Abstract:
A smart mine includes a pair of He 3  magnetic sensors exteriorly connected to a head compartment of a mine housing. A torpedo compartment for housing a torpedo completes the mine housing. The head compartment 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 He 3  magnetic sensors is produced for processing in a signal processor for determining the presence of large ferromagnetic bodies and producing a torpedo firing signal.

Description:
The Government has rights in this invention pursuant to Contract No. 60291-80-0161 awarded by the Navy Department. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to smart mines and more particularly to a mine utilizing He 3  magnetic sensors. 
     This invention relates to U.S. patent application Ser. No. 762,478, Filed Aug. 5, 1985 For &#34;Improved Buoy Detection System Using He 3  Magnetic Sensor&#34; (TI-11182) and to U.S. patent application Ser. No. 762,477, Filed Aug. 5, 1985 For &#34;Remote Magnetic Field Monitoring System Using He 3  Sensor Array&#34; (TI-11183). 
     Prior art &#34;Smart Mines&#34; 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. 
     SUMMARY OF THE INVENTION 
     Accordingly it is an object of the invention to enhance operational effectiveness of &#34;Smart Mines&#34;. 
     Another object of the invention is to provide a &#34;Smart Mine&#34; having increased target detection or tracking or both capability. 
     Briefly stated the invention comprises incorporation of He 3  magnetic sensors either to the exclusion of acoustic sensors or in addition to the acoustic sensors. The He 3   magnetic sensor is particularly suited owing to its sensitivity at low frequencies, size, low power consumption and low cost in quantity production. Further, the He 3  sensor has a higher sensitivity than other low power magnetometers and thus provides longer range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIGS. 1a and 1b are isometric views of the mine in the non-deployed state and in the deployed state, respectively; 
     FIG. 2 is a plan view of the He 3  magnetometer constituting sensors for the mine; 
     FIG. 3 is a block diagram of the sensor electronics and showing the relationship of the electronics and the sensors; and 
     FIG. 4 is a block diagram of the digital phase locked loops frequency synthesizers for the sensors. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1a and 1b, the He 3  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 He 3  magnetometers or sensors 12 and 14 (FIG. 2) are those of U.S. Pat. No. 3,206,671 issued Sept. 14, 1965 to Texas Instruments Incorporated, assignee, as improved by the improvement of U.S. Pat. No. 4,567,439 filed Sept. 23, 1983 (TI-9233) assigned to Texas Instruments Incorporated. As the sensors are identical in construction only one need be described. The sensor 12 (FIG. 2) includes an He 3  cell 32, an He 4  lamp 34, Fresnel lens 36 and polarizer 38, pickup coils 40 and Helmholtz (moment rotation) coils 42. 
     The He 3  cell 32 is either a spherical or cylindrical glass cell containing a working substance (He 3 ) under a pressure of about 1 to 10 Torr. In the cell 32 He 3  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 He 4  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 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 He 3  atoms to magnetize the He 3  atoms in a direction parallel to the direction of the ambient magnetic field (H). 
     After the cell magnetization has been established parallel to the Earth&#39;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&#39;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 FIG. 3, the sensor support electronics includes low noise preamplifiers 44 connected by leads 46 and 48, respectively, to He 3  sensors 12 and 14. The preamplifiers 44 are connected to digital phase locked loop 50. The phase locked loop 50 has inputs connected to a fluxgate magnetometer 52 and a one MHz clock 54. The phase locked loop 50 has output terminals including a 28 V power supply connected to a DC-DC converter 56 for converting the 28 V to a 36 V power source connected to a 36 V 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 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 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. 
     Although a single embodiment of the invention has been described, it will be apparent to a person skilled in the art that various modifications to the details of construction shown and described may be made without departing from the scope of this invention.