EP2673755B1

EP2673755B1 - Security system, controller for a security system, transducing cable for a security system and method for detecting a disturbance and determining the location of the disturbance

Info

Publication number: EP2673755B1
Application number: EP12726475.2A
Authority: EP
Inventors: Ian Macalindin; Colin BATHE
Original assignee: Detection Technologies Ltd
Current assignee: Detection Technologies Ltd
Priority date: 2011-06-01
Filing date: 2012-05-24
Publication date: 2014-12-10
Anticipated expiration: 2032-05-24
Also published as: GB2490179A; GB2490179B; WO2012164260A1; GB201109234D0; EP2673755A1

Description

The present invention relates to security systems and their components, in particular the field of electronic perimeter security, where intrusion activity is detected by means of a vibration-sensitive sensor deployed along the perimeter of a site.
Most perimeter intrusion detection systems available on the market today that are based on vibration-sensing technology operate by dividing the perimeter into a number of discrete 'zones' the length of each zone being determined by the technology in use and requirements particular to the site itself.
For example, a perimeter intrusion detection system suitable for a high security application such as a prison will be designed with relatively short zone lengths (40 - 60 metres typically) primarily because such systems are used in conjunction with closed circuit television cameras to investigate and verify the cause of alarms generated by the detection system. In applications such as this, each detection zone would be viewed by one (or more) camera(s).
In UK prison applications, the cameras and lenses must be designed to ensure that a mansized target appearing anywhere within the field of view of the camera is clearly identifiable on a monitor screen. This requirement restricts the distance the camera can cover and hence the length of the detection zone. Conversely, on lower security sites where financial considerations are paramount, the perimeter zones tend to be longer as this means that the total number of zones is reduced and hence the system cost is also reduced.
Conventional systems such as those described above usually require some electronic means of analysing the signals produced by the sensors to ensure that genuine intruder activity is detected while non-hostile activity caused, for example, by wind and rain does not generate false alarms. The cost of these analysers contributes significantly to the overall system costs.
Furthermore, systems that rely on electronic analysis on a zone by zone basis may incur significant installation costs resulting from the need to provide power and an alarm communication network to route alarm information from the perimeter of the site to security staff working within a site control room.
Figure 1 depicts a typical schematic layout of a conventional perimeter intrusion detection system. As can be seen from the diagram, the protected perimeter 1 is physically split into a number of discrete zones 6. Each of the zones 6 requires an electronic analyser 2 to determine whether the signals detected by the zone sensors is of hostile or benign origin. In a typical arrangement, each electronic analyser 2 may monitor a zone on either side of the analyser. Adjacent zones 6 not sharing an electronic analyser 2 may be separated by a zone termination 3.
Each analyser 2 provides an alarm signal, often by means of a simple relay contact that opens when an alarm is detected. It is the provision of this signal on a zone-by-zone basis that indicates to the operator which part of the perimeter is under attack by an intruder. However, it is therefore required that each analyser 2 be connected to the control room 4 by a signal line 5. Furthermore, a power line must also be provided to each analyser 2.
Clearly, in instances where the zone length may be in the order of a few hundred metres, identification of the exact point of intrusion is not possible given that only one alarm signal per zone is provided. Furthermore, the duplication of analyser electronics for every zone clearly adds to the cost of the system. Reducing the zone length to increase the accuracy of identification of the point of intrusion results in significant cost increases due to the requirement to provide further electronic analysers 2. In addition, the use of a plurality of analysers results in a reduction in reliability of the system because the use of many electronic components increases the likelihood of a failure.
It would be desirable to have a security system that overcomes or reduces some of the above problems.
According to an aspect of the present invention, there is provided a security system, for detecting a disturbance and determining the location of the disturbance along a boundary, the security system comprising:

a section of transducing cable, arranged along said boundary and configured such that a disturbance at one location on the boundary generates a signal at a corresponding location in the transducing cable that propagates from said location along the transducing cable towards first and second ends of said section of transducing cable; and
a controller, configured to receive the signal from said first and second ends of the section of transducing cable and to determine the location in the transducing cable at which the signal was generated based on any difference in the time at which the controller receives the signal from said first and second ends;
wherein said section of transducing cable is divided into a plurality of portions; and
each portion of transducing cable is separated from an adjacent portion by a time delay unit, configured to delay the transmission of said signal between the portions by a predetermined time delay.

According to a further aspect of the invention, there is provided a controller for a security system, comprising:

first and second inputs for receiving signals from first and second ends, respectively, of a section of transducing cable that is configured such that, in response to a disturbance at one location along the transducing cable, it generates at said location a signal that propagates from said location along the transducing cable towards said first and second ends, the section of transducing cable being divided into a plurality of portions, each of which is separated from an adjacent portion by a time delay unit, configured to delay the transmission of said signal between the portions by a predetermined time delay; and
a processor, configured to determine from the signals received at said first and second inputs the location in the transducing cable at which the signal was generated based on any time difference in the time at which the signals are received at the first and second inputs.

According to a further aspect of the invention, there is provided a method for detecting a disturbance and determining the location of the disturbance along a boundary, comprising:

providing a section of transducing cable, arranged along said boundary and configured such that a disturbance at one location on the boundary generates at a corresponding location in the transducing cable a signal that propagates from said location along the transducing cable towards first and second ends of said section of the transducing cable, the section of transducing cable being divided into a plurality of portions, each of which is separated from an adjacent portion by a time delay unit, configured to delay the transmission of said signal between the portions by a predetermined time delay; and
receiving at a controller the signals from said first and second ends of the section of transducing cable and determing the location in the transducing cable at which the signal was generated based on any difference in the time at which the controller receives the signals from said first and second ends.

An advantage of the invention discussed above is that a single controller may be used to identify the location of a disturbance at any part along a boundary. Accordingly, only a single analyser system may be required to discriminate between genuine disturbances, such as an intrusion at the boundary, and non-hostile activity, such as wind and rain. This may reduce the cost of the system.
Furthermore, such a system may not require power to be supplied to a plurality of analysers distributed around the boundary and/or may not require a plurality of separate alarm signalling cables to be provided from the plurality of analysers back to the control room. This may reduce the cost of the overall system. Furthermore, this may increase the resilience of the system, namely by reducing the likelihood of a fault within the system.
It should be appreciated that a system of present invention may be specifically configured to detect and locate an appropriate disturbance, depending on the use to which the security system is put. For example, the system may be configured to detect and locate a localised vibration of the transducing cable. Such a vibration may be caused, for example, by an intruder directly interacting with the transducing cable and/or by an intruder making contact with the structure along the boundary, such as a fence or wall, to which the transducing cable is attached. In that case, the vibration may pass through the structure to the transducing cable, resulting in the detection and location of the disturbance.
The transducing cable is divided into a plurality of portions, each separated from an adjacent portion by a time delay unit that delays transmission of signals between the portions by a predetermined time delay. Optionally, each of the time delay units may delay the transmission of these signals by the same amount. Such an arrangement may permit a simpler controller to be used and/or may make it easier to detect the location of a disturbance to a desired accuracy range.
In this context, it will be appreciated that the signals may propagate along the transducing cable at a significant proportion of the speed of light. Therefore, the difference in the time at which the signals reach the first and second ends of the second of transducing cable caused by the difference in lengths of transducing cable along which these signals have propagated may be very small. This may require a very sensitive controller to measure the time difference and therefore determine the location of the disturbance. Therefore, the controller for a system without time delay units may be relatively expensive.
However, time delay units are provided and introduce time delays that are significantly greater than the time taken for the signal to propagate along the section of transducing cable or the portions thereof. Therefore, the time difference between the signals received at the first and second ends of the section of transducing cable will be primarily due to a difference in the number of time delay units that each signal has passed through. Based on this, the controller may identify in which of the plurality of portions of the transducing cable the disturbance occurred. A controller configured to do so would not need to be as sensitive due to the significantly greater size of the time delays. Accordingly, the controller may be significantly cheaper, offsetting any increase in cost by the provision of the time delay units.
In an embodiment, the time delay units may be formed from passive components, such as inductors and capacitors. This may have the advantage that no power needs to be provided to the time delay units, which may be distributed along the boundary. Furthermore, such time delay units may be relatively cheap.
In an embodiment, the controller may comprise a correlator used to assess the time difference between the two signals received. For example, the correlator may be configured such that it provides a maximum output value when provided with two identical signals, theoretically, or, in practice, with two similar signals with little or no time difference between them. The controller may then use the correlator to perform a correlation of the received signals for a plurality of different time delays introduced to one of the signals. It will be appreciated that the introduced time delay corresponding to the correlation having the largest output value corresponds to the time different between the two signals.
In an embodiment of the invention utilising time delay units as discussed above, the controller may compare the time delay determined using the correlator to multiples of the time delays introduced by each time delay unit to determine the difference in the number of time delay units that the signals received at the first and second ends of the transducing cable have passed through. From this, the controller may determine the portion of the transducing cable in which the disturbance occurred.
It will be appreciated that the comparison of the time delay between the signals determined by the correlator and the multiples of the time delays introduced by the time delay units may be performed in a number of ways. For example, the controller may include a processor that simply divides the time delay between the two signals by the time delays introduced by the time delay units and identifies the closest integer. Alternatively, for example, the controller may use a look-up table to directly identify the portion of the transducing cable in which the disturbance occurred based on the time delay between the signals determined by the correlator. Other arrangements may also be used.
In embodiments of the invention, the signals received from the ends of the section of transducing cable may be amplified and/or high-passed filtered. High-pass filtering the signals may remove relatively low frequency components of the signals but pass relatively high frequency components of the signals to the correlator. This may be advantageous because it may remove interference signals that may be common to both, such as interference from power supplies, in particular AC power supply lines, for example.
In an embodiment of the present invention, the output of the correlator may be low-pass filtered, removing relatively high frequency components from the output but passing relatively low frequency components. This may assist in removing spurious peak values before the controller determines the peak correlation having the largest value. This may improve the accuracy of determining the time difference between the two signals.
In an aspect of the invention, there is provided a transducing cable for use in a security system as discussed above, comprising a plurality of portions of transducing cable, each configured to convert a localised vibration of the transducing cable to an electrical signal and to propagate said signals along the transducing cable; and
a plurality of time delay units, each used to separate a portion of the transducing cable from an adjacent portion, and configured to delay the transmission of said signals between the adjacent portions by a predetermined time delay.
The present invention will now be described by way of non-limiting examples with reference to the accompanying drawings, in which:

Figure 1 depicts an arrangement of a previously known security system;
Figure 2 depicts the arrangement of a security system according to an embodiment of the present invention;
Figure 3 depicts, in cross-section, a transducing cable that may be used in an embodiment of the present invention;
Figure 4 depicts a further embodiment of the present;
Figure 5 depicts a time delay unit that may be used in an embodiment of the present invention; and
Figure 6 depicts the functional components of a controller according to an embodiment of the present invention.

As shown in Figure 2, a security system of 10 of the present invention may include a section of transducing cable 11 that may, for example, be arranged around the boundary of an area 12 to be secured. First and second ends 11a, 11b of the section of tranducing cable 11 may be connected to a controller 13.
The transducing cable 11 is configured such that a disturbance at the boundary, for example corresponding to an individual attempting to breach the perimeter, results in the localised generation of a signal that propagates in both directions along the section of transducing cable 11, namely towards both the first and second end 11a,11b of the transducing cable 11. Accordingly, the controller 13 receives two signals, one from each end 11a,11b of the transducing cable 11. The controller 13 is configured to determine the time difference between the receipt of the two signals. The time difference between the receipt of the two signals corresponds to the difference in the length of the transducing cable 11 that the signals have propagated along in order to reach the respective ends 11a,11b. Accordingly, the controller 13 may determine the location of the disturbance along the transducing cable 11 based on this time difference.
The transducer cable 11 used in the present invention may be based on a linear format electrical induction transducer cable described in British Patent GB 2,175,771A . Figure 3 depicts a cross-section of such a cable. The transducing cable comprises a pair of copper conductors 21,22 (or conductors of other materials) which are free to vibrate within a static magnetic field produced by a pair of flexible ceramic magnetic profiles 23. A central stress member 24 may also be provided to minimise the effects of expansion of the thermoplastic elements of the cable relative to the metallic elements as a result of temperature effects. An outer sheath 25 may be included, providing weather protection. Furthermore, RF shielding may be provided by a metallic tape wrap 26, which may be formed from aluminium foil, for example. The transducing cable 11 may, for example, be produced in continuous lengths of up to 1500 metres.
In practice, the transducer cable 11 may be deployed by mechanically fixing it to the structure of a perimeter fence or wall such that the cable vibrates in response to vibrations transmitted through the structure of the fence or wall. As the core of the cable, largely comprising the flexible magnetic profiles 23, is effectively attached to a vibrating surface, it too vibrates with the same frequency and amplitude characteristics of the fence or wall itself. It will be appreciated that other deployments may also be used. For example, the transducer cable 11 may be deployed such that an intruder directly interacts with the cable, causing it to vibrate.
The pair of copper wires 21,22 within the core of the sensor are free to move, not being tightly constrained within the core. Accordingly, mechanical inertia dictates that these wires 21,22 will lag behind any movement of the core caused by vibration impinging on the cable core. This lag results in displacement of the copper wires 21,22 relative to the cable core and, since the displacement of the wires within the core occurs within the static magnetic field generated by the ceramic magnet profiles 23, a current is induced within the wires themselves.
In a previously known use of such a transducing cable, electrical connections are provided such that, at one end of the cable, the wires 21,22 are connected together, while at the other end of the cable, the wires are terminated with a resistor. A current loop is therefore created with induced currents passing around the loop and appearing as a voltage across the terminating resistor. A disadvantage of the previously-known use of the transducing cable 11 discussed above, in which the wires 21,22 are connected together at one end is that any current induced as a result of intrusion activity will be the same at every point along the length of the cable. This prevents identification of the source of such a signal on a long length of the cable.
Accordingly, an arrangement such as that depicted in Figure 1 is required, with a plurality of analysers.
Therefore, according to the present invention, an arrangement such as that depicted in Figure 2 may be used, in which at both ends of the transducing cable 11, both conductors 21,22 are connected to the controller 13, which is configured to detect the signals reaching the respective ends 11a,11b of the transducing cable 11 resulting from a disturbance at one location along the cable 11.
It should be appreciated that alternative arrangements of transducing cable may be used that generate locally a signal as a result of a disturbance and are configured such that the signal propagates along the transducing cable such that a controller may determine the location of the disturbance from a time difference between the receipt of the signal from the two ends of the transducing cable 11. However, the use of a transducing cable 11 as discussed above may offer a number of significant advantages over other transducing cables, such as microphonic cable sensors. An important advantage is that the cable offers a very low source impedance which therefore results in a high signal to noise ratio. The system is therefore capable of detecting low level disturbances without these signals being lost in the inherent (Johnson) noise that may be present at a much higher level in high impedance sensor devices.
It should be appreciated that a controller for such an arrangement may need to be very sensitive, namely capable of detecting very short time differences between the receipt of the signals at the two ends.
The propagation velocity of electrical signals along a cable of this type is in region of 0.6 C where C is the speed of light in a vacuum. Therefore, a signal generated within a length of such cable would propagate along the cable at a speed of 0.6 x 3 x 10⁸ m/s = 1.8 x 10⁸ m/s. Translating this into times, a signal generated in the middle of a 1000 metre length of sensor cable would arrive at the ends of the cable within (500 / 1.8 x 10⁸ ) seconds = 2777 nS.
This translates to a propagation time per metre of cable of 2777/500 nS = 5.5nS. Assuming that it is a requirement to provide disturbance point location to an accuracy of 10 metres, the controller measuring the time differences must resolve time intervals of 10 x 5.5 = 55nS.
For example, the controller may use very high speed analogue to digital converters in order to provide the required time sensitivity. It should be appreciated, therefore, that such a controller 13 may be relatively expensive.
An alternative embodiment of the present invention may enable the use of a less sensitive and therefore less complex and less costly controller. Such an embodiment is depicted in Figure 4.
As shown, the security system 10 of this embodiment is formed from a plurality of portions 31 of transducing cable 11, each separated by a time delay unit 32. As before, the ends 11a,11b of the transducing cable 11 are connected to a controller 33.
Figure 5 schematically depicts an arrangement of a time delay unit 32 that may connect the wires 21,22 of adjacent portions 31 of the transducing cable 11. As shown, such a time delay unit 32 may be formed from passive components, such as inductors L1, L2 and capacitors C1, C2. Therefore, the additional cost of the components used to form the time delay units 32 may be significantly less than the cost saving by providing a simplified controller 33.
It should be appreciated that the portions 31 of transducing cable 11 may be the same length. Alternatively, the portions 31 of transducing cable 11 may have different lengths. Accordingly, the portions 31 of transducing cable 11 may be connected to the time delay units 32 within a factory, for example if the time delay units 32 are to be provided at regular intervals. Alternatively, the time delay units 32 may be connected to the portions 31 of transducing cable 11 during installation of the security system 10 at a site at which it is to be used. In such an arrangement, the portions 31 of transducing cable may be cut to a length that is desirable for each zone of the security system 10.
In an embodiment, the time delay units 32 may, for example, introduce a 17µs delay per unit and are matched to the characteristic impedance of the transducing cable 11 (ca. 140 ohms) in order to minimise spurious reflections caused as the signals pass along the transducing cable 11. In an embodiment, a time delay unit such as that depicted in Figure 5 may be formed using inductors of approximately 1.15mH and capacitors of approximately 68nF.
Optionally, all of the time delay units 32 may be configured to introduce substantially the same time delay. This may simplify the arrangement of the controller 33. However, it should be appreciated that this is not essential.
In an arrangement as depicted in Figure 4, a disturbance, such as an intrusion, may result in the generation of a signal within one portion 31 of the transducing cable 11. The signals produced propagate along the cable to either end 11a,11b, however time delay units 32 inserted in series with the cable 11 cause these signals to be delayed.
These time delay units 32 are distributed along the cable such that the time delay between the signal being produced and it reaching either end of the cable is governed by the number of time delay units 32 between the point of origin of the signal and each end 11a,11b of the cable.
In this respect, it should be appreciated that time delays introduced by the time delay units 32, for example as discussed above, may be significantly longer than the time that it would take for the signals to propagate along the transducing cable 11 if there were no time delay units 32. Accordingly, the variation in time taken for the signal to propagate from its point of origin to one end of the transducing cable 11 is not significantly affected by the length of any of the portions 31 of the transducing cable but substantially determined by the number of time delay units 32 between the point of origin of the signal and the end 11a,11b of the cable.
It has been found that signals within the 1kHz to 10kHz frequency band are most useful for deriving the position of a disturbance along the length of a cable 11. Furthermore, with a passive time delay unit comprising the components described previously, signals of a higher frequency will be subject to a greater delay than signals of lower frequency. Therefore, it is preferable to utilise signals within such a relatively narrow frequency band. Therefore, the transducing cable 11 for use in the security system 10 of the invention may be selected such that is capable of generating frequencies within the 1kHz - 10kHz band and it is only signals within this band that are analysed for the purposes of deriving position location information.
Figure 6 depicts the functional arrangement of a controller 33 that may be used in an arrangement of the present invention. As shown, the signals reaching the two ends 11a,11b of the cable may be amplified by respective amplifiers 41,42 before being converted into the digital domain by analogue to digital convertors 43,44. The resultant digital signals are passed through high- pass filters 45,46 to remove low frequency noise which may include spurious signals such as power line interference which would otherwise degrade the accuracy of location of the point of disturbance, such as an intrusion. For example, the filters 45,46 may remove or attenuate signals below 1kHz.
After filtering, the two signals are then correlated with each other by a correlator 50. The design of the correlator 50 may be such that the maximum output will be achieved when there is effectively no time difference between the two input signals, namely when it receives two identical signals. It should be appreciated that the correlator 50 may be configured in the opposite sense, namely that it provides a minimum output when it receives two identical signals. However, this is generally considered to be less convenient.
As discussed above, the different paths of the signals between the point of origin of a signal, namely the location of a disturbance, and the two ends 11a,11b of the transducing cable 11 may result in time differences between the two signals. For example, delays will have been introduced by the time delay units. The correlator may therefore be configured to successively add time delays into the leading signal until the effective time difference is minimised resulting in maximum output from the correlator (or minimum).
Once the correlator 50 output is maximised, inspection of the value of the introduced time delay indicates the time difference between the incoming signals received at the two ends 11a,11b of the transducing cable 11. From this, the controller may determine the point at which the signal was introduced into the chain of time delay units 32. For example, the controller 33 may divide the value of the time delay introduced by the correlator 50 by the value of the time delays introduced by each of the time delay units 32 in order to determine the difference in the number of time delay units 32 through which each of the signals received at the two ends 11a,11b of the transducing cable 11 have passed. From this, the controller 33 may determine in which portion 31 of the transducing cable 11 the disturbance occurred, which may correspond to a zone of the security system 10.
In an alternative arrangement, the controller 33 may include a look-up table that equates a range of time delays introduced by the correlator 50 to a particular portion 31 of the transducing cable 11 or a particular zone within the security system 10.
As shown in Figure 6, the correlator 50 output may also be subjected to a low-pass finite impulse response filter 51 which is designed to remove spurious peaks which can result from any electrical interference beyond the 1kHz - 10kHz band which may have been induced within the transducing cable.
Since the correlation process involves, at high speed, introduction of a wide range of delay values, the correlator 50 output during the correlation process may comprise a series of peaks.
A peak detector 52 is therefore used to identify the largest peak which will correspond to the point at which the time delay introduced by the correlator 50 equals the time delay between the two signals received at the two ends 11a,11b of the transducing cable 11, namely the difference in time delays introduced by the time delay units 32. From this, as discussed above, a processor 53 in the controller 33 may determine the portion 31 of the transducing cable 11 in which the disturbance occurred.
As shown in Figure 6, optionally the controller 33 may further include a signal generator 61, digital to analogue convertors 62, 63 and amplifiers 64, 65, which may be used to generate test signals that may be transmitted along the transducing cable 11 to detect the position of any breaks or short-circuits within the transducing cable or connections to the delay elements.

Claims

A security system (10), for detecting a disturbance and determining the location of the disturbance along a boundary (1), the security system comprising:
a section of transducing cable (11), arranged along said boundary and configured such that a disturbance at one location on the boundary generates a signal at a corresponding location in the transducing cable that propagates from said location along the transducing cable (11) towards first and second ends (11a,11b) of said section of transducing cable (11); and

a controller (13,33), configured to receive the signal from said first and second ends (11a,11b) of the section of transducing cable (11) and to determine the location in the transducing cable at which the signal was generated based on any difference in the time at which the controller receives the signal from said first and second ends;

wherein said section of transducing cable is divided into a plurality of portions (31); and

each portion (31) of transducing cable (11) is separated from an adjacent portion by a time delay unit (32), configured to delay the transmission of said signal between the portions by a predetermined time delay.
A security system according to claim 1, wherein each of said time delay units delays the transmission of said signal by the same time delay.
A security system according to claim 1 or 2, wherein the controller comprises a correlator configured to provide a maximum or minimum output value when provided with two identical signals;
the controller is configured to use the correlator to perform a correlation of the signal received from the first end of the section of transducing cable with the signal received from the second end of the section of transducing cable for a plurality of different time delays introduced to the signal received from one of said ends prior to correlation; and
the controller is configured to identify a peak correlation having the output value closest to the output value for two identical signals and to determine the location of said disturbance from the corresponding time delay introduced by the controller to the signal received from said one of the ends prior to said correlation.
The security system according to claim 3, when dependent from claim 2, wherein the time delay introduced by each time delay unit is significantly greater than the time taken for the signal to propagate along said portions of the transducing cable; and
the controller determines the location of the disturbance by comparing the time delay introduced to the signal received from said one of the ends of the transducing cable in order to provide the peak correlation with multiples of the time delay introduced by each time delay unit to determine the difference in the number of time delay units the signals passed through from said location of the disturbance to the first and second ends of the transducing cable and therefore to determine in which portion of the transducing cable the disturbance is located.
The security system according to claim 3 or 4, further comprising high-pass filters configured to remove relatively low frequency components from the signals received from the first and second ends of the transducing cable prior to performance of the correlation.
The security system according to any one of claims 2 to 5, further comprising a low-pass filter configured to remove relatively high frequency components from the output of the correlator before the controller identifies the correlation having the largest value.
The security system according to any one of the preceding claims, wherein said section of the transducing cable forms a loop provided along the boundary of an area to be secured.
The security system according to any one of the preceding claims, wherein said transducing cable is configured to convert a localised vibration of the transducing cable caused by said disturbance into an electrical signal.
The security system according to any one of the preceding claims, wherein said time delay units are formed from passive components.
A controller (13,33) for a security system, comprising:
first and second inputs for receiving signals from first and second ends (11a,11b), respectively, of a section of transducing cable (11) that is configured such that, in response to a disturbance at one location along the transducing cable, it generates at said location a signal that propagates from said location along the transducing cable towards said first and second ends, the section of transducing cable being divided into a plurality of portions (31), each of which is separated from an adjacent portion by a time delay unit (32), configured to delay the transmission of said signal between the portions by a predetermined time delay; and

a processor (53), configured to determine from the signals received at said first and second inputs the location in the transducing cable at which the signal was generated based on any time difference in the time at which the signals are received at the first and second inputs.
A controller according to claim 10, further comprising:
a correlator configured to provide a maximum or minimum output value when provided with two identical signals;

wherein the controller is configured to use the correlator to perform a correlation of the signals received at the first and second inputs for a plurality of different time delays introduced to the signal received at one of said inputs prior to correlation; and

the controller is configured to identify a peak correlation having the output value closest to the output value for identical signals and to determine the location in the transducing cable at which the signal was generated from the corresponding time delay introduced to the signal received from said one of the ends prior to said correlation.
A transducing cable for use in the security system of any one of claims 1 to 9, comprising:
a plurality of portions of transducing cable, each configured to convert a localised vibration of the transducing cable to an electrical signal and to propagate said signals along the transducing cable; and

a plurality of time delay units, each used to separate a portion of the transducing cable from an adjacent portion, and configured to delay the transmission of said signals between the adjacent portions by a predetermined time delay.
A transducing cable according to claim 12, wherein each of said time delay units delays the transmission of said signal by the same time delay.
A method for detecting a disturbance and determining the location of the disturbance along a boundary (1), comprising:
providing a section of transducing cable (11), arranged along said boundary and configured such that a disturbance at one location on the boundary generates at a corresponding location in the transducing cable a signal that propagates from said location along the transducing cable towards first and second ends (11a,11b) of said section of the transducing cable, the section of transducing cable being divided into a plurality of portions (31), each of which is separated from an adjacent portion by a time delay unit (32), configured to delay the transmission of said signal between the portions by a predetermined time delay; and

receiving at a controller (13,33) the signals from said first and second ends of the section of transducing cable and determing the location in the transducing cable at which the signal was generated based on any difference in the time at which the controller receives the signals from said first and second ends.