This paper presents a general object-oriented model for measurement systems. The limitations of the conventional function-oriented models are examined in the light of the generalized concept of measurement and its theoretical framework proposed previously by the authors. The proposed model identifies five classes of objects, i.e., measured object, measuring instrument, reference standard, human observer, and operating environment. Each is characterized by its own attributes and operations or functions at three levels, i.e., internal, operational, and environmental. The interactions between them are also modeled, including the coupling between the measured object and the measuring instrument, the human-instrument interface, the calibration, and the interference. It serves as both a modeling framework and a practical tool for description, analysis and design, and, in particular, for computer-aided analysis and design of a measuring system. It will find applications in instrumentation engineering and measurement research and education

Butler, C

Yang, QP

IEEE Transactions on Instrumentation and Measurement

English

Qingping Yang, 

C. Butler

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An object-oriented model of measurement systems

Brunel University Research Archive

104 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 47, NO. 1, FEBRUARY 1998An Object-Oriented Model of Measurement SystemsQingping Yang, Member, IEEE, and Clive ButlerAbstract—This paper presents a general object-oriented modelfor measurement systems. The limitations of the conventionalfunction-oriented models are examined in the light of the gen-eralized concept of measurement and its theoretical frameworkproposed previously by the authors. The proposed model iden-tifies five classes of objects, i.e., measured object, measuringinstrument, reference standard, human observer, and operatingenvironment. Each is characterized by its own attributes andoperations or functions at three levels, i.e., internal, operational,and environmental. The interactions between them are also mod-eled, including the coupling between the measured object andthe measuring instrument, the human–instrument interface, thecalibration, and the interference. It serves as both a modelingframework and a practical tool for description, analysis anddesign, and, in particular, for computer-aided analysis and designof a measuring system. It will find applications in instrumentationengineering and measurement research and education.Index Terms—Measurement science, measurement system datahandling, modeling, object-oriented methods, system analysis anddesign, virtual instrumentation.I. INTRODUCTIONMEASUREMENT systems may be modeled using var-ious representation schemes at different levels of ab-straction, e.g., linguistic, graphical, and mathematical. Asan abstraction of some measurement system, a model onlycaptures the essential system characteristics, with irrelevantdetails omitted to reduce the system complexity. A well-developed model does not only function as a useful descriptionof a measurement system, it also greatly facilitates its anal-ysis and design. Modeling has played an increasing rolein instrumentation and measurement systems, especially incomplicated measurement systems and, more recently, virtualinstrumentation.A model is inevitably limited by the explicit and/or implicitassumptions made in its design and use. While the explicitassumptions are usually justified to reduce the system com-plexity, the implicit assumptions often reflect an underlyingconceptual framework and originate from a priori propositionsabout measurement systems. Practical difficulties will oftenarise if its underlying theoretical framework is inadequate orflawed.The underlying conceptual framework has been addressedin [1], in the epistemological context of measurement. Ageneralized concept of measurement based upon a “knowledgepyramid” was also proposed in [1], i.e., measurement trans-forms the measured properties or attributes through internalManuscript received June 1, 1997; revised May 1, 1998.The authors are with The Brunel Centre for ManufacturingMetrology, Brunel University, Middlesex, UB8 3PH, U.K. (e-mail:emstqqy@brunel.ac.uk).Publisher Item Identifier S 0018-9456(98)06311-6.Fig. 1. Functional representation of a measurement system.symbolic representation directly into knowledge: measuredproperties symbolic data knowledge, where the acquiredknowledge may be low-level facts (e.g., the magnitudes ofmeasurands), or in a structural or relational form (e.g., empir-ical models). The former is essentially the conventional resultof measurement.This paper will discuss the modeling of measurement sys-tems, in particular, an object-oriented model of measurementsystems. But the discussions will be made in the light of thegeneralized knowledge-driven concept of measurement and itsconceptual framework [1].II. CONVENTIONAL MODELS AND THEIR LIMITATIONSTraditionally, measurement systems are usually modeledwith functional representations. The architecture of these rep-resentations typically consists of three basic stages, the inputstage including a sensor or transducer, the signal processingstage, and the output stage, each of them represented asinput–output functional blocks or elements, as shown in Fig. 1.Functional representations are well established. This kindof model is often used as the basis for the classificationof measuring instruments. They are very useful for studyingstatic and dynamic characteristics through transfer functionsof each element. They can also be computer-based models,using conventional functional programming languages.Common to the modeling in various problem domains,the intrinsic limitations of a functional model arise from itslow level of abstraction. Its reusuability is very poor as itis intended for specific systems. Despite of the wide use ofstructural or modular design concept, it is rather inflexibleand often difficult to modify. The size of this kind of model,especially a computer-based one, often increases exponentiallywith complexity.In the modeling of measurement systems, functional modelstend to be oversimplified, neglecting some important aspects ofa measurement system, e.g., the coupling interactions betweenthe instrument and the measured object, although work hasrecently been reported to extend these models [2]–[3]. Asoriginated mainly from control engineering, functional modelsof measurement systems have often failed to address someissues which are of theoretical and practical importance tomeasurement, e.g., operating space of measurement [4]. Asa result, incomplete descriptions of measuring systems are0018–9456/98$10.00  1998 IEEEYANG AND BUTLER: OBJECT-ORIENTED MODEL OF MEASUREMENT SYSTEMS 105widely used, and the distinctiveness of measurement sciencehas been seriously obscured. This is probably one of the mostimportant reasons why measurement science has attracted littletheoretical interests, compared with control engineering.III. OBJECT-ORIENTED MODEL OF A MEASUREMENT SYSTEMA. Object-Oriented MethodsThe object-oriented approach views a system as a collec-tion of discrete objects that contain both data structure andbehavior. The data structures and behavior or operations of anobject are defined by a class. An object is an instance of aclass. Objects can interact with other objects.The history of object-oriented methods is closely associatedwith computer programming. The use of object-orientation firstappeared in the development of a discrete event simulationlanguage (Simula) in Norway 30 years ago [5]. As they beganto mature in the late 1980’s, object-oriented methods shifted toother areas, e.g., system analysis and design. Indeed, they havebecome a general modeling approach in the past ten years [6].The power of the object-oriented approach lies largely in itscloseness to the natural view of the real-world. By raising thelevel of abstraction from the function-level to the object-level,it focuses on the real-world aspects of a system and providesa better model of the problem space. Its consistency with thereal-world also provides a unifying platform for all the relatedactivities, e.g., analysis, design, and implementation.The essential characteristics of the object-oriented approachinclude information hiding or encapsulation, inheritance, andpiecewise refinement. Objects encapsulate both their datastructures and operations and they thus work at a higherlevel of abstraction. This makes the object-oriented modelmore stable than the convectional functional model becausethe changes in operations or functions are decoupled and arelocalized within objects.The inheritance comes from the class hierarchy structure, inwhich a class can have its own data structure and operations,and can also inherit them from its parent class(es). Althoughthe concept originated in artificial intelligence, its inherentexistence in class hierarchy structure is unique with the object-oriented approach. The reusability of similar objects is one ofthe most important benefits of object-oriented methods.The feature of piecewise refinement results from the object-orientation and, in particular, its separated external and internalviews of an object. Together with encapsulation, it providesa powerful mechanism to achieve high productivity, goodmaintainability, system integrity, and reliability.B. Object-Oriented Model of a Measurement SystemAlthough there is much literature regarding object-orientedmethods, it is nontrivial to develop a sound object-orientedmodel for measurement systems. This is partly because theobject-oriented methods have been strongly influenced bycomputer programming, which has often obscured the natureof the approach, and it is partly also because of lacking a soundconceptual framework in measurement science, as discussedabove. The authors have developed an object-oriented modelFig. 2. Architecture of an object-oriented model for measurement systems.for measurement systems, which may become a practicaltool to solve the problems with the existing function-orientedmodels.Shown in Fig. 2 is the architecture of an object-orientedmodel for measurement systems. It captures the importantrelations in a measurement system and embraces five objectsor subsystems, i.e., measured object, measuring instrument,human observer, referencing standard, and operating envi-ronment. The model represents a measurement system atthree levels internal (measuring instrument), operational andenvironmental. The measurement can be affected by all thefive subsystems and their interactions, with the measuringinstrument as the core of the model. The model allows for thesystematic study of the objects or subsystems in a measuringsystem and their interrelations.Although the model architecture has revealed importantaspects of a measurement system, it is necessary to refine it inmore detail. The object model shown in Fig. 3 gives furtherdetails about the relations between the five classes of objectsin a measurement system. Notations used here are based onRumbaught et al. [6]. A class is represented by a box withthree regions, the top region is the class name, the middleone represents the list of attributes, the bottom one the list ofoperations. For simplicity the lists of attributes and operationsare not given in Fig. 3 (some examples given in Figs. 4–6).The associations between object classes are modeled usingmultiplicity symbols. A solid ball is for “many,” meaning zeroor more; a hollow ball for “optional,” meaning zero or one;a line without multiplicity symbols represents a one-to-oneassociation unless otherwise specified (e.g., 1 ).Due to their importance, four associations have been mod-eled as four classes in Fig. 3, namely, the coupling betweenthe measured object and the measuring instrument, the hu-man–instrument interface, the interface and the calibration.An association class is represented as a box attached to theassociation by a loop.106 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 47, NO. 1, FEBRUARY 1998Fig. 3. Object model of a measurement system.Fig. 4. Object model of a measuring instrument.It is clear from the model that the operating space of themeasurement system is not only determined by measuringinstrument, but also by the object instances of the four associ-ation classes. It has highlighted problems such as the validityand boundary conditions of a measurement [4].Of course, each object in the model can be further brokendown to more detailed objects according to piecewise refine-ment. For example, the measuring instrument object is furthermodeled in Fig. 4, in which a small diamond is drawn at theassembly end of an aggregation relationship. A triangle is usedfor specialization or inheritance relationship, with the parentclass connected to the apex of the triangle and the child classto the base of the triangle. The knowledge extraction classis introduced in accordance with the generalized concept ofmeasurement.The coupling between the measured object and the mea-suring instrument has often been neglected in traditionalmodels and may be modeled as shown in Fig. 5. Althoughlooking familiar, loading and sampling, the latter in particular,have profound implications in a measurement system. Allmeasurands go through a natural sampling, both spatially andFig. 5. Object model of the coupling between measured object and measur-ing instrument.Fig. 6. Object model of the measured object.temporally. This indicates that many practical measurementproblems are inadequately defined and specified. This is par-ticularly true if the measured object is modeled according tothe generalized concept of measurement, as shown in Fig. 6,where the relational knowledge about the measured object isusually the purpose of a measurement.Other classes in Fig. 3 can be modeled using similar tech-niques. Together with Fig. 3, they represent the object modelof a measurement system. An object-oriented model also dealswith functional and dynamic aspects of a measurement system[6]. They are compatible with the conventional concept ofmeasurement, and an object-oriented model has thus accom-modated conventional functional models.IV. ADVANTAGES AND LIMITATIONS OF THE PROPOSEDMODELAs an instance of an object-oriented model class, the pro-posed model will inherit the benefits of object orientation.These include a higher level of abstraction, good reusability,high productivity, good maintainability, system integrity, andreliability.YANG AND BUTLER: OBJECT-ORIENTED MODEL OF MEASUREMENT SYSTEMS 107The proposed model is suitable for both simple and compli-cated measurement systems. In fact, it is capable of modelingnetworked or distributed measurement systems. As indicatedin Fig. 3, the model generally supports multiple measuringinstruments, multiple measured objects, or human observers.With the encapsulation (or information hiding) and a higherlevel of abstraction, the model is closer to the nature view ofthe real-world and has highlighted the knowledge extraction ofa measurement system. The proposed model is thus inherentlyconsistent with the generalized concept of measurement, wherethe knowledge about the real-world is the ultimate purpose ofany measurement.The model reusability is a very important feature of anobject-oriented modeling. Obviously, the potential reusabilityrelies upon the availability of a library of previously developedobjects or classes. Thus, the benefit may not be very clear atthe early stage of the object-oriented modeling as only a smallnumber of components have become available. Also difficultymay arises from the standardization for the use of theseobjects [7]. The specification of an object-oriented componentis usually more complicated than functional procedures.V. APPLICATIONSIt should be pointed out that there is no unique implementa-tion using an object-oriented approach. The actual model as anabstraction of a practical measurement system varies accordingto the purpose of the modeling and the understanding of systemcharacteristics.Serving as both a framework and a practical tool, the pro-posed model will certainly find applications in instrumentationengineering, and measurement research and education. As aframework, it has integrated various aspects of a measurementsystem. It also helps to identify and highlight the criticalaspects of a measurement system.As a practical tool, it can be used for the classification andsystematic organization of measuring devices or systems, e.g.,in the development of a relational database of measurementsystems. The model also potentially allows for the evaluationof the performance of a measurement system. It will begenerally useful in the description, analysis, and design of ameasuring instrument or system.As object-oriented methods are closely related to the soft-ware development, the proposed model can be readily appliedto virtual instrumentation, where its use may be further ex-tended to the full life-cycle of a virtual system includingsystem implementation and maintenance. The model will alsobe applicable in networked or distributed virtual measurementsystems, e.g., through the Internet and the World Wide Web.VI. CONCLUSIONSAn object-oriented model for measurement systems hasbeen discussed in connection with the generalized conceptof measurement [1]. The proposed model has the advantagesof an object-oriented method. In addition, in the modelingof measurement systems, it has captured the system charac-teristics in a wider view. Its inherent consistency with theknowledge-oriented measurement concept has the potentialto solve the problems associated with traditional function-oriented models. It will provide a more satisfactory solutionto the modeling of a measurement system.With the model as both a conceptual framework and a prac-tical tool, its applications include classification and systematicorganization of measuring devices; description, analysis, de-sign and performance evaluation of measurement systems; andvirtual instrumentation.REFERENCES[1] Q. Yang and C. Butler, “On framework of measurement science,” inProc. XV IMEKO World Congr., Tampere, Finland, June 1997, vol. 5,pp. 101–106.[2] F. Abdullah, L. Finkelstein, S. H. Khan, and W. J. Hill, “Modeling inmeasurement and instrumentation—An overview,” Measurement, vol.14, pp. 41–53, Sept. 1994.[3] P. H. Sydenham, “Unsolved problems of measurement. An internationalstudy,” in Proc. XIII IMEKO World Congr., Torino, Italy, Sept. 1994,pp. 827–831.[4] P. K. Stein, “The unified approach to the engineering of measure-ment systems for test & evaluation-A brief survey,” in Proc. IEEEInstrumentation Measurement Tech. Conf., Brussels, Belgium, 1996, pp.1–28.[5] K. Bjørn, Object-Oriented Programming with Simula. Reading, MA:Addison-Wesley, 1989.[6] J. Rumbaugh, M. Blaha, W. Premerlani, F. Eddy and W. Lorensen,Object-Oriented Modeling and Design. Englewood Cliffs, NJ:Prentice-Hall, 1991.[7] H. J. W. Spoelder, A. H. Ullings and G. C. A. Groen, “Virtualinstrumentation: A survey of standards and their interrelation,” in Proc.IEEE Instrumentation Measurement Tech. Conf., Ottawa, Ont., Canada,1997, pp. 676–681, 1997.Qingping Yang (M’92) received the Diploma ininstrumentation and measurement from ChengduAeronautical Polytechnic, Chengdu, China, in 1983.He was awarded a scholarship to study at BrunelUniversity, Middlesex, U.K., in 1988, and receivedthe Ph.D. degree in 1992.He was an Assistant Engineer, Deputy Head ofDepartment of Measurement and Testing in theAircraft Structural Strength Research Institute (Min-istry of Aerospace), Shaanxi Province, China. Hecurrently lectures on instrumentation design, man-ufacturing metrology, and quality control. He has published more than 20papers on sensors, transducers, instrumentation and measurement.Clive Butler received the B.Sc .degree in physicsfrom the University of Manchester, Manchester,U.K., in 1963 and the M.Sc. and Ph.D. degrees fromImperial College, London, U.K., in 1968.He has held a number of industrial posts includ-ing Chief Physicist, OMT Ltd., Director, MetronicTechnology Ltd. and Digital Metrology Ltd. Hewas also Director of Research at Watford Collegeof Technology, Watford, U.K. He is now Readerat Brunel University, Middlesex, U.K. His currentresearch interests are in dimensional metrology andquality systems. He has presented more than 60 papers in applied optics,metrology, quality management, and printing technology.Dr. Butler is a fellow of the Royal Society of Arts and member of theInstitute of Quality Assurance.

He was awarded a scholarship to study at Brunel University,

Object-Oriented Modeling and Design. Englewood Cliffs,

Object-Oriented Programming with Simula.

On framework of measurement science,” in

Qingping Yang (M’92) received the Diploma in instrumentation and measurement from Chengdu Aeronautical Polytechnic,

received the B.Sc .degree in physics from the University of Manchester,

The uniﬁed approach to the engineering of measurement systems for test & evaluation-A brief survey,” in

Unsolved problems of measurement. An international study,” in

An object-oriented model of measurement systems

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