Retail Theft Detection Devices and the Alloys That Make Them Work

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Theft of consumer products at the retail level is causing growing concern worldwide. According to the National Retail Federation, retailers of all types in the U. S. alone lost more than 27 billion dollars to sales "shrinkage" in 1995.

Advanced societies everywhere are coping with the same problem. Studies have confirmed shoplifting losses in Canada and the U.K. amounting to billions of dollars. France, Italy and the Netherlands, among other nations, are also documenting huge losses.

This unwanted tribute to mass marketing and expanding consumerism has fostered an entire industry devoted to electronic article surveillance (EAS). That industry has developed a variety of sensors, to frustrate would-be thieves, that are made from highly sophisticated materials and components.

EAS systems for retailers employ either radio frequency technology or magnetic technology. Both systems provide a means of deactivating a tag placed on an article for sale - anything from automotive to women's wear - prior to the customer leaving the store. At or near the exit, the departing customer passes through an interrogation zone, or walk-through pedestal, which sounds an alarm if the tag is not deactivated.

Radio frequency tags, or sensors, are made from aluminum, plastic and paper - all nonmagnetic materials - that are finely layered in foil. The aluminum serves as an antenna that picks up and transmits signals back to the interrogation zone that monitors the exit.

Magnetic Tags 

Magnetic systems are based on either one of two concepts - harmonic magnetics or magnetoacoustic effect. Both systems use a soft magnetic alloy in combination with a hard magnetic alloy.

With a magnetic type EAS system, a customer typically carries the tagged merchandise to a checkout counter. There, a clerk sweeps the item(s) with a laser scanner for price and, using a separate device, deactivates the tag. If the tag is not effectively deactivated, the departing customer will trip a false alarm that embarrasses the store and leaves the customer with negative feelings.

When a harmonic tag is used, a magnetic signal of a pre-determined, or primary frequency is emitted from the exit pedestal as the customer passes through. This tag on the purchased item becomes magnetized and sends a signal back to the pickup coils in the pedestal at harmonics of the primary frequency. The pickup coils are designed to search for specific harmonics. If the coils detect these frequencies, an alarm is tripped.

Soft Magnetic Alloy

Carpenter Technology HyMu "80"® alloy has been commonly used for the soft side of harmonic tags. This is an 80% nickel-iron molybdenum alloy which offers extremely high initial permeability and maximum permeability with minimum hysterisis loss. It produces a signal which, in the presence of an applied magnetic field, is recognized by the detection instrument.

Usually, a soft magnetic alloy like Carpenter Technology HyMu "80" alloy is magnetized along a preferred axis to exhibit a square loop. The more square the loop, the better the harmonic content in the signal response function.

Shape of the tag can minimize loop shearing and roundness. This is one reason why strip of this alloy is usually converted into thin, narrow ribbons. Annealing the alloy in a longitudinal field also can increase loop squareness.

It is a well known fact that high permeability in magnetic materials is very sensitive to alloy processing and handling. If great care is not exercised in manufacture and shipping, magnetic response can be seriously impaired.

Magnetoacoustic tags rely on two magnetic components. One is an amorphous alloy known to have a strong magnetoacoustic effect. An amorphous alloy is somewhat like glass, with no crystalline structure. It is quite different from most metals, which typically have such structure.

The other magnetic component could be made of a semi-hard magnetic alloy like CarTech MagneDur® 20-4 alloy, a cobalt-free, nickel-molybdenum-iron material. This alloy has been designed primarily as a candidate for use in theft detection tags. It is malleable and ductile, lending itself to the manufacture of strip, foil and wire.

The resonator inside magnetoacoustic tags is made of the amorphous alloy, and the adjacent magnetic strip is made of the semi-hard magnetic alloy. In this design, the resonator inside the tag begins to vibrate as the tagged merchandise passes through the pedestal. The magnetic strip controls the frequency of the resonator.

Inside each tag, the magnetic strip is magnetized by an alternating magnetic signal from the pedestal at the store entrance or exit. This magnetized strip ensures that the acoustic oscillations of the resonator are identical to the low radio-frequency signal transmitted by the pedestal. The selected frequency minimizes interference from any store or nearby electronics.

A scanning device at each store checkout counter deactivates the tag (when the merchandise is paid for) by demagnetizing the semi-hard magnetic strip or altering its magnetic properties. Subsequently, the resonator will not oscillate, or it will do so at the wrong frequency. Thus, no alarm is initiated.

The electromagnetic fields transmitted from the pedestals can penetrate the shielding of the tag on an item hidden inside a coat pocket. However, if they are not sealed inside a hard plastic casing, the tags can be disabled if they are bent or removed by a shopper. This is why more tags have been affixed recently inside the package or product.

The role of the two major components in magnetic surveillance systems is clear. One is the sensor, the most active part of the tag. The other is a semi-hard magnetic alloy which acts as the switch that turns the sensor off and on.

Currently, the derivation of such tags is changing. While many tags are still attached by clerks in participating retail establishments, the trend is decidedly moving toward source tagging by product manufacturers. This is a trend that retailers warmly embrace.

Installing tags at the source of manufacture is much more cost effective than hand affixing at the retail level. Source tagging also permits placement of tags where they cannot be seen or easily removed. That is expected to further reduce the massive retail cost of "shrinkage"


By Leslie L. Harner

Carpenter Technology Corporation
Reading, PA

Leslie L. Harner