DETECTOR

BACKGROUND

[0001] The present invention is related to a metal detector and a method to

assemble a metal detector.

[0002] In the industry where machinery is involved in the production of goods, there is always a probability that a piece of metal, such as a screw or a bolt, does break away from the machinery, and finally ends up in the processed product. Therefore metal detection systems are used at various stages of a production process to detect products that are contaminated by metal. Metal detection systems are also often used for inspecting finished product, in order to ensure consumers' safety and quality standards.

[0003] A typical metal detector used for detecting metallic presence in products generally comprises an electromagnetic coil arrangement on a former or also called a frame assembly having an pass-through aperture. The former assembly is surrounded by a housing or case and the remaining space between the former assembly and the case is filled with a potting medium.

[0004] A metal detection system as mentioned above comprises a former

assembly with an electromagnetic coil arrangement that usually comprises three coils, one transmitter coil and two receiver coils that are aligned in parallel. Since the two receiver coils are identical and at the same distance from the transmitter coil, an identical voltage is induced in each of the receiver coils. When the receiver coils are connected in opposition, these voltages cancel out resulting in a zero output when there is no product present in the metal detector. This is a balanced coil system. During the inspection process, the product, typically transported on a conveyor belt, is passed through the coils of the balanced coil system. In the transmitter coil which is usually placed between the receiver coils, flows an electrical current that generates an alternating magnetic field that induces an electrical signal in the two receiver coils. In most metal detectors like mentioned above, the receiver coils are positioned symmetrically to the transmitter coil, so that identical signals are induced in both receiver coils when no product is present in the balanced coil system. In addition, the receiver coils are coupled together in such a manner that the signals induced therein are subtracted from each other. In that way, when no product is present in the balanced coil system, there is a zero signal at the output of the receiver coils. However, a piece of magnetically and/or electrically conductive material that passes through the balanced coil system will disturb the magnetic field and will cause modifications of the electrical signal that is induced in the receiver coils. These perturbations occur first in the first receiver coil and then in the second receiver coil, when the product approaches it.

[0005] Metal detector assemblies are known which comprise a coil-carrying

former or a plurality of laminated frames that form the former between which the transmitting and receiving coils are located surrounded by a case or housing. Such assemblies are generally complicated and costly to manufacture; additionally and/or alternatively they are often subject to tension due to the instability between the case, former and the coils. Such instability in the assembly leads to problems like electronic drift and reduction in repeatability which in turn affects the yield of the detector.

[0006] To ensure a stable design and construction of a metal detector, it is of high importance that the physical connections made between the various components of a metal detector have to be stable avoiding vibrations and cause minimum if not eliminate the tension between them.

[0007] The document WO 95/1 1462 A1 discloses a method of joining the case and the former of a metal detector. The method includes using mechanical fixings like aluminium dowels. These aluminium dowels, however, themselves not being a part of the case or former (it is an external entity) introduce a mechanical tension between the case and the former in case of vibrations or shocks. In an another embodiment, the document discloses a tongue-groove joint to join the parts of the case to each other and also the case to the former in certain adjoining parts. As mentioned in the document, the tongue-groove joint ensures a minimum magnetic leakage from the joint faces of the case. The document however, does not emphasize on the means taken to ensure the tension free physical connection or physical joint between the former and the case which in turn affects the stability of the metal detector.

[0008] The document GB 2267351 A discloses a method for assemblying a metal detector. In the document, the former is constructed from three frames and held together with the use of a glue or an interlock, after which the coils are arranged on the former. A layer of plastic coating which acts like a potting medium is formed on the former followed by an application of a metal layer coating using methods like metallic painting, arcspraying, plating, or vacuum depositing. This metal layer coating upon hardening forms the case or the housing of the metal detector. The disadvantage of the disclosed method in terms of case construction is that using methods like metallic painting, arcspraying, etc. can be an expensive affair in terms of resources and labour.

[0009] It is known in the art that the stability between a former and a case is

further ensured by introducing a potting or a plastic medium in the empty space between the two. The potting medium used is generally resin or foam or in some cases concrete, which holds the assembly together.

Examples of using a potting medium for the above purpose are discussed in documents GB 2267351 A, GB 2232254 A and WO 95/1 1462 A1.

However, the contact surface between the former and the potting medium is limited to a flat horizontal plane which leads to a weak connection between the two. In presence of mechanical vibrations or shocks, the potting medium may move with respect to the former due to the weak contact surface area between them.

[0010] In addition, known prior-art-formers usually have smaller widths than that of the case. In case of such formers with a small width, they are generally covered with a potting medium and then encompassed by a case, increasing the chances of instability.

Metal detectors usually require their pass-through apertures to be lined with a conductive material like an electrostatic screen to protect the coils from a phenomenon called bulk effect, the more scientific term of which is capacitive effect. It is known that at low operating frequencies, the electrostatic screen is applied across each side of the pass-through aperture leaving a small gap to the edge of the case to ensure no contact with the case in order to avoid intermittent eddy currents to flow through the screen picked up by the metal detector receiver coils creating unwanted nuisance detection signals. Further, at higher operating frequencies, it becomes necessary to remove the gaps and hence the electrostatic screen is fitted directly across onto the case. However, although this is effective as a screen, it presents a risk to the stability of the metal detector. Any cracks appearing at the screen transition onto the case has the potential to cause the abovementioned problems.

SUMMARY OF INVENTION

[0012] It is an objective of the present invention to provide a stable assembly of a metal detector by ensuring a tension free physical connection between the former and the case.

[0013] It is a further objective of the present invention to provide a stable and tension free assembly of a metal detector which ensures increased stability between the former and the potting medium which in turn stabilizes the former with respect to the case.

[0014] It is a further objective of the present invention to provide an easy and cost-saving method for assembling a metal detector. [0015] With the above objectives in view, the present invention discusses an improved and stable assembly of a metal detector according to the independent claims.

[0016] According to the present invention in one aspect, there is provided a metal detector comprising a case having at least four closed sides and two openings, a first opening on the rear side of the case and a second opening on the front side of the case. The metal detector comprises a non- conductive former that is inserted within the case and means for securing the former to the front and rear sides of the case. The former has a pass- through aperture coaxial with the first and the second opening of the case, wherein the width of the former is the distance from the front side to the rear side of the case and is equal to the inner width of the case in a direction along the pass-through aperture. About the circumference of the former are provided balanced transmitting and receiving coils which operate to produce and detect within the former an electromagnetic field which is disturbed by a metallic body passing through the aperture.

[0017] The case and former are physically connected wherein protruding means, on the case are inserted into recesses of the former which are further toughened with the help of an adhesive. This physical connection offers minimum if no tension between the case and the former as they are not fixed using mechanical fixing like screws. An advantageous feature of the present invention is the provision of an increased contact surface area between the former and a potting medium filling the space between the former and the case. The former comprises keying slots on at least one of its sides facing towards the respective closed side of the metal case wherein the potting medium resides. Preferably the keying slots are arranged on two sides of the former or alternatively on all four sides of the former facing the respective closed sides of the case. The potting medium is filled into the keying slots of the former ensuring higher stability in the metal detector. [0018] The former can be made from a single piece or be constructed using several parts and assembled before inserting it into the case. The former constructed in the present invention has a width equal to the inner width of the case which rules out having to fill the space with potting medium width- wise and adds to the stability of the system.

[0019] In the context of the present invention the term inner width of the case and width of the former are defined to extend in a direction along the pass- through aperture through which products to be inspected are transported.

[0020] A further advantageous feature of the present invention is that the former is inserted into the case from the aperture side. It is known from prior art, that the former was either inserted inside the case from the side

perpendicular to the aperture or the case was constructed around the former using methods like metallic painting, arcspraying, plating and the like as mentioned in GB 2267351 A. The former used in the present invention has a width equal to the inner width of the case and therefore, cannot be inserted from the side perpendicular to the aperture.

[0021] A method of assembling a metal detector according to the present

invention makes it cost and labour effective as using methods like metallic painting, arcspraying, plating, etc. on a former covered with potting plastic can be an expensive affair due to usage of additional resources like machinery required to carry out the painting, spraying, etc. The present method avoids the usage of expensive machinery to construct the case. In the present invention, the case and former are constructed separately and then put together instead of constructing the case around the former. The present invention discloses a method of inserting a former into a case from the aperture side. If the case-former assembly according to the present invention is viewed in a way that the viewers eye can look through the aperture of the assembly, the case has two openings wherein the opening on its rear side (back side of the aperture) is slightly smaller than the opening on its front side (front side of the aperture). This provision makes it possible to slide the former from the front opening of the case. The sliding former is stopped by the opening on the rear side of the case due to it being smaller than the front opening of the case as discussed above, and thus the former is held on the rear side of the case. The former comprises recessing means which receive protruding means present on the rear side of the case for a stable fit.

[0022] A further developed embodiment of the present invention comprises a retainer which holds the case and former stable once the former is inserted into the case. The retainer comprises of protruding means which are inserted into the recessing means of the former and held together with the use of an adhesive.

[0023] A further developed embodiment of the present invention comprises an electrostatic screen which is basically a conductive material which lines the pass-through aperture to protect the coils from the so-called bulk effect phenomenon. The proposed method discusses fitting the screen across the former in such a way that the screen considerably overlaps across the protruding means of the case rather than making a direct contact with the case. The screen only contacts the case on four corners ensuring that the conductive material is still applied to the case but the lengths of contact are significantly reduced. Thus, a small gap exists between the screen and case and also the screen is earthed in at least one position.

[0024] A further advantageous embodiment of the present invention comprises the addition of a liner which encompasses the case and the former once the former has been inserted into the case and the retainer has been fixated on the case. The liner is used to provide environmental protection like protection against humidity, temperature, ingress or impacts to the coil system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Reference is made to the accompanying schematic drawings from which its novel features and advantages will be apparent. 6] In the drawings:

Fig. 1 is an isometric view of a typical metal detector;

Fig. 2 is a 3-D perspective of the case-former assembly according to the present invention in which a corner is cut out;

Fig. 3A, 3B show two different embodiments of the former with recessing means and keying slots according to the present invention in a 3-D perspective;

Fig. 4 is a side view of the case-former assembly cut along the Y-Z plane shown in Figure 2, wherein the former is inserted into the case from the front side of the case;

Fig. 5 is a side view of the case-former assembly cut along the Y-Z plane shown in Figure 2, after the insertion of the former in the case from the front side of the case and the filling of the empty space with a potting material;

Fig. 6 is the top part of the side view of an exploded view of the case- former assembly cut along the Y-Z plane as shown in Figure 2;

Fig. 7 is a top part of the side view of an exploded view of the case- former assembly cut along the Y-Z plane as shown in Figure 2, which is basically shown in Figure 6 including details like the keying slots and coil slots on the former ;

Fig. 8 is a top part of the side view of a mounted view of the case- former assembly cut along the Y-Z plane as shown in Figure 2, with additional details like the keying slots and coil slots on the former; Fig. 9 is a side view of retainer comprising second protruding means running along its perimeter;

Fig. 10 is a 3-D perspective of the case-former assembly seen from

the front side of the case according to an alternative

embodiment of the present invention; and

Fig. 1 1 is a 3-D perspective of the case-former assembly seen from

the rear side of the case according to another alternative embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

[0027] A typical metal detection system 10 is shown in Figure 1 of the drawings which comprises a metal detector 12 having electromagnetic coils arranged on a former (not shown in the figure) enclosed by a case 16. The metal detector has an aperture 13 through which packaged products may pass for inspection purpose using the transport equipment 18 which usually is a conveyor. An electronic module 20 is included for two-way communication with the user.

[0028] The case-former assembly 15 according to the present invention is shown from Figure 2, wherein a 3-D perspective of the physical connection between the case 16 and former 22 is seen in the cut-out corner as well as one side of the case which typically carries the electronic module (see Figure 1). The former 22 is inserted in the case 16. The case 16 has four closed sides 17 and two openings (as seen in Figure 1). The width W1 of the former 22 is equal to the inner width W2 of the case 16. The case 16 has first protruding means 28 on its rear side 24 that fits into first recessing means 32 of the former 22. A retainer 36 is fixated on the case 16 after the former 22 has been inserted. The retainer 36 has second protruding means 30 as seen in Figure 2 that fit into second recessing means 34 of the former 22. An electrostatic screen 23 is fitted on the former 22 and retainer 36 on the side facing the aperture 13. A liner 50 is then fixed in a way that it encompasses the case 16, the former 22, the retainer 36 and the electrostatic screen 23.

[0029] The former 22 is inserted in the case 16 as discussed above. The former

22 is generally made of non-conducting material and may be constructed from a single piece or can be made of four separate side pieces and be assembled to a single-piece former before being inserted into the case 16.

[0030] Figure 3A and Figure 3B each show a 3-D perspective of a former 22 with first recessing means 32 and second recessing means 34. The former 22 having a pass-through aperture 14, has recessing means 32, 34 on either sides of its open ends. The form of the recessing means 32, 34 can clearly be seen on the front side in Figures 3A and 3B. The electromagnetic coils (not shown in the figure) which create and detect an electromagnetic field inside the former 22 in presence of a metal, for example in a packaged product, are wound on the coil slots 46. Keying slots 42 are seen in Figure 3A and Figure 3B, wherein these keying slots 42 have a trapezoidal cut into the body of the former 22. According to Figure 3A the keying slots 42 are running on all four sides of the former 22 facing the respective closed sides 17 of the case 16 and according to Figure 3B, the keying slots 42 are only present on the top side and the bottom side of the former 22. According to the present invention, it is necessary that the keying slots 42 be present on atleast one side of the former 22 facing the respective closed side 17 of the case 16 to increase the contact surface area between the former 22 and a potting medium 54 (potting medium shown in Figure 5) present between the former 22 and the case 16. Alternative embodiments can be considered wherein the keying slots 42 can be shaped circular, rectangular, conical, or polygonal.

[0031] Figure 4 and Figure 5 are sections of the case-former assembly 15 cut along the Y-Z plane of Figure 2. Referring Figure 2 and Figure 4, it can be seen that the case 16 has a front side 26 and a rear side 24. Further in Figure 4, the case 16 has two openings, namely first opening 38 of length d1 on the rear side 24 of the case and a second opening 40 of length d.2 on the front side 26 of the case, wherein the length d.2 of second opening 40 is slightly greater than the length d1 of first opening 38, so that the former 22 can pass through the second opening 40 of the case 16, but be retained on the rear side 24 of the case 16. The former 22 is inserted from the front side 26 of the case 16 through the second opening 40 of the case 16. The physical connection between the case 16 and the former 22 is facilitated using protruding and recessing means as explained further. The case 16 has a set of first protruding means 28 on its rear side 24 which fits into the corresponding first recessing means 32 of the former 22. The former 22 has a second recessing means 34 upon which is fitted a retainer 36 having a set of corresponding second protruding means 30.

[0032] Figure 4 shows the method of inserting the former 22 inside the case 16 from the front side 26 of the case 16. Adhesive means 62 are applied on the first recessing means 32 of the former 22 before inserting the former 22 inside the case 16. As a result, the rear side 24 of the case comprising of the set of first protruding means 28 receive the first recessing means 32 of the former 22. The adhesive means 62 holds the bond of the former 22 and case 16 together on the rear side 24 of the case 16. Adhesive means 62 are then applied on the second recessing means 32 of the former 22 after inserting the former 22 inside the case 16. After applying adhesive means 62, a retainer 36 is fixated on the case 16 on the front side 26 of the case 16. The second recessing means 34 of the former 22 receive the second protruding means 30 present on the retainer 36. These second protruding means 30 are inserted into the second recessing means 34 and the bond is strengthened by adhesive means 62 as mentioned above. Upon the insertion of the second protruding means 30, the retainer 36 is held together with respect to the case 16 using inserts 43 (for example Rivnut® fasteners). The retainer 36 is then welded onto the case 16 using a weld fillet 35 (as shown in Figure 8).

[0033] Figure 5 shows the side view of the case-former assembly 15 cut along in the Y-Z plane of Figure 2. After the former 22 has been inserted into the case 16 and the retainer 36 is fixed onto the assembly, a potting medium 54 is then filled in the space between the case 16 and the former 22 from the opening meant for the electronic module 20 as seen in Figure 1. After filling the case 16 with the potting medium 54, the opening is closed with the electronic module 20 (as seen in Figure 1 ). The keying slots 42 on the former 22 (seen in Figure 4 and Figure 5) help in increasing the contact surface area between the former 22 and the potting medium 54, thus ensuring the stability of the case-former assembly 15 in case of

mechanical vibrations or shocks. The potting medium 54 is generally a resin filler with a composition of 80% filler material (for eg. Chalk, quartz, polymer, etc.) and 20% resin or foam or any other similar material.

[0034] Figure 6 is the top part of the side view of an exploded view of the case- former assembly 15 which is cut along the X-Z plane of Figure 2. The electrostatic screen 23 is fitted onto the former 22 in such a way that it covers some part of the case 16 and the retainer 36 as seen in Figure 6. The case-former assembly 15 is then encompassed by a liner 50 whose functionality is to provide environmental protection to the electromagnetic coils (not shown in figure). The liner 50 is usually made up of thermoplastic material like polypropylene, ABS (Acrylonitrile Butadiene Styrene) to name a few. A combed multi-seal 48 is a layer of silicon having a comb structure which is used to tighten the space between the liner 50 and the case 16 to prevent water or any other external material entering the liner 50.

Fastening means 44 are used to fix the liner 50 to the case 16 outside of the area where the former 22 is present within the case 16 in a way that it does not touch the former 22. The former 22 is thus not under any mechanical tension which makes the metal detector tension-free. The clamping bezel 56 is used to hold the liner 50 in place with respect to the case-former assembly 15.

[0035] Figure 7 and Figure 8 show the top part of the side view of an exploded and mounted view of the case-former assembly 15 respectively cut along the X-Z plane of Figure 2. Figure 7 and Figure 8 is a detailed figure compared to Figure 6. The added details include coil slots 46 and keying slots 42. The electromagnetic coils (not shown in figure) are wound on these coil slots 46 shown in Figure 7 and Figure 8. The keying slots 42 are present to increase the contact surface area between the potting medium 54 (not seen in Figures 7 and 8) and the former 22.

[0036] As mentioned earlier, once the former 22 has been inserted into the case

16, the retainer 36 is fixated onto the case 16. The retainer 36 retains the entire assembly and closes it from the front side 26 of the case 16. Figure 9 shows the top view of the retainer 36 that comprises of the second protruding means 30. These second protruding means 30 comprises of flanges as protruding means running along the perimeter of the retainer 36 except at the corners as seen in Figure 9. The flanges can be with or without indentations 52.

[0037] Figure 10 and Figure 1 1 show two alternative embodiments of the present invention wherein the former 220, 220" has a circular cross-section creating therein a circular pass-through aperture. Both these embodiments also have a retainer 360, 360" having a circular cross-section and fitting onto the former 220, 220' from the front side 260 of the case 160. The first and second opening of the case 160, 160" also have a corresponding circular cross-section as seen in the figures 10 and 1 1.

[0038] As seen in Figure 10, the case 160 has first protruding means 280 on its rear side 240 that fits into first recessing means 320 of the former 220 when the former 220 is slid inside the case 160 from the front side 260 of the case 160. A circular cross-sectional retainer 360 corresponding to the circular cross-sectional former 220 is fixated on the case 160 after the former 220 has been inserted into the case 160. The retainer 360 has second protruding means (not seen in the Figure) that fit into second recessing means 340 of the former 220. As seen in Figure 10, the first protruding means 280 run non-continuously in parallel around the first opening on the rear side 240 of the case 160.The second protruding means also run non-continuously along the perimeter of the retainer 360 facing towards the front side 260 of the case 160. [0039] The metal detector as shown in Figure 1 1 , has first protruding means (not shown in the Figure) on its rear side 240" of the case 160" that fits into first recessing means 320" of the former 220" when the former 220" is slid inside the case 160" from the front side 260" of the case 160". A circular cross-sectional retainer 360" corresponding to the circular cross-sectional former 220" is fixated on the case 160" after the former 220" has been inserted into the case 160". The retainer 360" has second protruding means 300" that fit into second recessing means 340" of the former 220". As seen in the Figure 1 1 , the first protruding means run continuously in parallel around the first opening on the rear side 240" of the case 160" and the second protruding means 300" run continuously along the perimeter of the retainer 360" facing towards the front side 260" of the case 160".

[0040] It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, the cross-section of the former, the pass-through aperture, the first opening and the second opening of the case 16, 160, 160" and the retainer can be typically of rectangular, quadratic, circular, or elliptical shape, but can also have any other shape that may be required in a practical application. In addition, "front side" and "rear side" are relative terms and are not necessarily related to the direction of movement of a conveyor. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein. The terms front side and rear side, for example, can be exchanged without changing the scope of the invention. LIST OF REFERENCE NUMERALS Metal detection system

Metal detector

Aperture

, 140, 140' Pass-through aperture of former

Case-former assembly

, 160, 160' Case

Closed sides of the case Transport equipment

Electronic module

, 220, 220' Former

Electrostatic Screen

, 240, 240' Rear side of the case

, 260, 260' Front side of the case

, 280 First protruding means

, 300' Second protruding means

, 320, 320' First recessing means

, 340, 340' Second recessing means

Weld fillet

, 360, 360' Retainer

First opening of case

Second opening of case Keying slots

Insert

Fastening means

Coil slots

Combed multi-seal

Liner

Indentations

Potting medium

Clamping Bezel 62 Adhesive means

W1 Width of former

W2 Inner width of case

d1 Length of first opening of case d2 Length of second opening of case