CN105954806B - A kind of metal probing method and device modulated based on giant magnetoresistance and quadrature bias - Google Patents

Abstract

The invention discloses a metal detection method and device based on giant magnetoresistance and quadrature bias modulation. The method of the present invention includes two parts: the first part proposes an "orthogonal bias" method for overcoming the magnetic field component of the excitation coil, that is, the bias current of the giant magnetoresistive sensor is the same frequency as the excitation coil current, and the phase difference is 90°, so that The DC signal output by the giant magnetoresistive sensor is only related to the metal eddy current magnetic field; the second part proposes a "phase-shift modulation" method to overcome the DC offset of the amplifier circuit, through a low-pass filter, thereby eliminating the DC offset of the amplifier circuit , to obtain a signal output related only to metal eddy currents. The device of the invention is composed of an excitation coil, a giant magnetoresistance sensor and a detection circuit. The giant magnetoresistance sensor is placed in the excitation coil, and the excitation coil is used to generate an alternating magnetic field, the giant magnetoresistance sensor is used for sensitive metal eddy current magnetic fields, and the detection circuit is used for detecting metal eddy current signals.

Description

A metal detection method and device based on giant magnetoresistance and quadrature bias modulation

Technical field:

The invention relates to a metal detection method and device, in particular to a giant magnetoresistance-based metal detection method and device.

Background technique:

A metal detector is an instrument specially used to detect metal. Metal detectors are widely used in industrial processes, safety precautions, exploration and surveying, traffic control and consumer life.

Existing coil metal detectors are divided into two types: single coil and balanced coil. For single-coil metal detectors, when metal approaches the detection coil, the equivalent inductance and equivalent resistance of the coil change, causing a change in resonant frequency or impedance. Metal is detected by detecting changes in resonant frequency or impedance. When the metal volume is small, the ratio of the change in coil inductance to the initial test inductance of the coil is extremely small, requiring extremely high frequency resolution or impedance resolution, so it is difficult to detect tiny metals. For balanced coil metal detectors, one set of exciting coils and two sets of receiving coils are required. When the number of turns and size of the two sets of receiving coils are exactly the same, and the installation space of the two sets of receiving coils is completely symmetrical, the amplitude and phase of the induced potential generated by the current of the exciting coil in the two sets of receiving coils are exactly the same, and the induced potential of the two sets of receiving coils The difference is zero. When metal passes through the detector, the differential potential of the two sets of receiving coils only includes the induced potential caused by the metal eddy current, so that the gain of the signal amplification circuit is fully improved, thereby improving the metal detection sensitivity. However, the number of turns and size of the two sets of receiving coils are difficult to be exactly the same, and the installation positions of the relative excitation coils are difficult to be completely symmetrical. Therefore, the sensitivity of metal detection is still limited.

Existing metal detectors based on giant magnetoresistance sensors consist of excitation coils, giant magnetoresistance sensors, and detection circuits. The excitation coil generates an AC excitation magnetic field. When metal passes through the detector, an eddy current is generated in the metal, and the eddy current generates an eddy current magnetic field with the same frequency as the excitation magnetic field. Signal. The frequency of the excitation magnetic field and the eddy current magnetic field are the same, and the two cannot be separated by a filter, and because the excitation magnetic field is much stronger than the eddy current magnetic field, the eddy current magnetic field is completely submerged, so that the gain of the detection circuit is limited, so the sensitivity of metal detection is limited.

The invention provides a new metal detection method, that is, a metal detection method based on giant magnetoresistance and quadrature bias modulation.

Invention content:

Aiming at the problems existing in the existing metal detection, the present invention proposes a metal detection method and device based on giant magnetoresistance and quadrature bias modulation, which is used to eliminate the magnetic field generated by the excitation coil current, the environmental magnetic field, and the equivalent of the giant magnetoresistance sensor. The influence of zero bias resistance and DC offset of the amplifier circuit improves the sensitivity of metal detection.

The metal detection device based on the giant magnetoresistance and orthogonal bias modulation of the present invention is composed of a giant magnetoresistance sensor, an excitation coil, and a detection circuit. The giant magnetoresistance sensor is placed in the excitation coil, and the excitation coil is used to generate an AC magnetic field. The sensor is used for sensitive metal eddy current magnetic field, and the detection circuit is used for detecting metal eddy current signal.

The detection circuit includes a sine wave signal generator, a power driver, an excitation coil current sampling resistor, a quadrature phase shift circuit, a square wave generator, a modulation switch, a voltage buffer, a low-pass amplifier circuit, an inverter, a demodulation switch, a low pass filter.

in:

(1) The sine wave signal generated by the sine wave signal generator is amplified by the power driver to drive the excitation coil;

(2) The excitation coil current sampling resistor is used to convert the AC current of the excitation coil into a voltage signal;

(3) The quadrature phase-shifting circuit shifts the voltage at both ends of the excitation coil current sampling resistor by +90° and -90° respectively, which are called +90° phase-shifting signal and -90° phase-shifting signal respectively;

(4) The square wave generator outputs a square wave signal with a duty cycle of 50%;

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

(6) After the output of the modulation switch is buffered by the voltage buffer, it provides the bias current of the giant magnetoresistive sensor;

(7) The low-pass amplifier circuit filters out the AC component and amplifies the signal;

(8) the inverter carries out unity gain inversion to the output of the low-pass amplifier circuit;

(9) The high level and low level of the output signal of the square wave generator respectively select the output of the demodulation switch as the output of the low-pass amplifier circuit and the output of the inverter, or as the output of the inverter and the output of the low-pass amplifier circuit output;

(10) The output of the demodulation switch is filtered out by a low-pass filter to obtain a signal output only related to the metal eddy current.

Principle of the present invention is as follows:

The metal detection device of the present invention is composed of a giant magnetoresistance sensor, an excitation coil and a detection circuit, wherein the giant magnetoresistance sensor is placed in the excitation coil. The excitation coil is connected with an alternating current to generate an alternating magnetic field. When a metal passes through the excitation coil, the metal generates an eddy current under the action of an alternating magnetic field, and the eddy current then generates an eddy magnetic field. The output voltage of giant magnetoresistance sensor 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 _e I _a (1)

In formula (1), k, B _a , B _ε , B _b , γ, and I _a are the magnetic field sensitivity coefficient of the giant magnetoresistance sensor, the AC magnetic field generated by the excitation current, the metal eddy current magnetic field, the environmental magnetic field, and the magnetic field independent of the magnetic field. Equivalent zero bias resistance, bias current of giant magnetoresistance sensor. It can be known from formula (1) that the AC magnetic field generated by the excitation current, the ambient magnetic field, and the equivalent zero bias resistance independent of the magnetic field will affect the sensitivity of metal eddy current signal detection.

Let the inductance of the excitation coil be L ₁ , the resistance of the excitation coil is R ₁ , the equivalent inductance of the metal eddy current is L ₂ , the equivalent resistance of the metal eddy current is R ₂ , the effective value of the metal eddy current is I ₂ , the mutual inductance between the excitation coil and the metal eddy current is M, the effective value of the excitation coil voltage is E ₁ , the angular frequency of the excitation voltage is ω, and the effective value of the excitation current is I ₁ , then the electrical equation:

E ₁ +jωMI ₂ ＝I ₁ R ₁ +jωL ₁ I ₁ (2)

jωMI ₁ ＝I ₂ R ₂ +jωL ₂ I ₂ (3)

From formulas (2) and (3), we can get:

The magnetic field generated by the excitation current is:

B _a =λI ₁ (5)

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

The magnetic field generated by the metal eddy current is:

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

Bring formulas (5), (6) into formula (1) to get the following formula;

The first, second, and third terms on the right side of equation (8) are interference terms that have nothing to do with metal eddy currents, and the fourth term is a signal term that is only related to metal eddy currents. In order to eliminate the interference term, the present invention adopts the following method:

Let the giant magnetoresistive sensor bias current _Ia have the same frequency as the excitation coil current _I1 , the phase is quadrature, and the amplitude is proportional, that is:

I _a = jμI ₁ (9)

The first, second, third, and fourth terms on the right side of formula (10) are all AC components, and only the fifth item is a DC component, which is only related to metal eddy currents.

Use a low-pass filter to filter out the AC component in formula (10), and get:

Equation (11) shows that using the current with the same frequency and phase quadrature as the excitation coil current as the bias current of the giant magnetoresistive sensor can effectively eliminate the interference items irrelevant to the metal eddy current signal, and create conditions for fully improving the gain of the amplifier circuit .

The signal after the aforementioned low-pass filtering is amplified, and the output of the amplifying circuit is:

In formula (12), σ and Respectively, the gain of the amplifier circuit and the output DC offset voltage of the amplifier circuit.

Although through the above method, the metal eddy current signal is converted into a DC signal, and the interference term is eliminated, so that the gain of the amplifier circuit is improved, but the sensitivity of metal detection is still limited due to the DC offset component of the amplifier circuit.

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

Make the giant magnetoresistive sensor bias current I _a switch between juI ₁ and -juI ₁ at an angular frequency ω ₀ , which is equivalent to using a duty cycle of 50% with a unit amplitude of +1 and unit amplitude of -1 at an angular frequency of ω ₀ After multiplied by the signal of juI ₁ , it is used as the bias current I _a of the giant magnetoresistance sensor, that is, to modulate the quadrature phase of the bias current of the giant magnetoresistance sensor relative to the excitation coil current, then the output of the amplifying circuit is:

Multiply the signal with the unit amplitude +1, unit amplitude -1 duty cycle 50% angular frequency ω ₀ and the output of the amplifier circuit for demodulation, and get:

The demodulated output is low-pass filtered to obtain:

Equation (15) shows that after modulating the quadrature phase of the bias current of the giant magnetoresistive sensor relative to the excitation coil current, and synchronously demodulating and low-pass filtering the output of the amplifier circuit, the DC offset of the amplifier circuit is eliminated. Thereby improving the sensitivity of metal detection.

The metal detection method based on giant magnetoresistance and orthogonal bias modulation of the present invention comprises the following steps:

1. Apply an AC current with an angular frequency ω to the excitation coil;

2. Phase-shift the AC current of the excitation coil by 90° to obtain the signal Ia1, and shift the phase of the AC current of the excitation coil by -90° to obtain the signal Ia2;

3. Generate a selection signal with an angular frequency ω ₀ and a duty cycle of 50%, and periodically select the bias current of the giant magnetoresistance sensor as Ia1 or Ia2, that is, to modulate the quadrature phase of the bias current of the giant magnetoresistance sensor relative to the excitation coil current ;

4. Perform low-pass filtering and amplification on the output signal of the giant magnetoresistance sensor;

5. The amplified signal is divided into two channels, one channel remains unchanged and is U1, and the other channel is unity gain inverted and is U2;

6. Select U1 or U2 as the demodulation output with the selection signal period of the aforementioned angular frequency ω ₀ and a duty cycle of 50%;

7. Perform low-pass filtering on the demodulation output above to obtain an output signal related only to the metal eddy current.

Description of drawings

Figure 1 Block diagram of metal detection device

Figure 2 Detection circuit block diagram

Figure 3 Detailed connection block diagram of the metal detection device

Embodiments of the invention

The composition of a metal detection device is shown in Figure 1. The detection device is composed of an excitation coil 1, a giant magnetoresistance sensor 2, and a detection circuit 3. The giant magnetoresistance sensor is placed in the excitation coil, and the excitation coil is used to generate an AC magnetic field. The magnetoresistance sensor is used for sensitive metal eddy current magnetic field, and the detection circuit is used for detecting metal eddy current signal.

The composition of the detection circuit in accompanying drawing 1 is shown in accompanying drawing 2. Including sine wave signal generator 1, power driver 2, excitation coil current sampling resistor 3, quadrature phase shift circuit 4, square wave generator 5, modulation switch 6, voltage buffer 7, low-pass amplifier circuit 8, inverter 9. A demodulation switch 10 and a low-pass filter 11 .

The detailed connection of the device in accompanying drawing 1 is shown in accompanying drawing 3. Including sine wave signal generator 1, power driver 2, excitation coil current sampling resistor 3, quadrature phase shift circuit 4, square wave generator 5, modulation switch 6, voltage buffer 7, low-pass amplifier circuit 8, inverter 9. Demodulation switch 10 , low-pass filter 11 , excitation coil 12 , giant magnetoresistance sensor 13 .

in:

(1) The sine wave signal generated by the sine wave signal generator 1 is amplified by the power driver 2 to drive the excitation coil 12;

(2) The excitation coil current sampling resistor 3 is used to convert the AC current of the excitation coil 12 into a voltage signal;

(3) The quadrature phase-shifting circuit 4 phase-shifts the voltage at both ends of the excitation coil current sampling resistor 3 by +90° and -90° respectively, which are called +90° phase-shifting signal and -90° phase-shifting signal respectively;

(4) square wave generator 5 outputs a square wave signal with a duty cycle of 50%;

(5) The high level and low level of the output signal of the square wave generator 5 respectively select the output of the modulation switch 6 as +90° phase-shifting signal or -90° phase-shifting signal, or as -90° phase-shifting signal or +90° phase-shifting signal °phase-shifted signal;

(6) After the output of the modulation switch 6 is buffered by the voltage buffer 7, the bias current provided to the giant magnetoresistance sensor 13;

(7) The low-pass amplifier circuit 8 filters out the AC component and amplifies the signal;

(8) Inverter 9 carries out unity gain inversion to the output of low-pass amplifier circuit 8;

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

(10) The output of the demodulation switch 10 is filtered out by the low-pass filter 11 to obtain a signal output only related to the metal eddy current.

The above presentation describes the basic principles and main features of the invention. The present invention is not limited by the above embodiments. Without departing from the principle and scope of the present invention, there will be various changes and improvements in the present invention, and these changes and improvements also belong to the rights of this patent.

Claims (2)

1. A metal detection device based on giant magnetoresistance and orthogonal bias modulation, consisting of a giant magnetoresistance sensor, an excitation coil, and a detection circuit. The giant magnetoresistance sensor is placed in the excitation coil, and the excitation coil is used to generate an AC magnetic field. The giant magnetoresistance sensor is used for the sensitive metal eddy current magnetic field, and the detection circuit is used for detecting the metal eddy current signal. Generator, modulation switch, voltage buffer, low-pass amplifier circuit, inverter, demodulation switch, low-pass filter; The sine wave signal generated by the sine wave signal generator is amplified by the power driver to drive the excitation coil; The excitation coil current sampling resistor is used to convert the AC current of the excitation coil into a voltage signal; The quadrature phase-shifting circuit shifts the voltage at both ends of the excitation coil current sampling resistor by +90° and -90° respectively, which are called +90° phase-shifting signal and -90° phase-shifting signal respectively; The square wave generator outputs a square wave signal with a duty cycle of 50%; The high level and low level of the output signal of the square wave generator respectively select the output of the modulation switch as +90° phase shift signal or -90° phase shift signal, or as -90° phase shift signal or +90° phase shift signal; After the output of the modulation switch is buffered by the voltage buffer, it provides the bias current of the giant magnetoresistive sensor; The low-pass amplifier circuit filters out the AC component and amplifies the signal; The inverter performs unity gain inversion on the output of the low-pass amplifier circuit; The high level and low level of the output signal of the square wave generator respectively select the output of the demodulation switch to be the output of the low-pass amplifier circuit and the output of the inverter, or to be the output of the inverter and the output of the low-pass amplifier circuit; The output of the demodulation switch is filtered by a low-pass filter to remove the AC component, and the signal output related only to the metal eddy current is obtained; The output voltage of giant magnetoresistance sensor 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 _e I _a (1) In formula (1), k, B _a , B _ε , B _b , γ, and I _a are the magnetic field sensitivity coefficient of the giant magnetoresistance sensor, the AC magnetic field generated by the excitation current, the metal eddy current magnetic field, the environmental magnetic field, and the magnetic field independent of the magnetic field. The equivalent zero bias resistance and the bias current of the giant magnetoresistive sensor; from formula (1), it can be seen that the AC magnetic field generated by the excitation current, the ambient magnetic field, and the equivalent zero bias resistance independent of the magnetic field will affect the sensitivity of metal eddy current signal detection. influences; Let the inductance of the excitation coil be L ₁ , the resistance of the excitation coil is R ₁ , the equivalent inductance of the metal eddy current is L ₂ , the equivalent resistance of the metal eddy current is R ₂ , the effective value of the metal eddy current is I ₂ , the mutual inductance between the excitation coil and the metal eddy current is M, the effective value of the excitation coil voltage is E ₁ , the angular frequency of the excitation voltage is ω, and the effective value of the excitation current is I ₁ , then the electrical equation: E ₁ +jωMI ₂ ＝I ₁ R ₁ +jωL ₁ I ₁ (2) jωMI ₁ ＝I ₂ R ₂ +jωL ₂ I ₂ (3) From formulas (2) and (3), we can get: <mrow><msub><mi>I</mi><mn>2</mn></msub><mo>=</mo><mfrac><mrow><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msub><mi>ML</mi><mn>2</mn></msub><mo>+</mo><msub><mi>jR</mi><mn>2</mn></msub><mi>&amp;omega;</mi><mi>M</mi></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><msub><mi>I</mi><mn>1</mn></msub><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>4</mn><mo>)</mo></mrow></mrow> The magnetic field generated by the excitation current is: B _a =λI ₁ (5) In formula (5), λ is the structural constant of the metal detector; The magnetic field generated by the metal eddy current is: <mrow><msub><mi>B</mi><mi>e</mi></msub><mo>=</mo><msub><mi>&amp;rho;I</mi><mn>2</mn></msub><mo>=</mo><mfrac><mrow><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msub><mi>ML</mi><mn>2</mn></msub><mo>+</mo><msub><mi>jR</mi><mn>2</mn></msub><mi>&amp;omega;</mi><mi>M</mi></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><mo>&amp;times;</mo><msub><mi>&amp;rho;I</mi><mn>1</mn></msub><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>6</mn><mo>)</mo></mrow></mrow> In formula (6), ρ is the structural constant of metal detector; Bring formulas (5), (6) into formula (1) to get the following formula; <mrow><msub><mi>U</mi><mi>o</mi></msub><mo>=</mo><mi>k</mi><mo>&amp;lsqb;</mo><mrow><mo>(</mo><mi>&amp;lambda;</mi><mo>+</mo><mfrac><mrow><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msub><mi>ML</mi><mn>2</mn></msub><mo>+</mo><msub><mi>jR</mi><mn>2</mn></msub><mi>&amp;omega;</mi><mi>M</mi></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><mo>&amp;times;</mo><mi>&amp;rho;</mi><mo>)</mo></mrow><msub><mi>I</mi><mn>1</mn></msub><mo>+</mo><msub><mi>B</mi><mi>b</mi></msub><mo>&amp;rsqb;</mo><mo>&amp;times;</mo><msub><mi>I</mi><mi>a</mi></msub><mo>+</mo><msub><mi>&amp;gamma;I</mi><mi>a</mi></msub><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>7</mn><mo>)</mo></mrow></mrow> <mrow><msub><mi>U</mi><mi>o</mi></msub><mo>=</mo><msub><mi>k&amp;lambda;I</mi><mn>1</mn></msub><msub><mi>I</mi><mi>a</mi></msub><mo>+</mo><msub><mi>kB</mi><mi>b</mi></msub><msub><mi>I</mi><mi>a</mi></msub><mo>+</mo><msub><mi>&amp;gamma;I</mi><mi>a</mi></msub><mo>+</mo><mfrac><mrow><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msub><mi>ML</mi><mn>2</mn></msub><mo>+</mo><msub><mi>jR</mi><mn>2</mn></msub><mi>&amp;omega;</mi><mi>M</mi></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><mo>&amp;times;</mo><msub><mi>k&amp;rho;I</mi><mn>1</mn></msub><msub><mi>I</mi><mi>a</mi></msub><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>8</mn><mo>)</mo></mrow></mrow> The first item, the second item, and the third item on the right side of the equation (8) are interference items irrelevant to metal eddy currents, and the fourth item is a signal item only relevant to metal eddy currents; in order to eliminate interference items, the present invention adopts the following method : Let the giant magnetoresistive sensor bias current _Ia have the same frequency as the excitation coil current _I1 , the phase is quadrature, and the amplitude is proportional, that is: I _a = jμI ₁ (9) <mrow><mtable><mtr><mtd><mrow><msub><mi>U</mi><mi>o</mi></msub><mo>=</mo><msub><mi>k&amp;mu;&amp;lambda;I</mi><mn>1</mn></msub><mo>*</mo><msub><mi>jI</mi><mn>1</mn></msub><mo>+</mo><msub><mi>k&amp;mu;B</mi><mi>b</mi></msub><msub><mi>I</mi><mn>1</mn></msub><mo>+</mo><msub><mi>j&amp;mu;&amp;gamma;I</mi><mn>1</mn></msub></mrow></mtd></mtr><mtr><mtd><mrow><mo>+</mo><mfrac><mrow><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msub><mi>ML</mi><mn>2</mn></msub><mi>k</mi><mi>&amp;rho;</mi><mi>&amp;mu;</mi></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></mrow>msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><msub><mi>I</mi><mn>1</mn></msub><mo>*</mo><msub><mi>jI</mi><mn>1</mn></msub><mo>-</mo><mfrac><mrow><msub><mi>R</mi><mn>2</mn></msub><msup><msub><mi>&amp;omega;Mk&amp;rho;&amp;mu;I</mi><mn>1</mn></msub><mn>2</mn></msup></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac></mrow></mtd></mtr></mtable><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>10</mn><mo>)</mo></mrow></mrow> The first, second, third, and fourth items on the right side of formula (10) are all AC components, and only the fifth item is a DC component, which is only related to metal eddy currents; Use a low-pass filter to filter out the AC component in formula (10), and get: <mrow><msub><mi>U</mi><mn>1</mn></msub><mo>=</mo><mo>-</mo><mfrac><mrow><msub><mi>R</mi><mn>2</mn></msub><msup><msub><mi>&amp;omega;Mk&amp;rho;&amp;mu;I</mi><mn>1</mn></msub><mn>2</mn></msup></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>11</mn><mo>)</mo></mrow></mrow> Equation (11) shows that using the current with the same frequency and phase quadrature as the excitation coil current is used as the bias current of the giant magnetoresistance sensor to eliminate the interference term irrelevant to the metal eddy current signal and create conditions for fully increasing the gain of the amplifier circuit; The signal after the aforementioned low-pass filtering is amplified, and the output of the amplifying circuit is: <mrow><msub><mi>U</mi><mn>2</mn></msub><mo>=</mo><mo>-</mo><mfrac><mrow><msub><mi>R</mi><mn>2</mn></msub><msup><msub><mi>&amp;omega;Mk&amp;sigma;&amp;rho;&amp;mu;I</mi><mn>1</mn></msub><mn>2</mn></msup></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><mo>+</mo><mi>&amp;Delta;</mi><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>12</mn><mo>)</mo></mrow></mrow> In formula (12), σ and ▽ are the gain of the amplifier circuit and the output DC offset voltage of the amplifier circuit, respectively; Although through the above method, the metal eddy current signal is converted into a DC signal, and the interference term is eliminated, so that the gain of the amplifier circuit is improved, but the sensitivity of metal detection is still limited due to the DC offset component of the amplifier circuit; In order to eliminate the limitation of the DC offset voltage of the amplifying circuit to the metal detection sensitivity, the present invention further adopts the following method: Make the giant magnetoresistive sensor bias current I _a switch between juI ₁ and -juI ₁ at an angular frequency ω ₀ , which is equivalent to using a duty cycle of 50% with a unit amplitude of +1 and unit amplitude of -1 at an angular frequency of ω ₀ After multiplied by the signal of juI ₁ , it is used as the bias current I _a of the giant magnetoresistance sensor, that is, to modulate the quadrature phase of the bias current of the giant magnetoresistance sensor relative to the excitation coil current, then the output of the amplifying circuit is: <mrow><msub><mi>U</mi><mn>3</mn></msub><mo>=</mo><mfrac><mrow><msub><mi>R</mi><mn>2</mn></msub><msup><msub><mi>&amp;omega;Mk&amp;sigma;&amp;rho;&amp;mu;I</mi><mn>1</mn></msub><mn>2</mn></msup></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><mo>&amp;times;</mo><mfrac><mn>4</mn><mi>&amp;pi;</mi></mfrac><munderover><mo>&amp;Sigma;</mo><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mi>&amp;infin;</mi></munderover><mfrac><msup><mrow><mo>(</mo><mo>-</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msup><mrow><mn>2</mn><mi>n</mi><mo>-</mo><mn>1</mn></mrow></mfrac><mi>c</mi><mi>o</mi><mi>s</mi><mrow><mo>(</mo><mn>2</mn><mi>n</mi><mo>-</mo><mn>1</mn><mo>)</mo></mrow><msub><mi>&amp;omega;</mi><mn>0</mn></msub><mi>t</mi><mo>+</mo><mi>&amp;Delta;</mi><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>13</mn><mo>)</mo></mrow></mrow> Multiply the signal with the unit amplitude +1, unit amplitude -1 duty cycle 50% angular frequency ω ₀ and the output of the amplifier circuit for demodulation, and get: <mrow><mtable><mtr><mtd><mrow><msub><mi>U</mi><mn>4</mn></msub><mo>=</mo><mo>&amp;lsqb;</mo><mfrac><mrow><msub><mi>R</mi><mn>2</mn></msub><msup><msub><mi>&amp;omega;Mk&amp;sigma;&amp;rho;&amp;mu;I</mi><mn>1</mn></msub><mn>2</mn></msup></mrow><mrow><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup></mrow></mfrac><mo>&amp;times;</mo><mfrac><mn>4</mn><mi>&amp;pi;</mi></mfrac><munderover><mo>&amp;Sigma;</mo><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mi>&amp;infin;</mi></munderover><mfrac><msup><mrow><mo>(</mo><mo>-</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msup><mrow><mn>2</mn><mi>n</mi><mo>-</mo><mn>1</mn></mrow></mfrac><mi>c</mi><mi>o</mi><mi>s</mi><mrow><mo>(</mo><mn>2</mn><mi>n</mi><mo>-</mo><mn>1</mn><mo>)</mo></mrow><msub><mi>&amp;omega;</mi><mn>0</mn></msub><mi>t</mi><mo>+</mo><mi>&amp;Delta;</mi><mo>&amp;rsqb;</mo></mrow></mtd></mtr><mtr><mtd><mrow><mo>&amp;times;</mo><mfrac><mn>4</mn><mi>&amp;pi;</mi></mfrac><munderover><mo>&amp;Sigma;</mo><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mi>&amp;infin;</mi></munderover><mfrac><msup><mrow><mo>(</mo><mo>-</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>n</mi><mo>+</mo><mn>1</mn></mrow></msup><mrow><mn>2</mn><mi>n</mi><mo>-</mo><mn>1</mn></mrow></mfrac><mi>c</mi><mi>o</mi><mi>s</mi><mrow><mo>(</mo><mn>2</mn><mi>n</mi><mo>-</mo><mn>1</mn><mo>)</mo></mrow><msub><mi>&amp;omega;</mi><mn>0</mn></msub><mi>t</mi></mrow></mtd></mtr></mtable><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>14</mn><mo>)</mo></mrow></mrow> The demodulated output is low-pass filtered to obtain: <mrow><msub><mi>U</mi><mn>5</mn></msub><mo>=</mo><mfrac><mrow><mn>8</mn><msub><mi>R</mi><mn>2</mn></msub><msup><msub><mi>&amp;omega;Mk&amp;sigma;&amp;rho;&amp;mu;I</mi><mn>1</mn></msub><mn>2</mn></msup></mrow><mrow><mo>(</mo><msup><msub><mi>R</mi><mn>2</mn></msub><mn>2</mn></msup><mo>+</mo><msup><mi>&amp;omega;</mi><mn>2</mn></msup><msup><msub><mi>L</mi><mn>2</mn></msub><mn>2</mn></msup><mo>)</mo><msup><mi>&amp;pi;</mi><mn>2</mn></msup></mrow></mfrac><munderover><mo>&amp;Sigma;</mo><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mi>&amp;infin;</mi></munderover><mfrac><mn>1</mn><msup><mrow><mo>(</mo><mn>2</mn><mi>n</mi><mo>-</mo><mn>1</mn><mo>)</mo></mrow><mn>2</mn></msup></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>15</mn><mo>)</mo></mrow></mrow> Equation (15) shows that after modulating the quadrature phase of the bias current of the giant magnetoresistive sensor relative to the excitation coil current, and synchronously demodulating and low-pass filtering the output of the amplifier circuit, the DC offset of the amplifier circuit is eliminated. Thereby improving the sensitivity of metal detection. 2. A metal detection method based on giant magnetoresistance and quadrature bias modulation, for the metal detection device based on giant magnetoresistance and quadrature bias modulation as claimed in claim 1, it is characterized in that comprising the following steps: 1. Apply an AC current with an angular frequency ω to the excitation coil; 2. Phase-shift the AC current of the excitation coil by 90° to obtain the signal Ia1, and shift the phase of the AC current of the excitation coil by -90° to obtain the signal Ia2; 3. Generate a selection signal with an angular frequency ω ₀ and a duty cycle of 50%, and periodically select the bias current of the giant magnetoresistance sensor as Ia1 or Ia2, that is, to modulate the quadrature phase of the bias current of the giant magnetoresistance sensor relative to the excitation coil current ; 4. Perform low-pass filtering and amplification on the output signal of the giant magnetoresistance sensor; 5. The amplified signal is divided into two channels, one channel remains unchanged and is U1, and the other channel is unit gain inverting and is U2; 6. Select U1 or U2 as the demodulation output with the selection signal period of the aforementioned angular frequency ω ₀ and a duty cycle of 50%; Seventh, perform low-pass filtering on the above-mentioned demodulation output to obtain an output signal related only to the metal eddy current.