WO1986005870A1

WO1986005870A1 - Target

Info

Publication number: WO1986005870A1
Application number: PCT/SE1986/000154
Authority: WO
Inventors: Karl Yngve Axner
Original assignee: Saab Training Systems Aktiebolag
Priority date: 1985-04-03
Filing date: 1986-04-02
Publication date: 1986-10-09
Also published as: FI874301A0; FI874301A; DK158855C; DK578286D0; SE451504B; DE3667325D1; SE8501649D0; SE8501649L; EP0248018B1; DK158855B; EP0248018A1; DK578286A

Abstract

A target with an acoustic chamber (4), within which inner microphones (5) are arranged to receive the sound-waves, that are generated by a projectile (6), fired against the target. The inner microphones (5) are connected to means adapted to establish the hit position of the projectile on the basis of differences in the transit time of sound-waves to said microphones (5). One outer microphone (9) is arranged at the front of the target, substantially in the front plane of the target, to receive the sound-waves, that are generated by a projectile (6), fired against the target with supersonic velocity. The outer microphone (9) is connected to means for determining the velocity of the projectile, which is provided to determine the supersonic velocity of the projectile, fired against the target, on the basis of the hit position of the projectile and the transit time of the sound-waves to the outer microphone (9).

Description

TARGET

The present invention relates to a target of the type having an acoustic chamber, within which inner microphones are arranged to receive the sound-waves that are generated by a projectile, fired against the target, when entering said chamber, the micro- phones being connected to means adapted to establish the hit position of the projectile on the basis of differences in the transit time of the sound-waves to said microphones.

Under some circumstances it may be of interest to know the velocity of the projectile, fired against the target. The most common way of determining this velocity is to measure, by the aid of two sensors, the time difference for the passages of the projectile through two measure¬ ment planes, arranged in a distance from each other. The velocity of the projectile is then calculated by dividing the known distance between the measurement planes by the measured time difference.

The object of this invention is to provide a target, in which the velocity of a projectile, fired against the target with supersonic velocity, can be determined in a simple way uti- lizing one single sensor, arranged in front of the target, close to it.

This object is achieved in a target of the type mentioned intro- ductorily, in accordance with the present invention, by arranging one outer microphone at the front of the target substantially in the front plane of the target to receive the sound-waves that are generated by a projectile, fired with supersonic velocity against the target, which microphone is connected to means for determining the projectile velocity, which are arranged to determine the supersonic velocity of the projectile, fired against the target, on the basis of the hit position i the projectile and the transit time of the sound-waves to the outer microphone.

Preferably means are provided for determining the sound velocity within the acoustic chamber and means are provided for main- taining the air temperature in the acoustic chamber substantially equal to the air temperature in the environment of the target.

In a preferred embodiment four inner microphones are arranged in the acoustic chamber connected to the means adapted to estab¬ lish the hit position of the projectile, which in this case are provided for determining the hit position of the projectile as well as the sound velocity in the acoustic chamber.

The means adapted to establish the hit position of the projec¬ tile and the means for determining the projectile velocity are preferably the same means.

The invention shall now be described more in detail, reference being made to the enclosed figures.

Fig 1 shows a front view of a target in accordance with the in¬ vention.

Fig 2 shows the lower part of this target sectionally in an en- larged scale.

Fig 3 illustrates the conical wave propagation for a projec¬ tile with supersonic velocity.

The target shown in fig 1 and 2 has a front sheet 1 of rubber or plastic material, which is of such a character, that the sheet 1 cannot be torn open or be frayed as a result of being pierced, but will close up again each time it has been pierced. The target is also provided with a rear sheet 2 of the same material as the i ront sheet 1. The sheets 1 and 2, which are impermeable <_., sound, are fastened to a circumferential frame 3, which together with the sheets 1 and 2 defines an acoustic chamber 4. Four microphones 5 are arranged in the lower part of chamber 4 to receive the sound-waves, that are generated by a projec¬ tile, fired against the target, entering the chamber 4. The pro¬ pagation of these sound-waves in chamber 4 is circular with the projectile in the centre. The microphones 5 are connected to means (not shown) which are adapted to determine the hit position of the projectile on the basis of differences in the transit time of the sound-waves to the four microphones.

In figure 2 there is shown a projectile 6 fired against the tar¬ get, and its trajectory 7. The projectile 6, which is shown in the moment as its point enters into chamber 4, is supposed to possess supersonic velocity, and the sound-waves, emanating from the point of the projectile, thus propagate against the target in the form of a cone with the cone angle 2*- . These outer sound¬ waves do not enter chamber 4, as they are stopped by the sheet 1 , impermeable to sound. In chamber 4 the sound-waves, that are generated by the projectile 6, propagate, as mentioned above, circularly with the projectile as a centre. These inner sound-waves reach the four microphones at different points of time, which are dependent on the hit position of the projectile, that is to say the distance to each microphone, and on the sound velocity in chamber 4. From these differences in transit time and the mathematical formula given below (equation I) the hit position of the projectile is determined by said means, not shown, mentioned above, to which the microphones 5 are connected. The hit position determined is registered, and the marksman obtains an indication hereof on a screen or strip.

An assumed hit position in figure 1 has the coordinates (x;y) in the coordinate system provided, whilst the four microphones 5 have the coordinates (x.-jy,-), (x₂;y₂)_> (xy V--) ^and .x^y^- The following relationship is true:

l/(χ - _j_)² + (y - y^² = v (t., ₊ t_o) (I)

where i = 1, 2, 3, 4; v is the sound velocity in chamber 4; t is the transit time for the sound-waves from the hit position point (x;y) to the microphone nearest to it; and t1. + to is the transit time from the hit p^rosition point (x_.y) to each microphone. This equation system thus contains four equations with four unknown quantities, that is x, y, t and v. The hit position (x;y) and the actual sound velocity in chamber 4 thus can be determined.

In fig 3 the conical propagation of the projectile 6 in front of the target is illustrated. The supersonic velocity of the projectile is marked by p and its wave propagation velocity parallel to the plane of the target is marked by w. Equal sound velocity v is assumed to be prevailing in acoustic chamber 4 and outside the target. As the sound velocity in air is dependent substantially exclusively on the temperature and is practically independent on air pressure, air density and air humidity, thus the same temperature is assumed to be prevailing within acoustic chamber 4 and outside the target. The target is provided with means, adapted to maintain the air temperature within the acoustic chamber equal to the air temperature in the environment of the target. These means here consist of ventholes 8, formed in the frame 3.

The following relationship between the velocities p, w and v is true:

P = ^wv (ID

- v The sound velocity v is determined, in accordance with the above mentioned equation I, and by determining the wave propagation velocity w parallel to the plane of the target the projectile velocity p can thus be determined.

The target in accordance with the present invention is provided, for this object, with an outer microphone 9 arranged at the front of the target, substantially in the fron plane of the target, that is to say immediately in fron of it, fastened to the lower part of the frame 3 by a holder 10. The outer microphone 9 is adapted to receive the sound-waves, that are generated by a projectile, fired against the target with supersonic velocity and is, like the inner microphones 5, connected to the above mentioned means, not shown. The outer microphone 9 has the coordinates (x_t-jy,-) in the coordinate system in fig 1, the following relationship being true:

s/ (x - ^■V^' (y - y. : ) ² - <-, .' (III)

where like above (equation I), (x;y) is the hit position point and t is the transit time for the sound-waves from the hit o position point to the inner microphone 5 nearest to it, and where t-. + t is the transit time of the sound-waves from the hit 5 o position point to the outer microphone 9. The hit position (x;y) and the sound velocity v are determined in accordance with the equation I above, and the wave propagation velocity w parallel to the plane of the target can then be calculated by the aid of equation III. The velocity p of the projectile is then, finally, calculated by the aid of equation II.

In another type of targets only three microphones are utilized for the determination of the hit position. In such targets the sound velocity v cannot be determined by the aid of equation I above but a separate device is utilized for this determination. Such a device may comprise a transmitter and a receiver. By know¬ ledge of the sound velocity v the hit position (x;y) is deter¬ mined. Then the wave propagation velocity w and the projectile velocity p are determined as described above by the aid of equations III and II.

With the position shown of the outer microphone 9 at the lower part of the target and with the definition of t given, t,- is normally negative, as the outer microphone 9 in this case is normally activated before all the inner microphones 5. Of course t may be defined in another arbitrary way to form a fixed time reference, to which the time points of activation for the indi¬ vidual microphones are related.

The calculated projectile velocity p is registered and can, like the hit position, be indicated to the marksman at a screen or strip.

Claims

1. A target of the type having an acoustic chamber (4) within which inner microphones (5) are arranged to receive the sound¬ waves that are generated by a projectile (6), fired against the target, when entering said chamber, the microphones being connected to means adapted to establish the hit position of the projectile on the basis of differences in the transit time of the sound-waves to said microphones, c h a r a c¬ t e r i z e d in that one outer microphone (9) is arranged at the front of the target, substantially in the front plane of the target o receive the sound-waves, that are generated by a projectile (6) fired with supersonic velocity against the target, which microphone is connected to means for determining the projectile velocity, which is arranged to determine the supersonic velocity of the projectile, fired against the target, on the basis of the hit position of the projectile and transit time of the sound-waves to the outer microphone (9).

2. A target as claimed in claim 1, c h a r a c t e r i z e d in that means are .provided for determining the sound velocity within the acoustic chamber (4) and that means (8) are provided for maintaining the air temperature within the acoustic chamber substantially equal to the air temperature in the environment of the target.

3. A target as claimed in claim 1 or 2, c h a r a c t e r- i z e d in that four inner microphones (5) are arranged in the acoustic chamber (4) and connected to the means adapted to establish the hit position of the projectile, which in this case is provided for determining as well the hit position of the projectile as the sound velocity in the acoustic chamber.

4. A target as claimed in any of claims 1 - 3, c h a r a c¬ t e r i z e d in that the means for determining the hit position and the means for determining the projectile velocity are preferably the same means.