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Patent No. 4591788 Magnetic field sensing device (Mohri, et al., May 27, 1986)
Abstract
A magnetic field sensing device for sensing low levels of magnetic intensity is composed of a ribbon-shaped strip of amorphous magnetic material having at least one twist along the length thereof between the ends of the strip. Subsequent to imparting the desired number of twists to the strip, the strip is supported in the twisted condition. The device is capable of generating a pulse in response to the presence of an external magnetic field similar to the Wiegand wire effect.
Notes:
Magnetic
field sensing device. Filed September 1982, granted May 1986. Says it is capable
of detecting magnetic fields in the .001 Hz to 6 Hz frequency range. Basically
just detects that the magnetic field is there and sounds like something from
a Junior High or High School general science class demonstration.
BACKGROUND
OF THE INVENTION
The present invention is directed to a magnetic field sensing device and more
specifically to a twisted, ribbon-shaped strip of amorphous magnetic material
which is supported in the twisted condition and exhibits an effect similar to
a Wiegand effect.
A Wiegand wire is a bistable magnetic device having specific characteristics
obtainable by processing a thin ferromagnetic wire to form a central relatively
"soft" core portion and an outer relatively "hard" magnetized shell portion
with relatively low and high coercivities respectively. Such a bistable ferromagnetic
wire is disclosed in the Wiegand U.S. Pat. No. 3,820,090, granted June 25, 1974
and entitled "Bistable Magnetic Device". The magnetized shell portion is operable
for magnetizing the core portion in a first direction, the magnetization of
the core portion is reversible by application of a separate magnetic field and
the shell portion is operable to remagnetize the core portion in a first direction
upon removal of the separate magnetic field. As a result of the changing magnetic
field, there is a net change in the flux outside of the wire and an appropriately
placed pick-up coil will generate a pulse-shaped voltage having a peak value
and a narrow width with no relation to the changing rate of the magnetic field.
Thus, the Wiegand wire is useful as a magnetic sensor. However, a conventional
Wiegand wire is capable of only being able to detect a fairly high external
magnetic field.
German Offenlegungsschrift No. 28 06 249 discloses a transmitter for emitting
an electrical signal during the magnetic reversal of a ferromagnetic wire within
a coil wherein the wire is kept under tensile stress and/or torsional stress.
SUMMARY OF THE INVENTION
The present invention provides a new and improved magnetic field sensing or
Wiegand effect device which is operable by a low magnetic field and is thus
capable of detecting low magnetic fields. This is accomplished by twisting a
ribbon-shaped strip of amorphous magnetic material and supporting the strip
in twisted condition. Such a twisted strip of amorphous magnetic material is
capable of generating pulses similar to a Wiegand wire in response to a low
magnetic field.
The foregoing and other objects, features and advantages of the invention will
be apparent from the following more particular description of a preferred embodiment
of the invention as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a ribbon-shaped strip of amorphous magnetic
material and its associated casing prior to twisting.
FIG. 1b is a perspective view of the device in FIG. 1 subsequent to being twisted
a number of times.
FIG. 2a is a perspective view of another embodiment of a twisted ribbon-shaped
amorphous magnetic strip and associated casing.
FIG. 2b is an enlarged perspective view showing the details of the embodiment
illustrated in FIG. 2a.
FIG. 3 is a perspective view of still another embodiment of the present invention.
FIG. 4 is a graph showing magnetic field intensity plotted against the number
of twists.
FIG. 5 is a graph showing magnetic field intensity plotted against the temperature
of the atmosphere.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 a strip of amorphous magnetic material 1 which is an alloy of Fe.sub.79
Cr.sub.2 B.sub.17 Si.sub.2 by atomic weight (%) is about 1.0 cm in width, 40
.mu.m in thickness and 10 cm in length. A casing 2 made of thermoplastic material
serves as a supporting means for the strip 1 of magnetic material and is provided
with a slit 2a extending the length thereof into which the amorphous magnetic
material strip is inserted. Upon softening of the casing 2 by heating, the casing
and the amorphous magnetic material strip 1 are easily twisted by applying twisting
forces to both ends of the casing 2 in opposite directions as indicated by the
arrows A and B in FIG. 1a. After the strip 1 is twisted a predetermined number
of times, the casing 2 is cooled and hardened so that the strip of amorphous
magnetic material 1 is supported in a twisted condition as shown in FIG. 1b.
In the embodiment of FIG. 2a, a hollow casing 3 is provided in the form of a
cylinder 3a having a threaded plug 3b screwed into one end of the cylinder 3a.
The lefthand end of the strip 1 as viewed in FIG. 2a is secured to the respective
end of the cylinder 3a by any suitable means and the righthand end of the strip
is secured by any suitable means to the end of the plug 3b. For example, as
seen in FIG. 2b, a slit 11 could be formed in the plug 3b for receiving the
end of the strip 1 while the other end of the strip 1 could be secured by welding
or the like to a rod 12 extending through the end portion of the cylinder 3a.
Thus, upon rotation of the threaded plug 3b, the strip 1 of amorphous magnetic
material can be twisted a predetermined number of times and supported in a twisted
condition. The casing 3 is made of non-magnetic, electrically insulating material.
The embodiment shown in FIG. 3 is similar to the embodiment shown in FIG. 2a
but utilizes a smooth cap 3c instead of the threaded plug 3b. The ends of the
strip 1 may be secured to the cylinder 3a and the cap 3c in a manner similar
to that disclosed in the embodiment of FIGS. 2a and 2b. After turning the cap
3c a predetermined number of times, the cap may be secured to the casing 3a
by means of an adhesive so that the strip of amorphous magnetic material will
be supported in the twisted condition.
The foregoing devices are capable of sensing low magnetic fields. FIG. 4 is
a graph showing the results of measuring the detectable magnetic field intensity
H[Oe] with the device according to the present invention against the number
of twists in the strip 1 of amorphous magnetic material. Thus, it is seen that
the level of the minimum detectable magnetic field is very low in comparison
with the minimum detectable magnetic field for conventional Wiegand effect wires
which is on the order of 3.0 [Oe]. The amorphous magnetic material used in the
measurement of the values for the graph of FIG. 4 is alloyed of Fe.sub.81 B.sub.17
Si.sub.2 by atomic weight (%).
FIG. 5 is a graph which illustrates the results of measuring the minimum detectable
magnetic field intensity H[Oe] with respect to the temperature of the atmosphere
T[.degree.C.] for an amorphous magnetic material alloyed of Fe.sub.81 B.sub.17
Si.sub.2, Fe.sub.79 Cr.sub.2 B.sub.17 Si.sub.2 and Fe.sub.78 Cr.sub.3 B.sub.17
Si.sub.2 by atomic weight (%). This property is measured 10 minutes after each
change in temperature. The strip of amorphous magnetic material is 10 cm in
length and is provided with 10 twists. Upon examination of the graph in FIG.
5, it can be seen that the level of the detectable magnetic field intensity
is stable over a wide range of temperatures especially for Fe.sub.79 Cr.sub.2
B.sub.17 Si.sub.2.
Also, as a result of the measurements, the magnetic field sensing device according
to the present invention has a sharp detectability so that the output voltage
of the magnetic sensor using the magnetic field sensing device according to
the present invention is 20 to 50 .mu.s in pulse width at the applied magnetic
field above 1.2 Oe oscillating in the frequency range of 0.01 Hz to 6 kHz.
The magnetic sensor using the magnetic field sensing device according to the
present invention may be in the form of the transmitter disclosed in German
Offenlegungsschrift No. 28 06 249 wherein a coil surrounds the magnetic field
sensing device.
In summary, the magnetic field sensing device according to the present invention
provides a highly efficient device for sensing low magnetic field levels over
a wide range of temperatures. The magnetic field sensing device according to
the present invention is provided by simply twisting a strip of amorphous magnetic
material which thereby eliminates the need for complicated, time consuming processing.
While the invention has been particularly shown and described with reference
to preferred embodiments thereof, it will be understood by those in the art
that the foregoing and other changes in form and details may be made therein
without departing from the spirit and scope of the invention.