WO1991004502A1

WO1991004502A1 - Induced field mineral value detector

Info

Publication number: WO1991004502A1
Application number: PCT/AU1990/000415
Authority: WO
Inventors: Brian Kenneth Thompson
Original assignee: Bayliss Electronic Industries Pty. Ltd.
Priority date: 1989-09-12
Filing date: 1990-09-12
Publication date: 1991-04-04

Abstract

A mineral value detector consisting of a search head having an aperture therein, at least one primary field inducing means to provide a primary field, and at least two secondary field receiving means to receive said primary field, the search head being configured in a balanced arrangement and adapted to provide a detection signal (1) having at least one peak therein when said field is disturbed by a mineral value proximate the search aperture, and a discriminator to determine if the detection signal is a bona fide signal (3). The discriminator may operate on a comparison of sequential, opposite-polarity peak valued detected (5, 6) from passage of subject material through the aperture.

Description

INDUCED FIELD MINERAL VALUE I___T__CT0_.

Field of Invention

The present invention relates to the field of metal or mineral value detectors. In particular, the present invention relates to a discriminator adapted to discern a metal or mineral value. Background Art

Metal detectors are used in many different applications, for example, in a production line situation to locate product contamination such as a piece of metal or machinery therein. The detectors may be constructed to have a conveyor belt assembly pass through the search aperture, such that product on the belt can be scanned. If contamination is detected, the detector may signal an automatic ejector to remove the contaminated product from the production line, or mark the product to indicate contamination.

The metal detector may produce a detection signal that will typically locate the contamination somewhere within the search aperture, an area bound by the conveyor belt width (or chute width) and a length along the belt or chute of approximately 0.7 times the aperture height. For large aperture metal detectors, automatic rejection of contaminated product may result in rejection of a large amount of uncontaminated product together with a small amount of contaminated product. There exists a need to more accurately locate the position of a contaminated product.

Furthermore, signals originating from external sources create an environment of spurious noise signals, in which a metal detector must operate. A conveyor belt may produce many different types of spurious noise, for example, partially magnetized moving metal parts may produce magnetic fields, variations in eddy current may occur due to flexing of the conveyor system, electrical motors and bus-bars proximate the detector may produce intermitant or alternating fields, and metal objects moving past the detector such as fork-lift vehicles may influence the detectors fields. There also exists a need to provide a discriminator able to distinguish between bona fide mineral value signals and spurious signals. Terms

Throughout this specification, the term "mineral value" includes metal. Object of Invention

An object of the present invention is to provide a discriminator for a mineral value detector adapted to discriminate or discern bona fide mineral value contamination signals from spurious signals produced by external electrical, electromagnetic, magnetic or mechanical origin.

A further object of the present invention is to provide a discriminator for a mineral value detector which can discern the location of a mineral value within the search aperture relatively more precisely. Summary of Invention

The present invention provides in a mineral value detector consisting of a search head having an aperture therein and at least one primary field inducing means adapted to provide a primary field, and at least two secondary field receiving means adapted to receive said primary field, the search head being configured in a balanced arrangement and adapted to provide a detection signal having at least one peak therein when said field is disturbed by a mineral value proximate the search aperture, a discriminator adapted to determine whether the detection signal is a spurious signal or a bona fide signal. The discriminator may comprise discriminator means for determining whether the detection signal includes a first peak of substantially opposite polarity to a second peak of the detection signal.

The present invention may also provide a discriminator wherein the location of said mineral value within said aperture corresponds substantially to an apex of said at least one peak.

The present invention may also provide a discriminator wherein the location of said mineral value within said aperture corresponds substantially to a mid-way point between the zero crossover points of said at least one peak.

The present invention may also provide a discriminator means adapted to further determine whether the first peak has an amplitude within 20% of that of the second peak.

The present invention may also provide a discriminator means adapted to further determine whether the time duration between each zero crossover of the first peak is within 20% of that of the second peak.

The present invention also provides a mineral value detector including the discriminator described above.

The search head provides a signal output when a field disturbance is monitored therein. This signal output may result from a mineral value, thereby providing a bona fide signal, or from spurious noise, thereby providing a spurious signal which should be ignored.

Advantageously, analysis of the signal output from the search head to determine the presence of a first peak and a second peak of opposite polarity enables determination of a bona fide signal as opposed to a spurious signal. Further analysis of the output signal as to amplitude and time duration between zero crossover points enables further confirmation of a bona fide signal as opposed to a spurious signal which is to be ignored.

Determination of the point at which there is maximum amplitude of a peak of the output signal provides a corresponding location of a mineral value within the search head aperture.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, wherein:

Figure 1 shows schematically a mineral value detector;

F.gure 2 shows the discriminator of the present invention; Figure 3 shows further elements of the discriminator of the preferred embodiment of the present invention; and

Figure 4a shows a typical bona fide waveform associated with the discriminator of the preferred embodiment disclosed.

Figures 4b, 4c, 4d and 4e show various spurious waveforms associated with field disturbances.

Figure 1 shows schematically a mineral value

10 detector. The detector includes a search head having an aperture through which items or goods pass in order to locate the presence of a mineral value in the items or goods. The goods may pass through the aperture by means of a conveyor assembly, about which the search head is _{1 ς} disposed. The search head includes a primary coil coupled to a primary drive circuit and at least two secondary coils coupled to a secondary amplifier circuit. The primary coil radiates a field, a disturbance in which by a mineral value is received by the secondary coils and processed by the _₀ detector's circuitry. Prior to entry of contamination into the search head aperture, the field therein is substantially constant and balanced, and thus provides a constant, balanced or substantially zero amplitude signal at the secondary amplifier, -_t- As a piece of contamination enters the search head aperture, an output signal results from the secondary amplifier in the form of an amplitude modulated signal having a frequency substantially that of the primary drive signal. The output signal indicates a field disturbance

30 within or proximate the aperture.

The remaining description will be made in reference to a mineral value detector having at least a three coiled search head which is used in association with a conveyor assembly. The conveyor is adapted to move, for example,

35 product to be detected from right to left through the search head aperture. However, as would be understood by those skilled in the art, the present invention should not be limited to such applications. Clearly, the present invention can be used on many types of mineral value detector irrespective of size or application, in order to more accurately and reliably provide and locate a bona fide contamination signal. For example, the present invention i equally applicable to a small hand-held metal detector.

With reference to Figure 4a, as a piece of contamination enters the search head (see Figure 1), the right secondary coil will provide an increasing signal proportional to the amount of field disturbance (A). The signal will continue to increase as the contamination moves closer to the right secondary coil, peak (B) and then begin to decrease as the contamination also starts to influence the left secondary coil. When the contamination is midway the two secondary coils (C), the field disturbance will be relatively equal in each secondary coil and the output signal will again be balanced (zero amplitude). The effect on one secondary coil is cancelled out by the effect on the other secondary coil. As the contamination continues towar the left secondary coil, the signal will again peak (D), an later again balance (E) or null when the contamination has left the search aperture.

As can be seen from above, the output signal of figure 4a is a bona fide contamination signal. The present invention serves to determine when such a signal is provide by a mineral value detector, and furthermore from this determine the position of the contamination or mineral valu in the aperture.

The location of the mineral value corresponds substantially to the peak(s) (B,D) of the output signal. The location of the mineral value detector may also correspond substantially to a mid-way point between the zer crossover points of the peak(s) of the output signal, although the circuitry of the present invention "tracks" th amplitude rise or fall corresponding to the peaks (ABC or CDE), and provides an output indicating the points B and/or D.

A peak detector circuit (5,6 of Figure 3) measures the signal provided by the mineral value detector, in which the signal amplitude is followed and a monostable circuit i triggered to indicate the apex of the peak of the signal (points B or D) . This trigger signal serves to indicate th relatively precise location of the mineral value. An ejector or other device can then be enabled correspondingly to the speed of travel of the mineral value through the search aperture to eject or otherwise identify the product contaminated with a mineral value.

The present invention as described performs the function of discrimination between a bona fide signal and a spurious signal caused by an external event. To see why discrimination is possible, examples of the types of search head signals generated by external events which should be ignored must be examined. The types of events, for example can be described as follows:- a) Operation of a reject device, in the form of, say, a sweep arm, flap or other structure designed to remove or re-position a selected product after it has been passed through a metal detector aperture. b) Operation of an infeed controller, ie. a gate value, flap or similar device designed to control, deflect or re-position any material prior to entry into the search head of a meta] detector. c) Passage of machinery in proximity to the search head of a metal detector. The machinery may not be associated with either the metal detector or the associated equipment, eg. passing fork lifts, trolleys, etc.

Figures 4b and 4c show a spurious signal associate, with a typical electrical interference signal. Figures 4d and 4e show a spurious signal associated with a typical mechanical external event, which is not a bona fide signal.

The types of signals generated by the above events gnerally can be described as unipolar or unidirectional signals. Assume that the waveform is symmetrical about a point midway between the two peaks. The criterion of detection for the discriminator includes the requirement fo a peak in the signal to be followed by a second peak of the opposite polarity. Thus a signal generated by an external event that is unipolar will be ignored by the system.

The signals generated by the reject devices only affect one of the secondary coils, the closest coil, thus the signals will only be in one direction. Similarly, infeed control devices will only affect one secondary coil

10 producing unipolar signals.

Passage of machinery in proximity to the search head may produce unipolar signals. However, sometimes a trolley passing close to the search head may produce signal similar to that shown in figure 4a, as can be seen, in part ₁ ,. in figure 4b. Generally the signal, whilst being symmetrical, will consist of an initial positive peak followed by a period of zero signal and then a second peak in the negative direction. A pulse duration or timing circuit will discriminate between signals that have a rapid _- transition between the two peaks and a signal that has a delay between the peaks.

In addition to the previous circuitry a secondary system of discrimination may be included. This system is intended for use when a piece of machinery is moving in «_ close proximity to the search head and its movement is controlled and can be timed to occur at a point in time whe no product is passing through the search head aperture. Fo example, packaging machinery that weighs out a portion of the product and delivers the portion using a gate value,

30 flap or other device. Movement of the value/flap as it opens will produce a transient signal in one direction. A feedback signal can be used to reset the .F. Amplifier so that any contamination can be detected. The closure of the value/flap will produce another transient signal, however since it will be unipolar no detection will be produced. 5 The cycle will be repeated as the value/flap opens for the next portion of product. The discriminator of the present invention serves to verify a bona fide signal and from the above it can be seen that there are three points on the contamination signa which can be used to locate the position of the contamination with the search head. The first is the plane of the first secondary coil which can be identified by locating the first peak in the modulated coil output (B). The second is in the plane of the primary coil where the output signal returns to the quiescent or balanced value at

10 point (C) or crossover value. The third is in the plane of the secondary coil where the output signal peaks for the second time (D) .

The third point (D) is the point that is identifiec by the present invention to determine contamination locatio •J_E and the following criteria are used to ensure that a bona fide detection is generated.

1) Both peak signals must exceed a preset minimuii level.

2) Since the polarity of the first peak is

2_Q opposite to that of the second peak, a peak of one polarity must be followed by a peak of opposite polarity for a detection signal to be generated at the second peak point.

3) The relative amplitudes of the first and

25 second peaks must be within 20% or closer for the second peak to be considered part of a valid pair.

4) For an equal amplitude peak pair the time duration between the first peak and the

3_Q crossover (A-C) must be within _\ 20% of the time between the crossover and the second peal< (C-E). With regard to Figure 2, the discriminator of the present invention utilizes a peak detector 2 to determine -_jt- whether the input waveform received at 1 from the search head has two peaks of substantially opposite polarity and substantially equal proportion. If so, a bona fide detection signal is given at output 3. Spurious noise signals often generate a signal having large and relatively small peaks, the signal having influenced one secondary coil more than the other coil. This type of signal (figures 4d and 4e) will not meet the above criteria, and accordingly, the discriminator of the present invention will not output a bona fide detection signal.

Figure 3 shows in more detail the discriminator of the present invention. A threshold setting is provided at

10 to reduce unwanted noise being introduced. The threshold setting 4 serves to remove low level noise and allow passag of signals above a particular amplitude. The signal is the tested to determine conformity with the criteria as set out above, and with reference to Figure 4a.

15 Peak detectors 5,6 determine whether there are two peaks in the signal. The peak detectors each follow the signal transition from the zero crossover base by means of an op-amp circuit, comparator and edge detector and a monostable circuit serves to provide an output peak point

20 signal at points B and D.

Measurement of peak amplitudes by circuits 7,8 is carried out by means of an integrator, whos output voltage corresponds to the peak height multiplied by the peak duration. _,. Time measuring circuits 9,10 analyse the signal to determine whether a second peak (C,D,E) occurs within a predetermined time following the first peak (A,B,C) and determines whether the second peak has a similar amplitude to that of the first peak.

The threshold or predetermined levels of the discriminator circuit are self adjusting or variable in order to accommodate detection of small mineral values whic may provide a signal of a few millivolts to detection of large mineral values which may provide a signal of a few _ volts in amplitude. If the signal meets these requirements a bona fide detection signal is given at output 3 to indicate a bona fide contamination is present. ln the case of a detector having in association therewith an automatic ejection means for removing or marking the product having contamination therein, a timer can be initiated by the peak (D), and knowing the speed of the conveyor, the ejector can be timed to eject to coincide with the travel time of the contaminated product, without unnecessarily ejecting an un-contaminated product. In this way, the present invention is able to locate the relative location of the mineral value relatively precisely in the search aperture, and enable identification; marking or ejection of a contaminated product having a mineral value therein.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. In a mineral value detector consisting of a search head having an aperture therein and at least one primary field inducing means adapted to provide a primary field, an at least two secondary field receiving means adapted to receive said primary field, the search head being configure in a balanced arrangement and adapted to provide a detectio signal having at least one peak therein when said field is disturbed by a mineral value proximate the search aperture, a discriminator adapted to determine whether the detection signal is a spurious signal or a bona fide signal

2. A discriminator as claimed in claim 1 comprising, discriminator means for determining whether the detection signal includes a first peak of substantially opposite polarity to a second peak of the detection signal.

3. A discriminator as claimed in claim 1, wherein the location of said mineral value within said aperture corresponds substantially to an apex of said at least one peak.

4. A discriminator as claimed in claim 1, wherein the location of said mineral value within said aperture corresponds substantially to a mid-way point between the zero crossover points of said at least one peak.

5. A discriminator means as claimed in claim 1, adapted to further determine whether the first peak has an amplitude within 20% of that of the second peak.

6. A discriminator means as claimed in claim 1, adapted to further determine whether the time duration between each zero crossover of the first peak is within 20% of that of the second peak.

7. A mineral value detector including a discriminator as claimed in claim 1.

SUBSTITUTE SHEET