Magnetic pattern viewer

Abstract: This invention provides a device for visually displaying magnetic patterns assumed within or recorded on a planar medium, e.g. electrical steel sheet, comprising a transparent plate (1) and a membrane (4) secured to and spanning the plate (1) defining a sealed area therebetween housing magnetic particles suspended in a liquid carrier. A field is applied by permanent magnet(s) (3) disposed around the periphery (2) of the plate or by an energised coil for subjecting said particles to a magnetic field in a direction normal to the plate (1) whereby, when placed in juxtaposition with the planar medium the particles agglomerate to assume positions in conformity with the magnetic patterns.

Description: This invention relates to a viewer by which magnetic patterns assumed within or recorded on a planar medium may be visually displayed. The invention is particularly, but not exclusively, directed to visually displaying magnetic domain structures in electrical steel sheet. In this regard power loss in such steels is closely linked with the domain structure of the material; thus, it is important to be able to observe the structure as an aid to the assessment of quality and to initiate corrective action as necessary in the production control. Whilst various forms of non-destructive magnetic readers exist for e.g. viewing magnetic patterns in tubes, discs, card etc., these are not suitable or convenient for the primary purpose to which this invention is directed. For example there is a viewer sold by 3M called Plastiform (R) B-1022 Magnetic Viewer. This is shown in Automation : September 1973, p8, and is described in principle in US Patent Specification 3010206. This suffers from the disadvantage that it requires the gap between top and bottom plates to be within a relatively small range and so requires a rigid bottom plate that does not fit well against a steel sheet. Also, the area covered by the 3M viewer is a 30 mm diameter circle, and a larger viewing area, useful for viewing magnetic domains of the size usual in electrical steels, would require a larger, and therefore thicker bottom plate to give the same rigidity. The sensitivity would thus be significantly reduced. The present invention provides a device for visually displaying magnetic patterns assumed within or recorded on a planar medium, comprising a transparent plate, a flexible membrane secured to and spanning the plate defining a sealed area therebetween housing magnetic particles suspended in an aqueous detergent solution, and means for subjecting said particles to a magnetic field in a direction normal to the plate whereby, when placed in juxtaposition with the membrane adjacent the planar medium the membrane flexes when contacting the medium and the particles agglomerate to assume positions in conformity with the magnetic patterns. The field may be applied by permanent magnet(s) disposed around the periphery of the plate or by a coil energised as appropriate. The plate may conveniently be a circular disc of Perspex or the like and the membrane may be a plastics film e.g. a polymer, white or light coloured to enhance the contrast with the particles for ease of viewing. The particles themselves may be iron oxide or finely divided iron. In order that the invention may be fully understood one embodiment thereof will now be described, by way of example with reference to the accompanying drawings, in which:- Figure 1 is a sectional side elevation of the viewer, and Figure 2 is a typical structure of grain oriented electrical steel as viewed in the Figure 1 device. Referring now to Figure 1 a circular-section plate 1 of e.g. Perspex (RTM) 10 cm in diameter has a peripheral recess 2 formed in it within which is wound a coil 3 energised from a supply source, not shown. A white polyvinyl chloride membrane 4 e.g. 35µ thick, is secured to the underside of the plate which is slightly recessed so on to define a cavity about 0.5 mm deep between the plate and membrane, this is filled via port 6 with an aqueous solution incorporating a proprietary detergent in which is suspended carbonyl iron powder. The size of the cavity for this slurry is such that the membrane 'bellies out' because of its flexibility somewhat beyond the underside of the Perspex plate itself - this is to ensure adequate planar contact with the steel sheet under investigation. In operation, the device is first shaken to distribute the particles in solution evenly, making the viewer opaque. It is then placed - underside down - on to the steel sheet under investigation and the coil is energised, typically producing a field of up to 50 oersted. The iron particles agglomerate and migrate into 'mounds' to the areas of high field in conformity with the magnetic domain patterns in the sheet, e.g. the surface coated electrical steel as depicted in Figure 2 - the contrast is clearly seen against the white membrane. The pattern is largely retained even following the removal of the energised field. For subsequently viewing another area the device may be rotated through 90°, set down and gently tapped whereupon the particles adopt the fresh domain pattern. This invention thus provides a simple compact, portable device with a very short response time for non-destructively viewing magnetic domains clearly, even through surface coated steel sheet, and, as mentioned above, it could readily be utilised for purposed other than that described in the production of electrical steels. The 'cell' beneath the transparent plate may readily be made as a separable, replaceable item since it is envisaged that damage to the membrane could be occasioned. In this regard the membrane may be made from a stronger material than the pvc exemplified. Indeed a non-ferrous metallic foil could be employed e.g. dispersion hardened aluminium; any significant thickening of this membrane for strengthening purposes will unfortunately tend to reduce the sensitivity of course which cannot readily be overcome by increasing the applied magnetic field since this could swamp the field in the steel under investigation. Additionally, since as with most polymers the membrane is not entirely impermeable to water provision may be made to top-up the cavity from time-to-time with distilled water. Moreover, since air bubbles may be generated within the cavity a 'bubble trap' in the form of a reservoir adjacent the periphery of the plate may be provided such that, by holding the viewer vertically with the reservoir uppermost the bubbles migrate into same.

Claims: A device for visually displaying magnetic patterns assumed within or recorded on a planar medium, characterised by a transparent plate (1), a flexible membrane (4) secured to and spanning the plate defining a sealed area therebetween housing magnetic particles suspended in an aqueous detergent solution, and means (3) for subjecting said particles to a magnetic field in a direction normal to the plate whereby, when placed in juxtaposition with the membrane adjacent the planar medium the membrane flexes when contacting the medium and the particles agglomerate to assume positions in conformity with the magnetic patterns. A device according to claim 1, characterised in that the means for subjecting said particles to a magnetic field is an electrically energised coil disposed around the periphery of the plate. A device according to claim 1, characterised in that the means for subjecting said particles to a magnetic field is a plurality of permanent magnets disposed around the periphery of the plate. A device according to claim 2 or claim 3, characterised in that the plate is a circular disc of plastics material. A device according to any one of claims 1 to 4, characterised in that the membrane is a film of transparent or translucent plastics material. A device according to any one of claims 1 to 5, characterised in that the particles are iron oxide. A device according to any one of claims 1 to 5, characterised in that the particles are finely divided iron.

Inventors: TAYLOR ROBERT JOHN, WATT JOHN ALBERT, TAYLOR, ROBERT JOHN, WATT, JOHN ALBERT
Applicants: BRITISH STEEL PLC
Application year: 1990
Main classification: G01R33
IPC codes: G01R33