Microwave sensors. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sensors. Microwave sensors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

Relay pushbuttons are being replaced by microprocessor automatic process control systems (APCS) of the highest performance and reliability. Sensors are equipped with digital communication interfaces, but this does not always lead to an increase in the overall reliability of the system and the reliability of its operation. The reason is that the very principles of operation of most known types of sensors impose severe restrictions on the conditions under which they can be used.

For example, non-contact (capacitive and inductive), as well as tachogenerator speed control devices (UKS) are widely used to monitor the speed of industrial mechanisms. Tachogenerator UKS have a mechanical connection with a moving object, and the sensitivity zone of non-contact devices does not exceed a few centimeters.

All this not only creates inconvenience during the installation of sensors, but also significantly complicates the use of these devices due to dust that sticks to the working surfaces, causing false alarms. The listed types of sensors are not capable of directly controlling an object (for example, a conveyor belt) - they are tuned to the movement of rollers, impellers, tension drums, etc. The output signals of some devices are so weak that they are below the level of industrial interference from the operation of powerful electrical machines.

Similar difficulties arise when using traditional level detectors - sensors for the presence of a bulk product. Such devices are necessary for the timely shutdown of the supply of raw materials to production tanks. False alarms are caused not only by sticking and dust, but also by touching the product flow when it enters the hopper. In unheated rooms, the operation of the sensors is affected by the ambient temperature. False alarms cause frequent stops and starts of loaded process equipment - the main cause of its accidents, lead to blockages, breakage of conveyors, fire and explosion hazards.

The solution of these problems became possible due to the development of fundamentally new types of devices - radar sensors for speed control, motion and backwater sensors, the operation of which is based on the interaction of a controlled object with a radio signal with a frequency of about 1010 Hz.

Using microwave control methods monitoring the state of technological equipment allows you to completely get rid of the shortcomings of traditional types of sensors.

The salient features of these devices are:

lack of mechanical and electrical contact with the object (environment), the distance from the sensor to the object can be several meters;
direct control of the object (conveyor belt, chain), and not their drives, tension drums, etc.;
low power consumption;
insensitivity to sticking of a product due to big working distances;
high noise immunity and directivity of action;
one-time adjustment for the entire service life;
high reliability, safety, absence of ionizing radiation.

The principle of operation of the sensor is based on a change in the frequency of the radio signal reflected from a moving object. This phenomenon (Doppler effect) is widely used in radar systems for remote speed measurement. A moving object causes an electrical signal to appear at the output of the microwave transceiver module.

Since the signal level depends on the properties of the reflecting object, motion sensors can be used to signal an open circuit (belt), the presence of any objects or materials on the conveyor belt. The tape has a smooth surface and low reflectivity.

When the product begins to move past the sensor installed above the working branch of the conveyor, increasing the reflection coefficient, the device signals the movement, that is, in fact, that the belt is not empty. By the duration of the output pulse, it is possible to judge the size of the objects being moved at a considerable distance, to make selection, etc.

If it is necessary to fill any container (from the bunker to the shaft), you can accurately determine the moment when filling is completed - the sensor lowered to a certain depth will show the movement of the filler until it is filled.

Specific examples of the use of microwave motion sensors in various industries are determined by its specifics, but in general they are able to solve a wide variety of problems of trouble-free operation of equipment and increase the information content of automated control systems.

Author: Koryakin-Chernyak S.L.

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