by Dr. Giselher Herzer, Vacuumschmelze GmbH & Co. KG, Hanau
10.07.2018, 17:00 h, TF, Aquarium
Retailers lose billions of Euros per year to shoplifters. Department store detectives and video cameras are therefore increasingly being assisted by electronic article surveillance (EAS). Hundred thousands of such systems are meanwhile installed and millions of disposable security labels are being produced on a daily base. Basically all EAS-systems operate on the same principle: Articles are affixed with security labels which, if not deactivated at the cash register, respond to electromagnetic fields generated from pedestals at the store's exits. The response is picked up by an antenna in the pedestals, thereby triggering an alarm. Today’s security labels are disposable items which are also used to secure inexpensive articles. Moreover, EAS labels are increasingly integrated directly into products or packaging during the manufacturing or packaging process. One major requirement therefore is that the labels are small and cheap. Further requirements are that the labels are reliably detectable and deactivatable and, as one of the major requests, that they cause no false alarms.
One of the most wide-spread EAS systems is based on magnetoelastic sensors which represent the latest and most sophisticated technology. The sensor element is a short magnetostrictive amorphous alloy ribbon which is housed in a small cavity such that it can vibrate freely. It is excited by magnetic field pulses to longitudinal, resonant vibrations. Once an exciting tone burst is over, the mechanical vibrations ring down exponentially over a time period of several milliseconds, hereby inducing a characteristic voltage in the receiver antenna while the exciting field is off. The detection electronics traces these echo voltages and triggers alarm if it recognizes the typical characteristics (like resonant frequency and ring-down time) of the resonator.
The talk surveys the physics behind magnetoelastic EAS labels and illustrates how to customize the sensor material by appropriate alloy design and thermal treatment.