Electricity: measuring and testing – Magnetic – Magnetometers
Patent
1992-01-14
1994-02-15
Snow, Walter E.
Electricity: measuring and testing
Magnetic
Magnetometers
324260, G01R 3304, G01C 1728
Patent
active
052870596
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a magnetism detecting apparatus incorporating a saturable type magnetic sensor and, more particularly, to a magnetism detecting apparatus in which a detection coil wound around a core is incorporated, as a resonance circuit, in an oscillation circuit so that the oscillation circuit acquires both an excitation function of exciting a magnetic sensor and a detection function of extracting a detection signal from the magnetic sensor.
BACKGROUND ART
A magnetism detecting apparatus incorporating a saturable type magnetic sensor can detect magnetism with high precision (Published Unexamined Japanese Patent Application No. 1-308982).
FIG. 5 is a block diagram showing a schematic arrangement of a magnetism detecting apparatus using a saturable type magnetic sensor. A rectangular wave generating circuit 1 outputs an AC rectangular signal e.sub.1 having a predetermined period T.sub.1, as shown in FIG. 6. A 0-volt line is located in the middle of the signal waveform of the rectangular signal e.sub.1. The rectangular signal e.sub.1 output from the rectangular wave generating circuit 1 is input to a differentiating circuit 2. The differentiating circuit 2 differentiates the rectangular signal e.sub.1. As a result, the differentiating circuit 2 outputs a pulse signal having a trigger waveform, which is synchronized with the leading and trailing edges of a rectangular wave included in the rectangular signal e.sub.1.
The pulse having the trigger waveform, output from the differentiating circuit 2, is applied, as an AC excitation signal e.sub.2, to a magnetic sensor 4 through an impedance element 3 constituted by a resistor. This magnetic sensor 4 is constituted by a rod-like ferromagnetic core 5 and a detection coil 6 wound around the core 5. The AC excitation signal e.sub.2 is applied to one end of the detection coil 6 of the magnetic sensor 4 through the impedance element 3. The other end of the detection coil 6 is grounded. The terminal voltage of the detection coil 6 is extracted as a detection signal e.sub.0 obtained by the magnetic sensor 4 and is input to a voltage detecting circuit 7.
The voltage detecting circuit 7 comprises a positive detector for detecting a positive peak voltage +Va of the detection signal e.sub.0 shown in FIG. 6, a negative detector for detecting a negative peak voltage -Vb of the detection signal e.sub.0, and an adder for adding the peak voltages +Va and -Vb detected by the respective detectors. An output voltage V.sub.0 corresponding to a magnetic field strength detected by the magnetism detecting apparatus is obtained by the voltage detecting circuit 7.
An operation principle of the magnetic sensor 4 will be described below with reference to FIGS. 7 to 11.
AC power having an AC voltage waveform shown in FIG. 7 is applied to the detection coil 6 of the magnetic sensor 4 through the resistor of the impedance element 3. In this case, a voltage e0 generated across the two ends of the detection coil 6 is determined in correspondence with a resistance R of the resistor and an impedance Zs of the detection coil 6. That is,
Since the detection coil 6 is wound around the ferromagnetic core 5, the voltage e.sub.0 changes in proportion to the impedance Zs and the magnetic permeability of the core 5.
Assume that an AC current is supplied to the detection coil 6 while no external magnetic field is applied to the magnetic sensor 4. In this case, the magnetic permeability characteristic of the core 5 changes in accordance with the hysteresis characteristic of the core 6, as shown in FIG. 10. Note that reference symbol n denotes the number of turns of the coil; and i, a coil current.
For this reason, an output voltage generated across the two ends of the detection coil 6 has a waveform shown in FIG. 8. In the absence of an external magnetic field, positive and negative waveforms are symmetrical, and a positive voltage V.sub.1 is equal to a negative voltage V.sub.2.
If an external magnetic field is applied in this state, a magnetic flux crossing the core
REFERENCES:
patent: 4503395 (1985-03-01), Kratzer et al.
Ando Seigo
Murakami Yoshihiro
NKK Corporation
Snow Walter E.
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