ANGULAR MOTION MEASURING DEVICE

Abstract: A measuring device such as a gravity gradiometer has a levitation coil (26) and a float body (12). The float body (12) is capable of being magnetically levitated by the coil (26) on a levitation axis (L) about which angular movement of the float body (12) relative to the coil (26) is to be detected. Angular motion of the float body (12) is effected using field-modulating strips (36) which are carried by the float body (12) and which are rotatable with the latter relative to pairs of coils (38, 40; 42, 44). The levitation coil (26) lies on the surface of a sphere. The float body (12) has a concave lift surface sensitive to levitational forces exerted by the levitation coil (26) in use. The lift surface also lies on the surface of a sphere.

Description: ANGULAR MOTION MEASURING DEVICE This invention relates to an angular motion measuring device and is more particularly, although not exclusively, concerned with a gravity gradiometer, a device used for measuring gravitational attraction. A traditional form of gravity gradiometer is a torsion balance comprising a bar which carries a mass at each end and which is suspended by a fine torsion fibre. The bar carries a mirror which is used to reflect light from a source to a light detector so that minute changes in the angle of the mirror about the axis of the fibre can be measured. Thus, the effect of gravitational attraction on the masses on the ends of the bar by suitably positioned large masses, for example, can be measured, whereby the gravitational constant can be measured. The sensitivity of such a device is not high however because of the inherent torsional stiffness and inelasticity of the torsion fibre. It has been previously proposed by Karen et al in Rev. Sci. Instrum. 64, 283 (1993) to measure angular motion of a niobium-coated hollow spherical shell which is magnetically levitated using a toroidal superconducting levitation coil. However, the sensitivity of such a device is comparable to the torsion balance described above. According to the present invention, there is provided an angular motion measuring device comprising a levitation coil, a float body which is capable of being magnetically levitated by the coil on a levitation axis about which angular motion of the float body relative to the coil is to be detected, and means for measuring angular motion of the float body; wherein the levitation coil lies on the surface of a sphere, and wherein the float body has a lift surface sensitive to levitational forces exerted by the coil in use, said lift surface also lying on the surface of a sphere. The device of the present invention is designed so as to be used in a terrestrial laboratory with operation of the levitation coil in persistent mode in a vacuum. This leads to an essentially loss-less suspension exhibiting negligible inelasticity and low thermal noise. As the magnetic levitating pressure acts radially on the lift surface of the float body, rotation of the coil about a horizontal axis due to ground tilt should not couple torques about the levitation axis. In a first embodiment, the levitation coil lies on a convex surface and the float body has a concave lift surface. In a second embodiment, the levitation coil lies on a concave surface and the float body has a convex lift surface. In cases where there is a risk of rotational acceleration of the levitation coil due to ground vibrations about a vertical axis, a device in accordance with the first embodiment and a device in accordance with the second embodiment may be provided and arranged to operate in coupled differential mode. Preferably, the spherical surfaces on which the coil and the lift surface lie are substantially concentric when the float body is levitated in use. Preferably also, the float body is a part-spherical shell. When used as a gravity gradiometer, the float body preferably has a plurality of masses suspended from its periphery. Typically, two or three equi-angularly spaced masses are suspended from the periphery of the float body on rigid suspension wires. Most preferably, the combination of the float body and the masses has a centre of mass and a centre of buoyancy (centre of figure) which are substantially coincident. This enables coupling of the float body to horizontal ground vibrations to be minimised. The levitation coil is preferably a unipolar coil and more preferably has turns whose pitch increases outwardly away from the levitation axis. The levitation coil may be provided on a part-spherical surface of a coil support. Azimuthal symmetry of the coil support and the levitation surface of the float body enables negligible restoring torque about the levitation axis to be achieved. The coil and the lift surface are preferably formed of a material which is capable of superconducting. Low Tc materials such as lead and, preferably, niobium may be used. Alternatively, high Tc materials may be used if they have sufficiently homogeneous superconducting properties at the operating temperature of the device. The means for measuring angular motion of the float body preferably comprises (i) at least one field-modulating element which is carried by the float body, and (ii) coil means disposed adjacent said at least one field- modulating element, said coil means being disposed adjacent the field- modulating element and providing an output signal which is dependent upon the angular position of the field-modulating element about the levitation axis. The field-modulating elements may be arranged to modulate the inductance of the coil means. The or each field-modulating element may be a strip formed of a material which is capable of superconducting. The coil means may be pick-up coils, preferably in a persistent superconducting circuit. The coil means may be positioned under the levitation coil. The coil support may have an aperture through which the or each field- modulating element extends so as to be angularly movable relative to the coil support. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic exploded perspective view of a float body and a levitation coil which form part of an embodiment of a gravity gradiometer according to the present invention, Fig. 2 is a schematic view of angular motion measuring coils forming part of the gravity gradiometer, and Fig. 3 is a circuit diagram showing how the coils of Fig. 2 are coupled to an input (pick-up) coil of a flux-gate magnetometer. Referring now to Fig. 1 of the drawings, the gravity gradiometer comprises a support indicated generally by arrow 10 made from a machinable ceramic (for example that sold under the tradename "MACOR"), and a float indicated generally by arrow 12. The support 10 comprises a head 14, a smaller diameter cylindrical lower region 16 and an intermediate region 18. The head 14 has an upper support surface 14a lying on a spherical surface centred on the longitudinal axis of the support 10. A cylindrical passage 20 extends axially through the support 10. Thus, the passage 20 opens onto the upper support surface 14a centrally of the latter. A pair of mutually perpendicular transverse bores 22 and 24 extend diametrically through the cylindrical lower region 16 of the support 10 so as to intersect the passage 20. Thus, the bores 22 and 24 are perpendicular to the passage 20. The spherical surface upon which the upper support surface 14a lies has a centre which itself lies in the horizontal plane containing the axes of the bores 22 and 24. A coil 26 is formed on the upper support surface 14a by thermal evaporation of a lead film followed by coating with a photoresist, patterning onto the resist using a computer-controlled laser beam, developing the photoresist and then wet-etching to remove unwanted lead . As an alternative, the coil 26 could be formed of niobium which is sputtered onto the surface 14a and then patterned, developed and etched. Thus, the coil 26 lies on the same spherical surface as the surface 14a. In this embodiment, the coil 26 is approximately helical and comprises lengths of constant latitude extending over approximately 120° of longitude. The adjacent ends of constant-latitude lengths are connected by inclined lengths extending over a few degrees of longitude. The extent in colatitude of the coil 26 (in the present embodiment, between 15° and 60°) is chosen as a compromise between lift force/unit area and transverse stability. A planar geometry would maximise the lift force but would give no transverse stability to the float 12. In this embodiment, the thickness of the lead film forming the coil 26 is 30 ± 4 μm. The width of the windings of the coil 26 is 150 μm. The coil 26 is unipolar and is formed on the surface 14a so that the pitch between the windings progressively increases from the centre to the outer edges of the coil 26. In this embodiment, the minimum pitch is 350μm, this being at the centre of the coil 26. The float 12 comprises a part-spherical shell 28 having a concave inner surface 28a which lies on the surface of a sphere having a radius which, in this embodiment, is 1 mm greater than the spherical surface upon which the support surface 14a lies. The shell 28 has a surrounding rim 30 from which are suspended three equi-angularly spaced masses 32 on relatively extensible support wires 34. Adjacent to the centre of the concave surface 28a of the shell 28, there are mounted pair of opposed strips 36. Each strip 36 extends downwardly parallel to the axis of the shell 28. Each strip 36 is arcuate in cross-section. The strips 36 are relatively disposed so that they lie on a pitch circle P (see Fig. 2) centred on the longitudinal axis of the shell 28. The spherical surface upon which the surface 28a lies has its centre which is coincident with the longitudinal axis of the shell 28. Diameter of the pitch circle P is less than that of the passage 20 so that the strips 36 can extend with clearance into the passage 20. The strips 36 are of a length such that they extend beyond the horizontal plane in which the axes of the bores 22 and 24 lie. Thus, the strips 36 extend for a distance which exceeds the radii of the spherical surfaces associated with the support 10 and the float 12. The gravity gradiometer further comprises a first pair of coils 38 and 40 and a second pair of coils 42 and 44. Coils 38 and 40 are mounted in the first bore 22 opposite sides of the passage 20. Likewise, the coils 42 and 44 are mounted in the bore 24 on opposite sides of the passage 20. The coils 38, 40, 42, 44 are shown flat-wound (Fig 2), but could alternatively be solenoidally wound. Coils 38 and 40 are connected together in series with an input (pick-up) coil 46 of a flux-gate magnetometer indicated schematically by a dotted line 48. The coils 42 and 44 are connected with the coil 46 so as to be electrically connected in parallel circuit with the coils 38 and 40. Heat switches 50 and 52 are respectively connected in series with the pairs of coils 38,40 and 42,44. In this embodiment, the shell 28 was constructed from copper sheet of initial thickness 400 μm spun into the final shape and then etched to an average thickness of 1 10 μm. The mass of such shell was 9.7 g. The inner radius of the shell 28 was 43.4 mm and the shell extended to a colatitude of 67°. The sphericity of the inner surface 28a was approximately 1 %. The inner surface 28a had a lead film of average thickness 20 μm thermally evaporated thereon. The masses 32 were each of a mass of 1 g. The support wires 34 were formed of copper with a diameter of 0.9 mm and a length of 120 mm. The mass of the complete float 12 was 17.0 g. its centre of gravity was approximately 8 mm from its centre of figure, and its radius of gyration was 34 mm. However, ideally, the centre of gravity is at most 2mm from the centre of figure. The spherical support surface 14a had an outer radius of 42.4 mm and a sphericity of 0.25%. In use the gravitometer is used in vacuum at an operating temperature of 4.2k maintained by liquid helium. The coil 26 is used in persistent current mode at a maximum persistent current of 3.65 A. The coil 26 acts as a levitation coil to suspend the float 12 1 mm above it so that the float 12 and the upper support surface 14a are concentric. It will be appreciated that the torsional effect produced by large masses (not shown) positioned adjacent the suspended masses 32 will result in rotary movement of the float 12 and thereby orbiting of the strips 36 about the centre of the pitch circle P which is on the axis of levitation L. Such movement of the strips 36 affects the inductances of the coils 38, 40, 42 and 44 which act as pickup coils. Thus, the electrical signal applied to the pick-up coil 46 of the magnetometer 48 is dependent upon the angular positions of the strips 36 relative to the coils 38, 40, 42 and 44. In this way, the angular motion of the float 28 relative to the support 10 can be measured. In order to create a persistent current, for example through the coils 42, 44 and 46, a current source is connected into the loop at points A and B on either side of the heat switch 52. Whilst both the heat switches 50 and 52 are active (hot), current flows from the source and around the loop. When the coils have been cooled to below their Tc, the switch 52 is de-activated (switched off) so that the circuit segment between points A and B becomes a short circuit allowing persistent current to flow around the loop including the coils 42, 44 and 46. Switch 50 is then allowed to cool. Current can be stored in the loop containing the coils 38, 40 and 46 in a similar way. Other configurations of charging wires and heat switches are possible to produce a persistent current. In an alternative embodiment (not shown), the flux-gate magnetometer is replaced by a SQUID (Superconducting Quantum Interference Device) displacement transducer in a manner known per se, for example see H. J. Paik, J. Appl. Phys. 47, 1 168 (1976) and P. W. Worden Jnr., Stanford University, PhD Thesis (1976).

Claims: CLAIMS 1. An angular motion measuring device comprising a levitation coil, a float body which is capable of being magnetically levitated by the coil on a levitation axis about which angular motion of the float body relative to the coil is to be detected, and means for measuring angular motion of the float body; wherein the levitation coil lies on the surface of a sphere, and wherein the float body has a lift surface sensitive to levitational forces exerted by the coil in use, said lift surface lying on the surface of a sphere. 2. A device as claimed in claim 1, wherein the spherical surfaces on which the coil and the lift surface lie are substantially concentric when the float body is levitated in use. 3. A device as claimed in claim 1 or 2, wherein the float body is a part-spherical shell. 4. A device as claimed in any preceding claim, wherein the float body has a plurality of masses suspended from its periphery. 5. A device as claimed in claim 4, wherein the combination of the float body and the masses has a centre of mass and a centre of buoyancy which are substantially coincident. 6. A device as claimed in any preceding claim, wherein the levitation coil is a unipolar coil having turns whose pitch increases outwardly away from the levitation axis. 7. A device as claimed in any preceding claim, wherein the levitation coil is provided on a part-spherical surface of a coil support. 8. A device as claimed in any preceding claim, wherein the coil and the lift surface are formed of a material which is capable of superconducting. 9. A device as claimed in any preceding claim, wherein the means for measuring angular motion of the float body comprises (i) at least one field-modulating element which is carried by the float body, and (ii) coil means disposed adjacent said at least one field-modulating element, said coil means being disposed adjacent the field-modulating element and providing an output signal which is dependent upon the angular position of the field-modulating element about the levitation axis. 10. An angular motion measuring system comprising two devices as claimed in any preceding claim which are arranged to operate in coupled differential mode.

Inventors: SPEAKE CLIVE CHRISTOPHER, SPEAKE, CLIVE, CHRISTOPHER
Applicants: SPEAKE CLIVE CHRISTOPHER, UNIV BIRMINGHAM, SPEAKE, CLIVE, CHRISTOPHER, THE UNIVERSITY OF BIRMINGHAM
Application year: 1998
Main classification: G01V7
IPC codes: G01C9, G01V7