AU652967B2 - Search head for metal detectors - Google Patents

Description

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AUSTRALIA

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A S 55 ~j 0 550 Name of Applicant: Actual Inventor: Address for Service: Invention Title: DETECTION SYSTEMS PTY. LTD.

15 Glare Street Bayswater 3153 in the State of Victoria Commonwealth of Australia Anthony John MAHER DAVIES GOLLISON GAVE, Patent Attorneys, 1 Little Collins Street, Melbournc, 3000.

Search Head for Metal Detectors The following statcnment is a full description of this invention, including the best method of performing it known to us: 911 106,gcpda1lflO,dcc.c, -2- SEARCH HEAD FOR METAL DETECTORS This invention relates to a search head for a metal detector.

The general object of the invention is to provide a novel search head for induction type metal detectors, the head being of relatively simple construction and convenient to use.

According to the invention there is provided a search head for a metal detector 15 for detecting metal in fluids flowing in a conduit comprising a generally U-shaped support body and a plurality of induction coils mounted on the body, each of said coils being U-shaped and conforming to said U-shaped support body and each of said coils Shaving the same effective area.

e* bo* 20 Preferably, the coil is formed from ribbon cable having a plurality of conductors in a single layer.

Preferably further, there are first, second and third loops, the first and second being formed at respective edges of the cable and the third loop being formed at or near the centre of the cable. In this arrangement, an oscillating signal can be applied to the third loop and the outputs of the first and second loops monitored so as to operate as an induction-balance rAetal detector. It is preferred that the first and second coils are coupled together so that induced voltages therein subtract from one another, the arrangement being such that a difference will appear in the subtracted voltages when a metal object is closer to one or other of the first or second loops.

940718,q:Aope cp,87019.,2 -3- The invention will now be further described with reference to the accompanying drawings, in which: FIGURE 1 is a perspective view of a search head of the invention, FIGURE 1A is a diagrammatic cross-section showing the head on a conduit, FIGURE 2 is a schematic view showing the location of the three coils of the head, FIGURE 3 shows a cross-section through a 0 0* '6 a .6 06 91 1028,gcpspcW8,dc~cdItdlv,3 I I 4 typical ribbon cable, FIGURES 4 to 9 show stages in folding the ribbon cable, FIGURE 10 shows connections at the abutting ends of the ribbon cable, and FIGURE 11 is a circuit diagram for a metal detector constructed in accordance with the invention.

The search head 2 illustrated in Figure 1 comprises a generally U-shaped non-metallic support body 4 which has conductive paint applied to its outer surface 6. The conductive paint functions as an electro-static shield. A loom 8 of ribbon cable is mounted on the outer surface 6, the cable having a single layer multi-conductor structure as shown in cross-section is Figure 3. The cable 10 is folded and connected in such a way that there are first, second and third loop coils 12, 14 and 16 formed within the loom The coils are diagramatically illustrated in Figure 2, the first coil 12 being shown in solid lines, the second coil 14 is broken lines, and the third coil 16 being shown in chain lines. The loom 8 includes connections at 18 in order to form the coils 12, 14 and 16 and to couple the coils to cables 20 for input and output f signals. The body 4 and loom may be located within a generally U-shaped housing 9 so as to protect the coils and electrically shield the coils as shown in Figure 1A., It is preferred that the housing is made from stainless steel. It is also preferred that the space between the housing and the body 4 is filled with epoxy resin (not shown) in order to fix the body 4 and loom firmly in position relative to the housing.

5 Because the head is generally U-shaped it can very readily be placed upon a conduit 11 which is to be monitored for particles of metal passing therethrough. It will be appreciated that the open side of the head permits it to be placed on the conduit 11 without the need to have access to a free end of the conduit. In most known induction type metal detectors, the coils need to encircle the conduit and therefore it is necessary to have a free end of the conduit in order to place and remove the head. These closed loop detectors are therefore much more cumbersome and inflexible to use than are the heads of the invention.

In the preferred embodiment of the invention, an oscillating signal is applied to the third coil 16 so that voltages will be induced in the first and second coils 12 and 14. The coils 12 and 14 have inherently the same effective area because of the ribbon cable construction and are symmetrically disposed with respect to the third coil 16 and thus the voltages induced in the first and second coils i' will be equal. The voltages in the coils 12 and 14 can be subtracted so that in a normal condition there S. is a zero resultant output from the combined output S* •of the coils 12 and 14. If however a metal object S. passes through the head it will first be closer to one or other of the coils 12 or 14 (depending on the direction in which passes through the head) and then i loser to the opposite coil. This leads to an imbalance in the subtracted voltages induced in the coils 12 and 14, thereby generating an output signal which can be detected by suitable circuitry. The existence of the output signals indicates the passage 6 of a metallic object through the head. The most sensitive part of the head is that region near the curved apex because, near the lower ends of the coils 12 and 14, the disturbances caused by a metal object tend to cancel out. It will be appreciated that it is important that the coils 12 and 14 have the same effective area so that the voltages induced therein will be equal.

Figures 4 to 9 show a preferred way of folding the ribbon cable 10 in order to achieve an equality of areas for the coils 12 and 14, as well as coil 16.

Figure 4 shows a strip of the ribbon cable 10 having peripheral conductors 22 and 24 and ends and 42. The cable 10 is folded at transverse fold lines 26 and 28 and diagonal fold lines 32, 34, 36, and 38. The first step is to rotate the end through 90" by first folding along line 26 then along line 32 so as to assume the configuration as shown in I. Figure 5. The next step is to rotate the end 42 through 90° about folC, line 34 so as to assume the configuration shown in Figure 6. The next step is to rotate the end 42 through 90° by folding about line 28 and then along line 36 so as to assume the 6 configuration shown in Figure 7. Finally, the end 42 is rotated towards the end 40 folding about the line 38 so that the two ends 40 and 42 abut or are adjacent to one another so as to assume the configuration shown in Figure 8.

It will be appreciated that the edge conductor 22 forms a loop which is effectively 7 rectangular in shape. It has however an additional, overlapping portion indicated by numeral 44 but any voltages induced in the overlapping part 44 will be very small as the wire forms a low inductance hairpin so that the effective shape of the overall loop is a rectangle. Similarly, the other edge conductor 24 forms an effectively rectangular loop. It too has additional overlapping portion indicated by reference numeral 46 but again the voltages in the overlapping portion 46 will be very small so that the effective shiape is rectangular. As can be seen from Figure 8, tithe effective rectangular loops formed by the edge conductors 22 and 24 are identical in area. It will Ole be appreciated that the central conductor of the cable 10 (not marked in Figures 4 to 8) will have the same area as the peripheral conductors 22 and 24 and em,. will be disposed symmetrically between those conductors.

The planar configuration of the coils as shown in Figure 8 can be used in some circumstances.

In the preferred arrangement of the invention however, the coils are generally U-shaped and accordingly the loom 8 is rotated generally about axis 48 so as to assume the three dimensional S* configuration of the loom 8 as shown in Figure 9.

Other coil configurations could be used such as spirals.

In the preferred embodiment, the overlapped portions of the cable 10 are adhered together so as to minimize the possibility of unwanted relative movements. The loom 8 can be mounted on the outer surface 6 of the body 4 by means of an adhesive or 8 double sided adhesive tape.

Figure 10 illustrates the preferred technique for forming connections 18 at the abutting ends 40 and 42 of the cable 10. It will be seen that the ends of the conductors are bent through about so as to extend generally parallel to one another away from the plane of the conductor 10. Selected pairs of conductors can have the insulation stripped therefrom and solded so as to form connections as illustrated by connections 50, 52 and 54. It will be appreciated that by this technique conductors can be connected in parallel or in series. Normally series connections are formed so that the coils 12 and 14 have a selected number of turns, usually from one to six. The third coil usually has more turns, say 12.

It is noted in passing that not all of the conductors C need be used in forming the coils 12, 14 and 16.

Those which are not used do not affect the electrical or magnetic properties of the coils but contribute substantially to the mechanical stability of the arrangement and the equality of area of the coils by fixing the position of the coils with respect to each other. Connections from the cable or cables 20 can also be made at appropriate places to the connections 52 and 54.

a Figure 11 shows one circuit configuration for a metal detector incorporating the head 2. The circuit includes an sine wave oscillator 60 which produces an output in the range from 1kHz to 1MHz.

The output is amplified in an amplifier 62 and applied to the primary winding 64 of an input transformer 66. The secondary winding 67 of the q -9transformer is coupled to a selected number of turns of the third coil 16. In the illustrated arrangement, the third coil 16 has twelve turns and the secondary winding 67 is coupled to the central six turns. A capacitor 68 is connected across the coil 16 and the value of the capacitor is selected so that the coil 16 is tuned so that the coil and capacitor form a resonant circuit at the output frequency of the oscillator 60. This minimizes the current to the coil, 16 and reduces distortion. The first and second coils 12 and 14 each include four turns, in the illustrated arrangement. One end of each of the coils 12 and 14 is earthed and the other ends are connected to respective ends of a winding of a coupling tansformer 72. The other winding 76 is coupled via an amplifier 78 to detection circuitry o. 80. The connection of the coils 12 and 14 to the winding 72 is such that voltages induced by the third coil 16 cancel and when the induced voltages are equal there will be zero volts across the winding 72 and hence zero input to the amplifier 78. When however one of the coils 12 and 14 is closer to a metallic object or particle the voltage induced therein will be different to the voltage in the other •coil. Consequently a voltage difference will appear '":across the winding 72 thereby constituting i signal el ,which is detected by the detection circuitry 80. The detection circuitry 80 can be the usual type used for monitoring induction balance type metal detectors and need not be described herein.

The arrangement of the invention has significant advantages. The main advantage is that previously great care had to be taken in the 10 positioning of the coils with respect to each other so that each of the pickup coils received the same voltage from the central transmitting coil.

Accurately machined narrow grooves had to be used and the coils wound into these and then individually encapsulated. Accurate balancing was hard to achieve. With the use of ribbon cable and the folding technique described the coils are inherently balanced because they are physically locked to each other within the ribbon cable. Second, it is very easy to form the loom 8 because it is simply folded in the manner described and mounted on the body 4.

b Any irregularities in the shape of the loom 8 affect all of the coils and hence tend to be of no consequence because the areas of all of the coils aro equally affected. Similarly should there by any displacement or vibration in use of the free ends of Se the body 4, the effects are the same for all coils and tend to cancel out. In any event the most effective area is adjacent to the curved part of the loom, as mentioned previously.

It will be further appreciated that the use of a ribbon cable provides substantially mechanical advantages because the wide flat shape gives the cable high lateral stability in contrast to the low lateral stability which would be inherent if a single or pair of conductors where used to form the coils.

Thus the loom 8 can be held on the body 4 merely by the use of an adhesive of adhesive tape whereas in many prior art arrangements, each conductor or the coil is located in a groove in the body in order to prevent lateral displacements.

11 Many further modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention.

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Claims (7)

1. A search head for a metal detector for detecting metal in fluids flowing in a conduit comprising a generally U-shaped support body and a plurality of induction coils mounted on the body, each of said coils being U-shaped and conforming to said U- shaped support body and each of said coils having the same effective area.

2. A search head as claimed in claim 1 wherein said coils are mounted on the outer surface of the body. 3, A search head as claimed in claim 1 or 2 wherein said coils comprise conductive loops which are formed from a single piece of ribbon cable which is folded such that selected conductors form respective coils. *6 i *66 *6 15 4. A head as claimed in claim 3 wherein the cable has a plurality of conductors in a single layer. 0*6

5. A head as claimed in any one of claims 1 to 4 wherein the support body and coils :0. f e* are located within a housing and a bonding medium is used to fix the support body and 20 coils within the housing.

6. A head as claimed in claim 5 wherein there are first, second and third induction coils, the first and second coils being formed at respective edges of the cable and the third coil being formed at or near the centre of the cable.

7. A head as claimed in claim 6 including means for coupling an oscillating signal to the third loop.

8. A head as claimed in claim 7 including means for monitoring signals induced in the first and second coils in response to said oscillating signal. 940718,qopgcP,8M7OjA12

13- 9. A head as claimed in claim 8 wherein the first and second coils are coupled together so that induced voltages therein subtract from one another, the arrangement being such that a difference will appear in the subtracted voltages when a metal object is closer to one or other of the first or second coils. A metal detector comprising: a search head as claimed in any one of claims 6 to 9; an oscillator coupled to apply an oscillating signal to the third coil; and monitoring means for monitoring signals induced in the first and second coils responsive to said oscillating signal and the presence of a metal obj ct adjacent to the first or second coils. a s *eg 0 6 .5so a .555 S S S *5 S DATED this 18th day of July, 1994 DETECTION SYSTEMS PTY. LTD. By its Patent Attorneys DAVIES COLLISON CAVE 940I 8,q:\opft'cp87019A13 14 ABSTRACT A search head for a metal detector comprising a generally U-shaped support body having an induction coil mounted on the body, the search head d*O* having the advantage that it can be placed on and removed from a conduit without the necessity to uncouple the ends of the conduit. 911 1O1,gcpipe.0G8,deect~div,14