Development of a magnetoelastic resonant sensor using iron-rich, nonzero magnetostrictive amorphous alloys
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INTRODUCTION
M A G N E T I C materials have long been used as transducers and hydrophones, as well as in a variety of sensor applications, v-9] However, over the past 2 decades, a new sensor business has been growing, presently at a rate of 25 pet annually, that makes use of specialized magnetic materials to track and control the flow of articles. The technology is called electronic article surveillance (EAS), the process of placing tags or markers on items to deter theft and track and control the flow of articles and assets. Presently, EAS expands rapidly in a world market in excess of several billion dollars per year. All EAS sensors or tags should have a characteristic magnetic signature that can be deactivated at a checkout counter. Their signal should be distinguishable from that of other common magnetic articles, detectable from a distance without exposing customers or employees to harmful electromagnetic radiation, and should not interfere with or be subject to interference from other electronics in the environment. What is needed for EAS is to set up an interrogation zone (usually defined by a magnetic dipole antenna pair) near the entrance or exit to an area to be secured. When the magnetic field in the interrogation zone is perturbed by an active tag, the system is alerted. The tags of interest here are magnetoelastic (ME). Temporal perturbation of the excitation field can be affected by a magnetoelastic tag which returns a delayed signal, echo, or response after the excitation field is turned off. Such tags are called resonant or magnetoelastic tags. The material requirement here is a magnetically soft material having nonzero magnetostriction.
Principles of Magnetoelastic Tag Operation Magnetoelastic tags change the characteristics of the excitation field in time. When magnetoelastic tags are excited
C.K. KIM, Research Scientist, and R.C. O'HANDLEY, Senior Research Scientist, are with the Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted February 14, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
by a primary signal for a period of time, these tags show a response (ring-down) that is delayed in time relative to the drive signal (Figure 1). The ring-down of a resonant tag is detected with a proper pickup electronics. Working principles of magnetoelastic tags can best be thought of using magnetoelastic coupling between their magnetic and elastic properties. The coupling between these two modes is provided by a thermodynamically defined parameter, the magnetostrictivity, d = (0eJ0H)~ = (OBlOo')~. Application of an a.c. magnetic field from a transmitting antenna drives magnetization into oscillation and depending on the strength of d, some of this magnetic energy is transferred and stored in an elastic mode. While the drive antenna is on, the system finds an optimal response that includes both magnetic and mechanical oscillations. When the magnetic drive is turned off, the magnetic system would stop oscillating in a matter of nanoseconds because of the absence of an i
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