Abstract:
A fluxgate compass direction sensor in which a fluxgate is mounted relative to a receptacle by means of a flexible circuit. The fluxgate is also suspended in a liquid by means of a float. In use, if the receptacle tilts relative to the horizontal, the level of the liquid will move to retain a horizontal aspect and the float, being buoyant in the liquid, will retain the fluxgate in the horizontal plane. The flexible circuit acts to allow tilting of the float and fluxgate relative to the receptacle while retaining the fluxgate in lateral and directional alignment relative to the receptacle.

Description:
FIELD OF THE INVENTION 
     This invention relates to a direction sensing device and more particularly, but not exclusively, to a fluxgate compass. 
     BACKGROUND OF THE INVENTION 
     Fluxgate compasses give an indication of the direction of an object in which the compass is installed by sensing the horizontal component of the earth&#39;s magnetic field. Fluxgate compasses operate by alternately magnetically saturating a permeable core with a drive coil and detecting, using sensing coils, the asymmetry of the resultant magnetic field, the asymmetry having been caused by the earth&#39;s magnetic field. 
     In order to sense direction accurately, it is necessary both for the fluxgate (coils and core) to be retained in directional alignment relative to the object and also for the fluxgate to be retained horizontally, so that the true horizontal component of the earth&#39;s magnetic field is sensed. 
     In Great Britain Pat. No. 2107057, a suspension assembly for a fluxgate is proposed in which the fluxgate is connected to a supporting frame to be mounted relative to the object, by means of a spherically symmetrical mechanical coupling which permits both tilt and twist. Thus, if the object tilts away from horizontal, the fluxgate will remain, due to its weight, in the horizontal plane. A flexible circuit is provided which both keeps the fluxgate directionally aligned and provides electrical connections thereto. 
     The largest factor in the cost of this prior art teaching is the mechanism used to support the fluxgate. To a large extent, this mechanism is there to support itself, with the essential mechanism of the fluxgate only being 1/50 of the weight of the whole suspended assembly. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a direction sensing device including: a receptacle having a liquid received therein; and a direction sensor coupled to the receptacle, the sensor being arranged to be buoyant in the liquid received in the receptacle and to be free to move so as to remain in the horizontal plane but to be in all other respects constrained to follow the movement of the receptacle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 is a sectional view of an embodiment of the invention; 
     FIG. 2 is a plan view taken through the plane 2--2 of FIG. 1; 
     FIG. 3 is an expanded perspective view of some of the components of the embodiment of the invention depicted in FIGS. 1 and 2; 
     FIG. 4 shows detail of the flexible circuit used in the described embodiment of FIGS. 1 to 3; 
     FIG. 5 shows a plan view of the fluxgate of the embodiment of FIGS. 1 to 4; 
     FIG. 6 is a sectional view of a second embodiment of the invention having a float of a modified form; and 
     FIG. 7 is a perspective view of the float of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a direction sensing device generally designated 1 is shown which includes a receptacle 3 of hollow, toroidal construction. The receptacle 3 is preferably formed from noryl or acetal resin, although it may alternatively be formed from other plastic materials, for example acrylic or polycarbonate. The receptacle includes two cooperating halves 5, 7 sealed together. The receptacle 3 is filled to approximately half its volume with a liquid 9, which is preferably 50 cs silicone oil. Oil of this viscosity is chosen to give a reasonable amount of dampening. 
     A direction sensor, generally designated 11, is disposed within the receptacle and includes a float 13, flexible circuit 15 and a fluxgate consisting of a coil former 17 to which a core 19 and coils 21a-b, 22a-b and 23a-h are attached (see FIG. 5). 
     The float 13, circuit 15, former 17 and core 19 are better illustrated with reference to FIG. 3. 
     The float 13 is formed from material buoyant within the liquid and may be formed from expanded polystyrene or, more preferably, a composite of a plastic material e.g. a vinyl ester and a plurality of hollow glass spheres 26 (glass balloons), the entrapped air within the spheres giving the required buoyancy. However any buoyant material of uniform density may be used. The float 13 is of generally annular form, having a slanted top surface 27, to allow oil displaced onto the float to drain off. Three arcuate ribs 29, 31, 33 are disposed at equal spacing around the periphery of the float 13. With reference to FIGS. 1 and 2, when the float is in position, the ribs 29, 31, 33 are submerged within the liquid and in normal operation lie spaced from the inner side of the receptacle. The body of the float 13 is of a lesser outer diameter than the ribs 29, 31, 33 to reduce the effect of surface tension forces acting between the side of the receptacle 3 and the float 13. 
     The float 13 is connected to the flexible circuit 15. This flexible circuit is of a similar construction to that disclosed in the aforesaid Great Britain Pat. No. 2107057. The flexible circuit 15 is formed from a thermoset (polyamide) base, preferably from Kapton (trade name) of 50 microns thickness having electrical connections 32a, 32b, 34a, 34b, 36a, 36b, formed from copper upon the base 30 in three spiral pairs, which will be referred to as 32, 34 and 36. The connections may be formed by depositing a uniform copper coating on the base 30 and selectively chemically etching the coating to give the required pattern. Between the spiral pairs, three spiral slots 35, 37, 39 are cut by conventional means. 
     The flexible circuit 15 provides a flexible connection between outer terminals 41-46 and inner terminals 47-51. The outer terminals 41-46 are connected to the coils of the fluxgate (see below), while the inner terminals 47-51 are attached to connecting members 47&#39;-51&#39; for connection to driving and sensing circuitry. 
     The former 17 is formed from plastic material and is also of generally annular construction. The former 17 is configured to accept the core 19, which is preferably formed from Mu-Metal (Registered Trade Mark), within a channel 60 and is provided with wall sections 62, 64, 66 having slots disposed therein at regular intervals. In assembly, the core 19 is placed within the slot 60 and the coils 21a-b, 22a-b and 23a-h are wound around the former 17 by conventional means. As illustrated in FIG. 5, the coils are configured so as to provide two orthogonally disposed pairs of connected sense coils 21a, 21b and 22a, 22b, and eight drive coils 23a-23h connected together and disposed between the sensing coils at equal spacing. The terminals of coils 23a, 23h are connected to the outer terminals 42, 43 of the flexible circuit 15, by low temperature soldering, with the terminals from coils 21a, 21b, 22a, 22b being connected to the outer terminals 44, 46, 45, 41 in a similar manner. Although the terminals from the coils are shown non-symmetrically arranged, these terminals may be disposed symmetrically about the former 17, for example in three equispaced pairs. 
     The drive signals for the sense coils are then applied through the inner terminals 50, 51, with the sensing coil output being provided between the inner terminals 47 and 49 and 48 and 49. The terminals 47&#39;-51&#39; may be connected to any conventional sensing and drive circuit for this application with the number of turns of the sense and drive coils being chosen accordingly. 
     In use, the direction sensor 11, as assembled, is secured, at the central land 16 of the flexible circuit to the receptacle 3, being clamped between the receptacle halves 5, 7, between washers 71, 73. The receptacle 3 is then mounted within, for example, a ship or other object whose direction it is desired to sense and is aligned at a predetermined orientation. 
     If subsequent movement of the ship causes the receptacle to tilt relative to the horizontal, this will cause the liquid 9 within the receptacle 3 to adjust its position so that the surface 10 thereof remains in the horizontal plane. This will cause movement of the float 13, which will tend to retain the direction sensor 11 in a horizontal aspect. The flexible circuit 15 will allow tilting movement of the direction sensor 11 relative to the receptacle 3 out of the horizontal plane but will, due to its spiral construction, resist any twisting movement of the sensor relative to the receptacle 3 in the horizontal plane, so as to retain the direction sensor 11 at a predetermined directional orientation relative to the receptacle and thus to the ship. The flexible circuit 15 also acts to locate the direction sensor 11 laterally within the receptacle, preventing any direct contact between the sensor 11, in particular the ribs 29, 31, 33 of the float 13 and the receptacle 3 which could generate frictional forces to disturb the direction sensor from horizontal. 
     If large rotational forces are present, for example if the ship is sailing in rough seas, the flexible circuit can become entangled with itself and thus not provide the desired resistance to twist. In order to prevent this, the gaps between the annular ribs 29, 31, 33 of the float 13 are arranged to cooperate with projections 70, 72, 74 of the receptacle to prevent excessive rotation of the float, and consequently of the direction sensor 11 relative to the receptacle. 
     FIGS. 6 and 7 show a second embodiment of the invention which includes a modified float 113. The float 113 is of similar form to that shown in FIGS. 1 and 3 except that it is of reduced height and is arranged so that the floating assembly is only marginally buoyant. In this case the strong adhesion tendency of particular types of liquid 9, for example silicone oil, may be used to advantage to stabilize the top surface 117, of the float 113. The stabilizing action results from the tendency of the liquid surface to adhere to the sharp edges 115 of the float 113. If any displacement from horizontal occurs, the angles α, β adopted by the liquid surface 10 adjacent to the float differ on opposite sides of the float 113, causing a restoring torque by the surface tension of the liquid. The liquid surface 10 can be likened to an elastic membrane. 
     In static conditions, the top surface 117 of the float 113 remains free of liquid, apart from a surface film. The float 113 may become flooded following agitation of the assembly, but recovers rapidly and with no significant effect on accuracy.