GB2253057A - Search coil assembly with low resistance coils and method for making same - Google Patents

Abstract

A metal detector search coil assembly comprising a frame assembly having an aperture therethrough, electrically conductive strands wound upon the frame assembly, each of the strands comprising a conductor including a bundle of fine wires being so woven that each wire occupies all positions in the conductor to substantially the same extent, each wire being insulated from the remaining wires in the conductor except at end terminals. Multiconductor (eg litz) wire is used for windings 60 in a metal detector search coil assembly to achieve lower resistance than that experienced in solid coils. The wire comprises a bundle of fine filaments (figure 5) insulated one from another. Potting plastics material 80 is disposed on external surfaces of the frame assembly and embeds the windings, the polling plastics material being a selected one of (1) a binary resin and (2) a cast polyurethane. A metal housing go encloses the potting plastics materials. <IMAGE>

Description

SEARCH COIL ASSEMBLY WITH LOW RESISTANCE COILS AND METHOD FOR MAKING SAME This invention relates to metal detectors of the type used to detect metal fragments in foodstuffs and pharmaceuticals, and more particularly to a search coil assembly having low resistance coils, and a method for making the same.

In many industries, non-metallic materials, such as foodstuffs and pharmaceuticals, are conveyed automatically to various processing machines and stations. A problem arises when tramp metal is introduced into the material stream. Such tramp metal can constitute a hazard if sold to the public in foodstuffs or pharmaceuticals, and even in the processing area can constitute a hazard for some types of processing machinery.

It is known in the art that non-metallic material can be passed through an electromagnetic field of a search coil and that if tramp metal is present in the material stream such metal will disturb the electromagnetic field of the search coil and induce a detectable error signal. Responsive to the error signal, the material stream can be stopped or diverted to a waste container, accompanied, if desired, by visual and audible alarm means.

The sensitivity of such search coil metal detector systems is related to the size and shape of an aperture through which the non-metallic stream is directed and around which is disposed the search coil. The sensitivity is also dependent upon the intensity of the electric field through which the contaminant is passed. The intensity may be increased by increasing the electrical current in the coil. For a given voltage, the current may be increased by using, as search coils, wires having low electrical resistance. It is known in the art, for example, to use large solid wire coils in the order of No.16 to No.8 gauge wire in order to reduce the resistance or what is sometimes referred to as the "eddy current effect".In practice, it becomes difficult or impractical to fabricate metal detection coils with wire larger than No.8, particularly with metal detectors requiring physically small coil sizes. From an overall efficiency standpoint, it is important to create the most intense field with the minimum expenditure of electrical power and with the smallest coils practicable.

An object of the present invention is to provide for increased field intensity in a metal detection apparatus, while at the same time increasing efficiency in use of electrical energy and reducing the size of coils required.

With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of a metal detector search coil assembly comprising a frame assembly having an aperture therethrough, a plurality of electrically conductive strands wound upon the frame assembly, each of the strands comprising a conductor including a bundle of fine wires, the wires being so woven that each wire occupies all positions in the conductor to substantially the same extent, each wire being insulated from the remaining wires in the conductor except at end terminals thereof, potting plastics material disposed on external surfaces of the frame assembly and embedding the strands, the potting plastics material being a selected one of binary resin and a cast polyurethane, and a metal housing enclosing the potting plastics material.

A further feature of the present invention is a method of making the above-described metal detector search coil assembly.

The above and preferred embodiments of the invention, including various novel details of construction and combinations of parts, will now be more particularly described by way of example with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular device and method embodying the invention are shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.

Reference is made to the accompanying drawings in which is shown an illustrative embodiment of the invention from which its novel features and --advantageswill be apparent. In the drawings: FIG.1 is a perspective view of a search coil frame assembly provided in accordance with the invention; FIG.2 is a perspective view, similar to FIG.1, but showing potting plastics material applied to the frame end coils, to embed the coils in the plastics material; FIG.3 is a perspective view, similar to FIG.2, but showing a metal housing enclosing the potting plastics material; FIG.4 is a sectional view, taken along line IV-IV of FIG.3, and showing a conveyor belt and product thereon to be passed through the detector;; FIG.5 is a perspective view of a conductor of the type used in the search coil assembly of the present invention; and FIG.5A is a sectional view showing one of the fine wires making up the conductor shown in FIG.5.

Referring to the drawings, it will be seen that there is provided a frame 2 having a top member 4, a bottom member 6, and side members 8, 10, defining an aperture 12 therethrough. The frame is constructed of non-metallic material, such as moulded plastics material or formed wood. The members 4, 6, 8, 10 are preferably glued together, or moulded as one integral unit, or held together by screws (not shown). The top frame member 4 has an internal surface 14, an external major surface 16, and end surfaces 18, 20.

The bottom frame member 6 is similarly provided with an internal surface 22, an external major surface 24, and end surfaces 26, 28. The first frame side member 8 is provided with an internal surface 30, an external major surface 32, and end surfaces 34, 36.

In like manner, the second frame side member 10 is provided with an internal surface 40, an external major surface 42, and end surfaces 44, 46.

In the embodiment shown in FIG.1, each of the external major surfaces 16, 24, 32 and 42, is provided with three grooves 50, which are in alignment from frame member to frame member to provide continuous grooves around the exterior of the frame.

After completion of the frame 2, electrically conductive strands 60, are wound upon the frame, the strands 60 being disposed in the grooves 50.

Typically, the strands 60 are arranged in three groups, defining three coils, each coil made up of one or more strands. A "transmit" coil 62 is disposed centrally of the frame, and on either side thereof is disposed a "receive" coil 64, 66. Each of the coils 62, 64, 66 has two free ends, respectively, 62a, 62b, 64a, 64b, and 66a, 66b. The free ends 62a and 62b of the transmit coil lead away from the assembly, and the free ends 64a, 64b, 66a and 66b of the receiving coils lead away from the assembly.

FIG.5 is a perspective view showing a known Litz conductor, as described above. Referring to FIG.S, it will be seen that the Litz conductor includes several strands 72 twisted to form a completed conductor. The configuration of the completed conductor shown in FIG.5 is substantially circular. Each fine wire 72 (FIG.SA) is typically covered with an insulating layer 74 of polyurethane, and the insulating layer 74 is coated with a textile wrapping 76.

The fine wires 72 are woven in such a way that each strand occupies all possible positions in the conductor to approximately the same extent. In this way, the total flux linkages surrounding the individual wires, when totalled over an appreciable length, will be substantially the same for all the fine wires. The result of equalizing the flux linkages, and hence resistances, of the individual wires is to cause the current to divide uniformly between strands, reducing the effective resistance compared to a single large wire of comparable size.

Thus, in a Litz conductor the flux linkages, and therefore the reactance, cause the current to divide more evenly between the wires, in contrast to the condition of a single conductor whose cross sectional area of copper is equivalent to that of the Litz conductor.

The inventors have discovered that a single turn of Litz conductor produces an effective field twice the intensity of a comparable single turn of solid wire. The Litz conductor consisted of 420 individual fine wires of No.42 AWG, enamel coated, and woven in the Litz fashion with an outside diameter of .0625". The solid wire tested had an outside diameter of .064".

A number of other benefits have been derived in the metal detection system when using the Litz wire in the receiver coils as well as the transmitter coil. For example, it is customary to tune the receiver coil circuit either directly or through an impedance matching device such as a transformer. By achieving a resonance condition in the output circuit of the coil system, discrimination against spurious signals and noise is achieved. In a resonant circuit this discriminating ability is limited by the effective resistance of the conductors employed. The Litz wire has proven more effective by achieving a sharper resonance. The result is a greater signal to noise ratio with corresponding increasing in detection sensitivity ability.

In accordance with the invention, therefore, Litz conductors are used as coil strands 60 rather than the conventional solid wire coils. In operation of the search coil assembly, it is imperative that there be no relative movement between the strands 60 and the frame 2. To ensure that such movement does not occur, the strands 60 (less the free ends) are embedded in molten plastics material. It is also important that no moisture reach the strands. The embedding of the strands in plastics material further serves to keep the strands free from moisture.

Before applying plastics material to the assembly, the aperture 12 is filled by a block 70 which shields the frame internal surfaces 14, 22, 30 and 40 (FIG.1) and supports the frame to prevent distortion thereof during the plastics material application stage.

With the block 70 filling the aperture 12 and extending in both directions therefrom, the frame and strand assembly is covered with molten potting plastics material 80. It is known in the art to use epoxy, a foam-in-place plastics material, or any such plastic material, or plastics-like matter as will readily adhere to the frame and wires. A polyurethane foam-type plastics material has been used and accepted. However, as the sensitivity of the metal detector is increased, for example, by the use of the Litz conductor previously described, the sensitivity to both metal contaminants and vibration increases. In order to reduce the unwanted sensitivity to vibration, it is necessary to fill the space between the frame and a metal housing 90 with a material whose mechanical stiffness properties are optimized. The aforesaid prior art potting substances have been found wanting.Two materials have been tried and found to be successful: (1) Binary Resin (Resin XKP-0194) with a specific gravity of about 1.2 to 1.5 (density of 3 about 1.2 to 1.5 Mg/m and ) flex modulus of about and a flex modulus of about 200,000 to 300,000 p.s.i. (approximately 1.4 GN/m to 2 2 GN/m ), supplied by the Formulated Resins Div. of TACC International Corp.; and (2) Cast polyurethane with a specific gravity of about 1.1 to 1.3 (density of about 1.1 to 1.3 3 Mg/m ) and a flex modulus of about 200,000 2to 300,000 2 p.s.i. (approximately 1.4 GN/m to 2 GN/m ), supplied by Plan Tech Inc.

The thickness of the plastics coating must be such as to completely embed the strands and protect them from moisture. In addition, it is desirable to have the plastics layer thick enough to maintain the metal layer, still to be applied, a sufficient distance from the strands 60, it having been discovered that close proximity of the metal casing to the strands greatly diminishes the effectiveness of the assembly. A thickness of about one to six inches (approximately 2.5 to 15cm) is generally deemed sufficient. After application, the potting plastics material 80 is allowed to set up and harden (FIG.2). A cavity 82 is preferably moulded at this stage into one end of the assembly to form a compartment 84 for electronic circuitry.

The present state of the art in the manufacture of metal detector coil systems requires that a metal housing be placed around the electrical coils in all directions, except the inspection direction. The metal housing may take the form of a metal box formed of rigid plates, or may be formed directly onto the potting plastics material, as by metallic painting, arc spraying, plating, vacuum depositing, and the like. The metal housing 90 completely covers the plastics material (FIG.3), other than in the cavity 82, and adheres thereto. The metal preferably may be zinc, aluminium or stainless steel. If the metal is applied in fluid form, it is allowed to set up and harden and the block 70 is removed from the aperture 12. A cover plate 92 may be used to cover the compartment 84 (FIG.3).The cover plate 92 includes an inner surface of conductive material which is insulated by a suitable gasket 94, or the like, from the metallic housing 90.

The exposed inner surfaces 14, 22, 30, 40, of the frame 2 may then be spray coated with a conductive material 100, such as carbon, which is of reduced conductivity and which creates an electrostatic shield to prevent false readings which might otherwise be occasioned by the passage of a conveyor belt through the coil assembly. This coating 100 may be protected by the insertion of sheet plastics material 102. Alternatively, the carbon coating may be included within the structure of a laminate frame assembly, not shown herein. See U.K. Patent application number 91o3687 of even date by the same applicant, (Agents reference P10408) which corresponds to U.S. Patent Application Serial No. 07/557,626, filed June 24th 1990, in the name of James M. Moran.

Upon completion of the above series of steps, there is provided a metal detector search coil unit, as shown in FIGS. 3 and 4, including the frame 2, Litz conductor coils 60 wound on the frame, a coating 80 of plastics material comprising either binary resin or cast polyurethane adhering to and enveloping the frame and coils, and a metal housing 90 enveloping the plastics material.

The search coil assembly is then connected to the remainder of a metal detector apparatus including a conveyor belt B (FIG.4) extending through the aperture 12 to convey package P through the aperture for inspection for metal particles, in accordance with known procedures, and the coil free ends connected to electronic circuitry disposed in the cavity 82 and/or external to the search coil assembly.

It is to be understood that the present invention is by no means limited to the particular construction herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims. For example, the aperture 12 may be of any shape required by a given product, such as rounded, square, rectangular, and the like.

Claims (6)

CLAIMS:

1. A metal detector search coil assembly comprising a frame assembly having an aperture therethrough, a plurality of electrically conductive strands wound upon the frame assembly, each of said strands comprising a conductor including a bundle of fine wires, said wires being so woven that each said wire occupies all positions in said conductor to substantially the same extent, each wire being insulated from the remaining wires in said conductor except at end terminals thereof, potting plastics material disposed on external surfaces of said frame assembly and embedding said strands, said potting plastics material being a binary resin or a cast polyurethane, and a metal housing enclosing the potting plastics material.

2. The assembly in accordance with claim 1 in which said binary resin has a specific gravity of 3 about 1.2 to 1.5 (density of about 1.2 to 1.5Mg/m ) and a flex modulus of about 200,000 to 300,000 p.s.i.

2 (about 1.4 to 2.0 GN/m ).

3. The assembly in accordance with claim 1 in which said polyurethane has a specific gravity of 3 about 1.1 to 1.3 (density of about 1.1 to 1.3 Mg/m and a flex modulus of about 200,000 to 300,000 p.s.i.

2 (about 1.

4 to 2.0 GN/m 4. A method of making search coil assemblies for metal detectors, comprising providing a frame, said frame defining an aperture therethrough of a selected size and configuration for inspection of a selected product of a given size and configuration, winding coils of electrically conductive strands about said frame, providing an electrically conductive layer between said aperture and said coils, applying a potting plastics material in liquid form to external surfaces of said frame and said coils to embed said coils in said plastics material and applying a metal housing to the exterior of said potting plastics material, said electrically conductive strands comprising Litz conductors, each said conductor including a bundle of fine wires, said wires being so woven that each said wire occupies all positions in said conductor to substantially the same extent, each wire being insulated from the remaining wires in said conductor except at end terminals thereof, and said potting plastics material comprising a binary resin, or a cast polyurethane.

5. A metal detector search coil assembly substantially as hereinbefore described with reference to the accompanying drawings.

6. A method of making search coil assemblies for metal detectors, substantially as hereinbefore described with reference to the accompanying drawings.