Conversion of bias strip in a frequency-dividing transponder tag into a tripole bar magnet to deactivate the tag

Abstract: A method of deactivating a frequency-dividing-transponder (10) that includes an active strip (12, 14) of magnetic material that, when magnetically biased to be within a predetermined magnetic field intensity range, responds to excitation by electromagnetic radiation of a first predetermined frequency by radiating electromagnetic radiation of a second predetermined frequency that is a frequency-divided quotient of the first predetermined frequency; and a magnetized bias strip (16) of magnetic material having first and second ends and disposed in relation to the active strip of magnetic material for magnetically biasing the active strip of magnetic material to be within the predetermined magnetic field intensity range only when the bias strip of magnetic material is magnetized. The method includes the step of converting the bias strip of magnetic material into a tripole bar magnet, having a pole of one magnetic polarity in a predetermined region of the strip (16) located between the ends of the strip (16), and having a pole of a different magnetic polarity than the one magnetic polarity at each end of the bias strip (16) to thereby provide opposing magnetic bias fields in opposite longitudinal halves of the active strip (12, 14) for causing any electromagnetic radiation of the second predetermined frequency that is generated in one half of the active strip to be of equal and opposite polarity and thus cancelled by any electromagnetic radiation of the second predetermined frequency that is generated in the other half of the active strip. This is accomplished by laterally passing a magnet (32) across and in close proximity to the bias strip (16) of magnetic material, with the magnet having sufficient flux density to overcome the magnetic bias of the bias strip, and with the magnet being passed across the predetermined region of the bias strip.

Description: The present invention generally pertains to presence-detection-system tags that include frequency-dividing transponders and is particularly directed to deactivation of frequency-dividing transponders of the type that includes an active strip of magnetomechanical material that frequency divides when in the presence of a magnetic field within a predetermined magnetic field intensity range and a bias strip of magnetic material for biasing the active strip to be within the predetermined range. This type of frequency-dividing-transponder is described in United States Patent No. 4,727,360 to Lucian G. Ferguson and Lincoln H. Charlot, Jr., which is assigned to the assignee of the present application. According to the teaching of said patent, the frequency-dividing transponder described therein is deactivated by demagnetizing the bias strip of magnetic material. However, even after the bias strip has been demagnetized, the active strip of magnetomechanical material will still frequency divide if it is in the presence of an ambient magnetic field that is within the predetermined magnetic field intensity range. In certain locations, the ambient magnetic field resulting from the Earth's magnetic field is within the predetermined magnetic field intensity range. presence-detection-system tags containing the above-described type of frequency-dividing transponder are adapted for attachment to articles to be detected within a surveillance zone. If the ambient magnetic field within the surveillance zone is within the predetermined magnetic field intensity range, false presence detections may occur even after the bias strip has been demagnetized. The present invention provides a method of deactivating a frequency-dividing transponder that includes an active strip of magnetic material that, when magnetically biased to be within a predetermined magnetic field intensity range, responds to excitation by electromagnetic radiation of a first predetermined frequency by radiating electromagnetic radiation of a second predetermined frequency that is a frequency-divided quotient of the first predetermined frequency; and a magnetized bias strip of magnetic material having first and second ends and disposed in relation to the active strip of magnetic material for magnetically biasing the active strip of magnetic material to be within the predetermined magnetic field intensity range only when the bias strip of magnetic material is magnetized. The method is characterised by the step of converting the bias strip of magnetic material into a tripole bar magnet, having a pole of one magnetic polarity in a predetermined region of the strip located between the ends of the strip, and having a pole of a different magnetic polarity than said one magnetic polarity at each end of the bias strip to thereby provide opposing magnetic bias fields in opposite longitudinal halves of the active strip for causing any electromagnetic radiation of said second predetermined frequency that is generated in one half of the active strip to be of equal and opposite polarity and thus cancelled by any electromagnetic radiation of said second predetermined frequency that is generated in the other half of the active strip. Preferably this step is accomplished by the step of laterally passing a magnet across and in close proximity to the bias strip of magnetic material, with the magnet having sufficient flux density to overcome the magnetic bias of the bias strip, and with the magnet being passed across said predetermined region of the bias strip. Additional features of the present invention are described in relation to the description of the preferred embodiment. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a plan view illustrating the orientation of active strips and a bias strip in a preferred embodiment of a presence-detection-system tag that includes a deactivatable frequency-dividing transporter. Figure 2 is a perspective view illustrating the method of the present invention for deactivating the tag of Figure 1, and further illustrating additional features of the tag of Figure 1 and a preferred embodiment of the magnetic wand of the present invention. Figure 2A is an exploded perspective view illustrating further detail of the megnetic wand of Figure 2. Figure 3 is a diagram illustrating the magnetic fields created in the bias strip of the tag of Figures 1 and 2 by passage of the magnet included in the magnetic wand of Figures 2 and 2A during the forming of the tripole bar magnet of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figures 1 and 2, a preferred embodiment of a frequency-dividing transponder tag 10 of the type described in the aforementioned U.S. Patent No. 4,727,360, is constructed so that its performance is not affected by interference with the Earth's magnetic field. The tag 10 includes two active strips 12, 14 of magnetic material that, when magnetically biased to be within a predetermined magnetic field intensity range, respond to excitation by electromagnetic radiation of a first predetermined frequency by radiating electromagnetic radiation of a second predetermined frequency that is a frequency-divided quotient of the first predetermined frequency. Each active strip 12, 14 of magnetic material is a thin, flat ribbon of low coercivity magnetostrictlve amorphous magnetic material having a transverse magnetic anisotropy defining the same magnetomechanical resonant frequency f₁, which is equal to one-half the first predetermined frequency in accordance with the dimensions of the ribbon, wherein when the ribbon is in the presence of a magnetic bias field within the predetermined magnetic field intensity range, the ribbon responds to the detection of electromagnetic radiation of a frequency 2f₁ by transmitting electromagnetic radiation of the second predetermined frequency, which is a frequency-divided quotient of the frequency 2f₁. Both active strips 12, 14 are of the same magnetic material and of the same dimensions in order to define the same magnetomechanical resonant frequency f₁. Suitable low coercivity magnetostrictive amorphous magnetic materials and the treatment and dimensioning thereof for making them useful as the active strips 12, 14 are described in the aforementioned U.S. Patent No. 4,727,360. The tag 10 further includes a bias strip 16 of magnetic material. The bias strip 16 is positioned in the same plane as the two active strips 12, 14 and is located between the two active strips 12, 14, with all three strips 12, 14, 16 being oriented in the same direction. During the process of manufacturing the tag 10, the bias strip 16 is magnetized by passing it over a permanent magnet. A suitable material for the bias strip 16 is 0.65 to 1.0 percent carbon steel ribbon with a coercivity of approximately 45 gauss and 2 to 5 mils thick The bias strip 16 of magnetic material is disposed in relation to the first and second active strips 12, 14 of magnetic material for blasing the first and second active strips 12, 14 so that at least one of the active strips 12, 14 is biased to be within the predetermined magnetic field intensity range when the bias strip 16 is magnetized, notwithstanding the orientation of the tag 10 with respect to the Earth's magnetic field. The bias strip 16 is disposed at a distance d₁ from the first active strip 12 so that the first active strip 12 has an optimum magnetic bias field B₁ - B E resulting when the Earth's magnetic field B E is parallel with the length of the active strip 12 and opposing the magnetic field B₁ from the bias strip 16. The bias strip 16 is disposed at a distance d₂ from the second active strip 14 so that the second active strip 14 has an optimum magnetic bias field B₂ + B E resulting when the Earth's magnetic field B E is parallel with the length of the active strip 14 and aiding the magnetic field B₂ from the bias strip 16. This feature is described in greater detail in the aforementioned U.S. Patent No. 4,727,360. As seen in Figure 2, the tag 10 includes a housing 18 defining cavities 20, 24 and 22 for containing the active strips 12, 14 and the bias strip 16 respectively. The housing 18 includes a paper cover 26, a paper base 28 and paper spacers 30. The active strips 12, 14 are disposed within the cavities 20, 24 so that they can vibrate freely within the cavities without interference or restriction, and so that no mechanical stresses are impressed upon the active strips by the walls of the cavities. Referring to Figure 2, the active strips 12, 14 of the tag 10 are deactivated by laterally passing the tag 10 with a magnet 32 in order to convert the bias strip 16 of magnetic material into a tripole bar magnet (Figure 3), having a pole of one magnetic polarity S in a predetermined region 34 of the strip 16 located between the ends 36 of the strip, and having a pole of a different magnetic polarity N at each end 36 of the bias strip 16, to thereby provide opposing magnetic bias fields in opposite iongitudinai halves of each active strip 12, 14 for causing any electromagnetic radiation of said second predetermined frequency f₁ that is generated in one half of each active strip 12, 14 to be of equal and opposite polarity and thus cancelled by any electromagnetic radiation of said second predetermined frequency f₁ that is generated in the other half of the respective active strip 12, 14. The magnet 32 must have sufficient flux density to overcome the magnetic bias of the bias strip 16. The magnet 32 is laterally passed across and in close proximity to the bias strip 16 of magnetic material. The magnet 32 is included in a magnetic wand 38, that further includes a rod 40 of insulating material and a dome 42 of ferromagnetic material. The magnet 32 is a disc-shaped magnet disposed at one end 44 of the rod 40. The disc-shaped magnet 32 has two opposed broad surfaces of opposite magnetic polarity, with one broad surface 46 of the disc facing the one end 44 of the rod 40. The magnet 32 is a neodymium-iron-boron magnet, having an energy density of approximately 25 x 10⁶ gauss-oersteds, and a 3/8 inch diameter. The dome 42 of ferromagnetic material is disposed adjacent the other broad surface 48 of the disc-shaped magnet 32 for aligning the flux density produced by the magnet over a large portion of the rounded surface of the dome 42, so that the wand 38 can be inclined at an angle from perpendicular with respect to the tag 10 when passing the tag, while still enabling the magnetic field distributed from the magnet 32 to the tag 10 to be of sufficient strength to overcome the magnetic bias of the bias strip 16. The dome 42 has a degree of curvature that allows the angle of inclination with respect to perpendicular to be as much as approximately 30 degrees. Preferably, the bias strip 16 is disposed at least coextensive with the active strips 12, 14. In any event, the tag 10 is passed by the magnet 32 in a predetermined region 34 of the bias strip 16 that is adjacent the longitudinal center of the active strips 12, 14. A tripole magnet 16 (Figure 3), per se, was formed by laterally passing the predetermined region 34 of the bias strip 16 of magnetic material having first and second ends 36 with the magnet 32 contained in the magnetic wand 38, as described above. The above-described embodiments of the present invention are also useful for deactivating tags that include an active strip of material that generates predetermined harmonics of an interrogation signal, such as described in French Patent No. 763,681 to Picard. A technique for deactivating such a tag is described in United States No. 3,747,086 to Peterson. Peterson describes disposing a bias strip of magnetic material in relation to the active strip in order to alter the generation of harmonics when the bias strip is magnetized. However, this technique is not always effective because, the magnetic field of the magnetized bias strip is sometimes overcome by ambient magnetic fields or by fields generated by equipment for detecting the harmonics.

Claims: A method of deactivating a tag (10), said tag including a frequency-dividing transponder comprising an active strip (12, 14) of magnetic material that, when magnetically biased to be within a predetermined magnetic field intensity range, responds to excitation by electromagnetic radiation of a first predetermined frequency by radiating electromagnetic radiation of a second predetermined frequency that is a frequency-divided quotient of said first predetermined frequency; and a magnetized bias strip (16) of magnetic material having first and second ends and disposed in relation to the active strip of magnetic material for magnetically biasing the active strip of magnetic material to be within said predetermined magnetic field intensity range only when the bias strip of magnetic material is magnetized, the method characterised by the step of converting the bias strip (16) of magnetic material into a tripole bar magnet, having a pole of one magnetic polarity in a predetermined region (34) of the strip located between the ends (36) of the strip, and having a pole of a different magnetic polarity than said one magnetic polarity at each end of the bias strip to thereby provide opposing magnetic bias fields in opposite longitudinal halves of the active strip (12, 14) for causing any electromagnetic radiation of said second predetermined frequency that is generated in one half of the active strip to be of equal and opposite polarity and thus cancelled by any electromagnetic radiation of said second predetermined frequency that is generated in the other half of the active strip. A method according to Claim 1, wherein said step is accomplished by laterally passing a magnet (32) across and in close proximity to the bias strip (16) of magnetic material, with the magnet having sufficient flux density to overcome the magnetic bias of the bias strip, and with the magnet being passed across said predetermined region (34) of the bias strip. A method according to Claim 2 wherein the magnet is a disc disposed at one end (44) of a rod (40), and having two opposed broad surfaces (46,48) of opposite magnetic polarity: and with one broad surface (46) of the disc facing said one end of the rod. A method according to Claim 3 wherein a dome (42) of ferromagnetic material is disposed adjacent the other broad surface (48) of the disc for aligning the flux density produced by the magnet over a large portion of the rounded surface of the dome. A method according to Claim 2 wherein the predetermined region of the bias strip is disposed adjacent the longitudinal center of the active strip.

Inventors: CANIPE LARRY KEITH, CANIPE, LARRY KEITH
Applicants: SECURITY TAG SYSTEMS INC, SECURITY TAG SYSTEMS, INC.
Application year: 1990
Main classification: G08B13
IPC codes: G08B13, G01S13, H01F21