Roll angle determination

Abstract: The invention relates to an arrangement for determining the roll angle of a rotating projectile, shell, missile or the like as it leaves the barrel or launch tube. The projectile comprises a magnetised part (2) with a known polarisation direction, and the barrel or launch tube is provided with two pairs of windings (9, 10) mounted at the very front of the muzzle bell of the barrel in such a way that a voltage is induced in the windings when the projectile passes the mouth, and an evaluation unit (17) is designed to calculate, with the aid of the voltage signals, the roll angle position of the projectile upon firing.

Description: The present invention relates to an arrangement for determining the roll angle of a rotating projectile, missile or the like by magnetic means as it leaves the barrel, launch tube or the like. The invention is applicable to all types of projectiles, missiles or the like which are fired from a barrel or launch tube and which rotate in their trajectory. The invention can be used in particular in so-called terminal-stage-guided ammunition, i.e. projectiles which are fired in a conventional manner in a ballistic trajectory to the immediate vicinity of the target, where they receive a command for necessary correction. Due to the fact that the projectile rotates in its trajectory, its roll position must be determined when the command is executed. In the absence of members for determining the roll position, an error otherwise occurs in the course correction. It is already known from Swedish Patent Application 8801831-2 to determine the roll angle position with the aid of polarised electromagnetic radiation, comprising a transmitter arranged to emit a polarised radiation in the direction towards the projectile and a polarisation-sensitive receiver arranged in the projectile. By having the emitted polarised radiation consist of at least two mutually phase-locked radiation components with a wavelength ratio of 2:1 and/or multiples thereof, which are superposed and form an asymmetrical curve shape, the roll position of the projectile can be unambiguously determined. The abovementioned arrangement presupposes that a transmitter is placed in connection with the launching position of the projectile and that the projectile is provided with a rearward-directed receiving antenna in order to receive the transmitted radiation. Although an arrangement of the type described permits an unequivocal determination of the roll position with satisfactory precision and without ambiguity, it can be a disadvantage to be dependent on two mutually phase-locked frequencies since both the transmitter and receiver are more complicated. It is also already known to determine the roll angle position by magnetic means by sensing the earth's magnetic field, see EP 0 319 649. Such a system is, however, latitude-dependent and sensitive to interference. The aim of this invention is to provide an alternative to the methods described above for roll angle determination, in which the determination is carried out by magnetic means instead of with transmitted microwave radiation, and without being dependent on the earth's magnetic field. The way this has been achieved emerges from the characterising clause of Patent Claim 1. An embodiment of the invention is shown diagrammatically in the attached drawings, in which Figure 1 shows a projectile (ballistic high-explosive shell) provided with a permanent magnet, Figure 2 shows the magnetic field orientation, Figure 3 shows a gun barrel with muzzle bell provided with two pairs of windings, Figure 4 shows diagrammatically how an induced voltage is generated as the projectile passes the windings, and Figure 5 shows an example of an evaluation unit for the sensor signals. Figure 1 shows a projectile in the form of a ballistic high-explosive shell 1, intended to be fired in a conventional manner from a barrel. A circular permanent magnet 2 is mounted in a wedge-shaped groove 3 in the nose cone casing of the shell in such a way that the magnetic field is oriented transverse to the longitudinal direction 4 of the shell, see Figure 2. The position of the permanent magnet 2 is chosen by taking into consideration the temperature stresses and acceleration stresses. The magnet can be of ferrite material and magnetised upon assembly. The magnet is assembled in a fixed position in a plane orthogonal to the longitudinal direction of the shell so that correct angle information will be obtained (see below), in which respect an antenna in the rear plane of the shell may constitute a reference. Two non-magnetic rings 5, 6 are arranged in front of and behind the permanent magnet. The shell is in other respects conventional and is therefore not described in greater detail. The mouth of the gun barrel 7 is equipped with a muzzle bell 8 in the form of a truncated cone. Two pairs of windings 9, 10 are mounted on the outermost part of the muzzle bell, each pair of windings consisting of two series-coupled windings placed on each side of the projectile trajectory. As the shell passes the two pairs of windings, a voltage is induced in the windings and, by means of suitable signal processing, the roll angle of the shell upon passage through the mouth can be determined. The roll angle is conveyed to a central unit, from which the angle information and time after firing can be conveyed to the projectile via a command link. By means of suitable electronics, the projectile can then calculate the actual rotation position from this information. These parts - central unit, command link and projectile electronics - do not however constitute part of this invention and are therefore not described in greater detail. The pairs of windings are expediently arranged in their respective grooves 11 in a circular retainer 12 mounted at the very front of the muzzle bell. The windings themselves are designed as rectangular coils 14, 15 which follow the curve of the muzzle bell, see Figure 3. Non-conductive and non-magnetic material is used as a base for the mounting of the windings, and the material will additionally be resistant to temperature and acceleration shocks. When the projectile with its magnet passes the windings, e.m.f.'s in accordance with Fig. 4 are induced according to the formula: ê = N · d dt [V] where ê = induced voltage in volts N = number of turns on winding d dt = flux alteration per time unit. For winding 1 and 2, the following applies: ê₁ = K·V o ·cosα resp· ê₂ = K · V o · sinα [V] where K = constant depending on the design of the winding and the dipole moment of the magnet V o = initial velocity of projectile ( d dt ∼ V o ) α = angle to the centre line of the windings. As the windings are turned 90° relative to each other, the induced voltage peaks lie in relation to each other in the ratio sinα/cosα , which gives: ê₁ = K · V o · cosα [V] ê₂ = K · V o · sinα [V] The following derivation shows how K and V o are eliminated: The ambiguity in the arc cos function is eliminated by studying the signs of e₁ and e₂. An estimate of the voltage induced in a winding has been made, in which ê = 2.6 mV/turn. For an A-D converter with 8 bits and 5 mV resolution the following is required: where N = the number of turns in a pair of windings. The voltages ê (sensor signals) induced in the windings 9, 10 are conveyed via cabling 16 to an evaluation unit 17 (see Figure 5) situated on the barrel 7 in the vicinity of the mouth and advantageously suspended in a shock-absorbing manner. Voltage feed and two-way transmission to a central unit (not shown) is via a common coaxial cable 18, adapted for high transmission speed. The evaluation unit 17 comprises two A-D converters 19, 20, registers 21, 22 and comparators 23, 24 connected to a microprocessor 25 for calculating the angle value α. The microprocessor 25 is connected via a MODulator 26 to the central unit via the said coaxial cable 18. The function of the evaluation unit is as follows. Immediately before firing, the A-D converters 19, 20 and the registers 21, 22 are reset. Clock signals CLOCK A and CLOCK B sample the A-D converters at a considerably higher frequency than the highest component frequency in the measurement signal (over-sampling). When the measurement signals appear, the analog signals are converted to digital quantities and are clocked over to the digital registers 21, 22 with a clock pulse displacement. When the comparators 23 and 24 detect that the register values are greater than the value just converted in the A-D converter 19 and 20, CLOCK A or CLOCK B is blocked. The peak value now lies stored in register 21 or 22 and can be input to the microprocessor 25 for evaluation. The calculated value in the microprocessor 25 is transmitted in serial form via the MODulator 26 to the central unit (not shown) via the coaxial cable 18. The control command to the microprocessor 25 can also be transmitted from the central unit via a DEModulator 27. The supply voltage to the measurement unit 17 is dealt with by the central unit with the aid of the cable 18. The voltage is applied to the electronics with the aid of a choke 28. The modulated signal is blocked at its frequency by the choke, and the coupling capacitors 29 and 30 on DEM and MOD block the d.c. level on cable 18.

Claims: Arrangement for determining the roll angle of a rotating projectile, missile or the like as it leaves the barrel, launch tube or the like, characterised in that the projectile comprises a magnetised part (2) with a known polarisation direction, in that at least two pairs of windings (9, 10) are assembled in connection with the barrel or the launch tube in such a way that a voltage is induced in the windings when the projectile passes the mouth and an evaluation unit (17) designed to calculate, with the aid of the said voltage signals, the roll angle position of the projectile upon firing. Arrangement according to Patent Claim 1, characterised in that the projectile comprises a permanent magnet (2) which is assembled in such a way that its magnetic field is oriented transverse to the longitudinal direction (4) of the projectile and in a fixed position in a plane orthogonal to the longitudinal direction of the projectile. Arrangement according to Patent Claim 2, characterised in that the permanent magnet (2) is circular and arranged in a groove (3) in the nose cone casing of the projectile in a plane perpendicular to the longitudinal direction (4) of the projectile. Arrangement according to Patent Claim 1, characterised in that each pair of windings (9, 10) consists of two series-coupled windings placed on each side of the projectile trajectory and at 90° relative to each other. Arrangement according to Patent Claim 4, characterised in that the windings in each pair of windings (9, 10) are designed as rectangular coils (14, 15) which follow the curve of the muzzle bell. Arrangement according to Patent Claim 5, characterised in that the pairs of windings (9, 10) are arranged in a respective groove (11) in a circular retainer (12) mounted at the very front of the muzzle bell of the barrel. Arrangement according to Patent Claim 1, characterised in that the voltages (e) corresponding to the sensor signals induced in the pairs of windings (9, 10) are conveyed to the evaluation unit (17), sampled by A-D converters (19, 20) and evaluated digitally in comparators (23, 24) and thereafter conveyed to a microprocessor (25) for calculation of the angle value.

Inventors: HANSEN AKE, NILSSON BERNDT, HANSEN, AKE, NILSSON, BERNDT, Hansén, Ake
Applicants: BOFORS AB, AB BOFORS
Application year: 1991
Main classification: F41G7
IPC codes: F41G7, G01B7, F41A21