Extending BPMN for modeling resource aspects in the

Extending BPMN for modeling resource aspects in the domain of machine tools R. Braun1, W. Esswein1 1

Technische Universität Dresden, Chair for Business Informatics, esp. Systems Engineering, Germany

Abstract Engineering processes that use machine tools face the possible problem of inaccuracy and a loss of product quality because of position errors of the machine tool. Current research projects address this problem and provide so-called correction and compensation methods to reduce position errors in machine tools and thus enhance the product quality. However, since these optimization processes are very innovative and sophisticated, their description as well as their integration into engineering processes is not trivial. This research article aims to address this issue by a conceptual modeling approach. The popular Business Process Management Notation (BPMN) is adapted and extended by domain specific concepts to represent resource intensive engineering and optimization processes. The BPMN extension is evolved systematically on the base of the BPMN specification and previous research in the field of BPMN extensibility. Further, a literature review was conducted to identify relevant resource concepts for the domain of machine tools. Keywords: BPMN, BPMN extension, resource modeling, machine tools.

1

Introduction, motivation, and research design

Engineering processes that are using machine tools put high requirements on the accuracy and quality of the manufactured products as well as the efficiency of the entire process from a technical and an economical perspective. The pursuit of quality and accuracy is mostly confronted with the consumption of different production materials that leads to a trade off between quality and costs. Further, there are some intrinsic challenges within machine tools like the problem of inaccuracy resulting from overheating effects in the machine itself or at the interface between the machine spindle and the workpiece [1]. Research projects WIT Transactions on Engineering Sciences, Vol. 87, © 2014 WIT Press www.witpress.com, ISSN 1746-4471 (on-line) doi:10.2495/AMITP20130531

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try to solve that problem by the development of so-called correction and compensation methods [1]. Correction methods aim to adjust and configure the machine control to avoid inaccuracy by—for example—realigning the position of the machine spindle during execution. Compensation approaches aim to enhance the accuracy by (re-) designing or adapt single machine components (e.g., by implementing isolation materials). The application of both optimization approaches to a specific engineering process consists of many underlying activities, decision points, resources and responsibilities. Since these approaches are very innovative and affect different fields of research, they have sophisticated semantics and are very domain specific. Semi-formal diagrammatic models from the field of computer science and information systems research may help to reduce the complexity of the presentation of the mentioned approaches and may enable an assessment of each approach by a disclosure of all required resources. Because of the fact that not only technical aspects but also economical aspects need to be expressed, a process modeling language can facilitate that requirement. As stated in Ref. [2], the Business Process Management Notation (BPMN) [3], [4] can be generally used in the field of engineering and machine tools. In contrast to other process modeling languages like Event-driven Process Chains (EPC), BPMN provides a well-defined syntax definition and allows extensibility for the definition of domain specific extensions [5], [6]. Besides, BPMN is widely accepted in practice [7], [3] and many BPMN software tools exist [8]. This research paper intends to provide a BPMN extension for modeling supporting processes that aim to improve the quality and accuracy of products that are manufactured by machine tools within engineering processes. The research paper follows the design science paradigm [9], [10], [11]. First, the problem has to be presented and motivated in detail and all requirements had to be presented to ensure relevance [9]. By means of the presentation of the state of the art, it became obvious that there are currently no adequate solutions for the design problem. Anyhow, existing artifacts (e.g., generic resource elements [12]) were studied in terms of a possible adaption and integration. Afterwards, the solution artifact in the form of a domain specific set of elements and the final BPMN extension is developed. Therefore, “meta artifacts” (extension elements of BPMN meta model) and results from previous research work are used to ensure rigor [9]. Finally, the developed BPMN extension is demonstrated by means of a use case. Referring to Österle et al. [11] this research paper aims to develop instructions as “goal measure statements” that are explicated by semiformal models.

2

Analysis

As stated above, the aimed modeling language has to provide the possibility of modeling correction and compensation processes that try to improve engineering processes and machine tools regarding product quality and accuracy. While it is also important to model parts of the engineering process that are affected by the optimization methods, the focus of the modeling language should lie on the presentation of these approaches. It is necessary to depict specific data objects (e.g., simulation models, control data or measurement data), time aspects (e.g., WIT Transactions on Engineering Sciences, Vol. 87, © 2014 WIT Press www.witpress.com, ISSN 1746-4471 (on-line)

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working time, preparation time), cost aspects (e.g., cost type, values, ranges), or applications (e.g., software tools). The following objects associate the engineering process and the optimization method: the entire machine, a machine component, a material or an auxiliary material. Altogether, an engineering process is extended by an optimization method and the entire scope has to be modeled. Therefore, it is necessary to extend the BPMN by engineering concepts (e.g., machines, components, auxiliary) as well as “optimization method” concepts (e.g., measurement data, software, roles). For reasons of clarity, both concept parts are grouped together to one BPMN extension. During research, the specific concepts of the targeted extension were investigated by unstructured interviews with domain experts. Results were explicated by ontologies. With respect to the limited space of the paper, the ontology is not depicted separately. 2.1 Analysis of default BPMN elements Below the BPMN 2.0 specification is analyzed to figure out what domain concepts can be represented by BPMN default elements and what concepts force an extension. The following concepts can be represented by BPMN default elements: Activities (tasks) can be used to depict process steps. A specification regarding the manual or automatic execution of the process step can be realized by adding activity markers [4], p. 163]. “Tools” can be modeled as lanes since lanes are suitable “for modeling systems and applications” [4], p. 306]. The following domain concepts can be implemented partly by using BPMN elements and extending them by new properties: Referring to Schuster [13] resources are understood as objects that can be added to activities in processes to ensure their execution. Resources can be divided into material, immaterial, technical or nontechnical resource [13], [14]. BPMN provides the element “resource” as a concept for human resources and “all other resources” [4], p. 95]. A resource is defined by its “resource parameters” that are specified by the generic “item definition” element [4], p. 91]. However, BPMN meta model lacks in a detailed specification of a resource element; it remains abstract and imprecisely. The BPMN element “participant” outlines the responsibility of a process execution and is represented by a lane or a pool [4], pp. 113, 305]. However, BPMN meta model lacks in the description of any role properties. Generic data and models can be represented as “data objects.” Data objects are elements that can be manipulated, transferred or transformed and are stored within a process. A specialization of data objects is not provided by BPMN and requires the implementation of additional properties. The particular state of a data object can be represented with the object type “data state” [4], p. 206]. The following concepts need to be built as extension elements since there are no adequate BPMN default elements: Modeling objectives is not possible in BPMN. An explicit modeling of costs and measuring tools is not provided within the BPMN. 2.2 Modeling resources in general It became obvious that BPMN cannot meet all requirements of a resource specific context and needs to be extended. To achieve this goal, a literature review was conducted to investigate the current research in the field of modeling resources. WIT Transactions on Engineering Sciences, Vol. 87, © 2014 WIT Press www.witpress.com, ISSN 1746-4471 (on-line)

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Schuster [13] conceptualizes the domain of resources and proposes a resource modeling language that contains a competence model and a human resource model. The competence model describes all competences, capabilities and knowledge of human resources in an organization. A proficiency level quantifies each of these elements [13], p. 91]. The human resource model connects human resources and roles to the organizational structure [13], p. 91]. Ouyang et al. (2010) emphasize a research gap in the field of modeling non-human resources in processes and propose a conceptual resource model as well as concepts for the duration of resource bindings [14], pp. 5–7]. Awad et al. [15] address automatable workflows and provide a BPMN extension for resource allocation constraints on the base of object constraint language workflow resource patterns. Resource allocation constraints are associated to activities as textual annotations. Stroppi et al. [16] address that research field, too, and emphasize the lack of representation of resource concepts on the BPMN execution level. Hence, the BPMN meta model is extended by human resource elements for workflow models. Resource structure, work distribution as well as authorization are investigated [16], p. 2]. Since Stroppi et al. [16] focus on executable workflow models, the proposed meta model extension is hardly adaptable to a conceptual level. With respect to resourceconsuming and time-consuming aspects Korherr and List [17], p. 5] propose the concepts “quality,” “cost”, and “cycle time” (“working time” and “waiting time”. Magnani and Montesi [18] extend the BPMN by elementary concepts for the annotation of costs, cost intervals and average costs to determine the overall costs of a process instance. However, a detailed conceptualization is missing. Saedi et al. [6], p. 620] develop a BPMN extension to integrate quality requirements in BPMN models and introduce special task types like “response time” or “reliability”. In terms of the usage of BPMN models in mechanical engineering, the research articles of Zor et al. [19], [20] and Garcia-Dominguez et al. [2] have to be mentioned. Both Zor et al. [19] and Garcia-Dominguez et al. [2] emphasize that BPMN can be leveraged for modeling manufacturing or production processes. However, there is a lack of domain-specific concepts and elements (e.g., machine time [19], p. 2]. Zor et al. [20] introduce a range of domain specific elements that are derived from the business domain, for example: “operating data,” “machine data,” “raw materials,” or “auxiliary materials”. 2.3 Research gap As shown in the abovementioned chapters, not all requirements can be fulfilled by the current state of research. Indeed, single aspects are treated—but a useful consolidation and integration is still missing. In particular, there are only very few implementations [16] that integrate resources in the BPMN meta model what makes it extremely difficult to reconstruct and reuse the presented elements and results.

3

Design

3.1 Extending the BPMN BPMN provides an “extension by addition” mechanism [21], p. 3]) to extend default BPMN elements by additional properties and obtain a valid BPMN core WIT Transactions on Engineering Sciences, Vol. 87, © 2014 WIT Press www.witpress.com, ISSN 1746-4471 (on-line)

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that guarantees exchangeability [4]. BPMN extension consists of the elements “extension definition,” “extension attribute definition,” and “extension attribute value” [4], p. 57]. Due to the fact Meta Object Facility [22] defines the BPMN, literals like enumerations can be used for the specification of properties. However, BPMN specification lacks in the provision of a detailed procedure model and there is only one research article addressing this problem [21]. Stroppi et al. [21] propose a procedure for the development of BPMN extensions: First, an UML based independent conceptual domain model of the extension (CDME) has to be designed. Afterwards, CDME elements are typed as “BPMN concepts” or “Extension concepts” by using UML stereotypes. A rule based transformation of the CDME leads to the BPMN extension model (BPMN+X; [22], p. 10]). Subsequently, the BPMN+X model is transformed to a XML schema (that transformation step is not deepened as the derivation of executable workflow models is not the aim of this research article). The above steps have been completed to derive the BPMN extension elements (with respected to the limited space of the paper, CDME and BPMN+X model cannot be presented at this point). 3.2 Graphical representation of the extension elements The developed BPMN+X model was implemented in a generic modeling tool to realize the entire extension in a prototypical environment. Table 1 outlines all elements and gives a brief introduction to the semantics of each element. After presenting the definition of the language extension, the following chapter provides an example of the artifact in terms of demonstrating its usefulness.

4

Example

Figure 1 demonstrates the evolved BPMN extension by presenting the application of a simple correction method to some machine. The semantic of that method is as follows: Based on the geometric definition of the machine type a initial mathematical model of the machine is built (finite element model, FEM) for the calculation of position errors that lead to inaccuracy and a loss of quality. The initial FEM is re-configured based on experiments on a test machine to enhance the quality of the evolved model. The calculated position errors cause the implementation of correction values on the machine (e.g., a changed setting of machine control parameters). Therefore, position sensors need to be installed at the machine to evaluate the success of the method. After testing the new configuration the results are evaluated to assess whether a “roll out” to the engineering process is possible or a re-configuration is necessary. The most of the activities consume resources during their execution, e.g., human resources (“worker,” “engineer”), applications (“middleware,” “sensor monitoring,” “regression tool”), and auxiliaries (“refrigerant”). Also some cost and time aspects are presented. Produced data objects are specified by their type (see “Finite Element Model”). Further, the “HR” tag specifies pools whether a human resource is responsible for the entire process execution. WIT Transactions on Engineering Sciences, Vol. 87, © 2014 WIT Press www.witpress.com, ISSN 1746-4471 (on-line)

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Table 1: Graphical Representation of Each Extension Element Worker

Element (and Example)

Milling Machine Costs (Fixcosts) (low [55000-80000])

Semantics

New elements

Milling Machine [Occupied (5; 7)] Worker

Worker Worker

Human resources are used within a process (e.g., a worker). A human resource is specified byProduction is “specialStep knowledge,” “skills,” and 1 “competences” that 120 imply the required level of qualification. A min. (wait: 5 min. work: 115 min.) Milling Milling Machine Machine Costs Costs (Fixcosts) (Fixcosts) proficiency level types each property [13]. Milling Machine Costs (Fixcosts) (low [55000-80000]) (low [55000-80000]) (low [55000-80000])

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