Combined alarm sensors.

Combined security alarm sensors.

Andreev S.P.
COMBINED SECURITY ALARM SENSORS

Source: magazine «Special Equipment»

Combination sensors, also called dual technology sensors, are relatively new and are currently becoming increasingly popular. The advantage of such sensors is a significant reduction in the frequency of false alarms. This is achieved by using a combination of two different physical detection principles in one sensor. An alarm is only issued if both detectors are triggered simultaneously or within a short time interval. To reduce the frequency of false alarms, the detection principles used must be such that the interference that causes false alarms affects each detector in the combination differently.

The most widely used method at present is a combination of microwave active and IR passive detection principles. Much less commonly used is a combination of ultrasonic and IR detectors. There are also individual sensor models that use three different physical detection principles, but such sensors have not yet gained popularity. In this review, we will consider the most common group of dual-technology sensors — IR + microwave. Before moving on to a detailed analysis of the features of dual-technology sensors, it is advisable to dwell on the basic principles of the microwave detection method.

The principle of operation of the microwave active detection method is based on the emission of an electromagnetic field of the microwave range into the surrounding space and the recording of its changes caused by reflection from an intruder moving in the sensor's sensitivity zone. Microwave active sensors that implement this method belong to the class of motion detectors.

Microwave sensors consist of the following main elements:

• A microwave generator;
• An antenna system that creates an electromagnetic field in the surrounding space, receives reflected signals, forms the sensor's directional pattern and determines the shape of the spatial sensitivity zone;
• A microwave receiver that records changes in the characteristics of the received signal;
• a processing unit that identifies signals caused by a moving person against a background of interference.

The microwave sensor generator is designed to generate a microwave signal — usually in the 3-centimeter wavelength range (10 … 11 GHz), recently sensor manufacturers have begun to master shorter wavelength ranges (24 … 25 GHz). Initially, microwave sensors used generators on Gahn diodes, now manufacturers have switched to transistor generators. Modern microwave generators allow you to generate a stable signal with the required characteristics with small dimensions and low consumption.

A single combined receiving and transmitting antenna is usually used as an antenna system in microwave sensors. Most modern sensors use microstrip antennas, which have smaller dimensions, weight and cost compared to the previously widely used horn antennas. However, horn antennas continue to be used by some sensor manufacturers today, as they provide somewhat higher accuracy of directional pattern formation. Generally speaking, the shapes of microwave detector sensitivity zones are not as diverse as those of passive IR sensors. The configuration of the sensitivity zone of microwave sensors is a three-dimensional body resembling an ellipsoid in shape. Ideally, the antenna system is required to emit (and, accordingly, receive) only into the front half-space without noticeable rear and side radiation (in order to minimize false alarms). For such an ideal antenna system, the sensitivity zone is a volumetric body of a teardrop shape (see figure), characterized by viewing angles A (in the horizontal and vertical planes), length Rmax (maximum range) and width D(height). These parameters are usually given in the documentation for microwave sensors (sometimes supplemented by the values ​​of the area and volume of the room monitored by the sensor). Typical values ​​of the sensitivity zone sizes for microwave sensors are:

Rmax=10…15 m, D=5…10 m, A=60О…100О.

The sensitivity zone formed by a real antenna system differs from the ideal one – due to rear and side radiation/reception. The ratio of detection ranges in the rear and front half-spaces Rз/Rmax can be 0.03…0.1.

The above characteristics are valid for free space. When the sensor is located indoors, the shape of the sensitivity zone is significantly distorted. Due to reflection from enclosing structures (the reflection coefficient for the field from brick and reinforced concrete walls is 0.3 … 0.6), the electromagnetic field «fills» with a greater or lesser degree of uniformity almost the entire room, if the dimensions of this room do not exceed the dimensions of the sensitivity zone. On the other hand, thin partitions made of lightweight materials, wooden doors, glass, curtains are not a significant obstacle for the electromagnetic field, so the sensitivity zone can extend beyond the protected premises, which can lead to false alarms, for example, when people walk along the corridor or vehicles drive past the windows of the first floor.At the same time, large objects (cabinets, safes, etc.) located in the room create «shadows» (dead zones). All this should be taken into account when choosing the installation location and the number of sensors used.

The movement of the intruder leads to the appearance of a time-varying reflected signal. Here, two effects are distinguished: a change in the spatial pattern of standing waves and a frequency shift of the wave reflected from a moving person (the Doppler effect). Microwave sensors based on the registration of the first effect are called amplitude-modulation, the second — Doppler. Generally speaking, both of these effects are inextricably linked, have a common nature and the same manifestation, and therefore are practically inseparable. In essence, the difference is manifested in the structure of the construction and characteristics of the microwave receiver of the microwave sensor. Doppler microwave sensors, which have higher sensitivity, are the most widely used. The Doppler frequency shift df occurs when the intruder moves along the beam — the frequency of the reflected signal increases when moving towards the sensor and decreases when moving away from the sensor. The absolute value of df is proportional to the frequency of the probing signal f and the component of the velocity of movement along the beam Vл. Dependences of df on Vлare shown in the figure, from which it is evident that typical values ​​of the Doppler shift values ​​recorded by the sensor lie in the frequency range of the 50/60 Hz power grid interference and its harmonics. To combat this interference, modern microwave sensors are equipped with rejection filters (including adaptive ones) for power grid harmonics. Other sources of interference that cause false triggering of Doppler microwave sensors are reflections from vibrating, oscillating and moving highly reflective objects. Such sources of false alarms may be, for example:

• installation fittings of switched on fluorescent lamps;
• operating electrical equipment that creates vibration;
• streams of rainwater on glass;
• movement of water in plastic pipes;
• small animals and birds.

In previous years, before the widespread use of IR detectors, microwave active sensors were very popular. Now both the demand and supply of these sensors have significantly decreased. The main characteristics of microwave sensors of Russian production, intended for indoor installation, are given in Table 1. All these sensors have a continuous volumetric sensitivity zone, the possibility of adjusting the maximum detection range within wide limits is provided. The recommended installation height is 2…2.5 m. It is allowed to operate several sensors in one room – to eliminate mutual influence of signals, it is possible to select one of four operating frequencies.

Table 1.