Classifications

• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B13/00—Burglar, theft or intruder alarms
  • G08B13/22—Electrical actuation
  • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
  • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
  • G08B13/2405—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used
  • G08B13/2408—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting characterised by the tag technology used using ferromagnetic tags
  • G08B13/2411—Tag deactivation
• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B13/00—Burglar, theft or intruder alarms
  • G08B13/22—Electrical actuation
  • G08B13/24—Electrical actuation by interference with electromagnetic field distribution
  • G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
  • G08B13/2428—Tag details
  • G08B13/2437—Tag layered structure, processes for making layered tags
  • G08B13/2442—Tag materials and material properties thereof, e.g. magnetic material details
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
  • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
  • H01F1/147—Alloys characterised by their composition
  • H01F1/153—Amorphous metallic alloys, e.g. glassy metals
  • H01F1/15308—Amorphous metallic alloys, e.g. glassy metals based on Fe/Ni

Definitions

• This invention relates to metallic glass alloys; and more particularly to metallic glass alloys suited for use in mechanically resonant targets of article surveillance systems.
• An essential component of all surveillance systems is a sensing unit, or "target", that is attached to the object to be detected.
• Other components of the system include a transmitter and a receiver that are suitably disposed in an "interrogation" zone.
• the functional part of the target responds to a signal from the transmitter, which response is detected in the receiver.
• the information contained in the response signal is then processed for actions appropriate to the application: denial of access, triggering of an alarm, and the like.
• the target in such systems is a strip, or a
• the ferromagnetic material is preferably a metallic glass alloy ribbon, since the efficiency of magnetomechanical coupling in these alloys is very high.
• the mechanical resonance frequency of the target material is dictated essentially by th'e length of the alloy ribbon and the biasing field strength. When an interrogating signal tuned to this resonance frequency is encountered, the target material responds with a large signal field which is detected by the receiver. The large signal field is attributable to an enhanced magnetic permeability of the target material at the resonance frequency.
• the target material is excited into oscillations by pulses , or bursts , of signal at its resonance frequency generated by the transmitter.
• the target material will undergo damped oscillations at its resonance frequency, i.e., the target material "rings down” following the termination of the exciting pulse.
• the receiver "listens" to the response signal during this ring down period.
• the surveillance system is relatively immune to
• the present invention provides magnetic alloys that are at least about 70% glassy and are characterized by long ring down times in resonance target applications. Such alloys evidence a low rate of damping of resonant oscillations, following the termination of an exciting pulse.
• the glassy metal alloys of the invention have a composition described by the formula Fe a Ni b M c B d Si e C f , where M is one of molybdenum and chromium, "a” - “f” are in atom percent, "a” ranges from about 39 to about 41, “b” ranges from about 37 to about 39, “c” ranges from 0 to about 3, “d” ranges from about 17 to about 19, and “e” and “f” range from 0 to about 2, with the provisos that (i) only one of "c", "e” and “f” can be zero, (ii) "e” cannot be zero if "f” is not zero, and (iii) "f” can be zero only when M is Cr.
• Ribbons of these alloys when mechanically resonant at frequencies ranging f rom about 55 kHz to about 60 kHz , evi dence ring down times of at least about 3 ms.
• ribbons of these alloys when mechanically resonant at frequencies ranging from about 21 kHz to about 25 kHz, evidence ring down times of at least about 7 ms.
• the metallic glasses of this invention are:
• the glassy metal alloys of the invention have a composition described by the formula Fe a Ni b M c B d Si e C f , where M is one of molybdenum and chromium, "a" - “f” are in atom percent, “a” ranges from about 39 to about 41, “b” ranges from about 37 to about 39, “c” ranges from 0 to about 3, “d” ranges from about 17 to about 19, and “e” and “f” range from 0 to about 2, with the provisos that (i) only one of "c", "e” and “f” can be zero, (ii) "e” cannot be zero if "f” is not zero, and (iii) "f” can be zero only when M is Cr.
• M molybdenum and chromium
• compositions is that found in normal commercial
• Ribbons of these alloys at lengths ranging from about 35 mm to about 40 mm exhibit mechanical resonance in a range of frequencies from about 55 kHz to about 60 kHz. When thus resonant, such ribbons evidence ring down times of at least about 3 ms.
• ribbons of these alloys, at lengths ranging from about 85 mm to about 100 mm exhibit mechanical resonance in a range of frequencies from about 21 kHz to about 25 kHz; and, when so resonant, evidence ring down times of at least about 7 ms.
• Ribbons having mechanical resonances in the range from about 55 kHz to about 60 kHz are preferred. Such ribbons are short enough to be used as disposable target materials. In addition, the resonance signals of such ribbons are well separated from the audio and commercial radio frequency ranges.
• alloys of the present invention offer the advantageous combination of long ring down times and economy in production of usable ribbon.
• such longer time intervals provide an additional, and advantageous, feature in the detection system in that the receiver may "listen" to the sample response more than once during the same ring down cycle, for
• Examples of metallic glasses of the invention in elude Fe 40 Ni 38 Mo 2 B 18 Si 1 C l , Fe 40 Ni 38 Mo 3 B 18 Si 0.5 C 0 . 5 , Fe 40 Ni 38 Mo 1 B 18 Si 1.5 C 1.5 , Fe 40 Ni 38 Mo 2.5 B 17.5 Si 1 C 1 ,
• the target material is exposed to a burst of exciting signal of constant amplitude, referred to as the exciting pulse, tuned to the frequency of mechanical resonance, of the target material.
• the exciting pulse is outlined in thick dashed lines in the Figure, and the peak-to-peak amplitude of the pulse is denoted by the quantity V 0 .
• V 0 the peak-to-peak amplitude of the pulse.
• the physical principle governing this resonance may be summarized as follows: When a ferromagnetic material is subjected to a magnetizing magnetic field, it experiences a change in length.
• the fractional change in length, over the original length, of the material is referred to as magnetostriction and denoted by the symbol ⁇ .
• a positive signature is assigned to ⁇ if an elongation occurs parallel to the magnetizing magnetic field.
• the ribbon When a ribbon of a material with a positive magnetostriction is subjected to a sinusoidally varying external field, applied along its length, the ribbon will undergo periodic changes in length, i.e., the ribbon will be driven into oscillations.
• the external field may be generated, for example, by a solenoid carrying a sinusoidally varying current.
• the frequency of the ribbon oscillations will be twice that of the driving field, since the magnetostriction is insensitive to the direction of the driving field at any given instant. In other words, as long as the absolute magnitude of the driving field is non-zero, there will be a change in the ribbon length.
• Magnetomechanical resonance occurs when the frequency of the driving field is one-half of f r , the mechanical resonance frequency of the ribbon.
• L is the ribbon length
• E is the Young's modulus of the ribbon
• D is the density of the ribbon
• the biasing field serves other purposes as well.
• the biasing field places the material at, or beyond, the "knee” of hysteresis loop of the material, in which magnetic state the motion of plane parallel walls has been expended, and. further
• magnetization of the sample occurs mainly by domain rotation.
• the efficiency of magnetomechanical response from the material has thus been improved.
• a biasing field serves to change the effective value for E in a ferromagnetic material so that t he mechanical resonance frequency of the material may be modified by a suitable choice of strength for the biasing field.
• a ribbon of a positively magnetostrictive ferromagnetic material when exposed to a driving ac magnetic field in the presence of a do biasing field, will oscillate at the frequency of the driving ac field, and when this frequency coincides with the mechanical resonance frequency, f r , of the material, the ribbon will resonate and provide increased response signal amplitudes.
• the biasing field is provided by a ferromagnet with a higher coercivity than the target material present in the "target package".
• the amplitude of the response signal, or voltage, from the target material increases through the duration of the exciting pulse (as dictated by inertia), and eventually reches a stable, constant value if the exciting pulse lasts for a long enough time.
• the peak- to-peak height of this stable amplitude is represented as V r in the Figure.
• the response voltage amplitude reduces to zero over a period of time.
• the motion of the excited target material is damped.
• the profile of the amplitude of the target response voltage is outlined in thick solid lines in the Figure.
• the term "ring down time” means the time interval during which the amplitude of response from the ribbon is reduced to about 10% of that amplitude extant when an exciting pulse applied to the ribbon is terminated, such time interval commencing at the instant of termination of the exciting pulse.
• the ring down time, t r is approximately a linear function of the ribbon length, L; the longer the ribbon, the longer is the ring down time. Without being bound by any theory, it is believed that the increase in t r with increasing L is associated with the lowering of the mechanical resonance frequency in longer ribbons. The same amount of energy takes longer to dissipate at lower frequencies.
• the magnitude of the response voltage sensed by a receiver is dependent on how that receiver is disposed within the system. For example, a receiver in a system requiring the insertion of an identification card into a slot will perceive a magnitude for V r that is different from that perceived by a receiver in a system designed for employment at the exit doors of a department store, even though identical target materials are used in both systems.
• V r there are no requirements on the magnitude of V r as far as the choice of target material is concerned. It is, however, understood that the value for V r should be such that the response voltage is of sufficient strength to be detected by the receiver at the instant, during the ring down period, when the receiver "listens" to the target material. Henceforth, for the reasons detailed immediately above, no further reference to V r will be made in the description of this invention.
• Table I lists the values for t r obtained from various metallic glass alloys that are outside the scope of this invention but which happen to lie within the scope of compositions claimed in the '489 and '490 patents. With the exception of the last named alloy, the ring down times for the alloys in this Table are short. This last named alloy is prone to the
• Ring down times, t r obtained from ribbons of metallic glass alloys containing Fe, Ni, Mo, B, and Si, but no C. These ribbons have mechanical resonances in the range of frequencies from about 21 kHz to about 23 kHz.
• compositions described in the examples are nominal compositions.
• Ribbon samples cut to about 38 mm in length, were used for the characterization of ring down times in the various alloys. This ribbon length is appropriate to a mechanical resonance frequency ranging from about 55 kHz to about 60 kHz in these alloys.
• the biasing dc field and the driving ac field were obtained from two
• the biasing solenoid about 0.38 m in length, had a turn density of about 3400 turns/m, and the driving solenoid was about 0*3 m long with a turn density of about 1440 turns/m.
• the sample was placed on the axis of these solenoids, at about the middle of their length.
• the sample response was sensed through a pick-up coil comprising between about 100 and 120 turns of wire wound closely around the sample and covering the entire ribbon length.
• the sample response (pick-up signal) and the driving ac signal were simultaneously monitored on an oscilloscope screen.
• a pulse was sent through the exciting solenoid, which pulse contained a counted number of waves at the resonance frequency determined earlier as appropriate for the sample.
• the number of waves, or, equivalently, the duration of the pulse was adjusted to be sufficient to ensure that the sample response had reached a stable value.
• Adjustments to the driving frequency, and to the biasing field strength, were also made, when necessary, to obtain a maximum sample response signal. Peak sample responses were obtained with the biasing field ranging from about 400 A/m to about 600 A/m, the driving
• the exciting pulse comprising between about 80 and 100 waves.
• the traces on the oscilloscope screen were as schematically illustrated in the Figure.
• the ring down time was determined as the time required for the sample response amplitude to reduce to 10? of the amplitude at the instant the exciting pulse was turned off.
• Table IV below lists the ring down times obtained from these 38 mm long as-cast metallic glass ribbons.
• Ribbons of selected alloys from the above Table were subject to simple stress relief anneals, i.e., low temperature anneals in the absence of externally imposed magnetic fields.
• the anneal temperature ranged between about 473 K and 573 K, and the anneal time ranged between about 15 min. and 60 min. Ring down times from these annealed ribbons were found to be longer than in the corresponding as-cast ribbons.
• the extent of increase was dependent on the chemical composition of the metallic glass and on the anneal conditions for a given alloy.
• Other anneal conditions which may optimize the magnetomechanical coupling effects available in a metallic glass alloy ribbon, such as those including the presence of external fields applied along the ribbon width, can be employed to improve the resonance target response of the alloys of this invention.
• Example 3 Numerous casts were made of the alloy Fe 40 Ni 38 Mo 2 - B 18 Si 1 C 1 , which belongs to this invention, and of the alloy Fe 40 Ni 38 Mo 4 B 18 , which is outside the scope of this invention, following the procedures detailed under
• Example 1 Uniformity in width, and a lack of holes and kinks were used as the criteria for selection of "good" 38 mm long ribbons, usable as target material. About 10 to 15 such ribbons could be derived from a typical cast of the alloy belonging to this invention, whereas the alloy outside the scope of this invention yielded only 4 to 8 such ribbons from a typical cast.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Automation & Control Theory (AREA)
• Computer Security & Cryptography (AREA)
• General Physics & Mathematics (AREA)
• Chemical & Material Sciences (AREA)
• Dispersion Chemistry (AREA)
• Power Engineering (AREA)
• Soft Magnetic Materials (AREA)
• Burglar Alarm Systems (AREA)

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Cited By (19)

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Citations (4)

• 1989
  • 1989-08-16 DE DE89909913T patent/DE68908184T2/de not_active Expired - Fee Related
  • 1989-08-16 WO PCT/US1989/003513 patent/WO1990003652A1/en active IP Right Grant
  • 1989-08-16 EP EP89909913A patent/EP0435885B1/en not_active Expired - Lifetime
  • 1989-08-16 JP JP1509158A patent/JPH04500985A/ja active Pending
  • 1989-09-11 CA CA000610868A patent/CA1341071C/en not_active Expired - Fee Related
• 1991
  • 1991-03-19 DK DK048791A patent/DK48791A/da not_active Application Discontinuation

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