Infrared security detectors. Active and passive IR sensors: differences and features

home

Kinds

1.3.1. Passive optoelectronic infrared (IR) motion sensors

To create the system, I decided to select modules that would be suitable for creating the system and monitor the perimeter.

I chose the following components:

passive infrared motion sensor;
GSM module;
siren.

Let's take a closer look at them.

In the 21st century everyone is familiar with IR sensors– they open doors at airports and stores when you approach the door. They also detect movement and sound an alarm. burglar alarm.

Currently, passive electro-optical infrared (IR) detectors occupy a leading position when choosing to protect premises from unauthorized intrusion at security facilities. Aesthetic appearance, ease of installation, configuration and maintenance often give them priority over other detection means.

Passive optoelectronic infrared (IR) detectors(they are often called motion sensors or PIR sensors) detect the fact of human penetration into the protected (controlled) part of the space, generate an alarm signal and by opening the contacts of the executive relay (monitoring station relay) transmit the signal “ anxiety» to warning means.

Terminal devices (TD) of notification transmission systems (TPS) or a fire alarm control panel (PPKOP) can be used as warning means. In turn, the above-mentioned devices (CU or Control Panel) transmit the received alarm notification via various data transmission channels to the central monitoring station (CMS) or local security console.

Operating principle of passive optical-electronic IR detectors based on the perception of level changes infrared radiation temperature background, the sources of which are the body of a person or small animals, as well as all kinds of objects in their field of vision.

Sensor, sensitive to infrared radiation in the range of 5-15 microns, detects thermal radiation from the human body. It is in this range that the maximum radiation from bodies falls at a temperature of 20–40 degrees Celsius.

The hotter an object is, the more it emits.
infrared illumination spotlights for video cameras, beam (two-position) detectors " beam intersections"and TV remote controls operate in the wavelength range shorter than 1 micron, visible to humans the spectral region is in the region of 0.45–0.65 µm.

Passive sensors they are called this type because they themselves do not emit anything, they only perceive thermal radiation from the human body.

The problem is that any object at a temperature of even 0º C emits quite a lot in the IR range. Worse yet, the detector itself emits – its body and even the material of the sensitive element.

Therefore, the first such detectors worked if only the detector itself was cooled, say, to liquid nitrogen (-196º C). Such detectors are not very practical in Everyday life.

That is, it is important that radiation from a person is focused only on one of the sites, and moreover, it changes.

The detector works most reliably if the image of a person first hits one site, the signal from it becomes greater than from the second, and then the person moves so that his image now hits the second site and the signal from the second increases, and from the first decreases.

Such fairly rapid changes in the signal difference can be easily detected even against the background of a huge and variable signal caused by all other surrounding objects (and especially sunlight).

Rice. 1.

IN passive optical-electronic IR detectors infrared thermal radiation hits the Fresnel lens, after which it is focused on a sensitive pyroelectric element located on the optical axis of the lens.

Passive IR detectors receive streams of infrared energy from objects and are converted by a pyroelectric receiver into an electrical signal, which is supplied through an amplifier and a signal processing circuit to the input of the alarm notification generator ( rice. 1).

In order for an intruder to be detected by a passive IR sensor, the following conditions must be met:

the intruder must cross the beam of the sensor sensitivity zone in the transverse direction;
the offender’s movement must occur within a certain speed range;
The sensitivity of the sensor must be sufficient to register the difference in temperature between the surface of the intruder’s body (taking into account the influence of his clothing) and the background (walls, floor).

an optical system that forms the directional pattern of the sensor and determines the shape and type of the spatial sensitivity zone;
a pyro receiver that registers human thermal radiation;
signal processing unit of the pyro receiver, which separates signals caused by a moving person from the background of interference of natural and artificial origin.

Rice. 2.

Depending on the version Fresnel lenses Passive optical-electronic IR detectors have different geometric dimensions of the controlled space and can be either with a volumetric detection zone, or with a surface or linear one.

The range of such detectors ranges from 5 to 20 m. Appearance these detectors are presented on rice. 2.

– they open doors at airports and stores when you approach the door. They also detect movement and sound an alarm in the security alarm system. How they work: The sensor, sensitive to infrared radiation in the range of 5-15 microns, detects thermal radiation from the human body. If anyone has forgotten physics, let me remind you: it is in this range that the maximum radiation from bodies falls at a temperature of 20–40 degrees Celsius. The hotter an object is, the more it emits. For comparison: infrared illumination spotlights for video cameras, beam (two-position) “cross-beam” detectors and TV control panels operate in the wavelength range shorter than 1 micron; the human-visible region of the spectrum is in the region of 0.45–0.65 microns.
Sensors of this type are called passive because they themselves do not emit anything, they only perceive thermal radiation from the human body. The problem is that any object at a temperature of even 0º C emits quite a lot in the IR range. Even worse, the detector itself emits radiation – its body and even the material of the sensitive element. Therefore, the first such detectors worked if only the detector itself was cooled, say, to liquid nitrogen (-196º C). Such detectors are not very practical in everyday life. Modern mass detectors all work on the differential principle - they are not able to accurately measure the actual amount of infrared radiation flux from a moving person (against the background of parasitic fluxes from much closer objects), but (also, in fact, on the verge of sensitivity) are capable detect a CHANGE in the DIFFERENCE in IR radiation fluxes incident on two neighboring sites. That is, it is important that radiation from a person is focused only on one of the sites, and moreover, it changes. The detector works most reliably if the image of a person first hits one site, the signal from it becomes greater than from the second, and then the person moves so that his image now hits the second site and the signal from the second increases, and from the first decreases. Such fairly rapid changes in the signal difference can be easily detected even against the background of a huge and variable signal caused by all other surrounding objects (and especially sunlight).

How to fool an IR detector
The initial disadvantage of the IR passive motion detection method is that the person must be clearly different in temperature from the surrounding objects. At a room temperature of 36.6º, no detector will distinguish a person from the walls and furniture. Even worse: the closer the room temperature is to 36.6º, the worse the sensitivity of the detector. Majority modern devices partially compensate for this effect by increasing the gain at temperatures from 30º to 45º (yes, the detectors also work successfully at the opposite temperature difference - if the room is +60º, the detector will easily detect a person; thanks to the thermoregulation system, the human body will maintain a temperature of about 37º). So, when the outside temperature is about 36º (which is often found in southern countries) detectors open doors very poorly, or, conversely, due to extremely high sensitivity, they react to the slightest breath of wind.
Moreover, the IR detector can be easily blocked by any object room temperature(sheet of cardboard) or wear a thick fur coat and hat so that your arms and face do not stick out, and if you walk slowly enough, the IR detector will not notice such small and slow disturbances.
There are also more exotic recommendations on the Internet, such as a powerful IR lamp, which, if turned on slowly (with a regular dimmer), will drive the IR detector off scale, after which you can walk in front of it even without a fur coat. Here, however, it should be noted that good IR detectors in this case will give a malfunction signal.
Finally, the most well-known problem with IR detectors is masking. When the system is disarmed, during business hours during the day, you, as a visitor, come to the desired premises (a store, for example) and, seizing the moment while no one is looking, block the IR detector with a piece of paper, cover it with an opaque self-adhesive film, or fill it with spray paint. This is especially convenient for a person who works there himself. The storekeeper carefully blocked the detector during the day, climbed through the window at night, took everything out, and then removed everything and called the police - horror, they robbed, but the alarm did not work.
To protect against such masking, the following technical techniques exist.
1. In combined (IR + microwave) sensors, it is possible to issue a fault signal if the microwave sensor detects a large reflected radio signal (someone came very close or reached out directly to the detector), and the IR sensor stopped producing signals. In most cases in real life this does not mean the malicious intent of the criminal, but the negligence of the staff - for example, a high stack of boxes blocked the detector. However, regardless of malicious intent, if the detector is blocked, this is a disorder, and such a “malfunction” signal is very appropriate.
2. Some control panels have a control algorithm when, after disarming the detector, it detects movement. That is, the absence of a signal is considered a malfunction until someone passes in front of the sensor and it issues a normal “there is movement” signal. This function is not very convenient, because often all premises are disarmed, even those that no one is going to enter today, but it turns out that in the evening, in order to arm the premises again, you will have to go into all the rooms where no one was there during the day, and wave your hands in front of the sensors - the control panel will make sure that the sensors are operational and will graciously allow you to arm the system.
3. Finally, there is a function called “near zone”, which was once included in the requirements of the Russian GOST and which is often mistakenly called “anti-masking”. The essence of the idea: the detector must have additional sensor, looking straight down, under the detector, or a separate mirror, or a special tricky lens, in general, so that there is no dead zone below. (Most detectors have a limited viewing angle and generally face forward and 60 degrees downward, so there is a small dead zone directly below the detector, at floor level about a meter from the wall.) It is believed that a cunning enemy will somehow be able to get into this dead zone and from there block (mask) the lens of the IR sensor, and then brazenly walk around the entire room. In reality, the detector is usually installed so that there is no way to get into this dead zone without bypassing the sensor's sensitivity areas. Well, perhaps through the wall, but additional lenses will not help against criminals penetrating through the wall.

Radio interference and other interference
As I said, the IR sensor operates close to its sensitivity limit, especially with room temperatures approaching 35º C. Of course, it is also very susceptible to interference. Most IR detectors can give a false alarm if placed near them. cellular telephone and call it. At the connection establishment stage, the phone emits powerful periodic signals with a period close to 1 Hz (it is in this range that typical signals from a person walking in front of the IR sensor lie). A few watts of radio radiation are quite comparable to microwatts of human thermal radiation.
In addition to radio emissions, there may also be optical interference, although the lens of the IR sensor is usually opaque in the visible range, but powerful lamps or 100 W car headlights in the neighboring spectral range can again quite produce a signal comparable to microwatts from a person in the desired range. The main hope is that extraneous optical interference, as a rule, is poorly focused and therefore equally affects both sensitive elements of the IR sensor, thus the detector can detect the interference and not issue a false alarm.

Ways to improve IR sensors
For ten years now, almost all IR security detectors contain a fairly powerful microprocessor and therefore have become less susceptible to random interference. Detectors can analyze repeatability and characteristic parameters signal, long-term stability of the background signal level, which significantly increased resistance to interference.
IR sensors, in principle, are defenseless against criminals behind opaque screens, but are susceptible to the influence of heat flows from climate control equipment and extraneous illumination (through a window). Microwave (radio) motion sensors, on the contrary, are capable of producing false signals, detecting movement behind radio-transparent walls, outside the protected room. They are also more susceptible to radio interference. Combined IR + microwave detectors can be used both according to the “AND” scheme, which significantly reduces the likelihood of false alarms, and according to the “OR” scheme for especially critical premises, which practically eliminates the possibility of overcoming them.
IR sensors cannot distinguish little man from a big dog. There are a number of sensors in which sensitivity to the movements of small objects is significantly reduced through the use of 4-area sensors and special lenses. Signal from tall man and in this case it can be distinguished with some probability from a low dog. You need to understand well that you can absolutely distinguish a crouched teenager from a standing one. hind legs Rottweiler, in principle, is impossible. Nevertheless, the likelihood of a false alarm can be significantly reduced.
A few years ago, even more complex sensors appeared - with 64 sensitive areas. In fact, this is a simple thermal imager with an 8 x 8 element matrix. Equipped with a powerful processor, such IR sensors (calling them a “detector” is too much for me) are capable of determining the size and distance to a moving warm target, the speed and direction of its movement - 10 years ago such sensors were considered the pinnacle of technology for homing missiles, but now they are used for protection against common thieves. Apparently, we will soon get used to calling small robots that wake you up at night with the words “IR sensor” with the words: “Sorry, sir, but thieves, sir, they want tea. Should I serve them tea now or ask them to wait while you wash up and take your revolver?”

Passive optical-electronic infrared (IR) detectors (they are often called motion sensors) detect the fact of human penetration into the protected (controlled) part of the space, generate an alarm signal and, by opening the contacts of the executive relay (monitoring station relay), transmit an “alarm” signal to the warning equipment . Terminal devices (TD) of notification transmission systems (TPS) or a fire alarm control panel (PPKOP) can be used as warning means. In turn, the above-mentioned devices (CU or Control Panel) transmit the received alarm notification via various data transmission channels to the central monitoring station (CMS) or local security console.

The operating principle of passive optical-electronic IR detectors is based on the perception of changes in the level of infrared radiation of the temperature background, the sources of which are the human body or small animals, as well as all kinds of objects in their field of vision.

Infrared radiation is heat that is emitted by all heated bodies. In passive optical-electronic IR detectors, infrared radiation hits a Fresnel lens, after which it is focused on a sensitive pyroelectric element located on the optical axis of the lens (Fig. 1).

Passive IR detectors receive streams of infrared energy from objects and are converted by a pyroelectric receiver into an electrical signal, which is sent through an amplifier and a signal processing circuit to the input of the alarm driver (Fig. 1)1.

In order for an intruder to be detected by a passive IR sensor, the following conditions must be met:

Passive IR sensors consist of three main elements:

Depending on the design of the Fresnel lens, passive optical-electronic IR detectors have different geometric dimensions of the controlled space and can be either with a volumetric detection zone, or with a surface or linear one. The range of action of such detectors lies in the range from 5 to 20 m. The appearance of these detectors is shown in Fig. 2.

Optical system

Modern IR sensors are characterized by a wide variety of possible radiation patterns. The sensitivity zone of IR sensors is a set of rays of various configurations diverging from the sensor in radial directions in one or several planes. Due to the fact that IR detectors use dual pyroelectric receivers, each beam in the horizontal plane is split into two:

The detector sensitivity zone can look like:

The variety and complex configuration of the forms of the sensitivity zone are primarily due to the following factors:

small rooms

special form

It is advisable to dwell on the requirement of uniform sensitivity in more detail. The signal at the output of the pyroelectric detector, all other things being equal, is greater, the greater the degree of overlap by the intruder in the detector’s sensitivity zone and the smaller the beam width and distance to the detector. To detect an intruder at a large (10...20 m) distance, it is desirable that the beam width in the vertical plane does not exceed 5°...10°; in this case, the person almost completely blocks the beam, which ensures maximum sensitivity. At shorter distances, the sensitivity of the detector in this beam increases significantly, which can lead to false alarms, for example, from small animals. To reduce uneven sensitivity, optical systems are used that form several oblique beams, while the IR detector is installed at a height above human height. The total length of the sensitivity zone is thereby divided into several zones, and the beams “closest” to the detector are usually made wider to reduce sensitivity. This ensures almost constant sensitivity over distance, which on the one hand helps to reduce false alarms, and on the other hand increases detection ability by eliminating dead zones near the detector.

When constructing optical systems of IR sensors, the following can be used:

low price

Typically, each segment of the Fresnel lens forms its own beam of the radiation pattern. Usage modern technologies lens manufacturing makes it possible to ensure almost constant sensitivity of the detector for all rays due to the selection and optimization of the parameters of each lens segment: segment area, angle of inclination and distance to the pyro receiver, transparency, reflectivity, degree of defocusing. IN Lately The technology for manufacturing Fresnel lenses with complex precise geometry has been mastered, 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 long distances. The material from which modern lenses are made provides protection for the pyroelectric receiver from white light. The following effects can cause the IR sensor to not perform satisfactorily: heat flows, resulting from heating of the electrical components of the sensor, insects falling on sensitive pyre receivers, possible re-reflections of infrared radiation from internal parts detector. To eliminate these effects, the latest generation of IR sensors use a special sealed chamber between the lens and the pyro-receiver (sealed optics), for example, in the new IR sensors from PYRONIX and C&K. According to experts, modern high-tech Fresnel lenses are practically not inferior in their optical characteristics to mirror optics.

Mirror optics as the only element of an optical system are used quite rarely. IR sensors with mirror optics are produced, for example, by SENTROL and ARITECH. The advantages of mirror optics are the ability to focus more accurately and, as a result, increase sensitivity, which allows you to detect an intruder at long distances. The use of several specially shaped mirrors, including multi-segment ones, makes it possible to provide almost constant distance sensitivity, and this sensitivity at long distances is approximately 60% higher than for simple Fresnel lenses. Using mirror optics, it is easier to protect the near zone located directly below the sensor installation site (the so-called anti-sabotage zone). By analogy with replaceable Fresnel lenses, IR sensors with mirror optics are equipped with replaceable detachable mirror masks, the use of which allows you to select the required shape of the sensitivity zone and makes it possible to adapt the sensor to various configurations of the protected premises.

Modern high-quality IR detectors use a combination of Fresnel lenses and mirror optics. In this case, Fresnel lenses are used to form a sensitivity zone at medium distances, and mirror optics are used to form an anti-tamper zone under the sensor and to provide a very long detection distance.

Pyro receiver:

The optical system focuses IR radiation on a pyroelectric receiver, which in IR sensors uses an ultra-sensitive semiconductor pyroelectric converter capable of recording a difference of several tenths of a degree between the temperature of a person’s body and the background. The temperature change is converted into an electrical signal, which, after appropriate processing, triggers an alarm. IR sensors usually use dual (differential, DUAL) pyroelements. This is due to the fact that a single pyroelement reacts in the same way to any temperature change, regardless of what it is caused by - the human body or, for example, heating a room, which leads to an increase in the frequency of false alarms. IN differential circuit the signal of one pyroelement is subtracted from another, which makes it possible to significantly suppress interference associated with changes in background temperature, as well as significantly reduce the influence of light and electromagnetic interference. The signal from a moving person appears at the output of the double pyroelectric element only when the person crosses the beam of the sensitivity zone and is an almost symmetrical bipolar signal, close in shape to the period of a sinusoid. For this reason, the beam itself for a double pyroelectric element is split into two in the horizontal plane. In the latest models of IR sensors, in order to further reduce the frequency of false alarms, quadruple pyroelements (QUAD or DOUBLE DUAL) are used - these are two dual pyroelectric sensors located in one sensor (usually placed one above the other). The observation radii of these pyro receivers are made different, and therefore a local thermal source of false alarms will not be observed in both pyro receivers at the same time. In this case, the geometry of the placement of pyro receivers and their connection circuit is selected in such a way that signals from a person are of opposite polarity, and electromagnetic interference causes signals in two channels of the same polarity, which leads to the suppression of this type of interference. For quadruple pyroelements, each beam is split into four (see Fig. 2), and therefore the maximum detection distance when using the same optics is approximately halved, since for reliable detection a person must, with his height, block both beams from two pyroelectric detectors. The detection distance for quadruple pyroelements can be increased by using precision optics that form a narrower beam. Another way to correct this situation to some extent is the use of pyroelements with complex intertwined geometry, which the PARADOX company uses in its sensors.

Signal processing block

The signal processing unit of the pyro receiver must ensure reliable recognition of a useful signal from a moving person against a background of interference. For IR sensors, the main types and sources of interference that can cause false alarms are:

The processing unit's identification of a useful signal against a background of interference is based on an analysis of the signal parameters at the output of the pyroelectric detector. These parameters are the signal size, its shape and duration. The signal from a person crossing the beam of the IR sensor sensitivity zone is an almost symmetrical bipolar signal, the duration of which depends on the speed of movement of the intruder, the distance to the sensor, the width of the beam, and can be approximately 0.02...10 s with a recorded range of movement speeds of 0 ,1…7 m/s. Interference signals are mostly asymmetrical or have a different duration from the useful signals (see Fig. 3). The signals shown in the figure are very approximate; in reality, everything is much more complicated.

The main parameter analyzed by all sensors is the signal magnitude. In the simplest sensors, this recorded parameter is the only one, and its analysis is carried out by comparing the signal with a certain threshold, which determines the sensitivity of the sensor and affects the frequency of false alarms. In order to increase resistance to false alarms in simple sensors The pulse counting method is used to count how many times the signal has exceeded the threshold (that is, in essence, how many times the intruder has crossed the beam or how many beams he has crossed). In this case, an alarm is not issued the first time the threshold is exceeded, but only if, within a certain time, the number of exceedances becomes greater than a specified value (usually 2...4). The disadvantage of the pulse counting method is the deterioration of sensitivity, which is especially noticeable for sensors with a sensitivity zone such as a single curtain and the like, when an intruder can only cross one beam. On the other hand, when counting pulses, false alarms are possible due to repeated interference (for example, electromagnetic or vibration).

In more complex sensors, the processing unit analyzes the bipolarity and symmetry of the signal shape from the output of the differential pyroelectric receiver. The specific implementation of such processing and the terminology used to refer to it1 may vary from manufacturer to manufacturer. The essence of the processing is to compare a signal with two thresholds (positive and negative) and, in some cases, to compare the magnitude and duration of signals of different polarities. A combination of this method with separate counting of excesses of positive and negative thresholds is also possible.

Analysis of the duration of signals can be carried out either by a direct method of measuring the time during which the signal exceeds a certain threshold, or in the frequency domain by filtering the signal from the output of the pyro receiver, including using a “floating” threshold, depending on the range of frequency analysis.

Another type of processing designed to improve the performance of IR sensors is automatic thermal compensation. Temperature range environment 25°С…35°С the sensitivity of the pyro receiver decreases due to a decrease in the thermal contrast between the human body and the background; with a further increase in temperature, the sensitivity increases again, but “with the opposite sign”. In so-called “conventional” thermal compensation circuits, the temperature is measured, and when it increases, the gain is automatically increased. “True” or “two-way” compensation takes into account the increase in thermal contrast for temperatures above 25°C...35°C. The use of automatic temperature compensation ensures almost constant sensitivity of the IR sensor over a wide temperature range.

The listed types of processing can be carried out by analogue, digital or combined means. Modern IR sensors are increasingly using 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. Recently, there have been reports of the development of completely digital IR sensors that do not use analog elements at all.
As is known, due to the random nature of useful and interfering signals, the best processing algorithms are those based on the theory of statistical solutions.

Other protection elements for IR detectors

IR sensors intended for professional use use so-called anti-masking circuits. The essence of the problem is that conventional IR sensor can be disabled by an intruder by first (when the system is not armed) taping or painting over the sensor input window. To combat this method of bypassing IR sensors, anti-masking schemes are used. The method is based on the use of a special IR radiation channel, which is triggered when a mask or reflective obstacle appears at a short distance from the sensor (from 3 to 30 cm). The anti-masking circuit 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 issue an alarm 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 had cleared itself, the alarm signal is not issued.

Another security element that almost all modern IR detectors are equipped with is a contact tamper sensor, which signals an attempt to open or break into the sensor housing. The tamper and masking sensor relays are connected to a separate security loop.

To eliminate triggering of the IR sensor from small animals, either special lenses with a dead zone (Pet Alley) from floor level to a height of about 1 m are used, or special methods signal processing. It should be taken into account that special processing signals allows you to ignore animals only if they total weight does not exceed 7...15 kg, and they can approach the sensor no closer than 2 m. So if there is a jumping cat in a protected room, then such protection will not help.

To protect against electromagnetic and radio interference, dense surface mounting and metal shielding are used.

Installation of detectors

Passive optical-electronic IR detectors have one remarkable advantage over other types of detection devices. It is easy to install, configure and Maintenance. Detectors of this type can be installed either on flat surface load-bearing wall, and in the corner of the room. There are detectors that are placed on the ceiling.

Competent selection and tactically correct use of such detectors are the key to reliable operation devices, and the entire security system as a whole!

When choosing the types and number of sensors to ensure the protection of a particular object, you should take into account possible ways and methods of penetration of the intruder, the required level of detection reliability; costs for the acquisition, installation and operation of sensors; features of the object; performance characteristics sensors A feature of IR passive sensors is their versatility - with their use it is possible to block a wide variety of rooms, structures and objects from approaching and entering: windows, showcases, counters, doors, walls, ceilings, partitions, safes and individual objects, corridors, room volumes. However, in some cases it will not be necessary large quantity sensors to protect each structure - it may be sufficient to use one or more sensors with the desired sensitivity zone configuration. Let's take a look at some of the features of using IR sensors.

General principle using IR sensors - the rays of the sensitivity zone must be perpendicular to the intended direction of movement of the intruder. The sensor installation location should be chosen so as to minimize dead zones caused by the presence of large objects in the protected area that block the beams (for example, furniture, houseplants). If doors in a room open inwards, the possibility of masking an intruder should be taken into account open doors. If dead spots cannot be eliminated, multiple sensors should be used. When blocking individual objects, the sensor or sensors must be installed so that the rays of the sensitivity zone block all possible approaches to the protected objects.

The range specified in the documentation must be observed permissible heights suspension (minimum and maximum height). This especially applies to radiation patterns with inclined beams: if the suspension height exceeds the maximum permissible, this will lead to a decrease in the signal from the far zone and an increase in the dead zone in front of the sensor, but if the suspension height is less than the minimum permissible, this will lead to a decrease in range detection while simultaneously reducing the dead zone under the sensor.

1. Detectors with a volumetric detection zone (Fig. 3, a, b), as a rule, are installed in the corner of the room at a height of 2.2-2.5 m. In this case, they evenly cover the volume of the protected room.

2. Placing detectors on the ceiling is preferable in rooms with high ceilings from 2.4 to 3.6 m. These detectors have a denser detection zone (Fig. 3, c), and their operation is less affected by existing pieces of furniture.

3. Detectors with a surface detection zone (Fig. 4) are used to protect the perimeter, for example, non-permanent walls, door or window openings, and can also be used to limit access to any valuables. The detection zone of such devices should be directed, as an option, along a wall with openings. Some detectors can be installed directly above the opening.

4. Detectors with a linear detection zone (Fig. 5) are used to protect long and narrow corridors.

Interference and false positives

When using passive optical-electronic IR detectors, it is necessary to keep in mind the possibility of false alarms that occur due to various types of interference.

Interference of a thermal, light, electromagnetic, or vibration nature can lead to false alarms of IR sensors. Although modern IR sensors have high degree protection from these impacts, it is still advisable to adhere to the following recommendations:

air flow

open window

Thermal interference - caused by heating of the temperature background when exposed to solar radiation, convective air flows from the operation of radiators of heating systems, air conditioners, and drafts.
Electromagnetic interference - caused by interference from sources of electrical and radio emissions to individual elements of the electronic part of the detector.
Extraneous interference - associated with the movement of small animals (dogs, cats, birds) in the detector detection zone. Let us consider in more detail all the factors affecting the normal operation of passive optical-electronic IR detectors.

Thermal interference

This is the most dangerous factor, which is characterized by changes in the ambient temperature background. Exposure to solar radiation causes a local increase in the temperature of individual sections of the walls of the room.

Convective interference is caused by the influence of moving air currents, for example from drafts with an open window, cracks in window openings, as well as during the operation of household heating devices- radiators and air conditioners.

Electromagnetic interference

They occur when any sources of electrical and radio radiation are turned on, such as measuring and household equipment, lighting, electric motors, and radio transmitting devices. Strong interference can also be caused by lightning strikes.

Extraneous interference

Small insects such as cockroaches, flies, and wasps can be a unique source of interference in passive optical-electronic IR detectors. If they move directly along the Fresnel lens, a false alarm of this type of detector may occur. The so-called house ants, which can get inside the detector and crawl directly on the pyroelectric element, also pose a danger.

Installation errors

A special place in the incorrect or incorrect operation of passive optical-electronic IR detectors is occupied by installation errors when performing work on installing these types of devices. Let us pay attention to striking examples of incorrect placement of IR detectors in order to avoid this in practice.

In Fig. 6 a; 7 a and 8 a show the correct, correct installation of detectors. You only need to install them this way and no other way!

In Figures 6 b, c; 7 b, c and 8 b, c present options for incorrect installation of passive optical-electronic IR detectors. With this installation, real intrusions into protected premises may be missed without issuing an “Alarm” signal.

Do not install passive optical-electronic detectors in such a way that they are exposed to direct or reflected rays of sunlight, as well as the headlights of passing vehicles.
Do not point the detector detection area at heating elements heating and air conditioning systems, on curtains and curtains, which can fluctuate due to drafts.
Do not place passive optical-electronic detectors near sources of electromagnetic radiation.
Seal all holes of the passive optical-electronic IR detector with the sealant supplied with the product.
Destroy insects that are present in the protected area.

Currently, there is a huge variety of detection tools, differing in operating principle, scope, design and performance characteristics.

The correct choice of a passive optical-electronic IR detector and its installation location is the key to reliable operation of the security alarm system.

When writing this article, materials were used, among other things, from the magazine “Security Systems” No. 4, 2013

These security detectors are used to detect movement inside a protected premises, additional blocking of surfaces, passages, open areas, and external perimeters. Otherwise they are also called Motion Sensor. Let's start with classification. The detectors discussed here are classified according to:

type of detection zone - volumetric, surface, linear
operating principle - infrared (IR), radio wave, ultrasonic.
execution - wall, ceiling, for external, internal installation

Any specific detector is simultaneously characterized by each of these categories.

INFRARED (IR) VOLUMETRIC DETECTOR, PASSIVE

The detection zone is volumetric, see Fig. 1. It should be noted that such a volumetric detection zone is inherent in a wall-mounted detector. Above is a side view (vertical plane), below is a top view (horizontal plane).

An alarm signal is generated when an object with a temperature different from the room temperature crosses sectors that determine the configuration and size of the detection zone. Therefore, the characteristics indicate - volumetric, infrared (i.e. thermal). And such detectors are called passive because they work only “receive”, without emitting anything. Accordingly, the design is single-block. In general, any infrared volumetric detector is passive.

INFRARED (IR) SURFACE DETECTOR, LINEAR

In addition to the volumetric one, IR security detectors can also have a surface detection zone - “curtain”, and a linear detection zone - “beam”. The surface security infrared sensor has a detection zone shown in Figure 2 (all similar to Figure 1). For the linear zone of the diagram I do not give a ray; it is a ray, either from above or from the side - approximately like the bottom of Figure 2.

Operating principle of surface, linear infrared sensors similar to IR volumetric detectors. In addition, a number of linear detectors have active principle actions, i.e. consist of two security units - an emitter and a receiver. An alarm signal will be generated by the receiver when a foreign object crosses the IR beam generated by the emitter.

To summarize what has been said about infrared security detectors, we note the following features that can be classified as disadvantages:

Security infrared sensor is critical to rigidity load-bearing structure. If it is subject to vibrations it may generate false alarms. Security infrared (IR) sensors should be installed on permanent structures.
When convection (heat) currents or light sources of variable intensity are in the detection zone of an infrared sensor, spontaneous triggering is also possible. When installing infrared volumetric sensors, you should take into account the location of heaters and windows.
Any objects located in the detection zone of the IR volumetric detector form behind them (on the side opposite to the detector) a “shadow zone” where detection of a moving object is impossible. As a consequence, for example, rearranging furniture will cause a change in the configuration of the detection zone. The solution is to use IR volumetric detectors of a ceiling design (according to the installation method).
Can track animals in the detection zone. But there are volumetric sensors protected from this factor
May react to ingestion small insects. The solution is to seal all inputs to the sensor and periodically carry out appropriate sanitation of the premises.

When choosing the type of infrared security detector, you should take into account the opening angle of the detection zone (measured in degrees), the range of the infrared detector. Please note that the range of the infrared volumetric sensor is indicated along the main axis; it is shorter along the side axes. Also, if you plan to use the infrared detector in unheated room, select the appropriate operating temperature range.

SECURITY RADIO WAVE, ULTRASONIC DETECTORS

The detection zone is three-dimensional, a kind of solid three-dimensional spindle. The principle of operation of volumetric radio wave and volumetric ultrasonic sensors is the same, based on the Doppler effect, namely, a sound or radio wave, when reflected from a moving object, changes its frequency (or, if you prefer, length). Thus, these volumetric security detectors are also designed to detect movement inside the protected premises. I would like to note that all the security detectors discussed here (infrared, radio wave, ultrasonic, volumetric, linear), having the appropriate climatic design, can be installed outdoors.

As the name suggests, a radio wave security sensor emits and receives radio waves, while an ultrasonic volumetric detector produces ultrasound. Unlike IR security detectors, these are indifferent to light, heat, drafts, but, however, have their drawbacks:

First, a volumetric radio wave detector emits radio waves of a sufficiently high frequency (about 1 GHz) for which walls, windows, and doors are transparent. At wrong choice the size of the detection zone of the radio wave volume sensor, it will respond to what is being done outside the protected area. (Ultrasonic - no).
second, (relating to radio waves) possible interference with other radio-electronic devices
third, if there are several volumetric radio wave detectors nearby, they can cause mutual interference. The solution is to use devices with different frequency letters. But there are few letters, and it is impossible to place a large number of radio wave detectors nearby.
fourth, being in the same room with working detectors of these types, although not fatal, is not very useful. The solution is to turn off the power to the sensors if you are constantly near people.
fifth, a surface detection zone is not formed.

Among the wide variety of security detectors, the infrared motion sensor is the most common device. Affordable price and efficiency are the qualities that have made them popular. And all thanks to the fact that infrared radiation was discovered at the beginning of the nineteenth century.

It lies beyond the limit of visible red light in the range of 0.74-2000 microns. The optical properties of substances vary greatly and depend on the type of irradiation. A small layer of water is opaque to IR radiation. Infrared radiation from the sun makes up 50 percent of all emitted energy.

Application area

Infrared motion sensors have been used for security for a long time. They recorded the movements of warm objects in the premises and transmitted an alarm signal to the control panel. They began to be combined with video cameras and cameras. When a violation occurred, the incident was recorded. Then the scope of application expanded. Zoologists began to use camera traps to control the animals being studied.

Most of all IR sensors are used in the system smart House, where they play the role of a presence sensor. When a warm-blooded object comes into the range of the device, it turns on the lighting indoors or outdoors. It saves electricity and makes life easier for people.

In access control systems, motion detectors control the opening and closing of doors in public buildings. According to experts, the IR sensor market will grow by 20% annually over the next 3-5 years.

Operating principle of IR motion sensor

The work of the IR detector is to monitor the infrared radiation of a certain area, compare it with the background level, and issue a message based on the results of the analysis.

IR motion sensors for security use active and passive types of sensors. The former use their own transmitter for control, irradiating everything within the device’s coverage area. The receiver receives the reflected part of the IR radiation and, based on its characteristics, determines whether there was a violation of the security zone or not. There are active sensors combined type, when the receiving and transmitting blocks are separated, these are detectors that monitor the perimeter of the object. They have a longer range compared to passive devices.

A passive infrared motion sensor does not have an emitter; it reacts to changes in the surrounding IR radiation. In general, the detector has two sensitive elements capable of detecting infrared radiation. A Fresnel lens is installed in front of the sensors, dividing the space into several dozen zones.

A small lens collects radiation from a specific area of space and sends it to its sensing element. An adjacent lens that controls the adjacent area sends a flux of radiation to the second sensor. The radiation from neighboring areas is approximately the same. If the balance is disturbed, or a certain threshold value is exceeded, the device notifies the control panel that the security zone has been violated.

IR sensor circuit

Each manufacturer has a unique schematic diagram IR detector, but functionally they are approximately the same.

The IR sensor has an optical system, a pyrosensitive element, and a signal processing unit.

Optical system

The working area of modern motion sensors is very diverse due to various forms optical system. Beams diverge from the device in a radial direction in different planes.

Since the detector has a dual sensor, all beams are bifurcated.

The optical system is oriented in such a way that it will monitor only one plane or several planes at different levels. Can control space in a circular or beam manner.

When constructing the optics of IR sensors, Fresnel lenses are often used, representing many prismatic facets on a convex plastic cup. Each lens collects the IR flux from its area of space and sends it to the PIR element.

The design of the optical system is such that the selectivity across all lenses is the same. To protect insects from their own heat from the elements, a sealed chamber is installed in the device. Mirror optics are rarely used. This significantly increases the range of the device and the price of the device.

Pyrosensitive element

The role of the sensor in the IR sensor is played by a pyroelectric converter based on sensitive semiconductor elements. It consists of two sensors. Each of them receives a radiation flux from two adjacent beams. With the same uniform background, the sensor is silent. If an imbalance occurs, an additional heat source appears in one zone, but not in the other, the sensor is triggered.

To increase reliability and reduce false alarms, quadruple PIR elements have recently begun to be used. This increased the sensitivity and noise immunity of the device. But it reduced the distance of confident recognition of the intruder. To solve this, you have to use precision optics.

Signal processing block

The main task of the block is to reliably recognize a person against a background of interference.

They come in a wide variety:

solar radiation;
artificial IR sources;
air conditioners and refrigerators;
animals;
air convection;
electromagnetic interference;
vibration.

The processing unit for analysis uses the amplitude, shape and duration of the output signal of the pyroelectric converter. The impact of the intruder causes a symmetrical bipolar signal. Interference produces asymmetrical values to the processing module. In the simplest version, the signal amplitude is compared with a threshold value.

If the threshold is exceeded, the detector reports this by sending a certain signal to the control panel. In more complex sensors, the duration of exceeding the threshold and the number of these exceedances are measured. To increase the noise immunity of the device, automatic thermal compensation is used. It provides constant sensitivity over the entire temperature range.

Signal processing is carried out by analog and digital devices. The latest devices have begun to use digital signal processing algorithms, which has improved the selectivity of the device.

The effectiveness of using an IR detector in security alarms

Its effectiveness largely depends on the correct choice of sensor type and location at the security site. Passive IR motion sensors for street and internal use react to the movements of objects that are warm compared to the background at certain movement speeds. At low speeds, changes in infrared radiation fluxes in neighboring sectors are so insignificant that it is perceived as background drift and does not react to violation of the security zone.

If the intruder puts on a protective suit with excellent thermal insulation, then the IR motion sensor will not react, and there will be no disturbance in the balance of radiation in neighboring areas. The person will merge with the background radiation.

The intruder moves along the motion detector beams at low speed, in which case he is often silent.

Flow changes are not sufficient to trigger the device. This is especially true for detectors with animal protection functions. They reduce sensitivity to avoid reactions to the appearance of pets.

It is important to install the infrared sensor correctly. According to the configuration of the building, it is required to use a “curtain” type device, and this should be done. The manufacturer recommends installing the device at a certain height, this must also be observed.

To increase the efficiency of infrared sensors, they are used in conjunction with sensors operating on other principles.

Usually, additionally given radio wave detector with high sensitivity, which reduces the percentage of false alarms and increases the reliability of the security alarm. When protecting windows from penetration, an ultrasonic detector is additionally installed that responds to glass breakage.

Conclusion

Gradually, IR sensors become more complex, their sensitivity increases, and selectivity improves. Sensors are widely used in smart home, video surveillance, and access control systems. Sharing with various devices improved the consumer properties of sensors. They are destined for a long life.

Video: Motion sensor, operating principle

Sections