US2925582A - Acoustical firing indicator - Google Patents

Description

Feb. 16, 1960 Filed Feb. 18, 1957 J. I. MATTE] ETAL ACOUSTICAL FIRING INDICATOR FIG. 1

4 Sheets-Sheet 1 IITTOF/VEY Feb. 16, 1960 J. I. MATTEl ETAL ACOUSTICAL FIRING INDICATOR Filed m. 18, 1957 4 Sheets-Sheet 2 Q 9 5 3 ,2, E 6' '2', A,

3 ll 3 I 11 12 FIG.7 FIG.8

FIGA F165 INVENTOR JEANLMATTE/ JEAN BLA/S'E RENE MA/VGA NNE ATTORNEY Feb. 16, 1960 J. l. MATTEI ETAL 2,925,582

ACOUSTICAL FIRING INDICATOR Filed Feb. 18, 1957 4 Sheets-Sheet 3 FIGJO F16-" JEA/VZ MATI'E/ c/E/M/ 5L 4 I35 PEA/E MANG/M/NE ATTORNEY Feb. 16, 1960 J- l. MATTEI ETAL 2,925,582

ACOUSTICAL FIRING INDICATOR Filed Feb. 13, 1957 4 Sheets-Sheet 4 FIG-12 pa 2Q //VVf/V7'0?$ dEA/VI. M47767 JAN BLA/SE yf/VE MANGA/VA/E By ArroE/VEY United States Patent O ACOUSTICAL FIRING INDICATOR Jean I. Mattei, Vincennes, Jean Blaise, Paris, and Ren Manganne, Chatillon-sous-Bagneux, France, assignors to Ofiice National dEtudes et de Recherches Aeronautiques, Chatillon-sous-Bagneux, France, a French The present invention deals with an'air-air or airground firing indicator.

When a firing exercise has been carried out on a real target, the benefit derived therefrom is increased according to the amount of information gained about the respective positions of the target and of the projectiles that have been launched toward it. It is desirable to know the paths ofprojectiles that did not hit the target and which accordingly did not leave any trace on it.

The object of the present invention is a device which registers the target area, either in an instantaneous and approximate manner visible to the firing operator, or in a more exact fashion on a recording apparatus which may be inspected after firing, or in both ways.

This invention is applied to firing with supersonic projectiles.

It is known that an object moving in the atmosphere at a supersonic speed creates a disturbance in the form of a ballistic shock wave. The wave, in general, has the following structure:'

The atmosphere rises sharply from a static pressure P to a higher pressure, Pd-I-p. Then the pressure decreases linearly, both in time and space, to P5-p 'and returhs sharply to P This structurecommonlyjiscalled a shock N-wave in the case of small projectiles;the complex' disturbance may involve three or four pressure jumps. The amplitude of the pressure jump and the interval, separating the straight edges depend on the characteristics of the object, its caliber and speed, and on the normal distance of the trajectory from the point of detection. V

Miss distance indicators based on the amplitude variation of the N wave already are known. A device of such type is described, for instance in an article of Markus 'C. Eliaphon et a1. entitled Acoustic Firing Error Indicator, in Electronics, vol. 25, No. 10, pages 98-101, October 1952.

The present invention does not utilize the variation in amplitude for measurement, but the variation in time duration of the N wave as a function of distance. Measurements of time duration are less subject to error in collection and transmission of results than measurements of amplitude. On the other hand, the former require great precision in collection and transmission of the time-data of the two rigid edges of the N wave. For this reason, the microphones picking up the passage of the N wave have a very broad band; and the transmission of data. is effected by frequencymodulation.

The microphones are placed about the target. Four in number are shown and they are placed on a circular crown. .Such an arrangement makes possible a close-up presentation of the target area which is suitable for immediate. use by the firing personnel and later for the recording in a more exact manner which, however, is not immediately available to the firing personnel.

The immediateindication is made in the form of a parametric plane representation of the target area. This parametric representation" is conceived to' permit' an im- 4 2,925,582 Patented Feb. 16, 1960 ice mediate summary interpretation by the firing operator. It may be accomplished by simple means, permitting installation aboard a fighter plane.

Theinvention will be better understood in connection with the detailed description which follows and with the annexed drawings, in which:

Fig. 1 is a graphic representation of a ballistic N wave; Fig. 2 represents a perspective view of an array of four microphones;

Fig. 3 represents the electrical equivalent circuit of the microphone;

Fig. 4 represents a cross-section of the assembly of monoammoniac phosphate plates in a microphone;

Fig. 5 is a cross-sectional view of one of the electrodes;

Fig. 6 is a cross-sectional view of one of the micro phones;

Fig. 7 is aperspective view of one of the microphones;

Fig. 8 is a cross-sectional view of a microphone;

Fig. 9 is a schematic diagram of the firing indicator;

Fig. 10 represents the signals as produced in the receivers after detection;

Fig. 11 is an example or" a firing indication;

Fig. 12 is an example of a transparent graduated mask, permitting the interpretation and use of the immediate parametric result furnished for the use of the firing operator.

Fig. 1 is a representation of a ballistic shock wave produced by a projectile traveling at supersonic speed.

The abscissa line .is plotted either in units of time or in units of distance. If it is in units of time, T represents the duration of the N wave. 27 represents an N wave having two straight edges, 29 and 30. Further details on N waves are given for instance in an article entitled A Determination of the Wave Forms and Laws of Propagation and Dissipation of Ballistic Shock Waves, in the Journal of the Acoustical Society or America, vol. 18, -No. 1, July 1946.

. In Fig. l, 28 represents the signal reproduced by.the

microphone withlessthan 10% distortion.

'-.In Fig. 2, a crown 45 is shown on which are mounted four microphones 41', 42, 43, and 44, placed at the extremitiesof two diameters at right angles. Each of these microphones has an identical adaptor, shown as 51, for microphone 41. The other identical adaptors are omitted for reasons of simplicity. The terminals of the adaptors are connected each to its corresponding transmitter; a transmitter is represented schematically as 61.

Microphones should be selected of a type capable of reproducing a ballistic N signal having a duration of between 0.1 and a few milliseconds, with a distortion of less than 10%.

The following approximate characteristics are necessary: Sensitivity: 20 rnicrovolts per bar; and a pass band from 20 c./s. to kc./s.

These characteristics may be obtained with microphones containing an array of approximately cubic shapes of small flat square plates of monoammoniac phosphate. It is known that monoammoniac phosphate, often designated by the abbreviation A.D .P. for ammonium dihydrogen phosphate, is used in the form of a piezoelectric crystal. Reference is made particularly to "Piezoelectric Crystals and Their Application to Ultrasonics by Warren P. Mason, D. Van Nostrand Co., Inc., 1950, pages 137 to 164.

By adopting the approximately cubic form, the multiple oscillations due to resonance between the various modes of mechanical oscillations which may be set up in the crystal are eliminated.

he plates may be shaped according to cross-section Z 45, as shown on page of the above cited reference, in such a way that, once the ballistic shock pressure is applied to the field of the plates, the piezoelectric current will be collected on the large square faces, against which the electrodes are arranged.

In Figs. 6 to 8, 1 designates the block of crystals of monoammoniac phosphate, and 2 achamber of insulating material 'on the botto rnof which the block is mounted with a--'flexible adhesive. 'It is further kept in place by supple spacers 3. made for instance'of cork. i

Thecrystals have the form of parallelepipedic plates; there are six'mounted in the model described and their assembly forms a cube, which, in the model of the invention, is 1 m Asa result, the thickness of the plates is approximately 1.6 mm. The crystals'are shaped, as above, as per cross-section 2 45. Block 1 is positioned in the chamber in such a way that the ballistic shock wave falls on the edge of the plates.

The housing 2 is inserted into a cylindrical metallic mount '4, which is screwed to a coaxial contact socket 5.

Oneof the connecting wires 7 of the microphone passes I face of the spacers and the active face of the crystals.

Plates 13 to 18, shown in Fig. 4, are arranged in parallel and electrodes 19 to 25, made for instance of silver leaf, are placed on the outside faces of plates 13 and-'18 and interposed between the adjacent faces of the difierent plates. Two adjacent plates are mounted in such away that, while receiving pressure on their fields side by side, the piezoelectric charges gathered on the large adjacent faces, separated by an electrode, are of the same polarity.

QEach electrode in Fig. 5 comprises a snap contact, 26 ' for'electro'des 19, 21, '23, 25, and 26' for electrodes 20, 22, 24. Wire 7 is soldered to contact 26, and wire 8 tocontact 26' as shown in Fig. 4.

Referring to Fig. 3, an electric-circuit, suitable'for a complete crystal, comprises a signal source 35, an

inductancerL a resistance R and a condenser. C in series, a shunt condenser C and a leak resistance R The phantom C represents the stray capacity.

. If we assume that block 1 is formed of a single cubic crystal equipped with the-two electrodes '19 and 25,'and

if we designate the charge appearing on the electrodes for a given pressure as q the voltage generated is:

where C designates the capacity of the condenser constitutedby electrodes 19 and 25 separated by the block of r'nonoammoniac phosphate. For the'cubic block constituted by n plates in shunt, the capacity C becomes n C since the capacity of a plate is nC Moreover, the f'charge becomes nq Subsequently the voltage generated becomes:

i nCo+C If we assume we get:

v w-ill be maximum for:

apaassa which gives:

in the case of the block with n plates. The product:

has not varied in relation to its value in the case of the single block. It has to be of the order of 10 times the duration T of the longest signal in N, or 75 milliseconds. C being, as we have seen, of the order of 1 pf. for a cube ofl cm. and the resistivity of monoammoniac phosphate, equal to the resistance ofa block 1 cm beingof the order of 4.10 we obtain 40 milliseconds, which is easily of the order of the size desired for the product C R As has been indicated in relation to Fig- 2, each microphone is connected with an adaptor which permits passing the output from the high impedance of the microphone to the impedance of the following transmission circuit. It is, for example, a cathode-follower stage of conventional type. It also serves as support for thernicrophone. The ensemble is mounted on rubber to limit vibrations and transmission of sound through the inner surfaces. 7

An adapter 51 is linked with a corresponding transmitter 61.- The transmitters are frequency modulated. .A suitable characteristic is: excursion of frequency, 300 kc ./s., and output 2 watts, for example, which permits apractic al range of 30 km.

The receiver will now be desccribed in relation to Fig. 9. There are four receivers 71 to 74, each with frequency modulation. After demodulation, each furnishes a signal similar to the abscissa line 28 of Fig. l. The output signal is differentiated in circuits shown schematically as 81 to 84, having pulse shapes represented as 3 1 to 34 of Fig. 10. Each of these pulses has two short pips hereinafter defined as leading pip and trailing pip, respectively. The first leading pip is shown to origi nate from the microphone closest to the trajectory, in the present case microphone 41. Taking the arrival of the first leading pip as the beginning time of the sequence of arrival of the pulses, the leading pips of the remaining channels are retarded at t t and t respectively. The time interval between the leading and trailing edge pips of a same signal increases with the distance of the microphone from the origin of this signal.

A time base 101 corresponds to receiver 71. If a gate circuit 91 is open the time base may be triggered. At the 'rnoment when time base 101 is triggered, it closes gate circuit 93 of the time base 103, opposing it. The link is shown schematically as connection 75.

Similarly, there is a time base 103 corresponding to receiver 73. This receiver can trigger its time base when gate circuit 93is open. As soon as the time base is released'it closes the circuit of gate 91 through circuit connection 76, associated with time base 101. The time bases 1 02 and 104, and the gate circuits 92 and 94 operate in a manner similar to the above.

ifll .to 104 will use signals of positive polarity and act onthe deflection plates 1 11 to 114 of cathode-ray tube 110. 3 t

Assuming that for thepassage of a certain projectile the signals issued from computers 81 to 884 are those represented in 31 to 34 in Fig. 10, it is the time base 101, which is triggered first, then after time 1 time base 102. At period 1 time base 103 cannot be triggered, since when time base101 is triggered, this will close gate circuit 93. Similarly, time base 104 will not have been triggered since the gate circuit 94 is closed.

The time bases are conceived so as to be stopped and returned to their original situation by the trailing edge pip. Thus, when the signals of Fig. have run their course, condition of the circuitry of the indicator returns to its initial state, ready to function on the next shock wave.

Fig. 11 represents the indication given by the indicator which will have received a shock wave corresponding to Fig. 10. The spot has a traced curve 120 comprising a horizontal section representing the period from zero to t and an oblique section representing the period from I; to the instant of occurrence of the trailing edge pip of the N wave received by microphone 51. As this is the microphone reached first by the wave, the trajectory has accordingly passed to the left of the target and as microphone 42 was reached before microphone 44, the trajectory accordingly passed over the target. Microphone 41 has been reached before microphone 42; the assumed point of impact was below the plane midway between these two microphones. This appears on the screen shown in Fig. 11 placed under the eyes of the firing operator.

In order that the firing operator may evaluate the target-trajectory distance, a grid of the type shown in Fig. 12 may be conveniently placed upon the screen of the cathode-ray tube. The equidistant lines are closed. Their representation depends on the law of variation of the quantity T with the distance.

On the graphic mask of Fig. 12 is represented a traced design corresponding to mm. shells. The equidistant curves are squares, and the distances corresponding to them are indicated by numbers, in meters, for example. It is possible to color the different zones diiferently. A spot track is represented in 120, similarly to that of Fig. 11.

Working from this distorted parametric representation, it is possible to reconstruct the exact target area. This requires an installation which is somewhat bulky for a fighter plane.

The method of operation may occur in the following manner: Parallel to the immediate indicator of fire, there is an indicator the screen of which is registered photographically. According to circumstances, the second indicator may use either the same receivers as the first or separate receivers. 'The target area is reconstructed by a translator according to the results of the photographic records.

As a variation, it is likewise possible to register the signals of Fig. 10 directly. A fifth track furnishes the time base. These elements are furnished as data to a translator-calculator.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.

the said target in a plane at an angle to the trajectory of said bullet, each said transducer including electric circuitry means to separately detect the passing of said leading edges and of said trailing edges of a said N shock wave and the time-lag between them, source of energy means to produce electrical outputs in response to the detected said leading and trailing edges of said wave, and proportionally in time to the time-lag between said two edges, receiver means comprising separate means to receive the said electrical outputs of each said transducer, computer means for each one said transducer to calculate the duration of each said N shock wave from the time elapsed between its leading and trailing edges, means to compare the duration of each said N shock wave, means to compute the target miss distance and duration of travel of each said passing bullet, when closest to the said target, on the basis of the law of variation of the duration of said N shock wave as a function of distance.

2. A target miss distance and direction indicating system as defined in claim 1, further comprising a transmitter for each said transducer, electrical circuit means connecting each said transducer with each said transmitter.

3. A target miss distance and direction indicating system as defined in claim 2, further comprising a receiver means, one for each said transmitter.

4. A target miss distance and direction indicating system as defined in claim 1, comprising recording means for subsequent analysis of target scoring from the outputs of said transducers and electrical circuit means connecting said transducers with said recording means.

5. A target miss distance and direction indicating system as defined in claim 4, where the said each transmitter and said each receiver is frequency modulated.

6. An acoustical firing error indicator for indicating the distance and direction of a supersonic bullet relatively to a given artificial target comprising four microphones assembled on a circular crown secured to said target, four frequency modulation transmitters operating on separated radiofrequency carriers fed by said microphones, a re ceiver set including four frequency modulation receivers, each of them tuned at one of said transmitter radiofrequency carriers, four difierentiating circuit means each deriving a first pulse signal from the said leading edge of the N-wave signal, and a second pulse signal from the said trailing edge of said N-wave signal, four time base circuit means each triggered on by said first pulse signal and triggered ofl by said second pulse signal, electrical circuitry means to group said four time base means into two pairs with only one of each of the said pairs of said time bases capable of triggering on at once, a cathode ray tube indicator with two sets of deflecting plates, means to apply time base signals issuing from said time base means of the first said pair to the first set of deflecting plates and time base signals of the second said pair to the second group of deflecting plates.

7. A target score system comprising apparatus means as defined in claim 6 and further comprising associated means to video plot said target relative to said miss distance and direction of said bullet.

8. A system for detection of target miss distance and direction in target practice of firing bullets at supersonic speeds creating supersonic N shock waves having a leading and a trailing edge, said system comprising an array of four piezo electric crystal microphones spaced in a prefixed relationship to each other and relative to the said target in a plane at an angle to the trajectory of said bullet, four electric circuitries, one for each said microphone, to produce electrical outputs responsive to said leading and trailing edges, including transmitting means of said outputs, receiver means comprising separate means to receive said electrical outputs of each said microphone, computer means for each one said microphone to calculate the duration of each said N shock wave from the time elapsed between its leading and trailing edges, means to compare the durations of each said N shock 7 wave, means to compute the target miss distance of each said passing bullet, when closest to the said target, onthe basis of the lawof variation oft-he duration of-sa'id N shock wave as a function of distance.

9. A target miss distance and direction indicating system as defined in claim 8, further comprisinga transmitter for each said transducer, electrical circuit I means connecting each said transducervwith each said transmitter.

10. v A target miss distance and direction indicating systemaas defined in claim 8, comprising recording means for subsequent analysis of target scoring from the outputs of said stransducersand electricalicircuit means connecting said transducers with said recording means.

References 'Cited in the file of this patent I Journal ofthe Acoustical Society of America, vol. 18, No; 1; July 1946, pp. 97118.

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