 2, 1946 FIG. I

RELAXATDN FRA'OUCNCY FREQUENCY \rtsat wvawmp W L. BOND ATTORNEY Patented July 8, i952 UNITED STATES PATENT OFFICE Telephone Laboratories, Incorporated,

New

York, N. Y., a corporation of New York Application July 2, 1946, Serial No. 680,965

9 Claims. I

This invention relates in general to vibrating systems. and in particular to electro-acoustic vibrating systems which include contact media having frequency selective properties.

The primary object of this invention is to facilitate the transfer of elastic vibrations from one element to another in an acoustic system.

In accordance with the invention, acoustic contact is maintained between the cooperating elements of a vibrating system by means of a mass of substance interposed between such elements which has the property of conforming to the shapes of the respective contact surfaces in response to slow variations in the applied pressure and responding elastically to rapid variations in pressure. 'The slow variations in pressure are of such a nature as would occur in the ordinary process of maintaining two surfaces in contact; while the rapid variations in pressure are of the nature of applied sonic or supersonic vibrations. A preferred embodiment of such contact medium comprises a material known as bouncing putty which is described as a dimethyl silicone reaction product, and which is disclosed and claimed in application Serial No. 569,647, now Patent 2,541,851, entitled Composition of Matter" filed on December 23, 1944, in the name of James G. E. Wright.

The above described contact medium has certain important advantages over the contact media of the prior art, in that it is frequencyselective, transmitting only frequencies above a predetermined critical frequency range and suppressing frequencies below such range. It is easily applied, easily removed, and maintains eiiicient contact between any two surfaces, including those of odd or non-conforming shapes. Furthermore, it does not appreciably penetrate and thereby change the vibrational character of the contact surfaces. Moreover, vibrations transmitted in accordance with this invention sustain much lower energy losses than is usual in prior art systems utilizing oily or waxy contact agent in the conventional manner.

For the purpose of illustration, the present invention will be described as embodied in an acoustic system for detecting flaws in propellant explosive grains comprising cylinders of the order of 8 inches in length. Such a testing system comprises the following cooperating parts: a test propellant grain disposed on a support, a piezoelectric vibrations generator centrally located with respect to the ends of the cylindrical grain and acoustically coupled thereto by means of a mass of "bouncing putty or similar material interposed between the contacting surfaces, two piezoelectric vibration responsive devices symmetrically disposed with respect to the vibrations generator and contacting the grain by means of attached steel rollers. and a cathode-ray oscilloscope having horizontal and vertical deflection plates respectively connected to the piezoelectric responsive devices.

My invention will be better understood after a study of the detailed specification as set forth hereinafter and the attached drawings, of which:

Fig. 1 shows rigidity versus frequency characteristic for a typical acoustic contact material in accordance with the present invention;

Fig. 2 shows an enlarged view of an acoustic contact agent of the present invention interposed between the contact surface of a piezoelectric unit for generating or detecting elastic vibrations and the surface of a vibrating body; and

Fig. 3 shows a piezoelectric system for the acoustic testing of propellant explosive grains in which a piezoelectric vibrations generator is coupled to the test surface in a manner prescribed by the present invention and shown in the detailed drawing of Fig. 2.

One of the most difficult problems encountered in the eificient design of electro-acoustic systems is that of maintaining a constant and reproducible contact with the vibrating surface through which the vibrational energy can flow without excessive losses. In applying piezoelectric units to test surfaces for the purposes of generating or detecting vibrations, it has been found that the vibrational response varies over a wide range with relatively small changes in the pressure of application. Such a contact between respective vibrating surfaces actually takes place at a number of points, each of which consists of a small area, usually of microscopic dimensions, at which the material of the contacting bodies is elastically or plastically deformed. Up to a certain point, which depends on the materials and surface contours, this contact area increases rapidly with pressure and then less rapidly as the pressure is further increased.

The need for high and constant contact pressures in order to eliminate variations in vibration transmission between the elements of an electroacoustic system is largely obviated by the use of a contact medium that has a putty-like consistency to touch but behaves in the manner of a solid to quick deformations. Such a material is easily placed in position, and at frequencies in the audio and super-audio ranges behaves like a rigid connection of large area.

As hereinbefore stated, a preferred embodiment of such contact medium is a substance known as bouncing putty which is described and claimed in application Serial No. 569,647, supra. The substance there disclosed is a putty-like, elastic, plastic composition comprising a dimethyl silicone reaction product, more particularly a heat reaction product of a dimethyl silicone oil and a boron compound, specifically a pyroboric acid.

Another substance which serves suitably as a contact medium in accordance with this invention is a sodium silicate NazSiO; described in an article entitled Bounces Like Rubber appearing on page 199 of the Science News Letter of September 29, 1945. This substance is made from one of the highly silicious silicates from which water has been evaporated until it composes only about 65 per cent of the solution.

The teachings of the present invention are not limited to the two specific agents mentioned. but are equally applicable to other materials having similar properties.

In order for a material to be so characterized that it responds plastically to slowly applied stresses, and in the manner of a solid body to rapidly applied stresses, such as sonic or supersonic vibrations, it should preferably exhibit certain well-defined changes in rigidity with progressive changes in the frequency of impressed elastic vibrations.

For the purposes of this specification and the attached claims, rigidity may be defined as that property of a body by which it resists change in shape. The rigidity of a body is measured by the ratio of the applied tangential distorting stress to the distortion it produces.

Referring to Fig. 1, a substance such as bouncing putty," which is suitable for the uses of this invention, has a rigidity vs. frequency characteristic which assumes a relatively low, nearly constant value over the lower range of frequencies. When a certain critical range of frequencies is reached, the characteristic rises rather sharply to a relatively high saturation value which remains nearly constant for further increases in frequency.

That frequency at which the rigidity of a particular substance reaches half its ultimate value will be defined for the purposes of this specification and the attached claims a the relaxation frequency of such substance.

In one aspect, this invention comprises the use of a thin layer of material having a relatively low relaxation frequency as a vibrational contact agent in an acoustic system which is vibrating at a frequency or range of frequencies substantially above the relaxation frequency of such agent. All liquids have relaxation frequencie in accordance with the above definition; but in most cases, such frequencies are beyond the range of usefulness for th purposes of this invention. Although oils and similar materials have frequently been used in prior art systems as acoustic contact agents, such use has been at frequencies of vibration substantially below their respective relaxation frequencies. A significant feature of my discovery is that above their relaxation frequencies, such contact mediabehave in the manner of solids, while below their relaxation frequencies, they behave in the manner of liquids.

The application as a contact agent in an electroacoustic system of a material of the nature described in the foregoing paragraphs is shown in Figs. 2 and 3 of the drawings.

Fig. 2 shows a piezoelectric unit I, which may Ill) constitute an element of either a vibration generator or detector, maintained in contact with the surface of a vibrating test member 2 by means of an interposed mass 3 of bouncing putty or such similar acoustic contact medium as hereinbefore described. For optimum performance in the presently described system, a putty mass 3 about the size and shape of a pea is initially interposed between the contact surfaces. A pressure of from ten to twenty pounds per square inch is then applied for a period of about a minute in order to flatten the contact element 3 into a pancake shape having an approximate thickness of M of an inch-and an approximate diameter of of an inch. Once properly applied, the putty mass 3 acts in the manner of a glue in holding the piezoelectric unit I in contact with the surface 2, so that only enough pressure is required between the contacting surfaces to prevent the putty mass 3 from becoming dislodged during vibration. For this purpose, the weight of the piezoelectric unit I resting on the surface 2 is usually sufficient. Alternatively, the mass 3 may be shaped before application to the test surface 2.

Because of the nature of the putty contact element 3, the system is operable under a wide range of contact pressures. However, it is apparent that too great contact pressure will cause.

the contact layer 3 to spread too thin for satisfactory operation.

The piezoelectric unit I, which is used for the purposes of illustration, comprises a group of 45-degree Z-cut crystals 4, of ammonium dihydrogen phosphate, each provided with evaporated gold electrodes on both faces, and all cemented together to form a prism 1 inch square by 2.5 inches long. Odd and even electrodes are respectively connected to the terminals 5 and 6 comprising thin pieces of gold-plated nickelsilver on opposite ends of the prism, whereby the respective crystal plates 4 are connected in parallel to form a condenser of approximately 675 micro-microfarads capacitance.

When used in a fixed position as a vibrations generator or detector, the group of crystals 4, is first cemented to a thin ceramic late I, which is in turn cemented to an iron prism 8 which is 1.9 inches long and of the same cross-section as the crystal group and which serves as a high impedance to the vibrations of the crystal unit I. Integral with the prism 8 at the cemented end is a flange 9 which serves as a supporting member whereby the unit I may be attached to a carriage for maintaining it in contact wtih the surface 2.

The surface of the unit I, which is brought to bear on the test surface 2 through the contact medium 3, is surmounted by an iron shoe I0, 1 inch square by A-inch thick, cemented thereto. The entire crystal unit is then surrounded with a thin loose sheet of metal foil for electrostatic shielding. When in-firm contact with a nonresonant solid, the main longitudinal resonance of the unit I is about 17 kilocycles.

If merely clamped onto the surface 2 of the piece to be oscillated, as in prior art practice, the piezoelectric unit I, which oscillates by elongation and contraction along its length thousands of times per second, would push but not pull the test surface 2, thereby dissipating a large proportion of the vibrational energy. However, by means of the putty contact 3, the piezoelectric unit I is enabled to both push and pull, effectually oscillating as part of the test element, and thereby transmitting a much higher percentage of vibrational energy. Efiicient vibrational cog-r tact is maintained by the mass 3, notwithstanding the shapes of the contacting s1 irfaces of the piezoelectric unit I and the test'element 2 which may be odd or non-conforming.

A piezoelectric firfit' coupled to a test surface in the manner described in the preceding paragraphs with reference to Fig. 2 may be incorporated in different types of electro-acoustic com; binations. However, for the purposes of illustration, the piezoelectric unit I and the interposed contact medium 3 will be described as functioning parts of a vibration generator in a system for locating flaws in large propellant explosive grains, such as shown in Fig. 3 of the drawings. The typical acoustical test system of Fig. 3 and the component elements thereof are used merely for the purposes of illustrating the present invention; and it will be apparent to those skilled in the art that the teachings of this invention are equally applicable to acoustic systems of totally different construction and function.

In the process of manufacture of large propellant grains, which comprise a mixture of pulverized crystalline materials compressed hydraulically with a small amount of binder, flaws occasionally develop which result in dangerously high pressures arising during combustion in a motor chamber. The function of the apparatus shown in Fig. 3 is to locate such flaws by impressing sonic frequencies of from 2 to 40 kilocycles on the test grain, and observing irregularities in the vibrational pattern of the grain by means of a pair of symmetrically located compressional wave responsive devices which impress their respective outputs on the deflection plates of a cathode-ray oscilloscope.

A standard test grain II of the composition described, cylindrical in shape with a length of 8 inches and a diameter of 7 inches, has an axial perforation extending from one end to the other thereof, into which is inserted a supporting member I2 comprising a length of 2-inch machined iron pipe 2 which is rigidly supported in a horizontal position by means of an angle bracket I3 attached to a heavy wooden base It. Ifhe pipe I2 is provided with two perforated 3-inch tapered rubber stoppers Ia and I51) which are forced into the grain perforation from either side thereby providing a rubber vibration insulation so that the supporting structure does not appreciably interfere with the free vibrations of the grain II. The perforated stoppers I5a and I5b are shrunk into collars 16a and IE1), respectively, which fit over the pipe I2 forming bearings adapted to rotate and slide thereon. thus enabling the grain II to be rotated to different angular positions about the pipe I2.

The grain II is driven to vibrate elastically by means of the piezoelectric vibration unit I,

which is similar in construction to the piezoelectric vibration unit I described hereinbefore with reference to Fig. 2. Vibrational contact is maintained between the surface of the test grain II and the unit I by means of a mass of material 3' which may comprise bouncing putty or such other suitable acoustic contact material as hereinbefore described, and which is similar in size and shape to the mass 3 described with reference to Fig. 2.

The vibration generating unit I is supported in a central position with respect to the two ends of the grain II by means of a clamp attachment between the flange 9' and the bracket I1, which is rigidly attached to the collar I6a riding on the A- P 6 rod so that the watirsnaitdllmane rotated with the grain l l During ordinary conllififihs bf operation, the bracket I1 is preferably positioned so that the piezoelectric generating unit I is pressed against the surface of the test grain II with a force equal to its own weight. For the purposes of initial application and shaping of the contact element 3', somewhat greater pressures may be desired, as hereinbefore described, in which case suitable weights may be hung on the bracket I1, and removed once the contact has been secured.

The generating unit I is energized to vibrate 7 piezoelectrically by means of connections througli the electrode terminals 5 and its mate (not shown) to a circuit which includes a conventional oscillations generator I3 and the amplifier I9.

The longitudinal elastic vibrations which are induced inthegrain II through the contacting mass 3 by means of the centrally located generating unit I and its associated circuit are detected by vibration responsive units 20 and 2| which are symmetrically positioned with respect to the ends of the grain I I and the vibrations generating unit I.

The vibration responsive units 20,, a n d Zly single crowned steel rg ller h aving a diameter of V inch. and of an inch wide, and with its axis disposed parallel to the axis of the grain II, so that it moves over the surface as the grain rotates.

The units 20 and 2| are held in place by means of clamp attachments to the respective arms 24 and 25 rotatably disposed on the horizontal shaft 26 which is rigidly connected to the support I3. Thus, it is possible to bring the crystal units 20 and 2I in contact with the surface of the grain I I in a definite way which can be duplicated.

Cracks or non-homogeneities in the grain II produce dis 'rifiIa' s iii'tli"vibratiohal Ye fifasi'isiijiniififiia-znfa izfrrfi fi fi" w compared on a recording instrument such as the cathode-ray oscilloscope 21. The output of the left-hand responsive unit 20 is connected through its electrode terminals to the horizontal deflecting plates of the oscilloscope 21; while the output of the right-hand responsive unit 2| is connected through its respective electrode terminals to the vertical deflection plates 29 of the oscilloscope 21. If the grain is homogeneous, and the units 20 and 2| symmetrically positioned, their outputs will be equal, and will therefore produce a diagonal straight-line pattern on the luminous screen of the oscilloscope 21. If the grain is non-homogeneous, the elliptical pattern traced on the screen of the oscilloscope 21 will have coordinates indicative of the size and position of the discontinuity.

While the positioning of the recording units 20 and 2| on the curved surface of the grain II is not critical, excepting for the stipulation that they be symmetrically placed with respect to the ends thereof and to the vibration unit I, it has been found that optimum recordings are obtained when the generating device I is rotated through a vertical angle of degrees with respect to the responsive devices 20 and 2|.

In a system in which the responsive devices 20 and 2I are to be held stationary, rather than brational energy is transmitted ,from the test grain to the respective responsive devices 20 and 2| with a minimum of loss.

What is claimed is:

1. A system comprising a plurality of elastically \x \vihrator elements includifigavibiation generatcnreo'fiiiied in vibration transmitting relation to each other, two of said elements being coupled 'to each other by a contact agent comprising a putty-like mass having the property of conforming to the shapes of the contact surfaces of said elements in response to slowly applied stresses and reacting elastically in response to rapid variations in stress induced by the vibrations from said generator.

2. In a system in accordance with claim 1, said contact agent comprising a dimethyl silicone reaction product specifically a pyroboric acid.

BTAsystemcomprising in combination a vibrations generator, an elastic body, and interposed between said generator and said body a putty-like mass having the property of conforming to the shapes of the contact surfaces of said generator and said body in response to slowly applied stresses and reacting elastically in response to rapid variations in stress.

4. A system in accordance with claim 3 in which said mass comprises a dimethyl silicone reaction product specifically a pyroboric acid.

5. A system comprising in. combination an elastically vibrating body, means responsive to the vibrations of said body, and a contact agent interposed between said body and said responsive means, said contact agent comprising a puttylike mass having the property in response to slow variations in stress of conforming to the shapes of the contact surfaces of said body and said responsive means, and said mass having the property of reacting elastically in response to rapid variations in stress.

6. In a vibratory system, a contact agent for acoustically coupling the elements of said system interposed between said respective elements, said a ahnuwaw u contact agent having a relaxation frequency substantially below the minimum frequency of said vibratory systeru.

'7. A system in accordance with claim 6 in which said contact agent comprises a dimethyl silicone reaction prodract, spec-ifidally a pyroboric acid.

8. in a system vibrating longitudinally within a given frequency range, said system comprising at least two elements, a mass of material interposed between and contacting said elements, said interposed mass having a relatively high rigidity in response to impressed frequencies above a certain critical range of frequencies and a relatively low rigidity in response to impressed frequencies below said critical range of frequencies, wherein said critical range is below the range of vibration of said system.

9. A system comprising a plurality of vibratory elements in vibration transmitting relation, wherein one of said elements is connected in vibration driving relation to another of said elements through a contact. agent comprising a putty-like mass having the property of conforming to the shape of the contact surfaces of said elements in response toslowly applied stresses below a critical range of frequency and reacting elastically in response to rapid variations in stress above said critical range of frequency, wherein the vibrations applied by said driving element include a frequency above the said critical frequency range of said contact agent.

WALTER L. BOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,280,226 Firestone Apr. 21, 1942 2,431,878 McGregor Dec. 2, 1947 2,458,581 Firestone et al. Jan. 11, 1949 OTHER REFERENCES Publication: Rubber Age, November 1944,

pages 1'73, 1'74 and 175.

Publication: The Oil and Gas Journal," October 6, 1945, pages 86, 8'7 and 88.

Images