Patent ID: 12236763

DETAILED DESCRIPTION

Where the arrangement herein is intended for use with perimeter fence intrusion detection, there are multiple intrusion types which need to be detected which are:Fence fabric cut;Fence fabric lift and crawl below lifted section;Fence climb.

A significant challenge to monitoring a fence with a fiber optic vibration and motion detecting sensor is the detection of an intrusion in the presence of strong weather such as wind or rain. Typically, systems will suppress false alarms in the presence of strong weather, however that introduces a vulnerability wherein a nefarious operator with knowledge of the system would wait for a weather event for scheduling an intrusion.

This invention outlines the application of the detection algorithms developed for zone products for application to portions of data collected or reported by the locating system shown inFIG.1.

There is an optical sensing system shown inFIG.1provided by an optical fiber1mounted n a fence2covering the protected perimeter. This can cover the whole perimeter or may be divided into sections such as particularly sensitive areas.

The optical sensing system provided by the optical fiber1is sensitive to vibration and movement. Thus the fiber1acts to encode vibration and movement into the light passing through the monitoring fiber from a transmitter3so that the signals transmitted are modified and reflected to a receiver at the head end. In this arrangement known as DAS the receiver is arranged to be responsive to the intensity of the signal which is measured as a function of time after transmission of the laser pulse. When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fiber. Changes in the Coherent Rayleigh Noise (CRN) of successive pulses from the same region of fiber are caused by changes in the optical path length of that section of fiber. The magnitudes of the changes depend on the strength and type of disturbance acting on the fiber. This type of system is very sensitive to both strain and temperature variations of the fiber and measurements can be made almost simultaneously at all sections of the fiber.

As shown in Figure the signal is transmitted from the transmitter3into the fiber1so as to introduce a monitoring optical signal into the optical fiber1. The reflected signals are received by the receiver4so as to receive optical signals from the optical fiber which are modified by events which affect the optical fiber.

As is known in the art, the DAS receiver acts to divide the optical signals into a plurality of streams S1to SN, where each stream is associated with a specific respective portion P1to PN of the optical fiber with the portions P divided along the length of the optical fiber so that each stream is indicative of disturbances in the respective portion P. Each stream S1to SN comprises a series of data values indicative of the magnitude of the disturbances in the respective portion over time. This output is known as a “waterfall” and is a well-established output form a DAS system.

These portions or streams can be collected in a variety of ways, representing a variety of data sets. These collection methods known to persons skilled in the art and available in the data from system used in practice such as DAS can be:Streaming raw;Streaming Internally processed;Internally processing within the locating system itself;Recording and transporting to a processor.

Additionally, depending upon the application the data can represent subsets of data in either streaming data as a representation of distance or data as a representation of elapsed time.

The captured signal streams S1to S6are applied to an algorithm10which provides data to an intrusion detection system11for carrying out a frequency and/or time dependent analysis on each block of the streams to create at least one coefficient dependent on the data values, comparing the coefficients thus generated in the intrusion check11with a coefficient value such as a threshold and in response to said comparing generating an output12indicative of a detection of an intrusion event.

The algorithm can use known systems such as Fence Detect, Smart Filter Detection (SFD), or Intrusion Signature (IS) as identified above.

The algorithm is therefore used on the data vertically up the waterfall. This allows for zooming in on a specific location on the fiber or the object being monitored of any desired selected width and treat that data stream as though it were the solitary reading over time of a zone system. The nature of DAS contains a great dal of randomness and noise on a signal. Averaging of this signal at a zone or block of the streams of data on the waterfall can reduce the randomness while preserving the true signal.

Dependent upon the processing capabilities of the system, it might be advantageous or necessary to utilize time division multiplexing to scan from block to block to perform this detection analysis. This is of course less desirable than monitoring and detecting the blocks within all portions simultaneously but processing capability restrictions may require this to be adopted.

In the output, the horizontal axis of the waterfall represents signal verses distance. That is, left to right indicates distance from some origin to a linear sensor extending to a location or along the fiber. This can be divided or a sample used to act as a “zone” of interest. For example, in a 2 km installation it is possible that only the section of a gate, for example, from location spanning 1.2-1.3 km. It is possible to isolate just that portion for analysis.

The vertical axis indicates signal at each of these locations as it changes over time thus forming the streams S1to SN. Analyzing the vertical axis allows analysis of zones as small as the sampling rate of the interrogator, or as large as the entire span. These widths can be defined for areas such as a gate, and can be dynamically altered.

In the zone sensor systems, the aggregate data from the entire length of sensor is streamed into the detection algorithms over time, representing change over elapsed time without consideration of location along the sensor.

As shown inFIG.2, the receiver4is divided into or includes sections4A,4B and4C where section4C acts to receive and analyze the signal emitted from the fiber to extract the required components, provide suitable filtering and to generate the required output. At4A, the data is divided into streams where each stream is associated with a respective location on the fiber. At4B the data output om each steam is converted into a stream of digital data or values. Arrangements for these functions are well known and commercially available.

At step14, a selection is made of certain streams to be formed into a block of streams. As above, typically there is a plurality of blocks of the streams where each contains a plurality of streams. However the number of blocks can be smaller or larger and the number of streams can be larger or smaller. The blocks can have different number of streams depending on the location on the fiber. The selection step14can be carried out at installation depending on the geometry of the installation or can be carried out dynamically by changes detected during the analysis.

In step14for each block, the data values of the plurality of streams in the block are collated to form a single stream of data for the algorithm. This is typically done by averaging the data horizontally but other collations methods may be used.

The number of streams in each block is selected to select desired sections of the length of the optical fiber and the number of streams in each block is variable. In some cases at least one block is selected so as to monitor an entire length of the optical fiber or a portion thereof.

A width of the blocks defined by the number of streams therein, that is the number of streams in each block, is dynamically changed for example in response to changes in environment.

Thus the number of streams in at least one block is different from the number of streams in at least one other block and can be selected at installation or changed dynamically during operation. Thus the number of streams each block is varied depending on detected changes on the object to be monitored and/or changes in the environment at the object which can be different from different positions of the object.

Each block contains the streams of signals at each of these locations along the fiber as it changes over time and wherein analyzing the streams allows analysis of zones as small as the sampling rate of the interrogator, or as large as the entire span.

The streams in each block can comprise raw data from the received signals or the streams are pre-processed such as by filtering or averaging.

In step16the algorithm is applied to the selected block or to each block independently of other blocks and the data from the algorithms is used to check for intrusions at step11providing the output12.

As shown inFIG.3in one example of an algorithm, in a Short Time Fourier Transform step20, the sequence of digital samples from each block shown inFIG.2at step15is converted into a sequence of Fourier Transform coefficients. The incoming signal is first converted into a sequence of fixed-sized temporal sections. The temporal sectionjs are of fixed or constant length. Each fixed sized block of samples has the Fourier Transform applied to generate the Fourier transform coefficients shown inFIGS.4A to4D.

An intrusion event is sensed at step11by comparing the Fourier Transform coefficients against a series of Envelope coefficients. The comparisons are shown inFIGS.4B and4Cwhere the transform coefficient is shown at “signal” and the envelope with which is it compared is shown at “envelope”. The Envelope is a block of numbers or coefficients where the block is the same size as the Fourier Transform and where corresponding or associated coefficients in the Envelope and Fourier Transform are compared.

As shown at step11, an intrusion is sensed if one or more Fourier Transform coefficients exceeds its corresponding Envelope coefficient by a predetermined threshold which is set as a hard value in the programming or may be user configurable. If adjustable, the overall sensitivity of the system can be controlled by adjusting the threshold.

If an intrusion event is sensed by the comparison as shown inFIG.4C, no further manipulation of the envelope coefficients is performed. That is as shown at link12A inFIG.3where the indication of the intrusion event is communicated to the envelope coefficient management system described below so as to prevent further modification of the envelope values.

Any sensed intrusions are reported to the user along a link12thus bypassing the envelope management system described below.

The intrusion check system11may wait (not shown) after detection of an intrusion event for a short period of time to give time for further intrusion events to be detected thus allowing the system to absorb subsequent intrusion sense events into a single reported event.

The management of the Envelope coefficients in order to automatically change a sensitivity of the analysis to accommodate environmental noise on the fiber is shown by the steps17,18and19:

Thus the system can desensitize itself to accommodate increasing environmental noise conditions such as wind.

In step17and as shown inFIG.4B, if the Fourier Transform coefficient (signal) is greater than the envelope, but the difference is less than the threshold which would trigger an intrusion event detection, the envelope coefficient is changed to become less sensitive. That is for each corresponding coefficient in the Envelope and Fourier Transform, the Envelope coefficient is changed, as shown inFIG.4D, to adopt the larger value where the larger value is equal to the actual difference which was detected inFIG.4B.

As part of the same envelope management, the system makes itself more sensitive to accommodate decreasing environmental noise conditions such as the waning of wind. That is, as shown inFIG.4E, at each cycle of operation defined by analysis of the next block of data from the selected block of the signal, each coefficient in the set of the Envelope coefficients shrinks or is decayed and thus slowly becomes more sensitive over time on a step-by-step basis after each cycle. In other words, each coefficient in the Envelope is reduced by a small amount (Decay). The decay value can be a hard programmed value or may be user configurable. It will of course be appreciated that a change in the Decay value can be used to make the system become more sensitive faster or slower. As stated above this can be selected at an installation to best suit the system being monitored.

However to prevent the system from becoming too sensitive to avoid false alarms from small events, for each Envelope coefficient that falls below a present floor value, that coefficient is replaced with the floor value. In this way the envelope coefficients are gradually and repeatedly decayed for each cycle until they reach a pre-set floor value whereupon the floor value is held. In this way, small events can be discarded and do not trigger an intrusion event detection. Such small events can include vibration and movement from small rodents, thermal expansion, and impact from small objects including raindrops, small hail, snow, small flying debris, etc.

The floor value may be pre-set or may be user configurable. A larger floor value makes the system less sensitive to small events.

The balanced effects of the increase in the envelope value after a comparison and desensitization action at step17and after the gradual decay or decrease in the value at step18and the floor value control at step19thus act to provide new values or coefficients which are communicated to the intrusion check step11.

In accordance with one important feature the system is arranged so that changing of the envelope coefficients to increase the envelope coefficients to a larger value is delayed by a time of a plurality of cycles. This can be done in a first in first out buffer (FIFO) which acts as a buffer and holds each value for a number of cycles or a set period of time. Thus for example the system may be run at a rate of 10 cycles per second and the FIFO acts as a delay of 2 or 3 seconds so that the delay can be as much as 20 cycles. The purpose of this is to prevent intrusions with a slow start from desensitizing the system. For example, a person getting ready to climb a fence may wiggle it gently such that it does not trigger an alarm, but is enough to desensitize the system which could cause the immediately following actual intrusion event to be missed. Thus the FIFO buffer acts to delays desensitization steps to ensure that there was no intrusion associated with it. If an intrusion is detected then all desensitization steps awaiting in or stored in the FIFO buffer are cancelled and not applied to the envelope.

As shown at step17, the changing of the envelope coefficients to increase the envelope coefficients to a larger value is delayed by a time of a plurality of cycles by use of the FIFO described above.

The arrangement herein thus acts to monitor the entire distance or portion thereof as though it was one or more zones of a zone systems. The signal is fed over time to the detection algorithm. The system also monitors a block of the signal over time in the so-called waterfall. This signal can be the entire width, or one or more blocks of any size.

The width of the above blocks can be dynamically changed, for example widened in the event of rain,

Each location is treated as a zone and the detection algorithm is applied to each. This creates a near zero-tune DAS system as each location is independently monitored. Applying the above zone principles and algorithms to the DAS system also provides a high level of nuisance alarm and false alarm rejection.

Turning now to the examples of data shown in the images ofFIGS.5to9,

FIG.5is an image showing data from a DAS system as a DAS waterfall where the top portion displays the disturbance amplitude as a function of linear distance from left to right in real time, and the lower portion displays a rolling representation of the instantaneous levels over time in what is called a waterfall. The peak values of the instantaneous trace from the upper half of this display will be “written” as the highest line in the lower display. As each line is written, all traces below it decrement one position. Shown in real time, the lower trace flows like a waterfall, and trends are easily seen. For example, inFIG.5the start of a large dark area in the right-most third of the waterfall indicates a disturbance has initiated. The display went from quiet (light) to active (dark).

FIG.6is an illustration of the data set acquired for applying the detection algorithm across the entire length as one zone for a single sweep in real time. As above, the upper trace displays the real time instantaneous trace of amplitude of the signal along the sensing fiber. One method is to average all of the peak values within the box shown and assign a single value to it. These can then be fed in succession into the detection algorithms.

FIG.7is an illustration of the data set acquired for applying the detection algorithm across a finite length of one zone or area indicative of a single block in real time. This illustrates the portion of signal to be analyzed if one were to assert the method shown inFIG.6across a selected portion or zone of the sensing fiber.

FIG.8is an illustration of the data set acquired for applying the detection algorithm across multiple zones simultaneously. In the lower trace, the boxes indicate individual portions or zones which can be treated independently for analysis. In the shown example, moving from left to right is found first a narrow zone with no activity apparent. This could illustrate a narrow section, such as a gate or lock-box, which is monitored specifically and is currently not in alarm. Moving to the right is a wider path blocked off. This might indicate a portion of fence that will be monitored without need to localize a disturbance within it. An example might be a fence with susceptibility to wind, but is within view of personnel or other monitoring. Moving farther to the right is another blocked area, monitored concurrently with but independent from the other blocks described above. In this section, there is clearly a disturbance of some sort which will be detected while the other zones are not affected.

FIG.9is an illustration of the data set acquired for applying the detection algorithm across a finite length of one zone or area indicative of a single location. In this illustration, a narrow portion of the whole is monitored for disturbance. This might be a door or a network lock-box that is at a specific location and requires specific monitoring. In this illustration, only that narrow portion is shown to be evaluated.