WO2012100804A2

WO2012100804A2 - Metal detector

Info

Publication number: WO2012100804A2
Application number: PCT/EP2011/006496
Authority: WO
Inventors: Erich Bettwieser; Lothar Golla; Elmar OBERDÖRFFER
Original assignee: Sartorius Weighing Technology Gmbh
Priority date: 2011-01-25
Filing date: 2011-12-22
Publication date: 2012-08-02
Also published as: WO2012100804A3; DE102011000303A1

Abstract

The invention relates to a metal detector, comprising a transmitter coil unit (1) provided for generating a primary field, two series-connected receiver coil units (3, 4) which are switched in opposite directions to one another, and an evaluation unit connected to the receiver coil units (3, 4) for processing a sum induction voltage generated in the series connection of the receiver coil units (3, 4), wherein the receiver coil units (3, 4) are disposed and configured in such a way that the sum induction voltage is zero with an undisturbed primary field. The metal detector is characterised in that the two receiver coil units (3, 4) are at different distances from the transmitter coil unit (1). Due to this asymmetry the closed surface of the metal detector is pushed out of the plane of the transmitter coil unit (1) and thus the area of maximum field intensity is utilised in order to receive a detectable stray field.

Description

metal detector

The invention relates to a metal detector comprising a transmitter coil unit provided for generating a primary field, two receiver coils connected in series and in opposite directions and an evaluation unit connected in parallel to the receiver coils for processing a sum induction voltage generated in the series connection of the receiver coils, the receiver coils being arranged and configured in such a way. that the sum induction voltage is zero when the primary field is undisturbed.

A metal detector of the type mentioned is for example from the

DE-OS 148 353, there in particular from Figures 2, 5 and 6 and associated description known. According to this prior art, the receiver coils are connected in series in opposite directions and arranged symmetrically on both sides and concentric with the transmitting coil. Ideally, the induced in the receiver coils of the primary alternating field of the transmitting coil compensate each other

Voltages so that the measured across both receiver coils

Sum induction voltage is zero, as far as the induced voltage alone comes from the primary field, ie the alternating field of the transmitting coil, which represents the undisturbed case. Under a disturbed primary field is the case understood that in

Detection area of at least one of the receiver coils an electromagnetically interacting foreign body is present, which generates a primary field superimposed stray field. In asymmetrically disturbed primary field, those induced in the receiver coils according to the prior art differ

Single-induction voltages, and the sum-induction voltage is measurably different from zero. One determined by the sum induction voltage

Receiver output signal is detected by the evaluation unit, whereby the foreign body can be detected. With a concentric arrangement of transmitting coil and receiver coil, a tunnel-like structure is realized, so that a product stream to be examined or individual products can pass through the coils. At tunnel-like

Embodiments may use the coils e.g. be round or square. Of the

Product stream or product may e.g. be guided by a conveyor belt. There are constant speeds or a positive or negative accelerated movement of the product (electricity) possible. Vertical arrangements are also known in which the product or product stream falls freely through the coils.

However, the principle of countercurrent of the two receiver coils can in

be applied accordingly in non-tunnel-like versions, for example, in flat embodiments in which the coil surfaces in

Are arranged substantially parallel to each other. Also, arrangements in which the coil surfaces of the receiver coils have a clearly different from 0 ° or 90 ° angle, are conceivable with the counter circuit described.

A problem with the known embodiments of the counter-circuit according to the prior art is the fact that an influence of the alternating field of the transmitting coil by a particle to be detected can only be determined if the interaction of the primary alternating field of the transmitting coil is unbalanced disturbed. If the interfering body is in a tunnel-like structure in the plane of the transmitting coil surface, the interfering field generated by the interfering body as a result of the interaction with the primary field generally propagates completely symmetrically to this plane. The thereby induced in the respective receiver coils

Single induction voltages are equal in magnitude, so that the

Total induction voltage is again zero. The plane of the transmitter coil surface thus forms a blind area in the measuring arrangement. However, there is the magnetic field of the primary alternating field maximum and generated in the

electrodynamic interaction and the interfering body the maximum stray field. Due to the interaction with the magnetic primary alternating field, a magnetic dipole moment is generated in a conductive sphere as an exemplary disruptive body, which is also maximal at maximum primary field, ie within the blind area. By the opposite circuit of the two receiver coils and their Symmetrical arrangement to the transmitter, the maximum dipole moment can not be detected when running the sample.

An analogous situation is given for a non-conducting sample, e.g.

ferromagnetic is (for example, ferrite). The field enhancement via the relative magnetic permeability is maximum under the transmitter, but this can not be detected by a balanced receiver. Also for the action of the alternating electric field on a non-conductive, non-magnetic sample is symmetrical receiver / transmitter structure, the blind area at the site of the transmitter. The electric field generates a maximum displacement current below the transmitter, the magnetic field of which does not induce any sum voltage with receiver coils arranged symmetrically with respect to the transmitting coil.

The invention is based on the technical problem to provide a metal detector of the type mentioned, in which the maximum

Field strength of the transmitting coil unit is usable and thereby a higher

Sensitivity of the metal detector is achieved.

The object is achieved in a metal detector of the type mentioned by the features of claim 1. Advantageous embodiments of the metal detector result from the dependent claims.

The receiver coils are different than in the prior art with different distance to the transmitting coil unit and thus asymmetrically relative to

Transmitter coil unit arranged. With this measure, a given for interference fields in opposite series connection of the receiver coils blind surface of the receiver coils can be moved out of the plane of the transmitting coil unit, resulting in a use of the highest field strength of the located in this plane

Primary field allows. It is exploited that a generated by a foreign body noise field has a steeper spatial decline, as the primary field of the transmitting coil unit. This circumstance is explained quantitatively plausibly in the following with reference to a non-magnetic, electrically conductive ball as a foreign body. The secondary magnetic field Ηκ generated by the transmitting coil on the foreign body can be described by a dipole. One can therefore look at the foreign body in its effect as a small single-coil from the radius of the sphere in which a certain current Ικ flows. For the secondary magnetic field H _K in the symmetry axis, the following proportionality relationship applies

(1) ΗΚ - ΙΚ ^ / ^ + Γ ² ) ³

Where r is the radius of the foreign body and z is a coordinate on the

Center longitudinal axis of the search channel, wherein the foreign body is positioned as a secondary transmitter at z = 0. An analogous statement also applies to the primary transmitter, but then r is to be replaced by the radius of the transmitting coil R. In a typical

Distance from the primary transmitter of z «R follows with r« R from equation (1) for the magnetic field approximately:

(2) HK - IK ^ Z ³ ,

The primary field H, however, is still in the near range in an analog range and it is approximately:

(3) H ~ l / R {1 - 3/2 (z / R) ² + ...}.

This shows that for r «R the secondary magnetic field of the foreign body drops much faster with increasing z than the primary magnetic field.

Thus, if the receiver coils have different distances a and b, with a> b, to the transmit coil and thus also to the foreign body positioned at z = 0, the receiver located at a distance b closer to the primary transmitter will receive a much stronger secondary magnetic field than the one with the distance a remote receiver to the primary transmitter. The secondary magnetic field of the

Foreign body induced in the receiver unit sum voltage V _s , so the sum of the individual voltages of both receiver coils, at the central ball position z = 0, the foreign body is thus at the location of the transmitter, then with (4) V _s (Z = 0) ~ IK a ² / b ³ (1 - (b / a) ³ ). be estimated.

For b «a, the receiving unit is highly asymmetrical and the sum signal correspondingly strong. In the symmetric case (a = b), however, Vs (0) = 0.

In the undisturbed case, the sum induction voltage of the two must

Receiver coils are zero. In order to satisfy this condition, it is necessary to make the two receiver coils different in order to compensate for the different distances to the transmitting coil. The decisive factor is that the magnetic flux changes generated by the primary field in the two receiver coils and thus the induced voltages are the same. For this the two can

Receiver coils have different numbers of turns. In addition, it is possible to provide the receiver coil at z = a (with a> b) with a larger radius than the other. These two measures can easily be combined. In the language of the above quantitative approximation of equation (4), this means that the second term is to be multiplied by a compensation factor k> 1:

(4 ') V ~ IK a ² / b ³ (1 - (kb / a) ³ ), because the primary field H is weaker at the location of the more distant receiving turn (z = a) than at the place of the near receive turn (z = b). This is precisely what has to be compensated for by additional turns at z = a or / and by a larger receiving surface. This is described by k> 1. From equation (3) k can be estimated. The more asymmetrical the structure, the larger the factor k becomes.

More precise fine adjustments to compensate for different magnetic fluxes are possible by means known from the prior art measures. Thus, a fine adjustment of the compensation by a meander-shaped or S-shaped conductor structure of one of the base coils can take place, for example, by simple Bending can be adjusted. Alternatively, an electrical compensation can be provided, in which the affected base coil is connected to an RC element. The latter technique is particularly suitable for base coils printed by

Circuits are realized and do not allow bending of the conductor structure.

Usually, the transmitting coil unit consists of a coil. However, it is also conceivable to provide several windings for the transmitting coil unit.

Different distances of the receiver coils to the transmitting coil in the sense of

Invention are already given when the distances are more than the usual

Distinguish manufacturing tolerance from each other. The distances a and b of the receiver coils advantageously have a ratio of a / b 1, 1. The measurement results can be optimized even with larger distance ratios, so that advantageous

Distance ratios can also be at least 1, 2 or at least 1, 3 or including the further incremented in tenth steps intermediate values at least 2.0.

The distance b may also be zero, i. one of the receiver coils lies exactly at the location of the transmitter coil. Thus, the ratio a / b becomes infinite, which is also covered by the invention. In this case, the second receiver coil can move very close to the transmitting coil.

The following are exemplary advantageous embodiments of the

Metal detector according to the invention illustrated by figures.

It shows:

Fig. 1: A first coil arrangement in the lateral cross section and

2 shows a diagram of the sum induction voltage over time.

Figure 1 shows in cross-section schematically a transmitting coil 1 on a support body 4, on which also still a first receiver coil 2 and a second receiver coil 3 are arranged. The first and second receiver coils 2 and 3 are in series connected in opposite directions. The first receiver coil 2 has a distance b with respect to the transmitting coil 1, which is substantially smaller than the distance a of the second receiver coil 3 to the transmitting coil 1. The two receiver coils 2 and 3 have to satisfy the condition that their summation induction voltage at a undisturbed, generated by the transmitting coil 1 primary field is substantially zero. Since the two receiver coils 2 and 3 have the same coil radius R _s , the higher distance a is due to a higher number of turns, here two, the second one

Receiver coil 3 compensated. The distance a is in the present case thus to be chosen so that despite the double number of turns compared to the first

Receiver coil 2 in both receiver coils 2 and 3 through the primary field in

Substantially the same magnetic flux changes are generated.

Now occurs in the bounded by the support body 4 control room 5 z. B. by means of a conveyor belt in the direction of the coil longitudinal axis z a not shown here

metallic interfering body, a time-varying dipole and thus an interference field would be generated in the body by means of the primary field. This interference field in turn generates interference field induction voltages in the two receiver coils 2 and 3.

On its way through the control room 5, the disturbing body will pass through a blind area in which the magnetic flux changes generated in the receiver coils 2 and 3 by the disturbing field are equal in magnitude, whereby by the opposite series connection of the two receiver coils 2 and 3 the sum signal of the disturbance field induced disturbance field induction voltages is zero. However, this blind area is located clearly outside the center plane of the transmitting coil 1. The center plane of the transmitting coil 1 and the blind area fall apart because the interference field generated by the disturbing body with the distance to the disturbing body drops much faster than the primary field.

As a result of the arrangement presented in FIG. 1, the region of the so-called metal-free zone (MFZ) likewise becomes asymmetrical with respect to the transmitting coil 1. The MFZ defines a zone which is to be kept free of metal in order to avoid disturbances of the metal detector. On the right side of the transmitter coil, on the side with the receiver coil 3 further out, the extent of the MFZ is greater than on the left side. The left side is thus less sensitive to bluff bodies outside the search coil. There may be technical applications in which such M FZ asymmetry is desired. Often, however, the asymmetry of the MFZ will be undesirable. Then, the metal detector is to be constructed so that the receiver coils 2 and 3 have equal distances to the edge of the control room 5. The transmitting coil 1 is then positioned asymmetrically, ie outside the center of the control room 5. In Fig. 1 would then be the entirety of the transmitting coil 1 and receiver coils 2 and 3 relative to the housing 4 to move (ab) / 2 to the left. As a result, although the primary field is relative to the control room. 5

asymmetric, but this has more neutral effects on the MFZ than one

Asymmetry of the receiver. The optimal positioning of the whole

Receiver and transmitter can be determined empirically or by numerical experiments.

In Fig. 2, for example, the course of the summation induction voltage is plotted for the case that a bluff body at a uniform speed through the

Control room 5 is performed. At the time t = 0, the interfering body passes through the center plane of the transmitting coil 1. The decisive factor for the device according to the invention is the more developed curve 6. It starts on the left side at negative times with the entry of the obstruction into the control room 5. The summation induction voltage drops to a minimum in the immediate vicinity of the center plane of the transmitting coil 1 and passes before exiting the control room a maximum. The blind area in this example would happen at about + 0.45.

For comparison, a second curve 7 is listed, the course of the

Sum voltage in a device according to the prior art identifies, in which the receiver coils, each with one turn the same distance from

Have transmission coil. In this case, the blind area is exactly in the center plane of the transmitting coil, and the maximum amount of the sum voltage is less than 0.5 (scaled units). In comparison, the maximum amount of

Sum induction voltage with asymmetrical arrangement slightly above 0.6 (scaled units). Higher, resulting in a higher sensitivity of the

Measuring arrangement results. _Q

transmitting coil

first receiver coil

second receiver coil

support body

control room

Curve

Claims

claims

A metal detector comprising

a) provided for generating a primary field transmitting coil unit (1), b) two in series and in opposite directions to each other

Receiver coil units (3,4), as well

c) an evaluation unit connected to the receiver coil units (3, 4) for processing a sum induction voltage generated in the series connection of the receiver coil units (3, 4),

d) wherein the receiver coil units (3, 4) are arranged and configured in such a way that the sum induction voltage is zero when the primary field is undisturbed,

characterized in that

e) the two receiver coil units (3, 4) different distances to

Transmitter coil unit (1).

2. Metal detector according to claim 1, characterized in that the distance a of the first receiver coil unit (3) to the transmitting coil unit (1) and the distance b of the second receiver coil unit (2) to the transmitting coil unit (1) the ratio a / b> 1, 1, preferably a / b 1, 2, more preferably 1, 5, more preferably a / b> 1, 7, further preferably a / b have 2.0.

3. Metal detector according to claim 1 or 2, characterized in that one of the receiver coils is arranged at the location of the transmitting coil unit.

4. Metal detector according to one of claims 1 to 3, characterized

characterized in that the receiver coil units (3, 4) are different

Have coil surfaces.

5. Metal detector according to one of claims 1 to 4, characterized

characterized in that the receiver coil units (3, 4) are different

Windungszahlen have.