GB2439131A

GB2439131A - Method of activating an altimeter trigger and device for implementation thereof

Info

Publication number: GB2439131A
Application number: GB9306049A
Authority: GB
Inventors: Bruno Avignon; Yves Canal
Original assignee: Thales SA; Thomson CSF SA
Current assignee: Thales SA
Priority date: 1992-03-24
Filing date: 1993-03-24
Publication date: 2007-12-19
Anticipated expiration: 2013-03-24
Also published as: GB9306049D0; DE4309505A1; FR2903771A1; FR2903771B1; GB2439131B

Abstract

The invention relates to altimeter triggers used in particular for triggering devices for bomb braking or for deploying cargo bombs or rockets. The aim thereof is to achieve correct triggering in the event of low-altitude release both with and without pulling-up. It consists, for an altimeter trigger, in making provision for an activation phase, during which there is imposed a triggering set-point level changing progressively from an almost zero value to the nominal triggering level. In the altimeter trigger shown an altimeter probe (1) delivers an electrical voltage proportional to the measured level, a threshold voltage source (2) delivers a voltage proportional to the nominal triggering level, a threshold voltage progressive application circuit (3) produces an output which rises from zero to this level and a comparator (4) compares the measurement voltage with the output of the circuit (3).

Description

METHOD OF ACTIVATING AN ALTIMETER TRIGGER

AND DEVICE FOR IMPLEMENTATION THEREOF

The present invention relates to altimeter triggers used in particular f or triggering the delayed braking control bomb braking device or for releasing sub-munitions stored on board a cargo bomb.

An altimeter trigger includes essentially an altimeter probe providing a measurement of level and a comparator comparing the measurement of level provided by the altimeter probe with a nominal value of level beneath which triggering must occur. Once it is set going, the altimeter trigger has its altimeter probe activated like its comparator to which is applied the nominal value of the selected triggering level. This setting going generally takes place upon release by the carrier of the projectile fitted with the altimeter trigger. This is appropriate only to high-altitude bomb release, above the selected triggering level, in which case the altimeter trigger normally triggers whilst the bomb is falling at the instant it crosses the selected triggering level. By contrast, this is inappropriate to a low-altitude release, below the selected triggering level, performed while pulling-out with the aim of increasing the range in order to avoid the carrier overf lying the objective since the altimeter trigger is triggered prematurely whilst the bomb is still on the ascending path of its ballistic path below the selected triggering level.

One way of resolving the problem posed by low-altitude release whilst pulling-up would consist in equipping the altimeter trigger with a timer delaying its setting into operation beyond the bomb ascent phase.

However, in the absence of any link for transmitting parameters between the bomb and the carrier, this timer would have to be adjusted on the ground, before the mission, and its use would interdict any other type of release than that anticipated.

The aim of the present invention is to resolve this problem without requiring any link for transmitting parameters between the bomb and the carrier or limiting the choice of release mode.

According to one aspect of this /lm,ention there is provided a method of activating an al-timeter trigger fitted to a projectile, consisting, after releasing the projectile, in progressively activating the altimeter trigger after setting it going by providing it with a set-point level changing progressively from a near-zero level, below.the release level, to the level anticipated for triggering which is attained only after a certain period which is all the longer the higher the level anticipated for triggering.

By virtue of this progressive activation, the altimeter trigger can be adjusted constantly to a set-point level below that occupied by the projectile in the ascending part of its ballistic path when the projectile is released at low altitude whilst pulling-up, thus avoiding premature triggering thereof whilst not prevent-ing it from triggering relatively quickly in the event of low-altitude release without pulling-up since, in this case, its set-point level quickly attains the projecti]e's flight level.

The progressive change of the triggering set-point level of the altimeter trigger during its activation can be undertaken according to a linear or quadratic progression or any other law, the quadratic progression having the advantage of simpler implement-ation.

there is provided According to another aspect of this invention / an altimeter trigger enabling the abovementioned method to be imple-mented.

Other characteristics and advantages of the invention will emerge from the description below of an embodiment given by way of example.

This description will be supplied in connection

with the drawing in which: -Figure 1 is a net of curves illustrating as a function of the time elapsed since release and of the chosen nominal triggering level, the initial phase of adjusting the set-point level of an altimeter trigger consisting in bringing this set-point level progressively from an almost zero value to the nominal value following a law of linear increase; -Figure 2 is a net of curves illustrating as a function of the time elapsed since release and of the chosen nominal triggering level, the initial phase of adjusting the set-point level of an altimeter trigger consisting in bringing this set-point level progressively from an almost zero value to the nominal value following a law of quadratic increase; -Figures 3 to 6 show the path of a projectile and the change, in the course of this path, in the difference between the actual level of the projectile and the set-point level for triggering its altimeter trigger for two different modes of low-altitude release with and without pulling-up and for two different modes of initial change of the set-point level either according to a law of linear increase or according to a law of quadratic increase; -Figure 7 represents the functional diagram of an altimeter trigger enabling a method of activation is embodied in which the invention/to be implemented and, -Figure 8 details the constitution of certain blocks of the diagram of Figure 7.

To obtain correct operation of the altimeter trigger in the event of low-altitude release with or without pulling-up, it is proposed to activate it pro-gressively, after it has been set going upon release of the projectile, by providing it with a triggering set-point level changing progressively from a near-zero level, below the release level, to the nominal level anticipated for triggering. The contradictory require-ments of a triggering after low-altitude release without pulling-up which must be almost immediate, and of a low-altitude triggering with pulling-up which must be delayed so as to prevent it occurring within the ascending part of the path of the projectile, are thus reconciled.

Indeed, in the event of low-altitude release without pulling-up, the progressive raising of the triggering set-point level enables the latter quickly to attain the projectile's flight level whilst in the event of low-altitude release with pulling-up, the progressive raising of the triggering set-point level enables the latter to remain below the projectile's flight level whilst the latter traverses the ascending part of its ballistic path.

The period taken by the set-point level to attain the nominal triggering level must be as short as possible whilst enabling the projectile in the ascending part of its ballistic path to attain a flight level above the nominal triggering level or, at the very least, to leave the ascending part of its path. It will be shown below that it may be chosen on the basis of knowing only the nominal triggering level. To do this, the time r taken by a projectile released while pulling up below the nominal triggering level to attain the nominal triggering level is firstly calculated by assuming that it actually attains it when it describes its ballistic path and when its release level HI, its initial speed V and its initial angle ofascent a are known. The level H(t) attained by the projectile an instant t after release as well as the horizontal distance x(t) traversed since the point of release are given by the relations: H(t) = HI-1-t V sin a --g t2 (1) x(t) = t V cos a (2) where g is the acceleration due to gravity. From these relations, which parametrically define the ballistic path of the projectile, is deduced the equation 12 g H(x) = HI + xtan a --x 2 V2COS2a which makes it possible to write: dli g -= tan a -X dx V2COS2a or again, replacing x by its value taken in relation (2) dH g -= tana -t dx VCOSa which gives the slope of the path of the projectile as a function of the period elapsed since its release. If r is the time required by the projectile to attain the nominal triggering level Hd and rg tan9 = tana -V COS a it follows that: rg V sin a = 1 -tan 0 cot a Substituting this value into relation (1) and making T = i, we obtain: r2g 1 H( r) H1+ --gr2=Hd 1-tanecota 2 From which follows the expression for the value of the time r taken by the projectile since its release to attain the nominal triggering level: Hd-HI g(1/(1-tan 0 cot a) -1/2 This value of the time r depends on the release level HI, on the angle of ascent a at the moment of release and on the angle of ascent 0 at the moment when the projectile attains the nominal triggering level which depend on the conditions of release. It is therefore unknown a priori. However, it can be approximated above by a duration T obtained by replacing, in the expression for the time i, the release level HI by a minimum release level Ho chosen for example equal to 40 metres, the angle of ascent a at the moment of release by a minimum angle of ascent while pulling up, assumed 15 , and the angle of ascent o at the level of the crossing of the nominal triggering level by an angle of descent Gm of the order of +10 corresponding to the start of the descending part of the ballistic path of the projectile. It then follows that: r Hd-Ho 1 T = I (3) Lg(1/(1-ktan Gm) -1/2) J with K around cot 15 , Gm around +10 and Ho around 40m.

This duration T dependent on the square root of the nominal triggering level is sufficient, in the event of release during pulling-up, for the projectile to attain the nominal triggering level or, at the very least, for it to begin the descending part of its ballistic path. It is chosen as the period taken by the set-point level, in the course of the progressive activa-tion of the altimeter trigger, to attain the nominal triggering level.

The progressive increase in the triggering set-point level over the duration T must be undertaken at a lesser speed than the speed of ascent of the projectile over the start of the ascending part of its ballistic path whilst it has not attained the nominal triggering level. Indeed, over this first part of the ballistic path of the projectile, the.altimeter trigger must constantly see a level greater than its set-point level in order not to trigger. This condition is realised for all laws of increase below the linear progression since, when considering the most limiting case where T is equal to r, adopting a law of linear progression amounts to making the chord which subtends the arc drawn by the ascending part of the ballistic path followed by the projectile traverse the set-point level in order to attain the nominal triggering level.

Figure 1 shows a net of curves illustrating, as a function of the time elapsed since release, the varia-tions imposed on the set-point level of the altimeter trigger in the course of the activation phase in the case of a law of linear increase, in regard to three different nominal triggering levels of 200, 500 and 1000 metres attained at the end of the durations T1, T2, T3 respectively. In the course of this activation phase the set-point level grows linearly over the period T from a near-zero level to the nominal triggering level and then retains this latter value. It is observed that for two distinct nominal triggering levels the slopes of linear increase in the set-point level during the activation phase are different, which possibly causes a few embodiment difficulties, since it must be possible to choose the nominal triggering level at the start of a mission.

To round this difficulty, it is proposed to adopt, for the triggering set-point level, during the activation phase after release, a law of quadratic increase of the form: H(t) = t2[g(1/(1-ktan Om)-112)]+Ho (4) deduced from the law of variation of the duration T of the activation phase as a function of the nominal trig- gering level given by relation (3). There is thus ob-tained, as shown in Figure 2, a unique law of variation for the triggering set-point level during the activation phase, which is interrupted once the nominal triggering level is attained. On the other hand, some speed in triggering is lost in the event of low-altitude release without pulling-up, but to a fairly insignificant degree.

Figures 3 to 6 illustrate the manner of trigger-ing of an altimeter trigger subjected, during an activation phase, to a linear or quadratic increase in its set-point level, this in the case where the projectile to which it is fitted is released at low altitude with and without pulling-up. In these figures the full curve represents the actual path of the projectile taking into account aerodynamic braking and the dashed curve the difference between the level of the projectile over its path and the set-point level imposed on the altimeter trigger, which difference, when it vanishes, brings about the triggering of the altimeter trigger and the activation of a braking device.

The projectile is released by a carrier at A. Its altimeter trigger is set going a short instant afterwards at B. It then undergoes an activation phase in the course of which its set-point level changes progressively from an almost zero minimum release level to the nominal triggering level attained at C. It is triggered at D when the projectile's flight level attains its set-point level and actuates the braking device.

For all of Figures 3 to 6, the minimum release level has been chosen equal to 40 metres and the nominal triggering level to 2000 metres. The duration T of the activation phase has been calculated from relation (3) with a minimum angle of ascent a value at the moment of release of 15 and an angle of descent em of +10 .

Figures 3 and 4 show the effect of an activation phase with an increase in the set-point level according to a law of linear progression.

The case of Figure 3 deals with a low-altitude release with pulling-up. The carrier is assumed to be in level flight at a level of 20 metres and then to pull up, during which it releases the projectile at A at a level of 600 metres, with a speed of 350 metres per second and under an angle of ascent of 45 . It is observed that the duration T of the activation phase is slightly greater than the period required by the projectile to ascend above the nominal triggering level and that the set-point level reaches the nominal triggering level whilst having always remained below the projectile's flight level, illustrated by the fact that the broken curve represent- ing the difference between the actual level and the set-point level does not cut the abscissa axis in this part of the path.

The case of Figure 4 deals with a low-altitude release without pulling-up. The carrier is assumed to be in level flight at a level of 60 metres and to release the projectile without pulling-up, with a speed of 350 metres per second and under an angle of ascent of 1 .

Triggering occurs practically instantaneously with the setting going of the altimeter trigger since the set-point level which grows linearly in the course of the activation phase very quickly passes from the minimum release level of 40 metres to a level higher than 60 metres. It is observed that the delay in triggering due to the activation phase is negligible by comparison with the actuation time of the braking device.

Figures 5 and 6 show the effect of an activation phase with an increase in the set-point level following a law of quadratic progression duplicating the variation in the duration T as a function of the nominal triggering level.

Figure 5 takes up again the conditions of release of Figure 3. The carrier is assumed to be in level flight at a level of 20 metres and then to pull up, during which it releases the projectile at A at a level of 600 metres, with a speed of 350 metres per second and under an angle of ascent of 450 The duration T of the activation phase has not varied and remains slightly greater than the period required by the projectile to ascend above the nominal triggering level. By contrast, it is observed that the increase in the set-point level is much slower at the start of the activation phase, which gives a much more arched shape to the start of the broken curve representing the difference between the set-point level and the actual level.

Figure 6 takes up again the conditions of release of Figure 4. The carrier is assumed to be in level flight at a level of 60 metres and to release the projectile without pulling-up, with a speed of 350 metres per second and under an angle of ascent of 1 . Triggering occurs slightly after the setting going of the altimeter trigger. It is a little longer in coming than in the case of Figure 4 by virtue of the smaller rate of increase in the set-point level at the start of the activation phase.

It, however, remains compatible with the actuation time of the braking device.

Figure 7 shows the diagram of an altimeter -10 -trigger enabling the method of activation just described to be implemented. The latter includes an altimeter probe 1 (of radar, barometer or other kind) which delivers an electrical voltage proportional to the measured level, a threshold voltage source 2 memorising, in the form of an electrical voltage, the chosen nominal triggering level, a threshold voltage progressive application circuit 3 and a comparator 4 comparing the electrical voltage provided by the altimeter probe with that delivered by the thres-hold voltage progressive application circuit 3 and delivering a triggering signal when the electrical voltage provided by the altimeter probe 1 becomes less than or equal to the electrical voltage delivered by the threshold voltage progressive application circuit 3.

Figure 8 details a digital embodiment of the threshold voltage source 2 and of the threshold voltage progressive application circuit 3 of the altimeter trigger of Figure 7.

The threshold voltage progressive application circuit 3 consists of a read-only memory 30 storing the digitised values of the law of quadratic variation defined by relation (4), giving a change of set-point level as a function of time t, of a digital-analog converter 31 placed at the data output of the read-only memory 30 delivering the value of the set-point threshold, level to the comparator 4 in the form of an electrical voltage, of a programmable counter 32 with a counting output connected to the addressing input of the read-only memory 30 and a programming input, and of an oscillator 33 providing a clock signal to the programmable counter 32. The threshold voltage source 2 consists of a digital programmer 20 coding the chosen nominal triggering level into a maximum addressing value applied to the prograinm-ing input of the counter 32.

On setting the altimeter trigger going, that is to say on powering it up, the altimeter probe 1 begins to provide its measurement voltage to the comparator 4 whilst the latter receives a zero set-point voltage from the threshold voltage progressive application circuit 3.

-11 -The threshold voltage progressive application circuit is then set going and delivers a threshold voltage cor-responding to the possible minimum, that is to say to the minimum release level Ho. Its counter 32 starts up, bringing about the reading, from the read-only memory 30, of a set-point level growing according to a quadratic law and is then halted having arrived at the maximum address-ing value delivered by the digital progranmier 20, which maximum value corresponds to the chosen nominal trigger-ing level.

The threshold voltage source 2 and the threshold voltage progressive application circuit may likewise be designed in digital form so as to apply to the altimeter trigger, during the activation phase, a set-point level changing according to a law of linear progression. In this case, the read-only memory 30 is divided into pages each containing a digital sampling of the law of linear variation corresponding to a given nominal triggering level and possesses a part of its addressing input devoted to page partitioning addressed directly by the digital programmer 20 which determines, as a function of the chosen nominal triggering level, both the value of the maximum addressing of the counter 32 and the choice of page in the read-only memory 30 containing the proper law of linear variation.

Claims

-12 -

CLAIMS

1. A method of activating an altimeter trigger fitted to a projectile, consisting, after releasing the projectile, progressively activating the altimeter trigger by providing it with a set-point level changing progressively from a near-zero level to the level anticipated for triggering which is attained only after a certain period.

2. A method according to Claim 1, wherein the period during, which the triggering set-point level of the altimeter trigger changes progressively from a near-zero level to the level anticipated for triggering is proportional to the square root of the deviation existing between the said near-zero level and the said anticipated triggering level.

3. A method according to Claim 2, wherein the period T during which the triggering set-point level of the altimeter trigger changes progressively from a near-zero level Ho to the level Hd anticipated for triggering has a value determined as a function of the relation: r Hd-Ho 11/2 T[ I g(1/(1-k.tan Om)-0.5) in which g is the acceleration due to gravity, k a coefficient of around cot 15 , em an angle of around +10 representing the minimum slope allowing the path to be regarded as descending.

4. A method according to Claim 1, wherein the change in the triggering set-point level from a near-zero level to the level anticipated for triggering occurs, as a function of time, according to a linear progression.

5. A method according to Claim 1, wherein the change in the triggering set-point level from a near-zero level to the level anticipated for triggering occurs, as a function of time, according to a quadratic progression.

6. An altimeter trigger including an altimeter probe -13 -which delivers an electrical measurement voltage proportional to the ground distance or to the altitude, a threshold voltage source representing the ground distance or the altitude anticipated for triggering and a comparator comparing the threshold voltage provided by the threshold voltage source with the electrical measurement voltage provided by the altimeter probe and delivering a triggering signal when the threshold voltage becomes greater than or equal to the measurement voltage, wherein it furthermore includes a circuit for progressive application of the threshold voltage, inserted between the threshold voltage source and comparator.

7. An altimeter trigger according to Claim 6, wherein the threshold voltage progressive ap-plication circuit includes a read-only memory storing the digitised values of one or more laws of variation of the set-point level, a digital/analog converter placed at the data output of the read-only memory and delivering the value of the threshold set-point level to the comparator, a programmable counter with a counting output connected to the addressing input of the read-only memory and an oscillator providing a clock signal to the program-mable counter. and in that the threshold voltage source includes a digital programmer coding the chosen nominal triggering level into a maximum addressing value applied to the programming input of the counter of the threshold voltage application circuit.

8. An altimeter trigger according to Claim 7, wherein the read-only memory stores the digitised values of a law of variation of the set-point level H as a function of time t complying with the equation: H(t) = t2(g(1/(1-ktan Om)-1/2)]-Ho in which g is the acceleration due to gravity, Ho the minimum release level, k a coefficient of around cot 15 and em an angle of around +10 representing the minimum slope allowing the path to be regarded as descending.

-14 - 9. An altimeter trigger according to Claim 7, wherein the read-only memory is divided into pages each containing digitised values of a law of linear variation of the set-point level appropriate to a given value of the nominal triggering level; and in that the addressing of read-only memory at page level is carried out by the digital programmer of the thres-hold voltage source.

10. A method of activating an altimeter trigger fitted to a projectile as described hereinbefore with reference to the accompanying drawings.

11. An altitude trigger substantially as described hereinbefore with reference to the accompanying drawings and as illustrated in Figures 7 and 8 of those drawings.

Amendments to the claims have been filed as follows 1. A method of activating an altimeter trigger fitted to a projectile, consisting, after releasing the projectile, progressively activating the altimeter trigger by providing it with a set-point level changing progressively from a near-zero level to the level anticipated for triggering which is attained only after a certain period.

2. A method according to Claim 1, wherein the period during, which the triggering set-point level of the altimeter trigger changes progressively from a near-zero level to the level anticipated for triggering is proportional to the square root of the deviation existing between the said near-zero level and the said anticipated triggering level.

3. A method according to Claim 2, wherein the period T during which the triggering set-point level of the altimeter trigger changes progressively from a near-zero level Ho to the level Hd anticipated for triggering has a value determined as a function of the relation: r Hd-Ho J1/2

TL

g(1/(1-k.tan 9m)-0.5) in which g is the acceleration due to gravity, k a coefficient of around cot 15 , em an angle of around +10 representing the minimum slope allowing the path to be regarded as descending.

4. A method according to Claim 1, wherein the change in the triggering set-point level from a near-zero level to the level anticipated for triggering occurs, as a function of time, according to a linear progression.

5. A method according to Claim 1, wherein the change in the triggering set-point level from a near-zero level to the level anticipated for triggering occurs, as a function of time, according to a quadratic progression.

6. An altimeter trigger including an altimeter probe which delivers an electrical measurement voltage proportional to the ground distance or to the altitude, a threshold voltage source representing the ground distance or the altitude anticipated for triggering and a comparator comparing the threshold voltage provided by the threshold voltage source with the electrical measurement voltage provided by the altimeter probe and delivering a triggering signal when the threshold voltage becomes greater than or equal to the measurement voltage, wherein it furthermore includes a circuit for progressive application of the threshold voltage, between the threshold voltage source and comparator.

7. An altimeter trigger according to Claim 6, wherein the threshold voltage progressive ap-plication circuit includes a read-only memory storing the digitised values of one or more laws of variation of the set-point level, a digital/analog converter placed at the data output of the read-only memory and delivering the value of the threshold set-point level to the comparator, a programmable counter with a counting output connected to the addressing input of the read-only memory and an oscillator providing a clock signal to the program-mable counter; and in that the threshold voltage source includes a digital programmer coding the chosen nominal triggering level into a maximum addressing value applied to the programming input of the counter of the threshold voltage application circuit.

8. An altimeter trigger according to Claim 7, wherein the read-only memory stores the digitised values of a law of variation of the set-point level H as a function of time t complying with the equation: H(t) = t2[g(1/(1-ktan Om)-1/2)]-Ho in which g is the acceleration due to gravity, Ho the minimum release level, k a coefficient of around cot 15 and em an angle of around +10 representing the minimum slope allowing the path to be regarded as descending.

9. An altimeter trigger according to Claim 7, wherein the read-only memory is divided into pages each containing digitised values of a law of linear variation of the set-point level appropriate to a given value of the nominal triggering level; and in that the addressing of read-only memory at page level is carried out by the digital programmer of the thres-hold voltage source.

10. A method of activating an altimeter trigger fitted to a projectile as described hereinbefore with reference to the accompanying drawings.

11. An altitude trigger substantially as described hereinbefore with reference to the accompanying drawings and as illustrated in Figures 7 and 8 of those drawings.