WO1997024575A1

WO1997024575A1 - Projectile location system

Info

Publication number: WO1997024575A1
Application number: PCT/AU1996/000837
Authority: WO
Inventors: John Patrick Baird; Joseph Cho Sam Lai
Original assignee: Unisearch Ltd.
Priority date: 1995-12-28
Filing date: 1996-12-24
Publication date: 1997-07-10
Also published as: AUPN727995A0

Abstract

A projectile location system (10, 20, 30) includes a plurality of microphones (12, 13, 14) adapted to receive the acoustic wave generated by a projectile as it passes through a planar target region (11, 21, 39). Differences in position are resolved using a statistical technique, ideally a cross correlation technique, in order to determine relative distance of the projectile from various ones of the microphones. The relative distance information is then utilised in a triangulation operation to obtain an estimate of position of the projectile in the plane of the target (11, 21, 39). In particular, a baffle (26) is interposed between the microphones and at least a portion of the target (11, 21, 39). In another particular form, the microphones are aligned along at least two separate axes which intersect at an obtuse angle.

Description

PROJECTILE LOCATION SYSTEM

The present invention relates to a projectile location system, and more particularly, to a system for locating and reporting the position of bullets as they pass near or through a target at which the bullets have been fired.

Prior Art Discussion

Certain forms of projectile location systems are known. For example Australian Patent No 51 5075 to Australasian Training Aids Pty Ltd discloses a system which utilises at least three transducers (as defined in the specification of that patent) in order to sense the relative time of arrival of a wavefront generated by a bullet or like projectile at each transducer as the bullet or like projectile passes near or through a predetermined area comprising a target.

There are several commercially available projectile location systems. Generally, systems such as the Lomah/Superdart which rely on detecting the leading edge of the shock wave generated by supersonic bullets, do not work for subsonic rounds and will not detect oblique shots. On the other hand, systems which work for both supersonic and subsonic rounds (for example the Polytronic TG3000 available from Polytronic International Switzerland) require a box target made of special material which has to be replaced on a regular basis. Australian Patent No. 523897 provides good background information on the various means detecting ballistic projectiles.

The subject matter of Australian patent No 5 1 5075 is particularly concerned with locating bullets or like proj ectiles travelling at supersonic speed (that is, greater than around 1 100 feet per second in air) . In this specification reference is also made to subsonic speed being a speed less than that of sound in air - that is less than 1 100 feet per second in air. Reference is also made to trans-sonic speeds which are speeds around that of sound in air - that is around 1 100 feet per second in air.

The basic principle of operation of this prior art system is described in the specification of Patent No 515075 with particular reference to Figs. 2 and 3 thereof. This prior art system seeks to measure the relative time of arrival of a shock wave front at the transducers by selecting only one particular point on the output wave from each transducer resulting from impingement of a shock wave on the transducer by the passage of a bullet or like projectile.

In practice it appears that there is some difficulty in identifying consistently the appropriate point on the output wave with the end result being that position sensing by the system has not been as accurate or as reliable as the user might desire.

The specification of Australian Patent No 515075 suggests at least one method to improve the consistency of the wave output from the transducers with reference to Fig. 7, 8, 9 and 10. This method involves encasement of the piezo-electric transducer so as to remove a spurious leading edge of the wave form identified as 43 in Fig. 7 of the patent specification of Australian Patent No 51 5075. Another known method to improve consistency of the output waveform from the transducers involves building as a hollow volume the target through which the bullets are to pass. A problem with this approach is that as bullets pass through the target they also pass through the ^• volume edges and ultimately destroy the volume so that the entire target assembly must be replaced. It is an object of the present invention to provide a projectile location system which overcomes or ameliorates one or more of the abovementioned disadvantages.

BRIEF STATEMENT OF INVENTION

In one broad from of the invention there is provided a system for determining the position of a projectile as it passes through a planar region which lies generally at right angles to the line of flight of the projectile; said system comprising at least three sound sensors displaced in space relative to each other; said at least three sound sensors also placed so as to receive a sound wave generated by said projectile when said projectile passes through said planar region; the system further including processing means adapted to perform a statistical operation on signals received from said sound sensors so as to obtain a measure of the difference in time of arrival of said sound wave at each of said at least three sound sensors whereby the position of said projectile as it passes through said planar region can be estimated.

Preferably said statistical operation is a cross correlation operation.

In one preferred form the system is applied to a projectile moving at subsonic velocity.

In another preferred form the system is applied to a projectile moving at supersonic velocity.

In a further preferred form, the system is applied to a projectile moving at trans-sonic velocity. In a further broad form of the invention there is provided in a system for indicating or locating a position of a projectile relative to a target; a method of determining the relative time of arrival of wavefronts derived from said projectile at at least three wavefront sensors which are displaced relative to each other; said method comprising obtaining signals derived from said at least three sensors, performing a cross-correlation operation on pairs of said signals so as to obtain a statistical measure of the difference in time of wavefront arrival at respective pairs of said at least three sensors.

Preferably said step of performing a cross-correlation operation is done by digital computation.

Preferably said sensors comprise microphones.

Preferably said sensors are placed so as to lie on the same straight line.

In a particular form said straight line is aligned with a bottom edge of said target.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings wherein: -

Fig. 1 is a front and side edge view of a target arrangement according to a first embodiment of the invention, Fig 2 is a front view of the target arrangement of Fig 1 showing microphone and bullet hit locations for which experimental data is provided in the description, Fig 3 is a block schematic diagram of signal processing equipment for use with the target of Fig. 1 ,

Fig 4 is a graph of amplitude versus time of output from the microphones illustrated in Fig 2 in relation to a single bullet hit, Fig 5 is a graph of amplitude versus delay time of the difference output between respective microphones in Fig 2 for a single bullet hit based on statistical processing of the signals illustrated in Fig. 4, Fig 6 is a front and side edge view of a target arrangement according to a second embodiment of the invention, Fig 7 is a graph of signal versus time showing signals from microphones with and without baffles interposed between the microphones and the target,

Fig. 8 is a graph of signal magnitude against time both with and without filtering,

Fig. 9 is a front view of a target arrangement according to a third embodiment of the invention incorporating a multi-planar microphone array, and

Fig. 10 is a graphical illustration of a triangulation method of determining distance of projectiles 1 having first determined relative distances from microphone pairs

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A projectile location system 10 according to a first embodiment of the invention is illustrated in Fig. 1. The system comprises a planar target 1 1 along a bottom edge of which is located first microphone 12, second microphone 13 and third microphone 14. The system 10 can include a fourth microphone 15 a distance L in advance of the target 1 1 for the purpose of providing a trigger signal The first and second microphones 12, 13 are displaced a distance A with respect to each other whilst second and third microphones 13 , 14 are displaced a distance B with respect to each other. It will be observed in this example that all three microphones 12, 13 , 14, lie in the same straight line and are aligned with a bottom edge of the target 1 1.

The microphones 12, 13 , 14 can be placed in advance of the target although, in this instance, they are placed in line with the rear plane of the target 1 1 .

Fig. 2 illustrates a particular example set up which follows the target arrangement of Fig. 1. In this instance the first, second and third microphones are identified as Ml , M2, and M3 respectively. Microphone M l is displaced a distance with respect to Microphone M2 whilst Microphone M3 is displaced a distance B with respect to Microphone M2. In this particular example A equals B equals 150 mm. The target dimensions are 140 mm by 550 mm as illustrated in Fig. 2.

In this example a series of rounds are fired at the target 1 1 resulting in hits on the target 1 1 clustered at the crosses identified A, B, C, D, E.

Fig. 3 illustrates a block schematic diagram of signal processing equipment connected to microphones M l , M2, M3 whereby the results graphed in Figs. 4 and 5 are produced.

With particular reference to Fig. 3 the signals from microphones Ml , M2, M3 are sent via an analog to digital (A to D) converter into a cross-correlator 16 whereby a statistical measure of the difference in time of wavefront arrival at respective pairs of the three microphones M l , M2, M3 can be derived. The time differences are then sent to data processing means where additional data comprising physical constants, target dimensions, microphone displacement and the like are combined with the time of wavefront arrival information derived from cross-correlator 16 so as to calculate (in known manner) an estimate of the location relative to target 1 1 of the bullet or projectile which generated the wave fronts.

The resultant data can be output in many forms including but not limited to visual indication at or near the target 1 1 and/or visual indication near the point at which the bullet or projectile is fired.

The cross-correlation performed by cross-correlator 16, in this application, can be used to provide "difference in time of arrival" information for wavefronts impinging on microphones M l , M2, M3 as can be seen with reference to Fig. 5. The peaks observed in the waveforms in Fig. 5 represent the points of maximum statistical correlation between the respective waveforms derived from the wavefronts impinging on the respective microphones.

Cross-correlation can be viewed as a generalisation of autocorrelation. The cross correlation of two real sequences v[n]and w[n]is defined as oo

Φ [n] - ∑ v[n + m]w[m] m= - oo If v[n]and w[n]are equal, the cross correlation and autocorrelation are identical.

This operation can be used to determine when two sequences are best aligned in time. For example, if x[n]and v[n]are two different but similar sequences, one may wish to determine which value of 1 results in the best approximation

X[n]= v[n + 1] This can be done by computing the cross correlation and looking for the time index, n, of the maximum peak of Φ [n]

It has been found that adoption of this statistical method tends to reduce the sensitivity of results to the vagaries of the signal derived from the wavefront impinging on the microphones, particularly in relation to projectiles travelling at subsonic velocity.

Where projectiles travel at trans-sonic or super-sonic velocity at least fourth microphone 15 may be required to provide a trigger signal to the cross-correlator 16 so that an appropriate window for the signals from Microphones M l , M2, M3 can be obtained.

The basic arrangements illustrated with reference to Figs. 2, 3 , 4 and 5 can be varied to improve accuracy and/or reduce sensitivities to various parameters For example increasing the displacement between respective microphones M l , M2, M3 will improve the time resolution of the system and hence the accuracy of the system In addition a greater number of microphones or other sound sensors can be used to increase the amount of statistical information available to data processing means 17 to further minimise uncertainty in projectile position location

It will be observed that the system according to the above described example does not require a special construction of target 1 1 .

With reference to Fig. 6 a projectile location system 20 according to a second preferred embodiment of the invention is illustrated in front and side view. As for the first embodiment the system comprises a planar target 21 with three microphones comprising first microphone 22, second microphone 23 and third microphone 24 mounted in a horizontal line at the rear of and near a bottom edge 25 of the target 21.

In this system 20 there is installed a baffle 26 located so as to shield the microphones 22, 23 , 24 from sounds emanating from that region of target 21 closest to the microphones 22, 23 , 24. It has been found that this shielding by baffle 26 assists in achieving better accuracy and performance reliability from the system 20 as compared with system 10 of the first embodiment. The baffle has been found to be particularly effective for projectiles which have a relatively large blast wave signature.

In this embodiment the same processing rationale as described with reference to Fig. 3 was utilised. Specific hardware components are as follows:

The microphones used are inexpensive commonly available RS Components 250-479 miniature unswitched tie-clip microphones These are electret microphones with a frequency response up to 16kHz. A power spectrum analysis of the signals produced in use showed that the bulk of energy is contained in frequencies less than 10kHz which confirmed the suitability of this type of microphone in this implementation of the invention The microphones had impedance 600Ω, sensitivity minus 64dB at 1 kHz with frequency response 50Hz- 16kHz and were mounted directly behind the target and separated by 350mm from each other as illustrated in Fig. 6 A triggering microphone 27 was used to initiate data acquisition The microphone signals were conditioned using Tektronix AM502 differential amplifiers The three microphone signals were then digitised using a WIN-30 DS4 1 Mhz data acquisition card with simultaneous sample and hold The sampling rate was 250kHz per channel The data acquisition card was installed in a Toshiba 3200SXC laptop computer.

An uncertainty analysis based on the sampling rate of 250kHz and a speed of sound of 340m/s indicated a calculated uncertainty of 1 .36mm in the calculation of distance 1 based on the triangulation calculation illustrated in Fig. 10 and for a microphone separation of 350mm

Fig 7 illustrates, for this example, the effect that the baffle 26 has on reducing precursor noise 28 occurring prior to useful signal 29

In addition it has been found that a digital filter such as, in this instance, a 256 point digital filter with Hamming window can reduce the spurious noise by a factor greater than 2 while reducing the peak amplitude of the signal of interest by less than 10% as shown in Fig 8.

Trials of this system indicate that the following accuracies are possible:

Ref Weapon Velocity Del x (mm) Del y (mm) Error (mm) (m/s) a .22 Ruger 3 17 1 7 6.6 7.0 b 5.56 821 2.2 14 4 14 7 c 9mm SD 3 17 3.6 10.2 1 1 .0 d 9mm A3 410 3.5 14.9 16.2 It has been observed that accuracy in the x direction (i. e. the direction parallel to the line of microphones) is greater than that in the y direction (i.e. perpendicular to the line of the microphones). Accordingly, Fig. 9 illustrates an example of the invention according to a third preferred embodiment of a projectile location system 30.

In this third embodiment two aligned arrays of microphones are utilised with the alignments offset one to the other and wherein both alignments are offset from the x axis (illustrated in Fig. 9 and defined as aligned with a horizontal direction) .

This arrangement permits the mathematical weighting in favour of alignment XX of first array 3 1 and alignment YY of second array 32 more heavily than components at rightangles to these alignments XX and YY for both vector x and vector y resolved respectively in the horizontal and vertical directions as illustrated in Fig. 9 and thereby to obtain improved overall accuracy of calculated distance 1 (refer Fig . 10) than with the single aligned microphone array systems of the first and second embodiments.

With further reference to Fig. 9 the first microphone array 3 1 comprises first, second and third microphones 33 , 34, 35 respectively aligned along line XX. Second array 32 comprises first, second and third microphones 36, 37 and 35 respectively aligned along axis YY.

It will be noted that third microphone 35 is common to both arrays 3 1 and 32.

Both axes (and corresponding arrays 3 1 , 32) are inclined with respect to both the horizontal (x) and vertical (y) axes which define the co¬ ordinate system against which the location of the projectile in the plane of the target 39 is determined . This arrangement permits the more accurate direction determinations made in the XX direction and YY directions of the arrays 3 1 , 32 to be resolved in both the x and y co-ordinate directions thereby providing greater accuracy in the y co¬ ordinate direction than permitted under the first and second embodiments previously described.

As with the previous embodiments the displacement based on acoustic signals received by pairs of microphones are determined using cross correlation The output from this operation is then utilised in a triangulation operation of the type outlined in Fig. 10 to produce an estimate of position of projectiles resolved as vectors in the x and y directions.

The above describes only some embodiments of the invention and modifications obvious to those skilled in the art can be made thereto without departing from the scope and spirit of the present invention.

INDUSTRIAL APPLICABILITY

Embodiments of the invention can be applied in all forms of target shooting where it is desired to obtain a reliable and relatively accurate estimate of the location of projectiles passing through a target. The invention has particular application where it is desired to transmit the estimates of position to a remote location and display the estimates at that location.

Claims

1. A system for determining the position of a projectile as it passes through a planar region which lies generally at right angles to the line of flight of the projectile; said system comprising at least three sound sensors displaced in space relative to each other; said at least three sound sensors also placed so as to receive a sound wave generated by said projectile when said projectile passes through said planar region; said system further including processing means adapted to perform a statistical operation on signals received from said sound sensors so as to obtain a measure of the difference in time of arrival of said sound wave at each of said at least three sound sensors whereby the position said projectile can be determined as it passes through said planar region.

2. The system of claim 1 wherein said statistical operation is a cross correlation operation.

3. The system of claim 1 or claim 2 applied to a projectile moving at subsonic velocity.

4. The system of claim 1 or 2 applied to a projectile moving at supersonic velocity.

5. The system of claim 1 or claim 2 applied to a projectile moving at supersonic velocity.

6. The system of any one of claims 1 to 5 wherein a baffle is interposed between said sensors and at least a part of said planar region closest to said sensors.

7. In a system for indicating or locating a position of a projectile relative to a target; a method of determining the relative time of arrival of wave fronts derived from said projectile with respect to at least three wave front sensors which are displaced relative to each other; said method comprising of training signals derived from said at method comprising obtaining signals derived from said at least three sensors, performing a cross-correlation operation on pairs of said signals so as to obtain a statistical measure of the difference in time of wave front arrival at respective pairs of said at least three sensors.

8. The method of claim 7 wherein said step of forming a cross- correlation operation is done by digital computation.

9. The method of claim 7 or 8 wherein said sensors comprise microphones.

10. The method of claim 7 or 8 or 9 wherein said sensors are placed so as to lie on the same straight line.

1 1. The method of claim 10 wherein said straight line is aligned with the bottom edge of said target.

12. The method of any one of claims 7 to 9 wherein said sensors are aligned on two axes; said axes inclined with respect to each other.

13. The method of any one of claims 7 to 12 further including the step of interposing a baffle between said sensors and at least a portion of said target closest to said sensors.

14. A system as hereinbefore particularly described with reference to what is shown in the accompanying drawings.