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Transponder News

A news service reporting on developments regarding the use of radio based tagging transponder systems for commerce and scientific applications. Covering the RFID technologies, EAS technologies and magnetic coupled techniques.

EAS systems

EAS stands for Electronic Article Surveillance. Becoming more common in the retail industry nowadays, the EAS systems are used to electronically detect goods that have not been authorised when they are removed from a retailer. The systems comprise a tag attached to the goods and a sensor mechanism. The tag can be neutralised by the retailer when he wishes to authorise the removal of the goods, for example when the items have been legitimately purchased. In effect, EAS systems are single bit RFID systems, able to convey their presence, but not having sufficient data capabilities to convey an identity.

Presently there are four major technologies used for EAS systems. They are:

Market penetration is currently estimated at 6 000 million tags per annum at $0.12 each

The different EAS technologies have widely differing performance in the issues of price, reading range and reliability. The Magnetic and radio-frequency versions are very cheap and are generally attached permanently to the goods or their packaging, while the microwave tags are expensive and are removed by the store personnel when the item is paid for using a special removal tool.

Markers that are left on the goods and neutralised by the sales staff are called deactivatable.

One type of deactivateable marker is in the form of an electronic circuit comprising inductance and capacitance elements which resonate at radio frequencies.

Another type of marker - a magnetic marker - comprises a strip of soft magnetic material which interacts with a ferromagnetic element made of a hard magnetic material which can be magnetised or demagnetised. The soft magnetic strip resonates and generates harmonics in the presence of a magnetic field having a certain frequency. This allows the marker to be identified. The hard ferromagnetic element can be magnetised or demagnetised thereby deactivating or activating the marker.

Another type of marker is the acouto-magnetic or magneto-mechanical marker. This type of marker comprises a strip of magnetostrictive material and a strip of magnetic material of high coercivity. The magnetostrictive material resonates mechanically in the presence of a magnetic field of a particular frequency. This resonance can be detected by a receiver sensitive to the magnetic field created by the mechanical resonating magnetostrictive material. The marker is ordinarily deactivated by modifying the magnetic bias of the strip of magnetic material.

The above systems are commercially available from many competing suppliers.

EAS is a simple addition to electronic RFID systems whose developments have been announced but are as yet still not commercially available. The advantage of such systems with regard to EAS, is that

These systems are still in their infancy and have a long development path ahead.


Multibit EAS tags

The following article explains some experimental concepts in achieving multibit or multi-status from a modification of standard EAS techniques. Generally EAS tags are single bit devices and are not switchable in both the on and off direction using a programming signal.

A resonant circuit is one in which the values of circuit resistance, R, capacitance, C, and inductance, L, are chosen such that the reactance of the resonant circuit is a minimum at a resonant frequency.

One method that is used is for a resonant circuit to be disposed on a thin insulating dielectric substrate to form a tag for use in electronic article detection (EAS) schemes. Generally, the coil of the resonant circuit consists of a closed loop of a conducting element which has a certain value of resistance and inductance. A capacitive element which forms part of this closed loop consists of two separate areas of thin metal conducting film disposed on opposite sides of the dielectric. The tag is attached to articles to be protected from theft. An RF signal at or near the resonant frequency of the resonant circuit is emitted from a base station. When the tag is in the RF field, the tag's absorption can lead to a change in the tank circuit current of the base station and a power dip in a receiving coil. Either one of these two effects can be used to sense the presence of the tag and hence the item to which it is attached. Thus, an alarm can be made to sound when either of these effects are sensed by a pickup coil or by an amplifier, indicating improper removal of an item. To deactivate the tag, a relatively high RF power pulse can be applied at the counter at which the point-of-sale of the item takes place. This high power acts to short the capacitor or burn out a weak portion of the coil. In either case, the circuit is no longer resonant and will not respond to the RF interrogation from the base station. Therefore, the customer who has made a legitimate purchase at the point-of-sale counter can pass through the interrogation-sensing gate without setting off an alarm.

It is clear from this description that these tags, once deactivated, are not reusable. In addition, in the configuration just described, the tags are capable of only conveying one bit of information. Thus, they cannot give any information regarding the item's identification and are useful only for anti-theft applications. This kind of tag is normally classified as a single bit tag.

Some RF tags consist of a resonant coil or a double sided coil containing two thin film capacitors with the plate of each capacitor on opposite sides of the dielectric. Such tags can be used for source tagging and have an initial frequency that is different from the frequency used at the retail establishment for theft protection. For example the tag is designated as being in a deactivated state until the first capacitor is shorted by means of a high power RF pulse at the then resonant frequency. Disabling the capacitor shifts the resonant frequency of the RF circuit to the store interrogation frequency. A second deactivation pulse is used to disable the second capacitor at the point-of-sale when payment is received for the item to which the tag is attached. At this stage, the tag is no longer usable and has been permanently destroyed.

Some other systems have been proposed where two or more frequencies can be obtained on a RF coil tag by altering the capacitance of the circuit. In one case, a strong DC electric field is applied to change the effective dielectric constant of the capacitor. Thus, the circuit has two resonant frequencies depending on the value of the applied electric field. Due to the ferroelectric hysteresis, the tag can be deactivated by the application of a DC field. However, it can also be reactivated and hence re-used by applying a DC field of opposite polarity.

In another version, a set of capacitors connected in parallel attached to an inductance have been described in which each dielectric of the set of capacitors varies in thickness. In this manner, a series of resonant frequencies can be obtained by applying different voltages (electric fields). Each of the capacitors then changes capacitance at a different electric field (voltage) levels depending on the thickness of the dielectric.

Another concept consists of an array of series capacitors connected in parallel with an inductor. Here, the resonance can be altered by selectively shorting one or more of the capacitors, thereby changing the resonant frequency of the resulting circuit. A frequency code can thereby be established by disabling or burning out selective capacitors at the time of interrogation, those capacitors becoming disabled which at the time of manufacture of the tag were "edimpled"u. The tag is not reusable once scanned since the code relies on burning out a capacitor during the scan cycle and observing the change in frequency. Thus, once the tag has been queried its capacitive elements become irreversibly shorted and hence the tag cannot be scanned again.

An idea for a reusable tag comprises of two ferromagnetic elements, one soft (low coercivity) and one hard (high coercivity) both physically covering a portion of an R.F. coil. The ferromagnetic element with high coercivity can be magnetized to apply a bias field to the soft material to put the latter into saturation. In that state, the R.F. field generates very small hysteresis losses leading to a relatively high Q of the tag circuit. On the other hand, when the hard magnet is demagnetized, the RF field results in hysteresis losses in the soft material which lowers the Q of the circuit. This change in Q can be used to determine whether a tag is active or has been deactivated.

A reader apparatus for interrogating and sensing the presence of a RF resonant tag is realised where the interrogating frequency is swept around a center frequency. In general, there is very little radiation emitted except when the tag is present in the field of the emitter. Thus, when there is no tag in the antenna field, very little energy is lost from the antenna circuit. When the swept frequency coincides with the resonant frequency of an active tag, energy is absorbed and a sensing circuit detects a drop in voltage level in the interrogating antenna oscillator circuit. The tag absorption occurs twice with every complete sweep cycle resulting in a negative dip in the oscillator circuit. The negative dip causes pulse modulation which is filtered, demodulated and amplified to cause an alarm to be activated, indicating theft of an item. Thus, the basic detection is achieved by varying the interrogation carrier frequency to match the resonance of a tag whose center frequencies span a range depending on the type or make of tag.


Using wires for identification

The following is an extract from US Patent 5729201 invented by Jahnes, Christopher; Gambino, Richard;Paunovic, Milan ;Schrott, Alejandro; and von Gutfeld, Robert where there is a description of prior art that describes the operation of the magnetic based technologies physics which are relevant to many EAS systems

Retail tagging, tagging used in the road/air-freight package industry, personnel identification tagging, pallet tagging in manufacturing processes, etc., requires a tag for identifying a product, article or person in detail. With a sufficient number of bits, the tag can be interrogated to yield useful information such as what the product is, its date of manufacture, its price, whether the product, article or person has been properly passed through a check-out counter or kiosk, etc. Further, identifying a large number of products via tags can lead to a new type of check-out system for the retail industry giving rise to the much hoped for "no-wait check-out".

Conventional tags and tag systems have had a number of problems including: 1) having only one bit, typical of anti-theft tags, or 2) requiring a large amount of power to read the tag, thus requiring a tag battery (or other suitable power source), or 3) being relatively easy to defeat by tampering.

Multibit, remotely-sensed tags are needed for retailing, inventory control and many other purposes. For many applications, the cost must be low and the tags must be able to be individually encoded. Further, when the tag is interrogated it must produce a distinctive signal to reliably identify the article to which the tag is attached or coupled.

Some conventional tags have employed the Barkhausen jump effect. Generally, the Barkhausen effect is characterized by a tendency for magnetization to occur in discrete steps rather than by continuous change, thereby giving rise to a large temporal flux change, d.phi./dt, which is key for inducing a sizable voltage in a sensing or pickup cot.

For example, U.S. Pat. No. 5,181,020 describes a thin-film magnetic tag having a magnetic thin film formed on a polymer substrate and a method for producing the same. The thin film exhibits a large Barkhausen discontinuity without intentional application of external torsional or tensile stress on use. A particular disclosed use is as a marker or tag for use in an article surveillance system wherein articles may be identified by interrogating the tagged article in a cyclic magnetic field of a predetermined frequency in a surveillance area and detecting a harmonic wave of the magnetic field generated by the tag in the surveillance area.

This conventional system is only a single bit element using a single Barkhausen layer with no ability to develop a code to distinguish items.

U.S. Pat. No. 5,313,192 describes another single bit tag which relies on the Barkhausen effect. The tag of this invention is selected to include a first component comprised of a soft magnetic material which constitutes the bulk of the tag. A second component comprised of a semi-hard or hard magnetic material is integral with the first component. The tag is conditioned such that the second component has activating and deactivating states for placing the tag in active and deactivated states, respectively. Such conditioning includes subjecting the composite tag to predetermined magnetic fields during thermal processing stages.

By switching the second component between its activating and deactivating states the tag can be switched between its active and deactived states. A reusable tag with desired step changes in flux which is capable of deactivation and reactivation is thereby realized.

U.S. Pat. No. 4,980,670 describes a one bit magnetic tag formed from a magnetic material having domains with a pinned wall configuration. The resulting hysteresis characteristic for that material is such that upon subjecting the material to an applied alternating magnetic field, the magnetic flux of the material undergoes a regenerative step change in flux (Barkhausen jump) at a threshold value when the field increases to the threshold value from substantially zero and undergoes a gradual change in flux when the field decreases from the threshold value to substantially zero. For increasing values of applied field below the threshold, there is substantially no change in the magnetic flux of the material. The tag may be deactivated by preventing the domain walls from returning to their pinned condition by, for example, application of a field of sufficiently high frequency and/or amplitude.

U.S. Pat. No. 4,940,966 describes the use of a plurality of magnetic elements in predetermined associations (e.g. with predetermined numbers of magnetic elements and with predetermined spacings between said elements), for identifying or locating preselected categories of articles. When the articles are caused to move relative to a predetermined interrogating magnetic field, each particular association of magnetic elements gives rise to a magnetic signature whereby the article or category of article carrying each of the predetermined associations can be recognized and/or located.

U.S. Pat. No. 4,660,025 describes a marker for use in an electronic surveillance system. The marker, which can be in the form of a wire or strip of magnetic amorphous metal, is characterized by having retained stress and a magnetic hysteresis loop with a large Barkhausen discontinuity. When the marker is exposed to an external magnetic field whose field strength, in the direction opposing the instantaneous magnetic polarization of the marker, exceeds a predetermined threshold value, a regenerative reversal of the magnetic polarization of the marker occurs and results in the generation of a harmonically rich pulse that is readily detected and easily distinguished.

U.S. Pat. No. 5,175,419 describes a method for interrogating an identification tag comprised of a plurality of magnetic, thin wires or thin bands which have highly rectangular hysteresis curves and different coercive forces. The wires or bands are preferably of amorphous material, but means for obtaining the highly rectangular hysteresis curves and different coercive forces are not taught; nor is the concept taught of using a time varying magnetic field superimposed on a ramp field for interrogation.

Their invention is
An inexpensive multibit magnetic tag is described which uses an array of amorphous wires in conjunction with a magnetic bias field. The tag is interrogated by the use of a ramped field or an ac field or a combination of the two. The magnetic bias is supplied either by coating each wire with a hard magnetic material which is magnetized or by using magnetized hard magnetic wires or foil strips in proximity to the amorphous wires. Each wire switches at a different value of the external interrogation field due to the differences in the magnetic bias field acting on each wire.

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