GB2200215A - Determination of projectile velocity - Google Patents

Abstract

Apparatus 14 for determining the velocity of a shell 13 in, or leaving, a large calibre ordnance gun barrel 10 has coil means 15 wound around the barrel, adjacent the muzzle, coil energising means 16 to pass direct current through it to form a magnetic field in the air core of the muzzle and measuring means 19 to determine the back-emf generated in the coil when a shell passes through the field and temporarily replaces the air core. The large rate of change of magnetic flux caused by a ferromagnetic shell moving at high speed enables a coil of modest ampere turns to produce a back-emf of several hundred volts. The emf is directly related to the shell velocity and is measurable with such resolution that small changes in velocity can be resolved, enabling the velocity measurement to be used in operation in a ballistics computer for subsequent rounds. The coil means may take other forms, such as a pair of coils disposed on an axis across the bore and folded around the barrel as saddles (Fig. 5 not shown). The apparatus also responds to non-ferromagnetic conductive projectiles by virtue of their movement through its magnetic field in accordance with Lenz's law. <IMAGE>

Description

DETERMINATION OF PROJECTILE VELOCITY This invention relates to determination of the velocity of a projectile travelling along the barrel of a gun and is particularly, but not exclusively, concerned with velocity at the muzzle of a gun and to such determination with operation of the gun as a means of improving its aim accuracy.

The invention is particularly, but also not exclusively, concerned with large calibre guns for which known techniques of determining muzzle velocity have been either complex or ineffective.

Muzzle velocity is employed in ballistic ranging calculations as a factor in determining the range of a particular projectile in combination with other factors such as launch elevation angle and air resistance, the velocity value employed essentially representing the maximum velocity of the projectile before retardation effects define the remainder of its trajectory.

It is well known that for most gun fired projectiles the maximum velocity, but called the muzzle velocity, is reached only when the projectile has travelled some distance from the muzzle of the gun, being accelerated thereto by outflowing muzzle gases until they disperse.

Methods have been proposed for determining muzzle velocity for various purposes associated with projectile range, such as estimating gun barrel wear, charge temperature or monitoring the relative flight characteristics of different projectile types and these are divided essentially into two types. A first method gathers information from the projectile after it has left the immediate vicinity of the gun, such as by radar tracking, whereas a second method measures the actual velocity of the projectile as it emerges from tha muzzle.

Both of these methods measure other than the traditional muzzle velocity value but the known dynamics of tie projectile behaviour enables the velocity actually measured to be extrapolated to give the equivalent maximum velocity for calculations involving it.

This invention is particularly concerned with the second of the above broadly outlined methods and the measurement of actual muzzle velocity. Hereinafter in this specification the term muzzle velocity" is used to mean the actual muzzle velocity measured at the emergence of a projectile from the muzzle of a gun barrel with the understanding that any other known derivation of "muzzle velocity" can be extrapolated therefrom.

It is well known that projectile muzzle velocity is the major factor in determining the projectile range and is dependent upon such factors as propellant charge temperature, barrel wear, barrel temperature, projectile weight and shape as well as external environmental factors.

It is known to "calibrate" a particular gun by determining the muzzle velocity under controlled conditions for different projectiles and charges etc. in order to determine the likely muzzle velocity in any particular engagement to be used in range laying. With the introduction of computer-based fire control systems many parameters which influence the trajectory of a projectile can be monitored and the effects of these taken into account in off-aiming the gun from a particular sighted target.

Clearly it is desirable as part of such a system to include a measurement of muzzle velocity as one of the many parameters taken into account, and some known muzzle velocity measuring systems are clearly unsuited to exposed 'on-line' operation under battle conditions when the gun has to operate or move cvntinuously.

Muzzle velocity measuring arrangements have been proposed for direct attachment to the gun barrel involving strain guages, pressure transducers and electromagnetic coils.

Examples of such systems employing electromagnetic coils disposed adjacent the muzzle of a gun are given in Canaan Patent No. 950,700, U.S. Patents Nos. 4,228,397, 4,486,710 and 4,524,323 and Published PCT application No.

WO 84/01224.

These operate essentially either by the interaction of a ferromagnetic projectile body with a magnetic field of the apparatus or by disrupting a tuned oscillatory circuit to produce a small induced signal in a sensor coil to mark the passage of the projectile. The first numbered patent, intended basically for small bore arms, employs a pair of such coils located a predetermined distance part along the barrel axis and a time difference between occurrance of said induced signals is employed to provide a representation of the muzzle velocity.

Others use one or more coils and the time taken for the passage of a projectile of known dimensions is measured with the same result.

Whilst kndwn arrangements may enable a magnitude to be derived for the muzzle velocity with an ordinarily acceptable percentage error they are not in fact well suited to the above mentioned operational use where measurements made on a round-toround basis effect the corrections made for the firing of subsequent rounds.

In operation it is of less importance to know the absolute magnitude of the muzzle velocity of each projectile fired than to know how the muzzle velocity changes for each round in order to make suitable gun corrections for the next fired round, that is, the resolution of measured value is important. For example, a high explosive shell may be fired with a muzzle velocity of the order of 650 mlsec at a target at a range of 1500 metres and a muzzle velocity change of even of the order of Imlsec can effect a significant change in projectile range of the order of 2.3 metres.

Furthermore the measurement of projectile velocity at other points along the barrel before reaching the muzzle would enable the rate of wear along the barrel to be monitored but none of the known systems enable such measurement.

It is an object of the present invention to provide apparatus for determining the velocity of a projectile travelling along a gun barrel which mitigates the disadvantages of known projectile velocity measuring apparatus in fire control arrangements.

According to the present invention apparatus for determining the velocity of a projectile fired by a gun comprises electromagnetic coil means disposed adjacent the barrel wall and circuit means comprising coil energising means operable to provide a direct current through the coil means so as to generate a magnetic field extending into the path of the projectile and measuring means responsive to the generation of a back-emf in the coil means due to disturbance of the magnetic field by upon passage of a projectile fired by the gun to determine from the magnitude of the back-emf the velocity of the projectile past the coil means.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a sectional elevation through the muzzle portion of a gun barrel showing apparatus according to the present invention for determining the velocity of a projectile, fired by the gun, at the muzzle, the apparatus including d.c.

energised electromagnetic coil means in the form of a single coil wound coaxially with the barrel wall in which a back-emf is generated by passage of a ferromagnetic projectile, Figure 2 is a waveform of the back-emf generated in the coil means of Figure 1 by passage of a projectile, Figure 3 is a sectional elevation through a form of projectile not made of ferromagnetic material, Figure 4 is a sectional elevation through the muzzle portion of a gun barrel showing an alternative arrangement of coil means, Figure 5 is a perspective view of a preferred form of the coil means of Figure 4, and Figure 6 is a sectional elevation through a gun barrel showing apparatus according to the invention with coil means Df the form shown in Figure 1 disposed at points along the barrel relate from the muzzle.

Referring to Figure 1 a gun, for instance a large calibre ordnance piece, has a steel barrel 10, only a muzzle portion of which is shown, defining the muzzle 11. The barrel bore and muzzle centre are disposed upon the longitudinal axis 12 of the barrel tube.

The gun is intended to fire projectiles in the form of conventional high explosive shells, one of which is shown at 13, or, kinetic energy and chemical energy shells as discussed later. Apparatus 14 according to the present invention for determining the velocity of such a shell at the muzzle comprises electromagnetic coil means in the form of a single coil 15 wound about the periphery of the barrel adjacent the muzzle and through the centre of which the axis 12 extends.

The apparatus also comprises coil energising means 16 operable to supply direct current to the coil of such magnitude in relation the number of turns of the coil as to develop a magnetic field 17 at the muzzle portion. The flux lines of field 17 extend in part through the steel of the barrel and where the barrel ends spread through the air, extending at-the muzzle into a region 18 forming a notional extension of the barrel through which the projectile 13 will pass. The apparatus 14 also comprises measurement means 19 operable to monitor the voltage appearing across the coil and to determine the magnitude of a back-emf generated in the coil when the projectile passes through the muzzle and region 18.The measurement means comprises direct current decoupling means 20, such as a capacitor, and amplitude measuring means 21 operable to determine the magnitude of a transient back-emf pulse passed by the decoupling means 20. The amplitude measuring means may comprise sample and hold means 22 operable to store å representation of the maximum back-emf generated, analogue to digital (ADC) conversion means 23 operable to convert the magnitude of the stored emf into a digital number, scaling means 23 operable to multiply the emf value by a constant stored in constant store 23' and output means 24 operable to drive display means 25 and/or provide an output 26 for a ballistic computer representative of the magnitude of the generated back-emf.

It is useful at this point to consider the operational theory governing operation of the apparatus. Consider coil 15 as being of length L and wound with N turns on tube 10 of inner radius a and outer radius b, the relative permeability of the tube being 1 and relative permeability of the core being 2.

The magneto-motive force generated in the coil by direct current I is given by N.I and the magnetic flux = = N.I/SA (1) where 5A is the reluctance of the core and tube. This may be considered as separate reluctances S1 and S2 for the tube and core respectively in parallel so that equation (1) becomes #A = N.I(S1 + S2) /S1.S2 .......... (2) If the core material is replaced by another of relative permeability 3 and reluctance S3 the new magnetic flux #B = N.I.(S1 + S3)/S1.S3 ...............(3) The introduction of the new core material generates a back-emf in the coil proportional to the rate of change of flux (d/dt) and expressed as E = -N.d#/dt = -N.(#B - #A)/t or, E=-(N2,I/t).E S3)/S2S3 = -(N2.I/t).(S2-S3)/S.S3 .53 (4) Now reluctance S2 = L/(#0.#2.#.a ) and 53 = L/(#0.#3.#.a2), where #0 is the absolute permeability of air which substituted in equation (4) gives E = -N2.I.#0.#.a2.(#3-#2)/t.L ........(5) 2 = 1 and 0 = 4#.10-7 so the equation (5) gives E = -(N2.IIL).a2.(L3-1]It).4it2.1O7 (6) As the replacement core is provided by the projectile fired, the distance L covered in time t represents the velocity V of the projectile so that equation (6) can be expressed as E = -(N2.I/L2).a2.([ 3-1].V). 42.10-7 ..(7) It will be seen from equation (7) that the factor (N2.11L2) is defined by the coil, a2 is defined by the gun calibre and ([V3-1].V) by the projectile, and for a known value of constant 3 the generated back-emf E is directly proportional to the velocity V of the projectile. Furthermore as the coil is located at the muzzle the velocity V defines the muzzle velocity of the projectile.

Consideration of equation (7) will show that the magnitude of the back-emf generated is maximised for a short coil length L, operation with a large tube (barrel) radius a, a high relative permeability projectile and a high velocity.

For a typical installation with a 120mum calibre gun (giving a = 0.12 metres) firing a steel bodied (u3=300) HESH round at V = 667 metreslsec. a coil length L of 0.4 metres having a modest (N=) 10 turns and passing a current I of 0.5 amps produces a back-emf of some 450 volts. The wave form of the back-emf generated by such a projectile-coil combination is illustrated in Figure 2.

As stated above it is desirable in operation to determine the muzzle velocity of the projectile with sufficient degree of resolution to be able to quantify any changes in muzzle velocity due to changes in gun and/or projectile parameters which have an effect on projectile range, such as barrel wear or charge temperature, and alter the value of this parameter as produced for the next fired projectile.

Thus by providing an arrangement which by the generation of a back-emf enables a measured parameter of such an order of magnitude as described above a muzzle velocity value differing from that of a previous round by the order of 0.05 per cent is readily resolved.

It will be appreciated from the equation (7) that the large back-emf generated is due to the high muzzle- velocity of the projectile and relatively large barrel diameter as well as the permeability of the projectile body. However for smaller calibre weapons or slower projectiles the direct current in the coil and/or the number of turns can readily be increased without introducing problems of coil mounting or current generation.

It has been assumed for the above explanation that the projectile has a ferromagnetic body. It will be appreciated that gun-launched projectiles such as the kinetic energy and chemical energy rounds mentioned above are now being produced with conductive non-ferrous bodies, an example being the Armour Piercing Discarded Sabot (APDS) round as illustratpd in the sectional elevation of Figure 3.

The APDS projectile 27 comprises an aluminium sabot case 28 including aluminium sabot petals 29 between which is held for launch from the gun a penetrator 30 of tungsten or depleted uranium.

When such a projectile is fired by the gun the aluminium sabot case, upon reaching the muzzle, cuts the lines of magnetic flux 17 produced by the coil 16 and as can be readily determined from the Lenz Left Hand Rule, the movement causes a current to be generated within the projectile which in turn creates a magnetic field around the projectile in accordance with Flemings Right Hand Rule. The interaction of this induced field with that of the coil 15 induces a voltage change in the coil which can be detected in the same manner as the back-emf considered above.Although the voltage change produced by such ammunition material may be expected to be of somewhat smaller magnitude than with ferromagnetic ammunition APDS rounds, at least, are fired significantly (about 50 per cent) faster than ferromagnetic HESH shells, with correspondingly increased back-emf, and the coil parameters may be varied as indicated above to provide adequate resolution for a particular ammunition as desired.

If desired, such conductive non-ferrous ammunition as shown in Figure 3 may be provided with a ferromagnetic component, for example, a disc 31 as shown attached to the sabot petals 29. Such a ferromagnetic component, although aided by the Lenz law induction outlined, will produce a back-emf of greater level approaching that expected for a completely ferromagnetic projectile body.

The above description has related to coil means defined oy a single coil wound around the barrel such that the longitudinal axis of the coil is coincident with the bore axis of the barrel.

An alternative form of coil means is shown in Figure 4 32 comprising a pair of coils 33 and 33' spaced apart along a common longitudinal axis 34 extending orthogonally to the bore ?xis 12' of gun 10' and disposed adjacent the muzzle portion of the barrel. Each coil is provided with direct current from supply 15 as outlined above and the magnetic flux from each coil links with the barrel wall except at the muzzle where the flux from the two coils links across the muzzle as shown at 35.

It will be appreciated that the greater flux linkage across the air-filled core comprising the muzzle is subjected to a correspondingly greater change when that core is replaced by, a ferromagnetic projectile body or component or the flux is cut by any conductive body wherein a Lenz law current may be induced.

It will be appreciated from Figure 4 that the flux linkage across the muzzle is essentially due to those parts of the coils running tangentially to the edge of the muzzle.

Accordingly, although the coils may comprise planar coils extending in the planes shown in the sectional elevation of Figure 4, the coils are preferably formed with a saddle or yoke shape which extends partially around the periphery of the barrel as shown at 33" and 33"' in Figure 5, thereby extending the length of coil which runs along the edge of the barrel.

The theoretical relationship between ampere-turns of the coil means, back-emf generated and the projectile velocity is similar to that outlined above for the single coil, bearing in mind that the provision of two coils each producing magnetic flux enables a greater flux linkage to be obtained in the gun barrel, showing that the level of back-emf which may be expected to be generated may readily be made of such a value as to provide adequate resolution for small changes on a round-to-round basis which can be used advantageously in fire control ranging computations.Any non-linearity or undue complexity of calculations in the relationship caused by deforming the coils into the preferred saddle shapes may be accommodated for the gun by pre-calibration and provision of a look-up table instead of the constants store and scaling means 23' and 23 of Figure 1 whereby any back-emf generated can be related, for a particular ammunition type, to a muzzle velocity value.

As described above the apparatus of the present invention is arranged to measure a relatively large back-emf voltage from which to derive a muzzle velocity, or more particularly, detect a change from the last measured muzzle velocity. The measuring means 19 may be arranged such that instead of providing a muzzle velocity readingloutput,in a range extending from zero it provides a readingloutput representing a departure from a nominal or pre-calibrated muzzle velocity value, such a nominal or pre-calibrated value being determined from an actual firing of a projectile from the gun either by measuring means which does operate on a scale registering the absolute muzzle velocity value or by determining said nominal muzzle velocity by some independent measuring arrangement and applying the value to the measuring means of apparatus.

As stated at the beginning of the specification the muzzle velocity measured by the apparatus of the present invention is that of the projectile at the time it leaves the muzzle and is not the maximum value of velocity achieved somewhat later. It was suggested that known relationships exist between the velocity at the muzzle (the actual muzzle velocity) compared with the later maximum velocity employed in ballistic calculations (the ballistic muzzle velocity).Such relationship may be built into the apparatus (such as at 36 (Figure 1)), or a ballistics computer to which signals from the apparatus are fed, in the form of a look-up table whose values are produced by callibrating the apparatus after installation with any particular gun, for example, by firing the gun in order to obtain different projectile velocities and measuring the ballistic muzzle velocity by other arrangements and the actual muzzle velocity by the apparatus 14 and storing the relationships.

The above description has exclusively concerned the disposition of the coil means at a muzzle portion of a gun in order to determine the projectile velocity representing the actual muzzle velocity. The present invention is not limited to the determination of muzzle velocity and is able to determine the projectile velocity at any point at which it passes the coil means. The coil means may therefore be disposed adjacent any other part of the barrel, such as shown in sectional elevation in Figure 6 for a single coil means 15' (and 15") of the type shown in Figure 1.

The magnetic flux produced by the direct current in the coil extends weakly into the barrel bore whence it interacts with a passing projectile body to generate a back-emf of the form described above and related to the projectile velocity at that point.

The strength of flux extending into the bore will be considerably less than at the muzzle but may be increased by increasing the number of ampere turns of the coil to a value which provides a suitable magnitude of back-emf to enable resolution of operational velocity variations. Alternatively It may be arranged for the projectile body to have a much higher permeability than the gun barrel, that is, 3 V1i the presence of the projectile body may provide a particularly significant increase in flux linkage in addition to simple replacement of the normal core material of air and thus a larger level of generated back-emf.

By measuring the projectile velocity at a number of points along the barrel for example by further coil means 15" and circuit means at launch a more complete understanding'of wear patterns of the gun may be made.

Claims (14)

Claims

1. Apparatus for determining the velocity of a projectile fired by a gun comprising electromagnetic coil means disposed adjacent the barrel wall and circuit means comprising coil energising means operable to provide a direct current through the coil means so as to generate a magnetic field extending into the path of the projectile and measuring means responsive to the generation of a back-emf in the coil means due to disturbance of the magnetic field by upon passage of a projectile fired by the gun to determine from the magnitude of the back-emf the velocity of the projectile past the coil means.

2. Apparatus as claimed in claim 1 in which the coil and coil energising means are arranged to provide a magnetomotive force having a magnitude in operation related to an expected projectile velocity past the coil means whereby the resolution of variations in the back-emf is within the required resolution of changes in projectile velocity.

3. Apparatus as claimed in claim 2 in which the coil and the coil energising means are arranged to provide such a magnetic field strength in relation to the expected projectile velocity past the coil means that variations in the back-emf generated generated to variations in projectile velocity are readily resolved.

4. Apparatus as claimed in claim 3 in which the back-emf generated is in the order of hundreds of volts enabling resolution and measurement of variations therein under field conditions.

5. Apparatus as claimed in any one of claims 1 to 4 in which the coil means comprises a single helical coil coaxially disposed with respect to the barrel.

6. Apparatus as claimed in claim 5 in which the, or each, coil is wound on the barrel as a former.

7. Apparatus as claimed in any one of claims 3, 4 or 6 in which the coil means comprises a pair of coils wound on a common axis extending orthogonally to the bore axis of the barrel and spaced apart at opposite sides of the barrel.

8. Apparatus as claimed in claim 7 in which at least one of the coils of at least one pair is deformed to conform to a saddle shape extending partially about the periphery of the barrel.

9. Apparatus as claimed in any one of the preceding claims in which the coil means is disposed adjacent the muzzle of the barrel.

10. Apparatus as claimed in any one of the preceding claims including further coil means disposed at a different point along the barrel and circuit means therefore, said apparatus being operable to determine the velocity of a projectile at more than one point along the barrel.

11. Apparatus as claimed in any one of the preceding claims in which the measuring means includes scaling means operable to relate the magnitude of the back-emf generated to a constant factor relating back-emf to projectile velocity.

12. Apparatus as claimed in any one of the preceding claims in which the measuring means includes means operable to determine a difference between the measured back-emf and a known value representative of a nominal projectile velocity and to determine from said difference the departure of actual projectile velocity from the nominal projectile velocity.

13. Apparatus as claimed in any one of the preceding claims in which the measuring means include a look-up table operable to relate a measured back-emf value to a projectile velocity known to produce such a back-emf value to accommodate non-linearity in a relating factor.

14. Apparatus for determining the velocity of a projectile fired by a gun substantially as herein described with reference to Figures 1, 4, 5 or 6 of the accompanying drawings.