DIRECTION RESPONSIVE APPARATUS FOR USE IN A MAGNETIC FIELD

Abstract: Direction responsive apparatus for use in a magnetic field includes coil means (5, 6) adapted to be electrically energised to produce a rotating magnetic field the interaction of which with an external magnetic field in which the coil means is disposed, for example the earth's field, results in a time varying torque being exerted on the coil means (5, 6). The torque is measured, for example by means of a strain gauge (16), and the phase of a torque dependent signal is compared with that of a reference signal, for example the energising current applied to one coil (5) of the coil means, such that the angle between the coil means and the external magnetic field may be deduced.

Description: "Direction responsive apparatus for use in a magnetic field" This invention relates to direction responsive apparatus for use in magnetic fields and is particularly, but not exclusively, concerned with an electrical compass for determining directions relative to a fixed 5 field such as the earth's. magnetic field. The mos.t common form of compass comprises a rotatably mounted magnetised needle adapted to align itself with the earth's magnetic field and thus indicate the direction of magnetic north. This arrangement 0 has for many years proved adequate in every day use. However, in recent years there has become an increasing need for direction responsive apparatus capable of producing an electronic output dependent on the direction of the apparatus relative to the earth's field so - j _5 that the apparatus may be readily incorporated in, for example, computerised guidance systems and other electronic direction finding equipment. For example, in published International Patent Application No. WO 82/03132 there is described a hand held microprocessor 0 controlled device adapted to provide a Moslem with easily read information concerning his daily prayer times and the direction of Mecca in order to assist him in observing his rather demanding daily prayer routine. It has been proposed that the owner of such 5 a device can feed information into the microprocessor concerning ' his whereabouts in -the world, e.g. the longitude and latitude of the nearest large town or city, and that the microprocessor, which also receives an electrical output from an internal compass of the 0 device, will produce an output, which may be displayed by suitable means, indicative of the direction of Mecca. None of the existing forms of electrical compass is suitable for an arrangement such as this since all require rather intricate moving parts making them bulky and expensive to manufacture. According to the invention there is provided a direction responsive apparatus for use in a first magnetic field, comprising substantially static coil means adapted to produce a second, rotating magnetic field, the interaction of which with the first magnetic field results in a torque being exerted on the coil means, the magnitude of the torque varying as the instantaneous direction of the rotating magnetic field changes relative to the direction of the first magnetic field; means for providing a varying reference signal, the instantaneous characteristic of which has a fixed relationship to the instantaneous direction of the rotating magnetic field relative to a reference direction fixed with respect to the coil means; means for producing a varying torque dependent signal, the instantaneous characteristic of which is dependent on the instantaneous magnitude of the torque exerted on the coil means; and means for comparing the phases of the reference and torque dependent signals to provide a signal dependent on the relationship between the reference direction and the direction of the first magnetic field. It will be seen that such apparatus may conveniently take the form of an electrical compass in which case the first magnetic field is the ' earths magnetic field. The signal dependent on the relationship between the reference direction of the apparatus and the direction of the first magnetic field may be processed and/or dispalayed by suitable means so as to provide an indication of direction relative to the first field appropriate for the intended use of the apparatus. For example, the apparatus may be adapted for use as a simple compass in which case a display may be provided which simulates a compass needle or pointer. A microprocessor, which receives the direction dependent signal and also drives the display, may be appropriately programmed so that the pointer gives a direct indication of the direction of magnetic north. Alternatively, the apparatus may be adapted so that an indication is given of some other direction relative to the earth's field. In particular, if information concerning location on the earth's surface is additionally fed into the micro¬ processor, then the apparatus may be used to indicate directly the direction of a further location, for example a town or city. It will be seen that such an arrangement is particularly applicable to the hand held information device for Moslems discussed above. The user may simply feed into the microprocessor informat¬ ion dependent on his longitude and latitude and the apparatus will give a visual display directly indicative of the direction of Mecca. It is also envisaged that instead of- there being a pointer-type display, the apparatus may be adapted to produce either a visual or audible indication when ever the apparatus as a whole is pointed in a certain direction. Thus, in the case of the hand held device discussed above, the user could simply sweep out an angle of up to 360° with the device and the device would give an indication, for example a series of bleeps, when the device was pointed towards Mecca. Although the. invention has been discussed above principally in relation to a compass, it is also envisaged that apparatus in accordance with the invention may be adapted for use as a magnetometer. In this case, means are additionally provided for measuring the strength of the torque exerted on the coil means since this is dependent on the strength of the first field. Thus, both the direction and strength of the first field may be measured. Output means may be calibrated so as to provide a direct reading of field strength or alternatively two such devices could be used in combination so as to provide differential magnetometer. O PI - The coil means may conveniently comprise two coils mounted with their axes at 90 β with respect to each other and supported about a mutual axis transverse in use to the direction of the first field. The rotating magnetic field may then be provided by energising the coils with respective alternating currents which are out of phase with each other by 90°. In this case the periodically varying reference signal is preferably the signal applied to one of the coils, referred to below as the reference coil, since it will be seen that the magnitude of such a signal at any instant -in time has a" fixed relationship to the instantaneous direction of the rotating field relative to the coil means. In this embodiment the reference direction may be regarded as the axis of the reference coil and every time the reference current is zero, the rotating field is perpendicular to the axis, i.e. perpendicular to the reference direction, and every time the magnitude of the reference current is maximum, the field is parallel to the reference direction. The structure of the coil means may take any convenient form, although it will be appreciated that on the one hand there should be a sufficient number of turns to provide a rotating field of sufficient strength to interact with the first field such that a detectable torque is produced, while, on the other hand, the inertia of the coil mean is desirably relatively low such that torque exerted has measurable effect thereon. In one advantageous embodiment of the invention, the coil means comprises two coils, whose axes are at right angles, wound directly onto a ferrite core. Such a coil means provides a relatively strong rotating field. Any suitable means may be provided for measuring the torque exerted on the coil means. In one embodiment a .strain gauge or gauges adapted to produce an electrical torque-dependent output may be suitably coupled to the coil means. Alternatively, it is envisaged that the coil means may be suspended from support means including a piezo electric elastomeric element whose resistance changes with pressure exerted thereon. Thus, variations in the resistance of the elastomeric material are indicative of the torque exerted on the coil means. The coil means may be suspended from four such support means arranged around the coil means. With an arrangement comprising a ferrite core on which coils are wound directly lugs provided on the core may be received in elastomeric elements mounted on a support. The elastomeric elements are preferably connected in the manner of a bridge network, such that changes in resistance may be measured accurately, for example by means of a differential amplifier. In a further embodiment, it is envisaged that the coil means be mechanically coupled to the plates of one or more variable capacitative elements such that torque exerted on the coil means causes a small displacement of one of the capacitor plates and thus a detectable change in capacitance. It is also envisaged that optical means may be employed to measure coil displacements and thus torque variations; for example, a small mirror might be coupled to the coil means which is adapted to reflect a beam of light onto a suitable photo-sensor. In all the above embodiments it will be seen that in use the torque exerted on the coil means results in limited displacements of the coil means either side of an equilibrium position. Such displacements may cause slight peturbations in the torque dependent signal, and it might therefore be desirable that the phases of the torque dependent and reference signals are compared at the zero points of the torque dependent signal, i.e. at the equilibrum position of the coil means. For practical purposes, however, with small displacements, it may be satisfactory and simpler to compare the signals at maxima. A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which; Figure 1 is a diagrammatic perspective view showing two coils used in a compass arrangement and also illustrating, in schematic from, the circuit diagram of the compass arrangement; Figure 2 is a diagrammatic plan view of the two coils shown in Figure 1 (but omitting the remainder of the electrical circuitry) ; and Figure 3 shows wavefdor s occurring in the circuit arrangement. Direction responsive apparatus in the form of a compass comprises two coils 5 and 6 which are fixed relative to each other at 90° as shown schematically in Figure 2. They are mounted together as a unit and are supported by a suitable bearing arrangement - shown schematically at 8 and 10. Preferably a core of magnetic material (not shown) such as a ferrite core is provided within the coils 5, 6. Alternatively, the coils may be wound directly on a suitable ferrite core. The two coils are connected by a suitable mechanical connection 14 to a strain gauge 16 which restrains rotation of the coils about their central axis 12 and produces an electrical output signal on lines 18 and 20 which depends on the torque which the coils exert about the axis 12, as described in more detail below. The coils 5 and 6 are electrically energised with alternating currents at a suitable frequency, for example 100-500 HZ, the energising current applied to coil 6 being 90° out of phase with that applied to coil 5. In the illustrated example, the energising currents for the coils 5 and 6 are generated digitally, although it will be appreciated that any convenient current generating means may be used. Typically, the energising current may be 10-20 A. - - As shown in Figure 1, a counter 22 receives digital signals from an oscillator 24 and produces count outputs on a line 26 which are used as addresses for a read-only memory (ROM) 28. The ROM 28 stores predetermined electrical digital signals in response to the addresses on line 26 and produces a sequential digital output on line 30 in the form of a simulated sine wave. An input unit 32 applies the sine wave directly to a first coil 5, referred to hereinafter as the reference coil, and in addition passes the sine wave through a 90° phase shift circuit 34 which is connected to the second coil 6. In an alternative embodiment (not illustrated) the 90° phase shift circuit 34 may be omitted and instead the ROM 28 arranged to produce two simultaneous sets of digital outputs simulating two sine waves 90° out of phase with each other. Energisation of the coils * 5, 6 causes each coil ' to produce a varying magnetic field at right angles to the plane in which the coil lies i.e. along its axis. The magnitude and direction of each field at any instant in time depends on the instantaneous value of the respective energising sine wave. Since the coils are energised with sine waves 90° out of phase with each other, as described below the resultant of the two magnetic fields will rotate around the axis 12 and have a substantially constant magnitude. Figure 3A shows the energising current applied to the reference coil 5, referred to hereinafter as the reference current," and Figure 3B shows the energising current applied to coil 6, being 90° out of phase with the reference current. At time to, the reference current is zero while the current in coil 6 is at a maximum value. Therefore, the resultant magnetic field produced by the coils 5 and 6 will be directed along the axis of the coil 6, that is perpendicular to the axis of the reference coil 5. At time t -1-' the resultant magnetic field produced by the coils OMPI 5 and 6 will have turned through 90° and will be directed along the axis of the coil 5 and thus lie in the general plane of the coil 6. At this time the reference current is at a maximum. At time t j , the resultant magnetic field of the coils 5 and 6 will have turned through a further 90° and will now be at 90° to the coil 6 and thus lie in the general plane of the coil 5 i.e. perpendicular to its axis but pointing in the opposite direction to the resultrant magnetic field at time t . At this time the reference current is again zero. At time t 3 , the resultant magnetic field produced by the coils 5 and 6 will have turned through a further 90° and the reference current is at a minimum. At time t., the resultant magnetic field produced by the coils will be in the same direction as the field at time to and the reference current is zero. Thus, it will be seen that the axis of the reference coil may be regarded as a reference direction and the magnitude of the reference current at any instant in time has a fixed relationship to the instantaneous direction of the rotating field relative to the reference direction. Figure 2 illustrates the coils 5 and 6 in plan view, and shows the coil pair positioned relative to the direction of the earth's magnetic field, indicated by the line 40, so that the plane of the reference coil 5 lies ' parallel to the earth's magnetic field. At time t, , as described above, the resultant magnetic field produced by the coils 5 and 6 is perpendicular to the reference coil 5 i.e. along its axis and therefore lies at right angles to the earth's magnetic field direction 40. The direction of the resultant magnetic field at time T, , will of course depend on the actual direction of current flowing through the reference coil 5 at that time although in the illustrated example this is assumed to be such as to produce a North pole N, and a South pole S. as indicated in Figure 2. Therefore, interaction between the two poles .N χ and s pro duced by the magnetic field of the coils, and the two poles N and S of the earth's magnetic field will produce a torque tending to turn the coil pair in a clockwise direction (as viewed in Fig. 2) about the axis 12. At time t^, however, the magnetic field produced by the coil pair will be in the opposite direction and produces North and South poles N 2 and S 2 . Therefore, the interaction between these poles 2 and S 2 and the poles N and S of the earth's magnetic field will produce a torque acting in the anti-clockwise direction about the axis 12. It will be seen that at times T, and T 3 , i.e. when the resultant field produced by the coils is perpendicular to the earth's magnetic field, the magnitude of the torque acting on the coils is at a maximum and .at other instants in time the magnitude of the torque acting about the axis 12 of the coil pair will be less than the maximum value and will have a magnitude and direction depending on the instantaneous magnitudes and directions of the energising currents in the two coils. It will also be seen that the value* of the torque acting on the coil pair, when the coil pair has the position relative to the earth's magnetic field shown in Figure 2, varies as illustrated in Figure 3C, that is as a sine wave in phase with the reference current shown in Figure 3A. Similarly, if the coil pair is turned about the axis 12 through 90°, relative to the earth's magnetic field, then the torque exerted on the coil pair by the interaction between the resultant magnetic field produced by energisation of the two coils and the earth's magnetic field will vary as ' shown in Fig. 3D, that is, in phase with waveform 33 and 90° out of phase with the reference current. A further 90° angular movement of the coil pair relative to the earth's magnetic field will produce a torque variation which is 180° out of phase with reference current. A further 90° angular movement of the coil pair relative OMPI to the earth's magnetic field will produce a torque variation which is 180° out of phase with the reference current. A third movement through 90° will make the torque characteristic 270° out of phase with the reference current.. Finally, a further 90° movement of the coil pair will bring the torque characteristic back to that shown in Fig. 3C, that is in phase with the reference current applied to the first coil 5. It will be apparent that, as the coils are rotated about the axis 12, the phase difference between the sinusoidally varying torque, and the reference current 3A varies lineraly with changes in the angular position of the coil pair relative to the earth's field. Thus, a measurement of the phase difference between the reference current waveform shown in Fig. 3A and the torque will depend on and can be used to indicate the angular position of the coil pair relative to the direction of the earth's magnetic field. In the illustrated embodiment, the phase comparison is carried out by a phase comparator 42 (Fig. 1) which receives an output signal from the strain gauge 16 on the lines 18 and 20, this signal being proportional to the torque and therefore being a sine wave correspond¬ ing to variations in the torque. The phase angle of this waveform is compared with the phase angle of the reference current which is represented by a signal on line 44 dependent on the instantaneous count of the counter 22. The comparator 42 produces an output proportional to the bearing angle relative to ag " hetic North which may be displayed in any suitable manner, either visually or aurally. Various modifications may b.e made to the system. Clearly there may be more than two coils. There could be three coils, for example, energised with respective currents 120° out of phase with each other. However, more than three coils could of course be provided. As * discussed above various alternative means may be used to measure the torque for example piezo- ( OMPI electric elastomeric mounts for the coil means or varying capacitances mechanically coupled to the coil means. In these arrangements it is desirable that the frequency of the energising current is approximately equal to the resonant frequency of the coil means such that displacements of the coil means either side of its equalibrium position and thus the torque dependent signal are maximised. Such displacements will cause peturbations in the torque dependent signal, but do not effect the phase thereof relative to the reference signal at the equalibrium position of the coil means. Therefore, the phase comparisons is preferably made at the zero points of the torque dependent signal. It will be appreciated that apparatus in accordance with the invention is potentially very accurate, in that the direction is sampled many times per second in accordance with the frequency of the rotating field, and the resultant output is effectively an arithmetic integration averaged over the cycles. The apparatus may additionally include means to measure the strength of the torque in addition to its phase and thus may be used as a magnetometer.

Claims: CLAIMS: 1. A direction responsive apparatus for use in a first magnetic field, comprising substantially static coil means adapted to produce a second, rotating magnetic field, the interaction of which with the first magnetic field results in a torque being exerted on the coil means, the magnitude of the torque varying as the instantaneous direction of the rotating magnetic field changes relative to the direction of the first magnetic field; means for providing a varying reference signal, the instantaneous characteristic of which has a fixed relationship to the instantaneous direction of the rotating magnetic field relative to a reference direction fixed with respect to the coil means; means for producing a varying torque dependent signal, the instantaneous characteristic of which is dependent on the instantaneous magnitude of the torque exerted on the coil means; and means for comparing the phases - of the reference and torque dependent signals to provide a signal dependent on the relationship between the reference direction and the direction of the first magnetic field. 2. Apparatus as claimed in claim 1 wherein the coil means comprises two coils whose axes are disposed at substantially 90°. with respect to one another, the coils being -energised by respective alternating currents which are mutually out of phase by 90°. 3. Apparatus as claimed in claim 2 wherein, the coils are wound directly on a ferrite core. 4. Apparatus as claimed in claim 2 or 3 wherein the energising current applied to one of the coils constitutes the reference current. 5. Apparatus as claimed in any preceding claim — wherein means for measuring the torque exerted on the coil means comprises one or more strain gauges adapted to produce an electrical output dependent on the torque. 6. Apparatus as claimed in any of claims 1 to 4 wherein the coils means is suspended from support means including piezo electric elastomeric elements, the resistance of which vary in accordance with the torque exerted on the coil means. 7. Apparatus as claimed in any of claims 1 to 4 wherein one or more capacitative elements are mechanically coupled to the coil means whereby changes in capacitance are dependent on variations in the torque exerted on the coil means. 8. Apparatus as claimed in any preceding claim adapted for use as a compass. 9. Apparatus as claimed in any of claims 1 to 7 further comprising means adapted to measure the strength of the torque exerted on the coil means such that the strength of the first field may be measured.

Inventors: CHAPMAN ROBERT ARTHUR, CHAPMAN, ROBERT, ARTHUR
Applicants: DOULTON ROMM, DOULTON, ROMM
Application year: 1983
Main classification: G01C17
IPC codes: G01C17