28. Measurement Management Systems e Ploesuar

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List of Modules

1. History of metrology and measurement

2. Quantities and units

3. Measuring principles4. Measuring instruments

5. Physical principles of sensors

6. Design and manufacturing of measuringinstruments

7. Imaging and computer vision

8. Measurement of Temperature9. Pressure

10. Measurement of flow

11. Measurement of level

12. Length, Position, Dimension

13. Surface Roughness, Waviness and thePrimary Profile

14. Geometrical Properties

15. Angle

16. Frequency of rotation

17. Humidity

18. Measurement of force

19. Measurement of mass

20. Measurement of torque

21. Measurement of power and energy

22. Measurement of voltage

23. Measurement of current

28. Measurement managementsystems

Authors

Eva Kurekov, Rudolf Palenr

Long-term objectives

The long-term objective of this module is to enable thereader to obtain a general knowledge of the importancof properly designed and maintained confirmationsystems at the same time as keeping the measuremenprocess under statistical control. The topic is presentedfrom the two points of view, the metrological

confirmation system and the measurement processescontrol system.

Specific objectives

This module shows a sample procedure for theintroduction of the metrological confirmation systemduring design and manufacturing, putting into operatiothe service tasks of the measuring system. Afterreading this module, readers should master the tasksconnected with the introduction of the measurement

system designed to maintain a process under statisticacontrol. The readers can assess the capability of theprocess to fulfill the engineering requirements placed oit.

Content

28.1 Metrology policy and a quality

28.1.1 Key definitions

28.2 The metrological confirmation system28.2.1 Elements of the confirmation system

28.2.2 Disadvantages of the metrological confirmationsystem

28.3 System for the measurement process control

28.3.1 Elements of a measurement control system

28.3.1.1 Definition of the measurement process

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24. Measurement of resistance

25. Design of experiment (measurement)

26. Measurement models

27. Uncertainty of measurement28. Measurement management systems

29. International Organisations

30. Metrological System(s)

31. Accreditation and Certification

28.3.1.2 Methods and intervals used for the collectionof data on a measurement process

28.3.1.3 Analysis of data used for measurementprocess control

28.3.1.4 Correcting actions

28.3.1.5 Records on the measurement process control

28.3.2 Influence and importance of the measurementuncertainty

28.3.3 Statistical control of the measurement process

28.3.3.1 Shewhart control charts

28.3.3.2 Introducing measurement process control

28.3.3.3 Control of the established measurementprocess

28.3.4 Measurement process capability

28.3.4.1 Requirements put on the measurementprocess capability

28.3.4.2 Capability indices of the measurement proces

Last Updated: Maj 11, 2005

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Training Modules Content of Module

28.1 Metrology policy and a quality

The international trend of proving the quality of work lies in the credibility ofthe measurements of important factors of products and components, services,and processes. Measurements are the basis of that objective evaluation, and ofthe phenomenon and the foundation for accepting the decisions on the basis ofthe facts identified. The importance of the quality assurance of the measuringequipment, is a part of the standards order ISO 9000: 2001.

The new ISO 10012:2003 standard Measurement management systems Requirements for measurement processes and measuring equipment, replacesthe standard ISO 10012, Requirements for assurance of the quality ofmeasuring equipmentthat was divided into two parts, namely:

a) ISO 10012-1: 1992 Metrological confirmation system,

b) ISO 10012-2: 1997 Measurement processes control.

The new standard represents a coherent union of the two previous parts. Itspecifies the general requirements placed on the systems controllingmeasurement processes, and on the confirmation of measuring instrumentsused to prove the identity of requirements. The aim of the measurementcontrol system is risk management in the case when measuring equipment and

measuring processes could show false results that may influence the quality ofthe product of the organisation.

The ISO 10012 standard should support the ISO 9001:2001 standard. Theirstructures are very similar; the only difference is in their applications. ISO9001:2001 refers to the whole organisation, measurement control system.Standard ISO 10012 is aimed at the aspects of metrological function of theentire organisation. Both standards are focused on all aspects that couldinfluence the production quality. They are mutually linked to each other, theysupport themselves mutually, and motivate each other and cause a synergisticeffect.


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28.1.1 Key definitions

The following definitions explain several terms that occur in the text below:

- measurement management system: is a set of mutually connected and linkedelements that are inevitable to achieve the metrological confirmation andcontinuous measurement process control,

- measuring equipment: measuring instrument, software, metrologicalstandards, reference materials, auxiliary apparatus, or their combination, thatis inevitable for measurement process realisation,

- metrological characteristic (of measuring equipment): the character of themeasuring equipment that may influence the results of measurement(measuring equipment usually has several metrological characteristics that canbe the subject of calibration,

- metrological confirmation: a set of operations required to ensure that themeasuring equipment is in a state of compliance with the requirements madefor its intended use,

- measurement process: a set of operations that will establish the result ofmeasurement,

- metrological function: the responsibility of the organisation for definition andthe area introducing measurement process control,

- measurement process control: observation and the analysis of data obtainedin the measurement process, including the correcting actions. Their goal is tomaintain the measurement process to support manufacturing according to theproduct specification. Check standards, regulation charts, or other equivalentscan be used,

- calibration: a set of operations which establish, under specified conditions, therelationship between values indicated by a measuring instrument ormeasurement system, or alternatively, between values represented by amaterial measure or a reference material, and the corresponding values

realised by a reference standard. Calibration refers to the technical comparisonwith the standard only,

- verification: confirmation by review and audit of the fact that the specificrequirements of a product are fulfilled. They are all comparisons exceptcalibration, and include settings and necessary corrections.

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28.2 The metrological confirmation system

Part of the implementation of a measurement management system is theintroduction of a metrological confirmation system, abridged as confirmationsystem. Its aim is to ensure that the measuring equipment will work in themanner in which it was intended. That means, the risk that measuringequipment will show the results with unacceptable errors must be minimised.

In order to achieve a high level of measurement confidence it is necessary todevelop the confirmation system that contains:

- the individual items of the measuring equipment,

- the distribution of responsibilities and activities,

- the method to achieve the required accuracy.

The items of measuring equipment shall be in a state of compliance with therequirements made for its intended use - metrological confirmation isintroduced in this manner. It is a broader term than calibration because it alsoincludes adjustment or correction and following re-calibration, labeling, sealing,etc.

The methods used when introducing the confirmation system must be recorded.Therefore a quality handbook of the confirmation system has to be created. Themain parts of this handbook are presented schematically in the Fig. 28.1.

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Fig. 28.1Quality handbook of the confirmationsystem

28.2.1 Elements of the confirmation system

Putting important aspects for effective introduction of a confirmation systeminto practice include the following elements:

1) the requirements placed on measurement are unambiguously specified,

2) the requirements placed on the structure of the measuring instruments arespecified,

3) the metrological characteristics (requirements) of the measuring instrumentare clearly specified (e.g. accuracy, stability, range, resolution, etc.),

4) the measuring instruments must be maintained in such way that takes intoaccount all corrections, conditions of use, etc., all these being necessary toachieve the desired function,

5) the confirmation operations are clearly specified:

- whatshould be confirmed or verified, calibrated, adjusted, and marked,

- where will the operations be performed,

- who will perform the operations,

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- which methods and procedures will be used, etc.,

6) effectively a documented system shall be established (the metrologicalconfirmation system) for managing, confirmation, and the use of measuringequipment, including measurement standards used to demonstrate thecompliance of the instrument with specified requirements,

7) this system shall be regularly and systematically controlled (and maintained)to ensure that the risk of the measuring equipment producing results withunacceptable errors will remain within acceptable limits,

8) records must be kept and maintained about all elements of the measuringequipment and the confirmation results. Records should contain the following:

- a description and special identification of the measuring instrument,

- the date when each calibration was undertaken,

- the calibration results recorded after each adjustment or after the instrumentrepair, respectively,

- the calibration interval should be specified, providing an unambiguousidentification of the procedure employed,

- the determination of the limit for permissible error in the equipment,

- the calibration source used for traceability achievement,

- the corresponding conditions of the surrounding environment and data on allnecessary corrections undertaken,

- the uncertainties considered during calibration and their cumulative influence,

- details on each maintenance activity, e.g. service, repairs or realisedmodifications to the instrument,

- keep a record of any restriction of use,

- the identification of the person (persons) that are qualified to performcalibration,

- the identification of the person (persons) responsible for the correctness of allrecorded information,

- the consistent identification of all calibration certificates and other importantrelated documents,

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records must prove measurement capability for each element of the measuringequipment,

9) all elements of the measuring equipment that:

- were defective,

- were overloaded,

- were improperly functioning,

- cause doubts about their function,

- that exceeded the calibration interval,

- that have defective security mark

must be taken out of operation and must be notably marked,

10) all measuring equipment must be securely marked by a label or a code andmust be clearly visible:

- the state of confirmation with the date of most the recent confirmationrecorded,

- the period in which the confirmation is valid should be clearly stated,

- all limitation on the confirmation or use,

11) one must establish and maintain the system for measuring instruments andmeasurement standards that covers their:

- acceptance,

- manipulation,

- transport,

- storage,

- shipping

in order to prevent their abuse, misuse, damage and changes in dimensionaland functional characteristics,

12) each piece of measuring equipment shall be confirmed at relevant intervalsand at appropriate establishments on the basis of their stability, fitness for

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purpose, and use,

13) the intervals of the confirmation of standards and measuring equipmentshall be established on the basis of the following factors:

- the type of equipment,

- the manufacturer` s recommendation,

- trend data obtained from the previous calibration record,

- the recorded history of maintenance and servicing, the extent and severity ofuse,

- the tendency of the instrument to wear and drift,

- the frequency of cross-checking against other certified measuring equipment,particularly measurement standards,

- the frequency and formality of in/house check calibrations,

- the conditions of the surrounding environment,

- accuracy of the measurement sought,

- the penalty for an incorrect measured value being accepted as correctbecause the measuring equipment was faulty should be clearly set down,

14) all measuring equipment shall be calibrated or audited using measurementstandards that are traceable to international or national measurementstandards,

15) all measurement standards shall be supported by certificates, reports ordata sheets for equipment attesting, to the source. The date, uncertainty andthe conditions under which the results were obtained must be recorded,

16) measurement standards and measuring equipment shall be calibrated,adjusted and used in a controlled environment to the extent necessary to

ensure valid measurement results,

17) all confirmations shall be performed by staff having appropriatequalifications, training, experience, aptitude and supervision.

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28.2.2 Disadvantages of the metrological confirmation system

The task of the metrological confirmation system is to ensure sufficientaccuracy of measurements carried out by measuring equipment. It is expected

that measuring equipment (including measurement standards) shall beconfirmed at the appropriate intervals.

Though it can be supposed, on the basis of experience, that at the completionof the interval the activity will be correct, the occurrence of an accidentaldefect, or unexpected damage still cannot be totally excluded. The metrologicalconfirmation system alone does not provide any guarantee that measuringequipment is used accurately. Incorrect usage can be caused by any number ofthings including incorrect measurement procedures, improper measurementconditions or inexperienced personnel. Correctly described measurementproceedings shall serve as a safety net , although, it is impossible to ensure,all the time, that the prescribed procedure is really consistently followed.

The operations of a metrological confirmation system very often demands thatthe measurement equipment shall be moved from its place of work to thecontrol metrology laboratory for reassessment. In the metrology laboratory theequipment is subjected to calibration, adjustment or necessary correction. Theequipment may be subjected to audit and reconfirmation. It is often found thatequipment subjected to reconfirmation performed accurately in spite of theexpiration of the confirmation interval. As a consequence no intervention isnecessary. Such equipment could have been kept in the working environment

without being subjected to review and that situation would lead to considerablecost savings.

Vice versa, when measuring equipment is used within the confirmation intervaland its work is not accurate, the possibility of obtaining inaccurate datasignificantly increases and therefore the risk using incorrect results from themeasurement increases. The tools for assessing the state of measuringequipment that could exclude both the above possibilities is always sought.

Information obtained during the controlled measurement process whenchecking the object against measurement standards, can be used to establishcalibration intervals.

28.3 System for the measurement process control

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Measuring their important features continuously checks the quality of products,services and processes. It is inevitable that the equipment that is used toperform such measurements will show some form ofvariability., This variabilitywill in part come from the component and in part from the measuringequipment itself. It is important therefore that the quality of measuring processwill be well within the required limits of instrument uncertainty.

Under the term measurement system we understand the set of operations thatestablish the result of measurement. Those are the resources that areinextricably linked to each other (e.g. measuring equipment, measuringprocedure, operators, etc.) operations and environmental influences.

A measurement process like any other process can be controlled (see Fig.28.2). Therefore all significant quantities that could negatively influencefulfilling the requirements of the measurements procedure, should be identified.

It is recommended we monitor the measured data regularly and then analysethem through statistical methods.

This way the ability of the measurement process can achieve reliablemeasurements, For example that the quality of the measurement process willmeet the required uncertainty bounds indicated for the instrument.. When themeasuring equipment is discovered to be influenced by a defect, the systemshall be subjected to the necessary corrective actions to return it to its requiredstate. This measurement control system is especially suited to the cases ofrisky or very broad measurement systems (safety or economic purposes).

Fig. 28.2Scheme of themeasurement processcontrol

28.3.1 Elements of a measurement control system

Measurement control systems consists of the following main elements:

- the definition of the measurement process, i.e. the establishment of theproper plan of measurements with requirements placed on accuracy,

- establishing and maintaining an adequate confirmation system for the

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measuring equipment,

- collecting data that deals with the measurement process (method andintervals),

- analysis of obtained data used for measurement process control,

- correcting actions.

Procedures for measurement process control can serve for:

- the detection of atypical changes that occur during the duration of themeasurement process,

- the detection of problems with repeatability,

- the identification and quantification of compensating or correcting actionsaffecting characteristics of the measuring instrument,

- use of an aid to identify predictable periodical changes including cyclicalchanges of the system,

- obtaining a part of the documentation required for fulfilling the conditions thatare necessary for ensuring quality assurance.

A fault of the system that is used for the measurement process control can bedetected by:

- analysis of the control charts,

- consecutive inspections,

- interlaboratory comparisons,

- reclaiming of customers.

Fig. 28.3 shows main parts of a quality handbook used for measurementprocess control.

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Fig. 28.3Quality handbook of the system used formeasurement process control

28.3.1.1 Definition of the measurement process

A measurement process is understood as an integrated process. The processincorporates an analysis of the measurement principle, traceability to theappropriate standards and necessary calibration of the instruments employed.When necessary, it incorporates the adjustment, verification, metrologicalconfirmation and results obtained from the measuring instruments underspecified working conditions. A measuring instrument represents just one out ofmany factors that affect the measurement. Each element of the measuringequipment shall be metrologically confirmed.

A complete specification of the measurement process should contain:

- identification of all the important measuring equipment,

- the measurement methods,

- the measurement procedures,

- the measurement software,

- the conditions of use,

- personnel measuring skills required,

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- other factors directly affecting measurement accuracy and its reliability.

When designing the measurement process, planned experiments are oftenemployed. They enable the determination of the important factors acting on themeasurement process and directly affecting its results. If planned experimentscannot be performed (too expansive, too long lasting, etc.), data,

specifications, advice and limitations, provided by the instrument manufacturercan be used.

28.3.1.2 Methods and intervals used for the collection of dataon a measurement process

In order to control processes it is inevitable that it is necessary to assign themethods of collecting information about a controlled process. On this basis, itcan be assessed whether the controlled process fulfils the requirementsexpected of it, or it is out of required limits. Therefore, the system forsupervising the measurement process that includes the identification of thefollowing elements is implemented:

- measurement procedures,

- measuring tools,

- control intervals.

When collecting data from a controlled measurement process, the method ofcheck standardtends to prevail. In addition the methods that result from directmeasurement on the products or subjects of the measurement, or proceduresthat enable data collection of information concerning the measurement processare used.

Check standard is an item of measuring equipment, a product or any otherobject, or, the difference between two measurement standards. Its metrologicalcharacteristics must be well defined. It shall be verified, examined orcalibrated, relating to the reference standard, by a measurement process thatis independent of the controlling process, and is also more accurate thancontrolling process.

The use of a check standard enables the establishment of a database formeasurement process control. The process itself measures the standard andthe measured data are used in the control process. Random influences on the

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measurement uncertainty can be observed on the supporting charts. If there isany suspicion that the measurement process is influenced by aberrations, it isnecessary to examine these aberrations for their signification. Significantaberrations must be eliminated or compensated for, at reasonable costs.

The frequency used for measurements that are carried out on a check standard,

depends on following main factors:

- the dimension and tolerance being controlled,

- the required level of insurance,

- the allowed limits of measurement uncertainty.

For a new measurement process, shorter intervals of check standardsmeasurements are selected. After obtaining experiences, during the time untilthe measurement process is deemed stable, the intervals of measurements oncheck standard can be extended.

The time required for undertaking one measurement could, but should not,influence the choice for the frequency of measurements. A measurementprocess can sometimes last for several hours or several days. Complexity andthe difficulty of the measurement process can affect the duration of themeasuring process. Sometimes it is necessary to follow the influences on theprocess for a longer time, because when observing the process for a short timeit can lead to a rise in the significant intersectional changes. This does notnecessarily mean that these are unacceptable changes of the controlled

measurement process.

In addition to the measurement frequency, the selection of the appropriate timewhen the measurements are undertaken can also be very important

It is recommended that measurements of a check standard should beconducted randomly (in relation to time) during the working day or week,depending upon how long the manufacturing or operational process continues.Measurement that is repeated at the same time during the day (or is regularlyundertaken by the same person) can obscure random group errors.

28.3.1.3 Analysis of data used for measurement process control

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The measurement obtained from a check standard should be analysed andcompared to the stated requirements. Control charts represent an appropriatestatistical tool that can evaluate deviations caused by individual components ofthe measurement process.

Plotting data in a control chart enables the observer to find unexpected timedeviations or changes in the process, as well as long-term trends. Thus control

charts trace the actual status of the measurement process. Alternatively theycan be replaced by equivalent numerical methods.

Detailed analysis of the measurement process enables the observer todetermine:

- the reasons that can cause irregularities in a process and the sphere of theiractivity

- the influence of improper requirements in the specification,

- ways of controlling the process and the sphere of activities that requirecontrol,

- all other factors that can influence the measurement process.

28.3.1.4 Correcting actions

Correcting actions must be taken, if the analysis of measured data show thatthe particular data exceeds the prescribed limits or that data has anunacceptable trend. Correcting actions should return the measurement processunder control and the observer must prove that the measurement processremains under statistical control.

Correcting actions can consist of:

- decreasing the intervals between inspections,

- the repair or disposal of unstable or unreliable instruments,

- reduced uncertainties of the measuring instruments employed,

- an increased time interval between which the measurements are performed,

- an increased number of affecting quantities that are required to be controlled,

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- decreasing the maximum permissible error of the measuring instrument,

- increasing the capability of the personnel.

Correcting actions must eliminate the reasons that caused the discovereddeviation. The influence of correcting actions must be observed immediately

after their introduction.

28.3.1.5 Records on the measurement process control

When introducing measurement process control, in addition to observing thelevel of control, sufficient records must be maintained. The records shouldcontain:

- documents on the measurement process. This shall contain the specificationof the measuring instruments, the measuring procedures, directives forpersonnel in the use of the instrumentation, the range of responsibility of themeasuring personnel, the working conditions, the validity certificates (oncalibration of the measuring instruments and check standard), verificationrecords, the balance of the measurement uncertainties and the permissibledeviations.,

- the relevant data, collected by the system controlling the measurementprocess. That data should be plotted and kept as records Shewhart controlcharts are most often employed,

- all the interventions (correcting actions) that were performed on the basis ofthe data obtained from the system monitoring the measurement process,

- the identification of all important documents and their archiving,

- the identification of the person responsible for the preparation and delivery ofrespective data to be maintained as records,

- the requirements put on measuring and monitoring personnel, their requiredqualification and how information on how these requirements are fulfilled.

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28.3.2 Influence and importance of the measurementuncertainty

A calibration result must contain an expression of the measurementuncertainty. It is an important characteristic of the measuring instrument,

representing input to the evaluation of the overall measurement uncertaintythat is relevant to the process in which the measuring instrument is used.

All important measurement uncertainties must be taken into account whenrealising measurements and presenting their results. These uncertaintiescomprise the uncertainties of the measuring equipment (includingmeasurement standards), uncertainties resulting from the personnel makingthe measurement and the effect of the surrounding environment.

The best estimate of the measured quantity is considered as a measurementresult in conjunction with all the components of the uncertainty that contribute

to that variability, including those arising from systematic influences (e.g.components connected to the corrections made to the process or measuringsystem, and the reference measurement standards).

Some components of the overall uncertainty can be very small when comparedto other ones, and their accurate determination can cause technical oreconomic problems. A decision and its justification must be recorded in suchcases. The effort put on the determination and recording of the measurementuncertainty should be comparable to the measurement importance, with regardto the quality of final products made in an organisation.

Measurement uncertainty is usually stated at the beginning, when themeasurement system is introduced. One assumes that the uncertainty does notchange until the next confirmation. But conditions that affect measurement canvary everyday. Many influences contributing to the overall uncertainty manifestthemselves, and include:

- the operator,

- the measurement method,

- the environment,

- the measurement object,

- the measuring system.

Therefore continuous observation of the measurement process isrecommended. The correct use of statistical methods can be very advantageousin this context.

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Figure 28.4 introduces an example of complex influences that affectmeasurements of geometric properties. However, industrial practice does notgenerally pay satisfactory attention to such influences and their relation to themeasurement results.

Fig. 28.4Complex influences that affectmeasurement result formeasurement of geometry

properties

Utilisation of knowledge on technical performance of measuring instrumentsand measuring methods (namely for geometrical characteristics of products) is

often limited to the assignment of the measuring instrument to the geometricalcharacteristics of quality in technical practice. The ISO 14253-1:1998 standardevaluates in detail the restrictions for safe classification of products which haveactual dimensions close to the tolerance limit (see Fig. 28.5).

Fig. 28.5

Influence of the measurementuncertainty on safety of thedecision taken about productquality

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Decreasing the measurement uncertainty by correct operation, or according tothe agreement, recommended in stated standard for the quantitative evaluationof processes, increases the reliability of the decisions taken.

The first solution results in new principles for optimisation of the ratio betweentechnological and economical solutions. The second possibility assumes that

products are manufactured in a limited, so called manufacturing tolerance. Thedifference between the prescribed tolerance of the product (prescribed by thedesigner) and a manufacturing tolerance is usually recognised by expandedmeasurement uncertainty of the toleranced dimension.

One must take into account that functional and economical aspects affect thespecified size of the manufacturing tolerance, and there is often no fixedfunctional limit, the so calledsafe tolerance. Besides the simple addition of theuncertainty of the product tolerance, another methodology utilising the squareof the uncertainty was developed in the late nineteen-fifties. By means of that

method, Leinweber defined the so called golden rule of measurement. Thisrule states that for processes with a normal distribution, the ratio betweenuncertainty and tolerance should be in the range U/T = 0.1 to 0.2. For thiscase, it is valid to assume that the unstable range for decision on productsquality (their distribution into accuracy classes) can be neglected. IAlternatively if theoretical ratio U/T= 1 is considered, the quality of productscannot be assessed, because the whole tolerance range is overlapped by theuncertainty of the measuring equipment.

Modern production structures require an error rate at the level of ppm (partsper million). This requires a new basic approach to technical measurement and

statistical processing of measured data. Using statistical superposition, it ispossible to obtain satisfactory certainty when deciding on quality, ranking intoindividual accuracy classes. Based on statistical models of the process, one canjudge the number of improperly accepted or improperly rejected products forthe fixed ratio U/Tand the number of the defective products.

28.3.3 Statistical control of the measurement process

Data obtained from the measurement process are processed in the form of theso-called statistical characteristics. They express properties of the wholestatistical set or some of its parts and are processed in the form of statisticaltables or charts. Analysis of the results follows this stage, recognising context,and relations within the examined process.

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Statistical regulation of the measurement process directly follows the statisticalanalysis. Statistical regulation of the measurement process maintains that theprocess is under the statistical control at the respective specified level. Thisenables fast determination of laws exceeding the limits of permissible errors,their analysis and early correction by restorative actions. Therefore one mustachieve and maintain the measurement process under control by means of

continuous records of the measurement quality that was identified duringmeasurements.

Two types of variability are distinguished in the theory of control charts.

The first type is represented by the the term random variability: a situation thatis caused by random influences (sources). In such a situation a process isaffected only by influences that cannot be controlled and often ascribed adefinite origin. Each source contributes to the overall variability by only a verysmall part, but, in general, none of the sources contributes significantly to the

overall variation. Their sum can be measured and is considered as an inherent(own) feature of the given process. Random influences are a permanent part ofthe measurement process and affect all elements of the process. When theprocess is under statistical control, its behaviour can be predicted.

Processes under statistical control are considered as stable (see animation).

The second type of the variability represents a real change in the process. Thischange can be caused by certain identifiable reasons and are called definablereasons. Processes that are affected by random and definable reasons canbecome unstable and their behaviour cannot be completely predicted (see

animation). Definable reasons are not an inherent part of the measurementprocess and can be eliminated (theoretically at least). These reasons can beassigned to the measuring instruments being used, the measurement methodsand procedures, the measurement conditions, and the measuring ormanufacturing personnel, for example. When the definable reasons areidentified and eliminated, the measurement process becomes stable.

Statistical control of the measurement process contains three stages:

- a preparatory stage,

- a design stage,

- a check stage.

The measurement process is decomposed into individual phases in thepreparatory stage. Control charts are designed within the design stage, themeasurement process is monitored and when necessary and appropriate,control limits are adjusted. The process is continuously kept under statistical

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control during the check stage.

A control chart is the main statistical tool enabling measurement processcontrol. Control charts graphically represent the status of the measurementprocess. They enable comparison of information based on sequence ofselections that reflect the actual status of the measurement process.

Shewhart control charts of the arithmetic means and variances R areespecially suitable for measurement process control. Low sensitivity torelatively small deviations of the measurement process represents the maindisadvantage. Where this situation is unacceptable, charts of cumulative sums so called CUSUM charts are recommended for use.

Animations

The measurement process is not under the statistical control

The measurement process under the statistical control

Merac proces nie je v tatisticky zvldnutom stave

28.3.3.1 Shewhart control charts

Walter Shewhart first designed control charts as a graphical tool, utilisingprinciples of the statistical tests of confidence, in 1924.

Shewhart control charts operate with data obtained by the measurement of thecheck standard. Measurements are performed at approximately identical

intervals. Subgroups of data, having the same number of elements, arecreated. Certain characteristics the arithmetic mean and range R arecalculated in the majority of cases for each subgroup. The calculatedcharacteristics are regularly recorded in the control charts.

Central line (CL) is marked I on the Shewhart control chart. The central line isparallel to the xaxis at a distance equal to the reference value of the plottedcharacteristics. As the reference value is usually used the average value of the

means , are obtained from individual sub-groups.

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Control limits are of two types:

- Lower Control Limit- LCL,

- Upper Control Limit- UCL.

Exceeding of the control limits requires an intervention (realisation of thecorrecting action) to the process. If the process is under statistical control,approximately 99.73% values of the examined characteristics fits within thelimits of control limits (control limits).

Fig. 28.6 introduces a schematic of a typical Shewhart control chart.

Fig. 28.6

Draft of the control chart

An chart shows where the process is centred. As the nominal value of thecheck standard XKE is known, it also identifies the systematic error of the

measurement process and the stability of the meas-urement process. An chart shows the unwanted fluctuation among sub-groups identified from the'average of means' point of view. If sub-groups are created by different levelsof individual elements that belong to the measurement process (different

conditions, different personnel, etc.) and can be identified, an chart shows theinfluence of those elements on the measurement process.

An R chart uncovers each unwanted fluctuation inside the sub-groups andindicates the value of the repeatability error of the measurement process. Itrepresents a certain rate of consistency and homogeneity of the measurementprocess. If fluctuations inside the sub-groups are approximately the same, theR chart remains under statistical control.

An chart can also be influenced by conditions that relate to an R chart that is

not under statistical control. This is indicated by high fluctuation of data insideindividual sub-groups. The increased fluctuation is caused by either theoperator, the drift of the measuring instrument and also by unsteadymeasurement conditions.

Two types of errors can occur when using control charts:

- error of the first kind,

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A3

or

A2

X0 X0A10

R D4D3

R0 D2

0

D10

One must observe that the lower control limit of the R chart is not used in mostcases (for m < 7 is D3 = D1= 0).

Table 28.2 introduces values for the parameters A1,A2,A3, D1, D2, D3, D4. Thistable represents a limited version of the Table 2 as specified in the standardISO 8258. The stated parameters represent constants that depend on the sub-

group size m and these are tabulated with a probability (risk) = 0.27%. The

greater the number of measurements in one sub-group, the lower are thevalues of the constants and thereby the area between control limits decreases.

Tab. 28.2 Values of parameters that are used in the table 28.1

Sub-groupsize

Coefficients for the control limitsCoefficients for thecentral line

m A1 A2 A3 D1 D2 D3 D4 d2 C4

2 2.121 1.880 2.659 0.000 3.686 0.000 3.267 1.128 0.7979

3 1.732 1.023 1.954 0.000 4.358 0.000 2.574 1.693 0.88624 1.500 0.729 1.628 0.000 4.698 0.000 2.282 2.059 0.9213

5 1.342 0.577 1.427 0.000 4.918 0.000 2.114 2.326 0.9400

6 1.225 0.483 1.287 0.000 5.078 0.000 2.004 2.534 0.9515

7 1.134 0.419 1.182 0.204 5.204 0.076 1.924 2.704 0.9594

8 1.061 0.373 1.099 0.388 5.306 0.136 1.864 2.847 0.9650

9 1.000 0.337 1.032 0.547 5.393 0.184 1.816 2.970 0.9693

10 0.949 0.308 0.975 0.687 5.469 0.223 1.777 3.078 0.9727

Design of control charts

The situation when basic values are not stated, and therefore must bedetermined by preliminary observation of the measurement process, is morefrequent in a practical situation. It is recommended that one should performseveral measurements of the check standard under different statuses of the

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Two parameters must always observed when establishing measurementprocess control:

a) the value of the measured quantity (control chart of the arithmetic meansis used),

b) the variability of measured data (control chart of the variation R is used).

When the measurement process is stable, and range R of individual sub-groups change only randomly, and will only extremely rarely be lying out of thecontrol limits (approximately three out of one thousand)

The starting point for the analysis of the control charts; represents a set oftests shown in Tab. 28.3. This table shows atypical situations that require ananalysis of the measurement process. These tests show, apart from the basicsituations (the examined statistics are outside control limits), other situations.When the point falls outside the control limits, the process is unfit. This is too

late so it is preferable to anticipate such a damaging status by following trends.

The afore-mentioned tests consider the division of the control area into threeparts C, B and A. Area C is within a distance 1/3 of the control limit from thecentral line. Area B is within two-thirds (to both sides) and area A is just withinthe control limits (see Fig. 28.11). For a control chart with control limitsrepresented by the three sigma bands (three standard deviations), the area C,

has boundaries at a distance 1 from the central line, area B has distance

2 from the control line and area A has distance 3 from the central line.

These tests are suitable for chart has a normal distribution).

Fig. 28.11Division of the control area

Table 28.3 Description of tests for judging the control charts

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Test Description of the situationRepresentation of thesituation

1One point is outside the controllimits

2Nine successive points are below(above) the central line

3Six successive points registerdecline (growth)

4Fourteen successive pointsregularly fluctuate upwards anddownwards

5Two out of three successive points

fit within the A area

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upper tolerance limit UTL) and the lower boundary value (identical to the lowertolerance limits LTL):

T = UTL LTL (28.6)

The importance of the ratio U/Tis described in part 28.3.2 in more detail. In

general, convention states that the measurement processes should ensure theresults with an expanded uncertainty Ubeing 3 to 10 times lower than one-halfof the tolerance range

(28.7)

where

k= (3 to 10).

The values (of the product) determined by the measurement process, with anexpanded uncertainty U, must be achieved by Ubeing lower than UTL and by Ubeing greater than LTL (see Fig. 28.5). Therefore certain attempts are made toselect kas large as possible and thereby Uas low as possible. Another aspect issometimes the price of the measuring equipment and the cost of themeasurement process, this implies that a compromise must be introduced.

When evaluating the measurement process capability, we will further assumethat tolerance limits are fixed and unchangeable.

Therefore when evaluating the measurement process capability, it is importantto examine two factors variabilityof the measured values, caused by themeasurement process and the systematic deviation from the actual value of themeasured quantity. Both efforts the lowering of variability and processcentering are determined by the technical progress of modern measuringinstruments (see Fig. 28.14). Economical savings are a direct consequence tothe manufacturer. A solution to the problem might be improved control of theproduction process,, a consequential decrease in the number of unfit products,and therefore an increase in production efficiency.

Fig. 28.14

Influence of the loweredvariability of themeasurement processand effort for itscentering

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28.3.4.1 Requirements put on the measurement processcapability

Assuming that data are obtained by measurements of the check standard,requirements placed on the measurement process are most often described in

the following ways:

- the expanded uncertaintyU,

- the limits of the maximum permissible error dov,

- the tolerance rangeT.

The measurement process should ensure that measured values differ from theactual value of the measured quantity by less than the expanded uncertainty U,

and respectively, the maximum permissible error

dov. the actual value of themeasured quantity is represented by the value of the check standardXKE.

If the requirements placed on the measurement process are described by thetolerance range T, this range must be defined first. The tolerance range T isdefined as the difference between the upper boundary value (identical to theupper tolerance limit UTL) and the lower boundary value (identical to the lowertolerance limits LTL):

T = UTL LTL (28.6)

The importance of the ratio U/T is described in part 28.3.2 in more detail. Ingeneral, convention states that the measurement processes should ensure theresults with an expanded uncertainty Ubeing 3 to 10 times lower than one-halfof the tolerance range

(28.7)

where

k= (3 to 10).

The values (of the product) determined by the measurement process, with anexpanded uncertainty U, must be achieved by Ubeing lower than UTL and by Ubeing greater than LTL (see Fig. 28.5). Therefore certain attempts are made toselect kas large as possible and thereby Uas low as possible. Another aspect issometimes the price of the measuring equipment and the cost of themeasurement process, this implies that a compromise must be introduced.

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(28.13)

or if the difference is designated as

(28.14)

Relations between the stated values are graphically shown in Fig. 28.19.

Fig. 28.19Complex graphical representationof the measurement process

capability by the capability indexCPK

The CPK index already responds (unlike the CP index) to the deviation of the

average value of the measurement process is approaching any of the control

limits, and the standard deviation is decreasing at the same time.

When the process is capable, but not centred, it must be decided whether to tryto improve the process.. To decide whether the process is centred or not, thefollowing rules can be used:

1) ifCP=CPK, the process is centred in the middle of the tolerance area,

2) ifCP>CPK, the process is not ideally centred,

3) ifCPK = 0, the process is centred at one of the control limits,

4) ifCPK

28. Measurement Management Systems e Ploesuar

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