Despite the constant work of law enforcement agencies, the problem of theft of property of citizens and material values from enterprises remains relevant to this day. One of the ways to control the access of unauthorized persons to private or corporate property is a wireless motion sensor. What is remarkable about this device and how it is used in practice, we will consider in this article.
Types and principle of action
All wireless motion sensors, depending on their degree of protection against external factors, are divided into sensors for outdoor and indoor installation. The determining factor is the degree of dust and moisture resistance, indicated by the IP index and two digits. In accordance with sections 5 and 6 of GOST 14254-2015, digital indicators vary from 0 to 6 for dust resistance and from 0 to 9 for moisture resistance. The higher the value, the more aggressive conditions the motion sensor can work.
Depending on the principle of detecting a moving object, wireless motion detectors can be
passive, active and combined. Passive sensors are focused on their own radiation of the human body, their comparison with the state of the surrounding space. For active ones, their own generation of pulses into the controlled area and the subsequent measurement of the same radiation in the direct or reflected spectrum is natural. The combined models combine the principles of both active and passive devices.Depending on the principle of operation, motion sensors can be divided into the following categories:
- Infrared - based on the fixation of heat energy released by the human body.
- Ultrasonic - according to clause 2.1.1.3 of GOST R 50030.5.2-99, they work on the Doppler effect. When ultrasonic vibrations are generated into the surrounding space, in the event of a collision of a wave with an obstacle, part of the signal will be reflected and returned to the receiver.
- Microwave - is similar to the previous one, but instead of ultrasound, it produces microwaves with excellent sensitivity. However, in wireless models it is rarely produced due to the high power consumption.
- Radio waves - equipped with an antenna for generating radio waves, distinguished by the ability to penetrate through obstacles.
The most widespread among wireless motion sensors are infrared and radio wave models.
Infrared
The principle of operation of infrared wireless sensors is based on the operation of a sensitive pyroelectric element. Which is able to change its own conductivity depending on the temperature rise in the controlled area. The pyroelement here acts as an electronic key involved in supplying power to the actuator (relay coil or microcontroller output). The simplicity of the design provides the most affordable price among wireless motion sensors.
Due to the fact that the angle of coverage of the pitroelement is small, it is forced to expand due to the installation of a Fresnel lens, which allows the angle to be increased to 120º or more. The lens forms several sections, in some situations a sabotage protection section is provided. It forms a gripping zone under the sensor, eliminating the dead zone. The range of action of infrared sensors is about 12-15 m in the horizontal plane.
Radio waves
These are non-contact sensors that transmit an ultrasonic signal to the sensitivity zone and receive it in a reflected form. When a person moves in this area, the parameters of the ultrasonic signal will change, which will fix the measuring element of the sensor.
When movement occurs in the covered area, the principle of propagation of radio waves and the frequency of their oscillations will change. These changes are recorded by the sensitive element of the sensor and transmitted to the control circuit. After that, the control impulse is sent to the working body or sends an alarm signal to the security console.
Due to the use of a radio signal, it is able to penetrate walls, furniture and other obstacles. Thanks to this, the motion sensor itself can be safely hidden, in contrast to infrared. However, it should be remembered that the expansion of the working area is fraught with false alarms, therefore, the place of their installation and the length of the covered area are selected individually.
When installing several radio wave devices at once in one location, it is important to use models operating at different frequencies in order to eliminate mutual interference.