GB2200811A

GB2200811A - Monitoring of displacement

Info

Publication number: GB2200811A
Application number: GB08702855A
Authority: GB
Inventors: Ian Robert Fothergill
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1987-02-09
Filing date: 1987-02-09
Publication date: 1988-08-10
Also published as: GB8702855D0

Abstract

Displacement of a body (10) in three dimensions is monitored by providing a pair of mutually orthogonal line (eg wire) targets (22, 24) on the body so that they extend in x and z directions and scanning the targets using a laser beam (13) which can be projected selectively in the x, y, and z directions and scanned in the xy and xz planes, by displacement of the laser source (12), so as to intercept the targets transversely to produce reflected beams. The reflected beams are analysed in conjunction with the positions of the laser source and with respect to datum positions of the latter, in order to obtain measurements of the x, y and z displacement of the body. <IMAGE>

Description

Monitoring of displacement This invention relates to the monitoring of displacement of a body in at least one direction especially, but not exclusively in circumstances where the monitoring necessarily has to be conducted from a location remote from the body. Such circumstances may arise, for example, in the monitoring of components within the primary vessel of a nuclear reactor where the monitoring equipment is located externally of the primary vessel and access to the primary vessel interior has to be obtained via a penetration in the reactor roof, which penetration may be laterally offset from the component to be monitored.

According to one aspect of the invention there is provided a method of monitoring displacement of a body in which a beam of radiation is directed from a remote source towards the body; the source is moved to scan the radiation beam laterally with respect to a line target provided on the body until the beam intercepts the line target giving rise to a reflected beam, the reflected beam is detected and analysed to derive, with respect to a datum position, the position of the incident beam at the time the reflected beam is at its peak intensity or a predetermined intensity.

According to a second aspect of the invention there is provided apparatus for monitoring displacement of a body comprising means defining a line target, a source for producing a beam of radiation, means for scanning the beam laterally relative to the line target so as to intercept and thereby produce a reflected beam, means for detecting and analysing the reflected beam to derive, with respect to a datum position, the position of the incident beam at the time the reflected beam is at its peak intensity or a predet~ermined intensity.

The radiation source is advantageously a laser.

Preferably the arrangement is such that the beam can be scanned across the same line target in two different directions (which are preferably mutually orthogonal).

In this way, displacement of the body in two dimensions can be monitored using the same line target. It is not essential however, in two dimensional monitoring, for the same line target to be used; in an alternative arrangement separate angularly-related targets may be provided on the body each of which is scanned laterally by the laser beam.

Where displacement is to be monitored in three dimensions a suitably angularly-disposed second line target (or third target if a separate target is employed for each dimension) can be provided and, in this event, the beam will be scanned laterally relative to this further.target to enable data to be derived for the third dimension.

In a presently preferred embodiment, three dimensional monitoring of displacement is performed using two substantially mutually-orthogonal line targets, one of which can be scanned in a third direction which may be substantially orthogonal to the other two directions.

Scanning of the incident beam across the target or targets may be implemented by movement of the source and the arrangement is conveniently such that, by the use of reflectors, the beam may be scanned at least two angularly-related directions in response to movements of the source in a single direction.

The or each line target is conveniently in the form of a small diameter round-section wire, for example a platinum wire, which scatters the incident radiation and serves to provide a maximum scattered intensity when the incident beam axis intercepts the longitudinal axis of the wire. Other forms of line target are possible where the target presents a convex surface towards the incident beam, including an inverted V-shaped target with the apex of the V presented towards the incident beam so that the apex gives rise to a well-defined peak in the reflected beam cross-section. The line target may alternatively be constituted by a recess of narrow width which may be produced by engraving or etching a surface on the body itself or a component mounted on the body and which may be opaque or possess some translucency.

Any displacement of the body relative to a datum position or positions may be determined from the shift in incident beam axis necessary to locate the position at which the incident beam currently gives rise to a reflected beam with said peak or predetermined intensity.

This shift may be determined from the extent of the source displacement needed to produce said peak or predetermined reflected beam intensity, although the extent of the shift of the beam axis may be determined directly as described hereinafter.

To promote further understanding of the invention, reference is now made to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a displacement monitoring system in accordance with the invention; Figure 2 is a perspective view of a mounting block for the target wires of the monitoring system; Figure 3 is a diagrammatic perspective view of a mirror assembly for use in defining datum axes of the incident laser beam;. and Figure 4 illustrates an alternative form of monitoring system.

The system to be described below has been designed for the purpose of optically monitoring three dimensional displacements of a component, depicted by reference numeral 10, within the primary vessel of a nuclear reactor, the monitoring being effected at a remote location external to the primary vessel via.a penetration in the reactor roof. The ambit of the invention however is not restricted to this specific application and details of the reactor structure are therefore omitted from the drawings since these are not essential to the understanding of the invention.

As shown in Figure 1, the monitoring system comprises a laser source 12 which is displaceable both in the x direction and the z direction (into the paper) by means not shown. The laser 12 produces a beam 13/13a which, after passage through a beam splitter 14, is directed vertically downwards (y direction) through the reactor roof where it is reflected through 900 along a horizontal (xz) plane by fixed mirror 16. Depending on the point of incidence of the beam 13 on the mirror 16, the reflected beam is either reflected through 900 into the vertical (xy) plane by fixed mirror 18 or remains in a horizontal (xz) plane - contrast reflected beams 17, 17a. Thus, by appropriate positioning of the laser source 12 (which is movable conjointly with the beam splitter 14) along the x axis, the beam undergoes either two reflections at mirrors 16, 18, as in the case of beam 17, or a single reflection at mirror 16, as in the case of beam 17a. In both cases, the beams are directed into the vicinity of a target structure 20 mounted on the component 10. In addition to shifting of the beam in the plane of the drawing by displacement of the laser source along the x direction, the beam may also be shifted transversely of its direction of travel and in the z direction by displacement of the laser source in the z direction.

The target structure 20 comprises a pair of orthogonally related line targets, a target 24 extending in the y direction and lying in the xy plane and a target 22 extending in the z direction and lying in the xz plane, see also Figure 2 illustrating one form of mounting arrangement for the target structure, the mounting block 26 in use being secured rigidly to the component 10. The line targets in the illustrated embodiment are consituted by taut, bright, round section wires of small diameter which scatter the laser light when the beam impinges on the wires.

The laser beam may be scanned transversely of the targets 22, 24 by appropriate displacement of the laser source 12 and it will be understood that when the beam impinges on the target some of the scattered light will be reflected back along the path of travel of the incident beam and towards the beam splitter 14 which is arranged to transmit the incident beam but deflect the reflected beam from the target towards a detector 28, eg a photo-sensitive detector. The detector 28 produces an electrical output representative of the light intensityfollowing thereon and it will be understood that the intensity of the reflected beam from the target will vary according to the relationship between the incident beam axis and the axis of the target.In other words, the reflected beam intensity will be maximum when the incident beam axis intersects the wire target axis and will tail off as the incident beam axis is displaced to either side of the target axis. Thus, analysis of the output of the detector 28 allows the position of the incident beam axis to be determined when the reflected beam intensity is at a peak value or at some other predetermined value which is indicative of a known positional relationship between the incident beam and the target.

It will be seen that displacements of the component 10 in three dimensions can be monitored by scanning the targets 22, 24 in the x, y and z directions, ie by appropriate movements of the laser source 12 in the x and z directions, so as to cause the scanning beam to interesect each target 22, 24. Thus, displacement in the x direction is detected by moving the laser source 12 along the x axis until the incident beam it produces undergoes reflection by both mirrors 16, 18 and manipulating x-movement of the laser source so that the resulting y-direction incident beam scans in the x-direction and intercepts the target 22.Similarly y-direction monitoring is performed by moving the laser source 12 along the x axis until the incident beam undergoes reflection at only mirror 16 to produce an x-direction incident beam which is scanned in the y-direction, by movement of the laser source 12 in the x-direction, to intercept the target 22. To execute z-direction monitoring, x-direction incident beam from the source 12 is scanned in the z direction, by movement of the source 12 in the z-direction, so as to intercept the target 24.

In each case, a pre-selected positional relationship between the incident beam axis and the target (eg incident beam axis intersecting the target axis) can be found by monitoring the output of the detector 28 to locate the incident beam axis position corresponding to a peak reflected intensity or a predetermined reflected intensity and hence the corresponding position of the laser source relative to a datum position of the laser source for each dimension (which datum position may differ for each of the z, y, z dimensions and may be the same for two or more of- the dimensions). By determining, with respect to the datum position or positions, the shifts of the laser source necessary to secure the predetermined positional relationships between the x, y and z-direction beams and the targets 22, 24, the magnitude of any displacement of the component 10 in three dimensions can be obtained.

The output of the detector 28 is supplied to monitoring circuitry 30 for analysing the output to locate the peak intensity or indicate when the reflected intensity is at a predetermined value. The positioning of the laser source 12 may be controlled by control circuitry 32 which receives signals from the monitoring circuitry 30 indicating registry of each incident beam with the targets 22, 24 and is operative to record the corresponding positions of the laser source and translate these into an output representing displacement of the component 10.

In the embodiment of Figures 1 and 2, the position of the laser source 12, relative to datum positions of the source 12, is determined along the x and z axis in order to evaluate displacement of the component 10.

However, datum positions for the incident beam axis may be defined by other means. For example, as illustrated in Figure 3, datum positions for the beam axis may be defined with the aid of datum-position defining line targets 36, 38, 40 mounted on the fixed mirror 18, which targets may also be in the form of taut bright, round section wires. Thus, in the course of performing a monitoring cycle, a scan of the x, y and z-beams over the targets 22, 26 may be preceded by a scan to locate respective targets 36(x-datum), 38 (y-datum) and 40 (z-datum) using the reflected beams from these latter targets. The corresponding positions of the laser source 12 may then be recorded for enabling, in conjunction with the positions obtained during scanning of the targets 22, 24, determination of the displacement of the component 10.

The diameters of the wires 22, 24 (and 36, 38, 40 when used) may be selected with regard to the precision required. For example, appropriate selection of wire diameter readily permits determination of the position of maximum intensity with a precision better than imam, which is adequate for the particular nuclear reactor application described above. In practice, to improve signal to noise ratio and spatial resolution, it is appropriate to focus the incident beam so as to increase its intensity and cross-sectional area. Where the mirrors 16, 18 are to be located in the gas blanket above the molten sodium pool of a fast fission nuclear reactor, the reflective surfaces of the mirrors may be swept with a gas, eg Argon, to remove any sodium aerosol depositing on those surfaces.

In the embodiment described above with reference to Figures 1 and 2, the mirrors 16, 18 may have to be so widely spaced as to require separate mountings which may be inconvenient for some applications. In the embodiment of Figure 4, mirror 18 is replaced by a mirror 40 which is located above the mirror 16 thus enabling the mirrors to be fairly close to one another to allow their mounting as a single unit. Other components in the system generally the same as in Figure 1 and are identified by the same reference numerals as in Figure 1 (the circuits 30 and 32 being unshown).

In the system of Figure 4, z axis and y axis scanning is performed in the same way as in Figure 1 using the mirror 16 and displacing the laser source 12 over a suitable range of movement along the z and x axis.

In this case however, x axis scanning is performed by appropriate positioning of the laser source so that the mirror 40 comes into play. The resulting beam 42 can then be scanned transversely of the target 22 by displacing the laser source 12 in the x direction. Any displacement of the target 22 in the x direction can be readily calculated from the y displacement (as determined using mirror 16), the shift of the laser source, relative to a datum position, needed to detect the peak intensity (or a predetermined intensity of the reflected beam via mirror 40 and the angle A of the mirror 40.

Instead of scanning the beam by linear displacement of the laser source, the laser beam may be scanned by means of an angularly oscillating mirror or mirrors (which may scan the beam in two different, typically orthogonal, planes). In this way, it is possible to average out random variations in the peak (or predetermined) intensity position and thereby compensate for natural convection currents.

Claims

1. A method of monitoring displacement of a body in which a beam of radiation is directed from a remote source towards the body, the source is moved to scan the radiation beam laterally with respect to a line target provided on the body until the beam intercepts the line target giving rise to a reflected beam, and the reflected beam is detected and analysed to derive, with respect to a datum position, the position of the incident beam at the time the reflected beam is at its peak intensity or a predetermined intensity.

2. Apparatus for monitoring displacement of a body comprising means defining a line target, a source for producing a beam of radiation, means for scanning the beam laterally relative to the line target so as to intercept and thereby produce a reflected beam, and means for detecting and analysing the reflected beam to derive, with respect to a datum position, the position of the incident beam at the time the reflected beam is at its peak intensity or a predetermined intensity.

3. A method or apparatus as claimed in Claim 1 or 2 in which the arrangement is such that the beam can be scanned across the same line target in two different directions

4. A method or apparatus as claimed in Claim 1, 2 or 3 in which three dimensional monitoring of displacement is performed using two substantially mutually-orthogonal line targets, one of which can be scanned in a thirddirection which is substantially orthogonal to the other two directions.

5. A method or apparatus as claimed in claim 1, 2, 3 or 4 in which scanning of the incident beam across the target or targets is implemented by movement of the source.

6. A method or apparatus as claimed in any one of Claims 1-5 in which the arrangement is such that, by the use of reflectors, the beam is scanned at least two angularly-related directions in response to movements of the source in a single direction.

7. A method or apparatus as claimed in any one of Claims 1-6 in which the or each line target is in the form of a small diameter round-section wire which scatters the incident radiation and serves to provide a maximum scattered intensity when the incident beam axis intercepts the longitudinal axis of the wire.

8. A method or apparatus as claimed in any one of Claims 1-6 in which the or each target is of inverted V-shape with the apex of the V presented towards the incident beam so that the apex gives rise to a well-defined peak in the reflected beam cross-section.

9. A method or apparatus as claimed in any one of Claims 1-6 in which the or each line target is constituted by a recess of narrow width.

10. A method or apparatus as claimed in any one of Claims 1-9 in which the radiation source is a laser.

11. Apparatus for monitoring displacement of a body, substantially as-hereinbefore described with reference to, and as shown in, Figure 1 or 4 or Figure 1 or 4 when modified by Figure 2 or Figure 3 of the accompanying drawings.

12. A method of monitoring displacement of a body, substantially as hereinbefore described with reference to Figure 1 or 4 or Figure 1 or 4 when modified by Figure 2 or 3 of the accompanying drawings.