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Metrological infrastructure and improved measurement as a factor in economic development, product quality and the quality of life
A survey of the influence of technical diagnostics on progress in industry G. C. Knight Chairman of IMEKO Technical Committee on Technical Diagnostics (TC-IO) National Coal Board, Mining Research and Development Establishment, Stanhope Bretby, Burton-on-Trent, England and J. Kozak Scientific Secretary of IMEKO Technical Committee on Technical Diagnostics (TC-IO) Vyzkumny A. Zkusebni Letacky Ustav, Letnany, Prague, Czechoslovakia The ever-increasing complexity of equipment and systems in all branches of industry, together with the demand for higher efficiency and utilisation, requires a new approach to fault diagnosis, maintenance and repair. The paper examines the prospect of developing a maintenance strategy based on the regular or continuous measurement and interpretation of data indicating the state of repair or correctness of operational systems. Examples are given of the parameters presently measured to provide diagnostic information, and consideration is given to the factors influencing decisions based on these data. The paper closes by summarising the role of IMEKO in furthering the development of the technology. Introduction The optimisation of the maintenance procedures for plant and equipment to avoid unscheduled stoppages due to breakdowns, and at the same time minimise maintenance effort and maximise the intervals between repairs, is an obvious ideal. This aim can, However, only be realised if the user of the system has a complete and up-to-date knowledge of its condition. Technical diagnostics provide the method of obtaining this knowledge by the regular measurement of selected critical parameters and the interpretation of the data collected during routine operation. The application of technical dlagnosUcs does nevertheless involve the expense of the sensors, the signal processing equipment and the output devices, and it further increases the complexity of the total installed system. These aspects must be balanced against the type and scale of the system, its intensity of use and the consequences of its failure, when considering the cost-effectiveness of installing diagnostic equipment. In the early stages of development of the technology of diagnostics, the industries with the highest commitment - those with large and expensive plant used in an intensive manner, those with plant with high levels of interdependence, those with continuous process o u t p u t s , and those with critical safety a s p e c t s - will obtain the greatest benefit. In these situations very considerable practical savings will be derived from the exploitation of the technology.
Basis of technical diagnostics In the simplest approach to maintenance, machines are allowed to run until they fail to operate and then efforts are made to repair or overhaul them to make 42
them suitable for further use. This approach leads to unexpected stoppages, lost production, expensive secondary damage, lengthy periods of fault diagnosis and further delays in acquiring and installing replacement components or sub-assemblies; furthermore, the failure process may involve certain unwarranted hazards to operating personnel. As a consequence of the above problems it has been widely recognised that it is more economical to carry out some form of preventive maintenance, particularly in the case of large and expensive plant, and equipment with high levels of system dependence. This process involves the maintenance of equipment at some regular interval to forestall the possibility of a breakdown occurring at a time when the system is required to be operational. Preventive maintenance nevertheless has many problems. Much time and effort is spent dismantling equipment unnecessarily when no defect is found, and the frequent dismantling and re-assembling leads to damage to components and introduces the possibility of human error. There are fundamental problems in deciding the frequency at which maintenance should be carried out. If the interval is short enough to detect all of the possibilities of failure, examinations will be very frequent. This is expensive in terms of maintenance effort and the process may intrude into production time. The selection of intervals is further complicated in that the different components in the system may require different frequencies of examination and these may be incompatible with one another. It is therefore necessary to seek to develop a maintenance procedure that causes the minimum interruption to production schedules, the minimum maintenance effort, and permits the maximum operating time for each piece of equipment between repairs and Measurement Vol 1 No 1,Jan--Mar 1983
Knight and Kozak overhauls. Such a technique is attainable by carrying out maintenance on the basis of the actual measured condition of the plant, even though the resultant intervals between repairs may be irregular. For a maintenance strategy of this type to be effective it is necessary to measure certain parameters regularly or continuously and to compare the resultant data with expected normal values. The handling of the diagnostic data may take several forms; each progressive set of values may simply be compared with limiting values established from previous operational experience, or the rate of change of the values may be computed to show a changing operational condition. The latter approach is the most valuable since it gives the maximum amount of warning of an impending failure. This lead time can then be utilised to set up emergency actions, to schedule the necessary repairs into a non-critical period, or possibly to change the nature of the operational duty to avert a catastrophic failure. By providing quantitative information to the machine operator the need for human judgement is reduced, together with the risk of operational errors. The operator can also be provided with additional information concerning the machine's performance of which he would not otherwise be aware, for example, information on the interaction between his machine and other parts of the total system. Diagnostic
methods
The basis of all diagnostic techniques is first of all the capacity to measure critical aspects of machines and systems; secondly, to monitor the changes in the measurement with time; and, thirdly, to recognise defects from such data and to realise purposeful actions to effect control. To implement a condition-based maintenance system, the following questions need to be answered: • How is the item (eg, machine) likely to fail? • How can the symptoms before failure be predicted? • How can the presence of these symptoms be measured? • How often is it necessary to measure? • At what level of measurement, or change in measurement, is the item stopped for corrective action? It is not possible within the confines of a survey paper such as this to detail all of the situations in which technical diagnostics might be applied, but some typical industrial applications are summarised to illustrate the more commonly used parameters. Performance of machines The onset of a deterioration in performance is evidenced by detecting changes in, for example, electrical power, speed, torque, pressure, flow or rate of work. In many situations changes in condition are determined more effectively by measuring the consequences of power loss in terms of, say, temperature or electrical or hydraulic leakages. Changes in noise emission and vibration levels or spectral distribution are also effective indicators of performance changes. Measurement Vol 1 No 1, Jan-Mar 1983
Structural components Defects in structures such as bridges, oil rigs, ships' hulls and vehicle frames are detectable by measuring changes in structural stiffness, resonant frequencies or distortions in response to known loadings. Critical parts of structures may be monitored intermittently or continuously by electrical and/or mechanical strain gauges; or by the periodic examination of brittle surface films which will crack or craze in response to certain strain levels. The development of laser techniques over recent decades has presented new opportunities for monitoring deflections under stress or monitoring critical alignments upon which stability is dependent; while new developments in optical methods, and particularly those of fibre-optics, have substantially increased the capabilities of boroscopes for the internal inspection of structural members. Again, non-destructive testing techniques, such as ultrasonics, magnetic flux and dye penetrants, have been developed to high standards for the inspection of structures. Bearings The deterioration of anti-friction bearings by fatigue of the rolling elements or bearing tracks is detectable from the analysis of vibration data, or by sensing the shocks generated when the rolling member passes over the fatigue-damaged surface under load. Wear in both anti-friction and plain bearings is sensed by measuring radial changes in shaft position or shaft deflection by means of proximity sensors of various types. Cage failures in anti-friction bearings which allow the rolling elements to gather asymmetrically, or rollers to skew on their axes, cause frictional losses and are detectable by means of temperature sensors. Seizure damage caused by lubricant starvation or contaminants in the oil film are again detectable by temperature change. Lubricants and hydraulic fluids Regular sampling of lubricants and hydraulic fluids provides valuable indication of machinery condition in several ways. Degradation of the fluid evidenced by the increases in viscosity and acidity characteristic of thermal oxidation indicate the existence of high temperature zones in the s y s t e m - possibly arising from unusual frictional losses caused by component failure - or malfunction. Spectrographic analysis provides data on chemical composition and changes indicate the presence of dissolved metal from wear processes, or dissolved fuels from incomplete combustion or possibly increased carbon from ineffective combustion chamber sealing. The technique has also proved effective for the detection of the presence of iron oxides formed by such mechanisms as fretting or corrosion. The examination of solid particulate in fluids gives information on incipient failures and wear processes. Such checks are made by passing small sample quantities through fine pore-size membranes, possibly followed by high-magnification optical scanning. Alternatively, magnetic separation methods are used (ferrography) or continuous particle counting equipment (HIAC). Several of these methods allow subsequent 43
Knight and Kozak analysis of the exact composition of contaminant debris which may give indication of its possible source. Thermal contours
Macro scanning of the thermal contours of equipment provides valuable diagnostic data. Developments in high-resolution scanning of infra-red emissions by thermographic techniques have widespread application. Such methods have been well proved in detecting faults in electrical power transmission lines; thyristor banks in high-energy convertors; cupola furnaces for molten metals; electronic components in operating systems; and locating sources of energy loss in hydraulic systems. Examples of micro-scale thermal monitoring are too numerous to mention. Interesting developments have recently emerged from the capability to measure small thermal gradients with high resolution. An example of the value of such work is the monitoring of efficiency of high-capacity pumps; this can now be determined to within 1/4% with low-cost equipments. Research, too, is now progressing into extending the method to monitoring the efficiency of individual circuit components, such as valves and filters. Electronic systems
Conventional maintenance procedures can no longer be applied to electronic systems comprising many different equipments without unacceptable penalties in system down-time and cost. In former times a skilled technician would diagnose a fault down to a single component, using maintenance manuals and standard laboratory equipment. The higher the complexity of the electronic systems developed over recent years, and the more the degree of integration of equipments from different technological areas, the more obvious it becomes that conventional maintenance methods are no longer feasible. The users of complex systems - for example, in aviation, telecommunications and industrial p r o d u c t i o n - having now installed computercontrolled process automation, call for more efficient maintenance concepts. In modern electronic systems, therefore, a certain amount of special hardware is added to allow for system testing and self-diagnosis. Better still, if the system contains a digital computer and/or a microprocessor, special software is developed to detect instantaneously a failure condition and rapidly to locate the fault down at least to sub-assembly level. These may be systems of test diagnosis, in which special test input actions apply; or systems of functional diagnosis, when the item operates normally and only the working actions stipulated by the algorithm of the item functioning input to the system. Conventional test equipment generally includes a test-pattern generator applying test stimulii to the unit under test, an evaluation unit discriminating its response and a suitable display of test results. Accordingly, the test problem can be sub-divided into the aspects of test generation, test application, and evaluation of test data leading to fault detection.
Basing decisions on diagnostic data The techniques outlined above are clearly within the capability of modern instrumentation practices with 44
respect to the transducers, the signal processing, the detecting and computation, and the indication. The difficulties arise, however, in the decision-making process that follows and in the cause-to-effect understanding of the possible mechanisms of failure that leads to the outputs of the interrogation systems. This process is especially complex in the understanding of the severity of the fault that has led to the particular diagnostic information. Without this understanding, decisions to interrupt plant operation may be made for reasons that ultimately prove trivial; or conversely, problems may be ignored that subsequently lead to cataclysmic failures. In many existing applications the necessary supporting data have been determined empirically from past histories, or in cases where banks of similar machines are operating, by observing deviations from the average output of several machines. Diagnostic information derived from machines operating under normal production duties will be influenced by the nature of the load cycle, the amplitude of instantaneous values, the short-term history of load patterns and the long-term history of total usage. For the diagnostic techniques to be successful, intelligence systems must recognise these variations and respond accordingly. The problems outlined above are the subject of extensive international research and development at all levels - from the complex systems analyses using mathematical models to the experience being gained by plant operation. The design of the diagnostic system components themselves must be of the highest order. Each system must have the necessary capability in terms of its own performance and must be capable of checking its own integrity and indicating faults in its components. The availability must be such that it will operate promptly and accurately in response to a fault condition in the equipment it supervises; many systems may only be called upon to act once or twice in their whole working life, but those occasions may be vital. The unit must therefore have unquestionable reliability to carry out its assigned task in the environment in which it has to operate. The unit must be tolerant of variations in its supply voltage and to the transient effects of mainsborne or radiated transient disturbances. Without this standard of behaviour the diagnostic equipment is destined to failure through lack of confidence in the data it produces, and the decisions based upon those data.
The role of IMEKO in the further development of diagnostic methods So great are the potential advantages of technical diagnostics that it has already become a new multidisciplined branch of engineering science, and its associated technology is undergoing rapid evolutionary development. A number of different industrial branches are investigating the problems independently and progressing equipment and systems through the research, development, demonstration and exploitation stages. The sectors of the economy principally involved in the work include aviation, transportation, manufacturing, machine tools, mining, communications, power generation and chemical engineering. To promote a further increase in the rate of development of the technology, IMEKO has formed Measurement Vol 1 No 1, Jan--Mar 1983
Knight and Kozak Technical Committee TC-10, 'Technical Diagnostics', to facilitate a mutual exchange of information over a wide range of disciplines. These include electronics, computation and control systems, and mechanical and electrical engineering activities in structures and power transmissions. The objectives of this international collaboration are to seek the exchange of scientific and technical information by means of an on-going programme of symposia, and to promote co-operation between scientists and engineers in different countries working on diagnostic problems. A number of specific projects are also under development. These include the harmonisation of terminology, a bibliography of published work, the unification of systems, technological reviews of complete diagnostic systems, and a continuing international exchange of details of conferences, courses and publications. The Committee members also hold a register of specialists so that contact
Measurement Vol 1 No 1, Jan--Mar 1983
can be established through national delegates. Countries collaborating in this work include Czechoslovakia, France, German Federal Republic, German Democratic Republic, Hungary, Italy, Japan, Poland, UK, USA, USSR and Yugoslavia.
Acknowledgement The views expressed are those of the authors acting on behalf of the International Measurement Confederation (IMEKO) and not necessarily those of the organisations with which the authors are associated. The authors wish to acknowledge and thank Mr P. G. Tregelles, Director of Mining Research and Development for the National Coal Board, and the Director of Vyzkumny A Zkusebni Letacky Ustav for supporting their participation in the work of IMEKO.
45
Basis of technical diagnostics In the simplest approach to maintenance, machines are allowed to run until they fail to operate and then efforts are made to repair or overhaul them to make 42
them suitable for further use. This approach leads to unexpected stoppages, lost production, expensive secondary damage, lengthy periods of fault diagnosis and further delays in acquiring and installing replacement components or sub-assemblies; furthermore, the failure process may involve certain unwarranted hazards to operating personnel. As a consequence of the above problems it has been widely recognised that it is more economical to carry out some form of preventive maintenance, particularly in the case of large and expensive plant, and equipment with high levels of system dependence. This process involves the maintenance of equipment at some regular interval to forestall the possibility of a breakdown occurring at a time when the system is required to be operational. Preventive maintenance nevertheless has many problems. Much time and effort is spent dismantling equipment unnecessarily when no defect is found, and the frequent dismantling and re-assembling leads to damage to components and introduces the possibility of human error. There are fundamental problems in deciding the frequency at which maintenance should be carried out. If the interval is short enough to detect all of the possibilities of failure, examinations will be very frequent. This is expensive in terms of maintenance effort and the process may intrude into production time. The selection of intervals is further complicated in that the different components in the system may require different frequencies of examination and these may be incompatible with one another. It is therefore necessary to seek to develop a maintenance procedure that causes the minimum interruption to production schedules, the minimum maintenance effort, and permits the maximum operating time for each piece of equipment between repairs and Measurement Vol 1 No 1,Jan--Mar 1983
Knight and Kozak overhauls. Such a technique is attainable by carrying out maintenance on the basis of the actual measured condition of the plant, even though the resultant intervals between repairs may be irregular. For a maintenance strategy of this type to be effective it is necessary to measure certain parameters regularly or continuously and to compare the resultant data with expected normal values. The handling of the diagnostic data may take several forms; each progressive set of values may simply be compared with limiting values established from previous operational experience, or the rate of change of the values may be computed to show a changing operational condition. The latter approach is the most valuable since it gives the maximum amount of warning of an impending failure. This lead time can then be utilised to set up emergency actions, to schedule the necessary repairs into a non-critical period, or possibly to change the nature of the operational duty to avert a catastrophic failure. By providing quantitative information to the machine operator the need for human judgement is reduced, together with the risk of operational errors. The operator can also be provided with additional information concerning the machine's performance of which he would not otherwise be aware, for example, information on the interaction between his machine and other parts of the total system. Diagnostic
methods
The basis of all diagnostic techniques is first of all the capacity to measure critical aspects of machines and systems; secondly, to monitor the changes in the measurement with time; and, thirdly, to recognise defects from such data and to realise purposeful actions to effect control. To implement a condition-based maintenance system, the following questions need to be answered: • How is the item (eg, machine) likely to fail? • How can the symptoms before failure be predicted? • How can the presence of these symptoms be measured? • How often is it necessary to measure? • At what level of measurement, or change in measurement, is the item stopped for corrective action? It is not possible within the confines of a survey paper such as this to detail all of the situations in which technical diagnostics might be applied, but some typical industrial applications are summarised to illustrate the more commonly used parameters. Performance of machines The onset of a deterioration in performance is evidenced by detecting changes in, for example, electrical power, speed, torque, pressure, flow or rate of work. In many situations changes in condition are determined more effectively by measuring the consequences of power loss in terms of, say, temperature or electrical or hydraulic leakages. Changes in noise emission and vibration levels or spectral distribution are also effective indicators of performance changes. Measurement Vol 1 No 1, Jan-Mar 1983
Structural components Defects in structures such as bridges, oil rigs, ships' hulls and vehicle frames are detectable by measuring changes in structural stiffness, resonant frequencies or distortions in response to known loadings. Critical parts of structures may be monitored intermittently or continuously by electrical and/or mechanical strain gauges; or by the periodic examination of brittle surface films which will crack or craze in response to certain strain levels. The development of laser techniques over recent decades has presented new opportunities for monitoring deflections under stress or monitoring critical alignments upon which stability is dependent; while new developments in optical methods, and particularly those of fibre-optics, have substantially increased the capabilities of boroscopes for the internal inspection of structural members. Again, non-destructive testing techniques, such as ultrasonics, magnetic flux and dye penetrants, have been developed to high standards for the inspection of structures. Bearings The deterioration of anti-friction bearings by fatigue of the rolling elements or bearing tracks is detectable from the analysis of vibration data, or by sensing the shocks generated when the rolling member passes over the fatigue-damaged surface under load. Wear in both anti-friction and plain bearings is sensed by measuring radial changes in shaft position or shaft deflection by means of proximity sensors of various types. Cage failures in anti-friction bearings which allow the rolling elements to gather asymmetrically, or rollers to skew on their axes, cause frictional losses and are detectable by means of temperature sensors. Seizure damage caused by lubricant starvation or contaminants in the oil film are again detectable by temperature change. Lubricants and hydraulic fluids Regular sampling of lubricants and hydraulic fluids provides valuable indication of machinery condition in several ways. Degradation of the fluid evidenced by the increases in viscosity and acidity characteristic of thermal oxidation indicate the existence of high temperature zones in the s y s t e m - possibly arising from unusual frictional losses caused by component failure - or malfunction. Spectrographic analysis provides data on chemical composition and changes indicate the presence of dissolved metal from wear processes, or dissolved fuels from incomplete combustion or possibly increased carbon from ineffective combustion chamber sealing. The technique has also proved effective for the detection of the presence of iron oxides formed by such mechanisms as fretting or corrosion. The examination of solid particulate in fluids gives information on incipient failures and wear processes. Such checks are made by passing small sample quantities through fine pore-size membranes, possibly followed by high-magnification optical scanning. Alternatively, magnetic separation methods are used (ferrography) or continuous particle counting equipment (HIAC). Several of these methods allow subsequent 43
Knight and Kozak analysis of the exact composition of contaminant debris which may give indication of its possible source. Thermal contours
Macro scanning of the thermal contours of equipment provides valuable diagnostic data. Developments in high-resolution scanning of infra-red emissions by thermographic techniques have widespread application. Such methods have been well proved in detecting faults in electrical power transmission lines; thyristor banks in high-energy convertors; cupola furnaces for molten metals; electronic components in operating systems; and locating sources of energy loss in hydraulic systems. Examples of micro-scale thermal monitoring are too numerous to mention. Interesting developments have recently emerged from the capability to measure small thermal gradients with high resolution. An example of the value of such work is the monitoring of efficiency of high-capacity pumps; this can now be determined to within 1/4% with low-cost equipments. Research, too, is now progressing into extending the method to monitoring the efficiency of individual circuit components, such as valves and filters. Electronic systems
Conventional maintenance procedures can no longer be applied to electronic systems comprising many different equipments without unacceptable penalties in system down-time and cost. In former times a skilled technician would diagnose a fault down to a single component, using maintenance manuals and standard laboratory equipment. The higher the complexity of the electronic systems developed over recent years, and the more the degree of integration of equipments from different technological areas, the more obvious it becomes that conventional maintenance methods are no longer feasible. The users of complex systems - for example, in aviation, telecommunications and industrial p r o d u c t i o n - having now installed computercontrolled process automation, call for more efficient maintenance concepts. In modern electronic systems, therefore, a certain amount of special hardware is added to allow for system testing and self-diagnosis. Better still, if the system contains a digital computer and/or a microprocessor, special software is developed to detect instantaneously a failure condition and rapidly to locate the fault down at least to sub-assembly level. These may be systems of test diagnosis, in which special test input actions apply; or systems of functional diagnosis, when the item operates normally and only the working actions stipulated by the algorithm of the item functioning input to the system. Conventional test equipment generally includes a test-pattern generator applying test stimulii to the unit under test, an evaluation unit discriminating its response and a suitable display of test results. Accordingly, the test problem can be sub-divided into the aspects of test generation, test application, and evaluation of test data leading to fault detection.
Basing decisions on diagnostic data The techniques outlined above are clearly within the capability of modern instrumentation practices with 44
respect to the transducers, the signal processing, the detecting and computation, and the indication. The difficulties arise, however, in the decision-making process that follows and in the cause-to-effect understanding of the possible mechanisms of failure that leads to the outputs of the interrogation systems. This process is especially complex in the understanding of the severity of the fault that has led to the particular diagnostic information. Without this understanding, decisions to interrupt plant operation may be made for reasons that ultimately prove trivial; or conversely, problems may be ignored that subsequently lead to cataclysmic failures. In many existing applications the necessary supporting data have been determined empirically from past histories, or in cases where banks of similar machines are operating, by observing deviations from the average output of several machines. Diagnostic information derived from machines operating under normal production duties will be influenced by the nature of the load cycle, the amplitude of instantaneous values, the short-term history of load patterns and the long-term history of total usage. For the diagnostic techniques to be successful, intelligence systems must recognise these variations and respond accordingly. The problems outlined above are the subject of extensive international research and development at all levels - from the complex systems analyses using mathematical models to the experience being gained by plant operation. The design of the diagnostic system components themselves must be of the highest order. Each system must have the necessary capability in terms of its own performance and must be capable of checking its own integrity and indicating faults in its components. The availability must be such that it will operate promptly and accurately in response to a fault condition in the equipment it supervises; many systems may only be called upon to act once or twice in their whole working life, but those occasions may be vital. The unit must therefore have unquestionable reliability to carry out its assigned task in the environment in which it has to operate. The unit must be tolerant of variations in its supply voltage and to the transient effects of mainsborne or radiated transient disturbances. Without this standard of behaviour the diagnostic equipment is destined to failure through lack of confidence in the data it produces, and the decisions based upon those data.
The role of IMEKO in the further development of diagnostic methods So great are the potential advantages of technical diagnostics that it has already become a new multidisciplined branch of engineering science, and its associated technology is undergoing rapid evolutionary development. A number of different industrial branches are investigating the problems independently and progressing equipment and systems through the research, development, demonstration and exploitation stages. The sectors of the economy principally involved in the work include aviation, transportation, manufacturing, machine tools, mining, communications, power generation and chemical engineering. To promote a further increase in the rate of development of the technology, IMEKO has formed Measurement Vol 1 No 1, Jan--Mar 1983
Knight and Kozak Technical Committee TC-10, 'Technical Diagnostics', to facilitate a mutual exchange of information over a wide range of disciplines. These include electronics, computation and control systems, and mechanical and electrical engineering activities in structures and power transmissions. The objectives of this international collaboration are to seek the exchange of scientific and technical information by means of an on-going programme of symposia, and to promote co-operation between scientists and engineers in different countries working on diagnostic problems. A number of specific projects are also under development. These include the harmonisation of terminology, a bibliography of published work, the unification of systems, technological reviews of complete diagnostic systems, and a continuing international exchange of details of conferences, courses and publications. The Committee members also hold a register of specialists so that contact
Measurement Vol 1 No 1, Jan--Mar 1983
can be established through national delegates. Countries collaborating in this work include Czechoslovakia, France, German Federal Republic, German Democratic Republic, Hungary, Italy, Japan, Poland, UK, USA, USSR and Yugoslavia.
Acknowledgement The views expressed are those of the authors acting on behalf of the International Measurement Confederation (IMEKO) and not necessarily those of the organisations with which the authors are associated. The authors wish to acknowledge and thank Mr P. G. Tregelles, Director of Mining Research and Development for the National Coal Board, and the Director of Vyzkumny A Zkusebni Letacky Ustav for supporting their participation in the work of IMEKO.
45