Perimeter Detection Barrier System Demonstrates Excellent Performance
by Ian Macalindin

A jointly engineered solution to the problem of protecting perimeter walls and fences within highly secure areas demonstrates compliance with the stringent requirements of HM Prison Service in providing an effective delay factor combined with reliable electronic detection of escape attempts.

Geoquip Ltd. and Binns Fencing have worked together to produce the FST (Flexible Security Topping) solution which combines Binns extensive experience in prison fence construction and installation with Geoquip’s world-wide expertise in electronic intruder detection systems.

The FST solution comprises a mechanical barrier specifically designed to provide the maximum delay to escapees attempting to surmount the barrier. Integral with the mechanical barrier is Geoquip’s Defensor electronic detection system comprising Geoquip’s unique linear microphonic sensor cable deployed along the barrier and connected to a remotely located signal analyser system.

Key to the high performance of the FST solution is the way in which the mechanical properties of the barrier are optimally matched to the detection response of the sensor system.

The unique response of Geoquip’s ‘Alpha’ sensor cable means that maximum signal output, and therefore maximum inherent sensitivity, is produced only when mechanical vibrations which cause the sensor to be subjected to fast mechanical excursions occur. Such excursions may be caused, for example, by a mechanical impact on the barrier structure. The inherent response of the sensor cable is optimised exactly for such fast mechanical excursions.

Armed with this knowledge of the sensor response, the mechanical design of the barrier was optimised to ensure that, under normal non-escape conditions, such fast mechanical excursions cannot be generated by normal movement of the structure under, for example, severe weather conditions.

The design also ensures that during an escape attempt, the barrier experiences a large mechanical deflection caused by the weight of the escapee. Variations in load (caused by movement of the escapee) at this large deflection generates large numbers of fast mechanical excursions which ensure a very high probability of detection of such activity.

To achieve the appropriate mechanical response from the barrier structure for both attack and non-attack scenarios, the unique shape and mounting techniques employed within the barrier design are important design features. These design features take account of the fundamental requirement to delay the passage of the escapee for as long as possible. Tests carried out by special forces demonstrate that the barrier design provides the longest delay factor when compared with any system with similar application.

The mounting technique employed to attach the barrier to the fence or wall is deliberately designed to allow a high degree of flexibility of the barrier. This permits the large mechanical deflection of the barrier when the weight of an escapee is applied. Furthermore, movement of the escapee while the barrier is deflected in this way causes the elements of the barrier to generate sequences of micro-impacts as the barrier elements move relative to each other and absorb the energy of the moving escapee.

Micro-impacts are may be described as fast mechanical excursions - exactly those events for which the sensor cable response is optimised.

The sensor cable therefore easily detects the micro-impacts and passes the electronic signals resulting from these events to the analyser unit where they are filtered and digitally processed to ensure that their defining characteristics are indicative of hostile activity on the barrier.

Under non-escape conditions, the barrier construction ensures that micro-impacts are prevented from occurring by ensuring that the separate elements of the barrier are sufficiently well coupled mechanically to avoid micro-impact generation unless sufficient deflection of the barrier occurs due to the weight of an escapee.

The mechanical interconnection of the separate elements of the barrier is an important aspect of the installation process and the skill of the Binns installation team ensures that the barrier construction meets the design requirements in this respect.

The barrier structure, being flexibly attached to the wall or fence, will move naturally as a result of strong winds and other environmental disturbances. Such movements however are characterised by a completely different set of mechanical parameters when compared with the micro-impacts caused by an escape attempt.

For example, strong winds usually apply a relatively slowly changing mechanical stimulus to the barrier structure. While there may be an initial deflection of the barrier during steady wind conditions, the response of the sensor cable will largely ignore signals which do not exhibit rapidly changing characteristics.

Overlaid with the slowly changing mechanical movement of the fence is a band of higher frequencies resulting from airflow through the structure itself. Again, the characteristics of this band of frequencies show slow changes in amplitude and frequency shift which again do not fall within the optimal detection band of the sensor cable itself.

In electronic terms, the changes in signal levels and frequencies caused by gusting winds for example, are extremely slow and therefore easy to eliminate by the filtering and digital signal processing algorithms employed in the signal analyser unit.

When these techniques are combined with the inherently low sensitivity of the sensor itself to such signals, it becomes apparent why false alarms due to severe weather conditions are extremely low.

Additionally, it must be considered that the deflection of the barrier structure under even severe gale conditions is only a small fraction of that experienced when the weight of an escapee is applied to the barrier. Only under this relatively large deflection of the barrier is the system designed to produce the micro-impacts to which the response of the sensor cable and processing electronics is optimised.

Observations of the FST solution installed in prisons within the UK show significant movement of the barrier structure under severe weather conditions and it may be difficult for the lay person to appreciate why the system is stable under such conditions, yet provides high detection performance.

From actual information derived from alarm management systems within a number of UK prisons, the false alarm rate of the FST solution has been shown to be better than 1 / day / kilometre. Video verification of such alarms shows that a primary cause of such alarms can be attributed to windblown debris such as plastic bags being caught in the structure of the barrier.

A significant advantage of the detection system employed on the FST solution is that there is no requirement for power or data cables to be routed along the perimeter of the establishment.

The design of the sensor cable is such that each zone requires only a simple twisted pair of wires to route the sensor signal back to the analysis system located centrally within the control room of the establishment. The distance between the furthest zone and the control room can be upwards of 3km, which caters for the architecture and cable routing infra-structure of most high security establishments.

Furthermore, because all sensor signals are routed to a centralised analysis system, all system adjustments and tests can be made immediately by control room personnel. System reliability is enhanced because no electronic assemblies are exposed to extremes of weather and maintainability is simple given that all electronic assemblies are located centrally thereby eliminating the need for time consuming visits to the sterile areas of the establishment.

Electrical transients resulting from lightning discharges are heavily attenuated by the long cable lengths which connect the sensors themselves to the centralised analysis system, and as the analysis system is located at one point only, the cost of providing a suitable earth terminal for efficient operation of the integral lightning protection system is minimised compared with systems employing processing or communication electronics distributed around the perimeter of a site.