JP2002006016A - Magnetic sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic sensor which is stable against environmental change by removing error factors caused by temperature change and aging. SOLUTION: There are provided a magnetic impedance element 2 representing change in impedance affected by magnetic field, a constant voltage circuit for applying a high-frequency constant voltage to the magnetic impedance element 2, a bias coil for applying from outside a DC bias magnetic field to the magnetic impedance element 2, a constant current circuit to provide a DC constant current to the bias coil, and an output amplifying circuit 3 which amplifies both-end voltage of the magnetic impedance element. The magnetic impedance element 2 is applied with a bias magnetic field which alternates positive direction and reverse direction at each constant period.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic sensor using a magnetic impedance element (abbreviation: MI element), and more particularly to a magnetic sensor capable of obtaining a stable output with respect to a change in ambient temperature and a change with time.

[0002]

2. Description of the Related Art In recent years, miniaturization and high performance of electronic equipment have been rapidly advanced.
With the miniaturization and increase in capacity of hard disks, magnetic heads utilizing the magnetoresistance effect (abbreviation: MR heads) have been used instead of the conventional heads utilizing the change in magnetic flux density.
Is being used for reading.

However, when an element having such a magnetoresistive effect is used, as a basic characteristic, a change in electric characteristics (に 対 し て R / Rs) with respect to a change in an external magnetic field is evaluated as a performance. It is desired that the value is large.

On the other hand, for example, when performing measurement and detection of a minute magnetic field such as measurement of terrestrial magnetism or measurement of a magnetic field in the brain, the above-described MR head or a giant magnetoresistive sensor (abbreviated name) recently attracting attention. , GMR sensor) is used,
In the case of such an MR head or GMR sensor, there is a disadvantage that various characteristics such as detection magnetic field sensitivity, temperature stability, and hysteresis characteristics are not practically sufficient. Then, in order to solve such a problem, a magnetic sensor using a magnetic impedance effect has been proposed.

[0005] The magnetic impedance effect refers to a phenomenon in which the impedance of a magnetic material to which a high-frequency current is applied changes with respect to an external magnetic field due to its skin effect. The impedance Z of the magnetic body is as follows: R = DC resistance component, L = proportional constant of inductance, and ω = angular frequency of AC current flowing through the magnetic body. Z = R + jωL ... (1) In the magnetic impedance effect due to the change in magnetic permeability, the inductance component (ω
L) changes.

FIG. 5 is a principle diagram showing a magnetic detection operation in a magnetic sensor using a conventional magnetic impedance element. At present, a magnetic sensor using a magnetic impedance element applies a DC bias magnetic field in a predetermined direction to the element, and determines a magnetic quantity and a magnitude according to the magnitude and polarity of the output with reference to the output of the magnetic impedance element at the time of the bias magnetic field. Detects magnetic polarity.

The principle will be described with reference to FIG.
+ H to the magnetic field of ias₁And the magnetic impedance at that time
Element output to V₁And In this state, the external magnetic field H₂si
When nωt is applied, the magnetic impedance element is marked.
The applied magnetic field is (H₁+ H ₂sinωt)
World (H₁+ H₂), The output of the magnetic impedance element is V
₂Then, the output Vout 'of the magnetoresistive element is expressed by the following equation.
Is done. Vout '= V₁+ (V₂-V₁) Sinωt ・ ・ ・ ・ ・ ・ ・ (2)

Accordingly, the output voltage V ₁ of the magneto-impedance element in the initial state of the magnetic sensor (state of zero external magnetic field).
, The second term (V ₂ −V ₁ ) sin of equation (2)
The magnitude and polarity of the external magnetic field can be determined based on ωt.

FIG. 6 shows an example of an amplifier circuit for amplifying an output from a magnetic sensor using a conventional magnetic impedance element. Normally, the magnetic field applied from the outside is not limited to the AC magnetic field, and there is naturally a DC magnetic field (the geomagnetism belongs to the DC magnetic field). For this reason, the magnetic sensor needs to have a DC response, and Vou expressed by the equation (2) is used.
It is necessary to DC-amplify t '.

The output of the magneto-impedance element of the magnetic sensor is usually very small, and is usually amplified by an amplifier circuit using an IC as shown in FIG. 6 for the above reasons. In the example of FIG. 6, the output of the magneto-impedance element is amplified by an amplifier circuit using IC ₁ ′, and the variable resistance VR
The offset voltage (output when the external magnetic field is zero) of the amplifier circuit is set by ₁ ′ and VR ₂ ′, and the amplification degree is set by the variable resistor VR ₃ ′.

[0011]

The magneto-impedance elements each have a resistance-temperature characteristic.
As a result, the output voltage of the magneto-impedance element (the output voltage of the magneto-impedance element when the bias magnetic field is zero) changes depending on the temperature. Similarly, with the aging,
The output voltage of the magneto-impedance element changes.

This change is caused by the fact that V ₁ and V
₂ contributes to both, and if the variation of the magnetic impedance output voltage is set to ΔV, the magnetic impedance output voltage Vou
t 'is represented by the following equation. Vout ′ = (V ₁ + △ V) + ｛(V ₂ + △ V) − (V ₁ + △ V)｝ sin ω t = (V ₁ + △ V) + (V ₂ −V ₁ ) sin ωt...・ ・ (3)

As is apparent from the equation (3), an error occurs in the reference voltage when the external magnetic field of the magnetic sensor is zero, and the error is amplified due to the configuration of the amplifier circuit (DC response is required). Has the disadvantage that the offset output error appears. The temperature change and the change over time of the output voltage of the magneto-impedance element are:
Since the polarity of the change can be both positive and negative, it cannot be corrected.

Therefore, if the calculation of the expression (3) is performed in a state where the error is included, an error occurs in the calculation of the magnetic quantity, and it is impossible to calculate the accurate calculation of the magnetic quantity.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a magnetic sensor that is stable against changes in the surrounding environment by eliminating the causes of errors due to temperature changes and changes over time of the magnetic sensor.

[0016]

The magnetic sensor of the present invention comprises:
A magnetic impedance element that causes a change in impedance when subjected to a magnetic field, a constant voltage circuit for applying a high-frequency constant voltage to the magnetic impedance element, and a bias coil for externally applying a DC bias magnetic field to the magnetic impedance element A constant current circuit for flowing a DC constant current to the bias coil, and a magnetic sensor including an amplifier for amplifying the voltage across the magnetic impedance element, as a bias magnetic field applied to the magnetic impedance detection element, By applying a bias magnetic field in the forward and reverse directions for a certain time, the output of the magnetic impedance element is amplified by an amplifier circuit including a high-pass filter circuit,
The amplified output voltage is compared by detecting the difference between the signals detected synchronously in the period in which the forward and reverse bias magnetic fields are applied, and detecting the difference between the signals synchronously detected in the period in which the forward and reverse bias magnetic fields are applied. It is intended to simply and easily remove a temperature drift and a change with time of an offset output voltage of a magnetic sensor to make the magnetic sensor stable with respect to a change in the surrounding environment.

Accordingly, the present invention provides a magneto-impedance element that changes impedance when receiving a magnetic field, a constant-voltage circuit for applying a high-frequency constant voltage to the magneto-impedance element, A magnetic coil comprising: a bias coil for applying a bias magnetic field; a constant current circuit for flowing a DC constant current to the bias coil; and an output amplifier circuit for amplifying a voltage across the magnetic impedance element. This is a magnetic sensor that applies forward and reverse bias magnetic fields for a certain period of time as a bias magnetic field applied to the element.

Further, the present invention is the magnetic sensor wherein the output amplifying circuit is constituted by an amplifying circuit including at least one high-pass filter circuit.

Further, according to the present invention, the output amplifier circuit includes a synchronous sensor circuit for synchronously detecting an output signal from the output amplifier circuit at a period in which a forward and a reverse bias magnetic field is applied. It is.

Further, the present invention is a magnetic sensor including an arithmetic circuit for detecting a difference between signals synchronously detected in a period in which forward and reverse bias magnetic fields are applied.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a magnetic sensor according to an embodiment of the present invention will be described.

FIG. 1 is a principle diagram showing the magnetic detection operation of the magnetic impedance element used in the magnetic sensor of the present invention when the external magnetic field is zero. Between two terminals of the bias coil disposed in the magnetic impedance elements, when a bias voltage is applied in forward and reverse directions to that predetermined time, the said magnetic impedance element, AC rectangular wave bias magnetic field polarity magnetic field amount H ₁ is Applied.

As shown in FIG. 1, the output characteristic of the magneto-impedance element with respect to the magnetic field is symmetric with respect to the output Vout axis of the magneto-impedance element. Then, the same value is obtained as the output of the magnetic impedance element.

Hereinafter, when expressed by a mathematical formula, the output of the magnetic impedance element when the AC rectangular wave bias magnetic field is + H ₁ is represented by V
out _1, when the AC rectangular wave bias magnetic field -H ₁ Vout ₂ the magnetic impedance element output o'clock, magnetic impedance element output in the case of FIG. 1 the external magnetic field is zero, positive and negative bias magnetic field at both the bias magnetic field H ₁ output V ₁ is obtained corresponding to. Vout ₁ = V ₁ (4) Vout ₂ = V ₁ (5)

At this time, the output representing the magnitude and polarity of the external magnetic field detected by the magneto-impedance element is expressed by the following equation.
It is represented by (6). Vout ₃ = Vout ₁ −Vout ₂ (6) Therefore, Vout _{3 in} the case of FIG. 1 (when the external magnetic field is 0)
Becomes Vout ₃ = 0, and becomes an output irrelevant to changes in the surrounding environment such as offset output voltage temperature drift and aging.

FIG. 2 is a principle diagram showing a magnetic detection operation of the magnetic sensor when an external magnetic field + H ₂ is applied to the magnetic impedance element used in the magnetic sensor of the present invention. With respect to the state of FIG. 1, the amount of the magnetic field applied to the magneto-impedance element changes due to the application of the external magnetic field + H ₂ , and the magnetic field applied to the magneto-impedance element at the time of the AC rectangular bias magnetic field + H ₁ is (H ₁ +
H ₂ ), the magnetic field applied to the magnetic impedance element at the time of the AC rectangular bias magnetic field −H ₁ is (−H ₁ + H ₂ ).

Assuming that the output of the magnetic impedance element at the time of the magnetic field (H ₁ + H ₂ ) is V ₂ and the output of the magnetic impedance element at the time of the magnetic field (−H ₁ + H ₂ ) is V ₃ , Vout ₁ and Vout
out ₂ is expressed by the following equation. _{Vout 1 = V 1 + (V} 2 -V 1) ········ (7) Vout 2 = V 1 + (V 3 -V 1) ········ (8)

That is, in both the magnetic field (H ₁ + H ₂ ) and the magnetic field (−H ₁ + H ₂ ), the output of the magnetic impedance element is centered on V ₁ (the output of the magnetic impedance element when the external magnetic field is zero). Is changed.

The output representing the magnitude and polarity of the external magnetic field detected by the magneto-impedance element is given by
Since it is expressed by (6), Vout ₃ in the case of FIG. 2 is as follows: Vout ₃ = V ₂ −V ₃ (9)

Further, due to a change in the external environment, V ₂ → V
₂ + ΔV, Vout when changing from V ₃ → V ₃ + ΔV
₃ is as follows: Vout ₃ = (V ₂ + ΔV) − (V ₃ + ΔV) = V ₂ −V ₃ (10)

As is apparent from equations (9) and (10), V
Out ₃ is always an output irrelevant to changes in the surrounding environment such as offset output voltage temperature drift and changes over time, so that magnetic detection can be performed without being affected by changes in the external environment.

FIG. 3 is a diagram showing an embodiment of an output amplifier circuit of a magnetic impedance element used in a magnetic sensor according to the present invention. The output of the magnetic impedance element is represented by a resistor R ₁ ,
R ₂ , first-stage amplification is performed by a differential amplifier circuit composed of an integrated circuit IC ₁ , and a high-pass filter circuit including a capacitor C ₁ and a resistor R ₃ connected to an output terminal of the integrated circuit IC ₁ is used to amplify the integrated circuit IC ₁ and the magnetic field. A DC amplification voltage component such as an offset voltage of the impedance element and a temperature change or a temporal change of the offset voltage is removed, and the signal is amplified by a common-mode amplifier circuit including an amplification degree adjustment resistor VR ₃ , a resistor R ₄ , and an integrated circuit IC _2. It is a two-stage amplifier circuit.

[0033] than the frequency of the AC rectangular wave bias magnetic field to the cut-off frequency of the high-pass filter circuit composed of a capacitor C ₁ and a resistor R _3, if fairly small set, amplifier output becomes a square wave output, the magneto-impedance element output Amplification becomes possible. VR ₁ and VR ₂ are adjustment resistors for arbitrarily setting the offset voltage of the circuit, and VR ₃ is an adjustment resistor for arbitrarily setting the amplification degree of the magneto-impedance element output amplifier circuit. It is.

FIG. 4 is a circuit functional block diagram from the magnetic impedance element 2 in the magnetic sensor of the present invention to the output (V ₂ ) of the magnetic sensor. An arithmetic unit 8 that performs synchronous detection on the output of the magnetic impedance element passed through the output amplifier circuit 3 when the AC rectangular wave bias magnetic field applied to the magnetic impedance element is positive and negative, and calculates the difference between the two synchronous detection outputs. Through which the output (V ₂ ) of the magnetic sensor is obtained.

Although the block function of FIG. 4 may be performed only by circuit processing, the number of components can be reduced by combining a microcomputer, and the size can be reduced.

As described above, as described above, a magneto-impedance element that causes a change in impedance when subjected to magnetism, a constant-voltage circuit for applying a high-frequency constant voltage to the magneto-impedance element, The magnetic sensor includes a bias coil for applying a DC bias magnetic field, a constant current circuit for flowing a DC constant current to the bias coil, and an amplifier for amplifying a voltage across the magnetic impedance element. As a bias magnetic field to be applied to the element, positive and reverse bias magnetic fields are applied for a certain period of time to amplify the output of the magneto-impedance element by an amplifier circuit including a high-pass filter, and to amplify the amplified output voltage in the positive and reverse directions. Synchronous detection is performed at the period when the magnetic field is applied. By detecting the difference between the signals synchronously detected in the cycle in which the bias magnetic field is applied, the offset output voltage temperature drift and time-dependent change of the magnetic sensor can be relatively easily removed, and stable with respect to changes in the surrounding environment. It is possible to provide a magnetic sensor.

[0037]

As described above, according to the present invention, an error factor due to a temperature change and a time-dependent change is eliminated, and an offset output voltage temperature drift and a time-dependent change of the magnetic sensor are removed, and the magnetic field is stable against a change in the surrounding environment. A sensor can be provided.

[Brief description of the drawings]

FIG. 1 is a principle diagram showing a magnetic detection operation of a magnetic impedance element used in a magnetic sensor of the present invention when an external magnetic field is zero.

[Figure 2] in the magnetic impedance element used in the magnetic sensor of the present invention, when the external magnetic field H ₂ is applied, the principle view of a magnetic detecting operation of the magneto-impedance element.

FIG. 3 is a diagram showing one embodiment of an output amplifier circuit in the magnetic sensor of the present invention.

FIG. 4 is a circuit functional block diagram of the magnetic sensor of the present invention.

FIG. 5 is a principle diagram showing a magnetic detection operation of a magnetic impedance element used in a conventional magnetic sensor.

FIG. 6 is a diagram showing an example of an output amplifier circuit in a conventional magnetic sensor.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 magnetic impedance element oscillation section 2 magnetic impedance element 3 output amplification circuit 4 magnetic impedance element bias circuit generation section 5 AC rectangular wave oscillation section 6, 7 synchronous detection section 8 arithmetic circuit

Claims (4)

[Claims] 1. A magneto-impedance element that changes impedance when receiving a magnetic field, a constant-voltage circuit for applying a high-frequency constant voltage to the magneto-impedance element, and an external DC bias magnetic field applied to the magneto-impedance element And a constant current circuit for supplying a DC constant current to the bias coil, and an output amplifier circuit for amplifying a voltage across the magnetic impedance element. A magnetic sensor wherein a bias magnetic field in a forward direction and a reverse direction is applied for a certain time as a bias magnetic field. 2. The magnetic sensor according to claim 1, wherein the output amplifier circuit includes an amplifier circuit including at least one high-pass filter circuit. 3. The output amplifying circuit includes a synchronous detection circuit for synchronously detecting an output signal from the output amplifying circuit at a period in which a forward and reverse bias magnetic field is applied. Item 3. The magnetic sensor according to item 1 or 2. 4. The amplifier according to claim 1, wherein said amplifier includes an arithmetic circuit for detecting a difference between signals synchronously detected in a period in which forward and reverse bias magnetic fields are applied. A magnetic sensor as described in