GB2197480A - Metal detectors - Google Patents

Abstract

A metal detector 1a comprises a transmitter coil TX flanked by a pair of interconnected receiver coils RX, the three coils defining a substantially horizontal conveyor path 3a for movement of material to be monitored for the presence of metal 20. The planes 22, 23 of the three coils are disposed at a substantial common angle A (20 DEG to 30 DEG ) to the vertical, for better detection of flat metal objects shaken down in cereal boxes. <IMAGE>

Description

SPECIFICATION Improvements in or relating to metal detectors This invention relates to metal detectors and is concerned with metal detectors of the kind comprising a transmitter coil flanked by a pair of interconnected receiver coils, the three coils defining a path for movement of material to be monitored for the presence of metal.

The transmitter coil generates a magnetic field and the receiver coils pick up induced voltages which are in balance until disturbed by the presence of metal.

According to the present invention, a metal detector comprising a transmitter coil flanked by a pair of interconnected receiver coils, the three coils defining a path for movement of material to be monitored for the presence of metal, is, in operation, characterised by the magnetic field lines generated by the transmitter coil being disposed generally at a substantial angle to the path.

The coils are preferably dispoded in planes disposed at 20 to 30 inclusive from the vertical.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a view in perspective of a presently-known form of metal detector, Figure 2 illustrates the various components of the detector, Figure 3 illustrates how the detector works, and Figure 4 illustrates the working of a metal detector modified according to the present invention.

In the figures, like reference numerals refer to like components.

With reference to Figs. 1 and 2, a metal detector 1 comprises a search head 2 housing a central transmitter coil TX flanked by a pair of interconnected receiver coils RX, the three coils defining a substantially horizontal path 3 for movement of material to be monitored for the presence of metal.

The receiver coils RX are equi-spaced from the central transmitter coil TX. All three coils are imbedded in an annular support structure (not shown) of "TUFNOL" (Registered Trade Mark) or like plastics material. The arrangement is known as a 3-coil, in-line system.

The search head 2 comprises a suitably screened metal case filled with pumice granules bonded together with resin to provide the case with solid but light reinforcement. The case is formed with a central aperture 4 through which the upper run 5 of an endless belt conveyor 6 passes to carry the material to be monitored. The upper run 5 (Fig. 1) is disposed in the substantially horizontal path 3 (Fig. 2).

The transmitter coil TX is driven by an oscillator 10, whereby a strong, high-frequency magnetic field is generated within the aperture 4. The magnetic field lines extend generally along the path 3.

The receiver coils RX each pick up a substantial induced voltage, the voltages being fed to a balancing circuit 11 wherein the coils are so interconnected in opposition that their induced voltages cancel and the output 12 of the circuit 11 is normally zero. The circuit 11 incorporates amplifying means whereby any signal present in the output 12 is increased in magnitude before leaving the circuit 11.

The output 12 is connected to a control unit 13 with a display 14. The unit 13 has a signal output 15 which is used to automatic cally arrest the conveyance of material being monitored, or to operate a reject device 16.

(Fig. 1). The signal indicative of the presence of metal, which is emitted by the unit 13, will be delayed, so as to give time for the contaminated material to reach the reject device 16. It will be appreciated that the material being monitored may be in bulk or contained in some kind of package.

The display 14 will also provide a signal indicative of the presence of metal.

With reference now to Fig. 3, if a disc 20 of metal is caused to travel along the path 3, eddy-currents are induced therein due to the presence of the high-frequency magnetic field.

The eddy-currents induced in the disc 20 upset the voltage balance in the circuit 11 whereby a signal proportional to the "signature" of the disc is sent to the display 13 by way of the output 12.

Detection results from the presence of magnetic metals (which increase coupling between the coils RX) or by non-magnetic metals which have an opposing effect.

When the disc 20 is disposed so that its surfaces extend at right angles to the general direction of the magnetic field lines, the eddycurrents 21 a flow over the full cross-sectional area thereof, and the "signature" of the disc will be similar to a sphere of the same diameter. However, when the disc 20 is disposed so that its surfaces are parallel to the general direction of the magnetic field lines, the cross-sectional area presented to the field is much reduced and the out-of-balance signal passing along the output 12 can be very small indeed.

Reference is made herein to the "general direction" of the magnetic field lines, as of course, the field lines, which are endless, are curved.

In some materials to be monitored, flat metal objects and wire are more likely contaminents than pieces of spherical form, especially in the manufacture of cereals where the material, (grits), are rolled.

In the case of loose materials such as cornflakes, flat metal contaminents may shake down so as to end up in close proximity and parallel to, the plane of the conveyor belt run 3. (Fig. 1). This is the worst possible orientation in which flat metal contaminents could present themselves to the metal detector 1.

However, the present invention solves this problem by a simple modification.

With reference to Fig. 4, the metal detector 1 a illustrated thereby has the planes of its coils TX, RX tilted over so that the magnetic field lines 25 generated by the coil TX are disposed generally at a substantial acute angle to the substantially horizontal path 3a of material to be monitored.

The planes of coils TX, RX are indicated by dotted lines 22, 23 respectively.

The angle of tilt A of this embodiment, which is common to all three coils, is 30 from the vertical. An angle of 20 to 30 inclusive is preferred.

The field lines 25 are of course endless and their cuvatures vary, being of relatively small radius in the proximity of the coil TX, and increasing in radius towards the axis of the coil. The general direction of the field lines 25 is indicated by the axis PQ.

The magnetic field will now intersect any flat metal object (presented by the disc 20) disposed face up/down as it moves along the path 3a. Even when disposed at an angle to the path 3a, the disc 20 will cut the field at entry and exit positions X an Y.

Claims (5)

1. A metal detector comprising a transmitter coil flanked by a pair of interconnected receiver coils, the three coils defining a path for movement of material to be monitored for the presence of metal, characterised in that, in operation of the detector, the magnetic field lines generated by the transmitter coil are disposed generally at a substantial angle to the path.

2. A metal detector as claimed in Claim 1, wherein the planes of the coils are disposed at an angle of 20 to 30 inclusive from the vertical.

3. A metal detector as claimed in Claim 2, wherein the planes of the coils are disposed at 30 from the vertical.

4. A metal detector as claimed in Claim 1, 2 or 3, provided with automatic material stop or reject means operable by a signal indicative of the presence of metal in the material being moved along the path.

5. A metal detector comprising a transmitter coil flanked by a pair of interconnected receiver coils, the three coils defining a substantially horizontal path for movement of material to be monitored for the presence of metal, said coils also surrounding said path, and conveyor means for transporting the material along the path, the three coils being disposed at an acute angle to said path.

5. A metal detector comprising a transmitter coil flanked by a pair of interconnected receiver coils, the three coils defining a substantially horizontal path for movement of material to be monitored for the presence of metal, and conveyor means for transporting the material along the path, the three coils being disposed at an acute angle to said path.

6. A metal detector as claimed in Claim 5, wherein the three coils are disposed at 200 to 30 from the vertical.

7. A metal detector substantially as hereinbefore described with reference to the accompanying drawings.

8. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.

CLAIMS Amendments to the claims have been filed, and have the following effect: Claims 1, 5 and 8 above have been deleted or textually amended.

New or textually amended claims have been filed as follows:

1. A metal detector comprising a transmitter coil flanked by a pair of interconnected receiver coils, the three coils surrounding a path for movement of material to be monitored for the presence of metal, characterised in that, in operation of the detector, the magnetic field lines generated by the transmitter coil are disposed generally at a substantial angle to the path.