Fluxgate magnetometer circuit with earth's field elimination

• US 4,293,815 A
• Filed: 05/01/1978
• Issued: 10/06/1981
• Est. Priority Date: 05/01/1978
• Status: Expired due to Term

• Current Assignee: Century Geophysical Corporation
• Original Assignee: Century Geophysical Corporation
• Inventors: Fung, Bing-Fai, West, Jerry B., Kretzschnar, John L.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/901,778
• Time in Patent Office: 1,254 Days
• Field of Search: 324/8, 324/3, 324/254, 324/255, 324/256, 324/225, 324/244, 324/249, 324/253, 324/345, 324/346
• US Class Current: 324/254
• CPC Class Codes: G01R 33/045 in single-, or multi-aperture elements

Abstract

A flux gate magnetometer system utilizes three independent flux gates for measuring the three mutually orthogonal components of the earth'"'"'s magnetic field. A sensor for each flux gate includes a ring core with a toroidal winding as the drive winding and a differential winding for sensing the difference in saturation due to the earth'"'"'s flux linkages in the core. The operation of the flux gate magnetometer system is stabilized by a second harmonic servo type null circuit. The output of the differential sense winding is beat with a signal twice the frequency of the drive signal and in phase with the sensor output. This demodulated signal is the error signal for an integration stage. The integration stage drives a high impedance current source which sums into the flux gate sense winding and eliminates the effect of the earth'"'"'s field in the sensor. An error signal is generated only when the high impedance current source does not null the earth'"'"'s flux linkages in the core. The flux gate magnetometer system provides a means for determining the components of the earth'"'"'s magnetic field for use in a digital mineral logging system.

Claims

1. A stabilized fluxgate magnetometer, circuit, comprising:

a fluxgate sensor having a ring core, a drive winding for the core excited by a frequency generator, and a sense winding differentially wound about the ring core for sensing the component of the earth'"'"'s magnetic field normal to said sense winding;

means for demodulating the output signal from said sense winding to produce a rectified DC signal having a polarity and magnitude determined by the phase and amplitude of the output signal from said sense winding;

means for integrating said rectified DC signal to provide the phase stability and memory for the magnitude of current to eliminate the earth'"'"'s field to indicate the direction and magnitude of the component of the earth'"'"'s magnetic field normal to said sense winding;

an operational amplifier driven by said integrating means for feeding a DC current signal back to the output of said sense winding in a direction and magnitude determined by said demodulating means to eliminate the effect of the earth'"'"'s magnetic field, such that the fluxgate magnetometer is stabilized in operation at a null point;

resonating circuit means for resonating in response to the second harmonic output signal from said sense winding;

means for filtering and amplifying the second harmonic output signal from said sense winding;

a phase shifting circuit for providing a signal at twice the frequency of the signal to the drive winding and in phase with the output signal from said sense winding; and

a synchronous demodulator for beating the filtered second harmonic output signal from said resonating circuit means with the output signal from said phase shifting circuit for providing a rectified DC output signal to said integrating circuit means.

2. A stabilized fluxgate magnetometer for use in measuring one of the three mutually onthogonal components of the earth'"'"'s magnetic field taken with respect to the central axis of a tool housing the magnetometer, comprising:

a fluxgate sensor having a ring core with a toroidal drive winding and a differential sense winding about the core for sensing the difference in saturation due to the component of earth'"'"'s magnetic field normal to said differential sense winding;

an oscillator and a driver for exciting said winding drive winding of said fluxgate sensor;

a phase shifting circuit connected to said oscillator and driver for providing an output signal twice the frequency of the signal to said drive windings;

a second harmonic tuning element responsive to the output of said sense winding;

circuit means for filtering and amplifying the output signal from said second harmonic tuning element to said synchronous demodulators whereby signal offsets from the output signal of said sense winding are reduced;

a synchronous demodulator for multiplying the second harmonic signal from said tuning element by an in phase second harmonic reference signal from said phase shifting circuit for providing an error signal having a polarity and magnitude determined by the phase and amplitude of the output signal from said sense winding as the output;

means for integrating the error signal from said synchronous demodulator to provide the phase stability and memory for the magnitude of current to eliminate the earth'"'"'s field; and

a high impedance current source driven by said integrating circuit means which sums a DC current signal into the output of said fluxgate sense winding in a direction and magnitude determined by said synchronous demodulator to eliminate the effect of the earth'"'"'s field in the sensor.

3. An improved fluxgate magnetometer for operating at a stabilized point including an oscillator and a driver, a fluxgate ring core sensor and a drive coil excited by the oscillator and driver, a sense winding differentially would around said core for sensing the difference in saturations of the earth'"'"'s flux linkages in the core and circuit means for determining the polarity and magnitude of an error signal to be fed through the sense winding to eliminate the earth'"'"'s field in the sensor for operating the magnetometer at a stabilized null point, wherein the improvement comprises:

means for integrating the error signal to be fed through the sensor to provide the phase stability and memory for the magnitude of current to eliminate the earth'"'"'s field, and

high impedance current source driven by said integrating means for summing a DC current into the output of the sense winding in a direction and magnitude determined by the error signal circuit means to eliminate the effect of the earth'"'"'s field in the fluxgate sensor to improve the operational stability of the magnetometer by operating at a null point.

4. The improved fluxgate magnetometer of claim 3, wherein said high impedance current source is an operational amplifier.