Microwave Sensors for Industrial Applications

Neither of production control systems can do without sources of original information, namely, sensors monitoring the behaviour of process equipment. Push-button-relay control panels are replaced with micro- processor-based process control systems demonstrating the highest performance and reliability characteristics. Sensors are furnished with digital communication interfaces, however, this does not always result in the improvement of the general reliability and veracity of the system operation. It can be explained by the fact that the very principles of operation of the majority of the known sensor types dictate the necessity for active constraints concerning the conditions of sensor application.

The main function of a process control system is in the precise observance of feedstock processing technology and feedstock conversion into end product. In addition to continuous equipment monitoring and prevention of emergency situations a competently designed system must keep track of the product handling within the complete chain of processing machinery.

Technological processes intended for the modification of the chemical composition of feedstock, in-stream blending of various materials, wetting, etc. in case of loss of any component must provide for the reliable cut-off of the rest. Spring-loaded plates with microswitches are still in use for the control of availability of a product flow. In operation they are exposed to continuous shock impacts of humid and aggressive environment that naturally results in "freezing" of contacts or mechanical failure of plates.

Thus, the absence of reliable and cheap sensors for monitoring of industrial machinery and equipment results in the decline of the efficiency of production control systems, and poorer return of funds invested in automation. Sophisticated and expensive process control systems based on conventional types of sensors are only suitable for convenient group switch-on/switch-off of processing circuits, and in many cases they are not able to improve the quality of products and to save feedstock and resources.

The necessity for continuous maintenance, control and adjustment of sensors results in downtime: for the removal of the stuck product from a capacitance sensor it is necessary to discharge a bunker, to prepare and to install a winch for the personnel access to the bunker, on completion of the maintenance process it is necessary to adjust the device; for the replacement of a velocity pick-up or drag-type sensor on a continuous-bucket elevator the complete disassembly of the elevator boot is required. As a rule it takes several hours that brings down the production rate of a facility as a whole.

Several years ago the above mentioned problems gave rise to the development of radically new types of devices, namely, radar velocity pickups, mechanical motion and backup-pressure transducers, which operation is based on the interaction of the controlled item with a nearly 1010 Hz frequency radio signal.

Application of microwave methods of process equipment monitoring is completely free from the disadvantages of conventional-type sensors. Moreover, new devices successfully eliminate the earlier unsolved problems faced by process engineers and automation departments, as well as control and measuring instruments and automatic equipment sections.

The peculiar features of these devices are as follows:
• absence of a mechanical or electrical contact with an item (medium), the distance between a sensor and an item may be several meters;
• direct monitoring of an item (conveyor belt, chain) itself rather than its drive, tension drum, etc;
• low energy intensity;
• insensitivity to product sticking due to large working distances;
• high interference immunity and precise directivity of operation;
• air-tight design;
• once-only tuning for the entire service life;
• high reliability, safety, the device is free from ionizing radiation.

The right part of the table presents a list of microwave sensors for industrial automatics, which are commercially manufactured by the PromRadar research and production company, while the left part reflects the application areas of these devices.



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