RU2262123C1 - Induction measuring converter for metal detector - Google Patents

Abstract

FIELD: means for detecting concealed objects.

SUBSTANCE: device has agitation lid with ferromagnetic core and at least a couple of oppositely serially coaxial receiving coils. Receiving coils are mounted coaxially on agitation coil with possible moving and holding on it. Length of ferromagnetic core exceeds diameter of it. Diameter of each receiving coil is greater than its length. Length of ferromagnetic core is greater than agitation coil diameter more than 10 times. Receiving coils are held on agitation coil in points of maximal converter sensitivity.

EFFECT: higher sensitivity.

4 cl, 6 dwg

Description

The invention relates to means for detecting hidden objects from diamagnetic, paramagnetic and ferromagnetic materials and directly relates to induction measuring transducers for metal detectors used in magnetometry, geological exploration, archeology, during the inspection of people carrying hidden metal objects through the control zone, tracing hidden communications.

Known induction measuring transducer for a metal detector, including an excitation coil, a pair of included counter-sequentially coaxial receiving coils symmetrically located with their central axes in a plane perpendicular to the central axis of the excitation coil (see the decision to grant a patent according to the application of the USSR, No. 4796455 of March 01 1990), and the receiving coils along their entire length are located within the internal circuit of the field coil.

Such an induction converter has increased noise immunity, however, as experimental studies have shown, its output signal changes sharply within the excitation coil contour depending on the position of the desired object relative to its central axis, namely, it is inverted (changes sign) already at small distances from this axis.

As a result of this, in practice, when the search usually does not know the position of the desired object relative to the center of the measuring transducer, the identification of the material of the desired object (for example, ferrous or non-ferrous metal) is much more difficult and only becomes definite in the central zone with a radius of about 1/3 ... 1/2 of the radius or distance from the central axis to the circuit of the field coil. The effective detection zone in this case is only 0.1-0.2 of the area covered by the circuit of the excitation coil. Given the real threshold of sensitivity, due to the residual level of interference and the signal-to-noise ratio, the effective detection zone is further narrowed.

Closest to the claimed one is an induction converter containing an excitation coil with a ferromagnetic core and at least a pair of counter-coaxial receiving coils included, also located coaxially with the central axis of the excitation coil. The receiving coils are mounted on the ends of a thin stainless steel tube installed in the core of the field coil coaxially to the last one and are separated from the field coil by a distance exceeding its length (Medical Technology Magazine, No. 1, 1992, pp. 42-43 )

The induction converter works with a metal detector of a known type (RF Patent No. 1827197), including an alternating current generator, the first output of which is connected to the excitation coil. The output of two receiving coils, included gradiometrically, is connected through a series-connected AC voltage amplifier, a synchronous detector and a DC amplifier to a dial indicator. The second output of the alternator is connected to the input of the phase shifter, the output of which is connected to the control input of the synchronous detector. The metal detector also includes a switch, connected in parallel by the first and second contacts to the phase shifter adjustment element, previously divided into two series-connected parts, and the connection of these parts of the adjustment element is connected to the third contact of the switch.

However, such an induction measuring transducer has insufficient sensitivity.

The known induction transducer corresponds to a graph of changes in the magnetic field under the influence of the object (figure 1), measured by this transducer at the locations of the receiving coils.

The ferromagnetic core is separated from the receiving coils at a distance determined by the length of the thin tube, and is used in the known construction only to increase the field of the field coil.

поле рассеяния от которого по оси индукционного измерительного преобразователя изменяется по законуThe object is magnetized by an excitation coil, as a result of which an induced magnetic moment arises in it.

the scattering field from which along the axis of the induction measuring transducer changes according to the law

where r is the distance from the object (V.V. Nikolsky. Theory of electromagnetic field. M., Higher school, 1964).

As can be seen from the graph (Fig. 1), the useful signal of this induction converter is determined from the relation

E = a (H ₁ -H ₂ ),

where a is the conversion coefficient for a given induction measuring transducer.

The indicated location of the receiving coils relative to the ferromagnetic core does not allow using it to increase the useful signal, providing an increase in sensitivity.

The direct supply of receiving coils with ferromagnetic cores when exposed to a constant inhomogeneous magnetic field will lead to an unequal change in their initial magnetic permeability, that is, to an imbalance of these coils (false signal), which will require constant compensation.

Based on the foregoing, the objective of the invention is to increase the sensitivity of the induction measuring transducer to the measurement of the magnetic field gradients of the detection object while reducing its size.

The problem is solved in that in an induction measuring transducer for a metal detector containing an excitation coil with a ferromagnetic core and at least a pair of gradient-coaxial receiving coils included, the receiving coils are mounted coaxially on the excitation coil, with the possibility of movement and fixing on it, and the length the ferromagnetic core exceeds the diameter of the field coil.

Wherein:

- the diameter of each receiving coil exceeds its length;

- the length of the ferromagnetic core exceeds the diameter of the excitation coil by more than 10 times;

- receiving coils are fixed on the excitation coil at the points of maximum sensitivity of the converter.

The location of the receiving coils on the excitation coil with the possibility of moving along it allows you to pre-determine the maximum sensitivity points for a given induction converter with subsequent fixation of the receiving coils at these points.

Since the ferromagnetic core is located inside the excitation coil, it is magnetically remagnetized, which eliminates the possibility of an unbalance of the receiving coils, and when the latter are located in places of maximum sensitivity, the signal can be increased by at least 10 times.

The implementation of the core with a length exceeding the diameter of the excitation coil by more than 10 times, also allows you to strengthen the magnetic field, which increases the accuracy of detection.

As can be seen from the graphs (figure 2 and 3), in the case of the location of the receiving coils on the excitation coil with a ferromagnetic core at the points of maximum sensitivity, which for this induction converter can be calculated by the method (V.V.Dyakin, V.A. Sadovsky “Defectoscopy”, No. 4, 1999, p. 47-55) or determined experimentally (moreover, practical results exceed the calculated ones due to the nonlinear properties of the ferromagnetic material of the core), the nature of the dependence changes sharply throughout the core and in comparison with the useful signal will be equal to

Calculations and experiments show that the useful signal E 'is at least an order of magnitude higher than the useful signal E of the known measuring induction transducer.

Figure 1 presents a graph of the magnetic field from the object for a known induction measuring transducer;

figure 2 is the same for the inventive induction measuring transducer;

figure 3 presents a fragment of the graph of figure 2, corresponding to the length of the ferromagnetic core;

figure 4 is a fragment illustrating a change in the lines of force of the magnetic field of the inventive induction measuring transducer under the influence of the object;

figure 5 presents the inventive induction measuring transducer in section;

figure 6 is a block diagram of a metal detector.

The induction measuring transducer for the metal detector comprises an excitation coil 1 and at least a pair of gradient-coaxially included receiving coils 2 and 3. The receiving coils 2 and 3 are mounted coaxially on the excitation coil 1 with the possibility of movement and fixing on it. The excitation coil 1 is provided with a ferromagnetic core 4, and the excitation coil 1 is wound evenly along its entire length. The length of the ferromagnetic core 4 exceeds the diameter of the excitation coil 1 (Fig. 5).

Moreover, the diameter of each of the receiving coils 2 and 3 exceeds its length.

The length of the ferromagnetic core 4 exceeds the diameter of the excitation coil 1 by more than 10 times. The receiving coils 2 and 3 can be fixed on the excitation coil 1 at the points of maximum sensitivity of the converter, which are determined experimentally.

Graphs of changes in the magnetic field from the object 5 of the search for the inventive induction measuring transducer are shown in figure 2, 3 and 4.

The induction converter according to the invention works with a metal detector of a known type similar to the metal detector of the prototype (Fig.6). The conclusions of the measuring coils, connected in opposite and in series, are connected to the terminals of a variable resistor 6, the movable contact of which through the pre-amplifier 7 is connected to the input of the synchronous detector 8. The output of the synchronous detector 8 through the amplifier 9 is connected to the indicator 10. The first and second outputs of the generator 11 are connected to the conclusions of the excitation coil 1, and the third output through a controlled phase shifter 12 is connected to a controlled input of a synchronous detector 8.

Induction measuring transducer in the metal detector operates as follows.

Preliminarily, the measuring transducer is tuned, for which it is placed in a homogeneous external alternating magnetic field and using a variable resistor 6 a zero signal is set at the output of the indicator 10. Then, a metal object 5 is placed at a predetermined distance from the induction transducer along the axis of the excitation coil 1 and by moving the measuring coils 2 and 3 achieve the maximum signal at the output of the indicator 10. When determining the points of maximum sensitivity in a uniform field, the cycle is repeated. The measuring coils 2 and 3 are fixed at these points L ₁ and L ₂ and thereby configure the maximum sensitivity of the measuring transducer for a given distance.

In the process, the harmonic electrical signal of the generator 11 is converted by the excitation coil 1 into an alternating magnetic field. This field acts on a hidden desired object 5 located at a certain distance within the loop of the excitation coil 1, generating eddy currents in it and the corresponding secondary magnetic field (re-radiation field).

The secondary magnetic field along with the field of the excitation coil 1 induces a corresponding alternating voltage in the measuring coils 2 and 3. Its value, which is the output signal of the induction converter, depends, other things being equal, on the size, shape and material of the desired object 5 and its location, and the phase and corresponding sign depends on the conductivity and magnetic properties of the desired object.

The output signal of the induction measuring transducer through a variable resistor 6 is fed to the input of the pre-amplifier 7 and then to the input of the synchronous detector 9. After synchronous detection, the signal is amplified by a DC amplifier 9 and fed to indicator 10.

Thus, by moving the induction measuring transducer, you can find the location of the hidden object 5 at the maximum of the output signal, since at this moment the search object will be located on the axis of the excitation coil 1. By the magnitude of the output signal, it is possible to roughly judge the size of the hidden object 5. The high sensitivity of the induction converter allows you to detect fairly small search objects 5.

The presence of a controlled phase shifter allows you to work in two modes. In the in-phase detection mode, when the component of the induced magnetic moment in phase with the magnetizing field in a ferromagnetic object changes sign with increasing frequency, but does not change in a non-ferromagnetic object, the material of the search object 5 can be determined. In the quadrature detection mode, when the quadrature component of the magnetic moment always has the same sign for both a ferromagnetic object and a non-ferromagnetic object, it is possible to accurately determine the location of the hidden target.

Claims (4)

1. An induction measuring transducer for a metal detector comprising an excitation coil with a ferromagnetic core and at least a pair of counter-coaxial receiving coils connected in series, characterized in that the receiving coils are mounted coaxially on the excitation coil with the possibility of moving and fixing on it, and the length of the ferromagnetic core exceeds its diameter. 2. The induction measuring transducer according to claim 1, characterized in that the diameter of each receiving coil exceeds its length. 3. The induction measuring transducer according to claim 1 or 2, characterized in that the length of the ferromagnetic core exceeds the diameter of the excitation coil by more than 10 times. 4. The induction measuring transducer according to claim 1 or 2, characterized in that the receiving coils are fixed on the excitation coil at the maximum sensitivity points of the transducer.

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