#alarm
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Sensors are the main component of the security alarm system and largely determine the reliability of the entire system. An analysis of the range of security alarm sensors offered by the largest manufacturers of security alarm systems, as well as those presented at recent exhibitions, shows that the most popular sensors for protecting premises are passive IR, combined (mainly IR + microwave), various modifications of contact (primarily magnetic contact) and acoustic glass breakage sensors. CONTENT
Microwave, ultrasonic active and inertial impact sensors are also used, although to a much lesser extent. Other detection means are practically not used in the protection of premises, and in general there is a tendency to reduce the range of sensors by the operating principles used. The main trends in the construction of modern sensors are:
Let's consider the principles of operation, nomenclature and application features of the most popular security alarm sensors. INFRARED PASSIVE SENSORSPassive infrared sensors react to thermal radiation of a moving person in the wavelength range of 8-14 microns. Currently, passive infrared sensors are the most popular, they are an integral part of the security system of almost every facility. These sensors are produced by all leading manufacturers of alarm equipment: PYRONIX (England), ARITECH (Europe), C&K (USA), PARADOX (Canada), ADEMCO (USA), DSC (Canada), CROW (Israel), VISONIC (Israel), SCANTRONIC (England), etc. IR passive sensors consist of three main elements:
Most IR passive sensors use Fresnel lenses as their optical system. The advantages of Fresnel lenses include:
Modern passive IR sensors are equipped with a large number of removable Fresnel lenses to provide a variety of directional patterns and greater versatility when equipping rooms with a wide variety of configurations. By replacing the lens, it is possible to obtain a detector sensitivity zone in the form of a single solid curtain or in the form of multi-fan curtains, while it is possible to change the sensitivity zone length (from 6 m to 50 m), the viewing angle (from 5O to 180O, for ceiling sensors 360O) and the angle of inclination of each curtain, the number of beams (from 2 to 64), it is possible to form a sensitivity zone of a special shape, for example, with an insensitivity zone (alley) for domestic animals near the floor, etc. Traditional Fresnel lenses used in cheap IR sensors have uneven sensitivity over distance. This is due to the fact that each curtain of the sensitivity zone has a fixed viewing angle in the vertical plane, which means that the height of this sensitivity zone increases with distance from the sensor. Since the signal at the output of the sensitive element is proportional to the degree of overlap of the sensitivity zone by the intruder, then in traditional detectors, signals from a person at 20 m, or from a mouse at 1 m, or from an insect on the surface of the lens may be close in magnitude. On the other hand, signals from a person at a great distance will be significantly lower than signals from a person at a short distance. Recently, IR sensors with special, more complex lenses have appeared, in which this drawback is partially eliminated. For example, lenses from the company PARADOXhave a more complex precise geometry, which gives a 30% increase in the collected energy compared to standard lenses and, accordingly, an increase in the level of useful signal from a person at large distances. The material from which these lenses are made provides protection of the pyroelectric receiver from white light. Another way to combat the phenomenon of uneven sensitivity is to use mirror optics, widely used, for example, by the company ARITECH. In this case, instead of Fresnel lenses or in addition to them, several mirrors (usually 3) of a special shape and a specially designed pyroelectric receiver are installed in the IR sensors. The total length of the sensitivity zone is divided into several zones, each of which is controlled using its own mirror (step focusing). Due to this, practically constant sensitivity by distance is ensured, and this sensitivity at long distances is approximately 60% higher than for Fresnel lenses. Mirror optics also provide protection for the near zone located directly under the sensor installation site. Similar to replaceable Fresnel lenses, IR sensors with mirror optics are equipped with replaceable detachable mirror masks, making it possible to adapt the sensor to various configurations of the protected premises. Poor performance of an IR sensor may be caused by effects such as heat flows resulting from heating of the sensor's electrical components, insects getting onto sensitive pyrodetectors, and possible infrared radiation reflections from the detector's internal parts. To eliminate these effects, the latest generation of IR sensors use a special sealed chamber between the lens and the pyrodetector (sealed optics), for example, in the new IR sensors from PYRONIX and C&K. The IR sensors use a highly sensitive semiconductor pyrodetector (usually made of lithium tantalate) as a sensitive element, capable of detecting a difference of several tenths of a degree between the human body temperature and the «background» temperature. The temperature change is converted into an electrical signal, which, after appropriate processing, triggers an alarm. IR sensors usually use dual pyroelements. In the latest models, quadruple pyroelements (type QUAD) are used in order to reduce the frequency of false alarms — these are two dual pyroreceivers located in one sensor (usually placed one above the other). The logic of joint processing of signals from these two pyro detectors is based on the fact that the observation radii of these pyro detectors are made different, and therefore the thermal source of false alarms will not be observed in both pyro detectors simultaneously. In this case, the geometry of the pyro receiver placement and the circuit for their connection are selected in such a way that the signals from a person are of opposite polarity, and electromagnetic interference causes interference signals in two channels of the same polarity, which leads to the suppression of this type of interference. Protection from electromagnetic and radio interference is also ensured by dense surface mounting and metal shielding (field strength up to 20-50 V/m in the range up to 1000 MHz). The pyro receiver signal processing unit reduces the false alarm rate and increases the detection capability of the IR sensor. Simple passive IR sensors use analog processing methods, which may include filtering in the useful signal band (in the range of approximately 0.6-10 Hz), pulse counting, threshold processing, etc. Modern IR sensors increasingly use digital processing methods using specialized microcontrollers with ADCs and signal processors, which allows for detailed processing of the fine structure of the signal to better distinguish it from the background noise. Thermal compensation circuits are used to ensure operability in the high temperature range (33OC-37OC), when the signal caused by human movement is sharply reduced by reducing the thermal contrast between the human body and the «background». In IR sensors intended for professional use, so-called anti-masking circuits are used. The essence of the problem is that a conventional IR sensor can be disabled by an intruder by preliminary (when the system is not armed) sealing or painting over the sensor's input window. To combat this method of bypassing IR sensors, anti-masking schemes are used. One of the most effective methods is implemented in the Mercury series of IR sensors from ARITECH. The method is based on the use of a special IR radiation channel that is triggered when a mask or reflective barrier appears at a short distance from the sensor (from 3 to 30 cm). The anti-masking scheme operates continuously while the system is disarmed. When the fact of masking is detected by a special detector, a signal about this is sent from the sensor to the control panel, which, however, does not generate an alarm signal until the time comes to arm the system. It is at this moment that the operator will be given information about masking. Moreover, if this masking was accidental (a large insect, the appearance of a large object for some time near the sensor, etc.) and by the time the alarm was set, it has disappeared, the alarm signal is not generated. Another protective element that almost all modern IR detectors are equipped with is a contact tamper sensor that signals an attempt to open or break into the sensor housing. Let's take a closer look at the capabilities and characteristics of IR sensors using products from well-known companies as an example. ARITECH. The sensors of this company use precision mirror optics that provide uniform sensitivity throughout the protected area (step focusing). Replaceable mirror masks make it possible to form various directional patterns. The temperature compensation circuit and automatic sensitivity adjustment ensure stable and reliable detection in the entire temperature range from -18°C to +50°C. When processing signals from pyro receivers, 3D or 4D analysis is used, which in the company's terminology means the following. During 3D processing, the presence of the following signal parameters characteristic of human movement in the protection zone is analyzed:
Signals that do not meet these criteria are ignored. For 4D— in addition to the analysis of the specified parameters, low-amplitude signal filtration is added to the processing. IR sensors with 4D processing are equipped with a built-in microprocessor that controls signal analysis. Some models of IR sensors from ARITECH are equipped with an autofocus system that reduces false alarms for small areas where conventional IR sensors give a high frequency of false alarms. An anti-masking system is used. In the most powerful EV-600 series, to further reduce false alarms, the ability to operate two or more IR sensors in a common protected area (Double Checker), including those directed at each other (Eye-To-Eye Checker), has been implemented. The main characteristics of IR sensors from ARITECH are given in the table.
Canadian PARADOX SECURITY SYSTEM produces two series of IR passive sensors: analog and microprocessor. These series are represented by both traditional technical solutions and new developments of the company. The lenses of the IR sensors have a complex precise geometry, which provides a 30% increase in the collected energy compared to standard lenses. The lenses used provide complete coverage of the space in the 110O sector at a distance of up to 14 m. Depending on the specific conditions of use, you can select one of 12 replaceable lenses. The IR sensors use automatic temperature compensation, which ensures the constancy of the sensor characteristics in the temperature range from -25 °C to +50 °C without loss in range and without increasing the likelihood of false alarms. The IR sensors use both dual and quadruple pyroelectric detectors, and for the quadruple configuration, special sensors of complex geometry have been developed, containing intertwined sensitive elements, which has made it possible to increase the range by two times compared to conventional quadruple pyroelectric detectors. The pyroelectric detector signal processing algorithm used in the PARADOX IR sensors has a feature associated with the fact that here, unlike traditional threshold methods, the energy of each detected signal is measured, stored in memory and accumulated. An alarm signal is issued if the accumulated energy exceeds a certain threshold level. For strong signals, the detector immediately issues an alarm signal, operating as a threshold, and for low-level signals, the detector automatically switches to pulse counting mode, which significantly reduces the likelihood of false alarms. The number of accumulated pulses depends on the energy level of the signals and can reach 25. The processing algorithm is practically identical for the analog and microprocessor series, the difference is only in the technical implementation — in the microprocessor series, processing is performed using a RISC processor. In its latest developments, the company PARADOX began to use an improved processing algorithm, which introduced Entry/Exit Analysis, the essence of which is as follows. When a person enters and exits the beam, the polarity of the signal changes to the opposite in each of the differential elements of the pyro receiver. Portions of energy corresponding to entering and exiting the beam are separately entered into memory and separately accumulated. In this case, an alarm signal is issued only if, over a certain period of time, the accumulated signals of both inputs and outputs exceed a certain threshold. This processing method allows for additional suppression of interference signals caused by the action of air heaters, fans, etc. The analog IR sensor AVANTAGE, which implements this method, is the most effective of the entire analog PARADOX series. SensorPARADOME is designed for ceiling mounting and has a very narrow flat curtain. Designed to protect windows, doors, display cases, paintings or other flat objects. Comparative characteristics of PARADOX passive IR sensors are given in the table.
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