WO1998007052A1

WO1998007052A1 - Anti-shoplifting security system

Info

Publication number: WO1998007052A1
Application number: PCT/US1997/014441
Authority: WO
Inventors: Jon Neal Weaver
Original assignee: Jon Neal Weaver
Priority date: 1996-08-16
Filing date: 1997-08-15
Publication date: 1998-02-19
Also published as: EP0919002A1; AU4070697A

Abstract

The present invention relates to methods and apparatus for 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. This invention utilizes a modulated transmitter carrier signal generated in an interrogation zone. Modulated harmonics of the transmitter carrier signal are detected when a marker is present in the interrogation zone. The modulation detected can be at the same frequency as the modulation of the transmitter signal or a harmonic of the modulation of the transmitter signal. Detection of the modulation allows higher signal to noise ratios and therefore reduces false detections.

Description

DESCRIPTION

ANTI-SHOPLIFTING SECURITY SYSTEM

Background of the Invention

This invention relates to electronic article surveillance (EAS) systems. These systems are often used to detect shoplifters or otherwise monitor the movements of tagged articles. The EAS systems of the type to which the subject invention is directed, are generally employed to detect the presence of a magnetic marker in a magnetic field. Such systems thus include a device, for example a generating coil, for generating the magnetic field, and another device, for example a receiving coil, for detecting signals generated when a marker is passed through the field.

EAS systems must comply with the rules of various agencies, e.g. the Federal Communications Commission. These agency rules can, for example, limit the amount of spurious emissions and the amount of electromagnetic interference (EMI) allowable.

Additionally, the performance requirements of EAS systems can include limitations on the number of false alarms from noise or metallic objects, transmission energy, and an ability to control large interrogation zones. Current EAS systems based on low frequency electromagnetic transmission can not meet all of these performance requirements while simultaneously satisfying the spurious emissions and EMI limitations, due to the rapid attenuation of the transmitted signal.

Important considerations in the design of EAS systems include, for example, power consumption of the system, cost of the system, signal-to-noise ratio of the received signal, and sensitivity to a marker in the detection field. Factors which may influence the signal-to-noise ratio and sensitivity to a marker in the field may include coupling of the generated magnetic field and/or marker generated fields in the receiver, metallic objects in the vicinity of the EAS system, spurious emissions, and electromagnetic interference. Improving the signal-to-noise ratio and sensitivity to a marker in the field generally involves increased power consumption of the EAS system transmitter and/or receiver circuitry. Prior disclosed EAS systems have utilized pulsed transmitter signals and corresponding detection of the signal generated by a marker located in the magnetic field produced by the transmitter. For example, both electromagnetic (EM) and radio frequency (RF) EAS systems have been disclosed which utilized a pulsed transmitter signal. These systems have received, during the "off cycle of the transmitter signal, the signal generated by a marker due to the marker's response to the transmitter signal generated duπng the "on" cycle of the transmitter In these systems, the pulsing of the transmitter resulted in somewhat better signal to noise ratios. However, no attempt was made to detect modulation as an indication of the presence of a marker. In addition, pπor EAS systems, for example, in U.S. Patent Nos. 4,710,752 and

5,005,001, have also been disclosed which create modulation via generation of two overlapping magnetic fields having different frequencies. These systems have found that the generation of two overlapping magnetic fields, having substantially different frequencies and substantially different amplitudes, results m a greater ability to detect a marker in the magnetic field Specifically, the ability of these EAS systems to detect the presence of a marker, in the presence of noise, is enhanced by generating the magnetic field with more than one frequency component The ferromagnetic marker, in the presence of this magnetic field, produces output pulses at the frequency of the higher frequency field component and its harmonics which are modulated by the lower frequency component and its harmonics. In these systems, the amplitudes of sidebands of the harmonics, generated by a marker, were compared as an indication of marker presence

However, these systems did not attempt to detect modulation as an indication of the presence of a marker. The use of two overlapping magnetic fields of substantially different frequency and amplitude thus enables detection of smaller tag signals and/or increased spacing between the transmitter and the receiver. Even so, these EAS systems can not provide transmitter-to-receiver spacing greater than about 1.2 meters.

The proliferation of electrical noise in retail environments greatly decreases the probability of detecting the weak signals from markers. Therefore, while improvements have been made in increasing the size of interrogation zones, current electromagnetic EAS systems have not been able to surpass about 1.2 meters m size of interrogation zone while maintaining reliable performance. Therefore, there exists a need for low frequency electromagnetic EAS systems which can utilize low transmission power, operate in an environment with high noise levels, and still detect a marker m an interrogation zone of two or more meters.

Brief Summary of the Invention The subject invention pertains to novel Electronic Article Surveillance (EAS) systems and methods of electronic article surveillance which are able to detect tagged articles with, for example, reduced incidence of false alarms in the presence of noise, lower power consumption, and lower costs than current systems. Advantageously, the subject invention allows significant improvement m noise reduction and greatly increased sensitivity to a marker in the detection field.

In a specific embodiment, the EAS system of the subject invention utilizes an amplitude modulated transmitter earner signal created by an electromagnetic generator and single coil When a soft magnetic marker is brought into the transmitter field, the marker generates a signal.

This signal generated by the marker contains harmonics of the transmitter earner signal which are amplitude modulated at the same frequency as the transmitter modulation signal. In addition, the marker signal contains harmonics of the transmitter carrier signal which are modulated at frequencies which are harmonics of the transmitter modulation signal Accordingly, the receiver coil and receiver signal processor of the subject EAS system can receive the signal generated by the marker

In a preferred embodiment, the receiver demodulates the received harmonics of the transmitter earner signal and detects the modulation having the same frequency as the amplitude modulation signal as an indication that a marker is present. In a more preferred embodiment, the receiver demodulates the received harmonics of the transmitter carrier signal and detects the modulation havmg frequencies which are harmonics of the amplitude modulation signal as an indication that a marker is present.

Advantageously, the subject EAS system can have a higher signal-to-noise ratio than other systems utilizing amplitude modulated transmitted signals The subject EAS system detects the modulated harmonics and/or the harmonics of modulation generated by the marker, where modulated harmonics and/or harmonics of modulation are only present when a marker, or similar object, is present in the detection field.

In one embodiment, the subject invention provides novel Electronic Article Surveillance (EAS) systems which are able to detect tagged articles with a reduced incidence of false alarms even in wide interrogation zones and in the presence of noise. Advantageously, the systems of the subject invention can operate at lower power than current systems. In a specific embodiment, an electromagnetic field is created as a superposition of two individual fields generated with two transmitters, close in frequency and approximately equal m amplitude When a ferromagnetic marker is brought into the interrogation zone, I e., into the magnetic field, the marker generates amplitude modulated harmonics of each of two individual fields, wherein each marker generated amplitude modulated harmonic is phase shifted The presence of a marker is then determined by detecting the presence of a phase shift m an amplitude modulated harmonic. The detection of this phase shift by the receivers, greatly reduces the probability of detecting a random noise signal, as well as minimizes false alarms due to ferrous objects having greatly different phase responses from that of an intended marker.

To insure that markers can be highly differentiated from other metallic objects and to maximize the transmission field intensity and marker detection, each transmitter and its associated receiver can be separated by the interrogation zone.

Brief Descnption of the Drawings Figure 1 shows a schematic representation of an embodiment of an EAS system transmitter and receiver in accordance with the subject invention. Figure 2A shows one representation of an amplitude modulated transmitter signal in accordance with the subject invention.

Figure 2B shows a representation of an amplitude modulated harmonic produced by a marker located in the field produced by the amplitude modulated transmitter signal of figure 2A, in accordance with the subject invention. Figure 2C shows a representation of a modulation waveform which can be used in accordance with the subject invention.

Figure 3 shows a schematic of an embodiment of an EAS system with the transmitter modulation frequency locked to the receiver modulation filter in accordance with the subject invention. Figure 4 shows a schematic representation of an embodiment of an EAS system havmg two transmitter-receiver pairs, in accordance with the subject invention.

Figure 5A shows a schematic of an EAS system having two transmitter-receiver pairs wherein the transmitters are turned on and off, out of phase, by a clock.

Figure 5B shows an example of clock, transmitter, receiver, and modulation signals in accordance with the EAS system shown in Figure 5A.

Detailed Disclosure of the Invention The subject invention pertains to novel Electronic Article Surveillance (EAS) systems and methods of electronic article surveillance which are able to detect tagged articles with reduced incidence of false alarms m the presence of noise, lower power consumption, and lower costs than current systems. Advantageously, the subject invention allows significant improvement in noise reduction and greatly increased sensitivity to a marker in the detection field. In a specific embodiment, the EAS system of the subject invention utilizes an amplitude modulated transmitter signal created by, for example, an electromagnetic generator and coil When a soft magnetic marker is brought into the transmitter field, the marker generates a signal This signal generated by the marker contains harmonics of the transmitter earner signal which are amplitude modulated at the same frequency as the transmitter modulation signal In addition, the marker signal contains harmonics of the transmitter earner signal which are modulated at frequencies which are harmonics of the transmitter modulation signal. Accordingly, the receiver coil and receiver signal processor of the subject EAS system can receive the signal generated by the marker. In a preferred embodiment, the receiver demodulates the received harmonics of the transmitter earner signal and detects the modulation having the same frequency as the amplitude modulation signal as an indication that a marker is present. In a more preferred embodiment, the receiver demodulates the received harmonics of the transmitter earner signal and detects the modulation having frequencies which are harmonics of the amplitude modulation signal as an indication that a marker is present.

Advantageously, the subject EAS system can have a higher signal-to-noise ratio than other systems utilizing amplitude modulated transmitter signals. For example, pnor radio frequency EAS systems utilizing amplitude modulated transmitters attempt to detect the modulated field produced by the marker, in the presence of coupling of the modulated field produced by the transmitter in the receiver. Specifically, the modulation is present whether or not a marker is present in the detection field, due to coupling in the receiver. Accordingly, these other amplitude modulated systems must then differentiate the signal due to the presence of a marker and the signal due to coupling of the transmitter generated field the receiver. With respect to the subject invention, the modulation itself is not a problem. This is because modulated harmonics and/or harmonics of the modulation generated by a marker are detected, which are present only when a marker is present.

The subject invention improves on previous designs by maximizing a modulated signal from the marker while the transmitter is generating its highest (peak) output signal and, simultaneously, reducing the power consumption of the system when the transmitter is generating its lowest output signal. In addition, modulating the transmitter output signal can further enhance detection by reducing the shielding of the marker by the transmitter This shielding effect occurs when the transmitter output signal is at a maximum and can be reduced or eliminated when the transmitter output signal is reduced. In a preferred embodiment, the minimum transmitter output signal magnitude is between about 30 percent and about 50 percent of the maximum transmitter output signal magnitude. When a marker is near the transmitter coil dunng the maximum transmitter signal the marker can, in effect, be shielded by the transmitter and, therefore, not be detected. However, when the transmitter signal drops to about 30 to about 50 percent of the maximum, the marker is no longer shielded and can be detected, thus enhancing detection. This effect can also be observed when the transmitter is pulsed on and off if the ns g and/or falling edge(s) of the pulse train reside at a value of about 30-50 percent of the maximum value for a sufficient duration, thus allowing detection of a marker near the transmitter coil.

Additionally, modulating the transmitter output signal, the subject invention allows the use of smaller markers, fewer loops on the transmitter antenna coil, and/or more peak power with the same components as previous EAS systems. The use of smaller markers can reduce the expense of marking items, as well as allowing more effective marking. For example, the smaller markers can be more discretely appended to merchandise Transmitter coils having fewer loops are less expensive to build, lighter, and more compact. It is a matter of design choice as to which of these advantages to exploit. In addition, by detecting modulation of harmonics of the transmitter earner signal, the effects of coupling of unwanted signals from the transmitter in the receiver are minimized, creating a very high signal to noise ratio.

In a specific embodiment, for example when extremely high noise environments are encountered, the subject EAS system can alternate the modulation frequency from a first frequency to a second frequency in order to produce a dual (two tone) modulation which is extremely difficult to duplicate in ordinary circumstances and environments. This reduces the probability of false detections. In a specific embodiment, refemng to Figure 3, different modulation frequencies are accomplished with the transmitter modulation frequency locked to the receiver modulation filter. This circuitry can act independently from the general EAS system frequency or circuitry.

Following are examples which illustrate procedures for practicing the invention. These examples should not be construed as limiting.

Exam le 1 Refernng to Figure 1 , a specific embodiment of the subject invention can compnse transmitting generator 1 and transmitting coil 2 for generating a magnetic field in the interrogation zone, and detector 3 and receiving coil 4 for receiving and detecting magnetic fields generated by a marker located in the interrogation zone. Figure 2A shows one representation of an amplitude modulated transmitter signal in accordance with the subject invention. This transmitter earner signal can have many different waveform shapes, for example sinusoidal. Referring to Figure 2C, the modulating signal can also have many different waveform shapes and/or parameters. For example, the amplitude modulating signal can have equal duration on and off cycles, or an on cycle to off cycle ratio which differs from one. The off cycle can be set to turn the transmitter signal completely off or set to lower the magnitude of the transmitter signal. For example, the amplitude modulating signal can vary between a peak value and some fraction, for example 50 percent, of peak value. Figure 2B shows a representation of a signal generated by a marker present in the interrogation zone, which is a harmonic of the transmitter carrier signal modulated by the modulating signal of Figure 2C. Detector 3 can receive and demodulate the modulated harmonic generated by a marker and utilize the modulation as an indication of the presence of a marker.

Example 2

A further embodiment of the subject invention is particularly advantageous when extremely high noise environments are encountered. The embodiment described in Example 1 can be utilized with additional circuitry added to allow the modulation frequency to alternate among at least two frequencies. For example, the frequency of the modulating signal can alternate between a first frequency and a second frequency m order to produce a dual (two tone) modulation This two tone modulation is unlikely to be duplicated by noise, thus lower the probability of false detections. Referring to Figure 3, different modulation frequencies can be accomplished by, for example, locking the transmitter modulation frequency to the receiver modulation filter Preferably, this circuitry can act independently from the general EAS system circuitry.

Example 3

In an additional embodiment of the subject invention, two transmitter-receiver pairs can be utilized to, for example, increase the width of the interrogation zone which can be monitored. Refemng to Figure 4, each pair can comprise a transmitting coil 11, 12 and a receiving coil 9, 10. In a specific embodiment, transmitting coil 11, preferably of a single coil design, is connected to a first field generator (transmitter) 13, and transmitting coil 12, preferably of a single coil design, is connected to a second field generator (transmitter) 16. Associated with transmitter coil 11 is receiver coil 9, which can be parallel to transmitter coil 11 and separated from transmitter coil 11 by the interrogation zone. Associated with transmitter coil 12 is receiver coil 10, which can be parallel to transmitter coil 10 and separated from transmitter coil 12 by the interrogation zone. Receiver coil 9 is connected to detector 15, which employs circuitry to detect a marker, and receiver coil 10 is connected to detector 14 which employs circuitry to detect a marker.

In a specific embodiment, referring to Figure 5A, transmitters 13 and 16 may be switched on and off out of phase with each other, for example, by clock 20. Accordingly, when transmitter 13 is on, transmitter 16 is off and vice versa. Referring to Figure 5B, when transmitter 16 is on, coil 12 generates an appropriate time- varying magnetic field in the interrogation zone, and when transmitter 13 is on, coil 11 generates an appropriate time-varying magnetic field in the interrogation zone. Advantageously, the coil pairs can be separated further, for the same power output, such that each pair only covers one half of the interrogation zone.

For example, when a marker is located in the half of the interrogation zone closest to coil 12, the marker will produce an amplitude modulated signal which can be detected and demodulated by detector 14. Likewise, when a marker is located in the half of the interrogation zone closest to coil 11, the marker will produce an amplitude modulated signal which can be detected and demodulated by detector 15. It can be observed, that this embodiment allows various advantages, for example, lower power consumption, smaller transmitter coils, and/or a wider interrogation zone.

Example 4 Referring to Figure 4, various alternative switching schemes for coils 11 and 12 can be realized according to the subject invention. For example, in a specific embodiment, coil 11 and coil 12 can be modulated at different frequencies. The signal from the marker can be detected and demodulated, with the amplitude modulation indicating the presence of a marker. Alternatively, coil 11 and coil 12 can be modulated at different frequencies and the signal from a marker can be detected and demodulated, with the phase modulation used as an indication of the presence of a marker.

In an additional embodiment coil 11 and coil 12 can be connected to transmitter 13 and 16, respectively, wherein the transmitters 13 and 16 operate at different frequencies. For example, as set forth in U.S. Application Serial No. 08/699,880 filed on August 16, 1996 which has been incorporated herein by reference, operating transmitters 13 and 16 at different frequencies can create an amplitude and phase modulated harmonic signal produced by a marker. Furthermore, transmitters 13 and 16 can be switched on and off, or pulsed, simultaneously in order to create an additional amplitude modulation within the harmonic generation of the marker. The result of amplitude modulation, or switching on and off, of the two earners and the phase modulation created by two different transmitter frequencies creates a marker harmonic signal having amplitude modulation at a specific frequency and a phase modulation at another unique frequency The combination of the amplitude modulation at a specific frequency and the phase modulation at another unique frequency are not easily realized by random noise sources.

In a specific embodiment, coil 11 can be dnven at 1000 Hz and coil 12 can be dnven at 1020 Hz, creating a marker signal with a phase modulation of 20 Hz or a multiple of 20 Hz. In addition, if both transmitters are simultaneously switched on and off, or pulsed, at 6 Hz, a marker signal can contain a 6 Hz amplitude modulation of the marker generated harmonics of the transmitter signals. Accordingly, phase detection and narrow band filtering of the 20 Hz, or multiple of 20 Hz, signal produce a 20 Hz phase modulation signal which can indicate the presence of a marker in the interrogation zone. In addition, amplitude detection and narrow band filtenng of the 6 Hz amplitude modulation results in a 6 Hz amplitude modulation signal which can also indicate the presence of a marker m the interrogation zone. In a preferred embodiment, both the 20 Hz phase modulation signal and the 6 Hz amplitude modulation signal can be detected and used as an indication of the presence of a marker wherein combination of both modulation signals is extremely difficult to replicate in any noise environment and therefore can reduce false alarms. Advantageously, the switching of transmitters on and off allows the use of smaller transmitter coils without reducing peak transmit field intensities.

Example 5

Again refemng to Figure 4, a specific embodiment of the subject invention incorporating two transmitter-receiver pairs, operating at different frequencies, wherein the transmitter and receiver of each pair are separated by the interrogation zone is shown. It is preferable for each transmitter to be of a single coil design. An EAS system of the subject mvention used to detect a marker in the interrogation zone is shown, schematically, in Figure 1 In a specific embodiment, the system of the subject invention can compnse a pair of antenna arrays each compnsing a transmitting coil 11, 12 and a receiving coil 9, 10. In a specific embodiment, transmitting coil 11, preferably of a single coil design, is connected to a first field generator 13, that can produce an output equivalent to less than about 50 Watts rms, and preferably less than about 25 Watts rms. In a specific embodiment, the frequency of a transmitting coil is approximately 1 kHz (FI). Associated with the transmitter coil 11 is a receiver coil 9, which is parallel to the transmitter coil 11 and separated from transmitter coil 11 by the interrogation zone. The receiver coil 9 is connected to detector 15, which employs circuitry to detect a marker

A second transmitter coil 12, preferably of a single coil design, is connected to a second field generator 16, which is about 3 to about 50 Hertz lower or higher m frequency (F2) than the frequency (FI) produced by the first field generator 13. The receiver coil 10 is connected to a detector 14, which is substantially equivalent to detector 15. Accordingly, there are components of two transmitter-receiver systems that operate at slightly different frequencies with associated transmitters and receivers separated by an interrogation zone.

The transmit fields developed by coils 11 and 12 and associated generators vectoπally add or cancel, for all onentations, in the interrogation zone. The ratio of the amplitudes of the transmit fields is in the range of about 1 : 1 to about 1.5: 1, and preferably about 1.1. A marker passing through the interrogation zone produces harmonics which are amplitude modulated at a rate determined by the difference in transmission frequency [F1-F2] and which are phase shifted due to inherent material properties of the marker Since all generators produce harmonics of the transmitted fundamental, the receives coils 9 and 10 are positioned away from their associated transmitters on opposite sides of the interrogation zone The detector 14 can utilize narrow band harmonic filters to minimize out-of-band signals such as the harmonics being transmitted by the adjacent coil 11. Detector 15 functions m an equivalent manner.

In-band signals passed through harmonic filters in each detector are demodulated and the modulation compared m phase with a reference signal. Since both detectors and their associated generators are phase locked together, phase compaπson is relatively simple and when present conditions are met, an alarm is activated. In a specific embodiment, the reference signal, both detectors, and both transmitters are phase locked

The advantages of this surveillance system are that the transmit field is maximized in all onentations creating a large interrogation zone and the detection of phase shifted amplitude modulated harmonics greatly reduces false alarms due to noise.

It should be understood that the examples and embodiments descnbed herein are for illustrative purposes only and that vanous modifications or changes m light thereof will be suggested to persons skilled in the art and are to be included within the spint and purview of this application and the scope of the appended claims

Claims

Claims 1. A system for detecting the presence of a ferromagnetic marker in an interrogation zone, compnsing. a generating means for generating an amplitude modulated magnetic field m the interrogation zone; and a detecting means for detecting a modulated harmonic signal produced by a ferromagnetic marker present within the interrogation zone, wherein said detecting means demodulates said modulated harmonic signal and utilizes a resulting modulation signal as an indication of the presence of said marker in the interrogation zone.

2. The system, according to claim 1, wherein said amplitude modulated magnetic field is produced by a transmitter coil driven at a earner frequency and modulated by an amplitude modulation signal, wherein said modulated harmonic signal is a harmonic of said earner frequency modulated at the frequency of the amplitude modulation signal or a harmonic of the frequency of the amplitude modulation signal

3. The system, according to claim 2, wherein the frequency of said amplitude modulation signal is less than about 50 hertz.

4 The system, according to claim 2, wherein the frequency of said amplitude modulation signal alternates between at least two frequencies in order to create a multi-tone modulation.

5. The system, according to claim 4, wherem said detecting means compnses a modulation filter wherein the frequency of said amplitude modulation signal is locked to said modulation filter.

6. The system, according to claim 2, wherein said amplitude modulation signal is a pulse tram

7 The system, according to claim 6, wherein said pulse train alternates in magnitude between a maximum value and a minimum value, wherein said minimum value is between about 30 percent and about 50 percent of said maximum value

8. The system, according to claim 7, wherein said minimum value is approximately zero.

9. A system for detecting the presence of a ferromagnetic marker an interrogation zone, compnsing 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; and a detecting means for receiving and demodulating a signal generated by a marker present m the interrogation zone, wherein a resulting modulation signal is used as an indication of the presence of said marker in the interrogation zone.

10. The system, according to claim 9, wherein said second frequency is about 3 to about 50 hertz lower or higher than said first frequency, and wherein phase modulation of said marker generated signal is detected.

11 The system, according to claim 9, wherein said marker generated signal is a harmonic of said first frequency or a harmonic of said second frequency, and wherein said detecting means detects amplitude modulation of said marker generated signal.

12. The system, according to claim 9, wherein said first generating means and said second generating means are simultaneously switched on and off at a third frequency, creating amplitude modulation of said marker generated signal, wherein said amplitude modulation occurs at said third frequency or a harmonic of said third frequency.

13 The system, according to claim 10, wherein said first generating means and said second generating means are pulsed out of phase with each other, wherem said first generating means and said second generating means are each pulsed between a maximum value and a minimum value between about 30 percent and about 50 percent of the maximum value.

14. The system, according to claim 12, wherein said detecting means detects phase modulation of said marker generated signal

15. A method for detecting the presence of a ferromagnetic marker in an interrogation zone, compnsing the following steps: generating an amplitude modulated magnetic field the interrogation zone; and detecting a modulated harmonic signal produced by a ferromagnetic marker present within the interrogation zone, wherein said modulated harmonic signal is demodulated and a resulting modulation signal is used as an indication of the presence of said marker in the interrogation zone.

16. The method, according to claim 15, wherein said amplitude modulated magnetic field is produced by a transmitter coil dnven at a earner frequency and modulated by an amplitude modulation signal, wherem said modulated harmonic signal is a harmonic of said earner frequency modulated at the frequency of the amplitude modulation signal or a harmonic of the frequency of the amplitude modulation signal.

17. The method, according to claim 16, wherein the frequency of said amplitude modulation signal is less than about 50 hertz

18. The method, according to claim 16, wherein the frequency of said amplitude modulation signal alternates between at least two frequencies in order to create a multi-tone modulation.

19. The method, according to claim 18, wherein the step of detecting a modulated harmonic signal compnses the use of a modulation filter wherem the frequency of said amplitude modulation signal is locked to said modulation filter.

20. The method, according to claim 16, wherein said amplitude modulation signal is a pulse tra .

21. The method, according to claim 20, wherein said pulse tram alternates in magnitude between a maximum value and a minimum value, wherein said minimum value is between about 30 percent and about 50 percent of the maximum value

22. The method, according to claim 21, wherein said minimum value is approximately zero.

23. A method for detecting the presence of a ferromagnetic marker m an interrogation zone, compnsing the following steps: 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; and receiving and demodulating a signal generated by a marker present m the interrogation zone,

wherein a resulting modulation signal is used as an indication of the presence of said marker in the interrogation zone.

24. The method, according to claim 23, wherein said second frequency is about 3 to about 50 hertz lower or higher than said first frequency, and wherein phase modulation of said marker generated signal is detected.

25. The method, according to claim 23, wherein said marker generated signal is a harmonic of said first frequency or a harmonic of said second frequency, wherein amplitude modulation of said marker generated signal is detected.

26. The method, according to claim 23, further comprising the step of simultaneously switching said first and second magnetic fields on and off at a third frequency, creating amplitude modulation of said marker generated signal, wherem said amplitude modulation occurs at said third frequency or a harmonic of said third frequency

27. The method, according to claim 24, further compnsing the step of pulsing said first and second magnetic fields out of phase with each other, wherein said first and second magnetic fields are each pulsed between a maximum value and a minimum value between about 30 percent and about 50 percent of the maximum value.

28 The method, according to claim 27, wherein phase modulation of said marker generated signal is detected.

29. A system for detecting the presence of a ferromagnetic marker in an interrogation zone, compnsing- a first generating means for generating a first magnetic field m 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

30. The system, according to claim 29, wherein the ratio of the amplitudes of said magnetic fields is in the range from about 1 : 1 to about 1.5: 1

31. The system, according to claim 30, wherein the ratio of the amplitudes of said magnetic fields is about 1: 1.

32. The system, according to claim 29, wherem said first and second generating means are each of a single coil design.

33. The system, according to claim 29, wherem said signal is an amplitude modulated signal

34. The system, according to claim 33, wherein said signal is an amplitude modulated harmonic.

35 The system, according to claim 34, wherem said signal is an amplitude modulated harmonic of either said first or said second magnetic field

36. A system, according to claim 29, wherein said first and second generating means and said detecting means are phase locked

37. A system, according to claim 29, wherem said detecting means comprises a reference signal for companson of the phase of signal to said reference signal.

38. The system, according to claim 29, wherein said interrogation zone is greater than about 1.5 meters.

39 The system, according to claim 29, wherein said first and second generating means produce an output of less than about 50 watts rms each.

40. The system, according to claim 39, wherein said first and second generating means produce an output of less than about 25 Watts rms each.

41. A method for detecting the presence of a ferromagnetic marker m an interrogation zone, compnsing the steps of: generating a first magnetic field 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; wherem said second frequency is about 3 to about 50 hertz lower or higher than said first frequency.

42. The method, according to claim 41, wherein the ratio of the amplitudes of said magnetic fields is in the range from about 1 : 1 to about 1.5: 1

43 The method, according to claim 42, wherein the ratio of the amplitudes of said magnetic fields is about 1 :1.

44. The method, according to claim 41, wherem the steps of generating a first magnetic field further compnses generating said first magnetic field with a single coil design generating means, wherein the step of generating a second magnetic field further compnses generating said second magnetic field with a single coil design generating means

45. The method, according to claim 41, wherein said signal produced by a ferromagnetic marker present within the interrogation zone is an amplitude modulated signal.

46. The method, according to claim 45, wherein said amplitude modulated signal is at a frequency which is an amplitude modulated harmonic of either said first or said second magnetic field

47. The method, according to claim 44, wherein the step of detecting a phase shift of said signal further compnses companng the phase of a reference signal to the phase of said signal.

48 The method, according to claim 47, wherem said first and second generating means and said reference signal are phase locked

49. The method, according to claim 41 , wherein said interrogation zone is greater than about 1.5 meters.

50. The method, according to claim 41, wherein said first and second generating means produced an output of less than about 50 watts rms each.

51. The methods, according to claim 50, wherem said first and second generating means produce an output of less than about 25 Watts rms each.