U.S. patent number 3,638,074 [Application Number 05/031,918] was granted by the patent office on 1972-01-25 for fluxgate magnetometer drive circuit including a sensor demagnetizer.
This patent grant is currently assigned to TRW Inc.. Invention is credited to George T. Inouye.
United States Patent |
3,638,074 |
Inouye |
January 25, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
FLUXGATE MAGNETOMETER DRIVE CIRCUIT INCLUDING A SENSOR
DEMAGNETIZER
Abstract
A drive circuit for a fluxgate magnetometer including a
demagnetizing system for the fluxgate sensor. The drive circuit is
arranged to minimize the second harmonic frequency of the drive
frequency in the drive signal to reduce interference with the
signal output from the magnetometer at the second harmonic
frequency. The circuit draws current only half the time to conserve
power and consequently reduce the weight of the power source. The
drive voltage is derived from a crystal oscillator having a
frequency four times that of the desired frequency and which is
connected to the drive circuit through a pair of series connected
divide-by-two flip-flops to develop a square wave. Demagnetization
is achieved by momentarily increasing the current in the fluxgate
sensor and then allowing it to decay back exponentially to a lower
value.
Inventors: |
Inouye; George T. (Palos Verdes
Peninsula, CA) |
Assignee: |
TRW Inc. (Redondo Beach,
CA)
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Family
ID: |
21862102 |
Appl.
No.: |
05/031,918 |
Filed: |
April 27, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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700396 |
Feb 25, 1968 |
3509424 |
Apr 28, 1970 |
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Current U.S.
Class: |
361/204; 324/254;
327/110; 361/149 |
Current CPC
Class: |
G01R
33/04 (20130101) |
Current International
Class: |
G01R
33/04 (20060101); G01r 033/02 (); H01f
013/00 () |
Field of
Search: |
;317/148.5,157.5
;324/43R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hix; Lee T.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This is a continuation-in-part of application Ser. No. 700,396,
filed Jan. 25, 1968 which will issue as U.S. Pat. No. 3,509,424 on
Apr. 28, 1970.
Claims
What is claimed is:
1. In a fluxgate magnetometer of the type wherein the sensor is
driven by a fixed oscillator, the improvement comprising means for
demagnetizing said fluxgate sensor including:
means for momentarily increasing the drive current flowing in said
fluxgate sensor and for allowing said increased current to decay
back exponentially to the value prior to increase; so that
said fluxgate is demagnetized without interrupting its ability to
make measurements.
2. An improvement for a fluxgate magnetometer as claimed in claim 1
wherein the means for momentarily increasing the drive current
flowing in the fluxgate sensor and for allowing it to decay back
exponentially to the value prior to increase comprises:
a load resistor in series with the current supply to the fluxgate
sensor; and
pushbutton means shorting said resistor to increase the drive
current flowing in the fluxgate sensor whenever said pushbutton is
depressed.
3. A fluxgate magnetometer drive circuit comprising:
push-pull power amplifier means connected to apply a drive signal
to the drive coil of the sensor of said magnetometer;
means for demagnetizing said fluxgate sensor including;
means for momentarily increasing the drive current flowing in said
fluxgate sensor and for allowing said increased current to decay
back exponentially to the value prior to increase, so that
said fluxgate is demagnetized without interrupting its ability to
make measurements;
first and second gate circuit means connected to control said
push-pull amplifier;
first control means to apply a signal of the fundamental drive
frequency of said sensor to a first of said gate circuits and means
to apply a signal of the same frequency but inverted phase to a
second of said gate circuits; and
second control means to apply to each of said gate circuits a
control signal to alternately open and close said gate circuits at
a frequency which is an even harmonic of said fundamental drive
frequency.
4. A fluxgate magnetometer drive circuit as claimed in claim 3
wherein said means for momentarily increasing the drive current
flowing in the fluxgate sensor and for allowing it to decay back
exponentially to the value prior to increase comprises:
a load resistor in series with the current supply to the fluxgate
sensor; and
pushbutton means shorting said resistor to increase the drive
current flowing in the fluxgate sensor whenever said pushbutton is
depressed.
5. A fluxgate magnetometer drive circuit as claimed in claim 3
wherein:
said push-pull power amplifier means comprises a pair of
transistors connected to form said amplifier;
said outputs of said first and second gate circuit means are
connected to the base electrodes of said transistors; and
said gate circuits each comprise logical AND gates.
6. A fluxgate magnetometer drive circuit as claimed in claim 3
wherein:
said first control means comprises a crystal controlled oscillator
stabilized at a frequency which is a harmonic of said fundamental
drive frequency and the output from which is applied through at
least one divide-by-two flip-flop in order to derive said signal of
said fundamental drive frequency which is applied to said gate
circuits; and
said second control means comprises circuitry to apply a signal of
one of said harmonic frequencies generated by said first control
means through phase shifting means to control each of said gate
circuit means to permit application of said signal of said
fundamental drive frequency through said gate means only during a
fraction of the time duration of said fundamental frequency signal.
Description
BACKGROUND OF THE INVENTION
This invention relates to fluxgate magnetometer drive circuits
which may, for example, be used in magnetometers of the type
described in a book entitled, Methods and Techniques in Geophysics,
edited by S. K. Runcorn and published in 1960 by Interscience
Publishers Inc. of New York. Reference is particularly made to
pages 139 through 147 of this book and more generally to all of the
article entitled, Measurement of the Geomagnetic Elements by K.
Whitham, beginning on page 104. Both the field of invention and the
relevant prior art are well set forth therein.
In a particular application the objective of the fluxgate
magnetometer drive circuit is to generate a sinusoidal current of
approximately 80 milliamperes peak to peak in the drive coil of the
fluxgate sensor. Typically, the drive frequency may be 11
kilocycles and the waveform should not contain any second harmonic
components since this is the signal frequency in the output of the
magnetometer which contains the information as to the magnitude and
direction of the magnetic field being measured. The manner in which
this second harmonic component of the output signal is analyzed for
such measurement is clearly set forth in the above-noted text.
One problem associated with a fluxgate sensor is that it may
acquire a "perm" or an offset bias of unpredictable magnitude of
the order of several gamma on exposure to fields on the ground
having a magnitude of one or two gauss.
A solution to the "perm" problem is to obtain a measure of this
offset, and then subtract it from the fluxgate reading. Several
experimenters have added mechanical devices to their fluxgate
sensors to periodically mechanically reverse the orientation of the
sensors by 180.degree.. This reversal permits extraction of the
offset by averaging the fluxgate readings before and after
reversal. Mechanical reversers, however, add unnecessary weight and
complexity to the magnetometer.
Another approach to the "perm" or offset problem is to demagnetize
the fluxgate sensor to eliminate the "perm." In the normal
demagnetization process such as is used for demagnetizing magnetic
tapes, watches or spacecraft, the object to be demagnetized is
subjected to an alternating magnetic field having an envelope which
decreases from a large value to zero at a rate which is much slower
than the alternating field. The peak amplitude of the envelope
should be larger than the field which originally magnetized the
object. The ambient magnetic field should be cancelled out.
In the case of the fluxgate magnetometer, the high-permeability
cores are already subject to a large oscillating field of several
tens of gauss at the sensor drive frequency. If the field were
increased and then reduced to zero to cause demagnetization, the
fluxgate sensor would be inoperable during the period of reduced
field.
It is desirable, therefore, to have a means for demagnetizing the
fluxgate sensor without rendering it inoperable during the period
of demagnetization. It has been found that if the magnetic field is
increased momentarily, and then allowed to decay back to the
previous level, fluxgate sensor demagnetization will occur without
an interruption in the operation of the fluxgate sensor.
SUMMARY
In accordance with an example of a preferred embodiment of the
present invention, a fluxgate driver is designed to minimize the
second harmonic content, operate on reduced power, and demagnetize
the fluxgate sensor without interrupting its operation.
In order to minimize the second harmonic content, the drive
waveform is derived from a divide-by-two flip-flop which is driven
by a 22-kilocycle waveform. A 44-kilocycle crystal oscillator is
normally used as the primary frequency source to eliminate
frequency variations which would cause undesired phase shifts
within the instrument. A 22-kilocycle reference frequency signal is
derived from a first divide-by-two flip-flop and is applied to one
input of a synchronous detector and to a second divide-by-two
flip-flop. The output of the second divide-by-two flip-flop is an
11-kilocycle signal which is applied to the drive circuit of the
magnetometer sensor. The second harmonic output of the sensor
provides the other input for the synchronous detector. Normally,
the drive circuit operates as a push-pull Class B amplifier with an
input which is a push-pull square wave and with an output
transformer tuned to 11 kc. The secondary of the transformer
matches the sensor drive coil. In the present circuit, both the 22
kilocycle and the 11-kilocycle square wave signals are combined and
are applied to the drive circuit in a manner described below such
that current is drawn only one-half of the time, thereby conserving
on the power consumption of the instrument.
To cause demagnetization of the fluxgate sensor, the current in the
sensor is momentarily increased, which in turn increases the field
of the sensor. The increased field is allowed to decay
exponentially back to its previous level. The increase in the field
and its subsequent decrease causes sufficient demagnetization of
the sensor for most purposes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an example of a fluxgate magnetometer
including a fluxgate sensor demagnetizer according to the present
invention;
FIG. 2 is a circuit diagram for the driver circuit of the
magnetometer and for the demagnetizing circuit;
FIG. 3 is a graph illustrating voltage waveforms which occur in the
drive circuit of FIG. 2 as a function of time; and
FIG. 4 is a graph of the axial field of the fluxgate sensor as a
function of the sensor current.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning to FIG. 1, there is shown a block diagram of a typical
fluxgate magnetometer as described in greater detail in the
above-referenced book by Runcorn. The magnetometer includes a
sensor 10 which is a magnetizable core which is driven in and out
of saturation by a driver 11. In the absence of any component of
ambient magnetic field, the peaks detected in the output voltage
from sensor 10 will be uniform. In the presence of a magnetic
field, the peaks vary in a manner which is well understood in the
art and which is measured by applying the output voltage through an
output amplifier 12 to one input of a synchronous detector circuit
13.
The other input to synchronous detector 13 is derived from a
crystal oscillator 14 through a divide-by-two flip-flop 15 which
has its output connected to the second input of synchronous
detector 13 and also through a second divide-by-two flip-flop 16 to
driver circuit 11. The flip-flop 16 develops a square wave which is
impressed on driver circuit 11. Such a square wave has only odd
harmonies, and hence, minimizes the second harmonic frequency of
the drive frequency. The output of synchronous detector 13 is a DC
voltage which affords a measure of the ambient field sensed by the
core of sensor 10.
The magnetometer also includes a fluxgate sensor demagnetizer 17.
When a pushbutton 124 is depressed, the current in sensor 10 is
increased momentarily by approximately 50 percent. The increased
current is then allowed to decay exponentially to its previous
value. The increase in current momentarily increases the envelope
of the magnetic field surrounding sensor 10. The envelope decreases
as the current decays back to the value prior to increase. The
decreasing magnetic field envelope demagnetizes sensor 10, thus
reducing the effect of ambient magnetic fields.
In the example shown, it will be noted that crystal oscillator 14
is tuned to a 44-kilocycle frequency so that the output of first
flip-flop 15 is at 22 kc. It is this 22-kilocycle voltage which is
applied to one side of synchronous detector 13 and also to the
second dividing flip-flop 16. The output of flip-flop 16 is an
11-kilocycle voltage which is applied through driver circuit 11 to
sensor 10. Thus, the fundamental frequency at which sensor 10 is
driven alternatively in and out of saturation is 11 kc. As has been
noted above, the information with respect to the ambient magnetic
field is contained in the second harmonic of this fundamental
frequency to which amplifier 12 is tuned. This second harmonic is,
of course, 22 kc. which is the operating frequency of synchronous
detector 13.
In FIG. 2 there is shown a detailed circuit diagram of driver
circuit 11 and demagnetizing circuit 17. Driver 11, it will be
seen, consists of a pair of transistors 110 and 111 which may be of
the NPN-type as shown and which are connected to operate as a
push-pull Class B amplifier with the primary winding 112 of an
output transformer driver, and tuned to the 11-kilocycle
fundamental drive frequency. Tuning may readily be achieved by
capacitor 113. This output transformer is connected through a
matching network to match impedances to the drive coil of the
fluxgate sensor 10.
Drive circuit 11 may be energized by a battery having its negative
terminal grounded and its positive terminal, V.sub.o, connected to
the midpoint of the primary winding 112 via a resistor 123. A
capacitor 122 is connected from the midpoint of primary 112 to
ground.
The push-pull-connected transistor amplifiers 110-111 are driven
through respective gate circuits 114 and 115 which have output
resistors 116 and 117 connected from the gate output to the base of
transistors 110 and 111, respectively. The gate circuits 114 and
115 may be any logical AND gate circuit, many types of which are
well known in the art.
The inputs to driver circuit 11 (which is shown enclosed in the
dash line block in FIG. 2) are derived over input lines 120 and
121. As can be seen by comparing FIGS. 1 and 2, line 120 carries
the 11-kilocycle output of flip-flop 16, whereas line 121 carries
the 22-kilocycle output of flip-flop 15. Line 120 is connected
directly to one input of first AND-gate 114 and is connected
through an inverter amplifier 119 to one input of second AND-gate
115. The 22-kilocycle voltage on input line 121 is connected
through a 90.degree. phase shifter 118 and thence to the second
input of each of AND-gate circuits 114 and 115. The effect of this
type of connection is illustrated graphically in the waveforms of
FIG. 3.
In FIG. 3, there is shown a graph in which volts on the vertical
axis are plotted as a function of time on the horizontal axis for
various waveforms in the circuit of FIG. 2, each of which has its
own separate zero level as indicated on the voltage axis. Starting
at the upper waveform in FIG. 3, the 11-kilocycle voltage applied
to input line 120 is first depicted. The next waveform is the
inverted 11-kilocycle voltage which is derived as the output of
inverter 119. Next there is shown the 22-kilocycle voltage which is
derived as the output from the phase shifter or delay network 118,
which is preferably used to introduce a small delay such as
90.degree. in the voltage applied over input line 121 in order to
avoid exact coincidence of the leading edges of the 11 and
22-kilocycle inputs applied to AND-gate circuits 114 and 115.
Finally, the next waveform represents the output of AND-gate 114,
which is applied to the base of driver 110, and the lower waveform
represents the output of AND-gate 115 which is applied to the base
of driver 11.
Demagnetizing circuit 17 is part of the energizing circuit for
driver 11. A pushbutton 124 is shunted across resistor 123. Voltage
V.sub.o, which supplies driver circuit 16, likewise controls the
current flowing in fluxgate sensor 10. Resistor 123 reduces voltage
v.sub.o and the current flowing in fluxgate sensor 10 to a
"desired" level. When pushbutton 124 is momentarily depressed, the
current flowing in fluxgate sensor 10 is increased. The increased
current enlarges the magnetic field envelope surrounding sensor 10
as shown in FIG. 4. The increased current in sensor 10 then decays
back to the "desired" level exponentially.
Assume the fluxgate sensor 10 is wound with 0.0035-inch-diameter
wire, which would create a field of 141 oersted per ampere if wound
in a single large solenoid. In a first test performed to measure
the offset caused by exposure to high fields, the solenoid creating
the field around the sensor inside the fluxtank also "permed" the
fluxtank itself to the extent of 10 gamma as measured by the sensor
after it was demagnetized. The net sensor "perm" was about .+-.1
gamma for a .+-.100,000-gamma exposure, and doubled to .+-.2 gamma
for a .+-.200,000-gamma exposure.
A second test performed was to decrease the peak-demagnetizing
current from 0.16 ampere until the effectiveness was degraded for
the 2-gauss magnetization. About 0.10 ampere was found to be the
minimum current required.
A third test measured the effect of a fixed field created by the
fluxtank solenoid while degaussing with a 0.12-ampere peak current.
The results were as follows:
Perming field (.gamma.) Offset (.gamma.) 300 > 1 600 1 1,000 2
2,000 3 3,000 3 5,000 4 7,000 4 9,000 4 11,000 4 21,000 3 40,000 2
80,000 2 110,000 1
The offset thus has a broad peak of 4 gamma for perming fields
between 5,000 and 10,000 gamma.
The results of these tests show it is possible to demagnetize the
fluxgate sensor in the magnetic field environment in which it is
making measurements, and still be sufficiently effective to reduce
any offset to less than a few tenths of a percent of the ambient
field. This order of accuracy is sufficient for the uses to which
magnetometers are put. The fact that the magnetometer can make
measurements while the sensor is undergoing demagnetization is a
decided advantage.
Since the peak power needed to cause demagnetization is only
momentarily needed, power dissipation in driver circuit 11 is
inconsequential. However, transistors 110 and 111 and the other
components of the driver should be designed to withstand the
increased voltages to prevent breakdown.
It will be noted that the 22-kilocycle voltage is in effect used to
gate a portion of both the original and the inverted 11-kilocycle
signal so that the push-pull arrangement is driven during only half
of the total time duration of these respective signals. Since each
excursion of the 22-kilocycle square wave has a width or time
duration only half that of the 11-kilocycle signal, it follows that
this must necessarily be so by virtue of the operation of the AND
gates in the circuit shown. Thus, in this manner, the 22-kilocycle
and the 11-kilocycle square wave signals are combined so that
current is drawn only one-half of the time, thereby conserving on
the power consumption of the instrument.
The fact that current is drawn only half of the time is an obvious
advantage. Furthermore, the power dissipated in the driver
transistors is small because the collectors are shorted to the
emitters when current is being drawn. Because they are used in a
switching mode, the tolerances on the transistor parameters are
relaxed. Since the drive circuit power consumption is a major
portion of the power required in the total instrument, this saving
is a great advantage for airborne instruments where weight
considerations place a limit on available power.
It is possible to combine the 44-kilocycle drive in addition to the
22- and 11-kilocycle drive signals to thereby further reduce the
power consumption by drawing current only during one-fourth of the
11-kilocycle signal waveform. Such an extension is by
straightforward analogy to that illustrated herein. Since the
22-kilocycle and 44-kilocycle signals are already available in the
circuit as originally designed, the only additional circuitry
required for such a power-saving arrangement are the gating logic
stages which are available in a single integrated circuit
component.
* * * * *