Abstract:
An applied magnetic field direction indicating device comprising a magnetic sensor (1) having an omnidirectional characteristic and an output which is non-linearly related to the magnitude of the applied field. The sensor (1) is subjected to two mutually perpendicular sinusoidal magnetic biassing fields of the same frequency and in time quadrature relationship. A characteristic of the output of the sensor (1) is indicative of the direction of the applied field.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to devices for indicating the direction of a magnetic field. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide such a device having no moving parts. 
     According to the present invention there is provided a device for indicating the direction of an applied magnetic field comprising: a magnetic sensor exhibiting an omnidirectional directional characteristic and producing an output which is non-linearly related to the magnitude of the applied magnetic field; and means for subjecting said sensor to mutually perpendicular sinusoidal magnetic biassing fields of the same frequency in time quadrature relationship, whereby the output of said sensor includes a component at the frequency of said biassing fields whose phase relative to said biassing fields indicates the direction of said applied magnetic field relative to a reference direction. 
     Preferably said sensor comprises at least one member of magnetostrictive material and transducer means responsive to deformation of said magnetostrictive member to produce said output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One device in accordance with the invention will now be described by way of example with reference to the accompanying drawings of which: 
     FIG. 1 shows a perspective view of a magnetic sensor forming part of the device; 
     FIG. 2 is a perspective view of part of the device; 
     FIG. 3 is a schematic diagram of the whole of the device; and 
     FIG. 4 is vector diagram of magnetic fields in the region of the magnetic sensor in operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 the device includes a magnetic sensor 1 of a kind forming the subject of United Kingdom patent application No.: 8705449 (now G.B. No. 2188157A). The sensor 1 comprises a planar circular disc of piezoelectric material 3 sandwiched between two planar circular discs of electrically conductive magnetostrictive material 5. The two discs 5 have the same radial dimensions as the disc 3 and are positioned on the opposite main surfaces of the disc 3, in register with the disc 3. The disc 3 is polarised so that when subject to compression or tension in any direction parallel to its main surfaces it will generate a potential difference between its main surfaces. The two discs 5 also serve as electrodes of the sensor 1 and are provided with leads 7. 
     When the magnetic sensor 1 is subjected to a magnetic field directed parallel to its plane the discs 5 will expand in the direction of the applied field and consequently stretch the disc 3 in the same direction. A dc potential difference is consequently generated between the main surfaces of the disc 3 whose value is representative of the strength of the applied field. This potential difference may thus be applied to an electric circuit via the leads 7. 
     It will be appreciated that the sensor exhibits the same sensitivity in all directions parallel to its plane and thus exhibits an omnidirectional characteristic in this plane. 
     Referring to FIGS. 2 and 3, the magnetic sensor 1 is subjected to a magnetic biassing field produced by an alternating current fed by a reference frequency generator 9 to two coils 11, 13 having the same dimensions and wound around a rectangular former 15 containing the sensor 1 so that the magnetic axes of the coils 11, 13 are perpendicular and lie in the plane of the sensor 1. 
     The phase of the alernating current fed to the coil 13 is arranged to be out of phase by π/2 with the alternating current fed to the coil 11. This is effected by the presence of the π/2 phase shifter 17. The voltage between the leads 7 is fed via a differential amplifier 19 to a bandpass filter 21. As explained below, in operation the output of the bandpass filter 21 comprises a phase shifted carrier at the same frequency as the reference frequency generator 9, amplitude modulated by a magnetic field applied to the sensor whose direction is to be determined. 
     The operation of the device when used as a magnetic compass to determine the direction of the magnetic meridian will now be explained in detail. For such the sensor 1 is placed with its plane horizontal. 
     Referring to FIG. 4, at any instant the total magnetic field H T  in the region of the plane of the sensor 1 will comprise a component H o  due to the horizontal component of the earth&#39;s magnetic field and a component h due to the alternating currents fed to the coils 11, 13. It may be therefore be written 
     
         H.sub.T =H.sub.o +h                                        (1) 
    
     The components H o , h may each be resolved into two components in the x and y directions respectively: 
     
         H.sub.o =H.sub.ox +H.sub.oy =|H.sub.o | cos θ+|H.sub.o | sin θ          (2) 
    
     
         and: 
    
     
         h=h.sub.x +h.sub.y =|h| cos wt+|h| sin wt                                                        (3) 
    
     where 
     H ox  is the sub-component of H o  in the x direction; 
     H oy  is the sub-component of H o  in the y direction; 
     h x  is the sub-component of h in the x direction, 
     h y  is the sub-component of h in the y direction; 
     θ is the angle between the vector H o  and the x-axis; 
     wt is the angle between the vector h and the x-axis; 
     |H o  | is the modulus or magnitude of the vector H o  ; and 
     |h| is the modulus or magnitude of the vector h. 
     Substituting the results of equations (2) and (3) in equation (1): 
     
         H.sub.T =|H.sub.o | cos θ+|H.sub.o | sin θ+|h| cos wt+|h| sin wt 
    
     
         H.sub.T =|H.sub.o | cos θ+|h| cos wt+|H.sub.o | sin θ+|h| sin wt 
    
     Therefore: 
     
         |H.sub.T |.sup.2 =(|H.sub.o | cos θ+|h| cos wt).sup.2 +|H.sub.o | sin θ+|h| sin wt).sup.2           (4) 
    
     The magnitude of the potential difference generated between the leads 7, is proportional to the magnetostrictive expansion of the discs 5. The magnetostrictive expansion of the discs 5 is proportional to the square of the magnitude of the magnetic field in the region of the sensor. It may therefore be written: 
     
         |V|=k|H.sub.T |.sup.2  (5) 
    
     where V is the potential difference between the leads 7; and k is a constant of proportionality dependent upon the sensitivity of the matrials used to fabricate the sensor 1. 
     From equations (4) and (5): ##EQU1## 
     In equation (6) k|H o  | 2  and k|h| 2  are dc terms and will therefore be filtered out by the bandpass filter 21. The output V out  of the bandpass filter 21 will therefore be given by: 
     
         V.sub.out =2k|H.sub.o ||h| cos (wt-θ)                                              (7) 
    
     From equation (7) it can be seen that V out  has an amplitude proportioned to the magnitude |H o  | of the horizontal component of the earth&#39;s magnetic field in the region of the sensor 1 and a phase shift θ with respect to the phase of the biassing alternating current applied to coil 11 (the coil 11 producing the component h x  =|h| cos wt of the biassing magnetic field, as shown in FIGS. 2 and 4). This phase shift θ is equal to the bearing of magnetic north from the direction of the axis of the coil 11. The phase shift may be detected by utilising the output of the bandpass filter 21 and a phase reference signal 23, obtained from the reference frequency generator 9, in a phase detector or phase and gain meter (not shown). 
     It will be appreciated that whilst in the embodiment of the invention described by way of example the relationship defined in equation (5) is a square relationship, in other embodiments this relationship may be any other non-linear relationship such that the output voltage of the sensor contains a cos (wt-θ) term. 
     In the particular embodiment described by way of example, this relationship exists by virtue of the fact that the applied magnetic field is non-linearly related to the radial deformation of the magnetostrictive discs 5 and an output voltage proportional to the deformation is produced by a transducer means constituted by the piezoelectric disc 3. However, in other devices according to the invention alternative forms of transducer means may be used. 
     For example, a fibre-optic measurement technique may be used wherein the transducer means comprises one or more turns of optical fibre wound around and bound to the circumference of a disc of magnetostrictive material. Upon radial deformation of the disc the circumference of the disc varies proportionally thus stretching the optical fibre. 
     Accurate detection of the change in length of the optical fibre can be carried out by making the optical fibre one arm of an interferometer (for example a Mach-Zehnder interferometer). The output of the interferometer will be of the form H o  cos (wt-θ). 
     Another alternative form of transducer means for measuring the deformation of a magnetostrictive disc is a strain-gauge having its resistive element wound around, but insulated from, the circumference of the disc. 
     It is further pointed out that in devices in accordance with the invention wherein the magnetic sensor comprises at least one member of magnetrostrictive material and transducer means responsive to deformation of the magnetostrictive member, the or each magnetostrictive member is not necessarily in the form of a disc, as in the embodiment described by way of example. For example, the or each magnetostrictive member may alternatively be of annular form.