Detection of metal objects in building structures.

Detection of metal objects in building structures.

Kargashin Viktor Leonidovich Candidate of Technical Sciences
Kolyada Sergey Egorovich
Mitrokhin Sergey Ivanovich

Detection of metal objects in building structures

When searching for hidden metal objects in building structures, microwave flaw detectors are used, for example /3/, which allow analyzing the internal structure of structures and detecting foreign metal inclusions. Effective operation of flaw detectors is possible with high training of specialists, the survey method implements low productivity and, in addition, the equipment itself has a fairly high cost.

The use of devices that allow detecting hidden metal objects is also widely used to search for mines and explosive devices installed in the ground. The sensitivity of modern metal detectors allows detecting metal objects of various sizes at depths of up to a few meters /1, 2/.

The following methods of detecting metal objects in the soil are known and are used in practice in mine clearance:

Magnetometric;
Induction;
Radar;
Mechanical probing;
Electric contact;
Seismoacoustic;
Biophysical, etc.

The first four methods have gained the greatest practical significance due to their greater efficiency and productivity.

It seems relevant to use the known advantages of induction methods for searching for metal objects for inspecting building structures, including reinforced ones. Such a task is necessary when searching for metal components of covert information control devices installed in building structures. In building structures that do not contain metal, for example, brick type, high sensitivity can be achieved, allowing to detect metal objects with dimensions of several square centimeters at a depth of up to 20 … 25 cm, which is quite sufficient for solving inspection problems. In reinforced building structures, for example, reinforced concrete, metal detectors implementing the traditional induction method are subject to a high level of interference caused by the metal elements of the structures themselves. But even in building structures that do not contain reinforcement, there may be false interference responses caused by foreign metal objects, for example, wires embedded in the thickness of the structure, invisible fastening elements (nails, screws, etc.). Combining the high sensitivity of induction methods for detecting metal objects with the ability to identify detected responses is an important search task and its solution can provide additional opportunities for the search methods and technical means used.

The possibilities of additional identification of responses from induction devices in building structures are determined by a certain classifiability of both signal and false interference objects by depth of occurrence, by thickness of the metal layer, by shape of the object. Classification of signal and interference responses gives the operator additional opportunities to distinguish objects even when examining metal-containing structures.

Interference signals include:

— reinforcement of reinforced concrete building structures, which can be modeled by a set of periodic linear structures;
— chain-link mesh, which also has a certain uniform structure;
— embedded wire and cable communications, which can be modeled by extended metal structures;
— metal reinforcement of window and door openings.

The indicated interference signals have a certain regularity (at least on a certain spatial interval of analysis), and it is fundamentally possible to use methods of spatial compensation of such signals.

Fig. 1. Examples of regular metal structures of building structures

The sought-after metal objects, as a rule, are local inclusions of limited size (units of centimeters) and of practically regular shape, which can be installed anywhere in the metal-containing environment, including at a short distance from the interfering metal structure. The geometric shapes of the sought-after objects can be modeled by a fairly limited set of figures, for example, as shown in Figure 2.

Fig. 2. Examples of sought-after objects (metal objects) of regular shape

It is obvious that in a real situation, the models of signal and interference metal structures may differ from ideal spatial forms. For example, the reinforcement pitch may not be strictly regular, but random quasi-periodic, wire communications may be laid along not quite straight lines, etc. The shapes of the sought objects may also have some differences from ideal geometric figures. However, such differences should not significantly reduce the expected average efficiency of detection and identification of metal objects in metal-containing environments due to compensation for regular interference and selection of signals from objects of a certain shape, size or depth.

Let's consider the possibilities of induction methods of searching for metal objects to detect unauthorized metal objects in building structures. Pulse induction methods are the most modern and have greater information content, therefore, for solving the problems of distinguishing interference and signals, they have significant advantages over harmonic ones. The essence of the detection method is simple and consists of creating a powerful magnetic field pulse using an exciting coil and receiving the magnetic field of secondary eddy currents using a receiving coil. The block diagram of a pulse metal detector is shown in Figure 3.

Using a pulse signal generator, a current pulse is generated in the excitation coil, which creates a magnetic excitation field. If there is a metal object in the excitation zone, it creates a secondary magnetic field, which is received by the receiving coil. The amplified signal is digitized in time and these time readings are fed to the computer, which implements a specified algorithm for detecting the object. The information block required by the operator is created on the indicator, allowing the operator to make the necessary decision.

Fig. 3. Block diagram of a pulse induction metal detector

The first pulse methods for detecting conductive objects were developed by Veit (1951) and Jost (1952), who studied the responses of conductors to the action of pulsed magnetic fluxes. The practical application of the method for detecting hidden metal objects was developed in detail by Wescott (1955) and Johnson (1956). However, the actual production of pulse metal detectors began with the advent of semiconductor technology. As early as 1962, Barringer used similar powerful metal detectors to detect conductive ore masses. Cowloni (1966) and Foster (1968) were able to optimize signal reception and minimize interference reactions due to the reception of signals with a time delay, which is extremely necessary when searching for metal objects in a semiconducting medium (water). In 1966, the first samples of pulse metal detectors were manufactured, and extensive field experiments and studies were conducted after the development of the PIMDEC and AQVDEC models in 1971. As a result of the studies, it was found that the time diagrams of the response of metal objects when exposed to a pulsed magnetic field depend significantly on the shape of the object and the type of metal. Figure 4 shows an example of time diagrams of changes in the secondary field from various metal objects.

As can be seen from the graphs shown in Figure 4, the time dependences of the response signals are significantly different for metal objects depending on their shape and size. This provides a fundamental opportunity to determine some parameters of the object based on the analysis of the shape of the time characteristic, i.e. to identify the detected object to a certain extent. This is the task that is always solved to one degree or another in pulse-type induction metal detectors.

1 – copper body measuring 2x3x1/4
2 – 3 nails
3 – 4 wrenches
4 – 1967 penny
5 – 1967 halfpenny
6 – 2 nail
7 – 1948 penny coin
8 – steel plate measuring 3×4
9 – half-crown coin

Fig. 4. Experimental data on the time dependence of responses from various
metal objects exposed to a pulsed magnetic field

A rigorous mathematical analysis of secondary magnetic fields for objects of arbitrary shape is a complex problem, the exact solution of which is possible only for objects of regular simple shape. Thus, if a coil in the form of a small cross-section turn, located above an infinite metal sheet, is excited by a current pulse, then the time characteristic of the vector potential of the magnetic field of eddy currents is determined by the geometric dimensions of the coil, the distance to the plate and the conductivity of the material /4/. The maximum magnetic vector potential and secondary magnetic flux is achieved immediately after the jump in the exciting current and its value does not depend on the conductivity of the material and its thickness, but is determined only by the geometric dimensions. The nature of the change in the magnetic field over time already depends on both the conductivity of the material and its thickness. This means that by measuring the magnetic flux immediately after the jump in the excitation current, it is possible to determine the distance to the metal sheet using known mathematical expressions, and by analyzing the shape of the signal over time, it is possible to determine other parameters of interest to the object.

The technical problem is the practical measurement of the magnetic field of eddy currents immediately after a current surge, since in any real excitation coil a non-stationary process of changing the current and the excitation field occurs for some time, against the background of which it is impossible to measure a relatively small magnetic flux of eddy currents. It is for this reason that the time dependences shown in Figure 4 begin at 50 μs, that is, with a time delay.

The metal detector «SMD-300», developed by the «Engineering and Commercial Multidisciplinary Center – 1», maximizes the advantages of the pulse induction method as applied to the tasks of examining building structures. The appearance of the metal detector is shown in Figure 5.

Fig. 5. Appearance of the metal detector «SMD-300»

The operation of the metal detector «SMD-300» is based on the following principle. Using the excitation coil, a pulse of a powerful magnetic field is created. The receiving magnetic coil receives the secondary magnetic field, the measurement of which is carried out with a minimum time delay, which in the device is 10 μs from the front of the excitation pulse. The received secondary signal is digitized and its shape is analyzed, and the value of the initial amplitude is calculated taking into account the introduced time delay. The computing device, taking into account the known parameters of the excitation coil, the value of the current in it and the parameters of the receiving coil, determines the distance to the metal object based on known mathematical expressions.

It can be shown that for metal objects with a small value of the thickness of the conductive layer, the duration of the processes of attenuation of the magnetic flux of eddy currents is proportional to the value of , where — metal conductivity, — metal layer thickness. This means that as a result of analyzing the time shape of the magnetic flux, it is possible to estimate the thickness of a metal sheet with a known metal conductivity. With an unknown conductivity, it is possible to roughly estimate the thickness of the metal layer based on the assumed type of the metal being sought.

Determining other characteristics of metal objects (dimensions, shape, depth of placement) is a complex mathematical problem that requires solving an electrodynamic problem with complex boundary conditions. Even for finite-sized objects of relatively simple and regular shape (parallelepiped, sphere, cylinder, etc.), an exact solution to the problem is problematic using microprocessor technology that allows implementing equipment in acceptable dimensions. Therefore, the SMD-300 equipment uses a simplified algorithm for estimating the geometric parameters of metal objects that provides results sufficient for practice. To obtain the basic relationships linking the excitation field, metal detector parameters, geometric parameters of the metal object, and the eddy current field, the equipment uses two excitation coils and one receiving coil. Figure 6 shows a simplified diagram of the arrangement of all components of the interaction of magnetic fields in the form of coaxial turns.

Fig. 6. Layout of coils

of the SMD-300 equipment

For the sought-after metal object, the radius of a certain turn can be considered as an average indicator of its geometric dimensions. For such a geometric model of representing the coils of the metal detector and the sought-after object, the voltage on the receiving coil can be represented as follows:

,

where — is the current in the excitation coil, — the number of turns in the excitation and receiving coils, respectively, — the mutual induction functions of the excitation and receiving coils with the desired metal object, respectively, — a normalized function displaying the time dependence of the magnetic flux of eddy currents.

The mutual induction functions of coaxial turns are determined by the expression:

,

where — are the radii of the coaxial turns, , , — elliptic integrals of the form:

.

Provided that Elliptic integrals can be approximated by the following series:

,

.

Using the above approximations and assuming that the condition , you can get expressions for calculating the size of the coil and the distance from the metal detector coils to the desired object:

,

.

Thus, it is possible to analytically link the geometric parameters of the metal detector and calculate some parameters of the desired object that are of interest to the operator and allow not only to detect objects, but also to identify them.

The following algorithm for calculating the parameters of metal objects is implemented in the SMD-300 equipment:

— voltages are measured and in the receiving coil with alternate excitation of the transmitting coils with radii and respectively with a time delay of 10 μs;
— the signals are digitized and the time form of the dependencies and is analyzed;
— the values ​​of and are restored by calculation;
— the functions and are calculated;
— the above approximations are used to calculate and ;
— the parameters of the detected metal object are calculated and ;
— based on the analysis of the time dependence of the signal and the magnitude of the thickness of the metal object and its shape are assessed.

The visualization of information for the operator in the SMD-300 device is carried out on the liquid crystal screen, on which it is displayed in tabular and graphical forms, as shown in Figure 7.

2

Fig. 7. Presentation of information on the screen of the device «SMD-300»

Figures 8 and 9 show the results of experimental studies on the detection of various metal objects by the device «SMD-300».

1 – a coil of wire with a diameter of 11.7 cm
2 – copper foil 10×10 cm
3 – a “Krona” battery 4.5×2.5 cm
4 – a 50 kopeck coin

Fig. 8. Dependences of measured sizes of objects on the distance of their location

– battery «Krona»
– a turn of wire with a diameter of 11.7 cm
— copper foil

Fig. 9. Dependences of measured distances on the true value of the location range

Table 1 shows the exact results of experimental studies for various metal objects.

Table 1

The errors in measuring the sizes and range do not exceed 15%, which for most practical applications seems sufficient when solving problems of detecting metal housings of units of covert information control devices installed in building structures.

In addition, the SMD-300 device has a PC output, which allows practical application of methods for compensating interference with a regular structure and methods for detecting metal objects with known limited geometric dimensions.

A metal detector with such new capabilities for the consumer in analyzing the sought-after objects will undoubtedly be widely used in surveys of building structures in order to find metal housings and parts of covert information control devices.

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