TD Aktiv-SB: video surveillance and security systems. The principle of operation and the purpose of the infrared motion sensor Sensors on the Doppler effect

Landscaping and planning 15.06.2019

Landscaping and planning

How to fool the IR detector
The initial drawback of the IR passive method of motion detection: a person must clearly differ in temperature from the surrounding objects. At a room temperature of 36.6º, no detector can distinguish a person from walls and furniture. Worse than that: The closer the room temperature is to 36.6º, the worse the sensitivity of the detector. Most modern devices partially compensate for this effect by increasing the gain at temperatures from 30º to 45º (yes, detectors work successfully even with a reverse drop - if the room is +60º, the detector will easily detect a person, thanks to the thermoregulation system, the human body will keep the temperature around 37º). So, at a temperature outside of about 36º (which is often found in southern countries), the detectors open doors very poorly, or, conversely, because of the extremely high sensitivity, they react to the slightest breath of wind.
Moreover, it is easy to block the IR detector with any object at room temperature (a sheet of cardboard) or put on a thick coat and hat so that your hands and face do not stick out, and if you walk slowly enough, the IR detector will not notice such small and slow perturbations.
There are also more exotic recommendations on the Internet, such as a powerful IR lamp, which, if turned on slowly (with a conventional 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 the daytime during business hours, you, as a visitor, come to the right place (to the store, for example) and, catching the moment while no one is looking, block the IR detector with a piece of paper, seal it with an opaque self-adhesive film or fill it with spray paint. This is especially convenient for the 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 techniques are available.
1. In combined (IR + microwave) sensors, it is possible to issue a malfunction signal if the microwave sensor detects a large reflected radio signal (someone came very close or extended a hand directly to the detector), and the IR sensor stopped emitting signals. In most cases, in real life, this does not mean at all the malicious intent of the criminal, but the negligence of the personnel - for example, a high stack of boxes blocked the detector. However, regardless of malicious intent, if the detector is blocked, this is a mess, and such a “malfunction” signal is very appropriate.
2. Some control panel devices have a control algorithm when, after the detector is disarmed, it detects movement. That is, the absence of a signal is considered a malfunction until someone passes in front of the sensor and it gives a normal “there is movement” signal. This function is not very convenient, because all premises are often disarmed, even those that no one is going to enter today, but it turns out that in the evening, in order to put the premises back on guard, 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 national GOST and which is often mistakenly called "anti-masking". The essence of the idea: the detector should have an 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 field of view and mostly look forward and 60 degrees down, 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 (disguise) the lens of the IR sensor, and then brazenly walk around the room. In reality, the detector is usually installed in such a way that there is no way to get into this dead zone, bypassing the sensor's sensitivity areas. Well, maybe through the wall, but against criminals penetrating through the wall, additional lenses will not help.

Radio interference and other interference
As I said before, the IR sensor works close to the limit of sensitivity, especially when the room temperature approaches 35º C. Of course, it is also very susceptible to interference. Most IR detectors can give a false alarm if you place a cell phone next to them and call it. At the stage of establishing a connection, the phone produces powerful periodic signals with a period close to 1 Hz (this is the range in which typical signals from a person walking in front of the IR sensor lie). A few watts of radio emission are quite comparable to microwatts of human thermal radiation.
In addition to radio emission, there may 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, again, may well give a signal comparable to microwatts from a person in the desired range. The main hope at the same time is that extraneous optical interference, as a rule, is poorly focused and therefore equally affects both sensitive elements of the IR sensor, so the detector can detect interference and not give a false alarm.

Ways to improve IR sensors
For ten years already, almost all security IR detectors contain a sufficiently powerful microprocessor and therefore have become less susceptible to random interference. The detectors can analyze the repeatability and characteristic parameters of the signal, long-term stability of the background signal level, which made it possible to significantly increase the resistance to interference.
Infrared sensors, in principle, are defenseless against criminals behind opaque screens, but they are affected by heat flows from climate equipment and extraneous light (through a window). Microwave (radio) motion sensors, on the contrary, are capable of generating false signals, detecting movement behind radio-transparent walls, outside the protected premises. 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 between a small person and a large dog. There are a number of sensors in which the sensitivity to the movements of small objects is significantly reduced due to the use of 4-area sensors and special lenses. Signal from tall man and from a low dog in this case can be distinguished with some probability. It must be well understood that it is, in principle, impossible to completely distinguish a crouching teenager from a Rottweiler standing on its hind legs. Nevertheless, the probability of false alarms 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 a matrix of 8 x 8 elements. Equipped with a powerful processor, such sensors (you can’t call them a “detector” at all) are able to determine the size and distance to a moving warm target, the speed and direction of its movement - 10 years ago, such sensors were considered the height of technology for homing missiles, and now they are used to protect from commonplace thieves. Apparently, soon we will get used to calling the IR sensor small robots that will wake you up at night 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?

– they open doors at airports and shops when you come to the door. They also detect movement and give an alarm in burglar alarm. How they work: A 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 at a temperature of 20-40 degrees Celsius falls. The hotter an object is, the more it radiates. For comparison: infrared spotlights for backlighting video cameras, beam (two-position) “beam crossing” detectors and TV remote controls operate in the wavelength range shorter than 1 micron, visible to man the spectral region is in the region of 0.45–0.65 μm.
Passive sensors of this type are called 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 infrared range. Worse, 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. Modern mass detectors all work according to the differential principle - they are not able to accurately measure the actual value of the 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 of to detect CHANGE in the DIFFERENCE of IR fluxes incident on two adjacent sites. That is, it is important that the 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 area, the signal from it becomes greater than from the second, and then the person moves, so that his image will now fall on the second area and the signal for the second will increase, and the first will fall. Such fairly rapid changes in signal difference can be detected even against the background of a huge and unstable signal caused by all other surrounding objects (and especially sunlight).

How to fool the IR detector
The initial drawback of the IR passive method of motion detection: a person must clearly differ in temperature from the surrounding objects. At a room temperature of 36.6º, no detector can distinguish a person from walls and furniture. Worse, the closer the temperature in the room is to 36.6º, the worse the sensitivity of the detector. Most modern devices partially compensate for this effect by increasing the gain at temperatures from 30º to 45º (yes, detectors work successfully even with a reverse drop - if the room is +60º, the detector will easily detect a person, thanks to the thermoregulation system, the human body will keep the temperature around 37º). So, at a temperature outside of about 36º (which is often found in southern countries), the detectors open doors very poorly, or, conversely, because of the extremely high sensitivity, they react to the slightest breath of wind.
Moreover, it is easy to block the IR detector with any object at room temperature (a sheet of cardboard) or put on a thick coat and hat so that your hands and face do not stick out, and if you walk slowly enough, the IR detector will not notice such small and slow perturbations.
There are also more exotic recommendations on the Internet, such as a powerful IR lamp, which, if turned on slowly (with a conventional 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 the daytime during business hours, you, as a visitor, come to the right place (to the store, for example) and, catching the moment while no one is looking, block the IR detector with a piece of paper, seal it with an opaque self-adhesive film or fill it with spray paint. This is especially convenient for the 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 techniques are available.
1. In combined (IR + microwave) sensors, it is possible to issue a malfunction signal if the microwave sensor detects a large reflected radio signal (someone came very close or extended a hand directly to the detector), and the IR sensor stopped emitting signals. In most cases, in real life, this does not mean at all the malicious intent of the criminal, but the negligence of the personnel - for example, a high stack of boxes blocked the detector. However, regardless of malicious intent, if the detector is blocked, this is a mess, and such a “malfunction” signal is very appropriate.
2. Some control panel devices have a control algorithm when, after the detector is disarmed, it detects movement. That is, the absence of a signal is considered a malfunction until someone passes in front of the sensor and it gives a normal “there is movement” signal. This function is not very convenient, because all premises are often disarmed, even those that no one is going to enter today, but it turns out that in the evening, in order to put the premises back on guard, 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 national GOST and which is often mistakenly called "anti-masking". The essence of the idea: the detector should have an 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 field of view and mostly look forward and 60 degrees down, 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 (disguise) the lens of the IR sensor, and then brazenly walk around the room. In reality, the detector is usually installed in such a way that there is no way to get into this dead zone, bypassing the sensor's sensitivity areas. Well, maybe through the wall, but against criminals penetrating through the wall, additional lenses will not help.

Ways to improve IR sensors
For ten years already, almost all security IR detectors contain a sufficiently powerful microprocessor and therefore have become less susceptible to random interference. The detectors can analyze the repeatability and characteristic parameters of the signal, long-term stability of the background signal level, which made it possible to significantly increase the resistance to interference.
Infrared sensors, in principle, are defenseless against criminals behind opaque screens, but they are affected by heat flows from climate equipment and extraneous light (through a window). Microwave (radio) motion sensors, on the contrary, are capable of generating false signals, detecting movement behind radio-transparent walls, outside the protected premises. 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 between a small person and a large dog. There are a number of sensors in which the sensitivity to the movements of small objects is significantly reduced due to the use of 4-area sensors and special lenses. The signal from a tall person and from a low dog in this case can be distinguished with some probability. It must be well understood that it is, in principle, impossible to completely distinguish a crouching teenager from a Rottweiler standing on its hind legs. Nevertheless, the probability of false alarms 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 a matrix of 8 x 8 elements. Equipped with a powerful processor, such IR sensors (you can’t call them a “detector” at all) are able to determine the size and distance to a moving warm target, the speed and direction of its movement - 10 years ago, such sensors were considered the height of technology for homing missiles, and now they are used for protection from banal thieves. Apparently, soon we will get used to calling the IR sensor small robots that will wake you up at night 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?

1.3.1. Passive optoelectronic infrared (IR) motion sensors

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

I chose the following components:

passive infrared motion sensor;
GSM module;
siren.

Let's consider them in more detail.

In the 21st century, everyone is familiar with IR sensors– they open doors at airports and shops when you come to the door. They also detect movement and give an alarm in the burglar alarm.

Currently, passive optical-electronic infrared (IR) detectors occupy a leading position in the choice of protection of premises from unauthorized intrusion at security facilities. Aesthetic appearance, ease of installation, configuration and maintenance often give them priority over other detection tools.

Passive optical-electronic 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, form an alarm signal and, by opening the contacts of the executive relay (monitoring station relay), transmit the signal " anxiety» on alerts.

As a means of warning, terminal devices (UO) of notification transmission systems (SPI) or a fire and security alarm control device (PPKOP) can be used. In turn, the above-mentioned devices (UO or PPKOP) broadcast the received alarm notification via various data transmission channels to the central monitoring station (CMS) or the local security console.

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

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

The hotter an object is, the more it radiates.
infrared spotlights for backlighting video cameras, beam (two-position) detectors beam crossing” and TV remote controls 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.

Passive sensors of this type are called 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 infrared range. Worse, 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 the 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 area, the signal from it becomes greater than from the second, and then the person moves, so that his image will now fall on the second area and the signal for the second will increase, and the first will fall.

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

Rice. one.

AT passive optical-electronic IR detectors infrared thermal radiation enters 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 infrared energy flows from objects and are converted by a pyro receiver into an electrical signal that enters through an amplifier and a signal processing circuit to the input of an alarm generator ( rice. one).

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

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

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

Rice. 2.

Depending on performance fresnel lens passive optical-electronic IR detectors have different geometric dimensions of the controlled space and can be both with a volumetric detection zone, and with a surface or linear one.

The range of such detectors lies in the range from 5 to 20 m. Appearance of these detectors is presented on rice. 2.

Infrared detectors are one of the most common in burglar alarm systems. This is explained by a very wide range of their application.

They are used:

to control the internal volume of the premises;
organization of perimeter protection;
blocking various building structures "on the way".

In addition to the climatic version (outdoor and indoor installation), they are also divided according to the principle of operation. There are two large groups: active and passive. In addition, infrared detectors are divided according to the type of detection zone, namely:

voluminous;
linear;
superficial.

Let's look in order for what purposes one or another of their types are used.

Passive infrared detectors.

These sensors incorporate a lens that "cuts" the controlled area into separate sectors (Fig. 1). The detector is triggered when temperature differences between these zones are detected. Thus, the opinion that such a security sensor reacts purely to heat is erroneous.

If a person in the detection zone stands motionless, the detector will not work. In addition, the temperature of the object, which is close to the background temperature, also affects its sensitivity downward.

The same applies to cases when the speed of movement of the object is lower or higher than the normalized value. As a rule, this value is in the range of 0.3-3 meters/second. This is enough to reliably detect the intruder.

Active infrared detectors.

Devices of this type are composed of an emitter and a receiver. They can be made in separate blocks or combined in one body. In the latter case, when installing such security device additionally, an element reflecting IR rays is used.

The active principle of operation is typical for linear sensors that are triggered when the infrared beam is crossed. Below are the principles of operation and features of the use of the main types of IR detectors.

VOLUME INFRARED DETECTORS

These devices are passive (see above for what it is) and are used mainly to control the internal volume of premises. The radiation pattern of the volumetric sensor is characterized by:

opening angle in vertical and horizontal planes;
range of the detector.

Please note - the range is indicated by the central lobe of the diagram, for the side ones it will be less.

What is typical for any infrared sensor, including volumetric - any obstacle for it is opaque, respectively, creates dead zones. On the one hand, this is a disadvantage, on the other hand, an advantage, since there is no reaction to moving objects outside the protected premises.

Also, the disadvantages include the possibility of false positives from such factors as:

convection heat flows, for example, from heating systems of various operating principles;
flare from moving light sources - most often car headlights through the window.

Thus, when installing a volumetric detector, these points cannot be ignored. According to the method of installation, there are two versions of "volumizers".

Wall mounted IR detectors.

Ideal for offices, apartments, private houses. In such rooms, furniture and other interior items are usually located along the walls, so there are no blind spots. If we take into account that the horizontal viewing angle of such sensors is about 90 degrees, then by installing it in the corner of a room, one device can almost completely block a small room.

Ceiling volume detectors.

For objects such as shops or warehouses, a characteristic feature is the installation of shelving or showcases throughout the area of the premises. The installation of a ceiling sensor in such cases is more effective, of course, if these elements have a height below the ceiling.

Otherwise, you will have to block each formed compartment. In fairness, it should be noted that such a need does not always arise, but these are the subtleties of signaling design for each specific object, taking into account all its individual features.

LINEAR INFRARED DETECTORS

By their principle of operation, they are active and form one or more beams, tracking their intersection by a possible intruder. Unlike volumetric sensors, linear sensors are resistant to various kinds of air currents, and direct illumination, in most cases, will not harm them.

The principle of operation of a linear single-beam infrared emitter is illustrated in Figure 2.

Range of active line devices ranges from tens to hundreds of meters. The most typical options for their use:

corridor blocking;
protection of open and fenced perimeters of the territory.

To protect the perimeter, detectors with more than one beam are used (it is better if there are at least three of them). This is fairly obvious as it reduces the chance of penetration below or above the control zone.

When installing and configuring infrared linear detectors, precise alignment of the receiver and transmitter is required for two-unit devices or reflector and combined unit (for single-unit). The fact is that the cross section (diameter) of the infrared beam is relatively small, so even a small angular displacement of the transmitter or receiver leads to its significant linear deviation at the receiving point.

From what has been said, it also follows that all elements of such detectors must be mounted on rigid linear structures that completely exclude possible vibrations.

I must say that a good "linear" is a rather expensive pleasure. If the cost of single-beam devices with a short range still lies within a few thousand rubles, then with an increase in the controlled range and the number of IR rays, the price increases to tens of thousands.

This is explained by the fact that security detectors of this type are quite complex electromechanical devices containing, in addition to electronics, high-precision optical devices.

By the way, passive linear detectors also exist, but in terms of maximum range they are noticeably inferior to their linear counterparts.

OUTDOOR INFRARED DETECTORS

It is quite obvious that an outdoor burglar alarm detector must have an appropriate climatic design. This applies primarily to:

operating temperature range;
degree of dust and moisture protection.

According to the generally accepted existing classification, the protection class of a street detector must be at least IP66. By and large, for most consumers, this is not important - it is quite enough to indicate "street" in the description technical parameters device. It is worth paying attention to the temperature range.

Of greater interest are the features of the use of such devices and factors affecting the reliability of protection.

By the nature of the detection zone, infrared security detectors designed for outdoor installation can be of any type (in descending order of popularity):

linear;
voluminous;
superficial.

As already mentioned, street linear detectors are used to protect the perimeter of open areas. Surface sensors can also be used for the same purposes.

Volumetric devices are used to control various kinds of areas. It should be immediately noted that in terms of range they are inferior to linear sensors. It is quite natural that the prices for outdoor detectors are much higher than for devices intended for indoor installation.

Now, with regard to the practical side of operation in burglar alarm systems of infrared outdoor detectors. The main factors provoking false alarms of security sensors installed on the street are:

the presence of various vegetation in the protected area;
movement of animals and birds;
natural phenomena in the form of rain, snow, fog, etc.

The first point may seem unprincipled, since, at first glance, it is static and can be taken into account at the design stage. However, do not forget that trees, grass and bushes grow and over time can interfere with the normal operation of security equipment.

Manufacturers try to compensate for the second factor by using appropriate signal processing algorithms, and there is an effect from this. True, whatever one may say, if an object even with small linear dimensions moves in the immediate vicinity of the detector, it will most likely be identified as an intruder.

As for the last point. Here everything depends on the change in the optical density of the medium. In simple terms, heavy rain, heavy snow or thick fog can make the infrared detector completely inoperative.

So, when deciding on the use of street security detectors in the alarm system, consider all that has been said. Thus, you can save yourself from many unpleasant surprises when operating an outdoor security system.

* * *

The site materials are for informational purposes only and cannot be used as guidelines or official documents.

Passive optical-electronic infrared (IR) detectors (they are often called motion sensors) detect the fact of a person entering the protected (controlled) part of the space, generate an alarm signal and, by opening the contacts of the executive relay (RCP relay), transmit an “alarm” signal to the warning means . As a means of warning, terminal devices (UO) of notification transmission systems (SPI) or a fire and security alarm control device (PPKOP) can be used. In turn, the above-mentioned devices (UO or PPKOP) broadcast the received alarm notification via various data transmission channels to the central monitoring station (CMS) or the local security console.

The principle of operation of passive optical-electronic IR detectors is based on the perception of a change in the level of infrared radiation of the 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.

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

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

In order for the intruder to be detected by the IR passive 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 beam 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 more planes. Due to the fact that IR detectors use dual pyro receivers, each beam in the horizontal plane is split into two:

The detector sensitivity zone can look like:

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

long corridors

It is expedient to dwell on the requirement of uniform sensitivity in more detail. The signal at the output of the pyro receiver, all other things being equal, is the greater, the greater the degree of overlapping by the violator of the detector sensitivity zone and the smaller the beam width and the 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 which 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 inclined beams, while the IR detector is installed at a height higher than human height. The total length of the sensitivity zone is thus divided into several zones, and the beams “nearest” to the detector are usually made wider to reduce sensitivity. This ensures almost constant sensitivity over the distance, which, on the one hand, helps to reduce false positives, and, on the other hand, increases the detectability by eliminating dead zones near the detector.

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

low price

Typically, each segment of a Fresnel lens forms its own beam pattern. Usage modern technologies The production of lenses makes it possible to ensure almost constant sensitivity of the detector for all beams by selecting and optimizing the parameters of each lens-segment: segment area, tilt angle and distance to the pyroelectric receiver, transparency, reflectivity, degree of defocusing. Recently, the technology of 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 a useful signal from a person at long distances. The material from which modern lenses are made protects the pyroelectric receiver from white light. The unsatisfactory operation of the IR sensor can be caused by such effects as heat fluxes resulting from heating of the electrical components of the sensor, the ingress of insects on sensitive pyro-receivers, and possible re-reflections of infrared radiation from the internal parts of the detector. To eliminate these effects in the latest generation of IR sensors, a special hermetic chamber is used between the lens and the pyro receiver (sealed optics), for example, in new IR sensors from PYRONIX and C&K. According to experts, modern high-tech Fresnel lenses are almost as good as mirror optics in terms of their optical characteristics.

Mirror optics as the only element of an optical system is rarely used. IR sensors with mirror optics are available, for example, from SENTROL and ARITECH. The advantages of mirror optics are the possibility of more accurate focusing and, as a result, an increase in sensitivity, which makes it possible to detect an intruder at long distances. The use of several specially shaped mirrors, including multi-segment ones, makes it possible to provide an almost constant distance sensitivity, and this sensitivity at long distances is approximately 60% higher than for simple Fresnel lenses. With the help of mirror optics, it is easier to protect the near zone located directly under the sensor installation site (the so-called anti-tamper zone). By analogy with interchangeable Fresnel lenses, IR sensors with mirror optics are equipped with replaceable detachable mirror masks, the use of which allows you to select the desired 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-sabotage zone under the sensor and to provide very long distance detection.

Pyro receiver:

The optical system focuses IR radiation on a pyro-detector, which is used in IR sensors as an ultra-sensitive semiconductor pyroelectric converter capable of registering a difference of several tenths of a degree between the temperature of the human body and the background. The change in temperature is converted into an electrical signal, which, after appropriate processing, triggers an alarm. In IR sensors, dual (differential, DUAL) pyroelements are usually used. This is due to the fact that a single pyroelement reacts in the same way to any change in temperature, regardless of whether it is caused by the human body or, for example, heating a room, which leads to an increase in the frequency of false positives. In the differential circuit, the signal of one pyroelectric element is subtracted from another, which makes it possible to significantly suppress interference associated with changes in the background temperature, as well as significantly reduce the effect of light and electromagnetic interference. The signal from a moving person appears at the output of the dual 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 dual pyroelement splits into two in a horizontal plane. In the latest models of IR sensors, in order to further reduce the frequency of false positives, quadruple pyroelements (QUAD or DOUBLE DUAL) are used - these are two dual pyro receivers located in one sensor (usually placed one above the other). The observation radii of these pyro receivers are made different, and therefore the local thermal source of false alarms will not be observed in both pyro receivers simultaneously. At the same time, the geometry of the location of the pyroelectric receivers and the scheme of their inclusion are chosen in such a way that the 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 quad pyroelectric elements, 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 block both beams from two pyro receivers with his height. To increase the detection distance for quad pyroelements allows the use of precision optics that form a narrower beam. Another way to correct this situation to some extent is the use of pyroelements with complex interlaced geometry, which is used by PARADOX in its sensors.

Signal processing unit

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

refrigeration units

solar radiation

The selection by the processing unit of the useful signal against the background of interference is based on the analysis of the signal parameters at the output of the pyro receiver. These parameters are the magnitude of the signal, 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 the intruder, the distance to the sensor, the width of the beam, and can be approximately 0.02 ... ,1…7 m/s. Interference signals are mostly asymmetric or have a duration different from 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 magnitude of the signal. In the simplest sensors, this recorded parameter is the only one, and its analysis is performed 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, simple sensors use a pulse counting method when it counts how many times the signal exceeded the threshold (that is, in fact, how many times the intruder crossed the beam or how many beams it crossed). In this case, the alarm is generated not when the threshold is exceeded for the first time, but only if, within a certain time, the number of exceedances becomes greater than the specified value (usually 2…4). The disadvantage of the pulse counting method is the degradation of sensitivity, which is especially noticeable for sensors with a sensitivity zone such as a single curtain and the like, when the intruder can only cross one beam. On the other hand, when counting pulses, false alarms are possible due to repeated interference (eg electromagnetic or vibration).

In more complex sensors, the processing unit analyzes the bipolarity and symmetry of the waveform from the output of the differential pyro receiver. The specific implementation of such processing and the terminology used to refer to it1 may vary from manufacturer to manufacturer. The essence of 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 polarity. It is also possible to combine this method with separate counting of excesses of positive and negative thresholds.

Signal duration analysis can be carried out both by a direct method of measuring the time during which the signal exceeds a certain threshold, and in the frequency domain by filtering the signal from the output of the pyrodetector, including using a “floating” threshold that depends on the frequency analysis range.

Another type of processing designed to improve the performance of IR sensors is automatic thermal compensation. Temperature range environment At 25°С…35°С, the sensitivity of the pyrodetector 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 the so-called “conventional” temperature compensation schemes, the temperature is measured, and when it rises, the gain is automatically increased. With “real” or “two-sided” compensation, an increase in thermal contrast is taken into account for temperatures above 25°С…35°С. The use of automatic thermal compensation ensures that the sensitivity of the IR sensor is almost constant over a wide temperature range.

The listed types of processing can be carried out by analog, digital or combined means. In modern IR sensors, digital processing methods are increasingly being used 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 noise. Recently, there have been reports of the development of fully digital IR sensors that do not use analog elements at all.
As is known, due to the random nature of useful and interfering signals, processing algorithms based on the theory of statistical decisions are the best.

Other protection elements of IR detectors

IR sensors intended for professional use use so-called anti-masking circuits. The essence of the problem lies in the fact that conventional IR sensors can be disabled by an intruder by preliminary (when the system is not armed) gluing or painting over the input window of the sensor. To combat this way of bypassing IR sensors, anti-masking schemes are used. The method is based on the use of a special IR channel that is triggered when a mask or reflective barrier appears at a small 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 signal until it is time to arm the system. It is at this moment that the operator will be given information about the 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 eliminated itself, the alarm is not generated.

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

To eliminate IR sensor triggers 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 signal processing methods are used. It should be noted that special signal processing allows animals to be ignored only if they are 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 the protected area, then such protection will not help.

For protection against electromagnetic and radio interference, a dense surface mounting and metal shielding.

Installation of detectors

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

A competent choice and tactically correct use of such detectors are the key to reliable operation of the device, and the entire security system as a whole!

When choosing the types and number of sensors to ensure the protection of a particular object, one should take into account the possible ways and means of penetration of the intruder, the required level of detection reliability; expenses for the acquisition, installation and operation of sensors; features of the object; performance characteristics of sensors. A feature of IR-passive sensors is their versatility - with their use it is possible to block from the approach and penetration of a wide variety of premises, structures and objects: windows, shop windows, counters, doors, walls, ceilings, partitions, safes and individual objects, corridors, room volumes. However, in some cases it is not necessary a large number sensors to protect each structure - it may be sufficient to use one or more sensors with the desired configuration of the sensitivity zone. Let us dwell on the consideration of some features of the use of IR sensors.

General principle the use of IR sensors - the rays of the sensitivity zone must be perpendicular to the intended direction of movement of the intruder. The location of the sensor should be chosen in such a way as to minimize the dead zones caused by the presence of large objects in the protected area that block the beams (for example, furniture, indoor plants). If indoor doors open inwards, the possibility of masking the intruder should be taken into account. open doors. If dead zones 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 of permissible suspension heights specified in the documentation (minimum and maximum height). This applies in particular to directional patterns with inclined beams: if the height of the suspension exceeds the maximum allowable, then 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 allowable, this will lead to a decrease in the range detection while reducing the dead zone under the sensor.

1. Detectors with a volume 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. Placement of 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 existing pieces of furniture affect their operation to a lesser extent.

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 the approach to any values. The detection zone of such devices should be directed, as an option, along the 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 thermal, light, electromagnetic, vibration nature can lead to false alarms of IR sensors. Despite the fact that modern IR sensors have a high degree of protection against these effects, it is still advisable to adhere to the following recommendations:

open window

sun rays

load-bearing structures

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

Thermal noise

This is the most dangerous factor, which is characterized by a change in the temperature background of the environment. The impact of 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 flows, for example, from drafts with an open window, cracks in window openings, as well as during the operation of household heating appliances - radiators and air conditioners.

Electromagnetic interference

They occur when any sources of electrical and radio emission are turned on, such as measuring and household equipment, lighting, electric motors, radio transmitting devices. Strong interference can also be created from lightning discharges.

Extraneous interference

Small insects, such as cockroaches, flies, wasps, can be a peculiar 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 danger is also represented by the so-called domestic ants, which can get inside the detector and crawl directly over the pyroelement.

Mounting errors

A special place in the incorrect or incorrect operation of passive optoelectronic IR detectors is occupied by installation errors during the installation of these types of devices. Let's pay attention to vivid examples of incorrect placement of IR detectors in order to avoid this in practice.

On fig. 6 a; 7 a and 8 a shows the correct, correct installation of the detectors. You just need to install them this way and nothing else!

In figures 6 b, c; 7 b, c and 8 b, c are options incorrect installation passive optical-electronic IR detectors. With this setting, it is possible to miss real intrusions into protected premises without issuing an “Alarm” signal.

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

Currently, there is a huge variety of detection tools that differ in the principle of operation, scope, design and performance.

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

When writing the article, materials from the journal “Security Systems” No. 4, 2013 were also used