Features of combined PIC/MV detectors.
A combined security alarm, which consists of a passive infrared (hereinafter referred to as PIR) and a microwave (hereinafter referred to as MV) alarm, is designed to detect an intruder moving within the protected area. Another name for the alarms is dual technology detectors. The key feature of combined devices is the operation of two detection channels with «I» logic, which is the main factor in reducing false alarms. The combination of PIR and MV channels is not the only one — there are alarms that combine, for example, a passive infrared and an active ultrasonic detector. But they all have two channels based on completely different physical principles. Currently, PIR + MV alarms have gained the greatest popularity worldwide. The ultrasonic channel in combined alarms has become less widespread, although it can be found in the range of some Russian and foreign manufacturers. This may be explained by the increased sensitivity of the alarms to the movement of air flows, which may limit the scope of their use. Due to the presence of two detection channels, the cost of combined detectors is at least twice as high. In developed foreign countries, this is not so critical, since most security structures are interested in reducing false alarms. For large monitoring companies with tens and hundreds of thousands of subscribers, the factor of reducing false alarms can lead to a financial gain compared to the use of standard PIR detectors. In Russia, there are not so many such companies yet, which determines the lesser distribution of such devices. But, in our opinion, this is only a temporary phenomenon and the share of combined detectors will gradually increase.
Microwave (MW) section
A few words about the operating principle of the microwave detector. The microwave electromagnetic energy emitted by the detector antenna is reflected from a moving object and fed to the receiver input. When a moving object is irradiated with electromagnetic waves, the Doppler effect occurs, which underlies the detection of a moving object against a stationary background — a change in the frequency of the reflected signal from a moving object. If the object approaches the receiver, the frequency of the reflected signal increases, if it moves away, it decreases. The frequency difference is proportional to the speed of the object. The amplitude of the signal from the receiver output depends on the reflectivity of the object, the effective surface area, the distance between the detector and the object, and the amplitude of the emitted signal. Since the detector is stationary, and the expected value of the intruder's speed lies in the range of 0.3-6 m/s, the frequency difference is in the range of 20 to 400 Hz. The decision to issue an alarm signal is made by the logical unit of the detector. The simplest threshold algorithm is based on the formation of a detection pulse when the signal in the specified range exceeds a certain threshold value. Microwave Doppler detectors are most sensitive to the longitudinal movement of the intruder in the detection zone, while infrared detectors are most sensitive to the transverse movement. In this regard, it is a mistake to think that by moving perpendicular to the radius, you can easily bypass the microwave channel without causing an alarm. In real life, the detector is installed not in an open space, but in a closed room. Therefore, the receiver receives not only signals directly reflected from the object, but also those that are the result of re-reflections from the walls of the room. As a result, the receiver will always receive a signal that is proportional to the speed of the person's movement.
Currently, manufacturers use microstrip technology to produce MW sections. Most of the unit components are manufactured by sputtering onto a single dielectric substrate. An asymmetrical strip line acts as a transmission line. Many traditional elements, such as capacitors and coils, are small thickenings of the microstrip line. In fact, all the unit components — matching transformers, couplers, resonator and antenna — are placed in a small-sized module metallized on the back and side surfaces. The detection area of the microwave detector is a rotating cardioid figure. Unlike the PIR detector, whose detection area is artificially divided into alternating sensitivity/insensitivity zones, the detection area of the MW detector is a continuous zone. It should be noted that due to the fact that the MW section is an active device — it constantly emits radio waves — more than one combined detector operating at the same frequency with others cannot be installed in one room. Therefore, many companies manufacture several identical devices with different frequency letters, which are separated by 50-100 MHz. Although some manufacturers have managed to remove the limitation on the number of detectors in one room using algorithmic methods.
PIC section
The case of a combined detector is slightly larger than a standard PIR detector: the IR section is usually located in the lower part of the case, and the MW section is located in the upper part. Unlike PIR detectors, combined devices are equipped with extended indication: one LED shows the status of the IR section, the second is responsible for the MW section, and the third LED turns on in case of an alarm. If the number of light indicators is less than three, then you need to be careful. The fact is that sometimes only due to the presence of extended indication can the installer check the operation of the microwave section, for which the customer pays a lot of extra money. Imagine a situation where the manufacturer, in pursuit of reducing the cost of its product, saved on the quality of the microwave section of the detector. What will he do first of all to hide this from the installer? Of course, he will do it so that the inspector can see only the result of the detector as a whole, and not its individual channels. Where is the guarantee then that the declared range of the MV section corresponds to the passport values? After all, it is possible to make detection in the far zone only by the PIR section, but in the middle and near zones on two channels by one indicator it will not be detected. In fact, even the presence of separate indication in the test mode is not a 100% guarantee that in the operating mode the detector will correspond to the declared characteristics.
Anti-masking function
An important feature of some models of combined detectors is the ability to detect an attempt to mask them. As is known, a simple PIR detector can be made «blind» if a transparent substance in visible light, but shielding infrared radiation, is applied to the lens using aerosol sprays. Such cases are possible if the system is disarmed, and the detectors can be freely approached, such as in museums, libraries or stores. It is in such places that it is preferable to install detectors with an anti-masking function. There are various methods for implementing this function in security detectors, which can be divided into three groups.
The first and simplest method is based on registering multiple triggerings of the microwave channel of the detector in the complete absence of a signal at the output of the IR sensor. This is a passive method that cannot detect the difference between a malfunction of the IR section and masking.
The second method is based on the analysis of the microwave field only in the near detection zone (from 0 to 1 m) and the analysis of the IR section operation. The presence of any obstacle inside this area during a set time interval (no more than a few minutes) causes the opening of the «Anti-masking» output, which is connected to a separate alarm loop.
The third method is similar to the method used in fire smoke detectors. Two (or more) diodes are placed in the detector: emitting and receiving. Normally, the receiver does not receive radiation from the diode, but if a reflective object appears near the detector, the receiver begins to receive reflected radiation, which is the basis for stating the fact of masking. This method is often used in PIR detectors, since they do not have a microwave section at all. The disadvantage of this method is that, although it can detect shielding of the detector by a foreign object, it may not register an aerosol (or varnish) sprayed on the lens, which absorbs IR radiation well. To solve this problem, a number of manufacturers have improved this principle. Special elements (for example, an optical waveguide) were added to the optical system in such a way that a signal of a given level from the emitting LED always arrived at the photodiode input under normal conditions, but when aerosol hit these elements, the signal decreased.
Causes of false alarms for the MW section
Combined detectors were specially developed to protect those premises where the probability of false triggering of PIR detectors is quite high due to strong sources of thermal radiation. Therefore, if a combined detector is used as a panacea in all difficult cases, then a conflict with customers is a matter of time. Unfortunately, sometimes you can hear puzzled questions from installers about the fact of false triggering of a combined detector. They believed that this class of detectors should not give false triggering at all, since they use two different physical principles. To clarify this issue, it is necessary to determine which environmental factors can lead to false triggering of the MV section of the detector.
1. The main factor is determined by the operating principle of the Doppler MW detector itself. For this reason, the detector may mistake the movement of various objects or their parts for the movement of an intruder, such as, for example, fan blades, unclosed doors or vents, curtains, blinds and blinds (if there is a draft). Water flowing in plastic pipes or on the surface of a window glass may also be the cause of a false alarm.
2. Penetrating ability of microwave radiation. When passing through an obstacle, the microwave radio signal is noticeably weakened due to reflection and absorption. Moreover, the weakening depends on the material of the obstacle and the frequency of the radio signal. The vast majority of detectors use two frequency ranges: S and X. The first corresponds to the frequency interval from 2.7 to 3.1 GHz and is characterized by greater penetrating ability than the X-range (10 GHz), which, in turn, has an even greater penetrating ability than the K-range radiation (22-24 GHz). Naturally, the greatest weakening of the signal due to reflection occurs in metal or metal-containing barriers. Wooden or glass barriers transmit microwave radiation well. Thus, a situation is possible when, having passed through the wall of the protected room, the signal will be reflected from a moving object and enter the receiving path of the detector. And if the thermal background in the room at this moment also changes, the combined detector will go into alarm mode. This circumstance served as the basis for equipping the detector with a smooth adjustment of the detection zone range. Another important feature of the MW section: the microwave antenna has rear lobes of the radiation pattern, which can serve as a similar reason for false triggering of the MW section. After all, it is assumed that the detector should protect the room itself, and not the adjacent corridor along which people can pass. Therefore, the smaller the rear lobes of the detector, the better.
3. Domestic animals. The movements of a cat, dog or rat on the floor of the protected area can be factors leading to false triggering of the detector.
4. The presence of fluorescent lamps in the immediate vicinity of the detector. The reason for this is as follows. The fluorescent lamp is filled with inert gas, which is periodically ionized during operation. Ionized gas reflects microwave radiation well. The lamp is switched on and off at a frequency of 50 Hz. For the microwave section of the detector, oscillations of the ionized gas can be perceived as human movement. If special measures are not taken, the microwave part of the detector will generate a false signal.
5. Impact of radio frequency signals. As in the case of the PIR detector, the MW section is subject to the induction of external signals caused by high-frequency electromagnetic fields emitted by household or industrial electronic devices. The simplest example is a cell phone. Special filters and digital signal processing algorithms are used to counteract such interference.
Most of the above factors do not affect the operation of the PIR section of the detector, except for the presence of moving objects (e.g. curtains), animals or strong radio frequency signals in the detection area. But even if no special measures are taken, the probability of a false alarm of the combined detector will always be less than for each channel separately, since the probabilities are multiplied for independent events.
Wireless combined IR + microwave detector JA-80W (Jablotron)
The wireless combined IR + microwave detector – JA-80W (OASiS series) is designed to detect motion in a protected area. Thanks to the combination of passive infrared and microwave (microwave) sensors, the sensor minimizes the likelihood of false alarms. When reacting to a change in temperature/motion in the protected area, the IR sensor is triggered, the triggering of which activates the microwave sensor. Only when the microwave (microwave) sensor confirms the motion detection, the alarm signal is sent to the system control panel. The device monitors the battery status and, if it is about to run low, notifies the user about it, but continues to work (up to 2 weeks). Like most Jablotron sensors, certain functions of the JA-80W can be adjusted to specific customer requirements using 4 DIP switches (DEL/INS, PIR NORM/HIGH, MW NORM/HIGH, MW NORM/TEST).
Wireless combined detector PIR + MW JA-80W (Jablotron)
The product is a wireless component of the OASiS system. It is designed to detect movement in a protected area. Thanks to the combination of passive infrared and microwave (UHF) sensors, the sensor minimizes the likelihood of false alarms. When reacting to a change in temperature/movement in the protected area, the passive infrared sensor is triggered, and the microwave sensor is activated. If the microwave (UHF) sensor confirms the detection of movement, an alarm signal is sent to the system central unit.
Thus, the alarm signal is generated only when both sensors are triggered, which ensures maximum immunity to false alarms.
The product monitors the battery status and, if it is about to run low, notifies the user. The detector continues to operate (up to 2 weeks) and, in addition, notifies of movement by briefly flashing the signal lamp.
Certain functions of the sensor can be adjusted to specific customer requirements using DIP switches.
Two-sensor Silver security sensors with digital signal processing unit and masking control function (Satel)
Silver digital security sensors detect motion using two types of sensors simultaneously: passive infrared (IR) and microwave (MW) sensors operating on the Doppler effect. The IR sensor is equipped with high-precision LODIFF lenses, and the MW sensor has a masking control function. These sensors are equipped with a digital signal processing unit, have an alarm memory and allow you to fine-tune the sensitivity zones of the sensors. In addition, Silver sensors monitor the voltage in the power supply network and, if a malfunction is detected, transmit an alarm signal to the control panel. To eliminate interference of various origins and reduce the frequency of false alarms, these devices are equipped with a dual pyroelement.
Prestige AMQD Plus
Prestige AMQD Plus is a detector for premises with a range of 15 m, using two methods simultaneously for higher detection accuracy — passive IR (double pyroelement) and microwave (10.525 GHz with the ability to adjust the range). Thanks to the microprocessor signal processing system, digital temperature compensation and pulse counter, as well as reliable protection from illumination, the detector ensures stable operation and a high level of protection against false alarms.
In addition, the detector is equipped with a built-in anti-masking system, the operating principle of which is based on the use of two additional infrared LEDs located in a protected section of the optical system. The radiation of these LEDs penetrates into the surrounding space through the lens, and upon receiving a return signal, the system determines that the detector has been covered with something or the lens has been painted over. In this case, a special alarm signal is generated, which is transmitted via a separate relay output.
Tower 12AM (Visonic)
Professional combined PIC + MW detector with mirror optics, digital signal processing and anti-masking function.
Long-range detection zone with lower sector protection. Grade 3 according to the European classification. There is reliable protection against masking of the detector by aerosol sprays or blocking objects. Replaceable mirrors: «curtain» or «wide angle». Volumetric detection area: 25m x 30m; 90°. «Curtain» detection area: 25 x 2 m.
Immune to animals weighing up to 18 kg. The detector has 2 levels of protection against masking. Immune to illumination: more than 15,000 lux. Operating temperature range: from -20 °C to +50 °C. Installation height: 1.5–4 m. There are front and rear tampers, three separate LEDs/remote activation.
The ultra-miniature protective screen made of hard plastic, resistant to mechanical impacts, reliably protects the optical system from vandalism.
КX15DT/AM (Pyronix)
The combined PIC + MV detector has Grade 3 according to the European classification (KX15DTAM). Active MV anti-masking creates reliable protection against masking of the detector by aerosol sprays or blocking objects. Replaceable lenses: «curtain», «wide angle» or «corridor».
Volumetric detection area: 15 m; 90°. Curtain detection area: 18 x 2 m. For KX15DTAM, protection against masking (microwave): 0–1 m. Operating temperature range: from -30 °C to +70 °C. The detector has a front tamper, three separate LEDs, is immune to electromagnetic radiation (80-2000 MHz): 75 V/m. Included: 3 replaceable lenses and 2 brackets (wall and ceiling).
The detector periodically performs self-testing, is equipped with digital automatic compensation and can operate in two AND/OR modes.
Combined detectors SOKOL-2 and SOKOL-4 (Argus-Spectrum)
SOKOL-2 (patent RU 2167432 C1): resistant to movement of animals weighing up to 20 kg. SOKOL series detectors have microprocessor signal processing with digital adjustment, a new universal housing: installation with/without a bracket (included in the delivery set). Discrete change of the maximum detection range is provided. They have high detection capacity and noise immunity, self-monitoring of channels during operation is implemented. It is also possible to operate several detectors of the same frequency letter in one room.
Protection against masking and unauthorized access. Full thermal compensation of the IR channel in the entire range of operating temperatures (from -30° to +50°C).
Resistance to radiation from fluorescent lamps.
Maximum range: Sokol-2 — not less than 12 m; Sokol-4 — not less than 10 m. Viewing angle in the horizontal plane — 90°. Range of detected speeds: 0.3-3.0 m/s. Supply voltage: 9-16 V. Current consumption: Sokol-2 — no more than 20 mA, Sokol-4 — no more than 35 mA.
OML-DAM (OPTEX)
Combined detectors of the Optimal series are designed for reliable and stable operation without false alarms at objects of any complexity. Multifocus optics and «quadruple zone logic» provide uniform distribution of sensitivity at each point of the detector's working area, and the double shielding system of the pyroelement and temperature compensation guarantee reliable protection against interference.
The microwave zone also has uniform sensitivity and can be adjusted according to the size of the room to avoid detection outside it.
The OML-DAM detectors use an active anti-masking system that generates an alarm when attempts are made to cover the lens with adhesive tape or spray paint. The sensitivity of the system automatically adjusts to compensate for changes in environmental conditions.