JPH09318719A - Magnetic sensor circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To raise sensing performance by comparing the frequency of a Colpitts oscillator using a magnetic impedance element with the frequency of a reference- signal oscillator which has been corrected for external electromagnetic wave noise. SOLUTION: A magnetic impedance element 1 is connected between the collector and the base of transistor Tr. Due to impedance variation of magnetic impedance element 1, the oscillation frequency of Colpitts oscillator 8 varies. By counting this oscillation frequency, external magnetic field can be measured. The frequency of the Colpitts oscillator 8 and the frequency of a reference signal oscillator 6 are compared with a frequency comparator 7. From the variation of the frequency, the variation of external magnetic field can be exactly read. In order to eliminate unnecessary electromagnetic wave noise, an antenna for detecting the electromagnetic noise or a hollow coil is assembled in the reference signal oscillator 6, which corrects the output of the reference signal oscillator 6.

Description

Description: TECHNICAL FIELD The present invention relates to a magnetic sensor circuit using a magneto-impedance element for detecting an external magnetic field, and to shielding unnecessary electromagnetic wave noise that enters the magnetic sensor circuit. .

[Prior Art] Magnetic sensors currently in practical use in measurement control equipment, AV equipment, computers and the like include Hall effect elements, magnetoresistive effect elements, and the like. Among them, the magnetoresistive effect element uses the characteristic that the electric resistance changes according to an externally applied magnetic field. The magnetoresistive effect element that is currently put into practical use has an electrical resistance (MR
The amount of change in (value) is small, and its application was limited to equipment in which a large change in externally applied magnetic field occurs. Further, it is weak against temperature environment and its use range is limited to environmental conditions.
On the other hand, when a high-frequency current is passed through the magnetic body, the skin effect acts to excite the magnetic body in the radial direction. Then, the magneto-impedance element having the property that the magnetic permeability and the electric resistance of the magnetic material as viewed from both ends of the magnetic material through which a high-frequency current flows are changed by an external magnetic field (Japanese Patent Laid-Open No. Hei 7 (1999) -83945).
181239) has been proposed.

[Problems to be Solved by the Invention] However, since a magnetic sensor circuit using this magnetic impedance element causes a current of a very high frequency to flow through a magnetic material, a high frequency electromagnetic wave is generated from the magnetic impedance element, the connection cable, and the oscillation circuit section. There is a problem that it radiates in the air as noise. Further, there is a problem that the magnetic sensor is easily affected by electromagnetic noise from the outside, the S / N ratio (signal-to-noise ratio) of the magnetic sensor is deteriorated, and the magnetic detection sensitivity is lowered. Therefore, in order to cancel the common mode electromagnetic wave noise that enters from the outside,
You can configure multiple magneto-impedance elements in a differential type,
It was designed by integrating the oscillator circuit and the magneto-impedance element to eliminate the part affected by electromagnetic noise as much as possible. In addition, radio waves were partially shielded by non-magnetic materials. Further, the recent demands for application have been such that the detected magnetic signal has become smaller, and the space space in which the magnetic sensor head is incorporated has become very small. Therefore, it has become difficult to use the circuit unit integrally with it. That is, it has become necessary to have a structure in which the magneto-impedance element and the circuit section are separated from each other. For this reason, it becomes more susceptible to electromagnetic noise, and countermeasures against it have become necessary.

[Means for Solving the Problem] That is, in the magnetic sensor circuit according to the present invention, a magnetic impedance element whose magnetic permeability and electric resistance are changed by an external magnetic field is provided between a base and a collector of a transistor constituting a Colpitts oscillator. It is to provide a magnetic sensor circuit that is connected and compares its oscillation frequency with a reference signal oscillator. Further, the reference signal oscillator has a configuration for detecting external electromagnetic noise and correcting the oscillation frequency. is there. Similarly, the magneto-impedance element is connected between the output circuit section of the multivibrator oscillator and the ground. Further, the Colpitts oscillator circuit section, the multivibrator oscillator and the magnetic impedance element are separated from each other, and the connection cable thereof is configured to combine radio wave shielding with a non-magnetic material and magnetic shielding with a magnetic material. Furthermore, the entire Colpitts oscillator and the entire multivibrator oscillator are shielded by a non-magnetic material and a magnetic material.
It is a magnetic sensor covered with a shield case composed of laminated plate materials.

[Operation] In the magnetic sensor circuit of the present invention, even if the magneto-impedance element is excited by a high-frequency current, it is possible to reduce the influence of the electromagnetic-wave noise dissipated by the high-frequency current or the electromagnetic-wave noise entering from the outside. The detection sensitivity of can be increased. Further, it is possible to separate the sensor head portion of the magneto-impedance element from the circuit portion and use it for detecting a magnetic field in a minute space.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows an external perspective view of a magneto-impedance element 1 according to an embodiment of the present invention. The magnetic body 2 is made of an amorphous material exhibiting ferromagnetic properties, and has a linear shape, a tubular shape, or a strip shape. In this example, the magnetic body 2 was an amorphous wire having a length of about 5 mm and a diameter of about 30 μm. Both ends of the magnetic body 2 having the above-described shape are welded and fixed to the substrate 3 by the solder fixing portions 4. A conductor land (not shown) of a through hole connected to the solder fixing portion 4 is provided on the board 3, and is guided to the back surface of the board 3. Then, on the back surface of the substrate 3, the printed wiring pattern is connected to the pin terminals 5. The size of the substrate 3 may be such that the magnetic body 2 can be mounted, but in this embodiment, a glass epoxy substrate having a length of about 10 mm, a width of about 4 mm and a plate thickness of about 0.8 mm was used. A ceramic substrate is suitable as the material of the substrate 3, but if the frequency of the high-frequency current flowing through the magnetic body 2 is low, the glass epoxy substrate will not affect the magnetic detection characteristics. A high-frequency current of several MHz or more is applied to the magnetic body 2 from the pin terminals 5 and 5'to generate a skin magnetic field on the surface of the magnetic body 2. By applying an external magnetic field in the axial direction of the magnetic body 2, the magnetic permeability and electric resistance of the magnetic body 2 change. That is, the impedance between the pin terminals 5 and 5'changes due to the external magnetic field. The magnetic field detection direction of the magnetic sensor circuit according to the present invention is the same as the major axis direction of the current flowing through the magnetic body 2. FIG. 2 shows a Colpitts oscillation circuit section 8 in which the magneto-impedance element 1 is connected between the collector and the base of the transistor Tr. The oscillation frequency of the Colpitts oscillator 8 changes due to the impedance change of the magneto-impedance element 1. The external magnetic field can be measured by counting the oscillation frequency. In this embodiment, the oscillation frequency is from 8 MHz to 2
0 MHz was oscillated. A frequency comparator 7 compares the frequency of the Colpitts oscillator 8 with the frequency of the reference signal oscillator 6. The amount of change in the external magnetic field can be accurately read from the amount of change in frequency. A minute magnetic field can be accurately detected by converting the voltage into a voltage by an FV converter or the like and further amplifying the voltage. Since the magneto-impedance element 1 also detects unnecessary external electromagnetic wave noise, it is necessary to remove the unnecessary electromagnetic wave noise in order to obtain highly accurate detection performance. Therefore, an antenna or an air-core coil for detecting unnecessary external electromagnetic wave noise is incorporated in the reference signal oscillator 6, and only unnecessary electromagnetic wave noise in the detection direction of the magneto-impedance element 1 is detected. It has a function to correct the output of. With this function, it is possible to cancel unnecessary external electromagnetic wave noise that enters the magneto-impedance element 1 and the reference signal oscillator 6 as common mode noise. Since a high-frequency current is passed through the magneto-impedance element 1 to excite the magnetic body 2 at a high frequency, it is possible to detect an external magnetic field up to a very high frequency. But on the other hand,
In order to prevent the high frequency external magnetic field from penetrating not only the magneto-impedance element 1 but also the connection cable and the circuit section, as shown in the block diagram of the sensor configuration of FIG. 3, the magneto-impedance element 1 and the circuit separated from the circuit section are provided. A connection cable shield 13 in which a strip of a thin copper plate or a thin aluminum plate and a permalloy thin plate is half-overlapped is spirally wound around the connection cable 9 with the portion 11. As shown in FIG. 4, the circuit portion is formed by laminating a copper plate and a thin plate of permalloy to form a shield case 10, which covers and shields the entire circuit portion 11. The output signal from the sensor circuit is output from the output terminal 14. In this embodiment, the output voltage is taken out by the FV converter. FIG. 5 shows the measurement results of this example. Frequency of 10 MH for magneto-impedance element 1
A high-frequency current of z 1 was passed, the magneto-impedance element 1 was installed in the east-west direction, an externally applied magnetic field was applied with a solenoid coil, and the output voltage was measured at the output terminal 14. Good data are also obtained on the output voltage with respect to the applied magnetic field and the linearity of the output voltage. Further, the noise levels were measured by comparing the connection cable 9 and the circuit section 11 with and without the application of a magnetic field applied with a shield measure. FIG.
The point A is the noise level when no shield measure is taken. Similarly, the point B is the noise level when the shield measure is taken, and is reduced from 1/5 to 1/10 of the level of the point A. That is, the detection performance is improved. On the other hand, there is a limit to the distance for separating the magneto-impedance element 1 from the circuit section 11, and in this embodiment, it is about 10c.
When m is the limit, the output signal due to the detected external magnetic field sharply decreases and the S / N ratio deteriorates when the distance is further increased. FIG. 6 is a circuit diagram in which the magneto-impedance element 1 is connected to the output part of the multi-vibrator oscillator 15 configured by an integrated circuit. The magneto-impedance element 1 oscillates a rectangular wave having a fast rising edge, and an external magnetic field applied to the magneto-impedance element 1 The peak value of the rectangular wave changes. The shield of the connection cable and the shield case that covers the entire circuit in this multivibrator oscillator have the same method as that of the Colpitts oscillator described above.

[Advantages of the Invention] As described above, according to the present invention, the frequency of a Colpitts oscillator using a magneto-impedance element is compared with the frequency of a reference signal oscillator in which external electromagnetic noise is corrected, and a connection cable and a circuit. High-performance detection performance can be obtained by providing radio wave and magnetic shielding of the part. Further, by separating the magnetic impedance element and the circuit portion from each other, the magnetic sensor head can be downsized and its application is expanded.

[Brief description of drawings]

FIG. 1 is an external perspective view of a magneto-impedance element according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a Colpitts oscillator configured by using the magneto-impedance element of FIG.

FIG. 3 is a block diagram of a magnetic sensor circuit according to an embodiment of the present invention.

FIG. 4 is an external perspective view of a circuit unit according to an embodiment of the present invention. (Part of the inside is shown broken.)

FIG. 5 is a characteristic correlation diagram of an external magnetic field applied to a magneto-impedance element and an output voltage according to an embodiment of the present invention.

6 is a circuit diagram of a multivibrator oscillator using the magneto-impedance element of FIG.

[Explanation of symbols]

 1 Magneto-impedance element 2 Magnetic material 3 Board 4 Solder fixing part 5 Pin terminal 6 Reference signal oscillator 7 Frequency comparator 8 Colpitts oscillator 9 Connection cable 10 Shield case 11 Circuit part 12 Shield case base 13 Connection cable shield 14 Output terminal 15 Multivibrator oscillator

Claims (5)

[Claims] 1. A magnetic sensor circuit using a magneto-impedance element whose magnetic permeability and electric resistance change by the application of an external magnetic field, wherein the magneto-impedance element is connected between the base and collector of a transistor constituting a Colpitts oscillator, A magnetic sensor circuit characterized by comparing the oscillation frequency of a Colpitts oscillator with a reference signal oscillator. 2. A magnetic sensor circuit in which a magnetic impedance element is connected between the output of the multivibrator oscillator and the ground. 3. A magnetic sensor circuit, wherein the reference signal oscillator according to claim 1 is provided with a circuit for detecting an external electromagnetic wave noise and correcting the oscillation frequency of the reference signal oscillator. 4. The magneto-impedance element and the Colpitts oscillator according to claim 1, and the magneto-impedance element and the multivibrator oscillator according to claim 2, which are separated from each other,
The connection cable of the connecting portion is shielded by spirally winding a thin strip of non-magnetic material and a laminated strip of thin strips of magnetic material in a spiral shape. Magnetic sensor circuit. 5. The radio wave shield and the magnetic shield according to claim 1, wherein the Colpitts oscillator and the multivibrator oscillator are covered with a shield case formed of a plate material in which a non-magnetic material and a magnetic material are laminated to provide radio wave shielding and magnetic shielding. Magnetic sensor circuit.