ANTI-SUBMARINE WARFARE MINE

Abstract: A smart mine 10 includes a pair of He³ magnetic sensors 12 and 14 exteriorly connected to a head compartment 20 of a mine housing 22. A torpedo compartment for housing a torpedo 26 completes the mine housing 22. The head compartment 20 contains the sensor support electronics which includes a phase locked loop means having a pair of digital phase locked loops connected between the sensors and a difference circuit whereby a difference signal representative of the difference between the pair of He³ magnetic sensors 12 and 14 is produced for processing in a signal processor for determining the presence of large ferromagnetic bodies and producing a torpedo firing signal.

Description: This invention relates to smart anti-submarine warfare mines and more particularly to a mine utilizing He³ magnetic sensors. Prior art anti-submarine mines have utilized acoustic sensors to detect the presence of ships and submarines prior to the launch of a homing torpedo encapsulated in the mine. The detection effectiveness of acoustic sensors is decreasing owing to the quieting of ships including submarines. GB-A-2 129 106 discloses an underwater mine according to the preamble of claim 1 designed to stand upright in sand or silt on the sea bed and which ejects a torpedo or other guided missile upwards on detection of a target vessel or in response to a command transmitted to the mine. US-A-3,135 199 describes a magnetometer for measuring the total magnetic field strength, the magnetometer having three mutually orthogonal coils each having a square law characteristic so that the sum of the inductances of the coils is proportional to the square of the total field strength. The coils are connected as one arm of a bridge driven by an alternating current signal so that the amplitude of the signal from the bridge is dependent on the total magnetic field strength. A suggested use of the magnetometer is as a detector of a magnetic mine or depth charge. US-A-3,206,671 discloses a magnetometer using optical pumping of the metastable 2³S₁ state of He³ gas in a capsule by circularly polarised resonance radiation to produce a polarisation of the magnetic sublevels of that state and the consequent polarisation of the magnetic sublevels of the 1¹S o ground state. The radiation is produced by an He³ or He⁴ gas discharge. The capsule of He³ gas is subjected to an alternating magnetic field of a particular frequency and having a component at right angles to the ambient field. The energy splitting of the magnetic sublevels of the ground state atoms of He³ provides a measure of the ambient magnetic field strength. Accordingly it is an object of the invention to enhance operational effectiveness of anti-submarine warfare mines. Another object of the invention is to provide an anti-submarine warfare mine having increased target detection or tracking or both capability. According to the present invention there is provided an anti-submarine warfare (ASW) mine comprising: a housing including a torpedo compartment and a head compartment: sensor support electronics operatively mounted in the head compartment responsive to the detection of a target to cause ejection of the torpedo, characterised in that the mine includes a plurality of magnetic sensor means externally attached to the housing and operatively connected to the support electronics for sensing the Earth's magnetic field, said sensor means includes a cell containing He³ atoms, some of which are temporarily excited to a metastable state, a He⁴ lamp, for magnetizing the He³ atoms in the cell in a direction parallel to the ambient magnetic field, a Helmholtz coil in operative association with the cell for establishing an oscillatory magnetic field normal to that of the ambient magnetic field, and pickup coils operative during free precession of the cell for generating a.c. induced therein by the magnetic field associated with the rotation of the cell magnetization characterized by further including in said sensor support electronics a phase-locked-loop, means for providing ignition and excitation, and a scanning frequency synthesiser, an ignition oscillator for each sensor operatively connected to the He³ cell and He⁴ lamp of each sensor for exciting the cell and preparing the lamp for excitation, an exciter oscillator operatively connected to the He⁴ lamp of each sensor for excitation of the lamp, said scanning frequency synthesiser being operatively connected to the Helmholtz coil of each sensor for establishing the oscillating magnetic field normal to the ambient magnetic field, preamplifiers operatively connected to the pickup coils in each He³ sensor for amplifying the electrical signal induced by the rotating magnetic field of the He³ atoms in the cell, the output of said preamplifiers being operatively connected to said phase-locked-loop, a fluxgate magnetometer for determining the motion attitude of the housing, and a signal processor operatively connected to said fluxgate magnetometer and said phase-locked-loop of the sensor support electronics for detecting the presence of ferromagnetic bodies and generating mine operating signals. Other objects and features of the invention will become more readily apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings in which: Figures 1a and 1b are isometric views of the mine in the non-deployed state and in the deployed state, respectively; Figure 2 is a plan view of the He³ magnetometer constituting sensors for the mine; Figure 3 is a block diagram of the sensor electronics and showing the relationship of the electronics and the sensors; and Figure 4 is a block diagram of the digital phase locked loops frequency synthesizers for the sensors. Referring now to Figures 1a and 1b, the He³ magnetic sensor mine 10 includes a gradiometer having a pair of shielded sensors 12 and 14 pivotally connected through tubular aluminum rods 16 and 18 to the head 20 and two digital phase locked loop frequency discriminators. The head is removably fixed to the body 22 of the mine by a spring 24 mounted between the head 20 and the body 22. The body 22 of the mine is, for example, a torpedo tube for housing a torpedo 26. The body includes a port with a removable cover 28 attached to a motor 30 mounted in the head 20. In operation the body is filled with air and water pressure assists in maintaining the head in place with the spring 24 compressed. When the mine is initialized for firing, the motor 30 removes the port cover to flood the tube 22; this equalizes the pressure and the spring 24 removes the head 20 for torpedo firing. Sensors 12 and 14 include split spherical containers made of fiberglass mounted inside polyethylene shells which are sealed with a clamped O-ring. The aluminum arms 16 and 18 are potted into the shells and connect the sensor heads to the sensor support electronics mounted in the head of the mine. The He³ magnetometer or sensors 12 and 14 (Fig.2) are an improved form of those disclosed in U.S. Patent No. 3,206,671 issued Sep. 14, 1965 to Texas Instruments Incorporated as assignee. As the sensors are identical in construction only one need be described. The sensor 12 (Fig.2) includes an He³ cell 32, an He⁴ lamp 34, Fresnel lens 36 and polarizer 38, pickup coils 40 and Helmholtz (moment rotation) coils 42. The He³ cell 32 is either a spherical or cylindrical glass cell containing a working substance (He³) under a pressure of about 1 to 10 Torr. In the cell 32,He³ atoms are excited to a metastable state by an electric discharge generated in the cell by either a 50 MHz or 100 MHz ignition oscillator of the sensor support electronics hereinafter described. The He⁴ lamp 34 radiates infrared spectral lines in response to a 50 Mhz lamp exciter oscillator of the sensor support electronics. The Fresnel lens 36 collimates the radiation from the radiant electric discharge and the polarizer 38 circularly polarizes the collimated radiation. Thus, the polarized radiation is directed through the cell where it interacts with the metastable He³ atoms to magnetize the He³ atoms in a direction parallel to the direction of the ambient magnetic field (H o ). After the cell magnetization has been established parallel to the Earth's field direction and the electric discharges of the lamp and cell extinguished, an oscillatory magnetic field is established normal to the direction of the ambient magnetic field by connecting the Helmholtz coils 42 to a frequency synthesizer of the sensor support electronics. The scanning frequency synthesizer scans through a frequency range containing the free precession frequency. Free precession of the cell magnetization in the Earth's magnetic field then begins with the lamp, cell and frequency synthesizer power off and lasts more than 10 hours. The pickup coils 40 generate an alternating current (a.c.) induced therein by the magnetic field associated with the rotation of the cell magnetization. Those persons skilled in the art desiring more information about the sensor are referred to the above mentioned references. Referring now to Figure 3, the sensor support electronics includes low noise preamplifiers 44 connected by leads 46 and 48, respectively, to He³ sensors 12 and 14. The preamplifiers 44 are connected to digital phase locked loop 50. The phase locked loop 50 has an input connected to a one MHz clock 54. The phase locked loop 50 has output terminals including a 28V power supply connected to a DC-DC converter 56 for converting the 28V to a 36V power source connected to a 36V battery 58. Other output terminals connect excitation and ignition signals to ignition and lamp regulators 60, and rotate signals to the sensors 12 and 14. The ignition and lamp regulators are powered by the battery 58 and regulate power to the 50 MHz exciter oscillators 62 and 64, and to the ignition oscillators 66 and 68. The exciter 62 and ignition oscillator 66 are connected to sensor 12 and exciter 64 and ignition oscillator 68 are connected to sensor 14. The phase locked loop 50 has a further output connected to a digital signal processor to which the output of a fluxgate magnetometer 52 is also applied. The digital signal processor is connected to torpedo firing electronics. The digital phase locked loop 50 includes a pair of loops 70 and 72 (Fig.4) one for each sensor 12 and 14. As each loop is identical only one need be described. The other loop elements will be designated with primed numbers. The phase locked loop 70 includes a band pass filter 74 connected to the sensor 12 for bandpass filtering (700 to 3000 Hz) the signal from its pickup coils. A multiplying D/A converter 76 is connected to the bandpass filter 74; the D/A converter serves as a phase detector. A low pass anti-aliasing filter 78 is connected to the D/A converter for passing the analog signal into an analog-digital converter 80 with a 2 Hz data rate. A discrete time loop filter 82 (digital filter) determines the performance characteristics of the phase locked loop. The natural frequency of the loop is 0.03 Hz with a nominal damping factor of 0.8. The digital filter acts as an integrator with phase lead correction. The output of the filter goes into an adder 84. A holding register 86 is connected to the adder; together they form an automatically rezeroing ramp generator with the instantaneous slope controlled by the input to the adder. A read only memory (ROM) 88 is connected to the holding register 86. The adder, holding register and ROM form a digital VCO for the multiplying D/A converter. The digital word coming out of the holding register is essentially the phase of the digital VCO. The ROM is programmed to perform the cosine function of the phase which goes into the multiplying D/A converter 76. The digital signal corresponding to the magnetic sensor 12 signal frequency constitutes the input to the adder 84. Digital phase locked loop 72 follows the signal for the magnetic sensor 14. Difference/offset circuits 90 receive the digital signals corresponding to the magnetometers signal frequencies and output the difference as a digital signal. The digital signals represent the output of the gradiometer. A signal processor 94 is connected to the difference/offset circuits for comparing the filtered output of the gradiometer with a threshold (either a preselected value or a calculated value based on the gradiometer)and when the threshold is exceeded generating a port release signal to flood the torpedo tube to equalize the pressure for the spring to remove the mine body head 20.

Claims: An anti-submarine warfare (ASW) mine comprising: a housing (22) including a torpedo compartment and a head compartment: sensor support electronics operatively mounted in the head compartment responsive to the detection of a target to cause ejection of the torpedo, characterised in that the mine includes a plurality of magnetic sensor means (12,14) externally attached to the housing and operatively connected to the support electronics for sensing the Earth's magnetic field, said sensor means includes a cell containing He³ atoms, some of which are temporarily excited to a metastable state, a He⁴ lamp (34), for magnetizing the He³ atoms in the cell in a direction parallel to the ambient magnetic field, a Helmholtz coil (42) in operative association with the cell for establishing an oscillatory magnetic field normal to that of the ambient magnetic field, and pickup coils (40) operative during free precession of the cell for generating a.c. induced therein by the magnetic field associated with the rotation of the cell magnetization further characterised by including in said sensor support electronics a phase-locked-loop (50), means for providing ignition and excitation, and a scanning frequency synthesiser, an ignition oscillator (66,68) for each sensor operatively connected to the He³ cell and He⁴ lamp (34) of each sensor for exciting the cell and preparing the lamp for excitation, an exciter oscillator (62,64) operatively connected to the He⁴ lamp of each sensor for excitation of the lamp, said scanning frequency synthesiser being operatively connected to the Helmholtz coil (42) of each sensor for establishing the oscillating magnetic field normal to the ambient magnetic field, preamplifiers (44) operatively connected to the pickup coils in each He³ sensor for amplifying the electrical signal induced by the rotating magnetic field of the He³ atoms in the cell, the output of said preamplifiers being operatively connected to said phase-locked-loop (50), a fluxgate magnetometer (52) for determining the motion attitude of the housing, and a signal processor operatively connected to said fluxgate magnetometer (52) and said phase-locked-loop of the sensor support electronics for detecting the presence of ferromagnetic bodies and generating mine operating signals. An ASW mine according to claim 1 further characterised in that the phase locked loop (50) includes a plurality of digital phase locked loop circuits corresponding to the plurality of magnetic sensors (12,14), said phase locked loop circuits producing digital signals corresponding to the sensor signal frequencies, a difference circuit (90) operatively connected to the outputs of the digital phase locked loops, the output of said difference circuit coupled to said signal processor (94). An ASW mine according to claim 2 wherein each digital phase locked loop (70,72) includes a bandpass filter (74,74') operatively connected to said sensor for removing noise signals, a digital to analog (D/A) converter (76,76') operatively connected to said bandpass filter and to a digital VCO for phase detection, a filter operatively connected to the D/A converter (78,78'), an analog to digital (A/D) converter (80,80') operatively connected to said filter, a digital loop filter (82,82') operatively connected to the A/D converter for integrating the signals with phase lead correction and determining the performance characteristics of the phase locked loop, and said digital VCO comprising an adder (84,84') operatively connected to the digital filter (82,82'), a holding register (86,86') operatively connected to the adder and a read only memory (ROM) (88,88') operatively connected to the holding register, said adder and holding register coacting to form an automatically rezeroing ramp generator with instantaneous slope controlled by the adder for producing a digital word representative of the phase of the digital VCO and the ROM for performing the cosine phase function for the multiplying D/A converter to complete the phase locked loop.

Inventors: COLEGROVE, FORREST D., MARTON, BELA, MCGREGOR, DOUGLAS D., RICE, JOSEPH A., JR.
Applicants: TEXAS INSTRUMENTS INCORPORATED
Application year: 1986
Main classification: F42B22
IPC codes: F42B19, F42C13, F42B22