Planar magnetic harmonic sensor for detecting small quantities of magnetic substances

A device is disclosed for detecting the presence and/or determining the configuration of a magnetic substance. The device includes a magnetic circuit having a magnetic core and an air gap. Coupled to the magnetic circuit are an excitation coil, for periodically driving the magnetic circuit into saturation, and a measurement coil, for measuring the effective appearing permeability of the magnetic circuit in the presence of the magnetic substance.

RELATED CASE 
This application contains subject matter which is related to the subject 
matter of an application entitled Planar, Core Saturation Principle, 
Low-Flux Magnetic Field Sensor filed for Thomas Seitz on even date 
herewith an assigned to the assignee hereof. The contents of the related 
U.S. application, which bears Ser. No. 07/851,443, are incorporated herein 
by reference. 
FIELD OF THE INVENTION 
The present invention relates to a device for detecting the presence and 
determining the form of small quantities of a magnetic substance. 
BACKGROUND OF THE INVENTION 
It is often desirable to detect magnetic substances which are present in 
small quantities such as magnetic ink or a paramagnetic gas such as 
oxygen. For instance, an automatic banknote recognition device may read 
and or detect an image written or printed with magnetic ink on banknotes 
of countries including the United States, Japan or Germany. Alternatively, 
a gas analyzer may detect the presence of a paramagnetic gas. 
Additionally, it is often desirable to measure the rotational speed of a 
toothed wheel made of a magnetic material or at least the speed of the 
teeth and teeth intervals which are coated with a magnetic material. 
U.S. Pat. No. 4,864,238 discloses a device for measuring low-flux magnetic 
fields of the type produced by the magnetized magnetic ink found on 
banknotes. This patent discloses an extremely sensitive sensor, 
functioning on the core saturation principle, for measuring the extremely 
weak magnetic fields produced by the magnetized ink of the banknote. Such 
magnetic fields typically have a value in the order of 10.sup.-3 Gauss. In 
comparison, the Earth's magnetic field intensity is 0.5 Gauss or 
approximately three orders of magnitude greater than the fields of 
magnetized banknote ink. As such, the above-mentioned sensor requires 
extensive and costly magnetic shielding to accomplish its task. 
Furthermore, the banknote must be magnetized by a strong permanent magnet 
before being read. 
It is the object of the present invention to provide a magnetic sensor for 
which no permanent magnet and no costly and extensive magnetic shielding 
against foreign magnetic fields is required. It is a further object to 
provide a sensor capable of static measurement, at least in detecting the 
presence of the magnetic substance, where the static measurement covers 
most of the overall image of the magnetic substance (e.g., most of a 
printed image on a banknote without relative movement between the magnetic 
substance and sensor). Such a feature would afford greater security 
against misuse. 
SUMMARY OF THE INVENTION 
These and other objectives are achieved by means of the present invention 
which is directed to a device for detecting the presence and/or 
determining the configuration of a magnetic substance. The device includes 
a sensor including a magnetic circuit which has a core and an air gap. The 
sensor operates on the core saturation principle. An excitation coil is 
magnetically coupled to the magnetic circuit which periodically drives the 
magnetic circuit into saturation. Additionally, a measuring coil is 
magnetically coupled to the magnetic circuit which measures the effective 
appearing permeability of the magnetic circuit. Thus, when a magnetic 
substances is located in the air gap of the magnetic circuit, the 
measuring coil measures a different appearing permeability thereby 
detecting the presence, and/or determining the configuration of the 
magnetic substance. 
Illustratively, the excitation coil, the measurement coil and ferromagnetic 
core are insulated from one another. In such an embodiment, each of those 
elements may be fashioned in a single, separate layer, the three separate 
layers being parallel to one another. 
The inventive device may illustratively be provided with an evaluation 
means for evaluating a harmonic of a voltage measured by the measuring 
coil. In such a case, the excitation magnetic field H[t], which induces 
the measured voltage, illustratively has an amplitude H.sub.max which is 
greater than the saturation magnetic filed H.sub.s of the magnetic circuit 
such that 2k.multidot.H.sub.max =n.multidot.H.sub.s where n is an integer 
representing the mode number of the evaluated harmonic.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, a magnetic field detector is depicted having a 
sensor 1 including a magnetic core 3 and air gap 2 which form the magnetic 
circuit 2;3. The sensor 1 also has two coils 4 and 5. The two coils 4 and 
5 are inductively coupled together and are installed on the same side of 
core 3 (the lower side, as shown in FIG. 1). 
A generator 6 supplies the first coil 4, of the two coils 4 and 5, with at 
least one excitation current i[t] of period T via a resistor 7. The 
excitation current, which is variable with respect to time, is preferably 
a sawtooth-shaped function of time t but can also be another periodic 
function such as a sinusoid function. A sawtooth-shaped excitation current 
i[t] induces a sawtooth-shaped excitation of the core 3 which produces 
fewer losses than a sinusoid excitation current i[t] with equivalent 
control. The coil 5 is a measuring coil having two terminals at which an 
output voltage u[t] of the sensor 1 appears. 
The detector shown in FIG. 1 is additionally provided with a device 8 for 
evaluating a harmonic of the output voltage u[t] of sensor 1. The output 
of the sensor 1 is connected to the input of the evaluation device 8 which 
illustratively comprises a band pass amplifier 9 and a voltmeter 10. If 
such a structure is used, the band pass amplifier 9 filters a desired 
harmonic from the frequency spectrum of the output voltage u[t] and 
amplifies it for evaluation. The voltmeter 10 is an a.c. voltmeter and 
illustratively measures the effective value of the output voltage of the 
band pass amplifier 9, i.e., of the filtered amplified harmonic. 
FIG. 1 illustratively depicts a device for use with a banknote 11 on which 
a magnetic ink is printed. The banknote 11 and the magnetic substance 
printed thereon are at least periodically located in the proximity of the 
core 3, inside the gap 2, specifically on the side of the core 3 away from 
the two coils 4 and 5, for purposes of evaluation. If the core 3 is a flat 
core, as is assumed in FIG. 1, the banknote 11 is positioned in parallel 
with the core 3, at a distance of up to approximately 1 mm from the core 
3. 
The sensor 1 operates on the core saturation principle. Since the latter is 
known and is also described in the above-mentioned prior art reference, it 
is described only briefly below for a better understanding of the 
invention. 
A graph is depicted in the upper left portion of FIG. 2 which plots an 
ideal characteristic line B[H] of a effective induction B of the magnetic 
circuit 2;3 as a function of the magnetic excitation field H[t] produced 
by the excitation current i[t]. In this graph, the hysteresis of the 
ferromagnetic core 3 is neglected and the characteristic line is assumed 
to be linear outside of the saturation. In FIG. 2, B designates the 
saturation induction corresponding to a magnetic saturation field H.sub.s. 
At the lower left of FIG. 2, the magnetic excitation field H[t], which is 
induced by the excitation current i[t] flowing through the excitation coil 
4, is plotted as a function of time t. In FIG. 2, it is assumed that the 
excitation current i[t] induces a sawtooth magnetic excitation field H[t] 
which is a periodic function of time t. The amplitude H.sub.max of the 
sawtooth-shaped magnetic excitation field H[t] is selected to be 
sufficiently large to periodically drive the ferromagnetic core 3 into 
saturation. As a result, the magnetic induction B[t] present in the 
magnetic circuit 2;3 has a trapezoidal shape with respect to time t (see 
the upper right portion of FIG. 2). The slanted edges of the trapezoidal 
waveforms produce the periodic voltage u[t] of period T in the measuring 
coil 5, the periodic voltage u[t] waveform comprising a series of 
alternating positive and negative rectangular voltage impulses. 
The characteristic lines shown in FIG. 2 by dots and dashes are produced by 
an external magnetic field H.sub.o, e.g., the magnetic field of the 
magnetized magnetic ink of a banknote 11 having a corresponding magnetic 
induction B.sub.o. In particular, the rectangular voltage impulses of the 
output voltage u[t] are asymmetrical with respect to the time axis. This 
asymmetry can be utilized to measure the strength of the external magnetic 
field H.sub.o. In each instance, the periodic output voltage u[t]comprises 
(in accordance with a Fourier analysis) a fundamental harmonic assigned 
the mode number one, and a plurality of continuously numbered harmonics 
modes, assigned mode numbers beginning with two. The mode numbers of the 
fundamental harmonic and the other harmonics are hereinafter designated by 
n. 
As described in U.S. Pat. No. 4,864,238, the strength of the 
external magnetic field H of the magnetized magnetic ink may be determined 
by evaluating the second harmonic wave of the output voltage u[t]. 
According to a Fourier analysis, the amplitude of the n.sup.th harmonic 
wave is equal to: 
EQU U.sub.n =[(16H.sub.max /(n.multidot..pi.H.sub.s)].mu..sub.o 
.multidot..mu..sub.r.sup.* n.sub.2 .multidot.f.sub.1 
.multidot.F.multidot.H.sub.s 
sin[(n.multidot..pi..multidot.H.sub.s)/(2H.sub.max)].multidot.sin[(n.multi 
dot..pi./2))1+H.sub.o /H.sub.max)] (1) 
where .mu..sub.o is the permeability of a vacuum, .mu..sub.r.sup.* is the 
effective appearing permeability of the magnetic circuit 2;3 (and also the 
slope tg.alpha. of the sensor curve B(H) outside the saturation range as 
depicted in FIG. 2), n.sub.2 is the number of windings of the measuring 
coil 5, f.sub.1 is the frequency of the fundamental mode of the excitation 
current i[t] (and therefore also the frequency of the fundamental mode of 
the output voltage u[t]), F is the surface area of the cross-section of 
the core 3 and H.sub.max is the amplitude of the magnetic excitation field 
H[t]. In an H-shaped ferromagnetic core 3 (see FIG. 4) which has two 
relatively wide main strips connected by one narrow oblong transversal 
strip, F is the surface area of a cross-section of the transversal strip 
taken perpendicular to the longitudinal axis of the transversal strip (see 
FIGS. 3-4). The effective appearing permeability .mu..sub.r.sup.* of the 
magnetic circuit 2;3 is known to be equal to 1/[N+(1/.mu..sub.r)] where N 
is the so-called demagnetizing factor and .mu..sub.r is the relative 
permeability of the ferromagnetic material of core 3. 
If the sensor 1 is subjected to a periodic magnetic excitation field H[t] 
whose amplitude H.sub.max is greater than the magnetic saturation field H 
of the magnetic circuit 2;3 and if the amplitude H.sub.max is an 
even-numbered multiple 2k.multidot.H.sub.max of the magnetic saturation 
field H.sub.s of the magnetic circuit 2;3, the core 3 is intermittently 
driven into saturation and the first sinus factor in the equation (1) (and 
thereby also the amplitude U.sub.n of the harmonic wave) is equal to zero 
despite the presence of any external magnetic field H.sub.o. This is if 
the factor k is any desired integral number. 
If a magnetic substance, such as magnetic ink, now appears in proximity of 
the sensor 1, e.g., in the air gap 2, the sensor 1 is detuned. This is 
because the demagnetizing factor N, and, therefore, also the slope of the 
sheared characteristic line B[H], are modified by the presence of the 
magnetic substance. This leads to a different value of the magnetic 
saturation field H.sub.s. Thus, the condition 2k.multidot.H.sub.max 
=n.multidot.H.sub.s for the zero amplitude value of the corresponding 
harmonic of interest is no longer met for a given value H.sub.max of the 
amplitude of the magnetic excitation field H[t]. For this reason, the full 
spectrum of harmonics once again appears. In the case of a small detuning, 
such as the detuning which occurs in the presence of magnetic ink, the 
value of the amplitude U.sub.n of the harmonic wave increases in an almost 
linear manner. 
In a first embodiment according to the invention, the evaluated harmonic is 
an even-numbered harmonic of the output voltage u[t] of sensor I. In order 
to obtain maximum sensitivity of the device in that case, a constant 
magnetic field H.sub.o, e.g., the magnetic field of a permanent magnet, 
should be present in addition to the magnetic excitation field H[t]. Such 
additions maximize the value of amplitude U.sub.n of the corresponding 
even-numbered harmonic. Instead of a permanent magnet, a d.c. current 
I.sub.o, which produces a constant magnetic field H.sub.o, flows through 
the first coil 4 during the operation of the invention in addition to the 
periodic time-variable excitation current i[t]. The amplitude U.sub.n of 
the even numbered harmonics reaches its maximum when the second sinus 
factor in the equation (1) has a value close to one. Hence, n times the 
constant magnetic field H.sub.o should preferably be equal to an odd 
multiple of the amplitude H.sub.max of the magnetic excitation field H[t], 
i.e., equal to (2k+1).multidot.H.sub.max, to maximize the value of the 
amplitude U.sub.n of the even-numbered harmonic. 
Because a sinus function is known to be relatively flat near its maximum, 
the sensor 1 is relatively insensitive to changes of external magnetic 
fields including interfering magnetic fields (e.g., the Earth's magnetic 
field). Thus, the sensor 1 may function without magnetic shielding. This 
is possible because the sensor 1 does not measure a magnetic field but 
merely indirectly determines, by way of the amplitude U.sub.n of a 
harmonic the effective appearing permeability .mu..sub.r.sup.* of the 
magnetic circuit 2;3. 
To avoid high frequency problems and/or high frequency losses, such as 
strong attenuation, strong eddy current losses etc., preferably a 
relatively low-frequency harmonic is selected for evaluation. If an 
even-numbered harmonic is used, it is preferably the fourth harmonic of 
the output voltage u[t] of sensor 1. The amplitude of the fourth harmonics 
is equal to: 
EQU U.sub.4 =(4/.pi.).multidot..mu..sub.o .multidot..mu..sub.r.sup.* 
.multidot.n.sub.2 .multidot.f.sub.1 .multidot.F.multidot.H.sub.max 
.multidot.sin(2.pi.H.sub.s /H.sub.max).multidot.sin(2.pi.H.sub.o 
/H.sub.max) (2) 
For the evaluation of the fourth harmonic, the amplitude H.sub.max of the 
magnetic excitation field H[t] is preferably twice as large as the 
magnetic saturation field H.sub.s of the magnetic circuit 2;3. In other 
words, by selecting H.sub.max =2.multidot.H.sub.s, the first sinus factor 
in the equation (2) is set equal to zero. In this case, the constant 
magnetic field H.sub.o should preferably have a value of H.sub.max /4, 
i.e., equal to one fourth of the amplitude H.sub.max of the periodic, 
time-variable magnetic excitation field H[t]. 
In a second embodiment according to the invention, the evaluated harmonic 
is an odd-numbered harmonic of the output voltage u[t] of sensor I. For 
the same reasons mentioned in regard to the evaluation of even-numbered 
harmonics modes, the harmonic serving for the evaluation is, again, 
preferably a relatively low-frequency harmonic. Preferably, in such a 
case, the third harmonic of the output voltage u[t] of sensor 1 is used. 
The amplitude of the third harmonic is equal to: 
EQU U.sub.3 =(16/3.pi.).multidot..mu..sub.o .multidot..mu..sub.r.sup.* 
.multidot.n.sub.2 .multidot.f.sub.1 .multidot.F H.sub.max 
.multidot.sin(3.pi.H.sub.s /2H.sub.max).multidot.cos(3.pi.H.sub.o 
/2H.sub.max) (3) 
For the evaluation of the third harmonic, the amplitude H.sub.max of the 
magnetic excitation field H[t] is preferably 1.5 times the magnetic 
saturation field H.sub.s of the magnetic circuit 2;3. In other words, by 
selecting H.sub.max =1.5.H.sub.x, the first sinus factor in the equation 
(3) is set equal to zero. Using the third harmonic therefore, is more 
advantageous than using the fourth harmonic because a smaller amplitude 
H.sub.max of the time dependent magnetic excitation field H[t] is required 
to cancel out the harmonics which appear in the absence of a magnetic 
substance. In order to obtain maximum sensitivity (i.e., to maximize the 
amplitude U.sub.n of the evaluated odd-numbered harmonic of the magnetic 
field sensor with an odd-numbered harmonic mode, no constant magnetic 
field H.sub.o should be utilized in conjunction with the magnetic 
excitation field H[t]. This is because the second sinus factor of the 
equation (1) is always a cosinus factor (see equation (3)). This cosinus 
factor is always equal to one if H.sub.o =0. The second embodiment 
therefore has the advantage that the value of the amplitude U.sub.n of the 
odd-numbered harmonic wave is maximized when no constant magnetic field 
H.sub.o is present. 
The cosinus function is just as insensitive to external magnetic fields 
when it is near its maximum as the sinus function. Thus, no shielding is 
needed in the second embodiment. In both embodiments, the sensor 1 does 
not measure a magnetic field but instead measures the effective appearing 
permeability .mu..sub.r.sup.* of the magnetic circuit 2;3 having a 
magnetic substance located within its air gap 2. For example, the peak 
value U.sub.4 of the fourth harmonic wave may have a value in the order of 
magnitude of 10.mu.V when a dollar note is 1 mm from the sensor 1. The 
frequency of the periodic time-variable excitation current i[t] is 
preferably 20 to 200 kHz and the measuring coil 5 may illustratively have 
100 windings. 
In FIG. 3, the magnetic ink is schematically shown as a small black 
rectangle located below the banknote 11. The sensor is preferably a planar 
flat sensor having the structure shown in FIGS. 3 and 4. In such a case, 
the two coils 4 and 5 are preferably single-layer coils which are 
insulated from each other by a first insulation layer 13. Preferably, the 
two coils 4 and 5 are also installed in two separate and parallel layers 
on a support material, e.g., on a substrate 12. Illustratively, the 
substrate 12 may be made of ceramic or some other inexpensive insulating 
material. 
The two single-layer coils 4 and 5 are preferably in the form of 
rectangular coils. Each is preferably disposed within its layer so that 
one fourth of the two single-layer coils 4 and 5, i.e., one rectangular 
side of straight and parallel conductors of each coil 4 and 5, at least 
partially overlap each other. 
The ferromagnetic core 3 is thin and has a nearly constant thickness. It is 
installed in a third layer which is parallel to an electrically insulating 
second insulation layer 14 formed on the layer which comprises the 
single-layer coil 5. The core at least partially overlaps the overlapping 
portion of the two coils 4 and 5. In FIGS. 3 and 4, the coil 4 is 
installed in a first layer on the substrate 12 while the measuring coil 5 
is in turn installed in a second layer on the first insulation layer 13. 
The core 3 is installed in a third layer on the second insulation layer 
14. The air gap 2 of the magnetic circuit 2,3 is defined by the space 
located outside the upper and lower parallel surfaces of the ferromagnetic 
core 3. 
In a first variation of the core 3, shown in FIG. 4, the ferromagnetic core 
3 has an H-shaped cross-section taken in parallel with the substrate 12. 
In a second variation of the core 3, shown in FIG. 5, the ferromagnetic 
core 3 has a rectangular figure eight cross-section, with its upper and 
lower cross-strips having an additional air gap 15 or 16. When a banknote 
is fed past the air gaps 15 and 16, each functions as a reading gap for 
reading the contents of information of one track of magnetic material 
present on the banknote. The air gap 2, on the other hand, serves to 
detect the presence of the magnetic material on the banknote. 
The H-shaped or figure-eight-shaped configuration of the core 3 serve to 
concentrate the magnetic flux. It is desirable to concentrate the magnetic 
flux as much as possible in the center of the core 3. To that end, the 
central transversal strip of the core 3 (of either core variation) is 
preferably kept as narrow as possible. This reduces the demagnetization 
factor N. For example, the width of the central transversal strip can 
measure 0.5 mm, and its length 4 mm. For the same reason, the core 
thickness t is preferably reduced as much as possible. This can be 
achieved with especially satisfactory results if the sensor is produced 
using planar micro-technology. The core 3 preferably has a minimum core 
thickness t of approximately 0.025 mm when produced using hybrid 
technology, and approximately 0.5 .mu.m using planar micro-technology. The 
material of the ferromagnetic core 3 is preferably an amorphous magnetic 
metal which is also known as "magnet glass". Illustratively, the width of 
the wide main strips of the H-shaped core 3 is 5 mm, while their length is 
40 to 60 mm so as to cover a large portion of the width of a banknote. 
Thus, although only a single measuring head is used, almost the entire 
width (at the least, 1/4 to 1/3 of the width) of the banknote is scanned. 
The demagnetization factor N of a planar sensor is substantially determined 
by the ratio t/L, i.e., the comparison of the core thickness t with the 
core length L of the central transversal strip of the H-shaped or 
figure-eight-shaped core 3. The changes of the demagnetization factor 
brought about by a banknote bearing magnetic ink can thus occur either as 
a result of a change in the core length L or in the core thickness t. By 
using an extremely thin core 3, however, the demagnetization factor N, and 
therefore also the sensor 1, are much more sensitive to changes in the 
core thickness t than to changes in the core length L, the former being 
caused by feeding the banknote 11 near the core 3. For this structure, the 
coil 4 may not be wound around the core 3 as would be the case if the 
demagnetization factor were to function in response to a change in the 
length of core 3. The single-layer coil 4 shown in FIGS. 3 and 4 meets 
this condition. Thus, in the preferred embodiment, the thickness t, rather 
than the length L, of the ferromagnetic core 3 is changed by the magnetic 
substance. 
Because the sensor 1 is made as a flat sensor, it requires very little 
space. Its operation is practically offset-free, i.e., its offset voltage 
is practically equal to zero. Also, there is practically no signal noise 
as only the known, extremely weak Barkhausen signal noise of core 3 is 
present. The output voltage u[t] of sensor 1 is independent of a relative 
speed between the magnetic substance and the sensor. Static measurement is 
thus possible. 
When the core 3 in the form of a figure eight is used, not only is it 
possible to detect a magnetic substance but it is also possible to read 
the configuration of the magnetic substance as the two outer transversal 
strips of the core 3 are able to read two lines of the banknote. In this 
manner, the value of the banknote can be determined with a high degree of 
reliability. 
In summary, a magnetic field sensor is disclosed capable of detecting the 
presence and/or measuring the strength of small quantities of magnetic 
substances. The embodiments disclosed are intended to be merely 
illustrative of the invention. Numerous other embodiments may be devised 
by those ordinarily skilled in the art without departing from the spirit 
and scope of the following claims.