psam 7 / esrel’ 04 m. f. ramalhoto - instituto superior técnico lisbon - portugal statistical...

PSAM 7 / ESREL’ 04

M. F. Ramalhoto - Instituto Superior TécnicoLisbon - Portugal

Statistical Process Control Procedures in Relation with

Reliability EngineeringM. F. Ramalhoto, Portugal; R. Göb, Germany;

A. Pievatolo, Italy; O. Evandt, Norway; and C. McCollin, UK. The full paper is in the book “Probabilistic Safety Assessment and Management”, vol. 5, Springer, Editors Spitzer, Schmocker and Dang, pp. 3048-3053 and in the PSAM 7 / ESREL ’04 CD-ROM.

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MOTIVATIONProcess maintenance is a topic of:• SPC (statistical process control) • Reliability engineering.In industrial practice and in academic literature these two

approaches used to be considered separately.

ABSTRACT

The full paper (i) establishes a unifying framework based on

section 3 for process maintenance and; (ii) outlines perspectives for research.

This presentation only addresses (ii).



Becker et.al (1927) in Europe and Dodge and Shewhart in USA (Outstanding contributions both to statistical product inspection/acceptance sampling and to statistical process control, SPC.)

In the seventies, Deming and others (Interest switched from inspection of finished product to the control of manufacturing, i.e. from acceptance sampling schemes to SPC

algorithms).

Nowadays (Quality has become a guiding theme for any industry including the service sector, and for any department in a company, including engineering, manufacture, marketing, distribution, logistics, after sales maintenance.)


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To cope with today industrial environment: •Exceed the classical patterns of SQC algorithms.•Adopt a trans-disciplinary view, bridge quantitative disciplines in research and application.•Put industrial reality in the first place instead of mathematical methodology. •Focus all quality related issues in an integrative view rather than isolated as currently done.

SQM is an attempt to address those issuesStochastics for the Quality Movement, SQM, is a new trans-disciplinary field that integrates into an extended quality management concept.

(Includes all quantitative disciplines relevant for quality, to bridge the gap between statistics, engineering, and management science, so as to identify industrial problems, establish realistic models, and find applicable solutions. Emphasis is on models that allow for sidestepping problems proactively instead of reacting on manifest problems.)


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To fulfil its mission SQM Needs: Interested parties in academia, and pathfinders to outline perspectives and define directions.

ENBIS (European Network for Business and Industrial Statistics) andPRO-ENBIS (a 36 months EC project started in 2002), are two pathfinder

organizations. The approach for propagating and structuring SQM which resulted from Pro-ENBIS work is inspired by the example of the medical community: organized Contributions from interested parties in academia and industry regionally and world-wide, disposing of clear and structured rules and schemes for diagnosis and treatment, assuming responsibility for treatment follow-ups, see papers [4; ESREL’03, 881-888 ] ( SQM Library which provides roadmaps, tools and aids for diagnosis and problem solving, and presents research on novel trans-disciplinary areas), [5; ESREL’03, 1289-1294] (develops a scheme to enhance interaction and continuous dialogue between the interested parties in industry and academia).


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I. How much of the defects shown in the output product are due to the equipment reliability and maintenance management strategies?

II. Vice-versa how can knowledge on the equipment reliability and maintenance strategies be incorporated into the SPC scheme used?

The full scope of trans-disciplinary relations of SPC, reliability and maintenance might be better illustrated by a tetrahedron figure.

The vertices represent SQM (top vertex), SPC, Reliability, Maintenance, respectively. The six edges represent all potential

links and interactions that need to be described and analysed. Targeting aspects of the questions I and II, the present paper

considers the two edges SPCR and SPCM.

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2. Aspects of Process Operation and Process Control

Operations of a manufacturing process has two aspects:• process output (product manufactured);• production facilities (machinery and equipment). Two approaches of process control can be distinguished focusing:• quality characteristics of process output (SPC whose basic

objective is to guarantee satisfactory product quality); • technical characteristics of machinery and equipment

(reliability and maintenance schemes which try to prevent against failures and breakdowns of machinery and equipment).

SPC models are based on stochastic parameters of output quality characteristics.

Reliability and Maintenance models are based on stochastic parameters of machinery and equipment characteristics.


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SPC/Reliability have coexisted alongside one another without a defined interface.

In well structured production environment, there are explanatory laws which relate variation in the output quality parameters to variation in the parameters of machinery and equipment.

Such a relationship is stipulated by SPC models which assume that unwelcome variation in the output quality parameters might also eventually be traced back to assignable causes, i. e., relevant variation in the technical parameters of machinery and equipment. In most SPC schemes this relationship is not modelled and explained explicitly.


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SPC schemes that might facilitate to model reliability related quantities explicitly

•Economic SPC schemes (economic design of sample size, alarm region, sampling interval, in particularly the choice of the latter parameter is an incentive to consider topics of machine and equipment reliability like inter-arrival time of failures or failure intensity).• VSS (variable sample size) and VSI (variable sampling interval) SPC schemes (in standard control charting the sample size and the sampling interval remain fixed along the time axis, however, it is natural to design and to evaluate such schemes under assumptions on time to failure in a non ad-hoc manner).

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4. Unified Approach to Process Maintenance

The process is assumed to be set up at an acceptable value.

Objective: To maintain the quality parameters in the in-control state.

Process maintenance relies on two types of actions:

• monitoring (intends to keep track of the actual state of the process quality parameter);

• preventive or corrective intervention (intends to protect or to restore the in-control state).

PSAM 7/ESREL’ 04


SPC emphasizes monitoring (if the inspection verifies the out-of-control state, presence of an assignable cause in machinery, equipment, environment, a corrective intervention follows, usually a renewal).

The design parameters of SPC schemes are:• test parameters (test statistic, rejection limits);• sample sizes ; • length of the sampling interval.

In SPC approaches, usually, neither the evolution of the process quality parameter nor the effect of corrective intervention is modelled explicitly.

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Preventive and corrective intervention in the form of repairs and renewals are the concern of reliability engineering and reliability theory.

•Monitoring is considered only in the form of inspections that detect the system state with probability 1•Statistical inference on the system state by output sampling plays no role.•The essential design parameter of repair or renewal strategies is the length of the repair or renewal interval.


Section 3 provides the basis for a Unifying Approach to Process Maintenance which integrates components of SPC and of Reliability Engineering.

The evolution of process quality and the effect of preventive or corrective intervention can be studied via the stochastic process of quality parameters, with particular interest in the failure time.


PSAM 7/ ESREL’ 04


(i) SPC schemes based on the simple shock model

should adopt realistic and flexible models for the failure time from reliability and repair theory.

VSI or VSS strategies should be studied under the full scope of failure rate functions like increasing, decreasing, bathtub, or upside-down bathtub.

Five Perspectives for Research



(ii)

More refined models of process quality parameter evolution, like the accumulative shock model and the non-stationary time series model described in section 3, should be studied.

The study of such models is of particular importance for the economic design of SPC algorithms (since the economic effect of process operation at each time t is measured as a function of the process quality parameter at time t).



(iii)

.

The only corrective intervention considered by most SPC schemes is a renewal of the manufacturting process after the detection of out-of-control operation.

The entire apparatus of reliability engineering (minimal repairs, preventive renewals and repairs, age replacement etc.) should be integrated into SPC models.


(iv)

The estimation of stochastic reliability parameters from SPC data should be studied (From control charting , lots of data

documenting process history failures are available and may be used for estimation).

However, the time of failure detection under control charting systematically exceeds the time of failure occurrence. The delay originates in the sampling distance and in the erroneous acceptance of the in-control hypothesis over some time. Methods to deal with this systematic bias should be developed.



(v)

In SPC models, account of information on the process of quality parameters naturally leads to a VSI or VSS strategy.

Efforts should concentrate on establishing simple algorithms for effective use on shop-floor (some of the VSI or VSS strategies currently discussed in literature, suggest rather complicated switching rules for changing sampling interval or sample size, inconvenient for industrial practice).




5 Conclusions.

We have in the paper: Reviewed and outlined different ways and perspectives of using information from reliability and lifetime analysis for the design of SPC procedures. Used SPC methodology as a tool to analyse information from the areas of reliability and lifetime.

The five issues listed in section 4 provide important remarks on future relevant trans-disciplinary work to be pursued.

PSAM 7/ESREL’ 04


Acknowledgement:

The work on the paper was funded by the European Community in the framework of the Thematic Network Pro-ENBIS (EC contract number GTCI-2001-43031).

PSAM 7/ESREL’ 04


ESREL’05 Poland

We are organizing a Section on

“Integrated Maintenance, Reliability and Quality Improvement Concepts”

INTERESTED RESEARCHERS Please Send Abstracts AS SOON AS POSSIBLE To ESREL’05 (http://esrel2005.am.gdynia.pl )WITH A COPY TO

[email protected]

psam 7 / esrel’ 04 m. f. ramalhoto - instituto superior técnico lisbon - portugal statistical...

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