Single winding saturable core magnetometer with field nulling

• US 5,124,648 A
• Filed: 08/25/1987
• Issued: 06/23/1992
• Est. Priority Date: 08/25/1987
• Status: Expired due to Term

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Webb, Spencer L., Brokaw, A. Paul
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US07/089,147
• Time in Patent Office: 1,764 Days
• Field of Search: 324/225, 324/253-255, 324/117 R, 331/65, 33/361, 33/355 R, 33/363 R
• US Class Current: 324/253
• CPC Class Codes: G01R 33/04 using the flux-gate principle

Abstract

A magnetometer having only a single coil winding for sensing the magnetic field along a single axis. Functionally, the invention comprises an oscillator, an integrator, and a voltage-controlled current source. The oscillator uses a saturating inductor which also serves as the magnetic-field-sensing element. The inductor is driven with a positive voltage and when the current through the inductor exceeds a value which indicates that the core is saturated, the driving voltage switches to an equal-magnitude negative value. This negative drive is maintained until the current again indicates the core to be saturated, at which point the driving voltages switches back to the positive value. With no externally applied field, the inductor current averages to a zero value. An externally applied field helps the core saturate in one direction and hinders it in the other, resulting in a change in average inductor current. An integrator is used in a force-balance feedback arrangement to provide a compensation current to the coil winding. The voltage output of the integrator, which is used to generate this compensation current, directly reflects the strength of the magnetic field causing the imbalance, and is the instrument'"'"'s output signal.

Claims

1. An instrument for measuring a magnetic field, comprising:

a. an inductor comprising a coil wound on a core of saturable material having first and second leads, the first lead being connected to a common ground;

b. a voltage source having a first input for accepting a reference voltage and a second input for accepting the voltage across the inductor, for comparing the two inputs, and an output that is switchable between a first, positive voltage and a second, negative voltage in response to a control condition, the first and second voltages being of substantially equal magnitude;

c. a first resistor connected in series between the output of the voltage source and the second lead of the inductor;

d. an integrator having an input and an output, the input of the integrator being connected to the output of the voltage source;

e. a second resistor, such second resistor being connected between the output of the integrator and said second lead of the inductor;

f. means, coupled to the first input of the voltage source, for providing a reference voltage relative to the output of the voltage source; and

g. the control condition being the difference between the reference voltage applied to the first input of the voltage source and the voltage across the inductor applied to the second input of the voltage source whereby the output voltage from the output of the integrator provides a signal representative of magnetic field strength detected by the inductor.

2. An instrument for measuring a magnetic field, comprising:

a. an oscillator employing a saturable core inductor as the principal element thereof for storage of potential energy and having a voltage source coupled to one terminal of the inductor, said voltage source having an output which oscillates between a first voltage source and a second voltage;

b. an integrator having an input connection and an output connection and providing at such output connection a signal substantially representing the time integral of the voltage applied to the input connection, the input connection of the integrator being connected to the output of the voltage source in the oscillator;

c. means, coupled between the inductor and the integrator, for applying to the inductor a current varying with the output from the integrator; and

d. the oscillator further comprising means, coupled to the voltage source and responsive to the saturation of the core, for causing the output of the voltage source to switch

(i) from the first voltage to the second voltage when the inductor core saturates in one direction and

(ii) from the second voltage to the first voltage when the inductor core saturates in the opposite direction, whereby the signal at the output connection of the integrator is directly related to the strength of the component of the magnetic field present along the axis of the winding at the location of the inductor.

3. The instrument of claim 2 wherein the means for applying to the inductor a current varying with the output of the integrator comprises a voltage-controlled current source.

4. The instrument of claim 3 wherein the voltage-controlled current source is a resistor connected between the output of the integrator and the inductor.

5. The instrument of claim 2 further including a second winding on the core and wherein the means for applying to the inductor a current varying with the output of the integrator comprises means responsive to the output of the integrator for driving the second winding.

6. The instrument of claim 2 wherein the means to cause the output of the voltage source to switch includes

(i) a resistor connected between the output of the voltage source and one lead of inductor, and

(ii) means for sensing a change of voltage on said one lead of the inductor indicative of inductor core saturation.

7. A magnetometer comprising:

a. a comparator having an inverting input, a non-inverting input and an output;

b. an inductor comprising a single winding wound on a saturable core material;

c. a first resistor connected in series between the output of the comparator and a first lead of the inductor winding;

d. the second lead of the inductor winding being connected to ground;

e. the non-inverting input of the comparator being connected to the first lead of the inductor winding;

f. means, connected between the output of the comparator and the inverting input of the comparator for establishing a bipolar reference voltage which switches polarity in response to the polarity of the voltage at the output of the comparator;

g. an integrator having an input and an output, the input of the integrator being connected to the output of the comparator so as to supply at its output a current which compensates for changes in average inductor current caused by an external magnetic field;

h. means operatively connected between the output of the integrator and the first lead of the inductor for supplying to the inductor the compensation current, the compensation current representing the component of the external magnetic field along a specified axis and being the output of the magnetometer.

8. The magnetometer of claim 7 wherein the means for supplying to the inductor the compensation current comprises a second resistor operatively connected between the output of the integrator and the first lead of the inductor winding.

9. The magnetometer of claim 7 further including a second winding on the core and wherein the means for applying to the inductor a current varying with the output of the integrator comprises means responsive to the output of the integrator for driving the second winding.

10. The magnetometer of claim 7 wherein the means for establishing a reference voltage comprises:

(i) a pair of Zener diodes connected back-to-back in series, one lead of the pair being connected to the output of the comparator, and

(ii) a resistor connected between the other lead of the Zener diode pair and ground,

the inverting input of the comparator being connected to the junction of the resistor and the Zener diode pair.

11. An instrument for measuring a magnetic field comprising:

a. an inductor comprising a coil wound on a core of saturable material having first and second leads, the first lead being connected to a common ground;

b. a comparator having a non-inverting input, an inverting input, and an output that is switchable between first and second voltages, the first and second voltage being of substantially the same magnitude and opposite polarity, and output controlled to be equal in sign to the difference between the first and second inputs;

c. a first resistor having first and second leads, the first lead connected to the output of the comparator and the second lead connected to both the second lead of the inductor and the non-inverting input of the comparator;

d. an integrator having an input and an output, the input of the integrator being connected to the output of the comparator, and the integrator output being equal to the time integral of the comparator voltage;

e. a second resistor having first and second leads, the first lead connected to the output of the integrator and the second lead connected to the second lead of the inductor;

f. means, coupled to the inverting input of the comparator, for providing a reference voltage responsive to the output of the comparator, and

g. the output of the integrator representing the net D.C. current in the inductor, which in turn represents the sign and magnitude of the magnetic field impinging upon the core of the inductor along the axis of the inductor'"'"'s winding.

12. A single axis saturable core magnetometer comprising:

a saturable core;

a sense coil wound about said saturable core;

a feedback coil coaxial with said sense coil;

oscillator means for supplying first and second drive voltages to said sense coil for successively saturating said core in opposite directions, said oscillator means including comparator means for sensing when the voltage across said sense coil reaches a preselected fraction of said first and second drive voltages relative to a center voltage lying halfway between said first and second drive voltages, said oscillator means changing the supplied drive voltage from one of said first and second drive voltages to the other of said drive voltages when said preselected fractional voltage is reached;

means, responsive to the output of said comparator means, for determining the relative duration of said first and second drive voltages; and

means, responsive to said means for determining, for introducing current into said feedback coil proportionate to the duration of said first and second drive voltages to generate a feedback field to null a field being sensed, the amount of feedback current being representative of the strength of the field being sensed.