# US 5,703,566 A · Anti-shoplifting security system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,703,566 A - Document ID: US5703566A - Status: Granted - Kind: A - Application number: 08/699,880 - Filed: 1996-08-16 - Granted: 1997-12-30 - Earliest claimed: 1996-08-16 - Inventors: Jon Neal Weaver - Web page: https://patentgrep.com/patent/US5703566A ## CPC - G08B 13/2471: {Antenna signal processing by receiver or emitter} - G01V 3/105: {forming directly coupled primary and secondary coils or loops} - G08B 13/2474: {Antenna or antenna activator geometry, arrangement or layout (loop antennae H01Q1/22)} - G08B: SIGNALLING SYSTEMS, e.g. PERSONAL CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS - G01V: GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS (means for indicating the location of accidentally buried, e.g. snow-buried, persons A63B29/02) ## Abstract This invention relates to detecting the presence of a soft magnetic marker in an interrogation zone. A low frequency electromagnetic field is produced utilizing dual frequency generators causing the magnetic marker to produce a phase shift of the modulation of transmission harmonics. These signals are readily distinguished from ferrous alloys and noise. ## Claims ### Claim 1 A system for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising: a first generating means for generating a first magnetic field in the interrogation zone at a first frequency; a second generating means for generating a second magnetic field in the interrogation zone at a second frequency; a detecting means for detecting a phase shift of a signal produced by a ferromagnetic marker present within the interrogation zone; wherein said second frequency is about 3 to about 50 hertz lower or higher than said first frequency. ### Claim 2 (depends on 1) The system, according to claim 1, wherein the ratio of the amplitudes of said magnetic fields is in the range from about 1:1 to about 1.5:1. ### Claim 3 (depends on 1) The system, according to claim 1, wherein the ratio of the amplitudes of said magnetic fields is about 1:1. ### Claim 4 (depends on 1) The system, according to claim 1, wherein said first and second generating means are each of a single coil design. ### Claim 5 (depends on 1) The system, according to claim 1, wherein said signal is an amplitude modulated signal. ### Claim 6 (depends on 5) The system, according to claim 5, wherein said signal is an amplitude modulated harmonic. ### Claim 7 (depends on 6) The system, according to claim 6, wherein said signal is an amplitude modulated harmonic of either said first or said second magnetic field. ### Claim 8 (depends on 1) A system, according to claim 1, wherein said first and second generating means and said detecting means are phase locked. ### Claim 9 (depends on 1) A system, according to claim 1, wherein said detecting means comprises a reference signal for comparison of the phase of said signal to said reference signal. ### Claim 10 (depends on 1) The system, according to claim 1, wherein said interrogation zone is greater than about 1.5 meters. ### Claim 11 (depends on 1) The system, according to claim 1, wherein said first and second generating means produce an output of less than about 50 watts rms each. ### Claim 12 (depends on 1) The system, according to claim 1, wherein said first and second generating means produce an output of less than about 25 Watts rms each. ### Claim 13 A method for detecting the presence of a ferromagnetic marker in an interrogation zone, comprising the steps of: generating a first magnetic field in the interrogation zone at a first frequency; generating a second magnetic field in the interrogation zone at a second frequency; detecting a phase shift of a signal produced by a ferromagnetic marker present within the interrogation zone; wherein said second frequency is about 3 to about 50 hertz lower or higher than said first frequency. ### Claim 14 (depends on 13) The method, according to claim 13, wherein the ratio of the amplitudes of said magnetic fields is in the range from about 1:1 to about 1.5:1. ### Claim 15 (depends on 13) The method, according to claim 13, wherein the ratio of the amplitudes of said magnetic fields is about 1:1. ### Claim 16 (depends on 13) The method, according to claim 13, wherein the steps of generating a first magnetic field further comprises generating said first magnetic field with a single coil design generating means, wherein the step of generating a second magnetic field further comprises generating said second magnetic field with a single coil design generating means. ### Claim 17 (depends on 13) The method, according to claim 13, wherein said signal produced by a ferromagnetic marker present within the interrogation zone is an amplitude modulated signal. ### Claim 18 (depends on 17) The method, according to claim 17, wherein said amplitude modulated signal is at a frequency which is an amplitude modulated harmonic of either said first or said second magnetic field. ### Claim 19 (depends on 16) The method, according to claim 16, wherein the step of detecting a phase shift of said signal further comprises comparing the phase of a reference signal to the phase of said signal. ### Claim 20 (depends on 19) The method, according to claim 19, wherein said first and second generating means and said reference signal are phase locked. ### Claim 21 (depends on 13) The method, according to claim 13, wherein said interrogation zone is greater than about 1.5 meters. ### Claim 22 (depends on 13) The method, according to claim 13, wherein said first and second generating means produce an output of less than about 50 watts rms each. ### Claim 23 (depends on 13) The method, according to claim 13, wherein said first and second generating means produce an output of less than about 25 Watts rms each.