GB2146128A

GB2146128A - Metal detection apparatus

Info

Publication number: GB2146128A
Application number: GB08421920A
Authority: GB
Inventors: Bruce Goring Kerr
Original assignee: Goring Kerr PLC
Current assignee: Goring Kerr PLC
Priority date: 1983-08-30
Filing date: 1984-08-30
Publication date: 1985-04-11
Also published as: GB8323246D0; GB8421920D0; GB2146128B

Abstract

Metal detection apparatus in which undesired effects of the permeability of an object under test are compensated, comprises an oscillator 10 providing oscillatory signals to search coils S1, S2 and capacitors 11, 12 arranged to respond to the presence of an object under test. The search coils and capacitors are arranged in a bridge arrangement and provide signals to an adder comprising an operational amplifier 14. The search coils provide a signal whose phase is indicative of the presence of metal in an object under test and the capacitors provide a signal for compensating for the effects of permeability of the object in the search coil signal. The search coils may be in the form of loops surrounding the intended path of the object and the capacitors comprise plates arranged on opposite sides of the path and of non-conductive material covered by a layer of conductive material. The coils and capacitors may be located on one side of the path e.g. for detecting metal in the ground. <IMAGE>

Description

SPECIFICATION Metal detection apparatus The present invention relates to improved metal detection apparatus, and particularly to such apparatus which is capable of effecting compensation for the magnetic permeability of the product being tested.

A known method of detecting metal particles, for example in insulated products such as food, pharmaceuticals or textiles, involves a system in which a high frequency oscillator drives a search coil of the balanced bridge type, the signal from which is amplified and fed to a phase detector. This produces a DC output dependent on the phase of the input signal with respect to a reference signal from the oscillator.

In many applications the phasing is set such that the detectable signal is measuring the electrical loss caused by the metal in the search coil, rather than its permeability effect which produces a signal at 90 to the loss effect. This sytem enables most types of metal to be detected, but some with low loss but small permeability are not detectable.

To render these metals detectable a slight adjustment of the phase setting of the reference voltage to the phase detector is required.

This then allows for permeability effects to be detected. However, it is then found that if sufficient bulk of product is passed through the search coil a signal is produced with no metal present. This is caused by the fact that the product itself has a permeability different from that of air.

The present invention provides metal detection apparatus for detecting metal in an object moving relatively to the apparatus, the apparatus including oscillator means providing an oscillatory signal to coil means, the coil means including search coil means arranged to provide a signal whose phase is dependent on the presence of metal in said object while the metal is in the region of the search coil means, and capacitive means arranged so as to respond to the presence of the object to provide a signal for compensating for the effect of the permeability of the object on said signal from said search coil means.

Features and advantages of the present invention will become apparent from the following description of embodiments thereof, given by way of example, and when taken in conjunction with the accompanying drawings, in which: Figure 1 is a circuit diagram of one embodiment of the invention; Figure 2 is a circuit diagram of a modified embodiment; Figure 3 is a cut-away perspective view of a detector head suitable for use with the circuit of Fig. 2; and Figure 4 is a side sectional view of another detector head.

Referring to Fig. 1, an oscillator 10 drives search coils S1, S2 and also capacitors 11, 12 made up of plates Al, B1 and A2, B2 respectively. The capacitor plates are arranged in the search zones of their corresponding coils, as will be described later. So that the presence of the plates will not have an adverse effect on detection of metal, suitable precautions need to be taken, the preferred arrangement being to make the plates conductive but non-metallic, for example of graphite coated plastics material. The signal from the common connection point of coils S1, S2 is fed via a resistor R2 to amplifier 14 which also receives a signal from the capacitive arrangement including capacitors 11, 1 2 and the others as shown for balancing and matching purposes, via a variable resistor R1.

When product is passed through the search zone, i.e. through the coil and between the capacitor plates, an additional capacitative current flows proportional to the amount of product in the coil. This produces a signal at point C which is mixed with the undesired permeability signal from the search coils S1, S2 arising from the presence of the product.

The capacitive signal can be arranged to cancel the undesired permeability signal by adjustment of the ratio of R1 and R2, i.e. in the arrangement as shown, by adjustment of variable resistor R 1. The output of amplifier 14 can then be fed to the appropriate detection indicator. The phasing of the signal at point C can readily be reversed by interchange of the connections to plates Al and A2 to accept positive or negative permeability effects from the product.

Fig. 2 shows a similar arrangement to that of Fig. 1, but utilising a separate oscillator coil OC connected to oscillator 10, the search coils S1, S2 only reacting inductively to the oscillator signals. In other respects, the circuit operates identically to that of Fig. 1.

Fig. 3 shows a search head 30 suitable for use with the Fig. 2 circuit. The search head is in the form of an elongate annular rectangle, product being passed through the opening 31 in the direction of arrows P. Part of the search head is shown cut away for clarity. The search coils S1, S2 are wound on former 32 as shown, with the oscillator coil OC wound in between. A search head as shown, but without the oscillator coil OC would be suitable for the Fig. 1 circuit. Capacitor plates Al, A2 are positioned within and adjacent to the respective coils S1, S2 facing into the opening 31.

It will be seen from Figs. 1 and 2 that capacitor plates B1, B2 are both earthed and thus, in Fig. 3, can be conveniently implemented by a single common plate B.

Fig. 4 shows a different arrangement of search head 40 which is suitable for detection of metal in an earthed product 41, for example detecting metal in the ground. In this case, the product 41 is only on one side of the coils S1, S2. The earthed product 41 draws current from the capacitor plates Al, A2 to compensate for any magnetic permeability effects it may have. The product 41 thus acts as the other (earthed) plates (B) of the capacitors 11, 1 2 of Figs. 1 and 2. It is found that the distance of the product 41 from the plates Al, A2 compensates in capacity effect for the magnetic effects at the same distance, i.e. the laws of magnetic and capacitive signals versus distance at least approximate to the same pattern.

Claims

1. Metal detection apparatus for detecting metal in an object moving relatively to the apparatus, the apparatus including oscillator means providing an oscillatory signal to coil means, the coil means including search coil means arranged to provide a signal whose phase is dependent on the presence of metal in said object while the metal is in the region of the search coil means, and capacitive means arranged so as to respond to the presence of the object to provide a signal for compensating for the effect of the permeability of the object on said signal from said search coil means.

2. Apparatus as claimed in claim 1 wherein the coil means are arranged as one or more loops surrounding the intended path of the object, and said capacitance comprise plates arranged on opposite sides of the intended path.

3. Apparatus as claimed in claim 1 wherein the coil means and the capacitive means comprise components all located on one side of the intended path of the object.

4. Apparatus as claimed in claim 1 wherein the capacitive means comprises one or more non conductive plates covered by a layer of conductive material.

5. Apparatus as claimed in claim 1 wherein the search coil means comprises a pair of coils located in adjacent arms of a bridge arrangement, and said capacitance means comprises a pair of capacitors located in the remaining arms of the bridge.

6. Apparatus as claimed in claim 5 wherein said signals are taken as outputs from the bridge arrangement and are summed together in an operational amplifier means.

7. Apparatus as claimed in claim 6 wherein one of said signals is applied to the operational amplifier means through variable impedance means.

8. Apparatus as claimed in claim 1 wherein the oscillator means provides said oscillatory signal to said search coil means or to separate oscillator coil means.

9. Metal detection apparatus substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.