WO2000065379A1

WO2000065379A1 - Metal detector

Info

Publication number: WO2000065379A1
Application number: PCT/SE2000/000770
Authority: WO
Inventors: Lennart Eliasson
Original assignee: Lr Eliasson Ab
Priority date: 1999-04-23
Filing date: 2000-04-20
Publication date: 2000-11-02
Also published as: SE9901480D0; AU4634500A; SE9901480L

Abstract

The invention concerns a detecting device for detection of introduction and/or removal of metal objects into/from a sensing area surrounding the detector, said detector comprising a first loop and a drive circuit to pass a current through said first loop. The device in accordance with the invention is characterised in that it further comprises a second loop arranged in such a manner relative to the first loop that, in a normal state without the presence of metal objects, substantially no current is generated in the second loop from the magnetic field of the first loop but that in a state of interference, in which a metal object is entered into the sensing area, a current is generated in the second loop, and in that the device further comprises a detecting unit for detecting modifications arising in the current in the second loop.

Description

METAL DETECTOR

Technical Field

The present invention relates to a detecting device arranged to detect introduction and/or removal of metal ob ects into/from a sensing area around the detector.

Background

Metal detectors are used m various circumstances for the purpose of detecting instances of introduction or removal of metal objects to and from, respectively, a specific sensing area. For example, it is advantageous to use metal detectors m connection with collection of soiled linen or similar washing articles m e.g. health-care and nursing establishments. Such sensing reduces the risk that personal belongings get mixed with the laundry articles and thus are lost to the owner. Likewise, the risk is diminished that pencils, syringes and similar sharp objects accompany the cloths and subsequently injure the personnel responsible for handling the laundry, or damage or alternatively discolour the laundry.

Several types of metal detectors are already known, and for instance in US 5 576 621 is described a metal detector which is located at the mouth of a laundry basket. These prior-art detectors make use of a loop through which current is conducted. A magnetic field is thus generated, and metal objects entered into this magnetic field affect the current m the loop. The resulting modification of the current can then be sensed as minor variations of the current intensity, and the object can be detected. A drawback found m these prior-art detectors is, however, that they exhibit low sensitivity, since small variations are generated from an initially comparatively strong signal. Consequently, it is not possible, by means of these prior-art solutions, to detect such comparatively small-size objects as syringes, ball- point pens, and so on. In addition, these prior-art detectors are highly sensitive to metal objects present m the immediate environment, which makes their location highly delicate. As a result, these detectors must be integrated in a specially designed laundry basket or similar receptacle, and be configured m adaptation thereto, in order to be able to function reasonably satisfactorily.

Obηect of the Invention In view of the above it is one object of the present invention to provide a metal detector that completely or at least partly obviates the problems found m prior-art solutions.

This object is achieved by means of a device as defined m the appended claims.

Summary of the Invention

The invention concerns a detecting device for detection of introduction and/or removal of metal objects into/from a sensing area surrounding the detector, said detector comprising a first loop and a drive circuit to drive a current through said first loop. The device m accordance with the invention is characterised m that it further comprises a second loop arranged m such a manner relative to the first loop that, m a normal state without the presence of metal objects, substantially no current is generated m the second loop from the magnetic field of the first loop but that m a state of interference, m which a metal object is entered into the sensing area a current is generated m the second loop, and m that the device further comprises a detecting unit for detecting such modifications arising m the current m the second loop that depend on modifications m the magnetic flow. The use of a separate sensing loop m this manner radically intensifies the sensitivity compared to prior-art structures, because m the normal state the sensing- loop current is zero, or at least very low. In addition, by locating the second loop m a suitable position, it becomes possible to achieve a high degree of sensitivity by means of two coils only.

Preferably, the detecting device m accordance with the invention is adapted for use m detecting the presence of metal objects among textile materials, and preferably together with laundry baskets. Many other areas of application are, however, also possible, such as for example m laundries using conveyor belts or m door passageways for checking the presence of metal objects on the body of passing persons. The detecting device further preferably comprises a balancing circuit to allow the detecting circuit to be balanced m accordance with environmental conditions. In th s manner the device can be adapted to different surroundings and environments m a convenient manner.

Brief Description of the Drawings In the attached drawings :

Fig 1 is a schematical representation of a detector m accordance with the invention;

Fig 2 is a schematic block diagram relative to the detector m accordance with a preferred embodiment of the invention;

Fig 3 is a more detailed circuitry diagram relative to the detector of Fig 2; and

Fig 4 is a view of a wall-mounted detector m accordance with the invention to be used together with laundry baskets.

Description of Preferred Embodiments

The invention concerns a detecting device as illustrated in F g 1 for detecting entrances and/or removals of metal objects into/from a sensing area around the detector, said detector comprising a first loop 11, a drive circuit 12 to pass a current through this first loop, a second loop 13 and a detecting unit 12 for sensing modifications in the current m the second loop. The second loop 13 is so arranged relative the first loop 11 that m a normal state, in which no interfering metal objects are present, substantially no current is generated in the second loop from the magnetic field of the first loop but wherein, in a state of interference, when a metal object is entered into a sensing area, i.e. moves relative to the detecting device, a current is generated in the second loop. In this manner the detector becomes highly sensitive, since in the normal state no current flows in the second loop, and the difference from the state of interference thus becomes considerable, although also in this case the intensity of the current is very low. The coils could consist for instance of enamel -insulated copper wire and be enclosed inside a protective rubber strip.

Preferably, the second loop 13 has an extension in a plane that is essentially perpendicular to that of the first loop 11, and is located essentially symmetrical relative to that plane. It is particularly advantageous if the loops 11 and 13 are mounted in such a manner that the planes of the loops form a right angle α, preferably exactly 90°, an angle at which the sensitivity of the device reaches a maximum. However, it is a considerable advantage to be able to adjust the angle between the loops, since differences in the characteristics of the coils 11 and 13 may arise in the manufacture, and consequently some adjustment may be needed to maximise the sensitivity. A preferred way of arranging the loops is to mount the first loop 11 horizontally and to configure it with a bevelled edge on one of its short sides, as shown in Fig 4. This short edge is attached, preferably pivotally, to a wall. Thanks to the provision of the bevelled edge the coil may be positioned at an angle of a few degrees . The opposite end of the horizontal coil preferably is suspended or supported by lengthwise adjustable stays 30, for instance by means of a stretching screw or the like. A stay 30 could likewise consist of a threaded rod, which is attached to the wall or the second, vertically mounted coil 13. The rod then runs through an aperture m the horizontally mounted coil . A nut or similar means formed with corresponding threads is mounted on the rod, an arrangement that allows setting of the relative angle between the horizontal coil and the vertically extending coil.

Preferably, indicating means are arranged to show when the adjusted angle α is optimal, i.e. the angle that provides the keenest sensitivity. The indicating means could be connected to the detecting unit 12 or be made integrally with that unit. Preferably, the indicating means is also arranged to assist the user in determining m which direction the angle α is to be adjusted. For example, the indicating means could comprise one or several lights or LEDs .

It is advantageous that the stays 30 allow adjustment of and lends support to the horizontal coil. In addition, the stays also afford some protection against external influence. Adjustment of the angle could also be effected by means of resilient deformed portions of the coil mounting means, an arrangement that provides bias and eliminates play.

However, it is possible to locate also the second loop 13 m essentially the same plane as the first loop 11 but somewhat offset relative thereto. This mounting alternative is suitable for example in the case of metal searches externally of the coils, for example m the case of below-ground treasure hunting, since in this case the scanned area is larger and is directed away from the coils.

When the coils are thus located it is, however, suitable to allow the coils to overlap only slightly, overlap, for example by less than 10%. Fig 2 schematically illustrates a block diagram concerning a preferred configuration of the detecting unit m accordance with the present invention, and Fig 3 is a more detailed circuit diagram relative to the same circuit . The oscillator component 21 comprises two operational amplifiers, 0P1 and OP2. The frequency is determined by R5-R6 and C2-C3. Other frequency- determining components could, however, be used. The output voltage, which is a square wave, is coupled via C4 to a power stage 22. The power stage 22 supplies a square-wave voltage to the primary coil, that is, to the first loop 11. It could for instance comprise two MOSFET transistors Tl, T2 , diodes Dl, D2 , resistances R2 , R3 , and a capacitor Cl . The signal of oscillator 21 is supplied to the gate of T2. R3 provides the input signal -15V as a reference. When T2 is conductive, Tl is blocked because the gate of Tl goes to -15V. When T2 is blocked, Tl is conductive, because its gate is drown towards +15V by R2 and Cl . D2 blocks towards the output. The primary coil 11 is connected to the output, thus the dram of Tl . Preferably, the primary coil is AC-driven although DC could be used instead. The other loop 13, the secondary coil, is mounted to ensure that the magnetic flow of the primary coil 11 does not pass through it. Only when the magnetic field is interfered with owing to the presence of a metal object, causing asymmetry, does the secondary coil 13 receive a magnetic field from the primary coil 11. The then induced signal voltage is connected to a sensitive filter-equipped signal amplifier 25. This block consists of the operational amplifiers 0P5 and 0P6. The amplifier is a conventional amplifier connection comprising

C6, C8, which filters away low frequencies, and C7,

C9 as high-frequency filters. The amplification within the pass band is determined by R10/R9 x R12/R11.

The signal is now amplified to a level, where rectification may be effected for further DC amplification. It is always desirable to first amplify the AC voltage as much as possible m order to reduce errors originating from DC offset errors in the various stages.

The phase-sensitive rectifiers 23, 24 preferably are of identical design but are supplied with different reference phases. Magnetically conductive materials generate a phase difference of 90 degrees compared with electrically conductive objects. The 90°-phase rectifier 23 consists of an operational rectifier, 0P7 , and a transistor, T3. Resistances R15 and R16 should be of equal magnitude. Resistance R13 should be considerable relative to the forward resistance of T4. T3 is controlled by the square wave m phase with the signal. Consequently, operational amplifier OP7 has -lx amplification when T3 is conductive and -1+2 =+lx when T3 is not. The 0-phase rectifier 24 consists of the operational rectifier OP8 and the transistor T4 , and functions m an identical manner. The output signals from the two rectifiers are connected to the respective one of amplifiers

26, equipped with low-pass filters, OP9 and OP10, by the capacitors C9-C10 and C12-C13. The amplification may be set by means of potentiometers

PI and P2. The capacitors C11-C14 form a low-pass filter. The diodes D5-D6 and D7-D8 accelerate the zero setting of the detector when the signals are strong . The signal is then forwarded to an alarm, such as an alarm circuit 2 and an audible alarm 28. The signals from the operational amplifiers OP9 and OP10 are converted into absolute values in D9-D13 and OP11. A frequency converter consisting of potentiometer P3 and resistances R26, R32 determines the conversion level. The audible alarm is in this case a connection to a frequency converter consisting of OP4 with terminals T5 , T6 for a small loudspeaker. It functions only for + signals, and the frequency is proportional to the input signal. An alarm comparator, OP3 , sounds an alarm that may be reset by a switch SI. The diode D16 indicates the alarm until it is reset by SI. A zero balance indicator measures the output control of the signal amplifier. Zero balancing preferably occurs mechanically on the coils themselves.

Obviously, the detection circuit may be designed in other ways, and it is for instance possible to use CMOS circuits instead of operational amplifiers. However, several advantages exist in using operational amplifiers, for instance because they are simple, inexpensive, precise and not particularly sensitive to interference. It is likewise possible to use other types of filtering means for the purpose of making the device less sensitive to error detection, the balancing circuit may be designed differently, and so on. The circuit cards of the electronic equipment preferably are mounted m a protective box.

The device is particularly useful m handling textiles, such as garments and the like. The device could for instance be positioned on laundry baskets or the like, and either be stationarily mounted on the basket m the manufacture thereof, or as an accessory on already existing laundry baskets. The detector is then located so as to allow the objects to be passed down through the first loop. Any metal objects that may remain among the clothes, such as pens, syringes or the like, may be easily detected, and thus the risk that objects left among the cloths cause injury to the laundry-handling personnel is eliminated, as is also the risk of damages to the textiles themselves or to the washing machines used for the treatment of the textiles, and so on. It should be understood that the device could also be used in other stages of the laundry-handling process, or be used in other context, where it is desirable to detect the entrance of or removal of metal objects, for example when a conveyor belt is made to pass through the coil, or m a door-frame.

The loops 11 and 13 may be arranged m a number of advantageous ways. One particularly advantageous way is, however, to mount the second loop 13 vertically relative to, or preferably on, a wall and the first loop on a holder 32 supporting the laundry bag 33 proper, said holder 32 in this case being mounted so as to project at right angles from the wall . This arrangement is illustrated in Fig 4.

The first loop 11 then preferably is mounted in such a manner as to extend around the mouth of the laundry-bag holder 32. According to another variety, both loops 11 and 13 are mounted on the laundry-bag holder.

Advantageously, a thus designed laundry bag holder 32 or laundry bag container could be equipped with raised, and preferably outwardly tapering rims 31 surrounding the mouth, said raised edge serving as a funnel . In addition to serving as a feed-in aid, the raised rim 31 also serves to prevent watches, rings and similar objects worn by the operator from interfering with the detection of metal objects present among the garments being passed through the sensing loop as the objects enter the sensing area. Preferably, the funnel may be secured separately on the wall in order to counter-act transfer of mechanical loads and impacts to the loops 11 and 13, during the feed-in of laundry. If such a raised rim 31 is used, the second loop 13 advantageously is applied adjacent the vertical walls of the raised rim.

The loops 11 and 13 could have a rectangular as well as a circular shape in order to suit different types of application.

Preferably, the detecting device is battery- operated, but could also be connected to the mains supply. The battery-operated alternative makes the device portable and easy to install in optional locations. A check light is advantageously used to indicate the occurrence of a too low voltage battery state. It is also preferable that the device be connected to a switch, allowing it to be switched off when not in use, and in this way save energy. This is particularly important when the battery-operated alternative is used. Preferably, the switch is arranged to allow the device to be switched on automatically, when detection is required. For example, a micro-switch may be located on the lid of a laundry basket, so that the device is activated, when the lid is opened to enter garments or the like.

Preferably, the detecting device comprises two coils only, but it could also comprise one or several additional sensing loops, arranged in a manner corresponding to that of the second loop. In this way, sensing signals from the second loop as well as from the additional sensing loops may be made use of, whereby the detecting process becomes safer while at the same time the sensitivity is increased.

In order to balance off the effects of stationary large-size metal objects present in the vicinity of the detector, a balancing circuit preferably is needed. A circuit of this kind may comprise a potentiometer to set the amplitude (P2) of the sensing signal and a potentiometer to set the phase position (P3) of the signal. Preferably, it is coupled direct to the signal amplifier. The detector is preferably set on the basis of the results from a test setting carried out in connection with the manufacture, whereupon fine- tuning is effected at the location of use by means of the potentiometer that balances the coils. When the coils are in balance, this fact preferably is indicated to the user, for instance by means of the activation of a green steady light, which indicates that the detector is ready for use. Naturally, it is possible to use a balancing circuit that automatically balances the coils on the basis of certain predetermined criteria.

The detecting device has been described herein with the aid of a preferred embodiment. Many varieties are, however, possible, such as the use of the detector in other context, for example as a detector in doors or similar passageways, or as a ground detector, arrangement of the coils in a different manner, design of the detection electronics differently, and so on. Such and other obvious varieties of the invention must be regarded to be within the scope of protection of the invention as the latter is defined in the appended claims.

Claims

1. A detecting device for detecting the presence of metal objects among textiles, and preferably m connection with laundry baskets, said detector comprising a first loop (11), a drive circuit (12) to pass a current through said first loop (11), and a second loop (13) so arranged relative to the first loop (11) that m a normal state, m which no interfering metal objects are present, substantially no current is generated in the second loop (13) from the magnetic field of the first loop (11) , c h a r a c t e r i s e d in that the first loop is arranged for passage-through of textiles, and in that in a state of interference, in which a magnetically or electrically conductive object, such as a metal object, has been passed through the first loop (11) , a current is generated m the second loop (13) , and m that the device further comprises a detecting unit (12) for detecting modifications in the current m the second loop (13) .

2. A detecting device as claimed in claim 1, wherein the second loop (13) extends in a plane, which is essentially perpendicular to the plane of the first loop (11), and the second loop (13) further is arranged essentially symmetrically relative to said plane.

3. A detecting device as claimed in claim 2, wherein the angle between the planes formed by said first and said second loops (11 and 13, respectively) is 90°.

4. A detecting device as claimed in claim 1, wherein the second loop (13) extends in essentially the same plane as the first loop (11) but is offset relative to said first loop.

5. A detecting device as claimed in any one of claims 1-3, wherein said detecting device comprises at least one operational amplifier (OP1-9) to reinforce the sensing signal, and filtering means (25, 26) to make the device less sensitive to faulty detection.

6. A detecting device as claimed in any one of the preceding claims, wherein the drive circuit (21, 22) is adapted to operate said first loop with alternating current.

7. A detecting device as claimed in any one of the preceding claims, wherein said device is battery-operated .

8. A detecting device as claimed in any one of the preceding claims, wherein it further comprises a switch preferably arranged to cause automatic switch-on and switch-off of the device.

9. A detecting device as claimed in any one of the preceding claims, wherein it further comprises alarm means (27, 28) , such as a LED diode or a buzzer, arranged to sound an alarm, when the detecting device detects a change.

10. A detecting device as claimed in any one of the preceding claims, wherein said detecting unit further comprises a balancing circuit to allow balancing of the detecting circuit on the basis of environmental conditions.

11. A detecting device as claimed in any one of the preceding claims, wherein it further comprises at least one additional sensing loop arranged in a manner corresponding to that of the second loop, and wherein said detecting device is adapted to detect modifications on the basis of sensing signals issued by the second loop as well as by the additional sensing loop.