Features and differences of two-position perimeter security systems.

Features and differences of two-position perimeter security means.

Features and differences of two-position perimeter security means.

An attempt to make a choice based on advertising materials leads to the conclusion that all detectors are practically the same. All have certificates of conformity. Prices for models with similar parameters also differ little.

Despite the fact that all the offered detectors are certified for compliance with the declared characteristics, the stability of their operation at a particular facility depends on many factors. It is impossible to analyze all of these factors within the framework of this article. Some of the most important factors are: the quality of the development of electronic units; the implemented algorithm of the detector; the ability to configure at the facility (adaptation). The ability to adapt to external conditions at a specific section of the perimeter is essential for the stable operation of the detector.

Conventionally, two-position detectors can be divided into two categories:
— devices that do not require configuration (automatic adaptation);
— devices that have configuration.

The first category of detectors requires mandatory compliance with the requirements for preparing the protected boundary. These requirements are provided in the Operating Manuals (OM). As a rule, the area must be leveled, have an exclusion zone, etc. After turning on the power, detectors of this category automatically set the signal level sufficient for further processing and the alarm threshold parameters. If the area does not meet the requirements of the OM, the detector operates unstably. The main reasons for this are: a low signal at reception and an interference level comparable in magnitude to the «useful» signal. A low signal at reception can be formed due to interference and scattering of radio waves in an area that does not meet the OM. In this case, automatic gain control does not cope with its task. An increased level of interference is formed in the presence of «extra» vegetation on the area and foreign objects moving in the exclusion zone, etc. At the same time, automatic setting of alarm thresholds is also problematic, since the electronics “do not understand” which of the signals is “interference” and which is “alarm”.

The second category of detectors should also be used in specially prepared areas. However, due to the possibility of adjustment, the requirements for the areas may be less critical. Some models of this category also have an automatic adaptation mode, which expands their operational capabilities.

To adapt the detector to external conditions in the area, it is required:

— to set the required level of the received signal, according to the recommendations of the RE;
— set the alarm threshold level (there may be one, two, three) during the «test» passes of the operator assistant when setting up the site. This stage of adaptation is the most labor-intensive, but allows you to accurately set the alarm threshold level. When the situation in the protected area changes, the threshold setting process must be repeated. As a rule, the signal for this is the appearance of frequently repeated «false alarms». If after reconfiguration the «false alarms» do not disappear, you should look for other reasons for this;
— eliminate the influence of neighboring areas (by selecting the modulation frequency of the transmitter signal, synchronizing the operation of the transmitter and receiver of the detector, by selecting the direction of polarization of radio waves).

When processing the signal in the detector receiver, important information about the intruder's speed is provided by the frequency band of the so-called «useful» signal. According to the standard, the detector must detect an intruder moving at a speed of 0.3 to 10 ms. This approximately corresponds to a frequency band of 0.1 Hz to 20 Hz. The band of these frequencies includes «industrial» electromagnetic interference. If the detector has a setting in the form of selecting the speed range of the suspected intruder, additional noise immunity can be achieved by narrowing the frequency band of the «useful» signal. Of course, the noise immunity of the detector is mainly achieved by circuit and design features.

The list of necessary adjustments is always specified in the operating instructions for a specific model of the detector. Also, the operating instructions contain recommendations for installing detectors on the ground surface, walls, fences, etc.

      Comparing the operating instructions for various models of radio wave detectors, one can notice common features of adaptation to different perimeter configurations, interference conditions, and seasonal changes. The following can be added to the recommendations set out in the operating instructions:
— do not try to install a set of detectors at the maximum possible distances. Reducing the length of the section by 10-15 percent will give you an additional reserve in signal level, which is very important if the protected section slightly does not comply with the requirements of the RE;
— do not estimate the height of the detector installation «by eye». Always use a measuring ruler and follow the recommendations set out in the RE. You should always remember that the method for setting up detectors has been developed by the manufacturer specifically for these recommended installation methods.
—  do not set the maximum sensitivity of the detectors. 
—  if «false» alarms appear, try to determine the cause. Review the EO again. See what has changed in the protected area, in the power supply system, alarm loops. Make additional adjustments.

Each specialist can compile a table of adjustment features for the most well-known two-position detectors in Russia. The table is compiled based on the information provided in the operating instructions for two-position detectors. Unlike advertising information, operating instructions are more specific in terms of the technical and operational features of detectors. By evaluating all possible adjustments for adapting detectors, one can always conclude whether the selected model is suitable for specific perimeter areas.

The data in this table will significantly save time in selecting the most suitable detector models. An example of information generation is given in Table 1.

Since manufacturers are constantly improving their products, the data in the table you compiled will have to be updated. At least annually. The example table does not reflect all detectors from Russian manufacturers. I would be very grateful for additional information regarding other detectors, as well as constructive criticism. My contact E-mail: bersenev@umirs.ru V.P. Bersenev, ZAO Firma YUMIRS

And finally, pay attention to the following. In advertising materials you may come across phrases such as «under certain conditions, our device is capable of…, which distinguishes it from its analogues.» Look in the operating instructions. If these «certain conditions» are not specifically indicated there, then this phrase is «from the evil one» or, as they say now, «an advertising ploy.» If advertising materials say that this type of detector does not require seasonal maintenance (i.e. reconfiguration when the seasons change), look in the operating instructions. If you are told «our device is capable of… and much more,» do not be shy, ask for documentary evidence of this.

Some explanations of terms.
1. PRD — transmitter unit. PRM — receiver block.

2. The modulation frequency of the signal of the transmitter of a two-position detector. As a rule, this is an amplitude pulse modulation of the «meander» type (the period of the modulation pulses is equal to the pulse time). The RX perceives the set type of modulation. In order to prevent neighboring detectors from interfering with each other, various combinations of the modulation pulse period and pulse time are used.
3. Synchronization of the operation of the transmitter and receiver of the detector is used in the case where the period of the modulation pulses is long and the pulse time is short. When synchronizing, simultaneous operation of the Tx and RX is ensured during the pulse time.

4. Direction of radio wave polarization. Radio wave detectors use a linearly polarized electromagnetic field. The plane of polarization of the RX and Tx antennas must match. If the planes of polarization of different detectors do not match, their mutual influence is weakened.

5. Crossing a protected area, a person causes amplitude modulation of the signal at the reception. In this case, the signal may increase or decrease slightly in relation to a certain constant level (the signal level at the reception in the steady-state mode — the «security» mode). «Small threshold» — the level of signal decrease when the «intruder» crosses the protected area. «Positive threshold» — the level of signal increase when the «intruder» crosses the protected area. These thresholds are adjusted when crossing zones located near the middle of the protected area. Crossing an area near the RX or Tx leads to a sharp decrease in the signal. This threshold is also adjustable in some detector models.