Method and apparatus for generating and detecting acoustic signals

A magnetic/acoustic transducer is disclosed. The transducer can be used in security/smart tag applications. The transducer includes a sensor tag made of magnetic metallic glass having a relatively high magnetostriction and a relatively low coercivity. Driving signals are provided by an rf dipole loop antenna. The tag responds to the rf signals and converts the exciting magnetic field into acoustic signals via magnetoelastic coupling. That is, the tag is forced to vibrate in unison with the incident electromagnetic signals generating longitudinal acoustic waves along a length of the tag. This results in radiation of ultrasound waves in air which can then be detected and characterized using an ultrasound microphone or a piezoelectric sensor. The tag is provided having a length equal to one half or one quarter long of an acoustic wavelength so that an acoustic resonance condition is established to maximize the generation of ultrasound waves in air. The measured ultrasound signal is locked in phase with the excitation or reference signal for sensitive long-range detection. The tag can operate in a magnetized or a demagnetized state to stimulate binary signals for security-tag applications. Tags of different length and/or geometry can be deployed in combination so that the tag transducer produces unique and distinguishable frequency spectrums to be used as smart tags.

RELATED APPLICATIONS 
This application is a continuation of provisional application No. 
60/029,077 filed, Oct. 23, 1996 and provisional application No. 
60/034,008, filed Jan. 2, 1997. 
BACKGROUND OF THE INVENTION 
This invention is generally related to the design and fabrication of 
security and smart tags that generates acoustic waves in air to be 
detected via acoustic sensors. More particularly, this invention relates 
to the design and fabrication of a novel and improved tag system which can 
respond to electromagnetic driving signals in terms of acoustic waves 
using an efficient scheme to facilitate sensitive long-range transmission 
and detection. 
As is known in the art, security-tag systems are used in libraries, grocery 
stores, clothing, video and other merchandise outlets, etc. . . . to 
monitor items and detect movement of equipment such as the movement of 
equipment in factories to location of infants in hospital nurseries. Smart 
tags are presently used for a number of applications in the civilian and 
military sectors, including item identification, toll passes, and barrier 
identification. For security-tag applications the tag can generate two 
levels of identification indicating the state of the tag being 
interrogated. For a security tag its state can be interchanged via some 
external means. For smart tags they are required to generate multi-levels 
of identification, usually a predetermined property of the tag not subject 
to change. For both tag-system applications the traditional approach 
always involves the use of electromagnetic dipole antennas for detection, 
detecting the response of the tag utilizing some nonlinear structure of 
the tag circuitry. As such, the response signal from the tag is very weak, 
being at best a second-order effect of the employed detection scheme. 
Thus, the detection can be relatively difficult or in some instances 
impossible due to the existence of noise in the surrounding environment. 
Furthermore, noise can cause a detector to falsely produce an alarm 
signal. Furthermore, current smart tags are relatively expensive and carry 
a limited amount of information. 
What is needed for operation of a security/smart tag system is to set up an 
interrogation zone (usually defined by a magnetic dipole antenna pair) 
near an entrance or an exit of an area to be secured or classified. When 
the electromagnetic field in the interrogation zone is perturbed by a 
suitable object (e.g. a "tag", "marker", or "label") the system detects 
the perturbation. The "tags" can be electrical or magnetic. The 
perturbation signal must be of a nature that it can be resolved from a 
signal produced by a drive antenna and distinguishable from noise signals 
generated by other equipment and objects in and around the interrogation 
zone. 
Conventional techniques of system design for security/smart tags involve 
the use of magnetic tags and/or other electronic elements including 
Doppler shifting circuits and varactors and diodes. Upon interrogation, 
the tag reacts with an input electromagnetic signal to generate 
electromagnetic radiation which differs from an original electromagnetic 
field either in frequency (frequency-domain characterization) or in 
waveform (time-domain characterization). For both frequency- and 
time-domain detections the employed tags are generally required to possess 
high degree of nonlinearity so that high-order harmonics or waveform 
distortions can be effectively generated and detected. 
For both systems, a transmit antenna must focus its energy into the 
interrogation zone, not in directions where it could interfere with other 
electronic equipment: cash registers, computers, scanners, or other 
electronic systems. The receive antenna must be sensitive to the weak 
response of a tag which may fill only one part in 10.sup.10 of the 
interrogation zone. It must not trigger an alarm in response to electrical 
signals from the transmit antenna, or from other electrical equipment or 
magnetic objects. Magnetic shielding is therefore required for these 
antennas to improve the efficiency of a traditional tag system. 
The shielding material should have none of the characteristics of the type 
of tag for which the system is designed. This is obvious, but it is not 
trivial to achieve because the shield is much closer to the antenna and 
has a volume 10.sup.7 to 10.sup.8 times greater than that of the tag. 
Specifically, the shields must be very linear in their electromagnetic 
response and especially free of harmonics in the frequency range of the 
tag for frequency-domain detection, or, the shields must not show much 
waveform distortion in the time-scale characterizing the imposed 
electromagnetic pulses for time-domain detection. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a 
security/smart tag system to overcome the aforementioned difficulties 
encountered in the prior art. 
Specifically, it is an object of the present invention to provide a 
security/smart tag system in which ultrasounds, instead of electromagnetic 
waves, are detected in an interrogation zone. This can be achieved via the 
use of magnetostrictive tags. Since ultrasound can effectively propagate 
in air, it provides an extended method in long-range detection for the 
interrogated signals. 
Another object of the present invention is to provide sensitive detection 
of the enhanced transducer signal. Through the use of a resonant structure 
of the tag, the generation of ultrasounds becomes actually a first-order 
effect, and, hence, its response can be much more readily detected. 
Another object of the present invention is to provide essentially 
noise-free detection. By locking-in the detector phase with the 
transmitter, one can effectively amplify the signal voltage to many orders 
without amplifying the accompanying noise. This facilitates greatly signal 
detection. 
Another object of the present invention is to provide a suitable choice of 
the tag material. In order to produce maximum magnetomechanical coupling, 
magnetic tags possessing maximum magnetostriction coefficient are 
preferred. Also, in order to fully saturate the magnetomechanical coupling 
effect, the tags are preferably provided having a magnetic coercive force 
which is selected to be relatively low. Another benefit from low coercive 
field tags is that the drive required from the exciting magnetic field can 
be significantly reduced which translates into lowering costs of a 
security system utilizing such tags. In order not to dissipate ohmic heat, 
and, hence, to increase the transducer efficiency, the tags are preferably 
provided having a relatively low conductivity characteristic. For the same 
reason, the tag shall exhibit minimal hysteresis loop. Based on the above 
considerations, a material such as amorphous Fe.sub.40 Ni.sub.40 B.sub.20 
may be used as the tag materials. Iron and/or nickel may be replaced by 
other transition metals or rare-earth metals to affect a high 
magnetostriction tag material. Other considerations for tag materials 
include low conductivity and small magnetic hysteresis loops, as required 
by maximum power-conversion (magnetic to mechanical) efficiency. 
Another object of the present invention is to provide a simplified 
detection scheme for the tag systems. Since noise is minimized in the 
detection scheme and multi-path reflection is much less important as 
compared with the traditional systems, the need for magnetic shield is 
therefore minimized and in some applications may even be totally 
eliminated. 
Another object of the present invention is to provide cost-effective 
production of the tags. Since the tags may be cut directly from 
cold-rolled amorphous magnetic foils, the tags can be manufactured 
relatively inexpensively. For example, in some applications the cost of a 
security/smart tag can be as low as only a few cents. Power input to the 
current driver can be reduced and, therefore, costs by utilizing high 
magnetostriction and low coercive-field magnetic foils. 
Another object of the present invention is to provide a smart tag which 
occupies a relatively small volume but which stores a relatively large 
amount of information. This is achieved by deploying many tags of 
different length and geometry in a single package. Each of the tags 
operate at a different frequency. Thus each package can provide a 
different frequency distribution in a frequency-domain characterization. 
Another object of the present invention is to provide more security over 
items seeking protection. For example, while metal sheets with high 
conductivity and permeability can conceal the radiation from a traditional 
security tag, such sheets cannot block the propagation of acoustic waves 
generated from tags manufactured in accordance with the present invention. 
Briefly, in a preferred embodiment, the present invention discloses a novel 
technique for converting an electromagnetic interrogation field into 
ultrasonic waves via the use of a magnetostrictive transducer tag. The tag 
is arranged in mechanical resonance with the source signal whose phase is 
locked to an acoustic detector to facilitate sensitive long-range 
detection. Since acoustic detection disclosed in the present invention 
minimizes noise interference, there is no need to use a magnetic shield as 
required by a traditional tag system. Also, the propagation of ultrasounds 
cannot be blocked by a magnetic metal sheet. Fabrication of the tag system 
is inexpensive, and the information contained in a smart tag unit can be 
abundant, limited only by the resolution of the acoustic detector. 
It is an advantage of the present invention that it provides a 
security/smart tag system in which ultrasounds, instead of electromagnetic 
waves (rf-magnetic field), are being detected in the interrogation zone. 
This can be achieved via the use of magnetostrictive tags. Since 
ultrasounds can effectively propagate in air, it provides an extended 
method in long-range detection for the interrogated signals. 
Another advantage of the present invention is to provide sensitive 
detection of the enhanced transducer signal. Through the use of a resonant 
structure of the tag, the generation of ultrasound becomes actually a 
first-order effect, and, hence, its response can be much more readily 
detected. 
Another advantage of the present invention is to provide essentially 
noise-free detection. By locking-in the detector phase with the 
transmitter, one can effectively amplify the signal voltage to many orders 
without amplifying the accompanying noise. This facilitates greatly signal 
detection. 
Another advantage of the present invention is to provide a suitable choice 
of the tag material. In order to produce maximum magnetomechanical 
coupling, magnetic tags possessing maximum magnetostriction coefficient 
are preferred. Also, in order to fully saturate the magnetomechanical 
coupling effect, the tags should preferably exhibit minimum magnetic 
coercive force. In order to dissipate less heat, the tags are preferably 
provided having a relatively low conductivity and minimum hysteresis 
loops. For these reasons amorphous B.sub.20 T.sub.40 R.sub.40 may be 
ideally used as the tag materials, where T can be iron, and R another 
transition metal, Ni, Co, and/or their alloys. It may also be possible to 
use rare-earth metal alloys for R and T. Other considerations for tag 
materials include low conductivity and small magnetic hysteresis loops, as 
required by maximum power-conversion (magnetic to mechanical) efficiency. 
Another advantage of the present invention is to provide a simplified 
detection scheme for the tag systems. Since noise does not participate 
actively in the detection procedure and multi-path reflection is much less 
important as compared with the traditional systems, the need for magnetic 
shield is therefore totally eliminated. 
Another advantage of the present invention is to provide cost-effective 
production of the tags. Since the tags can be cut directly from 
cold-rolled amorphous magnetic foils, their costs can be very low. For 
example, the cost of a security/smart tag can be as low as only a few 
cents. 
Another advantage of the present invention is to provide more information 
that a smart tag can carry in a reduced volume. This is achieved by 
deploying many tags of different length and geometry in one unit to arrive 
at different frequency distribution under frequency-domain 
characterization. 
Another advantage of the present invention is to provide more security over 
items seeking for protection. For example, while metal sheets with high 
conductivity and permeability can conceal the radiation from a traditional 
security tag, they can hardly block the propagation of acoustic waves 
generated from the present device. 
These and other objects and advantages of the present invention will no 
doubt become obvious to those of ordinary skill in the art after having 
read the following detailed description of the preferred embodiment which 
is illustrated in the various drawing figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Since environmental noise generated by electrical motors and traffic 
vehicles are generally below 50 kilo-Hertz (kHz), this dictates that a 
preferred acoustic system shall operate above 50 kHz to avoid these noise 
signals. Also, the tag is desired to have minimal length or size, which 
translates into high frequency operation of the transducer tag, since the 
tag is of a length equal to one half or one quarter of the imposed 
acoustic wavelength. However, for ultrasounds to propagate effectively in 
air, the operation frequency cannot be too high, since the attenuation 
constant of (longitudinal) acoustic waves, denoted as .alpha., is 
proportional to the square of the frequency, denoted as f. In air at room 
temperature, .alpha.=0.006 dB/m for f=60 kHz. Therefore, the optimal 
frequency range for a tag system to operate is from 50 kHz to about a few 
hundreds of kilo-Hertz. 
To achieve mechanical resonance, the length of the tag denoted as l, is 
required to equal one half or one quarter of the acoustic wavelength, 
denoted as .lambda.. That is, l=.lambda./2 if both ends of the tag are 
unloaded, or l=.lambda./4 if one end of the tag is unloaded and the other 
end of the tag is rigidly clamped. In solids .lambda. may be written as 
##EQU1## 
where C denotes the elastic modulus and .rho. the mass density of the tag 
material. 
For C=2.0.times.10.sup.12 erg/cm.sup.3, .rho.=7.8 g/cm.sup.3, Eq.(1) 
implies .lambda.=8.4 cm for f=60 kHz, for example. 
Upon interrogation the tag sample will vibrate in unison with the incident 
electromagnetic (rf-magnetic) field at the same frequency. As such, 
electromagnetic energy is converted into kinetic energy and this energy is 
transferring at a rate of 
##EQU2## 
where V is the volume of the tag and s is the induced strain equal to the 
value of magnetostriction at a particular bias field strength H.sub.0 as 
will be discussed below in conjunction with FIG. 2. In Eq.(2) the acoustic 
field has been assumed to be a sinusoidal distribution along the tag over 
a length of .lambda./2 or .lambda./4. Assume a fraction, F, of the total 
power is transferred into air as ultrasonic waves propagating in air away 
from the tag 20 sample. This implies the following relationship. 
##EQU3## 
where r denotes the distance from the tag, u.sub.air sound velocity in 
air, .rho..sub.air mass density of air, and T.sub.air the generated stress 
field in air. Combining Eqs.(2) and (3) one obtains 
##EQU4## 
As an order of estimate, the following values are assumed: 
V=1.45.times.10.sup.-2 cm.sup.3, C =2.0.times.10.sup.12 erg/cm.sup.3, 
s=10.times.10.sup.-6, r=100 cm, F=0.1, f=60 kHz, u.sub.air 
=3.43.times.10.sup.4 cm/s, and .rho..sub.air =1.24.times.10.sup.-3 
g/cm.sup.3. The generated stress field in air is, from Eq. (4), T.sub.air 
=1.71 dyn/cm.sup.2 =0.171 Pa. This stress field can be readily detected 
using a microphone probe equipped with a preamplifier, for example, probe 
type 4138, preamplifier type 2633/70, and adaptor type UA0160 (Bruel & 
Kjaer Instruments, Inc., Decatur, Ga.). 
In Eq. (4) the energy transferring factor F depends on the coupling between 
the tag and its surrounding air. It is expected that maximum coupling 
efficiency results if the tag is driven at mechanical resonance, say, the 
tag is of a length equal to one-half or one-quarter the acoustic 
wavelength. Also, F will increase if the tag is backed up by a cavity 
served as a cushion (transformer) layer as shown, for example, in FIGS. 3 
and 4 below. However, the particular dimension of a cavity should provide 
optimal coupling coefficient of F and may be determined in any particular 
system using empirical techniques including iterative empirical 
techniques. 
FIG. 1 is a plot of T.sub.air, or .sqroot.F, as a function of frequency f. 
In FIG. 1 the term f.sub.0 denotes the frequency at which mechanical 
resonance occurs and .DELTA.f denotes half the line width. The quality 
factor 
Q is defined as Q=f.sub.0 /.DELTA.f. It should be noted that Q is an 
important factor in designing efficient tag transducers for smart-tag 
system applications: Q relates to the resolution power of the tag system 
in the frequency domain. When a maximum amount of data is desired to be 
packed across a fixed frequency range, Q is preferably selected having a 
value which is as large as possible. 
FIG. 2 is a plot of magnetostriction of the tag, .eta., as a function of 
the bias magnetic field, H. In FIG. 2A h.sub.c ' denotes the magnetic 
field beyond which the magnetization value 4.pi.M and the magnetostriction 
value .eta. corresponds to a saturated magnetization value 4.pi.Ms and a 
saturated magnetostriction value .eta..sub.0. When the tag is biased at a 
magnetic field strength of H.sub.0 and the driving rf-field is of a 
magnitude h.sub.rf, the induced rf-strain field is then given as s. 
Therefore, for an efficient tag system design, it is desirable to maximize 
.eta..sub.0 to induce maximal strain and to minimize H.sub.c, the coercive 
force, or H.sub.c ', the saturation force, so that minimal h.sub.rf is 
required to generate a given amount of strain field, s. 
The tag is preferably provided from a material such as amorphous Fe.sub.40 
Ni.sub.40 B.sub.20 manufactured by (Metglass Products, Parsippany, N.J.). 
This material is provided having a saturation magnetization, 4.pi.M.sub.s 
=10 kG, H.sub.c .about.0.1-0.3 Oe, remanence, 4.pi.M.sub.r .about.1-2 kG, 
.eta..sub.0 =14.times.10.sup.6, and a conductivity, .sigma., which is 
about one tenth that of metal iron. It should be noted that low .sigma. 
value is advantageous, since the tag then dissipates less energy as ohmic 
heat. Those of ordinary skill in the art will appreciate of course that 
other materials or combinations of materials having similar material 
characteristics and electrical and magnetic properties may also be used. 
The amount of strain, s, induced by a fixed rf field, h.sub.rf, depends on 
the bias condition of the film, H.sub.0. In FIG. 2 it is seen that s is 
minimum if H.sub.0 is close to 0 (demagnetized state), whereas s is a 
maximum if H.sub.0 reaches .sub.c ' (magnetized state). Therefore, one may 
apply a piece of semi-hard magnetic material adjacent to the soft tag to 
control the magnetization state of the tag. For example, a strip of 
Arnokrome.RTM., Crovac.RTM., or Vically.RTM., of dimension comparable to 
that of the tag, can provide the bias field having a magnetic field 
strength typically of a few Oe which is required to hold the tag near its 
optimal magnetoelastic coupling point near H.sub.c ' shown in FIG. 2A. If 
the semi-hard magnet (H.sub.c .about.50-100 Oe) is demagnetized, the tag 
becomes demagnetized and hence the sensor is deactivated. 
Thus, the tag can provide two levels of acoustic radiation, Eq.(4), 
characterizes the state of the tag controlled by the semi-hard magnet. 
This is the situation that a security-tag applies. Therefore, say, when a 
merchandise item equipped with a security tag has not been authorized for 
checking out, the tag is in the activated state which will consequently 
alert the alarm. However, after checking out the semi-hard magnet is 
demagnetized and the tag is deactivated so that the alarm will no longer 
be alerted. 
Referring now to FIG. 3, a tag 108 includes a first tag portion 110 which 
may be provided, for example, from a material such as amorphous Fe.sub.40 
Ni.sub.40 B.sub.20, a semi-hard magnet portion 120 and a base portion 130 
which may be provided from a non-magnetic material and to which tag 
portions 110, 130 may be coupled using bonding via glue or epoxy or other 
fastening techniques. The base 130 may be provided, for example, from 
non-magnetic stainless steel, or plastic which may be injection molded or 
any other non-magnetic material from which a low cost, durable base may be 
provided. In FIG. 3 the tag 110 and the semi-hard magnet 120 are brought 
face to face, spaced by a predetermined distance. Here, the space between 
the facing surfaces is filled with air although in some embodiments it may 
be desirable to fill the space with some other dielectric. The tag 110 is 
affixed to the magnet 120 at one end of its length 130; the other end of 
the tag 110 is set free. This sets the boundary conditions for a 
mechanical .lambda./4-resonator. 
For smart-tag applications the requirement for interchangeable 
magnetization states of the tag element is relaxed. As such, the need for 
a semi-hard magnet is eliminated, H.sub.0 =0, and the driving field 
h.sub.rf in FIG. 2 is generally required to exceed H.sub.c ' to optimally 
excite the strain field in the tag. However, instead of using a single 
piece of magnetoelastic element, multi-elements shall be used. It should 
be noted that in some applications it may be desirable to provide a tag 
having multiple magnetic status and a detector having a sensitivity which 
allows detection of the different magnetic states. 
Referring now to FIG. 4 a smart-tag 208 contains five tag elements or 
resonators, denoted as 210, 220, 230, 240, and 250, respectively. Tag 
elements are coupled to a side wall 260 to form quarter-wave resonators. 
In one embodiment tag elements 210-250 are attached rigidly to side wall 
260 but those of ordinary skill in the art will appreciate of course that 
in some embodiments tag elements 210-250 may be removably coupled to side 
wall 260. For example, tag elements may be coupled to side wall 260 via a 
snap-on connection, a tongue and groove connection or any other connection 
technique known to those of ordinary skill in the art. Again, air cushion 
is formed between the tags 210-250 and a bottom plate 270 so as to enhance 
the Q values of the resonators. The length of the tag elements 210-250 
differ and thus they resonate at different frequencies, denoted as 
f.sub.1, f.sub.2, f.sub.3, f.sub.4, and f.sub.5, respectively. The tag 
elements 210-250 thus function as five mechanical resonators which are 
provided having Q values which are large enough to result in the 
(ultrasonic) radiations being unambiguously distinguished by a detection 
circuit such as the detection circuit described below in conjunction with 
FIG. 5. 
Therefore, upon interrogating these five tag elements using their 
respective resonant frequencies, f.sub.1 to f.sub.5, one is able to tell 
the existence of these tag elements and thus the existence of tag 208. The 
operation of one or more of the resonators 210-250 can be prevented via a 
resonator blocker 280, 290. In one embodiment the operation of some of the 
tag elements 210-250 can be blocked by using mechanical damping layers, 
say, rubber strips, disposed underneath one or more of elements 210-250 
the tag. This is shown in FIG. 4 where blockers 280 and 290 are used to 
block operation of tag elements 240 and 220, respectively. The blockers 
are disposed between and in contact with at least one portion of a 
resonator such as resonators 210-250 and a portion of a bottom plate 270. 
In one particular embodiment, the blockers project from a first surface of 
bottom plate 270. The blockers may be provided as pieces separate from 
bottom plate 270 in which case the blockers are preferably fastened to the 
bottom plate 270 utilizing glue, epoxy, ultrasonic welding or other 
welding techniques or any other fastening technique well known to those of 
ordinary skill in the art. Alternatively in some applications it may be 
desirable to provide the blockers as an integral part of base plate 270 
using injection molding, milling or any other manufacturing techniques 
known to those of ordinary skill in the art. 
The particular tag system of FIG. 4 includes five resonators 210-250 of 
which resonators 220, 240 are blocked by blockers 280, 290. Thus the tag 
208 of FIG. 4 stores binary information of (10101). In a system which 
includes five resonators, thirty-two different combinations of resonators 
may be provided by selecting different combinations of blockers. It should 
be noted that other systems may include fewer or greater than five 
resonators and thus other systems may be provided having fewer or greater 
than thirty-two different resonator combinations. 
Although the tag elements can be arranged side by side in a linear array of 
resonators as shown in FIG. 4, they can also be packed together one above 
another in a planar or non-planar array geometry to reduce the overall 
packaging volume. Also, it is not necessary to have a rectangular 
geometry. For example, a triangular tag or tag element, or a circular tag 
or tag element, or combinations of any shaped tags and tag elements 
including arbitrarily shaped tags and tag elements, can be readily 
characterized by scanning the frequency in the interrogation zone. As 
such, almost an unlimited amount of information can be stored in the tag, 
to be limited only by the resolution power of the detection circuit. The 
particular shape used for tags and tag elements depends upon a variety of 
factors including, but not limited to, the cost and ease with which such 
tags and tag elements can be provided as well as the required strength of 
signals provided by the tag. 
Referring now to FIG. 5, driving and detection circuits of the tag system 
are shown to include a function generator 310 which generates sinusoidal 
signals at frequencies dictated by a tag 370 which may be one of the types 
described in conjunction with FIGS. 3, 4 or 6-8. This signal is fed to a 
power amplifier 350 to drive a dipole antenna 330 which may, for example, 
be provided in the form of multiple loops. However, in order to 
effectively feed the antenna, a capacitor 360 with variable capacitance is 
inserted in the driving circuit to cancel the inductance of the antenna 
loop. 
Antenna 330 emits an interrogation signal in an interrogation zone. A dc 
bias 320 is also used in FIG. 5, which generates a dc field in the 
interrogation zone to offset any remanent field (earth field) there. The 
detection circuit includes a microphone 380 as the front end receiver. 
Microphone 380 may be one of the types manufactured by Polaroid 
Corporation and included as one of the 7000 series Electrostatic 
Transducers. Those of ordinary skill in the art will appreciate of course 
that other microphones having similar characteristics may also be used. 
The particular microphone selected should be able to detect signals 
emitted by a tag. The microphone is fed to a lock-in amplified 340 whose 
phase is locked with the source generator 310. The amplified signal can 
then be observed and then manipulated from a PC console 390. It should be 
noted that antenna 330 and microphone 380 may be disposed in physically 
separate areas of a location in which the system is disposed. 
Analogous to tag configurations shown in FIGS. 3 and 4, other possible 
alternatives are also suggested in FIGS. 6 to 8. 
Referring now to FIG. 6, a tag 408 includes a plurality of magnetoelastic 
tag elements, 410 and 420 coupled as a unit to act as a dipole source to 
excite acoustic waves in air. The tag elements 410 and 420 are provided 
having a length corresponding to one-quarter of a wavelength of the 
acoustic waves in the tag element material. The tag elements are separated 
by a distance l.sub.0 equal to one-half the wavelength of the ultrasonic 
waves as measured in air. Since the tag elements 410, 420 move in 
horizontal directions as indicated by arrows 411, in order to beat air 
efficiently, the tags' edges 413 are bent into vertical positions, since 
the tag elements 410, 420 are expanding/contracting in the horizontal 
direction when responding to the driven interrogation signals. The tag 
elements 410, 420 are attached to a fixed frame 430 to form .lambda./4 
resonators. A semi-hard magnet (here shown in phantom and denoted 440) may 
be disposed under the frame as for the security tag-system applications. 
FIG. 7 is derived from FIG. 6 where the dipole source is realized in the 
form of a resonant cavity cut as a slot 530 in a frame 520. The slot is of 
a depth .lambda..sub.0 /4, corresponding to a quarter wavelength of the 
ultrasonic waves in air. A magnetoelastic tag element 510 is suspended 
across the frame 520, fixed at both ends to serve as a .lambda./2 
resonator. Here .lambda. denotes the acoustic wavelength in the tag 
element material. Therefore, upon responding to the interrogation signals, 
the tag element expanding/contracting horizontally, as indicated by arrow 
511, converting into vertical vibrational motion of the tag, because the 
total horizontal length of the tag element 510 is fixed by the fixed frame 
520. This results in excitation of standing acoustic modes in the slot 530 
which then emits ultrasounds. The width W of the slot 530 is selected in 
accordance with a variety of factors including, but not limited to, the 
length of tag element 510 and the depth D of slot 530. The particular slot 
width W used in a particular application may be determined empirically 
using iterative techniques and selected to result in optimal detection 
characteristics in a detection system. A semi-hard magnet may be disposed 
under the frame as for the security tag-system applications, for example, 
as discussed above in conjunction with FIG. 6. 
Another variation of the embodiments shown in FIGS. 6 and 7 is shown in 
FIG. 8 where the magnetoelastic tag 610 is bent into a arc whose diameter 
is .lambda..sub.0 /2, one half the wavelength of ultrasounds in air. The 
tag 610 is of a length .lambda./2, one half the acoustic wavelength in the 
tag, which is attached to the frame 620 at both ends. As such, the tag 
resonates with the interrogation signals, converting the tangential motion 
of the tag into vibrational motion of the arc, results in ultrasonic 
radiation in air. The tag configuration shown in FIG. 8 may be ideally 
used for smart-tag applications, since many tags of different length may 
be bent into concentric arcs affixed to a common frame. When compared to 
FIG. 4, this can save the volume of the tag system drastically. 
Instead of utilizing the forced resonance driving condition of the tag 
system as described in FIG. 5, one may apply a similar technique involving 
the detection of beating frequencies at higher harmonics, as shown in FIG. 
9. That is, in FIG. 9 the signal generator 910 now generates pulses of 
short duration at a sub-harmonic frequency of the tag system 970, denoted 
as f.sub.0 /n. The pulses excite the tag system during the active cycle of 
the pulses, relaxing into intrinsic oscillations of acoustic waves at a 
frequency f.sub.0 when the pulses become inactive. As such, ultrasonic 
waves will transmit in air, which are most pronounced if the ultrasound 
frequency beats with the driving frequency. For this reason, we have 
included in FIG. 9 a frequency multiplier 999 which multiplies the source 
signal by a factor of n. The multiplied signals are then, as before, fed 
into the lock-in amplifier 940 to effectively enhance the signal-to-noise 
ratio of the detection scheme. Elements 920, 930, 950, 960, 970, 980, 940 
and 990 have operating characteristics and functions which are similar to 
elements 320, 330, 350, 360, 370, 380, 340 and 390 described above in 
conjunction with FIG. 5. 
The present invention thus discloses a preferred embodiment which comprises 
a source circuit excite sufficient rf-current to drive a dipole antenna. 
The antenna is placed in the interrogation zone and transmits 
electromagnetic signals to enquire/check the status of the tags. The tags 
are magnetomechanically reactive, and thus translate the incident 
electromagnetic waves into outgoing ultrasonic waves which are then 
detected using a microphone sensor. The detector circuit is phase locked 
with the source circuit so that background noise can be excluded. 
The present invention also discloses a method for optimizing the resolution 
power of the detector circuit. The quality factor, Q, of the acoustic 
radiator has been increased by incorporating a cavity cushion with the tag 
resonator. As such, chances for false alarms can be greatly reduced for 
the security-tag applications, and the storage capacity for information 
can be optimized for the smart-tag applications. 
Therefore, the present invention teaches the Electronic Article 
Surveillance (EAS) industry a new technique in fabricating security and 
smart tag systems. This invention discloses the use of ultrasonic waves in 
the detection of, say, the forced resonant states of magnetostrictive tag 
samples. The invented technique will provide sensitive detection of the 
tag status over long distance, simplify the detection circuit with added 
reliability, decrease the chances for false alarms, increase the amount of 
information that a smart-tag system can carry, and to lower the 
fabrication costs of the tags. 
In order to efficiently couple the electromagnetic field with the 
vibrational motion of the tag, a two-stage frequency conversion scheme may 
be used. The excitation field is provided having a relatively high 
frequency typically in the range of about 1 MHz to 10 GHz. The particular 
frequency is selected based upon a variety of factors including but not 
limited to the coupling efficiency of the selected transducer materials. 
The detection signal is provided having a frequency f.sub.2 which is in 
the ultrasound frequency range. For example, the detection signal may be 
provided having a frequency typically in the range of about 20 KHz 200 
KHz. This allows sensitive acoustic detection in air. Thus, the 
interrogation signal is composed of two frequencies, the carrier frequency 
f.sub.1 and the modulation frequency f.sub.2, and the waveform can be 
amplitude, phase, or frequency modulated. FIG. 10 shows the interrogation 
signal which is amplitude modulated. 
Three advantages follow as a consequence of using a two-stage frequency 
conversion scheme. Firstly, since f.sub.1 is much larger than f.sub.2, the 
detection circuit tuned at f.sub.2 filters out signals at f.sub.1, and, 
hence, reducing interference between transmitting and receiving 
electronics. Secondly, interrogation signals with carrier at f.sub.1 and 
modulation at f.sub.2 can be conveniently generated by using a 
conventional microwave source, for example, a Traveling Wave Tube (TWT) at 
desired power levels, say, from a few Watts to a few hundreds of Watts. 
Most importantly, the radiated electromagnetic energy can be confined in 
space near the interrogation zone to reduce power consumption as well as 
to avoid multi-path reflection arising from objects outside the 
interrogation zone. For this purpose the interrogation zone is constructed 
using a pair of disk-antenna reflectors and a ground plane arranged 
face-to-face so that electromagnetic waves are reflected back and forth 
between them to form standing modes within the interrogation zone. 
Thirdly, the transducer materials that generate acoustic waves can be 
conveniently chosen based upon either their electric or magnetic 
properties. For electric transducers piezoelectric materials, like 
piezoelectric ceramics (PZT-class) at low frequencies (f.sub.1 .ltoreq.10 
MHz), quartz crystals at intermediate frequencies (10 MHz.ltoreq.f.sub.1 
.ltoreq.1 GHz), and sapphire crystals at high frequencies (f.sub.1 &gt;1 GHz) 
can be used. For magnetic transducers magnetostrictive materials such as 
amorphous/poly-crystalline ferromagnetic alloys containing iron, nickel, 
cobalt, or boron, as well as rare-earth/transition metal compounds may be 
used to generate the acoustic wave in the magnetic film at high 
frequencies. A third material class which can also be used as the 
transducer material includes Ni.sub.2 MnGa, Co.sub.2 MnGa, FePt, CoNi, and 
FeNiCoTi, etc. For these materials, martensitic phase transitions can be 
magnetically induced near room temperature, and, hence, appreciable 
mechanical strains will result near phase transitions in these materials. 
The aforementioned transducers are shown in FIG. 11 as element 40, which 
is either acoustically bonded or evaporated/deposited on top of a 
ferromagnetic metal strip, element 10. The assembly of 10 and 40 vibrates 
as a unit with f.sub.2 being the normal-mode frequency. However, in order 
to transition smoothly from the transducer 40 to the vibrator substrate 
10, a buffer layer may be needed between them, which also serves as the 
matching layer to compensate the difference in (acoustic) impedance 
between the transducer and the vibrator substrate. 
Therefore, upon application of the interrogation signal of FIG. 10, for 
example, electromagnetic energy will drive the system to vibrate at 
f.sub.2, since f.sub.1 is too high for the tag assembly 10 and 40 to 
follow mechanically. That is, the mechanical tag system is set to resonate 
at f.sub.2 and not at f.sub.1. This results in the generation of 
ultrasounds, which can then be measured using a detection system similar 
to that illustrated in FIG. 9. Again, the measured ultrasound is 
phase-locked with the modulation signal at f.sub.2 to enhance its 
signal-to-noise ratio. 
In FIG. 11, element 50 represents a damper, which can be brought in contact 
with tag 10 to stop the vibrational motion of the tag. The damper can be 
made of a thin sheet of rubber glued on top of a second magnetic tag shown 
as element 20 in FIG. 11. Both tags, 10 and 20, are affixed to a common 
supporter, element 30, which is made of magnetic-soft metal, for example, 
permalloy. Tags 10 and 20 are provided from semi-hard materials, which can 
be magnetized externally/manually with respective magnetization either 
parallel or anti-parallel to each other. Thus, when magnetization of the 
tags is along the same direction, they repel each other, resulting in 
undamped vibrational motion of tag 10 at f.sub.2. 
However, when the tags are magnetized in opposite directions, they attract 
each other so that the damper 50 is in physical contact with the tag 10. 
This reduces or in some instances eliminates the vibrational motion of the 
tag 10. Thus, depending on the magnetization state of the tags, 10 and 20, 
the tag system can respond differently to the interrogation signal, 
corresponding to the checked and unchecked states of the merchandise that 
is intended to be protected by the tags. It should be noted that in some 
embodiments, both supporter 30 and the damper 50 can be omitted, resulting 
in a simpler structure. That is, when tags 10 and 20 attract each other, 
they join together to form one unit which exhibits different vibrational 
frequencies as previously assumed at f.sub.2. This gives rise to different 
reaction of the tag system in response to the interrogation signal when 
compared to that when they are repelling each other. 
The third advantage is that we can incorporate the enhanced magnetic 
resonance mechanism into the detection scheme to increase the coupling 
between the incident electromagnetic waves and the resultant spin motion 
in tag 20. This utilizes the so-called ferromagnetic resonance (FMR) or 
spin-wave resonance (SWR) In this case, tag 10 can be magnetized 
externally to different state providing various bias field to tag 20. 
Consequently, when tag 20 is biased with a signal having an FMR or SWR 
frequency coincident with the driving frequency of f.sub.1, the coupling 
efficiency is relatively high and in some cases may be maximized, giving 
rise to a relatively large amount of acoustic generation. However, by 
varying the bias field provided by tag 10, the FMR or SWR frequencies are 
altered. This results in reduced amount of acoustic generation, and hence 
it can be distinguished from the previous state involving FMR/SWR 
resonance. For this configuration there is no need for the transducer 40 
and the damper 50 shown in FIG. 10. Instead, the tag system resembles that 
shown in FIG. 3 with tag 110 being the ferromagnetic metal, for example, 
nickel, and tag 120 being a semi-hard magnet whose magnetization can be 
controlled externally. 
Thus, the system can employ a two-stage frequency conversion scheme in 
which a signal at one frequency is responsible for the generation of 
electromagnetic waves in space in the interrogation zone, and a signal at 
another frequency corresponds to the normal-mode vibrational frequency of 
the tag. 
Also, with the present invention, the interrogation zone can be confined in 
space involving electromagnetic radiation in standing modes constructed 
using a pair of reflectors and a ground plane. 
In the system of the present invention, the transducer can be realized 
either electrically or magnetically. For electrical transducers, 
piezoelectric materials can be used, whereas for magnetic transducers, 
magnetostrictive materials can be used. Materials involving magnetically 
induced martensitic transition near room temperature can also be used as 
the transducer materials. 
Also, in the present invention, FMR or SWR phenomena can be incorporated 
with the tag-detection scheme to further amplify the resolution power of 
the security-tag system. 
Having described preferred embodiments of the invention, it will now become 
apparent to one of skill in the art that other embodiments incorporating 
the concepts may be used. Thus, the invention is not be limited to the 
particular embodiments disclosed herein, but rather only by the spirit and 
scope of the appended claims.