How is the infrared motion sensor connected? The principle of operation and purpose of the infrared motion sensor Infrared security detectors

– 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. 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, 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. 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.

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. 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?

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

It is beyond the visible red light range 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 accounts for 50 percent of all radiated energy.

Application area

Infrared motion sensors for security have been used 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. In case of violation, an incident was recorded. Then the scope expanded. Zoologists began to use camera traps to control animals under study.

Most of all, IR sensors are used in the smart home system, where they play the role of a presence sensor. When a warm-blooded object enters the area of ​​the device, it turns on the lighting in the room or on the street. Save electricity and make life easier for people.

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

The principle of operation of the IR motion sensor

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

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

The passive infrared motion sensor does not have an emitter, it reacts to changes in the surrounding infrared 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 sensitive element. An adjacent lens that controls the adjacent area sends a beam of radiation to the second sensor. The radiations of adjacent sections are approximately the same. If the balance is disturbed, if some threshold value is exceeded, the device notifies the control panel about the violation of the protection zone.

IR sensor circuit

Each manufacturer has a unique IR detector circuit diagram, but functionally they are about 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 of the optical system. Beams diverge from the device in the 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 control only one plane or several planes at different levels. Can control space in a circle or along a beam.

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 an element to the PIR.

The design of the optical system is such that the selectivity for all lenses is the same. To protect themselves from the elements' own heat, insects, a sealed chamber is installed in the device. Rarely used mirror optics. 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 neighboring beams. With the same uniform background, the sensor is silent. If an imbalance occurs, an additional heat source appears in one zone, and not in the other, the sensor is triggered.

To improve reliability and reduce false positives, quad 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 unit

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

They are very diverse:

1. solar radiation;
2. artificial IR sources;
3. air conditioners and refrigerators;
4. animals;
5. air convection;
6. electromagnetic interference;
7. vibration.

The processing block for analysis uses the amplitude, shape and duration of the output signal of the pyroelectric transducer. The impact of the intruder causes a symmetrical bipolar signal. Interference gives unbalanced values ​​to the processing module. In the simplest version, the signal amplitude is compared with a threshold value.

When the threshold is exceeded, the detector reports this by sending a certain signal to the control panel. In more complex sensors, the duration of the threshold exceeding is measured, the number of these exceedances. 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. In the latest devices, digital signal processing algorithms began to be used, which made it possible to improve the selectivity of the device.

Efficiency of using an IR detector in a burglar alarm

Its effectiveness largely depends on the correct choice of the type of sensor, location on the security object. Passive IR motion sensors for outdoor and indoor applications respond to the movement of objects that are warm compared to the background at certain movement speeds. At a low speed of movement, changes in the fluxes of infrared radiation in neighboring sectors are so insignificant that it is perceived as a background drift and does not respond 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 respond, there will be no imbalance in the radiation in neighboring areas. The person will merge with the background radiation.

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

Changes in flows are not enough to trigger the device. Especially characteristic of detectors with an animal protection function. They reduce sensitivity to avoid reactions to the appearance of pets.

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

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

Usually, a radio wave detector with high sensitivity is additionally attached, which reduces the percentage of false alarms and increases the reliability of the burglar alarm. When protecting windows from penetration, an ultrasonic detector is additionally installed that reacts to breaking glass.

Conclusion

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

Video: Motion sensor, principle of operation

What is an electronic motion sensor? The answer is obvious - a sensitive device, as a rule, from the class of security systems devices. True, there are also designs designed, for example, to control lighting sources and other devices. The operation of the motion sensor is based on the principle of generating a signal in case of detection of any movement within the boundaries of the controlled area. Devices are made on the basis of different technologies. The use of such sensitive sensors is becoming more and more popular not only in the economic and industrial sphere, but also in the household sphere. Consider what devices are produced, as well as examples of use.

Considered depending on the method of detecting the movement of an object. There are two classifications of devices:

1. Active.
2. Passive.

Active action detectors

Active action detectors are devices that operate on the principle of a radar circuit. This type of device emits radio waves (microwaves) within the controlled area. Microwaves bounce off existing objects and are received by the motion sensor.

Simplified schematic of the design of the active sensor: 1 - source (transmitter) of microwave radiation; 2 – receiver of the reflected microwave signal; 3 - scanned object

If movement is detected in the control zone at the moment of transmission by the micro-radiation sensor, an effect is created - a Doppler (frequency) shift of the wave, which is perceived along with the reflected signal.

This shear factor indicates that the wave has bounced off a moving object. Being an electronic device, the motion scan sensor is able to calculate such changes and send an electrical signal:

• to the alarm system
• on the light switch
• to other devices

schematically connected to a motion detection sensor.

Active microwave motion scanning sensors are mainly used, for example, on automatically operating doors of shopping malls. But at the same time, this type of device is well suited for home security systems or indoor lighting switching.

This kind of electronics is not suitable for outdoor lighting switching or similar applications. This is due to the massiveness of active objects in the street, which are constantly moving.

For example, the movement of tree branches from the wind, the movement of small animals, birds and even large insects are recorded by an active sensor, which leads to a trigger error.

Detectors of passive action (PIR - passive infrared)

Passive motion sensors are the exact opposite of active sensors. Passive systems don't send anything. infrared energy.

The design of the passive type sensor: 1 - Multi lens; 2 – Optical filter; 3 - quadruple infrared element; 4 - metal case; 5 - infrared radiation; 6 - stabilized power supply; 7 - amplifier; 8 - comparator

Infrared (thermal) energy levels are perceived by passive detectors that continuously scan the control area or object.

Considering that infrared heat radiates not only from living organisms, but also from any object with a temperature above absolute zero, conclusions can be drawn about the suitability of the application.

These motion detection sensors would not be effective if they could be activated by a small animal or insect that moves within the detection range.

However, most existing passive sensors can be tuned to sense motion in such a way as to monitor objects with a certain level of emitted heat. For example, the device can be tuned only to the perception of people.

Sensors of hybrid (combined) design

Combined (hybrid) motion scanning technology sensor is a combination system of active and passive circuits. triggers an action only if motion is detected by both circuits.

Combined systems are seen as useful for use in alarm modules, as they reduce the likelihood of false alarms.

However, this technology has its drawbacks. The combined instrument is not able to provide the same level of security as PIR and microwave sensors taken separately.

This is obvious since the alarm is only triggered when motion is detected by the active and passive sensors at the same time.

For example, if an intruder succeeds in some way to prevent detection by one of the sensors of the combined instrument, the movement will go unnoticed.

Accordingly, the alarm signal will not be sent to the microprocessor of the central alarm system. Today, the most popular type of combined sensors is considered to be the design where the PIR and microwave sensor circuits are combined.

Execution of motion sensors

Motion scanning sensors, developed and produced at the current time, have various shapes and overall dimensions. Below are some examples of device designs.

Passive infrared designs (PIR) - an example

One of the widely used designs that are used as part of home security circuits.

Passive infrared detectors are aimed at monitoring the change in the level of infrared energy caused by the movement of objects (human, pets, etc.).

A common design of a passive sensor, which is distinguished by the simplest electronic circuit and does not create difficulties when connecting. Only three electrical contacts are used

Scanners are passive due to the variability of heat and sunlight sources, so PIR is more suitable for motion detection indoors or in other closed environments.

Active infrared sensors - example

Active infrared detectors use a bi-directional transmission structure. One side is a transmitter, used to emit an infrared beam.

The other side is the receiver, used to receive the infrared signal. An alarm action occurs when an interruption of the beam connecting two points is detected.

An example of a single-beam active motion detection detector. Meanwhile, there are designs of a more complex configuration, thanks to which it is possible to solve various problems.

Active motion scanning sensors such as "Infra Red Beam" are mainly installed outdoors (outdoors).

Detection occurs through the use of transmitter and receiver theory. It is important that the infrared beam passes through the scanning area and reaches the receiver.

Ultrasonic Detector - Example

Motion scanning sensors using ultrasound are available in designs that can operate in both active and passive modes. Theoretically, an ultrasonic detector operates on the transmit-receive principle.

One example of a design based on ultrasound. Versatile systems that support functionality in both active and passive modes

High-frequency sound waves are sent, which are reflected from objects and perceived by the scanning receiving device of the device. If the sequence of sound waves is interrupted, the active ultrasonic sensor gives an alarm.

Applications of Motion Detection Sensors

Some of the key applications of detectors when it comes to tracking motion are:

• intrusion alarms
• automatic gate control
• switching lighting at the entrance,
• emergency security lighting,
• toilet hand dryers,
• automatic door opening, etc.

Ultrasonic sensors are used to control a residential property security camera or, for example, to capture wildlife.

Infrared sensors are used to confirm the presence of products on conveyor belts

Below is a practical example of using active and passive motion detection sensors.

Liquid level controller on ultrasonic sensors

The diagram below shows how the controller () controls the liquid level using an ultrasonic sensor.

The system works by providing accurate levels of liquid in the tank, controlling the engine, determining the specified limits of the liquid.

A practical example of the implementation of a task based on an ultrasonic device and the popular Arduino kit, clearly demonstrating what an ultrasonic motion sensor is and how it works

When the liquid in the tank reaches the lower and upper limits, the ultrasonic sensor detects these limits and sends signals to the microcontroller.

The microcontroller is programmed in such a way as to control the relay, which in turn controls the pump motor. The signals of the limit conditions set on the ultrasonic motion sensor are taken as a basis.

Automatic door opening on PIR

As in the above system, an automatic door opening system using a PIR motion sensor. In this case, the presence of people is detected and the door operation (opening or closing) is performed.

Another scheme, where a passive device is already involved. The popular Arduino constructor is also used here - a tool convenient for experiments and building real electronic systems.

The presence of people is detected by the PIR detector, after which a motion detection signal is sent to the microcontroller.

Depending on the signals from the PIR sensor, the microcontroller controls the door motor in forward and reverse modes using an IC driver.

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 a security device, an element that reflects IR rays is additionally 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 a volumetric one, is that any obstacle for it is opaque, thus creating 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.

The range of active linear devices is 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 of the technical parameters of the 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.

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The principle of operation of passive IKSO. The principle of operation of passive ICSOs is based on the registration of signals generated by the heat flux emitted by the detection object. The useful signal at the output of the inertialess single-site radiation receiver is determined by the expression:

where S u is the voltage sensitivity of the radiation receiver, is the change in the magnitude of the heat flux incident on the input window of the optical system and caused by the movement of the object in the detection zone.

The maximum value corresponds to the case when the object is completely within the field of view of the ICS. Let's denote this value as

Assuming that the losses in the optical system are so small that they can be neglected, we can express them in terms of the object and background parameters. Let within the background, the surface of which has an absolute temperature T f and emissivity E f, an object appears whose absolute temperature Tob, and emissivity Eov. The area of ​​the object projection onto the plane perpendicular to the direction of observation is denoted as soe, and the background projection area in the field of view - B f. Then the value of the heat flux incident on the input window of the optical system before the appearance of the object is determined by the expression:

where is the distance from the input window to the background surface; 1. f - background brightness; S BX - area of ​​the input window of the optical system.

The value of the heat flux created by the object is determined in a similar way:

where t - distance from IKSO to the object; - the brightness of the object.

In the presence of an object, the heat flux incident on the input window is created by the object and that part of the background surface that is not shielded by the object, from which the total heat flux

Then the change in the heat flux AF is written as:

Assuming that the Lambert law is valid for the object and the background, we express the brightness Lo6 and b f through emissivity and absolute temperatures:

where is the Stefan-Boltzmann constant.

Substituting and into, we obtain an expression for the AF in terms of the absolute temperatures and emissivities of the object and background:

For given parameters of the optical system and radiation receiver, the signal value in accordance with is completely determined by the change in irradiance DE.

The emissivity of human skin is very high, on average it is 0.99 relative to a black body at wavelengths greater than 4 microns. In the IR region of the spectrum, the optical properties of the skin cover are close to those of a black body. The temperature of the skin depends on the heat exchange between the skin and the environment. The measurements carried out with the help of the Aga-750 thermal imager showed that at an air temperature of +25°С, the temperature on the surface of a person's palm varies within +32 ... + 34°С, and at an air temperature of +19°С - in within +28...+30°С. The presence of clothing reduces the brightness of the object, since the temperature of clothing is lower than the temperature of bare skin. At an ambient temperature of +25°C, the measured average body surface temperature of a person dressed in a suit was +26°C. The emissivity of clothing can also be different from that of bare skin.

Other parameters included in the expression may take on different values ​​depending on the specific situation and/or operational task.

Let us consider in more detail the process of signal formation and the main types of interference that affect the false operation of passive ICSOs.

Signal formation. For a better understanding of the methods and algorithms for improving the noise immunity of ICSO, it is necessary to have an idea about the main parameters of the signal - the shape, amplitude, duration, dependence on the speed of human movement and background temperature

Consider one beam detection zone 10 m long with a beam diameter at the base of the cone 0.3 m. It is assumed that a person crosses the zone normal to it with maximum and minimum speeds at a distance of 10, 5 and 1 m from the receiver. at a distance of 10 m has the form of a triangle with a maximum when the zone is completely covered. On fig. 4.8.6 shows the spectrum of this signal. When crossing the beam at a shorter distance, the signal takes the form of a trapezoid with steep fronts, and the spectrum of this signal takes the form shown in Fig. 4.9.6.

Obviously, the duration of the signal is inversely proportional to the speed of movement and the distance to the receiver.

The real signal differs from the ideal picture due to the distortions introduced by the amplification path and the imposition of chaotic noise created by background temperature fluctuations. Recordings of real signals obtained using the domestic pyro receiver PM2D are shown in fig. 4.10. Its spectral characteristics are also presented here, obtained by passing the actually recorded signals through the spectrum analyzer of the company

The analysis of the records allows one to determine the spectral "window" necessary for the transmission of signals generated when crossing the zone at any place in the entire speed range from 0.1 to 15 Hz. At the same time, signal weakening is possible at the edges of the range, since the pyroelectric receiver has an amplitude-frequency characteristic with a decline in the region of 5 ... 10 Hz. To compensate for it, it is necessary to introduce a special corrective amplifier into the signal processing path, which provides an increase in the frequency response in the region of 5 ... 20 Hz.

temperature contrast. The signal amplitude, as already mentioned, is determined by the temperature contrast between the human body and the background to which the beam is directed. Since the background temperature changes following the change in the room temperature, the signal proportional to their difference also changes.

At the point where the temperature of the person and the background coincide, the value of the output signal is zero. At higher temperatures, the signal changes sign.

The background temperature in the room reflects the state of the air outside the room with some delay due to the thermal inertia of the structural materials of the building.

The temperature contrast also depends on the temperature of the outer surface of a person, i.e. mostly from his clothes. And here the following circumstance turns out to be significant. If a person enters the room where the IKSO is installed from the outside, for example, from the street, where the temperature can differ significantly from the temperature in the room, then at the first moment the thermal contrast can be significant. Then, as the clothing temperature "adapts" to the room temperature, the signal decreases. But even after a long stay in the room, the signal strength depends on the type of clothing. On fig. 4.11 shows the experimental dependences of the temperature contrast of a person on the ambient temperature. The dashed line shows the extrapolation of experimental data for temperatures above 40°C.

Shaded area 1 is the range of contrasts depending on the form of clothing, the type of background, the size of the person and the speed of his movement.

It is important to note that the transition of the temperature contrast value through zero occurred only if, in the temperature range of 30...39.5°C, the measurements were carried out after the adaptation of a person in a heated room for 15 minutes. In the case of an intrusion into the CO sensitivity zone of a person who was previously in a room with a temperature below 30°C or in the open air with a temperature of 44°C, the signal levels in the temperature range of 30...39.5°C lie in area 2 and do not reach zero.

The temperature distribution over the human surface is not uniform. It is closest to 36°C on the open parts of the body - the face and hands, and the temperature of the surface of the clothes is closer to the background of the room. Therefore, the signal at the input of the pyro-receiver depends on which part of the body overlaps the beam zone of sensitivity.

Consideration of the signal formation process allows us to draw the following conclusions:

The signal amplitude is determined by the temperature contrast of the human surface and the background, which can range from fractions of a degree to tens of degrees;

The signal shape has a triangular or trapezoidal shape, the signal duration is determined by the intersection of the beam zone and, when moving along the normal to the beam, can be from 0.05 to 10 s. When moving at an angle to the normal, the duration of the signal increases. The maximum spectral density of the signal lies in the range from 0.15 to 5 Hz;

When a person moves along the beam, the signal is minimal and is determined only by the temperature difference between individual sections of the person's surface and amounts to fractions of a degree;

When a person moves between the beams, the signal is practically absent;

At a room temperature close to the human body surface temperature, the signal is minimal; the temperature difference is fractions of a degree;

Signal amplitudes in different beams of the detection zone can differ significantly from each other, as they are determined by the temperature contrast of the human body and the area of ​​the background to which this beam is directed. The difference can be up to ten degrees.

Interference in passive IKSO. Let's move on to the analysis of interference effects that cause false operation of passive ICSOs. By interference we mean any influence of the external environment or internal noise of the receiving device that is not associated with the movement of a person in the SO sensitivity zone.

There is the following classification of interference:

Thermal, caused by background heating when exposed to solar radiation, convection air flows from the operation of radiators, air conditioners, drafts;

Electrical, caused by pickups from sources of electrical and radio emissions on individual elements of the electronic part of the CO;

Own, due to the noise of the pyro receiver and the signal amplification path;

Outsiders associated with the movement in the CO sensitivity zone of small animals or insects on the surface of the CO input optical window.

The most significant and "dangerous" interference is thermal, caused by a change in the temperature of the background areas, to which the beam sensitivity zones are directed. Exposure to solar radiation leads to a local increase in the temperature of individual sections of the wall or floor of the room. At the same time, a gradual change in temperature does not pass through the filtering circuits of the device, however, its relatively sharp and "unexpected" fluctuations, associated, for example, with the shading of the sun by passing clouds or the passage of vehicles, cause interference similar to the signal from the passage of a person. The noise amplitude depends on the inertia of the background to which the beam is directed. For example, the temperature change time of a bare concrete wall is much longer than that of a wooden or wallpapered wall.

On fig. a record of a typical solar interference at the output of a pyro receiver during the passage of a cloud, as well as its spectrum, is given.

In this case, the change in temperature during solar interference reaches 1.0 ... 1.5 ° C, especially in cases where the beam is directed to a low-inertia background, for example, a wooden wall or a curtain made of fabric. The duration of such interference depends on the speed of shading and can fall within the range of speeds characteristic of human movement. It is necessary to note one significant circumstance that makes it possible to deal with such interference. If two beams are directed to neighboring areas of the background, then the type and amplitude of the interference signal from the sun exposure are almost the same in each beam, i.e. there is a strong interference correlation. This allows the appropriate design of the circuit to suppress them by subtracting signals,

Convective interference is caused by the influence of moving air streams, such as drafts with an open window, cracks in the window, as well as household heating appliances - radiators and air conditioners. Air flows cause chaotic fluctuations in the background temperature, the amplitude and frequency range of which depend on the air flow velocity and background surface characteristics.

In contrast to solar irradiation, convective interference from various sections of the background, affecting even at a distance of 0.2 ... 0.3 m, is weakly correlated with each other and their subtraction has no effect.

Electrical interference occurs when any sources of electrical and radio emission, measuring and household equipment, lighting, electric motors, radio transmitting devices are turned on, as well as current fluctuations in the cable network and power lines. Lightning discharges also create a significant level of interference.

The sensitivity of the pyroelectric receiver is very high - when the temperature changes by 1 ° C, the output signal directly from the crystal is a fraction of a microvolt, so interference from interference sources of several volts per meter can cause an interference pulse thousands of times greater than the useful signal. However, most of the electrical interference has a short duration or a steep edge, which makes it possible to distinguish them from the useful signal.

The inherent noises of the pyro receiver determine the highest sensitivity limit of the ICSO and have the form of white noise. In this regard, filtering methods cannot be used here. The noise intensity increases as the crystal temperature rises by about a factor of two for every ten degrees. Modern pyroelectric receivers have a level of intrinsic noise corresponding to a temperature change of 0.05...0.15°C.

Conclusions:

1. The spectral range of interference overlaps the range of signals and lies in the region from fractions to tens of hertz.

2. The most dangerous type of interference is solar background illumination, the effect of which increases the background temperature by 3...5°C.

3. Interference from solar irradiation for close areas of the background is strongly correlated with each other and can be attenuated when using a two-beam scheme for constructing CO.

4. Convective interference from thermal household appliances has the form of fluctuating random temperature fluctuations, reaching 2 ... 3 ° C in the frequency range from 1 to 20 Hz with a weak correlation between the beams.

5. Electrical interference is in the form of short pulses or step actions with a steep edge, the induced voltage can be hundreds of times higher than the signal.

6. Intrinsic noises of the pyroelectric receiver, corresponding to the signal when the temperature changes by 0.05...0.15°C, lie in the frequency range that overlaps the signal range, and increase in proportion to the temperature approximately twice for every 10°C.

Methods for improving the noise immunity of passive ICSOs.Differential reception method Zh-radiation has become quite widespread. The essence of this method is as follows: with the help of a two-site receiver, two spatially separated sensitivity zones are formed. The signals generated in both channels are mutually subtracted:

It is clear that two spatially separated sensitivity zones cannot be crossed by a moving object at the same time. In this case, the signals in the channels appear alternately, therefore, their amplitude does not decrease. It follows from the formula that the noise at the output of the differential receiver is zero if the following conditions are met together:

1. The forms of interference in the channels are the same.

2. The amplitudes of interference are the same.

3. Interferences have the same time position.

In the case of solar interference, conditions 1 and 3 are satisfied. Condition 2 is satisfied only if the same material serves as the background in both channels or the angles of incidence of solar energy on the background are the same in both channels or in both channels, the solar radiation flux falls on the entire area of ​​the background that limits the sensitivity zone. On fig. the dependence of the noise amplitude at the output of the differential stage on the noise amplitude at its input is shown.

The parameter is the ratio of the amplitudes of interference effects in the channels. In this case, we mean that conditions 1 and 3 are satisfied.

From fig. it can be seen that with a sufficiently good coincidence of the amplitudes of interference effects in the channels, a 5 ... 10-fold suppression of these interferences is achieved. For U B xi/U B x2> 1.2, the interference suppression decreases and the characteristic oui = / tends to a similar characteristic of a single receiver.

Under the influence of convective interference, the degree of its suppression by a differential receiver is determined by the degree of its correlation at spatially spaced points of the background surface. Estimation of the degree of spatial correlation of convective interference can be carried out by measuring its intensity with differential and conventional methods of reception. The results of some measurements are shown in fig. 4.14.

Optimal frequency filtering. Effective interference suppression by this method is possible with a significant difference in the frequency spectra of signals and interference. It follows from the above data that there is no such difference in our case. Therefore, the use of this method for the complete suppression of interference is not possible.

The main type of noise that determines the sensitivity of the ICSO is the intrinsic noise of the receiver. Therefore, optimizing the bandwidth of the amplifier depending on the signal spectrum and the nature of the noise of the receiver makes it possible to realize the limiting capabilities of the receiving system.

Optical spectral filtering. The essence of the method of optical spectral filtering is the same as in the case of optimal frequency filtering. With spectral filtering, noise is suppressed due to differences in the optical spectra of signals and noise. These differences are practically absent for convective interference and for the solar interference component arising due to the change in the background temperature under the action of solar radiation, however, the spectrum of the solar interference component reflected from the background differs significantly from the signal spectrum. The spectral density of the energy luminosity of a blackbody is determined by Planck's formula:

where is the wavelength; k - Boltzmann's constant; T - body temperature; h is Planck's constant; c is the speed of light.

A graphical representation of the function normalized to the contrast radiation of the object and solar radiation is shown in Fig. 4.15.

According to the classical theory of linear optimal filtering, in order to ensure the maximum signal-to-noise ratio, the spectral passband of the optical filter must be matched with the contrast radiation spectrum of the object and have the form shown in Fig. 4.15.

Of the mass-produced materials, oxygen-free glass IKS-33 satisfies this condition most completely.

The degree of suppression of solar interference by these filters for various backgrounds is shown in Table. 4.1. The table shows that the greatest suppression of solar interference is achieved by the IKS-33 filter. Black polyethylene film is somewhat inferior to IKS-33.

Thus, even when using the IKS-33 filter, solar interference is suppressed by only 3.3 times, which cannot lead to a radical improvement in the noise immunity of a passive optical detection tool.

Optimal spatial-frequency filtering. It is known that the characteristics of detection under conditions of optimal linear filtering are uniquely related to the value of the signal-to-noise ratio. To evaluate and compare them, it is convenient to use the quantity

where U - signal amplitude; - signal power spectral density; - interference power spectral density.

Table 1. The degree of solar interference suppression by various filters for various backgrounds

Physically, the value is the ratio of the signal energy to the interference power spectral density. Obviously, when the solid angle of the elementary sensitivity zone changes, the intensity of the interference emitted by the background and entering the receiving channel changes. At the same time, the signal amplitude depends on the geometric shape of the elementary sensitivity zone. Let us find out at what configuration of the elementary sensitivity zone the value of q reaches its maximum value, for which we consider the simplest detection model. Let the sensitivity zone of the ICSO be fixed relative to the background, and the object to be detected moves with an angular velocity Vo6 relative to the point of observation. The sensitivity zone and the object in the plane normal to the optical axis are rectangular, and the angular dimensions of the object and the field of view are so small that it can be considered with a sufficient degree of accuracy

where is the solid angle at which the object is seen; is the solid angle of the sensitivity zone; is the angular size of the object

responsible in the horizontal and vertical planes; the angular size of the sensitivity zone, respectively, in the horizontal and vertical planes;

The energy brightness of the object B about is the same over its entire surface, and the spectral density of the energy brightness of the background noise is the same over the entire background surface. The signal and background noise are additive. The movement of the object occurs uniformly in the plane of the angle a. The energy receiver is inertialess, quadratic. The signal from the receiver is fed to a tunable optimal filter. Then the spectral power density of the background interference at the output of the receiver will be determined by the expression:

where Copt- transmission coefficient of the optical system; To t- transmission coefficient of the signal propagation path; To P- receiver sensitivity.

When an object crosses the field of view, a signal pulse is generated at the output of the receiver, the shape of which and the spectrum, in the case where u, are determined by the expressions:

where U0 is a signal pulse of unit amplitude; - spectrum of a signal pulse of unit amplitude.

For a noise emitting background whose power spectral density has the form, the value of the output of the inertialess receiver in accordance with the expression is determined as

The nature of the dependence of the quantity o and has the form shown in Fig. 4.16. From the foregoing, it follows that in order to ensure the maximum signal-to-background noise ratio, the shape of the sensitivity zone should be associated with the shape of the object.

For the case of fluctuating background noise, the maximum value of the signal/background noise ratio is achieved when the geometric shape of the elementary sensitivity zone coincides with the shape of the object. This conclusion is also applicable for the case of impulse solar interference. This is confirmed by the obvious fact that when the solid angle of the sensitivity zone increases from a value equal to the solid angle under which the object is visible, the signal amplitude does not change, and the amplitude of solar interference increases in proportion to the solid angle of the sensitivity zone. That is, the method of optimal spatial-frequency filtering makes it possible to increase the noise immunity of a passive optical detection means to both convective and solar interference.

Dual-band method for receiving infrared radiation. The essence of this method lies in the introduction of a second channel into the ICSO, which provides reception of IR radiation in the visible or near-IR ranges, in order to obtain additional information that distinguishes a signal from interference. The use of such a channel in conjunction with the main channel in the conditions of one room is ineffective, since both the signal and the interference in the presence of illumination are formed in both spectral ranges. Much more effective is the use of a visible range channel when it is installed outside protected premises, in places inaccessible to blocking this channel with artificial light sources. In this case, when the solar illumination changes, the channel generates a signal that prohibits the possible operation of the ICSO under the influence of solar interference. With such an organization, the dual-band method makes it possible to completely eliminate false positives of the ICSO, which are possible due to the occurrence of solar interference. The possibility of blocking the thermal channel for the duration of the interference is obvious.

Parametric methods for improving the noise immunity of IKSO. The basis of parametric methods for improving the noise immunity of ICSO is the identification of useful signals by one or a combination of parameters characteristic of the objects causing the appearance of these signals. As such parameters, the speed of the object, its dimensions, the distance to the object can be used. In practice, as a rule, specific parameter values ​​are not known in advance. However, there is some area of ​​their definition. So, the speed of a person moving on foot is less than 7 m/s. The combination of such restrictions can significantly narrow the domain of definition of a useful signal and, therefore, reduce the probability of a false alarm.

Let us consider some ways of determining the parameters of an object during its passive optical detection. To determine the speed of the object, its linear size in the direction of movement and the distance to it, it is necessary to organize two parallel zones of sensitivity, spaced apart in the plane of the object's movement by a certain base distance L. Then it is easy to determine that the speed of the object is normal to the zones of sensitivity

where is the delay time between signals in the receiving channels.

Linear dimension of the object bob in the plane normal to the sensitivity zones is defined as

where thio .5 - the duration of the signal pulse at the level of U=0.5U max .

Provided, the distance to the object is determined by the expression

where is the angular size of the elementary sensitivity zone in radians; is the duration of the front of the signal pulse.

Obtained parameter values wob, b^, D o6 are compared with the areas of their definition, after which a decision is made to detect the object. In the case when the organization of two parallel zones of sensitivity is impossible, the parameters of the signal pulse can serve as identifying parameters: rise time, pulse duration, etc. The main condition for the implementation of this method is a wide bandwidth of the receiving path, which is necessary for receiving the signal without distorting its shape, i.e. in this case, the use of the optimal filtering method is excluded. The parameter that is not distorted in the process of optimal filtering is the duration of the delay between signals that occurs in space-diversified channels. Therefore, identification by this parameter can be performed without expanding the bandwidth of the receiving path. In order to identify a useful signal in an ICSO with a multi-beam sensitivity zone in terms of the parameter m 3, it is necessary that it be formed in the plane of movement of the object using independent receivers.

For example, consider the areas of definition of the parameters of the signal pulse and the value of m 3 for a single-position ICSO with a multi-beam sensitivity zone at real values ​​of the angular divergence of the elementary sensitivity zone a n = 0.015 rad, the size of the entrance pupil d = 0.05 m and the angle between the sensitivity zones a p = 0.3 rad.

The pulse duration at the zero level is determined by the expression

Pulse duration domain for velocity range V O 6 \u003d 0.1.7.0 m / s, is t io \u003d 0.036 ... 4.0 s. Dynamic Range

The domain for determining the pulse duration at the level of 0.5U max is already 0.036 ... 2.0 s, and the dynamic range

The duration of the front of the signal pulse is determined by the expression

Where is the domain of definition, and dynamic

range

The duration of the delay between pulses that occur in adjacent channels can be determined by the formula:

Delay value definition range0...30 s. For the accepted value d=0.05 m and the range D o6 = 1...10 m, the definition area is 4.5...14.0, and the dynamic range is 3.1.

With d=0 dynamic range for all range values Do6=0...10 m.

Thus, the most stable identifying parameter is the value of t 3 /tf.

Due to the synchronism of the appearance of solar interference in spatially separated channels, noted in Sec. 4.3, there is the possibility of complete detuning from it using the parameter

The use of independent channels makes it possible to increase the resistance of the device to convective interference, since the final decision on detection is made only if signals are detected in at least two channels during a certain time interval determined by the maximum possible delay of the signal pulse between the channels. In this case, the probability of a false alarm is determined by the expression

where RLS1. Рlsg - false alarm probabilities in separate channels.

Comparative analysis of methods for improving the noise immunity of IKSO. The above methods for improving the noise immunity of ICSO are quite diverse both in their physical essence and in the complexity of implementation. Each of them individually has both certain advantages and disadvantages. For the convenience of comparing these methods in terms of the combination of positive and negative qualities, we will compile a morphological table. 4.2.

It can be seen from the table that no single method can completely suppress all interference. However, the simultaneous use of several methods makes it possible to significantly increase the noise immunity of the ICSO with a slight complication of the device as a whole. According to the totality of positive and negative qualities, the most preferable combination is: spectral filtering + spatial-frequency filtering + parametric method.

Let's consider the main methods and means implemented in practice in modern ICSO, which allow to provide a sufficiently high probability of detection with a minimum frequency of false alarms.

To protect the receiving device from the effects of radiation outside the spectral range of the signal, the following measures are taken:

The entrance window of the pyromodule is closed with a germanium plate that does not transmit radiation with a wavelength of less than 2 μm;

The entrance window of the entire CO is made of high-density polyethylene, which provides sufficient rigidity to maintain the geometric dimensions and at the same time does not transmit radiation in the wavelength range from 1 to 3 microns;

Table 2. Methods for improving the noise immunity of IKSO

	Positive traits	Negative qualities
Differential		Low noise immunity to uncorrelated noise
Frequency filtering	Partial suppression of solar and convective interference	Complexity of implementation for multichannel systems
Spectral filtering	Ease of implementation. Partial suppression of solar interference.	Convective interference is not suppressed
dual band	Complete solar interference suppression, Easy processing path	Possibility of blocking means by external light sources. Convective interference is not suppressed. The need for an additional optical channel
Optimal Spatial Frequency Filtering	Partial suppression of background and solar interference. Ease of implementation	The need to use receivers with a special shape of the sensitive area
Parametric methods	Partial suppression of background noise. Significant solar interference suppression	Complexity of the processing path

Fresnel lenses are made in the form of concentric circles stamped on the surface of the entrance window from polyethylene with a focal length corresponding to the maximum radiation level characteristic of the human body temperature. Radiation of other wavelengths will be "smeared", passing through this lens and, thereby, attenuated.

These measures make it possible to reduce the impact of interference from sources outside the spectral range by thousands of times and ensure the possibility of functioning of the ICSO in conditions of strong sunlight, the use of lighting lamps, etc.

A powerful means of protection against thermal interference is the use of a two-platform pyro receiver with the formation of a two-beam sensitivity zone. The signal during the passage of a person occurs sequentially in each of the two beams, and thermal noise is highly correlated and can be attenuated using the simplest subtraction scheme. In all modern passive ICSOs, two-platform pyroelements are used, and in the latest models, quad pyroelements are also used.

At the beginning of the consideration of signal processing algorithms, the following remark should be made. Different terminology may be used by different manufacturers to designate an algorithm, since a manufacturer often gives a unique name to some processing algorithm and uses it under its own brand, although in fact it may use some traditional signal analysis method used by other companies .

Algorithm optimal filtration involves the use of not only the amplitude of the signal, but all its energy, i.e., the product of the amplitude and duration. An additional informative sign of the signal is the presence of two fronts - at the entrance to the "beam" and at its output, which allows you to tune out many interferences that look like "steps". For example, in IKSO Vision-510, the processing unit analyzes the bipolarity and symmetry of the waveform from the output of a differential pyro receiver. The essence of the processing is to compare the signal with two thresholds and, in some cases, to compare the amplitude 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. PARADOX has named this algorithm Entry/Exit Analysis.

Due to the fact that electrical noise has either a short duration or a steep front, to improve noise immunity, it is most effective to use the detuning algorithm - highlighting a steep front and blocking the output device for the duration of their action. Thus, stable operation of CO is achieved even under conditions of intense electrical and radio interference in the range from hundreds of kilohertz to one gigahertz at field strengths up to SE/m. Passports for modern IKSO indicate resistance to electromagnetic and radio frequency interference with field strengths up to 20 ... 30 V / m.

The next effective method for improving noise immunity is to use the circuit "pulse counts". The sensitivity diagram for the most common "volumetric" COs has a multipath structure. This means that when moving, a person crosses successively several rays. At the same time, their number is directly proportional to the number of rays that form the CO detection zone and the distance traveled by a person. The implementation of this algorithm is different depending on the modification of the CO. The most commonly used is the manual setting of the switch at the expense of a certain number of pulses. Obviously, in connection with this, with an increase in the number of pulses, the noise immunity of the ICSO increases. To trigger the device, a person must cross several beams, but this may reduce the detectivity of the device due to the presence of "dead zones". The PARADOX ICSO uses a patented APSP pyro receiver signal processing algorithm that automatically switches the pulse count depending on the signal level. For high-level signals, the detector immediately generates an alarm, while working as a threshold, and for low-level signals, it automatically switches to the pulse counting mode. This reduces the chance of false alarms while maintaining the same detectability.

The following pulse counting algorithms are used in IKSO Enforcer-QX:

SPP - pulses are counted only for signals with alternating signs;

SGP3 - only groups of pulses with opposite polarity are counted. Here, an alarm condition occurs when three such groups appear within the set time.

In the latest modifications of IKSO, a scheme is used to increase noise immunity. "adapted reception". Here, the threshold automatically monitors the noise level, and as it rises, it also increases. However, this method is not free from disadvantages. With a multipath sensitivity pattern, it is very likely that one or more beams will be directed to a site of intense interference. This sets the minimum sensitivity of the entire device, including those beams where the noise intensity is negligible. This reduces the overall detection probability of the entire device. To eliminate this shortcoming, it is proposed to “reveal” the rays with the maximum noise level before turning on the device and shade them using special opaque screens. In some modifications of the devices, they are included in the delivery.

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 "floating" threshold, range-dependent frequency analysis. The threshold is set at a low level within the frequency range of the desired signal and at a higher level outside this frequency range. This method is embedded in the IKSO Enforcer-QX and has been patented under the name IFT.

Another type of processing designed to improve the characteristics of IKSO is automatic temperature compensation. In the ambient temperature range of 25...35°C, the sensitivity of the pyro receiver decreases due to a decrease in the thermal contrast between the human body and the background, and with a further increase in temperature, the sensitivity increases again, but "with the opposite sign." In the so-called "conventional" thermal compensation circuits, the temperature is measured and, as it rises, the gain is automatically increased. At "real" or "two-sided" compensation, an increase in thermal contrast is taken into account for temperatures above 25...35°C. The use of automatic thermal compensation provides an almost constant ICSO sensitivity over a wide temperature range. Such thermal compensation is used in IKSO by PARADOX and С&К SYSTEMS.

The listed types of processing can be carried out by analog, digital or combined means. In modern ICSOs, digital processing methods are increasingly being used using specialized microcontrollers with ADCs and signal processors, which makes it possible to carry out 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 ICSOs that do not use analog elements at all. In this ICSO, the signal from the output of the pyro receiver is directly fed to an analog-to-digital converter with a high dynamic range, and all processing is done in digital form. The use of fully digital processing allows you to get rid of such "analogue effects" as possible signal distortion, phase shifts, excess noise. The Digital 404 uses SHIELD's proprietary signal processing algorithm, which includes APSP, as well as analysis of the following signal parameters: amplitude, duration, polarity, energy, rise time, waveform, time of appearance, and signal order. Each sequence of signals is compared with patterns corresponding to movement and interference, and even the type of movement is recognized, and if the alarm criteria are not met, then the data is stored in memory for analysis of the next sequence or the entire sequence is suppressed. The combined use of metal shielding and software interference suppression made it possible to increase the Digital 404's immunity to electromagnetic and radio frequency interference up to 30...60 V/m in the frequency range from 10 MHz to 1 GHz.

It is known that, due to the random nature of useful and interfering signals, processing algorithms based on the theory of statistical decisions are the best. Judging by the statements of the developers, these methods are beginning to be used in the latest models of IKSO from C&K SYSTEMS.

Generally speaking, it is quite difficult to objectively judge the quality of the processing used, based only on the data of the manufacturer. Indirect signs of the SO having high tactical and technical characteristics may be the presence of an analog-to-digital converter, a microprocessor and a large amount of the processing program used.