Single-coil Eddy-current Sensor Research Articles

Reducing the Impact of Influence Factors on the Measurement Results from Single-Coil Eddy Current Sensors

Single-coil eddy current sensors (SCECS) form a separate and independent branch among the existing eddy current probes. Such sensors are often used for aviation and aerospace applications where the conditions accompanying the measuring process are harsh and even extreme. High temperatures (up to +600 °C in the compressor and over +1000 °C in the turbine of gas turbine engines), the complex shape surfaces of the monitored parts, the multidimensional movement of the power plants' structural elements, restrictions on the probes number and their placement in the measuring zone are the main factors affecting the reliability and accuracy of the measurement results obtained by the sensors. The article provides an overview of the relevant approaches and methods for reducing the impact of influence factors on the measurement results from SCECS based on the extensive experience of more than 30 years of research and development being carried out in the Institute for the Control of Complex Systems of Russian Academy of Sciences. The scope of the solutions discussed in the article is not limited to SCECS measurement systems only but can also be extended to the systems with primary transducers of other designs or other physical principles.

МОНИТОРИНГ СОСТОЯНИЯ ПАР ТРЕНИЯ ЭНЕРГОСИЛОВЫХ УСТАНОВОК НА ОСНОВЕ КЛАСТЕРОВ ОДНОВИТКОВЫХ ВИХРЕТОКОВЫХ ДАТЧИКОВ

The tasks of on-line assessment of the state of friction pairs of high-powered power plants are considered. The approaches to the solution of the tasks are offered, too. It is noted that the development of the appropriate methods and means for monitoring the parameters of the condition of bearing units of this class of machines will allow not only to detect timely the bearing damage and appearance of the pre-load state of the power plant, but also to estimate the residual life of the friction pair at each current point in time. This, in turn, facilitates the transition from the traditional routine maintenance and operation of power plants to their condition-monitored maintenance and operation. The proposed solutions are based on the use of original single-coil eddy-current sensors with sensitive elements in the forms of a wire segment or a single current circuit, as well as their cluster compositions (sensor ‘s groups or sensors with a group of sensitive elements). The simplicity of the sensor’s design and the technological solutions embedded in it ensure high technical and operational characteristics of the primary transducer and give the opportunity to use the sensors in real operating modes of power plants. It is proposed to use the information about the presence of metal wear particles of the friction pairs, washed with oil, in the lubrication system of the power plant and the axial displacement of the shaft in a radial thrust bearing as the main monitored parameters. The estimation of the bearing residual life resource can be given on the base of the information about the state of friction pair. A brief description of the methods for obtaining the information about the desired parameters is given. The principles of the construction of monitoring systems implement the methods is provided on the base of generalized structural scheme.

Wear Diagnostics of the Thrust Bearing of NK-33 Turbo-Pump Unit on the Basis of Single-Coil Eddy Current Sensors.

The problem of early wear diagnostics of the combined journal-and-thrust bearing of the turbo-pump unit (TPU) of the liquid-propellant rocket engine NK-33 is considered. A feature of the problem is the significant restriction on modifications of the power plant’s design. The original solution based on replacing the standard induction sensors of the turbo-pump rotational speed currently used in TPU by single-coil eddy current sensors (SCECS) with sensitive elements in the form of a segment of a linear conductor is proposed. The SCECS provide the monitoring of the axial displacement of the shaft in the thrust bearing, which characterizes the state of the unit and increases with the bearing wear. The function of the TPU shaft’s rotational speed measuring also remains. The article describes the proposed approach as well as a laboratory prototype of the system for early detection of the TPU thrust bearing’s wear. The results of the prototype research that confirm the feasibility of the proposed approach are analyzed.

Monitoring the state of power plants’ friction pairs on the basis of single-coil eddy-current sensors

The tasks of “on-line” assessment of the state of friction pairs of high-power power plants are considered. They are based on the use of clusters (groups) of original single-coil eddy-current sensors. It is proposed to use the information about the presence of wear particles in the lubrication system of the power plant and the axial displacement of the shaft in a radial thrust bearing as the main monitored parameters. A brief description of the methods for obtaining the information about the desired parameters and the tools that implement them is provided.

System for Measuring of Radial Clearances in Gas-Turbine Engine with Self-Compensation of Temperature Effects on the Sensor

The paper presents the description of developed hardware, algorithms and software of the system for measuring of radial clearances between stator and blade tips in the compressor of gas-turbine engine. The measuring system implements the method with self-compensation of temperature effects on the eddy-current single-coil sensor with a sensitive element in the form of a conductor segment. Due to the self-compensation the number of used sensors and related mounting holes in the power plant stator was reduced. The core operations of the self-compensation are realized in real time on hardware-and-software level. This makes it possible to use the system for the detection of dangerous states of gas-turbine engines during power plants operation. The previously unexplored phenomenon of "not-complete compensation" of temperature effects is considered. The phenomenon is related to the special features of the conversion of the single-coil eddy-current sensor’s informative parameter in the measuring circuit. It manifests in the difference of conversion functions of the system’s measuring channels under normal and nominal temperatures. The paper provides the way of the effect elimination by means of program correction. The results of experimental researches of the working model of the measuring system are given. They characterize the metrological appropriateness and efficiency of the system. The experimentally obtained calibration characteristic approximated by polynomial function was used to determine the systematic part of the error as a difference between the given and the calculated radial clearance. The random error was evaluated by the deviation of measured codes from an average value in the sample that was obtained at the specified value of radial clearance for the fixed blade’s position relatively to the sensor’s sensitive element. The efficiency of the working model was evaluated on the laboratory equipment during the rotation of the electrically driven compressor wheel. The quantitative estimates of speed and accuracy of the working model of the measuring system were obtained during experimental researches. They confirmed the possibility of using the system for the detection of dangerous states of gas-turbine engines applied in the electrical power industry.

Single-Coil Eddy Current Sensors and Their Application for Monitoring the Dangerous States of Gas-Turbine Engines.

The creation and exploitation of gas turbine engines (GTE) often involve two mutually exclusive tasks related to ensuring the highest reliability while achieving a good economic and environmental performance of the power plant. The value of the radial clearance between the blade tips of the compressor or turbine and the stator is a parameter that has a significant impact on the efficiency and safety of the GTE. However, the radial displacements that form tip clearances are only one of the components of the displacements made by GTE elements due to the action of power loads and thermal deformations during engines’ operation. The impact of loads in conjunction with natural aging is also the reason for the wear of the GTE’s structural elements (for example, bearing assemblies) and the loss of their mechanical strength. The article provides an overview of the methods and tools for monitoring the dangerous states of the GTE (blade tips clearances, impellers and shafts displacements, debris detecting in lubrication system) based on the single-coil eddy current sensor, which remains operational at the temperatures above 1200 °C. The examples of practical application of the systems with such sensors in bench tests of the GTE are given.

Influence of Disturbing Factors in a System for Measuring Radial Clearances in Gas-Turbine Engines with Temperature Self-Compensation

This paper presents the results of analysis of the effect of disturbing factors on changes in the informative parameter of a single-coil eddy-current sensor, which are used as input data in studies of a radial clearance measurement system with self-compensation of temperature effects on the sensor. Conversion functions of the measuring circuit with the connected single-coil eddy-current sensor are obtained in analytical form. The conversion functions are dependences of the extreme code values at the measuring circuit output on radial clearances and temperature effects on the sensor taking into account disturbing factors (adjacent blades, blade temperature, axial displacements of the gas turbine engine compressor rotor). Quantitative estimates are obtained for influence functions of these disturbing factors on the calculated difference of the extreme code values in the system, which characterizes the measured parameter.

Analysis of temperature influence on the informative parameters of single-coil eddy current sensors

This paper describes the study of temperature in the flowing part of a turbine on the informative parameters (equivalent inductances of primary windings of matching transformers) of single-coil eddy-current sensors with a sensitive element in the form of a conductor section, which are used as part of automation systems for testing gas-turbine engines. In this case, the objects of temperature influences are both sensors and controlled turbine blades. The existing model of electromagnetic interaction of a sensitive element with the end part of a controlled blade is used to obtain quantitative estimates of temperature changes of equivalent inductances of sensitive elements and primary windings of matching transformers. This model is also used to determine the corresponding changes of the informative parameter of the sensor in the process of experimental studies of temperature influences on it (in the absence of blades in the sensitive region). This paper also presents transformations in the form of relationships of informative parameters with radial and axial displacements at normal (20 °C) and nominal (1000 °C) temperatures, and their difference is used to determine the families of dominant functions of temperature, which characterize possible temperature errors for any radial and axial displacements in the ranges of their variation.

Modeling the process of measuring radial and axial displacements of complex-shaped blade tips

This paper gives brief descriptions of the distinctive features of the method of measuring radial and axial displacements of complex-shaped blade tips (employed in turbines) using a distributed cluster of single-coil eddy current sensors (SCECS) with a sensitive element (SE) in the form of conductor strips, known models of the electromagnetic interaction of SCECS SEs with the controlled and adjacent blades, and a measuring circuit (MC) with a cluster of two SCECSs designed for computational experiments. Results of experiments in the form of families of conversion functions (dependences of the equivalent inductances of SEs or digital codes at the MC output on the desired radial and axial displacements of the blade tip) and families of influence functions of adjacent blades on the equivalent inductances of the SE and the MC output codes are considered..

Method for measuring radial and axial displacements of complex-shaped blade tips

A new method for measuring radial and axial displacements of complex-shaped blade tips with the help of a distributed cluster of two single-coil eddy-current sensors with sensitive elements made as conductor strips is described. The main distinctive features of the method and the results of an experimental analysis of families of calibration characteristics of both sensors are considered. This analysis proves a higher sensitivity to changes in radial displacements and an extended range of axial displacement measurements of the new method as compared to the existent one.

Conversion functions of a cluster single-coil eddy-current sensor

A set of conversion functions of a cluster single-coil eddy-current sensor as the sensor inductance dependences on the blade tip coordinate displacement is described. A simplified model of electromagnetic interaction between the sensitive elements and the object simulator, that is, a rectangular plate, is developed. The model ensures calculating the set of conversion functions in an analytical form. Results of calculating the set of conversion functions, which were obtained for the given geometrical parameters of the measurement object, sensor, and current directions of its sensitive elements, are illustrated.