There are thousands of miles of security fencing installed throughout the world today. The most common security fence structure consists of multiple strands of barbed or woven wire strung horizontally and secured to a series of vertical posts. An extension arm is often rigidly attached to the top of each vertical post. Extension arms are normally inclined towards the threat side of the fence and away from the area to be secured, at 45 degrees upwards from the fence post. These extension arms are used to secure additional strands of barrier material. The additional barrier material is strung horizontally and attached to a series of such extension arms.
Extension arms must be rigid and durable to be effective. By securing additional barrier material to the top of the security fence, they can be used to extend the height of the security structure. This is meant to make the security fence more difficult to scale.
The majority of improper intrusions into a site protected by security fencing are accomplished by climbing over the barrier structure rather than by cutting through the security fabric. A principal reason for this is that security fences are seldom alarmed and can be relatively easily climbed over. In fact, the extension arms of the typical woven wire security fence are often viewed as key support means for climbing by an intruder, rather than as an impediment. And the additional barrier material attached to the extension arm can be made ineffective by simply throwing a rugged fabric, such as blankets or carpeting, over the top of it. By such methods, a thoughtful intruder can transform the extension arm from an impediment into an asset.
It has long been recognized in the art that the timely detection of an intrusion into an area protected by security fencing is of great value. The prior art directed towards this end can often be classified as being of either mechanical or electronics means.
A fence length of 100 meters has been widely accepted by the international security industry as a standard zone length. Much prior art has therefore focused upon the task of providing reliable intrusion detection along this entire standard length while employing the fewest feasible number of detectors. This criterion for development has resulted in several designs and inventions which utilize both electrical and mechanical methodologies.
Conventional intrusion detection systems which rely upon electronic sensing techniques are typically susceptible to false alarms, and also require significant maintenance activity. The fence fabric itself must be kept taut, and the electronic sensing components must be readjusted on a constant basis.
One approach in electronic detection of intrusion consists of monitoring the motion of the fence fabric. It is an assumption of this method that any attempt to climb over or cut through a security fence will result in significant, and detectable, motion of the security fence material. As it is anticipated that the vertical posts supporting the fence fabric will exhibit far less motion than the barrier material during an intrusion, the electronic detectors are typically designed to be mounted directly onto the security fence fabric.
U.S. Pat. No. 4,365,239 issued on Dec. 21, 1982 to Ronald W. Mongeon, describes a detection system of this kind. The Mongeon invention employs microphonic coaxial cable stretched longitudinally along a fence as the sensing element. Electrical noise is generated in proportion to movement of the security fabric. This noise characteristic is monitored and analyzed with the hope of distinguishing movement caused routinely by wind, rain and other innocuous sources, from movement caused by intrusion efforts. This type of system requires constant maintenance on both the fence fabric and the electronics monitoring system, as the tension exhibited by the security fence fabric changes over time. The actual result is the generation of a large number of false alarms.
This prior art detection system is not installed along the barrier material attached to the extension arms. The extension arms can still be used by an intruder as a support in climbing, and the barrier material of the extension arms can still be overcome through the employment of a carpet or a blanket. An intrusion by such means would be accomplished without detection, as long as the intruder does not grasp, kick or cause excessive motion of the security fence fabric which is directly attached to the vertical posts.
An alternate electronic method of intrusion detection can be seen on the separate inventions of Kalmus (U.S. Pat. No. 3,237,105, issued Feb. 22, 1966) and Geiszler and Mongeon (U.S. Pat. No. 4,064,499, issued Dec. 20, 1977). Both of these systems employ long electrical wires suspended from fence posts. These long wires are used to create a quasi-stationary electrical field. A receiver is employed to monitor the strength of this field. It is expected that the physical presence and movement of an intruder within the protected area will result in a measurable perturbation of the imposed electrical field.
In practice, systems of this type evidence high false alarm rates due, in part, to the sensitivity of the detection method. Slight movements of the fence fabric, and hence the long wires, significantly vary the state of the imposed electrical field. Systems of this type also require constant maintenance for the adjustment of the tension of the long wires.
A third intrusion detection method employing electronic means can been seen in an invention by Ciordinik et al. (U.S. Pat. No. 4,680,573, issued Jul. 14, 1987). This method requires the installation of a wire component along the length of the barrier material of a security fence. This wire component includes electrically or optically conductive lines. These lines are positioned so as to be broken when subjected to excessive stress. Such breakage of the conducting line would be detected by an electrical or optical sensing monitor.
It is the apparent hope of the inventors that the conductive wire will break during an intrusion and seldom by any other innocuous cause. This method requires the employment of very special material that must be replaced after each incidence of breakage regardless of cause.
Mechanical design approaches have also been used in intrusion detection systems. One common mechanical apparatus requires the installation of long segments of trip wires along the length of the fence. Each strand of trip wire is firmly anchored at both ends of a detection zone and a tension sensing device is positioned towards the center of the detection zone. It is expected that an intruder will deflect a trip wire and thus be detected by the tension sensing device.
The trip wires must be constantly maintained in a high tension state. As a consequence the vertical support posts of the fence fabric to which the ends of the trip wires are usually anchored must be extremely massive and sturdy. This requirement can significantly increase the cost of construction and maintenance of the entire fence structure. The extension arms of more massive posts provide even more support to an intruder in climbing over a fence.
This system is prone to false alarms from several sources. For one, the trip wires are made from ferrous materials so as to be indistinguishable from the barrier material of the fence. As ferrous matter has a high coefficient of thermal expansion and the trip wires are typically exposed to daily and seasonal temperature variations the tension evidenced by the trip wires will change without external cause. This change in tension can by itself cause a tension sensing device to generate an alarm signal.
Additional sources of false alarms would include wind rain and landings by birds.
This type of mechanical detection of intrusion is commonly known as Taut Wire Fencing and is covered by a number of U.S. Patents. U.S. Pat. No. 4,533,906 issued on Aug. 6, 1985 to Yoel Amir covers the construction of Taut Wire Fencing. Tension sensing detectors are described in U.S. Pat. No. 3,634,638 issued on Jun. 11, 1972 to Ori Even-Tov and in U.S. Pat. No. 4,683,356 issued on Jul. 28, 1987 to Aric Stoler. A fiber optic variation of a Taut Wire Fencing detector is disclosed in U.S. Pat. No. 4,829,286 issued to Dank Zvi on May 9, 1989.
Taut Wire Fencing is also cited in a patent issued to Slaats et al (U.S. Pat. No. 4,155,083 issued on May 15, 1979). This patent describes the use of a composite wire of electrical coaxial configuration as a kind of trip wire. It is desired that the coaxial wire will be twisted or broken by an intruder so as to alter the electrical parameters (e.g. resistance or capacitance) of the effected segment of wire. Slaats et al points out that cutting the coaxial wire will also signal an intrusion event. This feature is presented as a significant improvement over previous inventions.
In all known Taut Wire Fencing methods the force needed to produce an alarm signal increases substantially as the distance from the application of force to the detector increase. Taut Wire Fencing is therefore mostly used in 50 meter segments as systems of this methodology become unreliable when used at 100 meter increments.