ROTARY SIGNAL COUPLER

Abstract: Signals are coupled between a shaft (20) and a housing (22) which rotate relative to each other by means of a primary coil (24) inductively coupled with a secondary coil (26). The primary coil (24) has a relatively large extent (A) and the secondary coil has a much smaller extent (B) and is positioned at the centre of the primary (24). Suitably A/B >/= 6.

Description: Rotary Signal Coupler This invention relates to a coupling for transmitting electrical signals between first and second members which undergo relative rotation. The invention is particularly, but not exclusively, of relevance to the inspection of long hollow cylindrical objects, such as drill collars for oil boreholes. A drill collar typically has a length of 10 m and an internal bore of about 78 mm; this makes iπ.ternal inspection for cracks and the like difficult- In our EP-A-0 033802 there is disclosed an electro.— magnetic inspection apparatus using a probe with twin windings. This apparatus is suitable for detecting defects of interest in drill collars and the like, but it is necessary to scan the probe over the internal surface. One way of doing this would be to draw through the bore on which the probe is rotatably mounted, so that spiral scan is performed. However, it is necessary to couple the signals from the rotating probe to a non-rotating instrument including a bridge circuit. Since the parameter of interest is a small out-of-balance quantity, it is very easy for this to be swamped by noise in the coupling. It has also been found that the signal of interest can be swamped by spurious signals arising from non-uniformity of rotational and translational movement. Accordingly, an object of the present invention is to provide a coupling of the kind stated and which provides low noise and a high immunity to non-uniform movemen . The invention provides a rotary coupling for transferring an electrical signal between first and second members arranged to undergo relative rotation, comprising a primary winding on the first member, and a secondary winding on the second member adjacent the primary winding for inductive coupling therewith, the primary winding having a given linear extent and the secondary winding being positioned within a minor part of and spaced from the ends of said linear extent. Preferably there are two primary and two secondary windings, both pairs being as specified in the preceding paragraph, for coupling two signals. In a preferred form, the first member is a shaft and the second member a surrounding sleeve. The primary winding is a'single-layer coil of length A on the shaft surface, and the secondary a multi- ^0 layer coil of length B on the sleeve adjacent the centre of the primary. Suitably A/B 6. Preferably, the face of the primary winding away from the secondary winding is covered with a ferrite material. Embodiments of the invention will now be described, by way l ~ - of example, with reference to the drawings, in which:- Fig 1 is a perspective view of an inspection apparatus incorporating the invention; Fig 2 is a diagrammatic cross-section of a coupling embodying the invention for use in the apparatus of Fig 1; 20 Fi 3 is a similar view of an alternative embodiment of the coupling; and Fig 4 is a side view, partly in cross-section, of a further embodiment. The inspection apparatus shown in Fig 1 comprises a cylindrical 25 body 10 dimensioned to be pulled through the bore of a tubular member by a cable 12. The body 10 has spring-biased wheels 14 for engagement with the bore. A probe 16, suitably of the type described in EP-A-0033802, is mounted in a holder 18 which is rotatable with respect to the body 10, to produce a helical scanning pattern. 30 Referring to Fig 2, the probe and holder are attached to a shaft 20 rotatable within a sleeve 22. The two windings of the probe 16 are connected each to a respective primary winding 24a, 24b, which are single-layer windings formed over ferrite layers 26a, 26b in circumferential grooves in the outer surface of the 35 shaft 20. Each primary winding 24a, 24b is inductively coupled with a respective secondary winding 28a, 28b, these being multi-layer windings formed in narrow slots in the sleeve 22. In accordance with the invention, the linear extent A of the primary windings 24 is considerably larger than the linear extent B of the secondary windings. The purpose is to position the secondary winding in an area of uniform flux from its primary, and to avoid coupling in the end zone of the primary where flux concentration occurs. This minimises noise induced by axial movement between the shaft and the sleeve, or lack of concentricity in the rotational movement. It has been found that A/B _ 6 is suitable, and that (while the coupling efficiency is poor) induced noise is very low. The alternative embodiment shown in Fig 3 operates in a similar manner and like parts are denoted by like references. In this case, however, the primary windings 24 are disc-shaped in a transversely extending flange 30 and are coupled with secondary windings 28 in an annular housing 32. Bearings.34 journal the shaft 20 for rotation in the housing 32. Fig 4 illustrates the invention applied to the inspection of a narrow-bore tube 40 having a bore too small to accept the rotary coupling. A probe 16 is mounted on the end of a rod 42 for rotation and translation within the tube 40. Bearing means indicated at 44 are provided for locating the probe 16 within the tube 40. The rod 42 is rotated b.y a drive assembly 46 embodying the coupling described above and located outside the tube 40. It will be understood that the rod 42 houses conductors connecting the probe 16 to the inductive coupling. Preferably, the rod 42 is sectional, the sections being provided with mechanical screw or bayonet connectors and mating electrical contacts. This permits long tubing to be inspected with the drive assembly requiring axial movement only by the section length.

Claims: 01096 ' -4- CLAIMS : 1. A rotary coupling for transferring an electrical signal between first and second members arranged to undergo relative rotation, comprising a primary winding on the first member, and a secondary winding on the second member adjacent the primary winding for inductive coupling therewith, the primary winding having a given linear extent and the secondary winding being positioned within a minor part of and spaced from the ends of said linear extent.. 2. A rotary coupling having two primary and twerrseeαπda-ry windings, arranged in pairs of a primary and a secondary winding, each pair being in accordance with claim ~ . r . for coupling two signals. 3. A rotary coupling according to claim 1, in which the first member is a shaft and the second member is a sleeve. 4. A rotary coupling according to claim 3, in which the primary winding is a single—layer coil of length A on the shaft surface, and the secondary a multi-layer coil of length B on the sleeve adjacent the centre of the primary. 5. A rotary coupling according to claim 4, in which A:B is equal to or greater than 6. 6. A rotary coupling according to claim 1, in which the face of the primary winding away from the secondary winding is covered with a ferrite material.

Inventors: HALE CHRISTOPHER CHARLES HAROL, HALE JOHN CHRISTOPHER, HALE, CHRISTOPHER, CHARLES, HAROLD, HALE, JOHN, CHRISTOPHER
Applicants: CONTROLOGY LTD, CONTROLOGY PRODUCTS LIMITED
Application year: 1987
Main classification: H01F23
IPC codes: E21B47, H01F38