RU2313109C1 - Mode of detection of metallic objects - Google Patents

Abstract

FIELD: the invention refers to the field of detection of electric conductive bodies in nonconductive or poor conductive electric current mediums.

SUBSTANCE: the mode is based on excitation of metallic objects with an impulse magnetic field and recording the currents of the magnetic field caused by transitional processes in them. In quality of the source of excitation they use impulse succession of linear changing impulses of a triangular form of high linearity with an-off time ratio of impulses equals or more than 1,5. The impulses has an accumulation phase, a decay phase and a phase of absence of the magnetic field. The measurements are made during each phase with gating signals in no less then in two timely correlated positions in each phase. Each first selection is removed from the beginning of the phase on a time period defined by duration of fading of own transitional processes of the sensor. Each second selection is removed from each beginning of the phase on a time period equal or larger then the constant time of the transitional process of the object. At processing signals the difference of signals between the first and the second selection is calculated in each phase, absolute values of received differences in the first and in the second phase are summed up, filtered, intensified and used for detection objects by way of comparison with the threshold level.

EFFECT: increases sensitiveness and reduces influence of the excited field.

3 dwg

Description

The invention relates to the field of detection of electrically conductive bodies in non-conductive or weakly conductive electric current environments using inductive methods, in particular to the field of creating detectors of metal objects, including during engineering surveys in construction and agriculture, for detecting mines in metal cases during engineering exploration detecting firearms and cold steel at checkpoints.

There is a method of detecting well-conducting objects, based on the transient method, in which objects are exposed to a pulsed magnetic field and the reception in pauses between pulses of an unsteady magnetic field of eddy currents arising in conductive objects [1]:

Electrical Exploration: A Handbook of Geophysics. In two books / Ed. V.K.Khmelevsky and V.K.Bondarenko. Book one. - 2nd ed., Revised. and add. - M .: Nedra, 1989. 391-405 p.

The closest technical solution is a method for detecting metal objects, based on the excitation of the investigated medium by a pulsed electromagnetic field, measuring the transient response of the medium by doubling the measured signal after switching the electromagnetic field and at the end of the transition process and determining the difference of these signals during each half-period of the exciting field, which is judged by the presence of metal objects [2]:

USSR copyright certificate No. 345619, class G01V 3/10, 1981.

The formation of a linearly changing field is performed by applying to the generator circuit formed by the inductor L _g (exciting coil of the sensor) and the total active resistance r in the switching circuit of the generator circuit, a bipolar rectangular supply voltage of the "meander" type, under the condition:

τ _g = L _g / r> T _slave ,

where τ _g is the time constant of the generator coil;

L _g is the inductance of the generator coil;

r is the active resistance in the switching circuit of the Civil Code;

T _slave - the period of the working frequency.

The signal arising in the receiving circuit (receiving coils of the sensor) is gated twice during each half-cycle of the supply voltage. Detuning from signals caused by a violation of the orthogonality of the receiving and exciting coil of the sensor (violation of the "geometry" of the sensor) is carried out by subtracting the integrated signals of the first and second gates. The difference signal judges the presence of a metal object.

However, this method has inherent disadvantages.

Since the nature of the change in current in the generator circuit in the absence of a conductive object is non-linear and is determined by the expression:

signal in the receiving coils of the induction differentiating sensor:

e _s (t) = G _p · d [i _g (t) · M _gp ] / dt;

G _p is the conversion coefficient of the receiving coils of the induction sensor;

M _gp is the coupling coefficient of the magnetic field of the generator coil of the induction sensor with the receiving coils of the sensor.

Due to the nonlinearity of the expression d [i _g (t) · M _gp ] / dt in the sensor, in the absence of a conductive object, a signal arises due to this nonlinearity, which does not allow one to completely detune from the signals caused by the violation of the orthogonality of the receiving and exciting sensor coils by subtracting the integrated signals of the first and second gates.

The aim of this invention is to increase the sensitivity and reduce the influence of the exciting field.

This goal is achieved by the fact that in the known method for detecting metal objects, based on their excitation by a pulsed magnetic field and registration of the magnetic field of the currents caused by transients in them, by sampling the measured signal by gating, a linearly varying field of a triangular shape high is used as a source of the exciting field linearity with duty cycle

Q = (T _H + T _C + T _P ) / (T _H + T _C ),

equal to or greater than 1.5, having successively a phase of increase in the magnetic field of duration T _N , a phase of decay of the magnetic field of duration T _C and a phase of absence of a magnetic field of duration T _P , measurements are made during each phase by gating signals at at least two temporal correlated positions in each phase wherein each first sample is removed from the beginning of the phase for a period of time determined by the length of the attenuation of its own transient processes of the sensor, every second sample is removed from each beginning phase for a period of time equal to or greater than the transient time constant of the object, when processing signals in each phase, the signal difference between the first and second samples is calculated, the absolute values of the obtained differences in the first and second phases are summed, filtered, amplified and used to detect objects by comparing with some threshold level V _por .

The essence of the invention is as follows: due to the fact that signals caused by a transient process in a metal object are recorded during exposure to an exciting pulsed magnetic field, in order to attenuate the effect of the exciting field caused by the nonlinearity of the current in the generator circuit, as a source of the exciting field using a linearly varying field of a triangular shape of high linearity, the measurement is performed by sampling the signal at the beginning and end of the rise and fall phases of the field, in each phase yayut signal difference between the samples obtained difference is summed with the correct sign. With a linear exciting field in the first and second samples in each phase in the receiving coils of the induction sensor, a constant signal caused by the primary field will be allocated:

e _s (t) = d [i _g (t) · M _gp ] / dt = const,

the value of which in the first e _s (t ₁ ) and second e _s (t ₂ ) samples is the same and when subtracting their difference signal will be zero:

e _s (t ₁ ) -e _s (t ₂ ) = 0,

what excludes or decreases the influence of the exciting field.

If there is a metal object in the sensitivity zone of the sensor that is equivalent to a closed circuit with inductance L _ob and active resistance R _ob , an emf E _ob is induced in it under the influence of the exciting field, which excites the current:

- постоянная времени контура объекта,Where

is the time constant of the contour of the object,

forming its own exponentially decreasing magnetic field proportional to the current in the object:

H _ob (t) = I _ob (t),

which, when differentiated by the receiving coils of the sensor at the output, they generate signals in the rise phase of V _1-1 and V _2-1 and the fall phase of V _1-2 and V _2-2 .

When processing signals in each phase, the differences between the first and second samples are calculated:

V1 = V _1-1 -V _2-1 ,

V2 = V _1-2 -V _2-2 ,

the absolute values of the differences obtained in the first and second phases are summed up:

V = | V1 | + | V2 |,

they are filtered, amplified, and used to detect objects by comparing with some threshold level V _por . When V> V _por , an object detection signal is output. Due to the fact that in the second phase the emf of self-inductance of the inductance of the circuit of the object is summed with the emf caused by the exciting field, V2 in the second phase exceeds the value of V1. Thus, V is larger in magnitude than each difference taken separately in each phase:

V> V1 and V> V2,

what explains the increase in sensitivity.

On figa is a figure explaining the principle of excitation of a metal object and registration of the magnetic field of the currents in it created by the exciting field. It shows an induction sensor consisting of an exciting (generator) coil L _g and receiving coils L1 and L2; But - the magnetic field of the exciting coil, Hob - the magnetic field of the object.

Figure 1b shows the equivalent electrical circuit of the conducting circuit of the search object: Lob is the equivalent inductance of the object, Rob is the active resistance of the circuit of the object, Eob is the excited emf in the circuit of the object, Iob is the current in the circuit of the object.

On figv shows diagrams of the intensity of the exciting magnetic field Ho, emf in the circuit of the object Eob, the current in the circuit of the object Iob and the magnetic field of the current Hob, as well as the signals at the output of the receiving coils Emp, explaining the essence of the invention.

Figure 2 and 3 shows a functional diagram of a variant of the device that implements the proposed method for detecting metal objects.

The circuit contains: 1 - sensor; 2 - object of detection; 3 - signal processing device; 4 - executive device; 5 - generator of exciting current; 6 - shaper of synchronizing pulses; 8 - preliminary amplifier: 9, 10 - storage capacitors of the residual voltage; 11, 14 - gating keys of signal samples; 12, 13 - keys for storing residual stresses; 15, 16 - signal storage and filtering devices; 17 - summing differential amplifier; 18 - output terminal amplifier.

The above figures illustrate a variant of the practical implementation of the claimed invention in a device designed to detect metal objects. The sensor, figa, consists of a drive coil Lg and two receiving coils L1 and L2, included according to the components Hob of the field of the object and differentially in the external field. The planes of the receiving and generating coils are orthogonal; coils are rigidly fixed on a dielectric rigid base.

The exciting coil Lg is connected to the generator of the exciting current 5 (Fig.2 and 3b), creating in it a triangular current of high linearity and, accordingly, a magnetic field Ho of a triangular shape (Fig.1c). The exciting current generator consists of a triangular voltage pulse generator, obtained using an integrator 22, alternately connected with a key 20 to a negative voltage source -U (increasing a triangular pulse) and a key 21 to a positive voltage source + U (tripping a triangular pulse). The keys are controlled by pulse sequences F1 and F2, coming from the shaper of the synchronizing pulses 6 (Fig.2 and 3b). The formation of the exciting current is carried out using a voltage-current Converter 23 (figb).

In phase 1, the field increases linearly; in phase 2, it decreases linearly; in phase 3 there is no magnetic field. The magnetic field Ho induces an emf in the object circuit Eob (figb, 1c), which creates a current:

- постоянная времени контура объекта,Where

is the time constant of the contour of the object,

forming its own exponentially decreasing magnetic field proportional to the current in the object: H _ob (t) = I _ob (t). In phase 1 (Fig.1c), the inductance of Lob is charged, in phase 2 the emf of Eob is summed with the emf of self-induction created by the Lob discharge, in phase 3 the energy stored in Lob is discharged.

The magnetic field of the Hob object is detected by the receiving coils L1, L2. The output of them arises EMF Emp (pigv), proportional to:

E _mp = G _p · dHob / dt,

where G _p is the conversion coefficient of the receiving coils of the induction sensor.

In the signal processing device 3 (FIGS. 2 and 3a), the output signals from the receiving coils 7 are amplified by a preamplifier 8 and, by gating with the keys 11 and 14, the signals V1-1 are sampled in phase 1 and V1-2 - phase 2. In phase 1, in at the end of the transient signal from the object using the key 12 in the sample V2-1, the residual voltage is stored on the capacitor 9. Since the processes of excitation of the field and signal samples are periodic, in each subsequent sample V1-1, the residual voltage accumulated on the capacitor 9 in the samples V2-1 is subtracted.

The residual voltage in phase 2 is stored with s on the capacitor 10 in the sample V2-2 using the key 13.

The signals from the outputs of the keys 11 and 14 are accumulated and filtered in devices 15 and 16 and their outputs are summed up taking into account the signs in the differential amplifier 17. From the output of the terminal amplifier 18, the signal 19 is supplied to the actuator 4, in which the threshold analysis and indication of operation in in case the signal exceeds the set threshold voltage.

Device 6 (Fig.2 and Fig.3c) is the formation of control pulse sequences.

Claims (1)

A method for detecting metal objects, including those located in masking or hiding non-conducting or weakly conducting electric current environments, based on their excitation by a pulsed magnetic field and registration of the magnetic field of the currents caused by transients in them, by sampling the measured signal during exposure to exciting pulses of magnetic field, characterized in that as the source of the exciting field using a pulse sequence of linearly changing field pulses of a triangle a high linearity form with a duty cycle of pulses equal to or greater than 1.5, each having a phase of increasing magnetic field, phase of decay of the magnetic field and phase of absence of a magnetic field, measurements are made during each phase by gating signals in at least two temporal correlated positions in each phase wherein each first sample is removed from the beginning of the phase for a period of time determined by the length of the attenuation of its own transient processes of the sensor, every second sample is removed from each beginning phase for a period of time equal to or greater than the transient time constant of the object, when processing signals in each phase, the signal difference between the first and second samples is calculated, the absolute values of the obtained differences in the first and second phases are summed, filtered, amplified and used to detect objects by comparing with some set threshold level.

Images