Magnetic sensor using the earth's magnetism

A direction sensor includes at least one sensor element including a ring-shaped core formed from at least one flexible wire made of zero-magnetostrictive amorphous magnetic material, two pairs of series connected coils spaced from each other at equal intervals about the core for sensing an external magnetic field, with the core being formed by threading the wires through the coils, the coils being excited by an alternating frequency f, wherein voltages of frequency f are produced across each pair of coils; and a process circuit for providing a differential voltage between the voltages of frequency f produced across each pair of coils.

BACKGROUND OF THE INVENTION 
The present invention relates to a direction sensor or a flux sensor, and 
in particular, relates to such a sensor for finding the direction of earth 
magnetism at a particular point. The sensor may be used for finding not 
only the direction but also the location of the particular point on the 
earth. 
The present invention is applicable to locate an automobile. 
A prior earth magnetism flux sensor has been shown in Japanese utility 
model publication 30411/84, and in an article entitled "Magnetic Field 
Sensor and Its Application to Automobies" in page 83-90, Society of 
Automotive Engineers, Inc, 1980. 
A prior earth magnetism flux sensor is shown in FIG. 1. In the figure, the 
detector DET has an O-ring shaped magnetic core C made of permalloy, on 
whic an exciting coil C.sub.D is wound. A pair of detections coils C.sub.x 
and C.sub.y which are perpendicular to each other are also wound on the 
core C so that the core C is completely included in those coils C.sub.x 
and C.sub.y as shown in the figure. The exciting coil C.sub.D is wound 
along the path of the flux in the core C as shown in the figure. The 
oscillator OSC provides a signal of the frequency f (for instance f=500 Hz 
-2 kHz) to the exciting coil C.sub.D. A signal having a double frequency 
2f is induced in the coils C.sub.x and C.sub.y according to the horizontal 
component of the earth's magnetism. The outputs of those coils C.sub.x and 
C.sub.y are applied to the respective synchronous detectors SYNC through 
respective bandpass filters BPF and respective amplifiers AMP. The 
synchronous detectors SYNC also receive the reference signal of the 
frequency 2f which is provided by said oscillator OSC through a frequency 
doubler DOUBLER and phase controller (PHASE CONT). The synchronous 
detectors SYNC provide a direct (DC) signal, the level of which relates to 
the amplitude of the double frequency 2f signal of the coils C.sub.x and 
C.sub.y. Those DC signals are applied to DC amplifiers AMP, which provide 
respective outputs E.sub.x and E.sub.y, relating to the direction of the 
coils C.sub.x and C.sub.y. 
The level of the outputs E.sub.x and E.sub.y depends upon the horizontal 
component of the earth's magnetism. When the detector DET rotates in the 
horizontal plane, the output level E of the signals E.sub.x and E.sub.y 
changes as shown in FIG. 2, in which the horizontal axis shows the 
direction (.theta.), and the vertical axis shows the level E. The curves 
are sinusoidal as shown in the figure. 
However, the device of FIG. 1 has the following disadvantages. First, the 
detector DET which must have a ring core is too large in size for 
practical use (For instance, a prior detector has a diameter of 20-30 mm, 
and a thickness of 5-10 mm). Further, in the manufacturing process of a 
detector, it takes a long time to wind a coil on a ring shaped core. An 
automatic winding on a ring shaped core is impossible. A permalloy core is 
easily broken by vibration and/or shock. Further, the low exciting 
frequency is unstable, and therefore, the output levels E.sub.x and 
E.sub.y are subject to drift over a period of time. The use of a high 
frequency is also impossible when a permalloy core is used. 
SUMMARY OF THE INVENTION 
It is an object, therefore, of the present invention to overcome the 
disadvantages and limitations of prior direction sensors by providing a 
new and improved direction sensor. 
It is also an object of the present invention to provide a direction sensor 
which is small in size, light in weight, simple in manufacturing process, 
and has high operational reliability without drift. 
The above and other objects are attained by a direction sensor having at 
least one sensor element having an elongated core made of magnetic 
material; a pair series connected coils wound on said coil with spacing; 
said coils being excited by alternate frequency f, and a process circuit 
for providing a differential voltage between voltages of frequency f 
induced on said coils.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows a structure of the direction sensor according to the present 
invention. The sensor of FIG. 3 has a pair of sensor elements 1A and 1B 
which are located perpendicular to each other. The angle between the 
sensors 1A and 1B is not restricted to 90.degree., but another angle 
except 0.degree. is possible. 
The sensor element 1A has core wires AF.sub.1 which are made of magnetic 
amorphous material with zero-magneto striction. For instance, said sensor 
element is comprised of three wires each of which is 20 through 30 mm in 
length, and 110 microns in diameter. An example of the composition of the 
wires is Co.sub.68 Fe.sub.4 Si.sub.13 B.sub.15 (atomic percent). A pair of 
coils 3A and 3B are wound on the extreme opposite ends of the wires as 
shown in the figure. The number of turns of each of coils 3A and 3B is 150 
through 300 turns. The diameter of the coil on the wires is about 1 mm. 
The coils 3A and 3b are coupled with the process circuit 2A. 
The other sensor element 1B has the same structure as that of 1A, and has 
wires AF.sub.2, a pair of coils 4A and 4B, and a related process circuit 
2B. 
The process circuits 2A and 2B are the same as each other, and one of them 
is shown in FIG. 4. The structure of the process circuit is essentially an 
astable multivibrator circuit having a pair of transistors T.sub.r1 and 
T.sub.r2, each base of which is coupled with the collector of the other 
transistor through a parallel circuit of a capacitor C.sub.B and a 
resistor R.sub.B. The collector of the transistor T.sub.r1 is connected to 
one end of the coil 3A through the line L.sub.1, and the collector of the 
transistor T.sub.r2 is connected to one end of the coil 3B through the 
line L.sub.2. The emitters of the transistors T.sub.r1 and T.sub.r2 are 
coupled with an active filter ACF which is a lowpass filter through the 
lines L.sub.3 and L.sub.4, respectively. Those emitters are bridged by a 
series circuit of a pair of load resistors R.sub.L, and a variable 
resistor VR. The junction point of said load resistors R.sub.L, and the 
variable terminal of the variable resistor VR is ground. The variable 
resistor VR adjusts the balanced condition of the circuit so that no 
output signal E.sub.01 is provided when no flux is applied to the device. 
The output of the active filter ACF is the output E.sub.01 of the sensor 
element. The coils 3A and 3B are connected in series so that the 
differential output of the coils is provided to the lines L.sub.1 and 
L.sub.2. The junction point of the coils is coupled with the power source 
E, which supplies the frequency f. The frequency f is in the range between 
100 kHz and 500 kHz, and is preferably 200 kHz. 
The higher the frequency f is, the more preferable it is to induce a higher 
voltage to a process circuit. The upper limit of the frequency f is 
determined by the operational characteristics of the amorphous material. 
When the coils 3A and 3B which are located at the opposite ends of the 
magnetic wires AF are excited by the signal of the frequency f, the output 
level E.sub.01 is zero if no bias flux is applied to the magnetic wire 
AF.sub.1. On the other hand, when the wire AF.sub.1 is magnetically biased 
by the earth's magnetism, the output of the coil 3A increases, and the 
output of the other coil 3B decreases. Therefore, the differential level 
between the lines L.sub.1 and L.sub.2 is not zero, but has some level 
except zero relating to the bias flux. Thus, the balance condition of the 
circuit is broken by the bias flux, and therefore, the output level 
E.sub.01 has some amplitude relating to the bias flux, or the earth's 
magnetism. 
The structure and the operation of the other sensor element 1B are the same 
as those of element 1A, and the second sensor element 1B provides the 
output E.sub.02 relating to the bias flux, or the earth's magnetism on the 
core AF.sub.2. 
When the sensor element 1A or 1B is positioned on a horizontal plane with 
some angle to the earth's magnetism, the output level E.sub.01 or E.sub.02 
depends upon that angle, and the level E.sub.01 and/or E.sub.02 is on the 
sinusoidal curve as shown in FIG. 5, in which the horizontal axis shows 
the direction (.theta.), and the vertical axis shows the output level. 
It should be appreciated that a single sensor element is not sufficient to 
provide the absolute value of the direction, because each output level 
E.sub.01 or E.sub.02 corresponds generally to two directions in 
0.degree.-360.degree.. A combination of a pair of sensor elements with 
some angle may provide the absolute single direction. That angle is not 
restricted to 90.degree., but any angle except 0.degree. is available. 
The output signals E.sub.01 and E.sub.02 are applied to an external circuit 
(not shown), which indicates the absolute value of the direction. 
It should be appreciated that the present sensor elements 1A and 1B must be 
located on the horizontal plane for accurate measurement of the earth's 
magnetism. In order to detect the horizontal condition, a third sensor 
element which is the same as that of FIGS. 3 and 4 may be used. The third 
sensor element is located vertically, and the vertical condition is 
detected when the output of the third sensor element is zero. 
Therefore, by combining the third vertical sensor element with a pair of 
horizontal sensor elements of FIG. 3, the horizontal sensor elements can 
be located on horizontal plane, and the accurate measurement of the 
earth's magnetism is carried out. 
FIG. 6 shows a modification of the sensor element. In this modification, a 
pair of amorphous wire bundles 10A and 10B are located parallel to each 
other. And, a coil 3A is provided at the extreme end of the wire bundle 
10A, and another coil 3B is provided on the extreme end of the wire bundle 
10b so that the coils 3A and 3B are positioned at opposite ends of each 
wire. Similarly, a second sensor element is comprised of a pair of 
amorphous wire bundles 10C and 10D positioned parallel to each other, and 
the coils 4A and 4B positioned at the extreme ends of the wire bundles so 
that the coils locate at opposite ends of the wires. The parallel wire 
bundles 10A and 10B (10C and 10D) are positioned very close to each other. 
The structure of FIG. 6 is magnetically equivalent to the structure of 
FIG. 3. The structure of FIG. 6 is used when it is convenient for 
manufacturing reasons, and/or mounting reasons of the elements. 
FIG. 7 shows an other embodiment of the sensor element according to the 
present invention. In the figure, the sensor element has a hollow tube 5 
made of non-magnetic material, like a plastic. An amorphous core AF with 
three wires in a circular ring shape, having four coils 3A, 3B, 4A and 4B 
are located in said tube 5. Preferably, the amorphous core AF is C- shaped 
so that a gap space G is provided between extreme ends of the core AF. In 
the preferred embodiment, the diameter of each wire of the core is 110 
microns, the number of wires is three, the composition of the amorphous 
material is Co.sub.68 Fe.sub.4 Si.sub.13 B.sub.15 in atomic percent, and 
the wire is made of zero-magneto striction amorphous material. The coils 
3A and 3B are located on the diameter of the ring, and the coils 4A and 4b 
are located at another diameter of the ring. The length of the gap space G 
determines the de-magnetizing effect, and said length is designed 
according to the range of the magnetic field to be measured, and the 
sensitivity of the sensor. The longer the gap G is, the lower the 
sensitivity of the device is, and the wider the operational dynamic range 
is. 
The sensor element is coupled with the process circuits 2A and 2B, which 
provide the output signals E.sub.01 and E.sub.02, as in the case of the 
embodiment of FIGS. 3 and 4. 
FIG. 8 shows modifications of the embodiment of FIG. 7. The modification of 
FIG. 8A has the feature that the core AF is in a rectangular shape instead 
of a circular ring shape, and the feature of FIG. 8B is that the core AF 
is in an octagonal shape. Generally, a polygonal core instead of a 
circular core is possible in the present invention. a polygonal core has 
the advantage as compared with a circular core that it has a linear 
portion which is convenient to mount a coil. Of course, a gap space G is 
provided in the modification of FIG. 8, although it is not shown for the 
sake of the simplicity of the drawing. 
It should be appreciated of course that a core may be comprised of a single 
amorphous wire, although the embodiments show a core with three wires. 
Finally, some specific advantages of the present invention are enumerated. 
(a) As the output voltage of a sensor element is a voltage of frequency f 
induced on the coil, the higher the operational frequency is, the higher 
the output voltage is. Therefore, the use of high frequency is possible. 
The upper limit of the operational frequency is about 500 kHz, which is 
restricted by the characteristics of the amorphous material. Due to the 
use of higher frequency than the prior art, the operation is stale, and 
the drift of the output signal over a long period of time is small, as 
compared with those of the prior art. 
(b) The sensor element is not affected by vibration, and/or shock, because 
of the use of zero-magneto striction amorphous material, which has no 
anisotropy. 
(c) The winding process of a coil on a wire is simple, as compared with the 
winding process on a ring in the prior art. Therefore, the manufacturing 
process is simplified, and the manufacturing cost is reduced, as compared 
with the prior art. 
From the foregoing it will now be apparent that a new and improved flux 
sensor or a direction sensor has been found. It should be understood of 
course that the embodiments disclosed are merely illustrative and are not 
intended to limit the scope of the invention. Reference should be made to 
the appended claims, therefore, rather than the specification as 
indicating the scope of the invention.