| Modern perimeter security systems.
Boris Sergeevich Vvedensky,
MODERN PERIMETER SECURITY SYSTEMS
(Continued. Beginning in No. 3, 1999) Introduction In the first part of the article we looked at some perimeter security systems using radio emission — radio beam and radio wave. This section describes security systems based on the use of optical (infrared) radiation, as well as so-called «capacitive» systems. 1. Infrared systems 1.1. Active beam IR systems Beam infrared systems (often also called linear active optical-electronic detectors) consist of a transmitter and a receiver located in the zone of direct mutual visibility. Such a sensor generates an alarm signal when the beam falling on the photoreceiving unit is interrupted. A distinctive feature of active beam systems is the ability to create a very narrow detection zone. In practice, the cross-section of the sensitive zone is determined by the size of the lenses used in the optical units. This is especially important for objects around which it is impossible to create an exclusion zone. However, like radio beam systems, IR beam systems can only be used on straight sections of perimeters or fences. The main problem with beam IR security devices is false alarms under unfavorable atmospheric conditions (rain, snowfall, fog), reducing the transparency of the environment. Reliability in such cases is ensured by multiple excess of beam energy over the minimum threshold value required for the sensor to operate. Direct illumination of the receiver by sunlight can also be a source of interference. This most often happens at sunset or sunrise, when the sun is low above the horizon. According to Russian standards, the sensor must remain operational at natural illumination of at least 10,000 lux and at least 500 lux from electric lighting fixtures. Most modern domestic and foreign beam sensors have special means of filtering background radiation and meet the above requirements. However, to ensure high noise immunity from illumination, it is very important to correctly align the sensor when setting it up and follow all the manufacturer's recommendations for installation. In addition, IR systems can be triggered by birds, leaves, tree branches, etc. falling into the beam. To increase the stability and reliability of IR beam systems, they are made multi-beam (usually 2 or 4 independent beams are used), and automatic signal processing circuits are also used to minimize the influence of the external environment. Special measures are taken to maintain the functionality of sensors in winter conditions, when there is a possibility of freezing or snow sticking to the optical surfaces of the units. Special visors on optical filters and internal heaters of optical-electronic units serve as fairly reliable methods of combating these phenomena. One of the most common domestic IR-beam security devices are the SPEK series detectors. SPEK-75 setcontains a transmitter unit, a photodetector unit and a mounting kit. The system provides an optical beam divergence angle of 3 degrees and allows for a single-beam security line up to 75 m long (outdoors). The transmitter generates in the near IR spectrum at a wavelength of 0.8…0.9 μm, an alarm signal is triggered when the beam is interrupted for a specified period of time. To ensure operation in adverse conditions (rain, snowfall, fog), the transmitter has a 100-fold reserve in radiation power. Measures have been taken to eliminate false alarms from solar illumination (10,000 lux). The electronic units are identical in design, they have dimensions of 140 x 145 x 65 mm. To adjust the system, you can use a voltmeter specially connected for this purpose. The nominal supply voltage is -12 V, the current consumption is no more than 60 mA. The operating temperature range is from -40O to +50OC. To organize a two-beam barrier, a second set of the detector is used. Modifications of the SPEK series for security zones up to 175 meters are also produced. A more advanced and powerful domestic IR beam detector is “Rubezh-3M”.The kit includes two pairs of receiving and transmitting units controlled by a common control unit. The device uses pulse modulation of IR radiation and synchronous reception, which increased the range and implemented parallel operation of several emitters in multi-beam barriers. The kit allows you to organize a two-beam security circuit on a section length of 300 m or two separate single-beam boundaries on a length of up to 600 m. Using two Rubezh-3M kits, you can also create a 4-beam barrier with increased noise immunity. The system is operational even in thick fog, when the meteorological visibility range is reduced to 180 m. The equipment generates an alarm signal if the beam is blocked for at least 100 milliseconds, which corresponds to a person moving at a speed of up to 5 meters per second (18 km/h). The Rubezh-3M system's emitter and photoreceiver units are housed in identical metal cases, mounted on swivel brackets. The overall dimensions of the unit (with bracket) are 275 x 190 x 120 mm. The units have heating devices inside, which ensures operation at temperatures down to -45 °C. In unfenced areas, the units are installed on special stands. The minimum recommended beam height above the ground is 0.3 m, which allows detecting a crawling intruder. If there are fences, the units are usually mounted along the top edge of the fence. Almost all foreign IR-beam security devices combine a dual-beam or four-beam synchronous system in a common housing. IR-beam sensors from C&K, Atsumi, Visonic, Optex, Alarmcom and others are widely represented on the Russian market. Photo 1 shows the design of one of the units of the dual-beam IR sensor of the AX-100/AX-200 series.by Optex (Japan). The design of the transmitter and receiver units is similar. The front cover is made of impact-resistant plastic, transparent only for IR radiation. The cover has a special protruding visor that prevents frost from settling on the outer surface. Under the cover is an electronic-optical unit with two lenses mounted on a rotary platform. The angular position of the platform is adjusted during adjustment within (+/- 90O) horizontally and (+/- 5O) vertically using screws. To facilitate adjustment, a special miniature viewfinder is built into the rotary platform, allowing you to accurately point the lenses at the second unit of the system. For precise adjustment of the system according to the level of the received signal, the receiver unit has sockets for connecting a voltmeter. The sensor response time regulator and LED indicators (“rough adjustment” and “alarm”) used when adjusting the device are also located here. The optical-electronic unit is fixed on a mounting plate, which is usually attached to a vertical rod using a clamp. The beam overlap time regulator installed in the receiver unit allows changing the response time from 500 milliseconds (relatively slow climbing over a fence) to 50 milliseconds (a very fast running person). It is usually recommended to set the beam intersection time to no more than 70-100 ms to ensure sufficient sensitivity of the system. OPTEX AX series sensors provide a detection range of 22 to 150 meters outdoors and 40 to 300 meters indoors. A DC source with a voltage of 10.5…28 V is used for power supply, the consumed current is no more than 46 mA, the operating temperature range is from -35O to +55OC with humidity up to 95%.
For objects with a high degree of protection, IR beam systems with a number of beams from 4 to 8 are sometimes used. Among such multi-beam systems, one can mention the IPS 600 sensor by GPS (Italy), the IS 400 series sensors by Alarmcom (Switzerland) or the IPID series sensors by ECSI (USA). Structurally, multi-beam IR sensors are usually made in the form of vertical rods up to 3.5 meters high. Multi-beam systems are most often used to protect military facilities, nuclear power facilities, and large industrial enterprises. 1.2. Passive IR Systems Such “single-position” systems are passive IR detectors with a spatial sensitivity diagram in the form of a beam. They are easier to install and configure than two-position IR beam systems and are used mainly where it is necessary to block short sections of the perimeter — vehicle entry zones, gaps in fences, gates, window openings, etc. Such sensors are characterized by a larger cross-section of the sensitive zone than beam optical sensors. Passive IR barriers IS 402 and IS 412 from Alarmcom (Switzerland) are designed for outdoor use in difficult atmospheric conditions. Sensor IS 402(Photo 2) is made in a durable aluminum case with a visor protecting from solar glare. The IS 402 sensor forms a sensitivity zone in the form of a “curtain” 100 m long and up to 4 m high. The IS 412 sensor has increased sensitivity and provides a zone 150 m long.
Single-position passive IR sensors for perimeter protection are manufactured by the English company Security Enclosures Ltd (SEL). In open space, the Redwall-100Q sensor, using the “quadruplex (four-channel) detection technology, provides a sensitivity zone of 100 m in length and a cross-section of 3 m. The improved two-section Megared-180Q sensor (photo 3a) allows you to protect a zone up to 180 m long. One of the sensor sections is designed for detection in the “near” zone, and the other — in the “far” zone. Signals from the sensor sections can be used, for example, to control a PTZ camera. One of the modifications of the SEL detector, the combined Redwatch-100Q sensor, combines a passive IR sensor and a built-in miniature video camera, the field of view of which coincides with the sensitive zone of the IR sensor (photo 3b). The ability to quickly visually check the situation in the “alarm” zone greatly increases the overall efficiency of security.
To increase resistance to external factors and reduce the frequency of false alarms, perimeter IR detectors are sometimes structurally combined with microwave sensors. An example of such a combined device (sometimes called dual technology sensors) is the DT-900 series detector from C&K (photo 4). Two detection channels — passive infrared and radio wave — provide high detection capability with good resistance to interference. The sensor is equipped with a triple self-diagnostic system; it has a special active optical sensor that signals an attempt to intentionally block the device by blocking the sensitive zone. A microprocessor with event memory allows you to select the optimal intrusion detection algorythm in various environmental conditions. Depending on the focusing optics used, the sensor's range is 37 m (zone cross-section 3 m) or 61 m (cross-section 5 m).
2. Fiber optic systems Fiber optic cables, which are usually used for transmitting information, can also be used as sensors for perimeter security systems. Deformation of a fiber optic cable changes its optical parameters (refractive index, etc.) and, as a consequence, the characteristics of the laser radiation transmitted through the fiber. Due to the specificity of the physical principles used, fiber optic systems are characterized by very low susceptibility to any electromagnetic interference, which allows them to be used in unfavorable electrophysical conditions. Fiber optic cables exhibit several physical effects that allow them to be used as perimeter sensors. In all cases, a miniature semiconductor laser generating coherent radiation is connected to one end of the cable. The opposite end of the cable is connected to a photodiode (receiver), which converts the optical signal into an electrical signal. The analyzer compares the received signal with the reference signal, which corresponds to the undisturbed state of the sensor, and detects external effects on the perimeter (displacement, vibration, or compression of the cable). The Model M106E security system from Fiber SenSys (USA) uses the method of recording intermode interference. The laser emits several dozen modes (spectral lines) close in frequency with a certain energy distribution over the spectrum. If the fiber optic cable is subject to mechanical impacts, then the spectrum of radiation recorded by the receiver at its output changes, which allows detecting cable deformations. The fiber optic system of the Sabreline company (USA) uses the effect of changing the radiation distribution over the cross section when the fiber is deformed. At the output of the multimode optical fiber, a so-called speckle structure is observed, which is an irregular system of light and dark spots. Spatially sensitive photodetectors are used here to detect cable deformations. The FOIDS series fiber optic systems (manufactured by Mason & Hanger, USA) use the principle of dual-beam interferometry. The laser beam is split into two and sent into two identical single-mode optical cables, one of which is the detecting cable and the other is the reference. At the receiving end, both beams form an interference pattern. Mechanical effects on the detecting cable lead to changes in the interference pattern, which are recorded by a photodetector. An interesting feature of fiber optic systems is the possibility of their use to protect not only fences, but also unfenced areas. In the latter case, the fiber is placed under the ground surface, in a ditch filled with gravel. At the same time, as tests at Sandia National Laboratories (USA) have shown, the system is capable of recording the steps of a walking or running person. Among domestic developments of fiber-optic perimeter systems, the Voron system can be noted. The system is based on serially produced detectors consisting of two sealed units of a laser transmitter and a photoreceiver. Between these units there is a sensitive element — a special fiber-optic cable. Signal processing is carried out using an analyzer or a special learning processor using the principles of artificial intelligence. The processor is trained after installation at a specific site with the imitation of real intrusion signals. The limitations of the use of fiber-optic systems include the complexity of the procedure for splicing and repairing cables in the field (requires the use of a microscope and an expensive device for welding fibers). The experience of practical use of fiber-optic perimeter systems is relatively small, but the potential tactical and technical characteristics of such devices in terms of immunity to electromagnetic interference are of serious interest. 3. Capacitive perimeter security systems The capacitive system sensor is one or more metal electrodes mounted on insulators along the fence, and is, in fact, an antenna system. Such a system is often made in the form of a metal canopy and is installed using special posts and insulators on an existing fence. They are most effective at sites equipped with strong rigid fences (reinforced concrete slabs, brick walls, welded metal panels, etc.). Figure 1 shows the design of the antenna system of a capacitive sensor in the form of a decorative metal lattice mounted on a concrete wall. All sections of the lattice are connected into a common electrical circuit and are isolated from the main fence. The antenna system is connected to an electronic unit that generates an electrical signal and measures the capacity of the antenna system. When a person approaches the electrodes or touches them, the capacity of the antenna system changes, which is registered by the electronic unit, which issues an alarm signal.
The configuration of the detection zone is determined by the method of mounting the electrodes. When the main electrode is installed along the upper end of the fence, the system effectively registers only attempts to climb over. If the electrodes are mounted along the middle line of the fence, the system is triggered when the intruder approaches the perimeter. The most widely used domestic perimeter security devices using the capacitive detection method are the Radian series devices. The Radian-M and Radian-13 systems are designed to protect metal fences and canopies in the form of a mesh, lattice or wire barrier. The detection zone length is from 10 to 500 m; the delivery set includes an electronic unit and special insulators for attaching the canopy. The electronic unit has dimensions of 326 x 207 x 540 mm; it is powered by a DC source with a voltage of 20 — 31 V, the consumed current is 70 mA, the operating temperature range is from -50O to +50OC. The system is provided with lightning protection measures. An improved modification of the devices of this series is the Radian-14 system. It is distinguished by the use of a two-channel signal processing algorithm with the analysis of the active and reactive components of the signal. This allows to tune out interference created by precipitation and reduce the probability of false alarms from pulsed radio interference. In addition, the system provides for a simpler fastening of sensitive electrodes on the fence. The designers refused special insulators-adapters and the conductors of the antenna system are attached to plastic brackets mounted directly on the fence. A typical sensitive element consists of three parallel conductors forming a barrier about 0.8 m high above the main fence. The delivery set of the Radian-14 system includes an electronic unit, plastic brackets, antenna system wire and a mounting kit. The length of the protected area is up to 500 m, the range of operating temperatures is from -50O to +50OC. The electronic unit has dimensions of 320 x 223 x 95 mm; supply voltage 20 — 30 V, power consumption 0.5 W. One of the most well-known foreign capacitive security devices is the “E-Field” system by Senstar-Stellar (USA). The sensor of such a system is a structure of 3 conductors mounted on brackets that are attached to a fence (roof) or installed around open unfenced areas. The central transmitting electrode of the antenna system is connected to the signal source, and the two side electrodes are connected to the analyzer (single-zone or dual-zone). Both the generator and the analyzer are mounted in a common housing. When an intruder enters the detection zone, the analyzer monitors changes in signals and, if the specified activity threshold is exceeded, issues an alarm signal. The E-Field system analyzer evaluates the signal based on three characteristics:
An alarm is generated when all three factors are present at the same time, which ensures a very low probability of false alarms. Fig. 2 shows a typical configuration of the E-Field three-wire antenna system and a cross-section of the sensitive zone. The E-Field system can be effectively used to detect the destruction of a fence or climbing over it, as well as to detect undermining or an intruder approaching the perimeter line.
Capacitive perimeter systems are very versatile and attractive due to their insensitivity to uneven soil profile or fence lines. Domestic capacitive security systems are generally quite reliable and have been widely used at various sites over the past 20 — 30 years. Conclusion The wide variety of perimeter security systems does not allow us to cover them all in one article. In the third, final section of this publication, we will consider various vibration-sensitive systems on sensor cables and discrete seismic sensors. Some contact-breaking perimeter detectors and «active» territory security systems will also be briefly described. |
