HK1173549B - Method and system for receiver nulling using coherent transmit signals - Google Patents

Description

Method and system for receiver nulling using coherent transmission signals

Technical Field

The present invention relates generally to methods and systems for improving signal-to-noise ratio in electronic theft detection systems, and more particularly to methods and systems for detecting power line fluctuations received by a metal detection system transmitter and using the detected power line fluctuations to cancel signal fluctuations at a metal detection system receiver to improve signal-to-noise ratio.

Background

Electronic article surveillance ("EAS") systems are detection systems that allow for the detection of markers or tags within a given detection zone. EAS systems have many uses. Most commonly, EAS systems are used as security systems for signaling the theft of items from stores or the removal of property from office buildings. EAS systems come in many different forms and utilize many different technologies.

A typical EAS system includes an electronic detection unit, a tag and/or label, and a lock opener or decoder. The detection unit includes a transmitter and receiver antenna and is used to detect any active markers or tags that are brought within range of the detection unit. For example, the antenna portion of the detection unit can be bolted to the floor as a pedestal, buried under the floor, mounted on a wall, or suspended from the ceiling. The detection units are typically arranged in high traffic areas, such as entrances and exits of shops or office buildings. The decoder issues a signal to detect and/or decode the tag.

The markers and/or tags have particular characteristics and are specifically designed to be affixed to or embedded within merchandise or other objects sought to be protected. When the active marker passes the detection unit, an alarm is sounded, a light is illuminated, and/or some other suitable control device is set to indicate the removal of the marker from the prohibited detection zone covered by the detection unit.

Most EAS systems operate using the same general principles. The detection unit includes one or more transmitters and receivers. The transmitter transmits signals at a prescribed frequency within the detection zone. For example, in a retail store, locating the transmitter and receiver on opposite sides of a check out aisle or exit typically forms a detection zone. When a marker enters the area, it can cause interference with the signal emitted by the transmitter. For example, the marker may alter the signal sent by the transmitter by using a simple semiconductor junction, a tuned circuit comprising an inductor and a capacitor, soft magnetic strips or wires, or a vibrating resonator. The marker may also alter the signal by repeating the signal for a period of time after the transmitter terminates the signal transmission. This interference caused by the marker is then detected by the receiver through reception of a signal having the desired frequency, reception of the signal at the desired time, or both. As an alternative to the basic design described above, the receiver and transmitter units, including their respective antennas, can be mounted within a single housing.

Magnetic materials or metals (e.g., metal shopping carts) disposed near the EAS marker or transmitter may interfere with the optimal performance of the EAS system. Furthermore, some unscrupulous individuals utilize EAS marker shielding to steal merchandise without detection by an EAS system. Metal can shield tagged items from detection by an EAS detection system by preventing interrogation signals from reaching the tag or by preventing reply signals from reaching the EAS system. When the shielded tag passes through the detection unit, the EAS system cannot detect the tag. As a result, shoplifters can remove merchandise from the store without activating an alarm.

Metal detection systems are used in conjunction with EAS systems to detect the presence of metal objects such as shielded bags or shopping carts. The metal detection system may use a transmitter and receiver in common with the EAS system. For metal detection, the transmitter transmits a signal within the detection zone at a predetermined metal detection frequency. When a metal object enters the detection zone, it creates interference with the signal emitted by the transmitter. This interference caused by the metallic object is then detected by the receiver through reception of the modified signal. Upon detection of the modified signal, an alarm is sounded, a light is illuminated, and/or some other suitable control device is set to indicate operation of the metal present in the detection zone.

Metal detection systems are sensitive to interference signals introduced through the power line, including spikes in current or other fluctuations on the power line. Conventional metal detection systems tend to generate false alarm signals when subjected to power line interference signals. What is needed is a system and method for detecting and eliminating power line interference signals in a metal detection system to reduce the occurrence of false alarm trigger signals.

Disclosure of Invention

The present invention advantageously provides a method and system for canceling interference signals introduced into an emitted wireless signal and propagated to a signal received at a corresponding receiver.

According to one embodiment, the present invention provides a system for canceling an interfering signal from a received signal. A ratio module (ratio module) receives as input a ratio of the filtered receiver output signal level and the filtered transmitter output signal level. A product module receives as inputs the output of the ratio module and the transmitter output. The product module calculates the product of the output of the ratio module and the transmitter output. The adjusted receiver signal module receives a difference calculated from the received signal level and the output of the product module.

In accordance with another embodiment, the present invention provides a method for canceling an interfering signal introduced within a transmitted wireless signal. The method includes determining a filtered transmitter value, determining a filtered receiver value, and calculating a ratio of the filtered receiver value to the filtered transmitter value. The real time transmitter value is multiplied by the ratio to obtain a multiplied value, and the difference between the multiplied value and the real time receiver value is calculated. The corrected receiver values are provided for canceling interference signals introduced into the transmitted signal.

According to yet another embodiment, the invention provides a security system. The security system includes at least one antenna, an electronic monitoring system for detecting the presence of an active tag using the at least one antenna, and a metal detection system for detecting objects of metal using the at least one antenna. The metal detection system includes a ratio module for receiving as input a ratio of the filtered receiver output signal level and the filtered transmitter output signal level. The product module receives as inputs the output of the ratio module and the transmitter output. The product module calculates the product of the output of the ratio module and the transmitter output. The adjusted receiver signal module receives a difference calculated from the received signal level and the output of the product module.

Additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The aspects of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

Drawings

A more complete understanding of the present invention and the attendant advantages and features thereof will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of an exemplary security system having EAS detection and metal detection capabilities constructed in accordance with the principles of the present invention;

FIG. 2 is a block diagram of an exemplary EAS system controller constructed in accordance with the principles of the present invention; and

fig. 3 is an exemplary schematic diagram of an interference detection and cancellation circuit according to the principles of the present invention.

Detailed Description

Before describing in detail exemplary embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of apparatus components and processing steps related to implementing a system and method for reducing false alarm signals by detecting and eliminating interference signals introduced in a metal detection system.

The components of the systems and methods are represented where appropriate by conventional symbols in the drawings. The drawings show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

As used herein, relational terms, such as "first" and "second," "top" and "bottom," and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements.

The metal detection system emits a signal within the detection zone at a predetermined metal detection frequency. When a metallic object is present in the detection zone, the signal transmitted by the transmitter is disturbed. This interference is then detected by a receiver of the metal detection system, which receives the modified signal. The metal detection system may process the modified signal and generate an alert, e.g., sound an alarm, illuminate a light, and/or generate other alerts.

The metal detection system may pick up interference signals that pass through the power line, including protruding spikes in current or other fluctuations on the power line. The signal received at the metal detection receiver may include an interference signal if the metal detection system transmits a signal into an interrogation zone having a dominant interference signal. The interfering signal may degrade the performance of the system, including causing the receiver to generate false alarm signals.

An embodiment of the present invention advantageously provides a method and system for detecting an introduced interference signal by a metal detection system transmitter and canceling the interference signal at a metal detection system receiver to reduce the occurrence of false alarm signals, improve signal-to-noise ratios, and provide other advantages. According to one embodiment, the interference signal detected at the receiver is synchronously correlated with the interference signal introduced at the transmitter. For example, the ratio between the receiver signal level and the transmitter signal level may be constant over the entire detection period. Also, a coherent relationship between the transmitted and received signals may be applied to reduce false alarm signals generated by power line sags (dips).

EAS systems detect markers that pass through a predetermined detection zone (also referred to as an interrogation zone). The marks may comprise strips of fusion cast amorphous magnetic tape, among other mark types. Under specific magnetic bias conditions, the tag receives and stores energy, e.g., acousto-magnetic field energy, at its natural resonant frequency. When the emitted energy source is turned off, the marker becomes the source of the signal and emits energy, e.g., acousto-magnetic ("AM") energy, at its resonant frequency. The EAS system is configured to detect AM energy emitted by the marker, among other energy sources.

One embodiment of the present invention advantageously provides a method and system for detecting the presence of metal within the interrogation zone of a security system and determining whether the detected metal is EAS marker shielding. The security system combines traditional EAS detection capabilities with metal detection to increase the accuracy of the security system, thereby reducing the likelihood of false alarms.

Referring now to the drawings in which like reference designators refer to like elements, there is shown in fig. 1 a security system constructed in accordance with the principles of the present invention and designated generally as "100". The security system 100 may be located at an entrance to a facility, among other locations. The security system 100 may include an EAS system 102, a metal detection system 104, and a pair of pedestals 106a, 106n (collectively referred to as pedestals 106) positioned on opposite sides of, for example, an entrance 108. The metal detection system 104 may include an interference detector and cancellation circuit 105. The operation of this circuit will be discussed in detail below. One or more antennas 107a, 107n (collectively referred to as antennas 107) may be contained within the base 106. The antennas may be positioned a known distance apart for use by the EAS system 102 and the metal detection system 104. A system controller 110 is provided for controlling the operation of the security system 100 and is electrically coupled to the EAS system 102, the metal detection system 104, and the antenna 107, among other components. Notably, while the jammer detector and cancellation circuit 105 is shown in fig. 1 as being part of the metal detection system 104, it is contemplated that the jammer detector and cancellation circuit 105 can be separate or included within other elements of the system 100, for example, as part of the system controller 110. Further, although the EAS system 102, the metal detection system 104, and the system controller 110 are shown as separate elements, such representation is for ease of understanding and is not intended to limit the scope of the present invention. It is contemplated that the EAS system 102, the metal detection system 104, and the system controller 110 can be incorporated into fewer than three or more than three physical housings.

According to one embodiment, the EAS system 102 applies transmit pulses and a listening arrangement to detect objects (e.g., markers). The sensing period may be 90Hz (11.1 milliseconds), among other sensing periods. The detection cycle may include four periods including an emission window, a tag detection window, a synchronization window, and a noise window. The detection period may operate according to principles known to those skilled in the art.

According to one embodiment, a metal detection system 104 is presented, and the metal detection system 104 may share hardware components with the EAS system 102. Thus, the metal detection system 104 may share an antenna 107 with the EAS system 102. For example, the antenna 107 may be used as a transmit antenna for both the EAS system 102 and the metal detection system 104. The metal detection system 104 may monitor a signal for induced eddy currents that indicate the presence of a metal object at a location near the antenna 107. Typically, for a good conductor, the induced eddy currents will dissipate in the order of tens of microseconds. By comparison, eddy currents dissipate approximately two orders of magnitude faster than the AM energy of an acoustic marker.

The EAS system 102 and the metal detection system 104 may be designed to operate at different frequencies. For example, the EAS system 102 may operate at 58kHz, while the metal detection system 104 may operate at 56 kHz. Those skilled in the art will readily appreciate that these systems may operate at other frequencies. To avoid interfering with each other during operation, the signals generated by the EAS system 102 and the metal detection system 104 are separated by at least a detection period, e.g., 1/90Hz or greater.

The metal detection system 104 may experience signal distortion due to interference signals, including radio frequency interference signals, magnetic interference signals, power line interference signals, among other interference signals. For example, power line interference signals may be caused by a number of sources, including, among other power line interference signals: protruding spikes in current, random fluctuations in alternating current, poor or damaged wiring, interference from other machines or appliances, fluorescent lighting, lightning strikes to the power grid, and severe weather conditions. Signal distortion caused by the interfering signal can cause the metal detection system 104 to operate incorrectly, including generating false alarm signals or other undesirable signals.

Referring now to FIG. 2, the system controller 110 may include a controller 202 (e.g., a processor or microprocessor), a power source 204, a transceiver 206, a memory 208 (the memory 208 may include non-volatile memory, or a combination thereof), a communication interface 210, an alarm unit 212, a real-time clock ("RTC") 214, an electronic article surveillance module 222, and a metal detector module 224. The electronic article monitoring module 222 is in communication with the electronic article monitoring system 102, while the metal detection module 224 is in communication with the metal detector system 104 and the interference detector and cancellation circuitry 105. The operation of the jammer detector and cancellation circuit 105 will be described in more detail below.

The system build may be of modular construction to facilitate adding, deleting, updating, and/or modifying modules therein and/or features within modules. It should be readily appreciated that any number of modules may be used, and that a module may be a software application executing on a processor. Those skilled in the art will readily appreciate that the present invention may be implemented using individual modules, a single module incorporating features of two or more separately described modules, individual software programs, and/or a single software program.

The controller 202 controls operations performed by the system controller 110, including, among other operations: radio communication, data storage of the memory 208, communication of stored data with other devices, and activation of the alarm unit 212. The power source 204 may provide a DC voltage or an AC voltage to the system 100, among other operations. For example, the power source 204 may power the system controller 110. The alarm unit 212 may include software and/or hardware for providing an alert (e.g., a visual and/or audible alert) in response to the detection of an EAS marker and/or metallic object within the interrogation zone of the system 100.

The transceiver 206 may include a transmitter 216 electrically coupled to one or more transmit antennas 107a and a receiver 218 electrically coupled to one or more receive antennas 107 n. According to one embodiment, a single antenna or a pair of antennas may be used as both the transmit antenna 107a and the receive antenna 107 n. The transmitter 216 transmits radio frequency signals using the transmitting antenna 107a to energize EAS markers and/or to detect the presence of metallic objects within the interrogation zone of the system 100. Receiver 218 detects the response signal from the EAS marker and/or the response signal from the metallic object using receiving antenna 107 n. According to one embodiment, all or part of the interference detector and cancellation circuitry 105 may be implemented on the receiver 218 using a digital signal processor ("DSP") or other hardware structure.

A communication interface 210 may be provided to facilitate communication between the various components of the system 100. For example, the communication interface 210 may transmit data between the receiver 218 and the metal detector system 104 through the metal detection module 224. According to one embodiment, the metal detection module 224 may include the metal detector system 104 and the jammer detector and cancellation circuit 105. The controller 202 may trigger the alarm unit 212 to activate an alarm signal if the measured value of the response signal from the EAS marker and/or the response signal from the metallic object exceeds a predetermined threshold. The communication interface 210 and/or alarm unit 212 may send an alert signal to a device for alerting store security or other authorized personnel that may be properly monitoring or in proximity to the individual.

According to one embodiment, a real time clock ("RTC") 214 may be electrically coupled to the controller 202 to monitor the passage of time. The RTC 214 may be used to generate a timestamp that allows the occurrence of an alarm event and/or the occurrence of other events to be logged.

Fig. 3 is a schematic diagram of an exemplary interference detector and cancellation circuit 105. According to one embodiment, the transmitter sampling module 303 extracts a predetermined number of sample values from the signal transmitted by the antenna 107 a. The signal sample values may represent the signal amplitude or some other measurable characteristic of the transmitted signal. The transmitter sampling module 303 may operate at a frequency of 46kHz to 96 kHz. For example, transmitter sampling module 303 may operate at a frequency of 46.296kHz and may extract sixteen (16) sample values representing signal amplitudes. Those skilled in the art will readily recognize that the transmitter sampling module 303 may operate at other frequencies and may extract a different number of sample values.

The transmitter normalization module 305 receives the plurality of sample values from the transmitter sampling module 303 and provides a single value to a transmitter n tap (n-tap) Low Pass Filter (LPF) 307. According to one embodiment, the transmitter n tap LPF 307 may include a 50 tap LPF. Those skilled in the art will readily recognize that different n tap values may be used for the n tap LPF 307 of the transmitter. The transmitter n tap LPF 307 performs an averaging function on the single sample value obtained by the transmitter normalization module 305 along with n-1 previously stored single sample values. The output of the transmitter n tap LPF 307 is referred to as the filtered transmitter output and is provided to a division module 309. The transmitter normalization module 305 also provides a single transient transmitter value to the steady-state or transient transmitter module 311.

A receiver sampling module 322 is provided for extracting a predetermined number of sample values from the signal received at the antenna 107 n. The signal sample values may represent the signal amplitude or some other measurable characteristic of the received signal. According to one embodiment, the receiver sampling module 322 may operate at a frequency of 46kHz-96 kHz. For example, the receiver sampling module 322 may operate at a frequency of 46.296kHz and may extract sixteen (16) sample values representing signal amplitudes. Those skilled in the art will readily recognize that the receiver sampling module 322 may operate at other frequencies and may extract a different number of sample values.

The receiver normalization module 324 receives a plurality of sample values from the receiver sampling module 322 and provides a single value to a receiver n-tap Low Pass Filter (LPF) 326. According to one embodiment, receiver n tap LPF326 may include a 50 tap LPF. Those skilled in the art will readily recognize that different n-tap values may be used for receiver n-tap LPF 326. Receiver n tap LPF326 performs an averaging function on a single sample value obtained from receiver normalization module 324 along with n-1 previously stored single sample values. The output of receiver n tap LPF326 is referred to as the filtered receiver output and is provided to a division module 309. The receiver normalization module 324 also provides a single transient receiver value to the steady-state or transient receiver module 328.

According to one embodiment, the division module 309 calculates the quotient by dividing the filtered receiver output by the filtered transmitter output or (filtered Rx/filtered Tx). The quotient is stored in the ratio module 330. A multiplication module 332 is provided for receiving the output from the ratio module 330 and from the transient transmitter module 311. The multiplication module 332 calculates the product (Tx (filtered Rx/filtered Tx)) between the values stored in the ratio module and the single transient transmitter value. The output of the multiplication module 332 is stored within the product module 334.

A subtraction module 340 is provided for receiving the output from the product module 334 and from the transient receiver module 328. According to one embodiment, the subtraction module 340 calculates the difference (Rx- (Tx (filtered Rx/filtered Tx)) between the value stored in the product module 334 and the single transient receiver value, which is stored in the adjusted receiver signal module 342.

Thus, the signal stored in the adjusted receiver signal module 342 is stripped of interfering signals introduced into the system 100 by power lines or other sources of interference and subsequently transmitted by the transmitter 216. The interference signal may include a spike in current or other fluctuations on the power line that may degrade system performance, including causing the receiver to generate false alarm signals.

The jammer detector and cancellation circuit 105 removes jammer signals detected at the receiver that are synchronously correlated with the jammer signals introduced at the transmitter. In other words, the interference detector and cancellation circuit 105 relies on the ratio between the receiver signal level and the transmitter signal level being constant over the entire detection period to remove the interference signal. Also, a coherent relationship between the transmitted signal and the received signal may be applied to reduce false alarm signals resulting from power line sags.

The jammer detector and cancellation circuit 105 detects jammer signals introduced by the transmitter of the metal detection system and cancels the jammer signals at the metal detection system receiver to reduce the occurrence of false alarm signals, improve signal-to-noise ratios, and provide other advantages. The noise reduced signal stored in the adjusted receiver signal module 334 is provided to the metal detection system 104 to minimize false alarm alarms and improve overall system performance.

The present invention can be realized in hardware, software, or a combination of hardware and software. Any kind of computing system, or other apparatus adapted for carrying out the methods described herein, is suited to perform the functions described herein.

A typical combination of hardware and software could be a specialized computer system having one or more processing elements and a computer program stored on a storage medium that, when loaded and executed, controls the computer system such that it carries out the methods described herein. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which-when loaded in a computer system-is able to carry out these methods. Storage media refers to any volatile or non-volatile storage device.

Computer program or application refers herein to any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) replicated in different material forms.

In addition, unless stated otherwise above, it should be noted that all of the accompanying drawings are not to scale. Significantly, this invention can be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be had to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.

Claims (20)

1. A system for canceling an interfering signal from a received signal, the system comprising:

a ratio module that receives as an input a ratio of a receiver output signal level filtered from the received wireless signal and a transmitter output signal level filtered from the transmitted wireless signal, and stores the input as an output;

a multiplication module that receives as inputs an output of the ratio module and a single transient transmitter value, the multiplication module calculating a product of the output of the ratio module and the single transient transmitter value, wherein the single transient transmitter value corresponds to the transmitted wireless signal; and

an adjusted receiver signal module that receives a difference calculated from a single transient receiver value and an output of the multiplication module, wherein the single transient receiver value corresponds to the received wireless signal.

2. The system of claim 1, further comprising:

an emitter normalization module that receives a plurality of sample emitter values and computes a normalized emitter value of the plurality of sample emitter values; and

a transmitter low pass filter in communication with the transmitter normalization module, the transmitter low pass filter to determine an average transmitter value using the normalized transmitter value to determine the filtered transmitter output signal level.

3. The system of claim 2, wherein the transmitter low pass filter comprises a 50 tap transmitter low pass filter.

4. The system of claim 3, wherein the 50 tap transmitter low pass filter stores previous sample transmitter values and averages the normalized transmitter value with the previously stored sample transmitter values.

5. The system of claim 2, further comprising:

a receiver normalization module that receives a plurality of sample receiver values and computes a normalized receiver value of the plurality of sample receiver values; and

a receiver low pass filter in communication with the receiver normalization module, the receiver low pass filter using the normalized receiver value to determine an average receiver value to determine the filtered receiver output signal level.

6. The system of claim 5, wherein the receiver low pass filter comprises a 50 tap receiver low pass filter.

7. The system of claim 6, wherein the 50 tap receiver low pass filter stores previous sample receiver values and averages the normalized receiver value with the previously stored sample receiver values.

8. The system of claim 1, wherein the adjusted receiver signal module stores a signal from which interference signals have been substantially removed, the resulting signal being a metal detection signal.

9. A method for canceling an interfering signal introduced into a transmitted wireless signal, the method comprising:

determining a filtered transmitter output signal level from the transmitted wireless signal;

determining a filtered receiver output signal level from the received wireless signal;

calculating a ratio of the filtered receiver output signal level to the filtered transmitter output signal level;

multiplying a single transient transmitter value by the ratio to obtain a multiplied value, the transient transmitter value corresponding to the transmitted wireless signal;

calculating a difference between the multiplied value and a single transient receiver value to provide a corrected receiver value, the transient receiver value corresponding to the received wireless signal;

wherein the corrected receiver value cancels an interference signal introduced within the transmitted wireless signal.

10. The method of claim 9, further comprising applying a transmitter 50 tap low pass filter to generate the filtered transmitter output signal level from the transient transmitter value.

11. The method of claim 9, further comprising applying a receiver 50 tap low pass filter to generate the filtered receiver output signal level from the transient receiver value.

12. The method of claim 10, further comprising:

storing a previous transient transmitter value within a 50 tap low pass filter of the transmitter;

averaging the single transient transmitter value and the previously stored transient transmitter value; and

a single output is provided for a 50 tap low pass filter of the transmitter.

13. The method of claim 11, further comprising:

storing previous transient receiver values within a 50 tap low pass filter of the receiver;

averaging the single transient receiver value and the previously stored transient receiver value; and

a single output of a 50 tap low pass filter of the receiver is provided.

14. A security system, comprising:

at least one antenna;

an electronic monitoring system that uses the at least one antenna to detect the presence of an active marker;

a metal detection system that detects objects of metal using the at least one antenna, the metal detection system comprising:

a ratio module that receives as an input a ratio of a receiver output signal level filtered from the received wireless signal and a transmitter output signal level filtered from the transmitted wireless signal, and stores the input as an output;

a multiplication module that receives as inputs an output of the ratio module and a single transient transmitter value, the multiplication module calculating a product of the output of the ratio module and the single transient transmitter value, wherein the single transient transmitter value corresponds to the transmitted wireless signal; and

an adjusted receiver signal module that receives a difference calculated from a single transient receiver value and the output of the multiplication module, wherein the single transient receiver value corresponds to the received wireless signal.

15. The security system of claim 14, the metal detection system further comprising:

an emitter normalization module that receives a plurality of sample emitter values and computes a normalized emitter value of the plurality of sample emitter values; and

a transmitter low pass filter in communication with the transmitter normalization module, the transmitter low pass filter using the normalized transmitter values to obtain an average transmitter value.

16. A safety system as in claim 15, wherein the transmitter low pass filter comprises a 50 tap transmitter low pass filter.

17. A safety system as in claim 16, wherein the 50 tap transmitter low pass filter stores previous sample transmitter values and averages the normalized transmitter value with the previously stored sample transmitter values.

18. The security system of claim 14, the metal detection system further comprising:

a receiver normalization module that receives a plurality of sample receiver values and computes a normalized receiver value of the plurality of sample receiver values; and

a receiver low pass filter in communication with the receiver normalization module, the receiver low pass filter using the normalized receiver values to obtain an average receiver value.

19. A security system according to claim 18 wherein the receiver low pass filter comprises a 50 tap receiver low pass filter.

20. The security system of claim 19, wherein the 50 tap receiver low pass filter stores previous sample receiver values and averages the normalized receiver value with the previously stored sample receiver values.