Magnetic proximity fuse

Abstract: The invention relates to a passive magnetic proximity fuse for initiating charging in a charge carrier (1), for example a guided missile, projectile, grenade or the like, when this passes at a certain distance from a ferromagnetic object. The proximity fuse includes one or more sensors (4) for sensing deviations in the terrestrial magnetic field and one or more position-sensing elements (6) (gyros or accelerometers) for sensing the charge carrier's own movement. The signal processor (5) is arranged to emit an active output signal (i out ) in association with a deviation in the terrestrial magnetic field such that the active output signal only occurs in dependence on deviations in the terrestrial magnetic field which are occasioned by ferromagnetic objects (3).

Description: The present invention relates to a magnetic proximity fuse for initiating the charging of a moving charge carrier, for example a guided missile, projectile, grenade or the like, when it passes at a certain distance from a ferromagnetic object. The proximity fuse comprises a first set of sensors for sensing deviations in the flux densities of the terrestrial magnetic field, a second set of sensors for sensing the carrier's own movement and evaluating means arranged to produce an active output signal in association with a deviation in the terrestrial magnetic field. Two types of magnetic proximity fuses are known, active and passive. The one hitherto most used has been the active magnetic proximity fuse. An example of such a proximity fuse is described in Swedish Patent Specification 77.06158-8. The proximity fuse has a transmitter unit with a generator coil which generates an electromagnetic field which is distributed in space in accordance with known laws. The proximity fuse also includes a receiver unit in the form of a sensor coil which is placed separately from the generator coil. When the sensor coil is affected by an electromagnetic field, an electromotive force is induced in the coil. When there is a metal object located in the field from the transmitter unit, eddy currents are induced in its surface. These eddy currents generate a secondary field which is detected by the receiver unit. This makes it possible to determine if a metal object is located in the vicinity of the proximity fuse. The range is determined by the output power of the transmitter unit and the sensitivity of the receiver unit. A "typical" range is 0.5 - 1.5 m. The active magnetic proximity fuse has a distance dependence which monotonically increases from r -3 to r -6 (r= distance between the proximity fuse and the target). A passive magnetic proximity fuse utilises the fact that the terrestrial magnetic field is deformed around ferromagnetic objects. for example large objects of iron, for example military tanks and bodies of iron ore. The proximity fuse includes a sensing system in the form of sensors for flux density, and a signal processing section for evaluating the signals. This is due to the fact that changes caused, for example, by a tank in the terrestrial magnetic field are comparable to signals which are obtained in the charge carrier. Moreover, a longer range can be obtained since the distance dependence only increases with r -3 . At a distance of 3 metres from an iron object of the size of a tank, the effect is of the order of magnitude of 5%, which is sufficient for detection. In addition to this, demands for systems which do not disclose themselves and for increased resistance to interference provide support for passive systems. Self-disclosure is built into an active system and such a system also detects all well-conducting objects, for example decoys of aluminium foil. A passive system does not disclose itself and requires large ferromagnetic objects in order to provide a signal. A magnetic proximity fuse according to the preamble is disclosed in DE-35,03,919. According to the disclosed passive fuse, the two sets of sensors are treated in two mutually independent channels. If the requirements for activation as defined in the two individual channel independently are fulfilled simultaneously, ignition is activated. An AND-gate is proposed to be used for detecting this simultaneous fulfilling. One problem with this proximity fuse is that the charge carrier's own movement affects the sensed terrestrial magnetic field. The object of this invention is to produce a magnetic proximity fuse without an active part, that is to say a passive magnetic proximity fuse with greater range than active and passive proximity fuses known earlier. As already mentioned. the passive magnetic proximity fuse must sense very small changes in the terrestrial magnetic field. Furthermore, the charge carrier's own movement in the terrestrial magnetic field will affect the signal. According to the invention, this problem has been solved by introducing a compensation for changes in the magnitude of the sensor signals of the first set of sensors caused by a change of position of the charge carrier. The magnetic proximity fuse is characterised in that he evaluating means comprises a signal processor arranged to compensate for changes in the magnitude of the sensor signals of the first set of sensors caused by a change of position of the charge carrier based upon the position of the charge carrier sensed by the second set of sensors, the active output signal being produced when the compensated sensor signals deviate from reference values of the terrestrial magnetic field, so that the active output signal only occurs in dependence on deviations in the terrestrial magnetic field which are occasioned by the ferromagnetic objects. The first set of sensors may be made up of flux gate sensors for sensing the flux densities or coils for sensing the time derivative of the flux densities. According to one embodiment the first set of sensors consist of Hall elements for sensing the flux densities. Furthermore, the second set of sensors may comprise gyros or accelerometers for measuring the roll and yaw movements of the charge carrier. Using a proximity fuse of this type, a greater range is obtained than with an active proximity fuse, and resistance to interference is improved. In the text which follows, the invention will be described in greater detail in connection with the attached drawings which, by way of example, show an advantageous embodiment of the invention. Figure 1 diagrammatically shows a moving charge carrier (guided missile) which is moving in the terrestrial magnetic field. Figure 2 shows a block diagram of the main parts of the proximity fuse, and Figure 3 shows a flow diagram of the signal evaluation. Figure 1 diagrammatically shows a moving charge carrier in the form of a missile 1 which is moving in the terrestrial magnetic field B. The front part of the missile is equipped with a proximity fuse 2 which is to sense if a ferromagnetic object, for example a tank 3, is located in the vicinity of the missile and then output an output signal for triggering the effective part of the missile. The proximity fuse 2 consists of a passive magnetic proximity fuse with sensors for the terrestrial magnetic field B. To facilitate the continued description, an orthogonal missile-fixed coordinate system with the XYZ axes according to the figure is introduced, that is to say the X axis coincides with the longitudinal axis of the missile, the Y axis is at right angles to the side and the Z axis is at right angles downwards. The position and movement of the missile can be described with the aid of the roll, pitch and yaw angles Φ,  and Ψ, defined as follows: The roll angle Φ specifies a turning around the X axis. The angle is positive with a Y-Z turning, that is to say clockwise seen from the back of the missile. The pitch angle  specifies a turning around the Y axis. The angle is positive with a X-Z turning, that is to say missile nose up. The yaw angle Ψ specifies a turning around the Z axis. The angle is positive with an X-Y turning, that is to say yawing to the right. For the sake of simplicity, it is assumed that the sensors are made up of three orthogonal sensors, that is to say the sensors directed in the X, Y and Z directions. The three sensors then sense the flux densities B X , B Y and B Z . These flux densities are changed with the movements of the missile in accordance with the following system of equations: dB X = -dB Z + dΨB Y dB Y = dΦB Z - dΨB X dB Z = -dΦB Y + dB X Certain sensors, for example flux gate sensors, provide B X , B Y and B Z directly. Other sensors of the coil type provide the time derivative of the B field and B X , B Y and B Z must then be calculated by solving the system of equations. As mentioned in the introduction, a ferromagnetic object gives rise to deviations in the terrestrial magnetic field. In principle, the disturbance of the terrestrial magnetic field by the target can be represented by a magnetic dipole. The orientation of the dipole depends on the direction of the terrestrial magnetic field. If the terrestrial magnetic field is horizontal the axis of the dipole becomes horizontal. If the terrestrial magnetic field is vertical, the axis of the dipole becomes vertical and if the terrestrial magnetic field is then horizontal the axis of the dipole becomes horizontal. The range of the dipole (defined as the distance at which the dipole gives a certain field strength) is longer in the direction of the axis than in the equatorial plane but the difference only amounts to a factor of 3√2 = 1.26. As also mentioned in the introduction, the missile's own rotational movements in the terrestrial magnetic field give rise to a sensor signal. According to the invention, the proximity fuse includes a signal processor 5 which is arranged to compensate for the missile's own movements in the terrestrial magnetic field so that an active output signal only occurs in dependence on those deviations in the terrestrial magnetic field which are occasioned by a ferromagnetic object (the target). The missile therefore includes position-sensing elements 6, for example gyros, which sense the movement of the missile and the output signal, the gyro signal, is supplied to the signal processor for evaluation, see Figure 2. Figure 2 shows a block diagram of the main parts of the proximity fuse. Three sensors 4 measure the magnetic flux densities B X , B Y and B Z . The sensor signals are supplied via amplifiers 7 and A/D convertors 8 to the signal processor in the form of a microprocessor 9 for evaluation. The microprocessor is also supplied with gyro signals from the gyro 6 which senses the missile's own movement. The proximity fuse is intended to operate as follows: On launching, the three components in the terrestrial magnetic field B are measured. From these values, the magnitude and direction of the terrestrial magnetic field are calculated. During the continued flying time of the missile, the magnitude of the magnetic field B X , B Y and B Z is continuously measured and compared with the original values. If a deviation occurs, that is to say a change in the magnetic field which cannot be explained by a movement of the missile, it is known that there is a ferromagnetic object in the vicinity, that is to say the target has been encountered, and the proximity fuse outputs an output signal to the effective part. The functional principle is illustrated in Figure 3 with the aid of a flow chart.

Claims: Magnetic proximity fuse for initiating the charging of a moving charge carrier, for example a guided missile, projectile, grenade and the like, when this passes at a certain distance from a ferromagnetic object, comprising a first set of sensors (4) for sensing deviations in the flux densities of the terrestrial magnetic field (B x , B y , Bz), a second set of sensors (6) for sensing the carrier's own movement and evaluating means (5) arranged to produce an active output signal in association with a deviation in the terrestrial magnetic field, characterized in that the evaluating means (6) comprises a signal processor arranged to compensate for changes in the magnitude of the sensor signals of the first set of sensors caused by a change of position of the charge carrier based upon the position of the charge carrier sensed by the second set of sensors, the active output signal (i out ) being produced when the compensated sensor signals deviate from reference values of the terrestrial magnetic field, so that the active output signal only occurs in dependence on deviations in the terrestrial magnetic field which are occasioned by the ferromagnetic objects (3). Proximity fuse according to Claim 1, characterized in that the first set of sensors are made up of flux gate sensors for sensing the flux densities. Proximity fuse according to Claim 1, characterized in that the first set of sensors are made up of coils for sensing the time derivative of the flux densities. Proximity fuse according to Claim 1, characterized in that the first set of sensors consist of Hall elements for sensing the flux densities. Proximity fuse according to Claim 1, characterized in that the second set of sensors comprise gyros (6) for measuring the roll and yaw movements of the charge carrier. Proximity fuse according to Claim 1, characterized in that the second set of sensors comprise accelerometers for measuring the roll and yaw movements of the charge carrier. Proximity fuse according to Claim 1, characterized in that the signal processor (5) is arranged to continuously compare the compensated sensor signals during the movement of the charge carrier along its track with the reference values of the terrestrial magnetic field measured on launch. Proximity fuse according to Claim 7, characterized in that the signal processor (5) includes a microprocessor (9) for signal processing.

Inventors: FORSHUFVUD RAGNAR, PETTERSSON ELIZABETH, FORSHUFVUD, RAGNAR, PETTERSSON, ELIZABETH
Applicants: BOFORS AB
Application year: 1993
Main classification: F42C13
IPC codes: F42C13