GB2203533A - Rotation sensor - Google Patents

Abstract

In order to sense the rotation of a shaft (10) it is provided with a permanent bar magnet (11) which passes close to a Faraday effect sensor element (12) upon rotation of that shaft. Light from a laser (13) is conducted by polarisation maintaining fibre (14) through the sensor element (12) and back to a detector (19) via a further length of polarisation maintaining fibre (17) and a polarisation analyser (18). The rotation sensor may be used in a high temperature well-logging environment. <IMAGE>

Description

ROTATION SENSOR This invention relates to rotation sensors, and finds particular application in the remote sensing of rotation in an environment unsuited, for instance by reason of excessive temperatures liable to be encountered in well logging, to the use of Hall-effect rotation sensors.

According to the present invention there is provided a rotation sensor for sensing the rotation about an axis of a first body with respect to a second body, wherein the construction of the first body is such that it has a magnetic field which is not circularly symmetric about its rotation axis, and wherein the second body includes a sensor element through which polarised light is directed by optical fibre from a remote station, the optical activity of which sensor element is modulated by the changing magnetic field resulting from rotation of the first body.

There follows a description of rotation sensors embodying the invention ' in preferred forms. The description refers to the accompanying drawings in which: Figure 1 is a schematic representation of a transmission type sensor system, and Figures 2 and 3 are schematic representations of reflex type sensor systems.

The basic elements of a preferred form of transmission-type rotation sensor are depicted in Figure 1. These comprise a rotating shaft 10 provided with a small permanent bar magnet 11. As this shaft rotates on its axis the bar magnet passes close to a transparent Faraday effect sensor element 12. Light from an optical source 13, typically a laser, is ducted to the sensor element by way of an optical fibre 14 and beam expanding lens 15. Then after its passage through the sensor element, the light is collected by a second lens 16 and directed into a second fibre 17 which directs the light to a polarisation analyser 18, and on to a photodetector 19.In the drawings these lenses 15 and 16 have been depicted for illustrative purposes only as biconvex lenses, whereas in practice it is generally preferable to use graded index lenses which can be directly butted against the sensor element 12 and the fibres 14 and 17.

The rotation sensor of Figure 2 is similar in many respects to that of Figure 1, but is a reflex type instrument instead of a transmission type one. For this reason the sensor element is provided with a reflector 20, and the optical fibre is provided with a coupler 21 which directs at least some of the returning light out of fibre 14 and into a fibre 22 which ducts light to the polarisation analyser 18, and on to the photodetector 19.

The rotation sensor of Figure 3 is similarly a reflex type instrument, being distinguished from that of Figure 2 in that its sensor element has been turned through 90" to be sensitive to changes in the radial component of the magnetic field that result from rotation of the shaft.

In each instance the sensor element is made of a material, such as YIG, that has a large Verdet constant.

Plane polarised light passing through a medium in which there is a component of magnetic field H in the direction of propagation suffers a rotation of the plane of its polarisation which is linearly proportional to the strength of the axial component of the magnetic field, to the length over which that field is applied within the medium, and to the magnitude of the Verdet constant for that medium. As the shaft rotates its permanent magnet passes repeatedly through close proximity with the sensor element, thereby introducing a modulation of the magnetic flux threading that element, and hence a modulation of its optical activity.

One convenient way of monitoring this modulation of optical activity is to use single mode high birefringence (polarisation maintaining) fibre for the optical fibres. In the case of fibre 14 the fibre is oriented with one of its principal axes aligned with the plane of polarisation of the laser emission, so that the light enters the sensor element with a known linear polarisation plane. In the case of the transmission type sensor of Figure 1, the fibre 17 is oriented with one of its principal axes aligned with the plane of polarisation of the light emerging from the sensor element when the magnet is furthest away from the element, or alternatively when the magnet is closest to that element.

In its turn the polarisation analyser is oriented to intercept this light in order to provide a null in the signal output from the photodetector each time the shaft passes through this orientation. In the case of the reflex type sensor of Figure 2 the equivalent approach involves the use of a polarisation maintaining coupler and orienting the fibres to which it is coupled so that their principal axes are oriented with the principal axes of the coupler.

A method of constructing a polarisation maintaining coupler is described in the specification of Patent Application No. 8612660 to which attention is directed. Basically this method is an adaptation of the biconical fused fibre coupler manufacturing method of the specification of Patent No. 2150703B. The method of that Patent cannot directly be satisfactorily employed on high birefringence fibre because the structure that produces the requisite birefringence introduces excessive optical loss in the coupling region. This problem is avoided by splicing a short length of circularly symmetric conventional single mode fibre into both fibres from which the coupler is made, these spliced-in lengths being long enough merely to accommodate the coupling region of the coupler.

In the foregoing description the sensor element has been constituted by a bulk optics device, but alternatively it may be constituted by a length of circularly symmetric optical fibre which has been suitably doped to provide it with a high value of Verdet constant. A further variation is to employ more than one magnet on the rotating shaft, for instance by employing a conventional magnetically poled shaft encoder. The radial field component of such an encoder decays rapidly with increasing distance from the encoder, typically reaching a value of about 70 Oersteds at lmm. In the case of a reflex Figure 3 type configuration using a 2mm long sensor element of YIG, positioned to within 25 microns of the shaft encoder surface, the average field seen by the YIG will be at least 100 Oersteds. As the Verdet constant for YIG is 4"/k Oe cm, the net rotation of the plane of polarisation will be in the region of at least 0.160.

Claims (4)

1. AIMS

   CLAIMS : 1. A rotation sensor for sensing the rotation about an axis of a first body with respect to a second body, wherein the construction of the first body is such that it has a magnetic field which is not circularly summetric about its rotation axis, and wherein the second body includes a sensor element through which polarised light is directed by optical fibre from a remote station, the optical activity of which sensor element is modulated by the changing magnetic field resulting from rotation of the first body.
2. 2. A rotation sensor as claimed in claim 1, wherein the light is directed between the sensor element and the remote station by way of polarisation maintaining high birefringence single mode~optical fibre.
3. 3. A rotation sensor as claimed in claim 2, wherein said high birefringence single mode optical fibre includes a portion in which light from the remote station is directed towards the sensor element and light from the sensor element is directed towards the remote station.
4. 4. A rotation sensor substantially as hereinbefore described with reference to Figure 1 or 2 of the accompanying drawings.