CN105954806A - Metal detection method and device based on giant magnetoresistance and orthogonal bias modulation - Google Patents

Abstract

The present invention discloses a metal detection method and device based on giant magnetoresistance and orthogonal bias modulation. The method comprises two parts. In the first part, for overcoming a magnetic-field component of an excitation coil, an 'orthogonal bias' method is proposed, that is, the bias current of a giant magnetoresistance sensor and the current of the excitation coil have the same frequency, with the phase difference being 90 DEC, so direct-current signals output by the giant magnetoresistance sensor is only related to the metal eddy current magnetic field. In the second part, a 'phase shift modulation' method is proposed to overcome direct-current offset of an amplifier circuit. By means of a low-pass filter, direct-current offset of the amplifier circuit is eliminated, and signal output only related to the metal eddy current is achieved. The metal detection device of the invention comprises the excitation coil, the giant magnetoresistance sensor and a detection circuit. The giant magnetoresistance sensor is arranged in the excitation coil, which is used to generate an alternating-current magnetic field. The giant magnetoresistance sensor is used to sense the metal eddy current magnetic field. The detection circuit is used to detect metal eddy current signals.

Description

A kind of metal probing method modulated based on giant magnetoresistance and quadrature bias and device

Technical field:

The present invention relates to metal probing method and device, particularly relate to a kind of metal detection side based on giant magnetoresistance Method and device.

Background technology:

Metal detector is a kind of instrument being specifically used to detect metal.Metal detector is widely used in industry mistake Journey, safety precaution, exploration exploration, traffic control and personal consumption field.

Existing coil type metal detector is divided into unicoil and balance coil two class.For unicoil metal detector, When there being metal approach detection coil, equivalent inductance and the equivalent resistance of coil change, and cause resonant frequency Change or impedance variation, detect metal by the change of detection resonant frequency or impedance.When metal volume is micro- Hour, the ratio of the variable quantity opposed coil preliminary examination inductance value of winding inductance quantity is minimum, needs high frequency to divide Resolution or impedance resolution, accordingly, it is difficult to detection minute metallic.To balance coil metal detector, need one Group excitation coil and two groups of receiving coils.When two groups of receiving coil numbers of turn, size are completely the same, and relative excitation When the installing space of coil is full symmetric, the induced potential width that excitation coil electric current produces in two groups of receiving coils Value and phase place are completely the same, and the difference of the induced potential of two groups of receiving coils is zero, passes through detector when there being metal Time, the differential potential of two groups of receiving coils only comprises the induced potential that metal swirl causes, so that signal amplifies The gain of circuit is fully enhanced, and then improves metal detection sensitivity.But, two groups of receiving coil numbers of turn, Size is difficult to completely the same, and the installation site of relative excitation coil is difficult to full symmetric, therefore, and metal detection spirit Sensitivity is the most limited.

Existing metal detector based on giant magnetoresistance sensor is by excitation coil, giant magnetoresistance sensor, detection Circuit forms.Excitation coil produces ac-excited magnetic field, when there being metal to pass through detector, produces electricity in metal Eddy current, this current vortex produces the eddy current magnetism with excitation field same frequency, and giant magnetoresistance sensor is by excitation field The signal of telecommunication is converted to the resultant magnetic field of eddy current magnetism.Excitation field is identical with eddy current magnetism frequency, it is impossible to pass through Both are separated by wave filter, are better than far away again eddy current magnetism due to excitation field, are thoroughly flooded by eddy current magnetism, The gain making testing circuit is restricted, and therefore, metal detection sensitivity is limited.

The present invention provides a kind of new method of metal detection, the metal detection side i.e. modulated based on giant magnetoresistance and quadrature bias Method.

Summary of the invention:

The problem existed for existing metal detection, the present invention proposes one and adjusts based on giant magnetoresistance and quadrature bias The metal probing method of system and device, for eliminating the magnetic field of excitation coil electric current generation, environmental magnetic field, huge magnetic Electric resistance sensor equivalence zero inclined resistance, the impact of amplifying circuit DC maladjustment, improve the sensitivity of metal detection.

The metal detecting device that the present invention modulates based on giant magnetoresistance and quadrature bias by giant magnetoresistance sensor, swash Encouraging coil, testing circuit composition, giant magnetoresistance sensor is placed in excitation coil, and excitation coil is used for producing friendship Stream magnetic field, giant magnetoresistance sensor is used for current vortex magnetic field, sensitive metal, and testing circuit is used for detecting metal swirl Signal.

Testing circuit includes sine wave signal generator, analog line driver, excitation coil current sampling resistor, just Hand over phase-shift circuit, square-wave generator, modulation switch, voltage buffer, low pass amplifying circuit, phase inverter, solution Tune switch, low pass filter.

Wherein:

(1) sine wave signal that sine wave signal generator produces carries out power amplification through analog line driver Rear drive excitation coil；

(2) excitation coil current sampling resistor is for being converted to voltage signal by the alternating current of excitation coil；

(3) orthogonal phase-shift circuit voltage phase shift+90 ° to excitation coil current sampling resistor two ends respectively With-90 °, be called+90 ° of phase shift signals and-90 ° of phase shift signals；

(4) square-wave signal of square-wave generator output duty cycle 50%；

(5) square-wave generator output signal high level, low level select modulation switch to be output as respectively + 90 ° of phase shift signals or-90 ° of phase shift signals, or be-90 ° of phase shift signals or+90 ° of phase shift signals；

(6) output of modulation switch is after voltage buffer buffers, it is provided that the biased electrical of giant magnetoresistance sensor Stream；

(7) low pass amplifying circuit filters AC compounent, is amplified signal；

(8) that the output of low pass amplifying circuit is carried out unit gain is anti-phase for phase inverter；

(9) square-wave generator output signal high level, low level select solution tune switch to be output as respectively The output of low pass amplifying circuit and the output of phase inverter, or be output and the output of low pass amplifying circuit of phase inverter；

(10) the low-pass filtered device of output solving tune switch filters AC compounent, obtains only electric with metal The signal output that eddy current is relevant.

The principle of the present invention is as follows:

The metal detecting device of the present invention is made up of giant magnetoresistance sensor, excitation coil, testing circuit, wherein Giant magnetoresistance sensor is placed in excitation coil.Excitation coil incoming transport electric current, produces AC magnetic field.When having When metal is by excitation coil, metal produces eddy current under AC magnetic field effect, and eddy current produces therewith Eddy current magnetism.Giant magnetoresistance sensor output voltage is equivalent to

U_o=k (B_a+B_e+B_b)I_a+γI_a

=k (B_a+B_b)I_a+γI_a+kB_eI_a (1)

In formula (1), k, B_a、B_ε、B_b、γ、I_aIt is respectively the magnetic field sensitivity system of giant magnetoresistance sensor The equivalence zero that AC magnetic field that number, exciting current produce, metal swirl magnetic field, environmental magnetic field are unrelated with magnetic field Resistance, the bias current of giant magnetoresistance sensor partially.From formula (1), the AC magnetism that exciting current produces , the environmental magnetic field equivalence zero inclined resistance unrelated with magnetic field can be to the sensitivity generation of metal swirl acquisition of signal Impact.

If the inductance of excitation coil is L₁, excitation coil resistance is R₁, metal swirl equivalent inductance is L₂, gold Belonging to eddy current equivalent resistance is R₂, metal swirl current effective value is I₂, excitation coil and the mutual inductance of metal swirl For M, the voltage effective value of excitation coil is E₁, the angular frequency of driving voltage be ω, exciting current virtual value For I₁, then electric equation:

E₁+jωMI₂=I₁R₁+jωL₁I₁ (2)

jωMI₁=I₂R₂+jωL₂I₂ (3)

Can be obtained with (3) by formula (2):

$I_2=\frac{{\mathrm{\ω}}^2{\mathrm{ML}}_2+{\mathrm{jR}}_2\mathrm{\ω}M}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}{I}_1---\left(4\right)$

The magnetic field that exciting current produces is:

B_a=λ I₁ (5)

In formula (5), λ is the structural constant of metal detector.

The magnetic field that metal swirl produces is:

$B_e={\mathrm{\ρI}}_2=\frac{{\mathrm{\ω}}^2{\mathrm{ML}}_2+{\mathrm{jR}}_2\mathrm{\ω}M}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}\×{\mathrm{\ρI}}_1---\left(6\right)$

In formula (6), ρ is the structural constant of metal detector.

Formula (5), (6) are brought into formula (1) and can be obtained following formula；

$U_o=k\[(\mathrm{\λ}+\frac{{\mathrm{\ω}}^2{\mathrm{ML}}_2+{\mathrm{jR}}_2\mathrm{\ω}M}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}\×\mathrm{\ρ})I_1+B_b\]\×I_a+{\mathrm{\γI}}_a---\left(7\right)$

$U_o={\mathrm{k\λI}}_1{I}_a+{\mathrm{kB}}_b{I}_a+{\mathrm{\γI}}_a+\frac{{\mathrm{\ω}}^2{\mathrm{ML}}_2+{\mathrm{jR}}_2\mathrm{\ω}M}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}\×{\mathrm{k\ρI}}_1{I}_a---\left(8\right)$

On the right of formula (8) equation, Section 1, Section 2, Section 3 are the distracters unrelated with metal swirl, Section 4 is only relevant with metal swirl signal terms.In order to eliminate distracter, the present invention uses such as Lower method:

Make giant magnetoresistance sensor bias current I_aWith excitation coil electric current I₁Frequency is identical, quadrature in phase, amplitude It is directly proportional, it may be assumed that

I_a=j μ I₁ (9)

$\begin{array}{c}{U}_o={\mathrm{k\μ\λI}}_1*{\mathrm{jI}}_1+{\mathrm{k\μB}}_b{I}_1+{\mathrm{j\μ\γI}}_1\\ +\frac{{\mathrm{\ω}}^2{\mathrm{ML}}_{2}k\mathrm{\ρ}\mathrm{\μ}}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}{I}_1*{\mathrm{jI}}_1-\frac{R_2{{\mathrm{\ωMk\ρ\μI}}_1}^2}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}\end{array}---\left(10\right)$

Formula (10) the right Section 1, Section 2, Section 3, Section 4 are AC compounent, only Section 5 For DC component and only relevant with metal swirl.

Use low pass filter to filter AC compounent in formula (10), obtain:

$U_1=-\frac{R_2{{\mathrm{\ωMk\ρ\μI}}_1}^2}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}---\left(11\right)$

Formula (11) shows, uses electric current identical with excitation coil power frequency, quadrature in phase to pass as giant magnetoresistance The bias current of sensor, can effectively eliminate the distracter unrelated with metal swirl signal, amplifies electricity for fully improving Road gain creates condition.

Being amplified the signal after aforementioned low-pass filtering, amplifying circuit is output as:

$U_2=-\frac{R_2{{\mathrm{\ωMk\σ\ρ\μI}}_1}^2}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}+\mathrm{\Δ}---\left(12\right)$

In formula (12), σ andIt is respectively the gain of amplifying circuit, amplifying circuit output DC offset voltage.

Although by preceding method, metal swirl signal being converted to direct current signal, and eliminates distracter, Amplifying circuit gain is improved, but owing to amplifying circuit exists DC maladjustment component, still can limit The sensitivity of metal detection.

In order to eliminate the restriction to metal detection sensitivity of the amplifying circuit DC offset voltage, the present invention adopts further Take following method:

Make giant magnetoresistance sensor bias current I_aWith angular frequency₀At juI₁With-juI₁Between switch, be equivalent to use Unit amplitude+1, dutycycle 50% angular frequency of unit amplitude-1₀Signal and juI₁As huge magnetic after being multiplied The bias current I of electric resistance sensor_a, i.e. the bias current relative excitation coil current to giant magnetoresistance sensor Quadrature phase be modulated, then amplifying circuit is output as:

$U_3=\frac{R_2{{\mathrm{\ωMk\σ\ρ\μI}}_1}^2}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}\×\frac{4}{\mathrm{\π}}\underset{n=1}{\overset{\mathrm{\∞}}{\Σ}}\frac{{(-1)}^{n+1}}{2n-1}cos(2n-1){\mathrm{\ω}}_{0}t+\mathrm{\Δ}---\left(13\right)$

By unit amplitude+1, dutycycle 50% angular frequency of unit amplitude-1₀Signal defeated with amplifying circuit Going out is multiplied is demodulated, and can obtain:

$\begin{array}{c}{U}_4=\[\frac{R_2{{\mathrm{\ωMk\σ\ρ\μI}}_1}^2}{{R_2}^2+{\mathrm{\ω}}^2{L_2}^2}\×\frac{4}{\mathrm{\π}}\underset{n=1}{\overset{\mathrm{\∞}}{\Σ}}\frac{{(-1)}^{n+1}}{2n-1}\cos (2n-1){\mathrm{\ω}}_{0}t+\mathrm{\Δ}\]\\ \×\frac{4}{\mathrm{\π}}\underset{n=1}{\overset{\mathrm{\∞}}{\Σ}}\frac{{(-1)}^{n+1}}{2n-1}\cos (2n-1){\mathrm{\ω}}_{0}t\end{array}---\left(14\right)$

Output after demodulation, through low-pass filtering, can obtain:

$U_5=\frac{8R_2{{\mathrm{\ωMk\σ\ρ\μI}}_1}^2}{({R_2}^2+{\mathrm{\ω}}^2{L_2}^2){\mathrm{\π}}^2}\underset{n=1}{\overset{\mathrm{\∞}}{\Σ}}\frac{1}{{(2n-1)}^2}---\left(15\right)$

Formula (15) shows, through the orthorhombic phase to giant magnetoresistance sensor bias current relative excitation coil current Position is modulated, and after the output of amplifying circuit is carried out synchronous demodulation and low-pass filtering, the direct current of amplifying circuit loses Tune is eliminated, thus improves the sensitivity of metal detection.

The metal probing method that the present invention modulates based on giant magnetoresistance and quadrature bias comprises the steps:

1. the alternating current of angular frequency is applied to excitation coil；

2. pair excitation coil alternating current phase shift 90 ° obtains signal Ia1, to excitation coil alternating current phase shift-90 ° Obtain signal Ia2；

3. produce angular frequency₀, the selection signal of dutycycle 50%, the cycle select giant magnetoresistance sensor biasing Electric current is Ia1 or Ia2, i.e. to giant magnetoresistance sensor bias current relative excitation coil current just Friendship phase place is modulated；

4. pair giant magnetoresistance sensor output signal carries out low-pass filtering amplification；

5. the signal after amplifying is divided into two-way, and a road keeps constant and be U1, another road unit gain anti-phase and For U2；

6. use aforementioned angular frequency₀, dutycycle 50% the selection signal period select U1 or U2 as demodulation Output；

7. pair above-mentioned demodulation output carries out low-pass filtering, it is thus achieved that only relevant with metal swirl output signal.

Accompanying drawing explanation

Fig. 1 metal detecting device block diagram

Fig. 2 testing circuit block diagram

Fig. 3 metal detecting device connects block diagram in detail

Embodiments of the invention

The composition of a kind of metal detecting device is shown in accompanying drawing 1, and this detection device is passed by excitation coil 1, giant magnetoresistance Sensor 2, testing circuit 3 form, and giant magnetoresistance sensor is placed in excitation coil, and excitation coil is used for producing AC magnetic field, giant magnetoresistance sensor is used for current vortex magnetic field, sensitive metal, and testing circuit is used for detecting metal whirlpool Stream signal.

Testing circuit composition in accompanying drawing 1 is shown in accompanying drawing 2.Including sine wave signal generator 1, analog line driver 2, excitation coil current sampling resistor 3, orthogonal phase-shift circuit 4, square-wave generator 5, modulation switch 6, electricity Compression buffer 7, low pass amplifying circuit 8, phase inverter 9, solution tune switch 10, low pass filter 11.

Device in accompanying drawing 1 connects in detail sees accompanying drawing 3.Including sine wave signal generator 1, analog line driver 2, excitation coil current sampling resistor 3, orthogonal phase-shift circuit 4, square-wave generator 5, modulation switch 6, electricity Compression buffer 7, low pass amplifying circuit 8, phase inverter 9, solution tune switch 10, low pass filter 11, excitation line Circle 12, giant magnetoresistance sensor 13.

Wherein:

(1) sine wave signal that sine wave signal generator 1 produces carries out power amplification through analog line driver 2 Rear drive excitation coil 12；

(2) excitation coil current sampling resistor 3 for being converted to voltage letter by the alternating current of excitation coil 12 Number；

(3) orthogonal phase-shift circuit 4 voltage phase shift+90 ° to excitation coil current sampling resistor 3 two ends respectively With-90 °, be called+90 ° of phase shift signals and-90 ° of phase shift signals；

(4) square-wave signal of square-wave generator 5 output duty cycle 50%；

(5) square-wave generator 5 output signal high level, low level select modulation switch 6 to be output as respectively + 90 ° of phase shift signals or-90 ° of phase shift signals, or be-90 ° of phase shift signals or+90 ° of phase shift signals；

(6) output of modulation switch 6 is after voltage buffer 7 buffers, it is provided that to giant magnetoresistance sensor 13 Bias current；

(7) low pass amplifying circuit 8 filters AC compounent, is amplified signal；

(8) that the output of low pass amplifying circuit 8 is carried out unit gain is anti-phase for phase inverter 9；

(9) square-wave generator 5 output signal high level, low level select solution tune switch 10 to be output as respectively The output of low pass amplifying circuit 8 or the output of phase inverter 9, or be output or the low pass amplifying circuit of phase inverter 9 The output of 8；

(10) the low-pass filtered device of output 11 solving tune switch 10 filters AC compounent, obtains only electric with metal The signal output that eddy current is relevant.

More than display describes ultimate principle and the principal character of the present invention.The present invention is not limited by above-described embodiment System, without departing under the principle of the invention and scope, the present invention also has various changes and modifications, these change and Improve the right falling within this patent.

Claims (2)

1. the metal detecting device modulated based on giant magnetoresistance and quadrature bias, by giant magnetoresistance sensor, swashs Encouraging coil, testing circuit composition, giant magnetoresistance sensor is placed in excitation coil, and excitation coil is used for producing friendship Stream magnetic field, giant magnetoresistance sensor is used for current vortex magnetic field, sensitive metal, and testing circuit is used for detecting metal swirl Signal, it is characterised in that testing circuit includes sine wave signal generator, analog line driver, excitation coil electric current Sample resistance, orthogonal phase-shift circuit, square-wave generator, modulation switch, voltage buffer, low pass amplifying circuit, Phase inverter, solution tune switch, low pass filter；

The sine wave signal that sine wave signal generator produces carries out power amplification rear drive through analog line driver and swashs Encourage coil；

Excitation coil current sampling resistor is for being converted to voltage signal by the alternating current of excitation coil；

The voltage phase shift+90 ° to excitation coil current sampling resistor two ends respectively of orthogonal phase-shift circuit With-90 °, be called+90 ° of phase shift signals and-90 ° of phase shift signals；

The square-wave signal of square-wave generator output duty cycle 50%；

Square-wave generator output signal high level, low level select modulation switch to be output as respectively + 90 ° of phase shift signals or-90 ° of phase shift signals, or be-90 ° of phase shift signals or+90 ° of phase shift signals；

The output of modulation switch is after voltage buffer buffers, it is provided that the bias current of giant magnetoresistance sensor；

Low pass amplifying circuit filters AC compounent, is amplified signal；

It is anti-phase that phase inverter carries out unit gain to the output of low pass amplifying circuit；

Square-wave generator output signal high level, low level select to solve tune switch respectively and are output as low pass amplification electricity The output on road and the output of phase inverter, or be output and the output of low pass amplifying circuit of phase inverter；

The low-pass filtered device of output solving tune switch filters AC compounent, obtains only relevant with metal current vortex letter Number output.

2. the metal probing method modulated based on giant magnetoresistance and quadrature bias, for such as claim 1 The described metal detecting device modulated based on giant magnetoresistance and quadrature bias, it is characterised in that comprise the following steps:

One, the alternating current of angular frequency is applied to excitation coil；

Two, excitation coil alternating current phase shift 90 ° is obtained signal Ia1, to excitation coil alternating current phase shift -90 ° obtain signal Ia2；

Three, angular frequency is produced₀, the selection signal of dutycycle 50%, the cycle select giant magnetoresistance sensor inclined Putting electric current is Ia1 or Ia2, i.e. to giant magnetoresistance sensor bias current relative excitation coil current Quadrature phase be modulated；

Four, giant magnetoresistance sensor output signal is carried out low-pass filtering amplification；

Five, the signal after amplifying is divided into two-way, and a road keeps constant and is U1, and another road unit gain is anti-phase And be U2；

Six, aforementioned angular frequency is used₀, dutycycle 50% the selection signal period select U1 or U2 as demodulation Output；

Seven, above-mentioned demodulation output is carried out low-pass filtering, it is thus achieved that only relevant with metal swirl output signal.