An Overview of the Major EAS Technologies

Electronic Article Surveillance is a system that protects merchandise from theft. An EAS system has three components: 1) labels and hard tags—electronic sensors that are attached to merchandise; 2) deactivators and detachers—used at the point of sale to electronically deactivate labels and detach reusable hard tags as items are purchased; and 3) detectors that create a surveillance zone at exits or checkout aisles.

The EAS process begins by attaching labels or hard tags to merchandise. When an item is purchased, the label is deactivated or the hard tag is removed. However, if merchandise with an active label or hard tag is carried past the detector, an alarm sounds.

Over 800,000 EAS systems are installed worldwide, primarily in the retail market. This includes Apparel, Drug, Discount, Home Centers, Hypermarkets, Food, Entertainment, and Specialty stores.

EAS systems operate from a simple principle regardless of the manufacturer or the specific type of technology used: a transmitter sends a signal at defined frequencies to a receiver. This creates a surveillance area, usually at a checkout aisle or an exit in the case of retail stores. Upon entering the area, a tag or label with special characteristics creates a disturbance, which is detected by the receiver. The exact means by which the tag or label disrupts the signal is a distinctive part of different EAS systems. For example, tags or labels may alter the signal by using a simple semi-conductor junction (the basic building block of an integrated circuit), a tuned circuit composed of an inductor and capacitor, soft magnetic strips or wires, or vibrating resonators.

By design the disturbed signal created by the tag and detected by the receiver is distinctive and not likely to be created by natural circumstances. The tag is the key element, for it must create a unique signal to avoid false alarms. The disturbance in the electronic environment caused by a tag or label creates an alarm condition that usually indicate someone is shoplifting or removing a protected item from the area.

The nature of the technology dictates how wide the exit/entrance aisle may be. Systems are available that cover from a narrow aisle up to a wide mall store opening. Similarly, the type of technology affects the ease of shielding (blocking or detuning the signal), the visibility and size of the tag, the rate of false alarms, the percentage of detection rate (pick rate), and cost.

The physics of a particular EAS tag and resultant EAS technology determines which frequency range is used to create the surveillance area. EAS systems range from very low frequencies through the radio frequency range. Similarly, these different frequencies play a key role in establishing the features that affect operation.

Acousto-magnetic EAS systems use a transmitter to create a surveillance area where tags and labels are detected. The transmitter sends a radio frequency signal at a frequency of 58 kHz (thousands of cycles per second), but the frequency is sent in pulses. The transmit signal energizes a tag in the surveillance zone. When the transmit signal pulse ends, the tag responds, emitting a single frequency signal like a tuning fork. The tag signal is at about the same frequency as the transmitter signal.

While the transmitter is off between pulses, the tag signal is detected by a receiver. A microcomputer checks the tag signal detected by the receiver to ensure it is at the right frequency, occurs in time synchronized to the transmitter, at the proper level, and at the correct repetition rate. If the criteria is met, an alarm occurs.

 

 

The electromagnetic EAS system creates a low frequency electromagnetic field (fundamental frequencies between 70 Hz and 1 kHz are typically used) between two pedestals at an exit or checkout aisle. The field continuously varies in strength and polarity, repeating a cycle from positive to negative and back to positive again. With each half cycle, the polarity of the magnetic field between the pedestals changes.

In response to the changing magnetic field created by the transmitter, the magnetic field domain of the tag material abruptly "switches" as the field strength varies past a particular point, whether positive or negative, during each half of the transmit cycle. This abrupt change in the magnetic state of tag material generates a momentary signal that is rich in harmonics (multiples) of the fundamental frequency. Using electronic signal processing techniques, the system identifies that the harmonics are at the right frequencies and levels, and that they occur at the proper time in relation to the transmitter signal. If the criteria are met an alarm occurs.

 

 

Like other EAS technologies, swept-rf uses a transmitter to create a surveillance area where tags and labels are detected. The transmitter sends a signal that varies between 7.4 and 8.8 MHz (millions of cycles per second), which is why it is called swept; it sweeps over a range of frequencies.

The transmitter signal energizes the swept-rf tag or label, which is composed of a circuit containing a capacitor and an inductor or coil, both of which store electrical energy. When connected together in a loop, the components can pass energy back and forth or "resonate." The frequency at which the circuit resonates is controlled by matching the storage capacity of the coil and capacitor. The tag responds by emitting a signal that is detected by a receiver.

In addition to the small tag signal, the receiver also responds to the much larger transmitter signal. By detecting a phase difference between these two signals, and other properties of the tag signal, the receiver recognizes the presence of a tag and generates an alarm.

 

The system is composed of a transmitter, a synchronous receiver, a microprocessor-controlled detector, and an alarm. The transmitter produces two signals to excite the tag. One is a high frequency carrier signal and the other is a much lower frequency electrostatic signal. In North America the high frequency signal is hopped over a band of 902-906 MHz in distinct groups of 50 frequencies to avoid interference from adjacent systems. In Europe the high frequency is between 2402 and 2486 MHz and is not hopped; each system has its own unique frequency assigned.

The lower frequency is modulation signal is 111.5 kHz. It is a non propagating electrostatic signal that limits the range of the high frequency RF field to the desired surveillance zone. This prevents the signals from traveling large distances and interfering with other systems or causing alarms due to tags on articles just outside the surveillance zone.

The tag is composed of a microwave diode and a combination antenna for the receipt of the high and low frequency. When introduced into the field of the transmitter, the tag combines or mixes the two fields and re-radiates the combined signal to the high frequency receiver. The resultant 111.5 kHz modulation of the high frequency signal is amplified and compared to a reference in the detector to ensure it is at the correct frequency and level. If correct, an alarm is initiated.