WO2006082521A2

WO2006082521A2 - Article sequencing for scanning and improved article screening for detecting objects and substances

Info

Publication number: WO2006082521A2
Application number: PCT/IB2006/000325
Authority: WO
Inventors: Warrick Paul Chisholm; Peter Alaric Hayes; Simon Bedford; David Kenny; John Harold Flexman; Taras Nikolaevitch Rudakov; Vassili Timofeevitch Mikhaltsevitch; Paul Hamilton; Benedict Joseph Sinagra; Ken Mann; Colin Charette; Amit Verma
Original assignee: Qrsciences Pty Ltd; Rapiscan Security Products (U.S.A.) Inc.
Priority date: 2005-02-01
Filing date: 2006-02-01
Publication date: 2006-08-10

Abstract

An apparatus is described for screening articles such as bags (13) passed therethrough for detecting prescribed substances or objects within the articles. A conveyor (17) conveys articles (13) in sequence in a given direction. The articles pass through a shield station (19) where they are screened to sense the presence of any shielding objects (SSV's) that would mitigate the efficacy of any subsequent or previous QR or X-ray screening. The articles are passed through a QR station (21) where they remain static for a prescribed period whilst screened, to sense the presence of any prescribed substances therein using QR detection technology. The articles are passed through an X-ray station (23) where they are dynamically screened to sense the presence of any prescribed substances or objects therein using X-ray detection technology. The X-ray station (23) comprises a single view dual energy linescan X-ray system for generating an X-ray image of articles (13) passed through the X-ray station. Methods and apparatus of sequencing the articles through the stations by pipelining to maximize throughput are also described. In this arrangement, the spacing between articles (13) is maintained to a critical threshold, determined by the effect that screening of an article at one station has on an adjacent article, and the effect that an adjacent article has on an article being screened, so that there is no or negligible interference between the two. A method of superimposing a shield localization image (87) obtained by the shield detector at the shield station upon the X-ray image (83) obtained by the X-ray system is also described.

Description

"Article Sequencing for Scanning and improved Article Screening for Detecting Objects and Substances"

Field of the Invention

This invention relates to, but is not limited to, the sequencing of articles through a plurality of stations involving momentary static and dynamic treatment of the articles at the stations during their passage therethrough. The invention has particular, but not exclusive, utility in the detection of explosives and narcotics located within articles such as mail, baggage, goods and other packages using nuclear quadrupole resonance, also known as quadrupole resonance (QR), where articles need to be statically screened for detection purposes, and X-ray or other equipment, where articles need to be dynamically screened for detection purposes.

The invention also relates to, but is not limited to, the augmenting of X-ray equipment with QR technology for screening purposes of articles, such that resulting combined systems integrating both types of screening are more effective and efficient in handling a sequence of articles passing therethrough for such purposes.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Background Art

The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application. There is a substantial threat to worldwide civil aviation and the public in general from bombers, suicide bombers and hijackers transporting explosive devices onboard aircraft undetected in carry-on and hold baggage. Existing carry-on X- ray baggage screening equipment used at airports is effective at preventing guns, knives and some complete improvised explosive devices being transported onboard aircraft. However, these same systems are much less effective at preventing explosives generally, and particularly sheet, distributed and plastic explosives from being smuggled. This weakness is of particular concern in scenarios where a terrorist or attacker can construct a bomb from components after passing through a security checkpoint. A combined screening system integrating Quadrupole Resonance (QR) and X-ray technologies; namely a combined "QR X-ray" system can be deployed to combat these threats more effectively. Such a QR X-Ray system provides good probability of detection for most threat categories of explosives and weapons and excellent detection in the categories where current security checkpoints are most vulnerable. Alternative solutions such as replacing conventional X-ray equipment with more sophisticated X-ray technology, such as computed tomography, to meet this capability shortfall, although possible, would be expensive in terms of both operational and capital costs. Enhancing currently deployed X-ray systems with QR technology addresses the capability shortfall at much lower capital and operating cost.

Similar vulnerabilities exist in screening baggage, mail, goods, packages and other items generally for entry into secure facilities such as government buildings, military installations, courthouses, prisons and other at risk areas, buildings or border crossings. This invention is equally applicable to these applications.

Importantly, the concept of operation of such a QR X-Ray system is consistent with current security checkpoint practice and the training currently delivered to screening personnel. Neither the throughput nor the false alarm rate of current checkpoints would change substantially with the implementation of QR X-Ray systems even though the probability of detection for a broad range of threats would be significantly improved. Combining orthogonal technologies, such as QR and X-ray that have both overlapping and complementary coverage, also makes it more difficult for those attempting to smuggle contraband to develop, plan and employ countermeasures with confidence that they will not be detected. Safety and security is improved when those attempting to smuggle contraband through a security checkpoint are uncertain about the detection method that will be used to screen their baggage and other items and are faced with more than one sensor technology.

There are several problems that need to be addressed and overcome with combining QR and X-ray detection systems into an integrated unit that is commercially viable. Accordingly, the subject invention is concerned with addressing and overcoming some or all of these problems in order to provide for an achievable and commercially viable integrated system combining the aforementioned or similar technologies.

Disclosure of the Invention

In accordance with one aspect of the present invention, there is provided an apparatus for sequencing articles to be screened through a plurality of stations at least one of which requires articles to be static during the screening process, and another of which permits the articles to be dynamic during the screening process, the apparatus comprising:

an infeed conveyor for infeeding articles to a plurality of stations;

a main conveyor for receiving articles from said infeed conveyor and passing articles through the plurality of stations;

a plurality of sensors disposed at different locations along the conveyors with respect to said stations to sense the relative position of discrete articles along the conveyors; and

control means having: - A -

(a) a sensor interface for receiving information from said sensors to ascertain the relative position and size of articles along the conveyors,

(b) conveyor control to operate the conveyors and speed thereof,

(c) station control to operate the stations to perform the screening,

(d) timing means associated with the conveyor control and the station control to synchronize operation therebetween, and

(e) processing means to relate timing associated with screening the articles at the respective stations to relative conveyor operation, speed of movement of the conveyors and rate of speed of movement to ensure a prescribed spacing between the articles and effective screening at the respective stations.

Preferably, the apparatus includes a user interface associated with the stations, said user interface being provided with a master control to permit a user to asynchronously perform a prescribed function related to the screening process at one or more stations; and

said processing means includes a contingency process to respond to operation of said master control;

wherein said master control is interfaced with said control means to allow said processing means to invoke said contingency process, and said contingency process operates a routine to perform the prescribed function specified by said master control;

whereby said contingency process effects remedial conveyor operation by said conveyor control and prioritizes the screening of articles performed at the respective stations, in conjunction with the operation of said routine, to ensure that said prescribed spacing of articles is maintained and that the screening at all stations is properly attended to at the completion of said routine. In accordance with another aspect of the invention, there is provided a method for sequencing articles to be screened through a plurality of stations, at least one of which requires articles to be static during the screening process, and another of which permits the articles to be dynamic during the screening process, the method comprising:

pipelining articles through said stations to maximize throughput; whereby the spacing between articles is maintained to a critical threshold, determined by the effect that screening of an article at one station has on an adjacent article, and the effect that an adjacent article has on an article being screened, so that there is no or negligible interference between the two.

Preferably, the method includes releasing an article for conveying through the stations at staged intervals and independently conveying released articles through the stations relative to the next article to be released.

In accordance with a further aspect of the invention, there is provided an apparatus for screening articles passed therethrough for detecting prescribed substances or objects within the articles, comprising:

conveyor means for sequencing articles therealong in a given direction;

a shield station through which articles are passed using said conveyor means and screened to sense the presence of any shielding objects therein that would mitigate the efficacy of any subsequent or previous QR or X-ray screening;

a QR station through which articles are passed using said conveyor means and screened to sense the presence of any prescribed substances therein using QR detection technology; and

an X-ray station through which articles are passed using said conveyor means and screened to sense the presence of any prescribed substances or objects therein using X-ray detection technology. Preferably, the conveyor means and stations are arranged so that articles initially pass through the shield station, then the QR station, and finally the X-ray station, for detection purposes at each of these stations, respectively.

Alternatively, the conveyor means and stations may be arranged so that articles initially pass through the X-ray station, then the QR station, and finally the shield station, for detection purposes at each of these stations, respectively.

Preferably, said X-ray station comprises a single view dual energy X-ray linescan imager.

Preferably, said conveyor is a single article translating conveyor.

In accordance with another aspect of the invention, there is provided a method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and an X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of:

initially the shield detector, then the QR detector and finally the X-ray detector.

In accordance with a further aspect of the invention, there is provided a method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and an X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of:

initially the X-ray detector, then the QR detector and finally the shield detector.

In accordance with a further aspect of the invention, there is provided a method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and an X-ray detector for detecting the presence of a prescribed substance or object, comprising: passing the article through the detectors in the order of:

initially the X-ray detector, then the shield detector and finally the QR detector.

passing the article through the detectors in the order of:

initially the shield detector, then the X-ray detector and finally the QR detector.

passing the article through the detectors in the order of:

initially the QR detector, then the X-ray detector and finally the shield detector.

passing the article through¹ the detectors in the order of:

initially the QR detector, then the shield detector and finally the X-ray detector.

Brief Description of the Drawings

The drawings accompanying the description are as follows: Figure 1 is a schematic plan view of the system in accordance with the first embodiment;

Figure 2 shows the combined CID and AID of the shield detector according to the first embodiment;

Figure 3 is a schematic diagram showing the arrangement for the CID within a shield as described in the first embodiment;

Figure 4 is a schematic perspective view showing the first arrangement of coils within the CID described in the first embodiment;

Figure 5 is a side-on view of the magnetic field generated by the B coil in the first embodiment;

Figure 6 is a side-on view of the magnetic field generated by a saddle coil in the first embodiment;

Figure 7 is a schematic showing the circuit diagram for the CID in accordance with the first embodiment;

Figure 8 is a flowchart showing the basic method for detecting a shielded volume according to the first embodiment;

Figure 9 is a set of graphs showing typical signals observed on the CID as described in the first embodiment; wherein:

Figure 9A plots the Frequency Shift against the Distance; and Figure 9B plots the Q Shift against the Distance;

Figure 10 is a flowchart showing one method used to process signal obtained from the coils in the CID according to the first embodiment;

Figure 11 shows the AID design in accordance with the first embodiment; Figure 12 shows the magnetic field generated between a transmit and receive pair of the AID design;

Figure 13 shows a 25 kHz image generated by the AID for a bag that has a reinforcing metal loop;

Figure 14 shows a 3.3 kHz image generated by the AID for a bag that has a reinforcing metal loop;

Figure 15 shows the difference between figures 12 and 13;

Figure 16 shows the signal processing flowchart for the vertical image for the AID according to the first embodiment;

Figure 17 is a schematic block diagram of the electrical and microelectronic controller hardware components of the system as shown in Figure 1 ;

Figure 18 is a block diagram of some of the functional aspects of the microelectronic controller as shown in Figure 2;

Figures 19a to 19g are schematic plan views of the system as shown in Figure 1 showing the staged progress of bags placed on and passed through the system;

Figures 20a and 20b are flowcharts showing the power on sequence of the system according to the first embodiment;

Figures 21a and 21 b are flowcharts showing the error response process of the system invoked during the power on sequence shown in Figures 5a and 5b;

Figures 22a to 22e are flowcharts showing the logic of the scanning sequence of the system in accordance with the first embodiment;

Figure 23 is a flowchart showing the infeed conveyor operation of the system in accordance with the first embodiment; Figure 24 is a perspective photograph depicting the exterior of the system of the first embodiment from the infeed end;

Figure 25 is similar to Figure 24, but depicting the exterior of the system from the discharge end;

Figures 26 to 35 are photographs of various images displayed on the user interface monitor depicting different alert conditions as described with respect to the first embodiment;

Figure 36 is a schematic side view showing the projection of an object on to an array of X-ray sensors in the X-ray subsystem in accordance with the first embodiment;

Figure 37 is a similar view to Figure 35, but showing the orientation of the projection in Cartesian co-ordinates;

Figure 38 is a schematic plan view of the system in accordance with the second embodiment;

Figures 39a to 39h are schematic planning views of the system in accordance with the third embodiment showing pipelining of articles through the system; and

Figure 40 is a schematic plan view of the system in accordance with the fifth embodiment.

Best Mode(s) for Carrying Out the Invention

Quadrupole Resonance (QR) is a branch of radio frequency (RF) spectroscopy that provides for chemically specific detection of the energetic materials commonly used in explosives. QR, a derivative of nuclear magnetic resonance, is a volumetric inspection technology which detects crystalline explosives through the interaction of the nuclear quadrupole moment of specific nuclei with the electric field gradient due to the distribution of the electric charge in the surrounding molecule. QR is therefore selective for the specific chemical structure of the material detected. Low intensity RF fields, at specific frequencies, are applied to detect a particular explosive material by perturbing the alignment of specific nuclei with respect to the local electric field gradient. As the RF field is removed the nuclei recover to their original state and a characteristic radio signal is emitted. This radio signal is measured and analyzed using a sensitive receiver. The QR inspection process is conducted while the item under test is stationary within a QR coil which serves as both a transmit and receive antenna for excitation of the item under test and receiving the return signal.

The best mode for carrying out the invention combines QR with single view dual energy linescan X-ray systems widely deployed at security checkpoints worldwide in a conveyor based integrated screening system for the screening of baggage, parcels, goods, mail and other items.

In broad terms the X-ray operator, using the conventional X-ray image, retains the responsibility for inspecting and flagging the presence of weapons and other prohibited items such as flammable materials and narcotics. The operator, also using the X-ray image, similarly inspects for the presence of bulk explosives either in isolation or assembled into an explosive device with control and initiation components. The QR subsystem automatically inspects the item screened for the presence of the energetic materials used in plastic and sheet explosives. The results from both subsystems are displayed on a single computer screen for an overall operator clear or reject decision. The standard X-ray user interfaces (computer screen and control panel) are retained, with minimal modification, to reduce the extent of operator retraining.

In a variation to the best mode, the X-ray image is initially inspected for the presence of bulk explosives, weapons and contraband or any subset thereof by using image processing algorithms (not the subject of this invention) rather than by operator inspection and the automated detection result displayed to the operator for alert resolution along with the QR scan result.

To ensure the screening system is robust to shielding material that may otherwise prevent or hinder QR inspection (which is analogous to high density or high atomic number materials that may prevent of hinder X-ray imaging of an object through a "dark alarm") the best mode incorporates a Shield Detection subsystem to alert the operator to the presence of items that may be intentionally or unintentionally shielding explosives from the QR inspection process. In the preferred embodiment of the best mode described below, the Shield inspection process is conducted as a moving scan while the baggage moves through the system on the conveyor before reaching the QR coil.

The Shield Detection subsystem is an advanced configuration sensitive, target localizing detector that is constructed to detect shielded explosives when present in a sheet configuration only. Various arrangements of shield detection subsystems are described in one of the assignees pending patent applications, namely Australian Patent Application 2005900425 and 2005906277, the contents of which are incorporated herein by reference.

It is not a requirement to detect shielded explosives in bulk configuration as these are effectively screened via the X-ray image with operator interpretation or automatic algorithm analysis. Tuning the Shield Detection system to detect only shielded sheet rather than shielded sheet plus shielded bulk explosives serves to minimize system reject rate while maintaining overall system detection capability. The coordinates of a possible shielded sheet explosive when detected are determined by the Shield Detector with respect to the item under test and the results displayed either as an overlay on the X-ray image indicating the approximate target location of the item under test or communicated via a simple alert light, lamp or icon on the user interface computer screen.

Several embodiments of the best mode will now be described with reference to the drawings. For the purposes of description, any possible shielded sheet explosive will be referred to in the context of describing the Shield Detector as a significant shielded volume (SSV).

The first and preferred embodiment is directed towards an integrated QR X-ray system 11 for scanning and screening articles in the form of bags 13. The configuration of the system 11 is shown in Figure 1 of the drawings and generally consists of a conveyor comprising an infeed conveyor 15 to stage and control the input of articles under test and a main conveyor 17 that translates the articles under test through a series of stations comprising a Shield Detector subsystem 19, QR subsystem 21 and X-ray subsystem 23 in sequence. The drawing also shows the location of a series of article position sensors in the form of optical bag sensors 25a to 25e that serve to sense article movement and allow control of bag movement through the system.

The Shield Detector subsystem 19 includes a combination of two shield detector designs, namely a clustered inductive detector (CID) for operating in the low MHz region and an arrayed inductive detector (AID) for operating in the low kHz region.

The specific detail of the CID and AID designs will be described in more detail later, but for the moment, the combined shield detector of the present embodiment comprises particular components of an AID detector that are added to the components of a CID detector to enhance the operation of the latter. In this manner, the CID design can effectively detect a planar SSV whose normal is vertical by combining it with the AID design. This is particularly useful for bags containing a significant amount of small clutter or conductive bag loops, where the combination of broad uniform detection and localised but non-uniform detection provides very different pieces of information from each detection system.

In the present embodiment, the vertical ferrite rod sensors of the AID design are combined with the A, B and C coils of the CID design to produce a subsystem, which is superior to either system alone.

As shown in Figure 2, the coils of the ferrite vertical rods 350 of the AID design are combined with the A, B and C coils 360 of the CID design, but are disposed in two separate chambers, separated by a metal plate 340 and surrounded by a shield 330.

The decision making process for the combined system of the present embodiment involves a number of different methodologies, which will be described in more detail later. If any of these methods detect an SSV, then an alarm is produced or the operator is alerted under the control of the integrated system, which will also be described later.

The X-ray subsystem 23 comprises a single view dual energy linescan X-ray detector as described above. The design of such a detector is known and will not be described in further detail, except in relation to the components thereof that are relevant to integrating with the QR and Shield Detector subsystems.

Now describing the Shield Detector in more detail, as shown in Figure 2, the CID component and the AID component of the detector effectively operate separately, with the results of each processed and combined to provide a cumulative result. Although bags will proceed through the AID component first, the CID component will be described first with respect to its operation.

As shown in Figure 3, the CID component comprises a multi-axis resonant coil cluster 039 enclosed inside a chamber formed within a metallic conductive shield 040 whose ends are open so as to allow baggage to be transported through the cluster via a conveyor 044, and whose length is around 600mm. An optical sensor 043 is provided to detect the beginning and end of a scanned bag conveyed to and through the coil cluster 039 by the conveyor 044 and the location of the bag in time.

The coil cluster 039 employs a system of coils that define a compact scanning volume within which a bag is temporarily disposed on transport by the conveyor 044 through the cluster. The coils are particularly designed so that the SSV detector is able to detect a shielded volume in a manner where the response of the shielded volume is not strongly dependent on its orientation within the compact scanning volume. This is achieved by using coil designs that have relatively uniform magnetic fields.

The SSV detector also comprises a transmitter/receiver unit 047 and a processing unit 048. The resonance parameters of each coil in the cluster are measured through transmitting a signal and receiving a signal through the transmitter/receiver unit 047 and then analysing the received signal with the processing unit 048. The results are recorded as a function of the distance of the bag conveyed through the coil cluster 039, relative to the coil cluster.

The signal output by the optical sensor 043 is passed through a driver unit 046 and into the processing unit 048. This signal can either be used to trigger a data collection cycle or detect the beginning or end of a transported bag in time, which then can be related to data collected from the coil cluster in time.

As shown in Figure 4, in the present embodiment, the multi-axis coil cluster 039 includes three high Q copper coils A 021 , B 022, C 023, which are tuned to 1.6MHz, 1.7MHz and 1.8MHz, respectively, by adding high Q ceramic chip capacitors 031 , 032, 033. The three main coils (A 021 , B 022, C 023) are orientated to detect objects along three orthogonal directions so that even if the object has a thin profile it will be detected.

The Q's of the resonant coil systems A, B and C are approximately 500, with the inductances of these coils within an external shield ranging from 1 μH to 3μH.

These numbers are only indicative; any values could be chosen depending upon the application, size of the coils required etc. Although in the present embodiment, the coils are single turn; in another embodiment (not shown) they are multi-turn to increase inductance. The multi-turn configuration, however, is viewed as less desirable as the complexity in building them would increase and the ultimate unloaded Q is lowered.

In the present embodiment, the resonant circuit is constructed so as to be parallel resonant with the applied capacitance. Consequently, the resonant circuit closely matches the properties of an NQR system with which it is associated, and thus utilises similar components and analysis software systems to that provided with the NQR system.

In an alternative embodiment, the resonant circuit is constructed so as to be series resonant with the applied capacitance; however, in the alternative embodiment, the resonant circuit is not able to closely match the properties of an associated NQR system, and so would not use similar components. Nonetheless, such an embodiment may have utility as an independent SSV detector, where it is not important for the resonant circuit of the SSV detector to be closely matched with the properties of an NQR detector.

In the present embodiment, the coils A, B and C are arranged symmetrically and orthogonally to each other. In this manner, the coils minimally interfere with each other by reducing the currents that are induced on each coil. The result of this arrangement is that all coils are basically decoupled from each other. By virtue of this decoupling, the resonant frequencies of the coils could be chosen to be the same frequency, if desired, although in the present embodiment they are chosen to be marginally different from each other, as described above.

As a result of this configuration of the coils, the magnetic field directions from coils A, B, and C in the detection volume mainly point vertically, along the conveyor belt 044 and across the conveyor belt, respectively. This system allows essentially three orthogonal measurements of the baggage. The coil layout includes shim coils (not shown) to change the field homogeneity and additional search coils (not shown) to this basic cluster.

The coils are adjustable by bolting straight electrical grade copper bar segments together (not shown). This is useful to adjust the coil dimensions to find an acceptable layout, and allow a design with high conduction in the coil structure. The bars have a series of holes drilled along their length so that the attaching bar can be moved along its length. Several bolts are used to ensure electrical contact and preserve the electrical Q of coil. In other embodiments, the bar is replaced by an alternatively shaped conductor such as pipe, rod or wire, which may be considered desirable depending on the required engineering.

In the present embodiment, coil A 021 is a two loop saddle-like coil that detects planar objects that lie flat within the detection volume. This single turn design enables a high Q to be achieved that is extremely sensitive to metal targets. It is shaped to create a magnetic field that is relatively uniform throughout the detection volume. In designing the coil, care is taken so that the saddle-like form does not create a field that couples in the vertical direction along the width of the luggage tube. This is also useful to avoid coupling to the metal reinforcing loops of a bag, which generally will lie in the same plane as and close to the conveyor belt, when a bag is conveyed through the coil cluster 039 of the SSV detector.

The saddle-like coil shape is most suitable for this purpose; however, in other embodiments other coil shapes are adopted, which also are adequate. One of these is specifically described in the second embodiment.

In the present embodiment, the B coil is a narrow single turn coil 022. A side view of the magnetic field 050 generated by this coil 051 is shown in Figure 5. This coil primarily senses objects that present their greatest surface area in the direction of the conveyor belt motion 034, although the signature of bags with metal reinforcing loops also makes the signal received from this coil extremely useful for detecting objects in these reinforced loop type bags.

Coil C is also a single turn, saddle-type coil 023 constructed by connecting two loops. The magnetic field 063 generated by this coil 023 is shown in Figure 6.

This coil detects objects that face across the conveyor belt. The coil structure consists of a continuous metal structure 023 and two disconnected electrically continuous rectangular loops 025, known as opposing loops. These opposing loops can be regarded as field shimming coils. The shape of the coils and the opposing loops are designed to create a field that is relatively uniform in the direction of the probe formed by the coil cluster and associated circuitry.

The opposing loops reduce the field from coil C in the area of the sides of the luggage tube, as well as helping to shape the fields from coil A and B so they are more parallel to the sides of the luggage tube.

In an alternative embodiment, coil C is constructed from simple rectangular loops, where the loops are wired in parallel. However, this is less desirable because there is some loss in field uniformity. As described above, Figure 3 shows the connection from the coil cluster probe to the electronics, which monitors the probe and receives bag position information. The components of the system used to transmit and receive signals from the coils form a modulator/demodulator circuit as shown in Figure 7, whereby the same lines that are used to transmit to each of the channels of the A, B and C coils, are used to receive signals from the A, B and C coils.

A modulator circuit is used to generate a signal on any of the coils by using a Direct Digital Synthesiser (DDS) 052 to generate sinusoidal waves at the required frequencies on a single transmit line. Accordingly, an N-channel transmit demultiplexer 053 is used to split the single transmit channel into N channels that are sequentially selected, so that a sinusoidal pulse of about 500μs can be applied to each resonant coil in turn. Element 054 is an isolating component to ensure the demultiplexer 053 doesn't significantly load the resonant circuit and cause a deterioration in Q.

A demodulator circuit is used on the receive side to receive signals from the coils. The receive signals are initially amplified by amplifiers 060 and fed into an N- channel receive multiplexer 059 that is used to multiplex the N receive channels into a single receive line connected to a receiver module 061. The multiplexed signals are mixed down to 30 kHz in the receiver module 061 and further amplified before being sent into a single channel ADC card 051 , where the signal is sampled at 360 kHz. The sampled data is then sent to the computer 050 for signal processing and is graphically displayed via the display 049.

The method of operation of the SSV detector will now be described with reference to Figures 3, 7 and 8.

In the present embodiment, the CID form of the SSV shield detector operates in the low MHz frequency range. This range is reasonably close to the QR frequencies of interest in an NQR detector system for detecting particular types of explosive. The reaction from the SSV detector at these frequencies mirrors the shielding ability of a shielding material during an NQR excitation. The shielding effect is dependent on the conductivity, permeability and/or geometry of the shielding material. The effectiveness of the shield therefore changes with applied probe frequency, because the conductivity is dependent upon the frequency.

The choice of frequency region can be refined to include other benefits. An example of a benefit might be to operate at frequencies where low RF interference occurs from external sources. In this regime the resonant coils can potentially be employed using limited shielding 040 from external RF sources or in alternative embodiments no shielding at all.

As provided for in the present embodiment, a small offset from the QR frequencies of interest is desirable so that the process can be carried out at the same time as any sensitive QR scan process. For people skilled in the art this frequency range has an advantage in that large coils needed for the volume to be scanned can be easily constructed to have high-Q factors. In general this property of high-Q allows very small changes on the electrical properties of the coil antenna to be identified quickly.

In the best mode of the invention using the present embodiment, the coil cluster 039, the external shield 040, and the electronic chains of the transmitter/receiver unit 047 are constructed in such a way so as to operate with the high-Q resonant probes. For instance, the electronics required for the transmit and receive modes are lightly electrically coupled to the resonant tank circuit so as not to load the coils 039, and the external shield 040 is a highly conductive material that allows sufficient space to maintain high inductance and a low reluctance return flux path.

The SSV detector essentially measures the response of a small group of predominantly orthogonal coils 039 as a function of bag travel distance so that recorded features related to shielding objects distributed throughout the baggage can be matched to their location in the bag. An analysis is performed on the modification of the measured electrical properties from this group of coils to discern significant shielded volumes (SSV) relative to their location in the bag. The analysis of recorded data discriminates the SSV from other less significant shielded areas. Figure 8 describes the process, where the bag travels into the coil cluster 039 and the responses are collected from the multi-axis system. The inductive and resistive characteristics change for each resonant coil as bags of varying magnetic and electrical character pass through it. Potentially the luggage within a bag is composed of objects that can be divided into two types: clutter and SSVs. Here "clutter" describes shielding items that don't appear to be SSVs to the applied NQR field. As the bag moves through the coil cluster 039, the change in electrical characteristics for each coil as a function of position or "responses" of bag travel is recorded 063. The responses from clutter are modelled based on measured parameters 065, being produced from computational models for the clutter based on the measured character of the bag. These clutter responses, drawn from the best models of clutter, are compared and removed by subtraction from the recorded responses 067. On subsequent processing 069 if a significant response still exists an alarm is generated 071, otherwise not 073 and the bag is clear to pass on to NQR or other scanning.

To scan a bag for the presence of an SSV therein, a bag is brought into a scan area via the conveyor belt 044. In the process of proceeding into the scan area the bag breaks an optical fence sensed by the optical sensor 043, which triggers the measurement process.

One aspect of the measurement process involves measurement of the length of the bag. The bag length is determined by measuring the difference in time between when the optical fence is broken and not broken and knowing the average velocity of the bag. Algorithms are applied to account for possible issues such as dangling straps causing the beam to broken multiple times on the same bag.

This length information is useful for the later signal processing of bags with metal reinforcing loops. The correlation between the signals from the optical sensor and the Q and frequency shifts allows magnetic features of the bag and its contents to be located. This aids in processing, as mentioned previously, for metallic baggage structures generally have fixed locations near the edges of the bag. The overlay of the magnetically identified suspect locations with the real dimensions of the bag enables the bag to be more quickly searched by other means which are able to reference these dimensions.

By knowing the position of the bag where the bag is not influencing the coils significantly, the baseline can be identified. This data region is then used as a baseline so that the processing is able to effectively recalibrate to an empty coil cluster for each bag. The recalibration largely eliminates the effects of drifts in absolute resonant values of the unloaded resonant system caused through, for instance, temperature.

Apart from this one length dimension, it is also desirable to measure the other dimensions of the bag so that the maximum width and height is known for the bag under measurement. These dimensions are used to provide limits to the processing algorithms so as to more accurately define a clutter bag model, in particular the outer dimensions of any contained structural conductive loops.

These dimensions are measured using an optical fence where light beams are broken to indicate the desired dimension. Alternatively, and more desirably, a camera is used and the resulting image is processed to find its physical outline dimensions.

After tripping the optical sensor 043, a signal is then generated on a single transmit line to the coils A, B, C by the DDS 052 in the form of sinusoidal waves at the required frequencies. The N-channel multiplexer 053 splits this single transmit channel into N channels that are sequentially selected, so that the sinusoidal pulse of about 500μs is applied to each resonant coil in turn.

A cyclical coil scan process is then applied, where each coil is in turn excited over a stepped narrow range of frequencies and the receive signal is received by the receiver unit 047 and recorded by the process unit 048. Typically there are a low number of frequency steps (NF) for each coil, which produce NF intensity values. A typical value for NF is 10 steps.

The process is then repeated for the frequency range of the next coil and so on. This whole process is then repeated to monitor the bags travel through and beyond the coil cluster 039. The frequency sweeps of NF steps are designed to cover a range just before the resonant frequency of the coil, through the resonant frequency and a short frequency range just after it.

In the method used in the present embodiment, the frequency is swept for coils A, B, and C through a range that is 10-30 kHz wide, the range being chosen to suit the responses of the coils. The choice in range depends on the shift in frequency expected from each coil as the baggage passes through. This range is ideally optimised to the population and the expected variation from each coil so as to provide efficient scanning.

For an ideal resonant system the received signal intensity after its transient behaviour follows the text-book Lorentzian shape as the frequency is stepped with fixed increments through the resonance frequency. The peak of the Lorentzian corresponds to the resonant frequency and the width allows the Q to be calculated.

The variation in amplitude of the received signal is thus recorded and related to the Q after processing according to the Lorentzian shape.

The pulses that contain successively increasing frequency sinusoids are an efficient method of excitation, in that three coils are able to be scanned every 2cm of baggage movement at a conveyor belt speed of 0.5m/sec. This method allows a reproducible amount of energy delivered to the resonant system that finally generates a signal well above possible noise sources from within the luggage or from external RF noise.

Alternative methods of excitation may be provided. Indeed different methods are described in the fourth and fifth embodiments. However, each method enables the electrical parameters of the coil cluster to be recorded and analysed to calculate the resonant frequency and Q of the varyingly loaded resonant system as a function of bag position to create responses for each coil. Once the resonant frequency and Q is known, the presence of an SSV having an area that is able to intercept the field perpendicular to the direction of a particular coil is identified by virtue of it causing a shift in the resonant frequency and generally a noticeable change in Q of the loaded resonant coil.

if coils A, B and C detect a frequency shift and/or effective Q shift above a prescribed threshold, then an alarm is generated for the coil or coils, otherwise the bag is passed as clear.

The most useful response is the frequency shift. This response is strongly correlated to the dimensions of the SSV object. The Q response is also useful in that objects that cause its deterioration could act as an SSV. Characteristics of the Q of the shielded item allow some aspect of its conductive character to be determined eg. a thin conductive sheet will have a lower Q than a thicker sheet for instance.

In the present embodiment, the two responses, resonant frequency and Q, are correlated to produced a third measure, which is useful to further refine identification. The function that combines them is their multiplication at each measurement distance to generate a new response with distance. The thresholds are a constant with position in one mode, and are shaped as a function of position in another mode to account for the difference in coupling geometry between the NQR coil and the shield probe coils to the bag.

Before comparison to this threshold the responses are filtered or modified mathematically to account for the variations in response caused by the object being either displaced vertically or across the conveyor belt relative to the measuring coil structure.

Even though most coils are designed for uniform response, there still exists a variation in sensitivity, which generally means a small object close to the coils will generate a greater response over smaller travel compared to an object in the centre which would tend to generate a smaller response over larger travel. This variation is mathematically corrected so that the response curves are less dependent on target position.

In the present embodiment, this is achieved by processing a piecewise weighted summation around each point in the response. Alternatively, in another embodiment, this is achieved by using a filter function with the coefficients chosen to best reduce this variation.

A further method employed by the present embodiment to improve the quality of the data for later processing is the application of deconvolution methods to each response. This partially removes the shaping introduced by the finite position resolution of coil. Applying a controlled deconvolution method aids in extracting distributed small clutter throughout the bag to reveal larger shielded volumes. A reliable deconvolution technique adopted in the present embodiment is the Van Cittert method.

For bags with a metal reinforcing loop, the response is particularly strong, given that the loop can generate large diameter circulating eddy-currents in response to the applied RF field. With placement of the plane of the loop in the plane of the conveyor belt, coil A is the most affected, resulting in a large response that is peaked with the bag at the centre of the coil. This is the typical orientation for trolley bags with loops because they are conveniently loaded in this direction, and the preferred direction for a transmission X-ray as this is the thinnest cross- section.

The loop itself can often be considered a cluttering object as it generally does not act as an effective shielded volume with an exciting QR field whose direction lies in the plane of the loop. Given the relative difference in magnitude of response from the baggage loop and small shielded packages, it is difficult to use the A coil alone to find a shielded package.

Coil B, because of its field, generates a characteristic "M" type shape response as it alternately couples, decouples and couples again with the loop as it travels with the conveyor belt. This "M" shape originates from the orientation of the magnetic field around the B coil 022, as shown in Figure 5. Within the narrow volume circumscribed by the B field, the field lines 074 are parallel with the conveyor belt 044. However just outside the volume of the B coil 022, the field lines 074 bend and in fact form a circular shape around the copper strip. Hence when a bag first encounters this field, a frequency shift is observed because the field lines are traversing down and a current is induced on the metal loop 02 of the bag. This induced current causes a strong resonant parameter shift on coil B.

When the bag is symmetrically placed with respect to this coil, there is a minimum of current induced on the metal loop 02, as equal numbers of field lines 074 will move into the loop on one side of the coil 022, as they will be leaving the loop on the other side of the coil.

When finally the bag loop 02 exits the coil again, there is a current induced on the bag loop as the field lines cut through the bag loop from underneath.

Because the bag loop 02 would be expected to produce a symmetrical response, this can be exploited to determine if a metal object is present. If a large metal object is present, this expected shape is altered and thus the metal object can be detected on this basis.

A useful method employed in the present embodiment to determine whether the expected shape of the magnetic field has been altered involves selecting two points either side of the "M" shape, and measuring the magnetic field at these points. The least change of these two is then chosen as the baseline.

If the response observed lies outside a prescribed threshold, then an alarm is generated, otherwise the bag is cleared from this stage.

If this resultant minimum is sufficiently large when the bag is symmetrically placed with respect to the B coil, and the responses from the other coils match that of a looped bag, in particular the A coil, the software of the processing unit 048 is designed to calculate the expected response for a looped bag at the recorded bag positions for each of the coils. The shape of the expected response curves is based on archived results of bag loops measured over a wide range of potential parameters, such as loop dimensions, loop height, electrical conduction of loop etc. These archived results are stored in memory of the processing unit 048, and accessed for comparison purposes using appropriate modelling software.

These archived results may be obtained in a number of ways. In one embodiment they are constructed by solving Maxwell's equations for the specific geometry of the coil and metal bag structures as a function of bag travel, loop dimensions and height.

In the present embodiment, however, these results are estimated mathematically from empirical measurements to first determine the amount of magnetic flux penetrating the bag loop, then the amount of flux disturbed by the metal bag structure, and finally considering this as proportional to the expected response.

To determine the amount of magnetic flux penetrating the bag loop, each coil is treated as a segmented line of conductors of current to give the total field at a point. The amount of disturbed flux is found through integration over the area of the bag loop.

A function based on this flux approach is developed for each coil. The equations for the function are refined through comparison of collected data from loops formed on a test jig of varying dimensions and height and actual baggage structures measured with the shield detector. The measured data contains features due to deflections from the outer shield 040, the physical dimensions of conductors involved in constructing the coils 039, additional baggage frame structure such as handles etc. The equations are corrected to produce response shapes and overall magnitudes that are realistic for typical looped bags.

A well known technical computing language for analysing scientific signals is Matlab™ produced by Mathworks in MA, USA. Matlab™ computer software code for the response from coil B for circulating eddy currents in just a horizontal loop is shown as an example below: Shift due to loop - approximate vertical field

b_dist = dist - bag_length/2; a_dist = dist + bag_length/2; scale_freq = loop_width^*scale_mag;

Calculate flux through loop from bottom of coil

Ioop_freq1 = abs(log(1./(loop_height^Λ2 + b_dist.^*b_dist)) - log(1./(loop_height^Λ2 + a_dist.*a_dist)));

Calculate flux through loop from top of coil

Ioop_freq2 = abs(log(1./((coil_height-loop_height)^Λ2 + b_dist.*b_dist)) - log(1./((coil_height-loop_height)^Λ2 + a_dist.*a_dist)));

Find approximate loop frequency shift based on scaled total flux through loop that has been corrected for influence of external shield

loop_freq = scale_freq*(loop_jteq1 - Ioop_freq2). *exp((dist.*dist)/ (2^*(coil_range)^Λ2));

where:

'dist' is an array of bag locations relative to the centre of the coil. 'coil_range' is the range of the coils fringe field, modified by shield. 'loop_heighf is the height of the loop relative to the bottom of the coil. 'coil_heighf is the ditsnce between the top and bottom of coil B. 'scale_freq' is a value used to scale the flux result to a frequency shift.

It can be seen from the above code that the bag loop signature "M" shape depends upon bag loop length, width and height. As the bag loop length is approximately known from the optical sensor measurements, with typically the loops traversing the entire length of the bag, then the other two parameters can be varied to determine the expected bag loop signature. Once the bag loop signature is determined, this signal is subtracted from the received signal, and if the remaining signal lies above a threshold, then an alarm is generated.

Figure 9 shows graphically the types of signals obtained from four coils (A, B, C and D) for a loop bag. As can be seen, the signal obtained from coil B shows the characteristic "M" shape produced by a looped bag. Coil D is a supplementary coil that will be explained in another embodiment described below.

The processing performed by the processing unit 048 uses information from the coils to predict the responses from clutter, and in particular, the structure of the metallic reinforcing loop 02 of the bag.

The change in response for loop dimensions and height control largely the magnitude and shapes of the responses for each coil in very different ways. As noted earlier, the response from each resonant coil is not entirely uniform with displacement along the sampling field direction.

These features allow a system of equations to be constructed that approximately describes the response as a function of the parameters: loop height and bag width, so that a mathematical fitting process can find a satisfactory loop bag model. This fitting process is bounded by the physical limit imposed by bag length, and employs observed real-world correlations in the dimensions of bag lengths and bag widths to help define the unknown bag width.

The processing then constructs a list of appropriate models from the measured data. The responses are then calculated for these models and adjusted based on loop height and width within ranges to fit to collected responses.

The fitting process is forced towards minimums of selected response regions with position so that the derived model does not over-emphasise clutter. This avoids an overestimate of the responses from just the bag structure alone so that any shielded volume will be more likely to trigger an alarm. In general the information from the responses of coils A, B and C is enough to find acceptable estimates of bag loop width, height and, to some extent, other structures such as metallic handles.

In the present embodiment, the bag width and height, as well as length, are measured. In addition, potential positioning errors are accounted for, where the centre of the loop is found to be displaced or the loop is of slightly different size than that calculated from the optical sensor information. Accordingly, the centre of the models are displaced and scaled in a small range of values to attain a good model.

A laptop computer or other relatively continuous metal target may trigger processing related to a looped bag based on the threshold comparison above, but its response shape will lead to a detection. For example, coil B won't create an M shaped response, producing an increasing frequency response and generally decreasing Q response as the object is about centred relative to B. This difference in response shape from the model responses of the coils for the looped bag is used to discern a significant shielded volume. These differences are examined and if determined to be sufficient then an alarm is generated.

Once the best model for clutter has been determined, the associated responses are derived and a compare/subtraction process is performed. Each coil response is compared to its own set of characteristic thresholds to decide if the difference is sufficient to trigger an alarm.

The threshold is uniform with position in the one mode, or it is shaped so as to pick out selected areas of the bag in the second mode. These individual coil alarms are mathematically combined to give an overall assessment on the likelihood that the bag contains an SSV.

The specific process flow adopted by the processing unit 048 using the various techniques described above is shown in figure 10. Firstly, responses are created by finding the resonance parameters comprising frequencies, amplitudes and Q's, and processing those 100 as a function of distance. The responses are then offset 102 relative to recorded parameters without luggage. They are then filtered for noise (if any) and adjusted 104 depending upon the sensed proximity of the bag to the coils.

A comparison is then performed to ascertain 107 whether the responses cross the thresholds. If so, an alarm value and a shield position value are calculated and inserted 109 into an alarm array.

The process then proceeds to the next stage where the minimum magnitude of responses from selected areas of the bag associated with selected coils is found 111.

A further comparison is performed 113 to ascertain whether the offset responses cross their thresholds, and if so, an alarm value and a shield position value are calculated and inserted 115 into the alarm array.

The responses are then checked 117 to see if they are consistent with a looped bag of the measured length. If not, the total alarm value from the alarm array is determined 119 and compared 121 to see if it is above a threshold, whereby if it is, it is concluded that a shield has been found and the position from the weighted alarm positions is calculated 123, whereas if not, the bag is cleared 125.

If the checked responses 117 are consistent with a looped bag of the measured length, a list of looped bag models is constructed 127, based on the measured bag length. A list of model responses is then constructed 129, and the model responses that best fit selected bag response characteristics are selected 131.

The best model responses are then subtracted 133 from the bag responses to obtain difference responses, and the difference responses are then compared 135 to determine whether they cross their thresholds. If so, an alarm value and a shield position value are calculated and inserted 137 into the alarm array. The total alarm value is then determined 119 from the alarm array, and compared 121 , to determine whether it is above a threshold. As before, if it is, a shield is concluded to have been found, and its position is calculated 123 from the weighted alarm positions. If it is not above the threshold, then the bag is cleared 125.

In general the model responses which may be effectively zero are subtracted from the responses and the response weighted distance is calculated. For simple objects this gives the position of the object to within a few centimetres. As the clutter and hence model becomes more complex, the error between the calculated position becomes greater to the most significant shielded target.

In a very high percentage of cases, as shown in real-world testing, the SSV is still found to be within 10 cm, allowing the bag to be usefully segmented to reduce search time and/or enhance detection which may involve other techniques.

Other processes may be employed that are ancillary or alternative to that described with respect to Figure 10, and are described in subsequent embodiments.

In the present embodiment a graphical response with position (not shown), before and after model subtraction, is displayed to an operator or overlayed with some other image such as optical or X-ray generated image to help in identifying areas of interest.

As shown in Figure 11 , the AID component comprises a probe consisting of 20 ferrite rods in the vertical direction, each with a coil wrapped around to produce 20 discrete coil assemblies, which are spaced around a rectangular frame 165. The frame 165 is made of some suitably non-metallic substance such as wood or plastic.

The ferrite rods are Amidon material type-33 with a length of 195 mm long and a width of 12mm. The Amidon type-33 material is particularly suited to operating in the low kHz region. The advantage of operating in the radiofrequency region is the magnetic field produced by the coil assemblies is able to penetrate the metallic reinforcing loop of a bag to some extent, unlike the situation with the higher frequencies of the CID, resulting in shielded objects being able to be detected within bag loops. The magnetic field is able to partially penetrate metal reinforcing loops because it is measured over a more confined volume relative to the size of the conductive structure, i.e. the field will induce eddy currents in the structure but because the structure is large, smaller eddy currents will exist to counter the input flux from the sensor.

There are ten transmitting coils 157 and ten receiving coils 159 in the vertical direction. All coils are equally spaced apart by approximately 60 mm. The vertical transmit coils are connected in parallel in a circuit.

The AID frame 165 is mounted inside the anterior chamber within the metal conductive shield with the conveyor belt passing through it as shown in Figure 2. In such an arrangement, before a measurement is begun using the AID, typically a bag is moved along the conveyor through the optical fence provided by the optical sensor 25a. The breaking of the optical fence triggers the measurement process described below, after a short time delay to allow the bag sufficient time to be located within the AID.

To generate a transmit burst on the vertical transmit coils, a digital sine wave is synthesised in software at two different frequencies 3.3 kHz and 25 kHz. The purpose of these two frequencies shall be discussed later. These two signals are added together digitally and then transmitted through two output channels of a 10 channel simultaneously updating 100 kHz ADC/DAC card 164. From here the signals are sent to an input channels of a high power audio amplifier 163 and from there to the vertical transmit circuit, which, as previously stated, is connected in parallel. The transmit bursts for the vertical coils take 5.12 ms to occur. Although the signals can be transmitted to all of the coils simultaneously, in the present embodiment they are transmitted in bursts to the vertical coils, in an interleaved manner, 25-40 ms apart. If the transmit bursts were all transmitted at the same time, then the signal seen on some coils of ferrite rod receivers would be overloaded by the magnetic field from the adjacent transmit rods. The magnetic field created by the coils of the transmit rods is received by the individual coils on the ferrite receiver rods and input into 10 pre-amplifiers 155, which in turn amplify the received signals into the volts range. These signals are input into the 10 ADC channels of the ADC/DAC card 164, which in turn is connected to a computer 156. The ADC card thus simultaneously samples all ten channels at once.

The simultaneous sampling of the channels is superior to multiplexing the channels because it is fast and enables the user to 'slice' through the bag simultaneously, much like an X-ray scanner. If the system were multiplexed then the pixels generated would be staggered in time (and hence in distance along the bag) and thus more difficult to interpret and process. Simultaneous sampling is also less problematic since issues such as finding sufficient time to multiplex so many transmit and receive signals to and from the coils on the rods makes the resolution more dependent on the conveyor belt speed. In the present embodiment, the belt speed is 0.5m/s and so a 60cm bag passes through the scanner in only 1.2 seconds.

The previously described step is repeated approximately 30 times as the bag moves through on a conveyor belt, with a 50-80 ms gap between the slices. This process results in slices which are spaced 3-4 cm apart and in the process an image of the metal contained within the luggage is built up for the operator to 'see' any metal objects within the bag.

As the system only has vertical rods, one image is formed of the bag. At each of the thirty slices, 512 data points are recorded, which corresponds to a sampling time of 5.12 ms if the sampling is performed at 100 kHz. To form the image these 512 data points are baseline corrected by subtracting off the thirtieth time slice. Then each of the other 29 slices for each receiver channel are DC detrended, windowed and Fast Fourier Transformed (FFT) into frequency space. In performing the FFT, the absolute value of Fourier co-efficients (i.e. peak heights) at the two frequencies of interest are calculated and input into two arrays, which form the two metal images at 3.3 kHz and 25 kHz. To aid the following description these will be called Amp25khz and Amp3khz. In separate images the two phases of the baseline-corrected signal are stored. These are called Phase25khz and Phase 3khz. There are also four corresponding amplitude and phase images of the raw received signal (not corrected for the 30^th time slice). These images are called Raw25khz, Raw3khz, RawPhase25khz, RawPhase3khz. The images containing the raw received signal phases are then background corrected after they have been transformed into a phase by subtracting off the phases calculated in the same process for the thirtieth time slice. In another series of two images, instead of taking the absolute value of the Fourier co-efficients, the raw complex co-efficients at the two frequencies of interest are input into two more images. These are called the CplxReal25khz, Cplxlmag25khz, CplxReal3khz and Cp!xlmag3khz.

While it may appear the system can only detect objects in one direction, the system can detect objects in other dimensions. This is because the transmitted field from the coils on the vertical rods expands out in three dimensions, which results in the detection of objects that lie in the vertical plane parallel to the vertical rods. Consequently, the system is capable of detecting any shielded object in any orientation.

The field homogeneity should ideally be entirely uniform across the gap between the rods. However, in order to achieve this in practice is very difficult, given that the magnetic field decreases as 1/r³ from most coil designs. The AID design does, however, have a reasonably uniform field, which is helped by the fact that the coils on both the transmitter and receiver rods tends to concentrate field in their vicinity and thus counteract the drop off in field intensity from the coils of the transmit rods. The shape of the magnetic field 167 generated between any two coils of transmit receive ferrite rods 158 and 160 is shown in Figure 12.

The system operates at low power levels not harming passenger's luggage and routinely at a 0.5 m/s conveyor belt speed The system is also relatively inexpensive, and emissions from the device are limited by adding an appropriate metal shield around the device, while leaving an opening for the luggage to pass into and out of the device. As the device is extremely thin (only -40-70 mm wide), the additional shield used is quite small and the overall dimension of the device adds only a small amount of length to the Shield detector.

As previously discussed, bag loops 02 are contained in most trolley type bags. As these are continuous around the bag structure, any magnetic field 03 that impinges upon the bag from above will induce an eddy current signal 04 in the bag loop. This signal counteracts the impinging field and tends to cancel it. This makes it very difficult to detect metal objects contained within metal loop bags.

This is further compounded with the latest trend of manufacturers to include three or more metal loops in bags and steel bars along the bottom of the bag for the extendable trolley handle, although generally this metal section does not form a closed loop.

if this type of bag were to pass underneath an ordinary metal detector then every looped bag would cause an alarm, which does not necessarily accurately represent the shielding ability of the bag.

To overcome this problem in the present system, two images of the bag are formed at two different frequencies. One image is formed at a relatively high frequency (25 khz) and the other image is formed at a relatively low frequency (3.3 khz). Both of these images contain the signal from a bag loop that, due to the bag loop's size, is large in both amplitude and area. Smaller objects that are in the bag also appear in these images, however the targets almost always induce a signal, which has a slightly different phase to the bag loop signal. Hence superimposed upon the bag loop signal in both images is a smaller signal from a smaller target. The small target can either slightly increase the height of the bag loop signal where it occurs in the image, or slightly decrease the height of the bag loop signal. Invariably most targets increase the height of the bag loop signal. In these two images if the bag loop signal is subtracted out of the images then the smaller target within will be revealed.

Accordingly, this technique is used in the present embodiment to cancel the signal of the bag loop to reveal metal targets contained within. To subtract out the bag loop signal a symmetrical three dimensional Gaussian like surface using a simplex search is fitted to lie just under the peak shape of each of the two images. This fitted peak is then presumed to represent the bag loop signal and is subtracted out of each image revealing any targets superimposed upon the bag loop signal.

Figures 13, 14 and 15 show the subtraction process graphically. Figure 13 is the image generated from a bag with a reinforcing loop at 25 kHz and Figure 14 is the corresponding image at 3.3 kHz. A 3D surface is fitted to each image (not shown) and then subtracted out to reveal the smaller target as shown in Figure 15, which shows the metal object revealed in the top right hand corner of the bag, whereas in either of the original images it was unable to be seen.

Some objects, such as steel targets, rather than adding to the bag loop signal, actually lower the peak locally. To overcome this problem, a pre-screening step is performed to identify if there is a "dip" in the peak. Once identified the dip is inverted into a "bulge" upon the bag loop signal and then the subtraction proceeds as per normal.

Bag loops can be distinguished from other large metal objects by examining several different parameters. These parameters can include:

(i) Size: If the number of points in the 25 kHz amplitude image above a threshold is larger than a minimal area threshold then the signal is deemed to be large enough to be possibly a bag loop.

(ii) 25 khz Peak Height: If the maximum of the Amp25khz is above another threshold then the bag is flagged as possibly containing a bag loop.

(iii) RawPhase25khz: At the corresponding point at which the 25 khz image reaches a maximum, the nine closest points in the RawPhase25khz image are averaged. This average is then compared to upper and lower bounds.

If it lies within these bounds then it is deemed to be possibly representative of a bag loop. (iv) Meanar: If the average of the image, formed when the Amp25khz image is divided by the Amp3kHz image, lies above another threshold, then the bag possibly contains a bag loop

If the received signal passes these four criteria then it is flagged as being a looped bag. Many other parameters or combinations of parameters derived from the initial signal processing can be used to distinguish a bag loop from a plain metallic object.

Presently in the luggage market, bag loops tend to fall into three different category types, which for the sake of simplicity will be referred to hereinafter as: Type I₁ Il & III. The first two types dominate the marketplace and Type III has very similar, but not the same, characteristics as Type II. Through experimentation it has been found that the three different types arise from the different arrangement of metal loops in bags. For instance Type I bags have typically a solid steel band around the centre of the bag, steel tubes for a trolley handle, and wire loops top and bottom of the bag for extra reinforcement. Type III on the other hand can have only wire loops top and bottom of the bag.

Bag loop types are detected by examining the magnitude of the Meanar parameter. If this is small in value then the bag loop type is a Type I, if it is medium sized then the bag loop type is Type II, and Type III has the largest values.

The computer 156 performs the aforementioned processes, as well as additional processes according to the specific flowchart shown in Figure 16.

Initially, in order to determine if a shield is present in the vertical image, the 25 kHz bag image signals derived from the sensors are processed 171 to determine if any peaks or maxima lie above a baseline threshold.

If all peaks or maxima lie under the baseline threshold, then the bag is passed as clear 173. If any of the peaks or maxima do exceed the baseline threshold, then the same peaks or maxima signals are checked 175 to see if they lie above a second higher bag loop threshold, which is what would be expected for a bag with a reinforcing loop or laptop. If any of the peaks lie below this second threshold, then it is concluded that it is not a bag loop or laptop, but is probably a metal shield and so an alarm is generated 177. If the peak lies above the bag reinforcing loop/laptop threshold, then the signal area 25 kHz peak height and the RawPhase25khz are used 179 to determine if the signal is a laptop or a bag loop using the aforementioned bag loop detection process. If the signal is not detected as a bag loop then the object is deemed to be a large metal object and an alarm is generated 181 , accordingly. If the object is found to be a bag loop then the aforementioned subtraction process 183 is used to cancel the bag loop signal and reveal any underlying objects. Another comparison is then performed 185 to determine if the residual signals lie above a third prescribed threshold representative of the high probability of a significant metal object being present. If any residual signals lie above this third threshold, a significant metal object is deemed to be contained within the bag and an alarm is signalled 187, otherwise the bag is passed as clear 189.

As a back up to the foregoing procedure, the operator can often 'see' which bags contain reinforcing loops and which contain laptops and therefore the operator can make a judgment call if he/she feels the computer based decision making process has failed.

The flowchart of the specific process followed by the computer 156 is shown in Figure 17.

Vertical targets, which lie in the same plane as the vertical rods, are detected by the pattern they produce in the vertical image. Upon entering the detection area they produce a signal on the vertical image with negative phase, similar to the bag loops and laptops above. Accordingly, the present embodiment is able to detect objects in all three directions by correlating the shape observed in the vertical and horizontal images in space and time. The use of phase and amplitude information is also used to determine the type of metal detected. For instance, it is well known in the field of metal detection that ferrous metal objects produce a negative phase in metal detectors, aluminium foil produces a slightly positive frequency and other metals such as gold, silver and copper produce larger positive phased signals.

When processing and combining the results of the CID and AID components, the processing occurs at a high level where the results from each system are combined using Boolean logic or a weighted system. In addition, the data is combined at the response level from each system to more correctly describe the bag for a combined result.

It is possible to pass information between the AID and CID designs, to enhance the detection process. Accordingly, the processing unit of the computer is adapted to transfer this information and process it to enhance the detection results that can be achieved by either detector design alone.

This is accommodated two ways. Firstly, because the field profiles are slightly different, the detection of an object in one image allows its precise size and location to be determined. For instance, an object, which lies close- to the coils of the AID design, produces a large signal, but this decreases as its relative location to the coils is further away from these coils. The CID, on the other hand, detects an object with a similar signal whether it is close or further away to the coils. Therefore the present embodiment combines these results to gauge the depth of the object and its size. This information provides a better determination of whether the object was an SSV.

Secondly, the results from these two systems are combined so that the CID design is provided with information on height, width and length of the reinforcing loop to correctly remove it from the signal observed on the B coil. All of these parameters are approximately derived from the AID design and consequently on passing them through to the CID processing software, better modelling of the reinforcing loop of the bag is achieved.

The outlet end of the infeed conveyor 15 and the inlet end of the main conveyor 17 are serially juxtaposed, and the junction 27 between the two conveyors is disposed at the threshold of the shield detector subsystem 19. The sensors are disposed along the conveyor, so that sensor #1 25a is disposed along the infeed conveyor 15, at the outlet end thereof proximate the junction 27. The remaining sensors are disposed along the main conveyor 17, most proximate to the entry of each station. As shown, sensor #2 25b is disposed within and near to the entry passage through the shield detector system 19, sensor #3 25c is disposed within and near to the entry passage of the QR subsystem 21 , and sensor #4 25d is disposed within and near to the entry passage of the X-ray subsystem 23. Lastly sensor #5 25e is disposed within the X-ray subsystem, but proximate to the leading side of the X-ray fan beam 29, which will be described in more detail later.

The operation of the various components of the system is controlled by control means in the form of a microelectronic controller 31 essentially comprising a master subsystem and a slave subsystem. As shown in Figure 17, the system 11 operates with an X-ray PC 33 functioning as the master subsystem controller and a QR / Shield PC 35 (referred sometimes below as just "QR PC") as the slave subsystem.

The X-ray PC 33 and the QR PC 35 are interconnected via a LAN bus 37, control and sensor bus 39, a serial communications bus 41 and a user interface including a KVM switch 43, which connects both PC's to a common keyboard, video monitor 45, mouse and a master control incorporated into a control panel 47. The master control permits a user to asynchronously perform a prescribed function related to the screening process at one or more of the subsystems, such as rejecting or clearing a bag that has been scanned by all subsystems of the system.

The system 11 is provided with a power circuit comprising various power supply 49 and power distribution 51 circuits connected to an AC inlet 53. These circuits supply power to the infeed conveyor 15 via an infeed power line 55, the main conveyor 17, and the PC's 33, 35. The main power distribution circuit 51a has an emergency stop circuit 57 connected thereto, which includes emergency stop switches 59. The functional arrangement of the control means is shown in Figure 18, and includes: a sensor interface 61 , a conveyor control 63, a station control 65, timing means 67, and a logic processing means 69.

The sensor interface is embodied in the X-ray PC 33 and receives opto-sensor information from the sensors 25a to 25e, to ascertain the relative position and size of articles along the conveyors.

The conveyor control 63 includes an encoder for providing various operating control signals to the respective conveyor motor circuits via control line 71 to the infeed conveyor 15 and control line 73 to the main conveyor 17. These operating control signals effect discrete starting and stopping of the conveyors and their speed of movement.

The station control 65 is embodied within the respective PC's 33 and 35, and operates the particular subsystems 19, 21 and 23 to perform the screening and scanning of articles passing therethrough.

The timing means 67 is associated with the conveyor control 63 and the station control 65 to synchronize operation therebetween and provide relevant timing signals to the processing means 69 to perform various processing functions.

The processing means 69 comprises a CPU that runs a computer program containing a plurality of processes 75 for performing a variety of different processing and control functions using the sensor interface 61 , conveyor control 63, station control 65, and timing means 67.

The main control process 75a performed by the processing means 69 involves invoking appropriate routines 77a that relate the timing associated with the screening of articles at the respective stations, as provided by the timing means 67, to: (i) relative conveyor operation and speed of movement of the conveyors, as provided by the conveyor control 63 and (ii) article positioning as provided by said sensor interface 61. This timing is then used to generate appropriate control signals for (i) the conveyor control 63 to ensure a prescribed spacing between the articles, and (ii) the station control 65 to ensure effective screening at the respective stations.

Another process involves a contingency process 75b, which is invoked by the processing means 69 in response to a master control operation initiated by an operator via the control panel 47. The contingency process 75b, when invoked by the processing means 69, operates another series of routines 77b to perform the prescribed function specified by the master control. An important example of this arises from the use of a single conveyor belt 17 for the QR/Shield/X-ray system 11. In light of this, it is important that the operation of the system be designed such that if the main conveyor belt 17 is stopped or reversed for any reason (including but not limited to operator discretion, system safety or process interlocks or an operator decision to stop and/or reverse the bag for screening purposes) that the Shield scan and Shield data acquisition is capable of being interrupted and that complete Shield data, undistorted by motion artefacts, can be acquired or corrected for subsequent analysis.

Accordingly, the contingency process 75b in such a situation effects remedial conveyor operation by the conveyor control 63 and prioritizes the screening of articles performed at the respective stations of the subsystems, in conjunction with the operation of the relevant routines 77b, to ensure that the prescribed spacing of articles is maintained and that the screening at all stations is properly attended to at the completion of the routine.

In the system described above this is accomplished by providing a motion encoder signal from the main conveyor belt 17 direct to a Shield data acquisition system such that bag motion can be correlated with continuously acquired Shield data and the corresponding data frame for each bag under test be subsequently derived.

Now describing the typical operation of the system 11, bags 13 under test are loaded manually or automatically onto the separate in-feed conveyor 15. When ready, the bag 13 is automatically moved onto the main system conveyor 17. More than one bag is processed through the QR X-ray system 11 simultaneously in a pipelined and parallel process methodology with the QR inspection process being conducted on a second bag while the operator is performing image interpretation protocols and reaching a clear / reject decision on a first bag using the appropriate master control button on the control panel 47. System throughput in most applications is dependent on the QR inspection time, as this is conducted with the article stationary and in parallel with the operator making a clear / reject decision on a preceding bag which will normally be shorter than the QR inspection time.

The overall system scan process is shown in Figures 19a to 19g:

1 ) Bag 1 is manually loaded onto the infeed conveyor 15 and automatically staged with leading edge at the end of the infeed conveyor as shown in Figure 19a. Position sensor #1 25a is used to detect the bag and to stage the bag.

2) When the system is ready Bag 1 automatically moves from the infeed conveyor 15 onto the main conveyor 17 and stops centred in the QR coil of the QR station 21 after completing a moving Shield scan at the shield station 19, as shown in Figure 4b. Position sensor #2 25b is used to control the timing of the Shield scan and Position sensor #3 25c is used to sense and stop Bag 1 in the QR coil.

3) Bag 2 is manually loaded onto the infeed conveyor 15 and staged as described initially with respect to Bag 1 , as shown in Figure 19c. Once stationary Bag 1 automatically undergoes a QR scan.

4) As shown in Figure 19d, once the QR scan has completed Bag 2 automatically moves from the infeed conveyor 15 onto the main conveyor 17 and stops centred in the QR coil after completing a moving Shield Scan as above. Bag 1 simultaneously moves on the main conveyor 17 through the X-ray fan beam 29 and an X-ray image is acquired. Initiation of the X-ray image capture process is automatically controlled by Position sensors #4 25d & #5 25e.

5) Bag 3 is loaded onto the infeed conveyor 15 and automatically staged as above while Bag 2 undergoes a static QR scan, as shown in Figure 19e. Bag 1 remains stationary while the operator interprets the X-ray image presented along with the QR and Shield scan results previously acquired. The operator makes a "clear" or "reject" decision on Bag 1 using the appropriate master control button on the control panel 47 before manually clearing the scan process to continue on completion of the QR scan of bag 2.

6) As shown in Figure 19f, once the QR scan has completed Bag 3 automatically moves from the infeed conveyor 15 onto the main conveyor 17 and stops centred in the QR coil after completing a moving Shield Scan as above. Bag 2 moves on the main conveyor 17 through the X-ray fan beam 29 and an X-ray image is acquired as above. Bag 1 (following an operator decision) exits the system either cleared or for further inspection.

7) As shown in Figure 19g, Bag 4 is loaded onto the infeed conveyor 15 and automatically staged while Bag 3 undergoes a static QR scan as above. Bag 2 remains stationary while the operator interprets the X-ray image presented along with the QR and Shield scan results previously acquired. The operator makes a "clear" or "reject" decision on Bag 2 before manually clearing the scan process to continue on completion of the QR scan of bag 3.

8) Steps 6 and 7 repeat as required for subsequent bags.

Should a new bag not be staged ready for loading on the infeed conveyor 15 when required the scan process can continue as above but there is no need to delay movement of the main conveyor 17 after an operator decision is made as no QR scan is required because no bag is present in the QR coil.

Described below is the flow of operation and error handling between the X-ray and QR / Shield subsystems for start up and normal operation processes performed by the processing means 69 of the preferred embodiment above.

The Power up sequence is shown in the flowcharts of Figures 20a and 20b. Note: in the flowcharts herein, the X-ray system 23 is referred to as the TRX system which is a particular embodiment of general linescan X-ray systems. Similarly the computer operating systems referred to (Windows™ and DOS™) are examples of operating systems that may be used and do not limit this invention. In addition, references to "Opto's" are references to optical sensors which are a particular embodiment of position sensors 25 generally and any suitable sensor may be used.

• The X-ray system 23 starts the power-up sequence by flushing the system of bags 13. This is done so that the system begins in a known reference state (empty).

o There are communications between the systems during this stage to verify that both systems are operational

o During normal operation data is passed from the QR system 21 to the

X-ray system 23. This data includes the scan results, status of the scan, date, time, and so forth. The clock settings of the timer means 67 for the systems are synchronized during the power-up sequence to ensure that there are no time-related synchronization errors in the data

o Error handling in the start up process consists of monitoring the communications between the QR and X-ray PCs 35 and 33, respectively, and carrying out specific actions if there are communication errors

^■ If there are no communications within a designated time an error message is displayed on the system monitor

^■ If there is a status indication from the QR system 21 during the start-up sequence, and there is no response after a date/time request, then the system goes into a standard error handling routine that: • Sends a reset to the QR system 21 (first attempt to fix the problem)

• If this is not successful, reboots the QR system 21, so that a complete reset and power up is done by the QR system

• The two computers 33, 35 may boot up at different times, and may be in different states during the start-up. The system boot up logic is designed to take this into account, and to respond suitably

o During the boot up, date and time, bag count, system mode and other settings are synchronised and configured on the QR and X-ray systems

o During the boot sequence the status of the boot sequence is displayed to the operator on the computer screen of the monitor 45.

The error handling sequence is shown in the flowcharts of Figures 21a and 21 b.

There are two levels to the error handling. In the first error handling routine, HandleRespError shown in Figure 6a, the system attempts a quick recovery from the error condition by sending a reset (or initialize command) to the QR software. If this is successful, the system goes back to normal system operation.

If the reset is not successful, the error handling logic calls RespErrorStage2 shown in Figure 21 b, which reboots the QR computer 21.

On the second call to the error handling routine, if the QR reset and reboot have not worked, the "reboot" Boolean flag is used to monitor that the reset was done previously. If "reboot" is equal to "TRUE", then the reset and reboot was not successful, and a "Maintenance" message is displayed.

The Scanning Sequence is shown in the flowcharts of Figures 22a to 22e. The system logic is broken down into logic for the infeed conveyor 15, and logic for the main conveyor system 17. The main conveyor logic is detailed first.

The main conveyor system logic is "event" driven by bag position sensor events.

For example, when the first bag is run into the system, it will first trigger position sensor 2 25b. The logic for each of the position sensors is outlined in this section.

The logical approach is to have control pass to the most important event occurring at any given time. For example, when a bag is being QR scanned, the position sensor logic for the QR scan controls the system (position sensor 3). So when position sensor 3 25c is triggered, the control logic is timed to position the bag in the QR coil, stop the belt, run the QR scan, and then start the belt (subject to other conditions). After this, it passes control to the main scanning sequence (as detailed below).

Similarly, when the bag goes into the X-ray system 23, it triggers position sensor 4 25d. This position sensor "takes control" of the system to ensure that the bag is run all of the way through the X-ray beam 29. If other position sensors are triggered, the system continues to run the bag through the X-ray beam until the X- ray image is completely acquired. This logic can be thought of as "position sensor 4 logic" taking control of the system until the "position sensor 4" process is complete. This implementation is detailed in the flowcharts.

It should be noted that if the distance traversed by a bag loading into position in the QR coil from its previous stationary location allows a preceding bag to completely or at least partially traverse an X-ray fan beam, an X-ray image of the preceding bag can be partially or completely acquired.

It is possible to stop such a preceding bag across the X-ray beam location while the QR scan executes but this requires additional measures to ensure the X-ray image is undistorted by motion effects: o Either the speed or position of the bag must be measured as the bag decelerates and accelerates to allow spatial correction of the acquired X-ray image or

o A "cut free" process can be implemented:

■ That part of the X-ray image acquired during deceleration can be neglected

• The bag is stopped for the completion of a QR scan

^■ The bag is reversed a suitable distance and accelerated in the normal conveyor direction to full speed

^■ X-ray image acquisition is restarted from a point prior to the region of the bag in the X-ray beam when deceleration occurred

Similarly, if the distance traversed by a bag loading into position in the QR coil from its previous stationary location allows the same or another bag to preferably completely or at least partially traverse a Shield Detector system, a Shield inspection can be completed without degrading system throughput.

It is possible to stop such a bag on the conveyor belt while the QR scan executes but this requires additional measures to ensure the Shield Detection scan is undistorted by motion effects:

o Either the speed or position of the bag must be measured as the bag decelerates and accelerates to allow spatial correction of the acquired

Shield data or

o A "cut free" process can be implemented:

^■ That part of the Shield data acquired during deceleration can be neglected ^■ The bag is stopped for the completion of a QR scan

^■ Shield acquisition is restarted from a point prior to the region of the bag in the Shield detector when deceleration occurred.

The infeed conveyor operation is shown in the flowchart of Figure 23. The objective of this logic is to cue the bags 13 for the main system, and to use the cueing to maintain the bag spacing in the system.

Standard linescan X-ray interfaces 81 displaying a two dimensional projection 83 of the article under test are modified to include the results from the QR and Shield Detection subsystems 21 and 19, respectively, as shown in Figures 26 to 35. If automated X-ray threat identification algorithms are implemented, organic objects identified as threat items (potential bulk, military or commercial explosives) may be indicated by placing an ellipse 85, rectangle 87 or other shape indicator over or enclosing the questionable object or region in the X-ray image. QR inspection does not provide localization of a threat object and the alarm indication is a simple alert indicator 89 on the X-ray computer screen which applies to the entire bag. Potential shielded sheet explosives are indicated by a rectangle or other shape indicator placed over or enclosing the questionable object or region in the image.

In the case of both a Shield Detector alert and X-ray image alert the indicator may in addition be a shaded area overlay, center spot indicator, range brackets or any other suitable indicator device. It is expected that the indication of these alarms, alone or in combination, will prompt specific image interpretation protocols that will enhance operator performance in identifying threats.

It is also preferred that a second alert indicator is displayed on the computer screen 45 or control console 47 indicating an X-ray, QR or Shield alert a second time for each article under test. Such a second alert indicator may be a simple lamp, colored box or colored text box 91a, 91b, 91c as illustrated in the aforementioned figures. Such second indicator is visibly separated from the body of the X-ray image to ensure that an operator will not fail to recognize an X-ray, Shield or QR alert particularly in the case where the Shield or X-ray alert might otherwise be superimposed on the X-ray image of a cluttered or complex bag.

The examples of Shield, QR and X-ray alert indicators under various alert conditions shown in Figures 26 to 33 are as follows:

• Figure 26 shows a benign Bag : No Alert Indicators

• Figure 27 shows a Bag with an X-ray Alert Only

• Figure 28 shows a Bag with a Shield Alert Only

• Figure 29 shows a Bag with a QR Alert Only

• Figure 30 shows a Bag with an X-ray & Shield Alert Only

• Figure 31 shows a Bag with an X-ray & QR Alert Only

• Figure 32 shows a Bag with a Shield & QR Alert Only

• Figure 33 shows a Bag with simultaneous QR, Shield & X-ray Alerts.

The system user interface for the QR X-ray system is a unified and integrated display or displays and control panel 47 or panels as described above based on standard X-ray interfaces supplemented with the functionality required by the additional QR and Shield systems 21 and 19. Specifically, the unified system interface provides the following additional capability and features which may be implemented through the computer screen(s) 45 or control panel(s) 47:

• Error reporting for the QR and Shield Subsystems 21, 19 preferably implemented (but not limited to) via on screen text based messages including:

o General warning and status messages o Reporting of the error condition when a bag is longer than the length of the QR coil and hence a full QR scan of the bag cannot be completed - under the normal operating process

o Reporting of the error condition where bags are not adequately spaced as outlined elsewhere herein and a full QR scan of the bag or bags cannot be completed under the normal operating process

• Archiving and storage of QR and Shield inspection history data in a manner that is readily accessible and correlated with the X-ray inspection history data and system status logs

• Single point of access for all system startup and shutdown actions and for conveyor belt control

• Allows automatic and manual operational configuration of the QR and Shield systems in concert with the X-ray subsystem from a single interface according to operator requirements, for example setting the QR scan times, signal processing methodology, alert thresholds, bag flow process mode or disabling either the QR or Shield subsystems or both

• Allows soft or hard reboot of the QR and Shield subsystems in the event of system error

• Provides QR and Shied system status to the operator

• Reports to the operator when a QR scan is being conducted via a suitable information message or indicator (as the belt is stopped and the system may otherwise have the appearance of being idle or in a fault state)

• May indicate to the operator via a box or rectangle on the X-ray image surrounding the bag (or any other suitable representation, device or indicator) that the bag is ready for review (having completed the QR and shield scan and a full X-ray image acquired). Once the operator makes a clear or reject decision the box or rectangle is removed or color coded differently to indicate the decision status. The user interface 81 showing a blue box 93 surrounding the bag representing that the operator must make a clear / reject decision on the bag enclosed is shown in Figure 34 and the user interface 81 showing a red box 95 surrounding the bag representing that the operator has made a reject decision on the bag enclosed is shown in Figure 35.

• Allow the operator to toggle on and off the Shield target indicator overlaid on the X-ray image

• Allows initiation, monitoring and the results of system diagnostics checks run on the QR or Shield subsystems either at startup, on request or automatically at predefined times, to and by the operator

• Allows automated or manual performance verification tests to be run on the QR and Shield subsystems to determine the functionality and performance of these systems to specification. Such tests may include the manual loading of QR active materials or test Shield objects into the system as a part of an automatic test sequence. The results of the verification scans of the test objects would preferably be reported either numerically or as a pass / fail response to the operator via the user interface

• Informs the operator on system startup that the system must be flushed of all articles under test to permit calibration of the QR and Shield systems. It is preferred that the operator would respond by reversing the main conveyor belt to clear all bags and verify the system is empty.

As described, the Shield detector subsystem 19 generates a 2D image which can be superimposed on the X-ray image 83 in the form of a rectangle 87 to provide for shield localization. Projection of this rectangle is useful to the operator of the machine because objects which have been automatically identified as Shields can be quickly identified and cleared or rejected in conjunction with the interpretation of the X-ray image. Quick identification results in a quicker clearance of the bag and thus a higher throughput rate can be achieved as compared to if the operator had to search the entire extent of the X-ray screen image. This higher throughput rate aids passengers and airport screening systems alike and reduces the stress upon the system operators. Similarly using the information from the X-ray image 83 and a shield localization image in the form of a rectangle 87 together allows some automatic alerts to be cleared with a resultant reduction in reject rate to secondary screening.

A specific process is invoked by the processing means 69 to project the shield localization rectangle 87 upon the X-ray scanner image 83 to ensure that the two images align correctly. In practice it is more difficult to achieve the alignment of the two sets of images than may be first thought. This is due to the fact that the X- ray camera and Shield detector imaging systems usually operate with different viewing perspectives with respect to the article under test. Typically the X-ray source in the X-ray scanner does not lie directly below the bag, but is off to one side. This is shown in Figure 21 , where an X-ray source 451 is located off to one side of the detection volume 453 defined by the walls 455 of the X-ray detection chamber, where a bag 457 is located therein. The X-ray sensors 459 for receiving X-rays transmitted from the source 451 are located in an L shape array on the opposing sides of the adjoining walls 455. The signals obtained from the L shape array of sensors 459 are used to form the X-ray image.

In contrast the shield detector subsystem 19 generates two dimensional rectangular co-ordinates which are viewed directly above the bag being scanned.

As the X-ray intensities from each detector in the L shape array are not processed in any way and simply displayed as the X-ray image, this image and the shield detector rectangle are viewed from differing viewpoints, thus creating two images that are hard to relate. The images must be compensated relative to one another so that they are effectively viewed from the same point and can be overlaid.

To achieve this task, the detection system is provided with projection means to project the rectangle representing a shield onto the X-ray sensors of the X-ray detector array and conversion means to convert the rectangle co-ordinates provided by the Shield detector into the same co-ordinates as the projection of the X-ray image. Thus, the image derived from the Shield detector is projected as it would be seen from the X-ray cameras viewpoint.

This is done as shown in Figure 36. Moreover, the left hand edge (A) of a detected shield 461 within a bag 457 disposed within the detection volume 453 of the X-ray scanner is projected onto the A¹ position of the L shape array of X-ray sensors 459. Similarly the B right hand edge of the shield 461 is projected onto B¹ of the L shape array 459.

In Figure 37, the same diagram is shown with the key points labelled with x and y co-ordinates. It can be shown using proportionality that the x and y co-ordinates of the point A¹ are:

xA¹ = (y1/yA)*xA (1)

yA' = yi (2)

And similarly for point B' the x and y co-ordinates are:

xB' = x3 (3)

yB¹ =(x3/xB)*yB (4)

Hence once the shield detector images have been converted into the same coordinates as the X-ray image projection, it can be processed the same way as the X-ray sensor information and create an image which can be superimposed over the X-ray image. Information from the L shape sensor 459 is data 'straightened' into a linear array and this forms one line of the X-ray image. Many of these straightened arrays are conjoined together to form the X-ray image.

The method of processing the 2D rectangles from the shield detector and comparing them to the X-ray image is as follows:

(i) The shield detector detects a target and generates rectangular co- ordinates representing the edges of the shielded target. (ii) If more than one target has been identified and the targets overlap, then they are merged into one single target.

(iii) If any of the targets are small in size they are enlarged to a minimum size.

(iv) The shield co-ordinates that lie along the direction of the belt are corrected to align with the X-ray image in that direction.

(v) The shield co-ordinates that are across the belt are converted into the X-ray's L shape array co-ordinates using the above correction method.

(vi) All co-ordinates are slightly enlarged so the object is surrounded on the X-ray screen.

(vii) These co-ordinates are superimposed over the X-ray image in the shape of a rectangle to enable the operator the ability to identify the shield.

(viii) From this information, the operator decides whether the shield is real or the shield is a false alarm. Alternatively the operator manually searches for the target in the bag.

In a variation to the present embodiment, as referred to in the best mode, instead of the operator deciding whether the shield is real or is a false alarm, detection is performed automatically by a computer program. The computer program comprises X-ray image recognition software suitably trained to recognise shielded targets which cause false alarms and automatically reject the object as being a false Shield or otherwise flag the object detected as a real Shield.

It should be noted that the sensor scan sequence (Shield, QR, X-ray) is preferred to other sequences, as it readily provides for all inspection information to be available to the operator at the completion of the scan sequence and for a decision to be made on the bag immediately to hand. This has the following benefits:

• The operator may use the X-ray image to resolve automatic QR or Shield alerts leading to an overall lower system reject rate to secondary screening

• The operator may use a QR or Shield alert to key into particular features of the X-ray image for threat objects that may have been unlikely to be positively identified otherwise. Overall system probability of detection can thereby be improved

It is understood that the preferred embodiment described above could also be implemented with more closely spaced or essentially co-located Shield, QR & X- ray subsystems. Accordingly, a second embodiment of the invention is directed towards such a system configured within the limits of the required bag spacing limitations discussed herein and is shown in Figure 38 of the drawings.

This approach to design of a system has the advantage of a smaller system footprint and requires less process time to be spent on translating bags between the various subsystem stations. A disadvantage of the system, however, is that the electronics, QR coil, Shield sensors, X-ray sources, X-ray detectors, X-ray power supplies and other system components must be more closely located or co-located and thus must necessarily be engineered to sufficiently compact and integrated packages. Closer spacing also potentially may increase subsystem interaction or cross-talk difficulties between, for example, the QR system and

Shield Detector units or between X-ray power supplies and QR or Shield systems.

The third embodiment is substantially similar to the first embodiment except that the control means is adapted to provide closer article spacing on the conveyor belt for increased system throughput. This is referred to herein as "pipelining" baggage movement, where the spacing between bags is kept as short as possible to a critical threshold within the limitations of the following constraints, and the apparatus is operated at the fastest practical conveyor acceleration, deceleration and speed. The layout of a system as described in the preceding embodiments and the adoption of pipelined bag movement processes are dictated in part by the minimum bag spacing required between any two bags. This in turn is dictated by the greater of two dimensions:

1. The minimum distance required between bags such that when a QR scan is performed on a first bag stopped in the QR coil the scan does not pre-excite QR active materials found in a second bag on the conveyor belt prior to a QR scan of the second bag.

^■ Such pre-excitation may degrade or entirely suppress the QR signal normally generated by QR active materials

^■ An example of QR active materials subject to such pre-excitation is PETN explosive (Pentaerithrytol tetranitrate) which has a Ti time constant for signal recovery of around 30 seconds under normal ambient conditions

- In practice such bag spacing may be of the order of 30 to 40cm

2. The minimum distance required between bags such that when a Shield detector scan is performed on a first bag transiting through the Shield detector sensor there is zero or negligible signal contributed to the response of the first bag by bags preceding or following the first bag

^■ In practice such spacing may be of the order of 20 to 60cm.

The embodiment is described below and shown in Figures 39a to 39h.

1. Bag 1 is loaded onto the infeed conveyor 15 and staged with leading edge at the end of infeed conveyor as shown in Figure 39a. Position sensor #1 is used to detect the bag and to stage the bag.

2. When the system is ready, as shown in Figure 39b: a. Bag 1 automatically moves from the infeed conveyor 15 onto the main conveyor 17 and stops centred in the QR coil of the QR station 21 after completing a moving Shield scan at the shield detector station 19. Position sensor #2 is used to control the timing of the Shield scan and Position sensor #3 is used to sense and stop Bag 1 in the QR coil.

b. Bag 2, also staged by the infeed conveyor 15, is also transitioned to the main conveyor 17 with appropriate timing to set the desired bag spacing between Bags 1 & 2. Bag 2 automatically moves into and stops in the region of the shield detector station 19, partially completing a moving Shield Scan while a QR scan is completed on Bag 1.

c. Bag 3 is loaded onto the infeed conveyor 15 and staged as above.

3. Bag 1 completes a QR scan, as shown in Figure 39c.

4. After the QR scan is completed on Bag 1 , as shown in Figure 39d:

a. Bag 1 automatically moves out of the QR coil and stops between the X-ray fan beam 29 and QR coil.

b. Bag 2 automatically moves into the QR coil after completing a moving Shield scan.

c. Bag 3, staged by the infeed conveyor transitions to the main conveyor with appropriate timing to set the desired bag spacing between Bags 2 & 3. Bag 3 automatically moves into and stops in the region of the Shield Detector station 19 partially completing a moving Shield scan while a QR scan is completed on Bag 2

d. Bag 4 is loaded onto the infeed conveyor 15 and staged as above.

5. Bag 2 completes a QR scan, as shown in Figure 39e. 6. After the QR scan is complete on Bag 2, as shown in Figure 39f:

a. Bag 1 automatically transitions through the X-ray beam and an X-ray image is acquired

b. Bag 2 moves out of the QR coil and stops between the X-ray fan beam 29 and QR coil.

c. Bag 3 automatically moves into the QR coil after completing a moving Shield scan.

d. Bag 4, staged by the infeed conveyor transitions to the main conveyor with appropriate timing to set the desired bag spacing between Bags 3 & 4. Bag 4 automatically moves into and stops in the Shield Detector region partially completing a moving Shield Scan while a QR scan is completed on Bag 3

e. Bag 5 is loaded onto the infeed conveyor and staged as above.

7. Bag 3 completes a QR scan while the operator interprets the X-ray image presented for Bag 1 along with the QR and Shield scan results previously acquired, as shown in Figure 39g. The operator makes a "clear" or "reject" decision on Bag 1 before manually clearing the scan process to continue on completion of the QR scan of bag 3

8. After the QR scan is complete on Bag 3, as shown in Figure 39h:

a. Bag 2 automatically transitions through the X-ray beam 29 and an X- ray image is acquired

b. Bag 3 moves out of the QR coil and stops between the X-ray fan beam and QR coil.

c. Bag 4 automatically moves into the QR coil after completing a moving Shield scan. d. Bag 5, staged by the infeed conveyor transitions to the main conveyor with appropriate timing to set the desired bag spacing between Bags 4 & 5. Bag 5 automatically moves into and stops in the Shield Detector region partially completing a moving Shield Scan while a QR scan is completed on Bag 4

e. Bag 6 is loaded onto the infeed conveyor and staged as above.

9. Steps 7 and 8 are repeated for subsequent bags.

If a bag is not loaded onto the infeed conveyor 15 in time for it in turn to load onto the main conveyor 17 with the requisite bag spacing, a conceptual "phantom" bag is deemed to be loaded onto the main conveyor 17 in the position that the missing bag would otherwise have taken. The "phantom" bag maintains the position in the pipelined bag flow of the missing bag to maintain correct bag spacing between subsequent bags and preceding bags.

In this instance if a phantom bag enters the QR coil it is not necessary to subsequently delay movement of the main conveyor until a QR scan is complete.

In fact no QR scan is required and the conveyor can be forwarded immediately or as soon as the operator has made a clear / reject decision on a preceding bag, following acquisition of an X-ray image. Accordingly, on the microelectronic controller 31 being alerted to there being a phantom bag, the processing means 69 invokes a suitable process 75 to effect conveyor movement to achieve such an outcome.

Similarly if a phantom bag passes through the X-ray beam and an "image" is acquired there is no need for an operator to make a clear/ reject decision, and the process 75 causes appropriate control signals to be asserted by the conveyor control 61 to advance the main conveyor 17 immediately or as soon as the QR scan of a following bag is completed.

In another case where only one bag is loaded into the system, the processing means 69 invokes another process 75 to control the main conveyor 17, enabling the bag to go directly to the QR chamber, receive a QR scan, and then move directly through the X-ray beam for an operator decision.

In all instances the system logic takes into account the position of other bags in the pipeline and is designed to maintain the highest bag throughput. A disadvantage of this embodiment is, however, that article residence time in the system is significantly greater than other embodiments.

The fourth embodiment is substantially similar to each of the preceding embodiments, except that the apparatus is operated in the reverse configuration. Moreover, the bag process flow commences on the main conveyor in the reverse direction and proceeds initially through the X-ray station, then the QR station and finally the shield scan station, before exiting onto what now becomes the outfeed conveyor. Such process flow has the operational advantage that a significant number of bags, after an X-ray image has been acquired, may be manually or automatically cleared and need not undergo a QR or Shield scan with a resulting increase in system throughput.

In a variation to the present embodiment, the conveyor configuration is altered, whereby the infeed conveyor is used to feed articles to the threshold of the X-ray station, whereinafter the main conveyor is used to convey bags through the various stations.

The fifth embodiment is substantially similar to the preceding embodiments except that the main conveyor is modified by dividing it into two. As shown in Figure 40 of the drawings, the junction 101 between a first conveyor 103 and a second conveyor 105 is provided between the QR coil of the QR station 21 and X-ray fan beam 29 of the X-ray station 23. Such a design has the advantage that a bag, having completed a QR scan, can transition to the second conveyor 105 to traverse the X-ray beam 29 and acquire an X-ray image independently. This allows following bags to be moved as rapidly as possible through the Shield detector and into the QR coil to begin a QR scan. This can be done at potentially greater speed than would be permitted in a single conveyor system where the speed of the conveyor must be restricted by the speed at which an X-ray image of adequate quality can be acquired by the X-ray camera. In addition such an arrangement allows a bag on the second conveyor to be moved forward and backward in the X-ray beam, if necessary multiple times, to reacquire and re- inspect an X-ray image. Also once an X-ray image has been acquired and a clear / reject decision made, a bag on the second conveyor 105 may be immediately ejected from the system without waiting for the QR scan to complete.

It is noted that a variation of the present embodiment, similar to that described in the fourth embodiment, provides for a system configured for reverse bag flow processing with similar benefits to those mentioned in relation to the fourth embodiment.

As can be seen, the aforementioned embodiments provide many advantages over previous discrete systems involving independent scanning or detection technology. Some of these advantages include:

• Design of the system such that the moving parts are minimized

• The user interface is extremely efficient and provides for increased discrimination of false alarms and between objects that are threatening and those that are not

• The design is flexible and can allow for:

o the deletion of the shield detector component

o the deletion of the X-ray component

o the addition of further static or dynamic sensors

• Timing and reporting of alert decision to the operator is integrated and simple, whereby all results are displayed to the operator at the same time to minimize confusion !t should be appreciated that the scope of the present invention is not limited to the specific embodiments described herein. Accordingly, minor variations and improvements to any of the embodiments in accordance with standard engineering practice that do not depart from the spirit of the invention are deemed to fall within the scope of the invention.

Claims

The Claims Defining the Invention are as Follows

1. An apparatus for sequencing articles to be screened through a plurality of stations at least one of which requires articles to be static during the screening process, and another of which permits the articles to be dynamic during the screening process, the apparatus comprising:

a conveyor for receiving articles and passing them through a plurality of stations;

one of said stations comprising an X-ray station through which articles are passed using said conveyor and screened to sense the presence of any prescribed substances or objects therein using X-ray detection technology;

a plurality of article position sensors disposed at different locations along the conveyor with respect to said stations to sense the relative position of discrete articles along the conveyors; and

control means having:

(b) a conveyor control to operate the conveyors and speed thereof,

(c) a station control to operate the stations to perform the screening,

(e) processing means to relate timing associated with screening the articles at the respective stations via said timing means, to: (i) relative conveyor operation, and speed of movement via said conveyor control, and

(ii) article positioning information provided by said sensor interface;

and generate appropriate control signals for:

(i) said conveyor control to ensure a prescribed spacing between the articles, and

(ii) said station control to ensure effective screening at the respective stations;

wherein said X-ray station comprises a single view dual energy linescan X- ray system.

2. An apparatus as claimed in claim 1 , including a user interface associated with the stations, said user interface being provided with a master control to permit a user to asynchronously perform a prescribed function related to the screening process at one or more stations; and

whereby said contingency process effects remedial conveyor operation by said conveyor control and prioritizes the screening of articles performed at the respective stations, in conjunction with the operation of said routine, to ensure that said prescribed spacing of articles is maintained and that the screening at all stations is properly attended to at the completion of said routine.

3. An apparatus as claimed in any one of the preceding clams, wherein said conveyor comprises an infeed conveyor and a main conveyor; the conveyors being automatically controlled by said conveyor control in response to operation of said processing means, whereby said conveyor control selectively operates said infeed conveyor and said main conveyor to stage the passage of an article sequentially from said infeed conveyor to said main conveyor and subsequently through the stations, maintaining said prescribed spacing between the articles.

4. An apparatus as claimed in claim 3, wherein the outlet end of said infeed conveyor and the inlet end of said main conveyor are serially juxtaposed, and the junction between the two conveyors is disposed at the threshold of the first station.

5. An apparatus as claimed in claim 4, wherein a first article position sensor is disposed along said infeed conveyor, proximate to said junction, and remaining article position sensors are disposed along said main conveyor, proximate to the entry of each station to feed article position information to said sensor interface.

6. An apparatus as claimed in any one of the preceding claims, wherein said control means selectively effects screening at a plurality of stations, simultaneously.

7. A method for sequencing articles to be screened through a plurality of stations, at least one of which requires articles to be static during the screening process, and another of which permits the articles to be dynamic during the screening process, the method comprising:

sequentially conveying articles:

(i) dynamically through a single view dual energy linescan X-ray station whilst simultaneously screening the articles thereat through a screening process to sense the presence of any prescribed substances or objects therein using X-ray detection technology; and

(ii) to and from a station at which the article is required to remain static for a prescribed period of time during another screening process; and

pipelining the articles through said stations to maximize throughput; whereby the spacing between articles is maintained to a critical threshold, determined by the effect that screening of an article at one station has on an adjacent article, and the effect that an adjacent article has on an article being screened, so that there is no or negligible interference between the two.

8. A method as claimed in claim 7, including releasing an article for conveying through the stations at staged intervals and independently conveying released articles through the stations relative to the next article to be released.

9. An apparatus for screening articles passed therethrough for detecting prescribed substances or objects within the articles, comprising:

conveyor means for sequencing articles therealong in a given direction;

an X-ray station through which articles are passed using said conveyor means and screened to sense the presence of any prescribed substances or objects therein using X-ray detection technology; wherein said X-ray station comprises a single view dual energy linescan X- ray system for generating an X-ray image of articles passed through the X- ray station.

10. An apparatus as claimed in claim 9, wherein the conveyor means and stations are arranged so that articles initially pass through the shield station, then the QR station, and finally the X-ray station, for detection purposes at each of these stations, respectively.

11. An apparatus as claimed in claim 9, wherein the conveyor means and stations are arranged so that articles initially pass through the X-ray station, then the QR station, and finally the shield station, for detection purposes at each of these stations, respectively.

12. An apparatus as claimed in any one of claims 9 to 11 , wherein the shield station comprises a shield detector subsystem for generating a shield localization image for superimposing on said X-ray image.

13. A method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and a single view dual energy linescan X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of:

14. A method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and a single view dual energy linescan X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of: initially the X-ray detector, then the QR detector and finally the shield detector.

15. A method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and a single view dual energy linescan X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of:

16. A method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and a single view dual energy linescan X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of:

17. A method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and a single view dual energy linescan X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of:

18. A method for sequencing articles through a plurality of stations comprising a shield detector, a QR detector and a single view dual energy linescan X-ray detector for detecting the presence of a prescribed substance or object, comprising:

passing the article through the detectors in the order of:

19. An apparatus for sequencing articles to be screened through a plurality of stations substantially as herein described in any one of the embodiments with reference to the accompanying drawings as appropriate.

20. A method for sequencing articles to be screened through a plurality of stations substantially as herein described in any one of the embodiments with reference to the accompanying drawings as appropriate.

21. An apparatus for screening articles passed therethrough substantially as herein described in any one of the embodiments with reference to the accompanying drawings as appropriate.