In-Place Traceability for Automated Production Systems

IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 15, NO. 6, JUNE 2019 3155 In-Place Traceability for Automated Production Systems: A Survey of PLC and SysML Tools Wentao Wang , Student Member, IEEE, Nan Niu , Senior Member, IEEE, Mounifah Alenazi , Student Member, IEEE, and Li Da Xu, Fellow, IEEE

Abstract—Automated production systems are critical aPS, tracing spans both mechatronic modeling and executive enablers for Industry 4.0 because these design-to-order, code design. Specifically, the software written in programmable custom-built mechatronic systems are not only capable of logic controller (PLC) and the mechatronics modeled in Systems delivering automation capabilities to satisfy the stakeholder requirements in the manufacturing/production domain, but Modeling Language (SysML) shall be linked in order to offer doing so for a long period of time (e.g., several decades) aPS a variety of support, such as impact analysis and reuse [6]. during which numerous changing needs shall also be ac- Prior work has explored ways to establish the aPS trace links. counted for. Although traceability has long been recognized Czauderna et al. [7] presented a probabilistic network method as key to sustain changes, little is known about how the to automatically generate candidate trace links across multi- traceability information is managed in place, i.e., in the native environments where the engineering artifacts reside. ple and potentially dispersed mechatronics models, as long as We contribute in this paper a survey of traceability support those models include meaningful textual annotations. Feldmann within state-of-the-practice tools: seven for programming et al. [8] proposed a rule-based approach where the mechatronic logic controllers and six for building models in systems model elements’ correspondences were built heuristically and modeling language. We draw the similarities and differences stored in the XML Metadata Interchange files. The stored trace- from our survey results, and further present a design by leveraging the in-place traceability to better support the de- ability information could then be used for consistency checking. velopment and evolution of automated production systems. Despite these advances, little is known about how the traceability information is managed in place, i.e., in the native en- Index Terms—Automated production systems, development environments, industry 4.0, programming logic vironments where the to-be-traced artifacts reside [9]. Lack of controllers (PLC), systems modeling language (SysML), such knowledge is a serious problem because aPS development traceability. is contributed by engineers from diverse disciplines [10] who use special-purpose tools to manage their artifacts. This gap can I. INTRODUCTION further present prohibitive adoption barriers to practitioners. NDUSTRY 4.0 depicts the use and advancement of We aim at addressing the gap in order to thoroughly under- I cyber-physical systems in the manufacturing/production stand and coherently codify how traceability is supported in domain [1]. Among the key enablers of Industry 4.0 are the each of the PLC and SysML developments. To that end, this automated production systems (aPS), which are design-to- paper surveys the traceability features in contemporary tools order, custom-built mechatronic systems delivering automation which are integral to aPS development. While PLC are key to capabilities while incorporating dependability [2], agility [3], automated control applications, SysML has been increasingly flexibility [4], and other qualities in industrial applications [5]. adopted by diverse manufacturers and service providers, such No matter what technological innovation is employed, the as Boeing, SAP, and HSBC [11]. Additionally, standards like primary goal of aPS is to fulfill the stakeholder requirements. ISO/IEC 19514 [12] help to specify and streamline the use of Tracing the requirements is a key activity that enables the SysML, thereby promoting a shared foundation for systems en- reasoning of the fulfillment of stakeholder concerns, and in gineers, tool vendors, and other stakeholders. The tool selection of our survey is driven by industry practices. We adapt and refine the four life-cycle areas, namely strate- Manuscript received April 7, 2018; revised May 18, 2018 and July 14, 2018; accepted October 24, 2018. Date of publication October 30, gizing, creating, maintaining, and using traces [13], to structure 2018; date of current version June 12, 2019. The work is funded by our survey of the seven PLC integrated development environ- the U.S. National Science Foundation (Award CCF 1350487). Paper no. ments (IDEs) and that of the six SysML IDEs. Although we keep TII-18-0862. (Corresponding author: Nan Niu.) W. Wang, N. Niu, and M. Alenazi are with the Department of Electrical our in-place traceability surveys separate for PLC and SysML, Engineering and Computer Science, University of Cincinnati, Cincin- comparing these two classes of IDEs allows for a comprehen- nati, OH 45221 USA (e-mail:, [email protected]; [email protected]; sive view of the tools’ complementarity as well as a principled [email protected]). L. D. Xu is with the Institute of Computing Technology, Chi- manner to enhance collaborative traceability management for nese Academy of Sciences, Beijing 100190, China (e-mail:, bjtuxu@ aPS. gmail.com). The main contributions of this paper are to, first, identify Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. the commonalities and differences of the state-of-the-practice Digital Object Identifier 10.1109/TII.2018.2878782 PLC and SysML tools for their in-place traceability support,

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Fig. 1. (a) Structural overview of the PPU (pick & place unit): 1 stack serving as the input storage, 2 ramp storing the outputs, 3 stamp processing the work pieces, and 4 crane transporting the work pieces between 1 , 2 , and 3 . (b) PPU PLCs showing an excerpt of sequential function chart (SFC) where the crane’s actions (e.g., Turn_Right) and the logical conditions (e.g., CraneOnStack) are programmed sequentially. (c) PPU PLCs illustrating instruction list (IL) where a program’s variables are declared and assigned. (d) PPU SysML depicting a block definition diagram (BDD) that specifies the crane is composed of three microswitches for location detection. (e) PPU SysML showing an activity diagram snippet that specifies the crane’s behavior of turning toward and picking a work piece from the stack. All diagrams are adapted from [14]. and second, derive from our survey results a concrete design To coordinate traces between various mechatronic models, to consolidate the traceability tooling for aPS development and Czauderna et al. proposed two centralized (push and pull) and evolution. In what follows, we review related work in Section II, one distributed architectures [7]. In the push architecture, an and present our surveys of PLC and SysML in Sections III engineer working with an individual model (e.g., PLC) pushes and IV, respectively. Section V discusses our design, and finally, data from that model to a shared repository, and then the data Section VI concludes the paper. becomes accessible for tracing in other models (e.g., SysML). In the pull architecture, a single trace engine pulls artifacts from the models (PLC, SysML, etc.) via the support of a scheduler II. BACKGROUND AND RELATED WORK defining the events and/or time intervals at which each model’s This section provides some preliminaries on PLC and SysML data are pulled. In the distributed architecture, an individual trace in the context of aPS, and reviews prior research on systems engine is located alongside each model’s tool/environment, and traceability. Because each aPS—either a machine or a plant— a centralized registry provides minimalistic logic coordination needs to meet specific manufacturing constraints and cooperate between those engines. with other cyber-physical systems and with humans in real time, In summary, aPS development involves engineers who build it is often custom-built rather than mass-produced, and once a multitude of heterogeneous artifacts by using their own tools. deployed, it is expected to operate for up to 30 years [5]. As a Due to the long-term sustainability in the face of the continuous result, aPS must account for the many on-site changes made to changes, the aPS artifacts evolve within their own native envi- the software and the mechatronic parts. ronments. While coordination architectures [7] and plug-ins [8] To situate the background discussion, we use a bench-scale are proposed, understanding the nuances of specific tools and aPS called the Pick & Place Unit (PPU) [14] as a running ex- environments is crucial for practical traceability solutions [9]. ample in this section. Fig. 1(a) shows four mechatronic com- Critically reviewing, organizing, and comparing the in-place ponents requiring control for operation. The PLC codes and traceability [9] of the PLC and SysML environments are pre- SysML models of Fig. 1 illustrate the heterogeneous artifacts cisely the focuses of our work. that need to be traced in aPS development. A trace link expresses a relationship between a source artifact and a target artifact, e.g., the Turn_Right instruction of Fig. 1(b) implements the crane’s III. TRACEABILITY WITHIN PLC ENVIRONMENTS ACT_TurnToStack() operation of Fig. 1(d). The life cycle of trace links has four primary areas: trace creation, maintenance, We surveyed seven PLC IDEs because of the widespread and usage, and strategy planning and management [13]. It is this set popular adoption in industrial practices. Clearly, our selection of areas that guide our study of in-place traceability support in is not exhaustive. Our objective is to draw similarities and dif- the PLC and SysML environments. ferences of in-place traceability support from a representative The use of information retrieval (IR) methods has received sample where both proprietary and open-source tools are in- much attention to automatically recover the traceability informa- cluded. To that end, we relied on product descriptions, market tion within a software project [13]. Beyond the software bound- reviews, white papers, case studies, and other online resources ary, the applicability of textual cues has also been examined [15]. for tool selection and traceability survey. Next is a brief descrip- Nejati et al. [16] presented a two-step approach to identify tion of our surveyed PLC tools. SysML-based impact of requirements changes on system de- 1) Siemens STEP 7 is a product that is rated as one of the sign. Czauderna et al. [7] extended the probabilistic network IR most widely used PLC IDEs worldwide in industrial au- method to generate candidate trace links of mechatronic models, tomation, e.g., a comparison of PLC tools by Pedersen regulatory codes, and product or contractual requirements. estimated its market share to be 28.8% [17]. Not only WANG et al.: IN-PLACE TRACEABILITY FOR AUTOMATED PRODUCTION SYSTEMS: A SURVEY OF PLC AND SYSML TOOLS 3157

TABLE I TRACEABILITY SUPPORT WITHIN PLC DEVELOPMENT ENVIRONMENTS

†Empty cell indicates the information that our survey fails to identify. Trace link types (source → target). (1) Symbol/variable → I/O device. (2) Symbol/variable → logic block. (3) Symbol/variable → memory address. (4) Symbol/variable → user-deﬁned data type. (5) Symbol → variable table. (6) Symbol/variable → IP address. (7) Global variable → local variable. Tasks: (a) simulating, (b) visualizing hierarchical structure, (c) visualizing logical structure, (d) Proﬁcy Vision, (e) scope view, (f) cross reference, (g) querying, (h) reporting, (i) reference call, and (j) direct access.

does STEP 7 comply fully with the IEC 61131-3 stan- manufacturer and aims to use online or fieldbus connec- dards, but it offers easy access to a broad portfolio of tions to implement real-time controllers [22]. Siemens controllers. 6) CODESYS is an IDE for programming controller appli- 2) RSLogix 5000 is the PLC IDE built by another global in- cations developed by the German software company 3S dustrial automation company, Rockwell. The tool allows that also released free licenses for the PLC tool. Over 250 for configuring, programming, and maintaining the entire manufacturers from different industrial sectors offer in- AllenÐBradley family of controller products and related telligent automation devices with a CODESYS program- devices. RSLogix 5000 has been successfully used to ming interface, and nearly 400 devices are programmable solve many industrial challenges. For example, the use of with CODESYS [23]. RSLogix 5000 helped reduce palletizers’ machine design 7) OpenPLC is an open-source PLC tool that supports and development time to more than half [18]. human-machine interface (HMI) interface as well as Wii 3) Logic Developer is the controller programming tool part remote controller. OpenPLC runs on several platforms of GE’s holistic Proficy platform providing solutions to like Raspberry Pi, arduino boards, Modbus remote I/O, assembly and fabrication, material and information man- PiXtend driver, and so forth [24]. agement, and the like. The platform is used extensively by Table I lists each of the seven PLC tools’ in-place traceability industries like automotive manufacturers (e.g., affecting support organized by the four life-cycle areas [13]. We refine over 60% of the cars produced in North America [19]) and the trace link creation, maintenance, and usage areas with a cou- oil producers (e.g., controlling about 25% of the world’s ple of specific dimensions. We discuss the overall traceability blowout preventers [20]). strategy for the PLC tools together in Table I. 4) TwinCAT 3 is a tool that Beckhoff creates to configure, Two main mechanisms are used to create trace links: “term program, and diagnose automation devices. Furthermore, as unifier” and “drag & drop”. Fig. 2 uses STEP 7 to illustrate the embedding into the Visual Studio IDE equips Twin- these mechanisms. The elements from different PLC models CAT 3 with such capabilities as multilanguage support could be linked by the same variable name or interface address, (C/C++, MATLAB/Simulink, etc.), object-oriented ex- both of which are instances of “term as unifier”. While IR- tensions of IEC 61131-3, and debugging via breakpoints, based methods are applicable, straightforward term matching watch lists, and call stacks [21]. may be more cost-effective to automate trace link recovery in 5) TwinCAT PLC is also a Beckhoff’s tool but differs from PLC environments. The “drag & drop” feature is extensively TwinCAT 3. Instead of the binding of a visual program- used to link between different interfaces (e.g., link 3 in Fig. 2). ming environment and object-oriented extensions, Twin- The dotted box of Fig. 2 shows that the linked artifacts (function CAT PLC supports IEC 61131-3 independently of the block and organization block) as a whole are further linked to the 3158 IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 15, NO. 6, JUNE 2019

Fig. 2. Trace creation mechanisms in STEP 7: 1 and 2 link variables and addresses by names, 3 establishes linking via drag & drop, 4 links multiple models to the simulator, and 5 helps close the feedback loop inside the PLC development environment. simulator. The simulation results could be fed back to modify tionships among the model elements. If the simulation fails to the PLC elements and to maintain the trace links. In addition run or generates anomalies, then the PLC models and specific to the two main link creation mechanisms, “tag to trace” (as in model elements may need to be updated and retraced. RSLogix 5000) and “interface control” (as in Logic Developer’s In a nutshell, the underlying traceability strategy within our quick panel) can also be used. The created trace links can be surveyed PLC IDEs is to create finer-grained trace links based represented in a symbol list (as in STEP 7), variable list (as in primarily on the names of the primitive constructs. Because such Logic Developer), or input/output table (as in CODESYS). links exist in a natural and straightforward way inside the PLC We analyze the in-place link maintenance by examining two tools, maintaining them could be automated to a large degree aspects: if the traces are persistently stored and what mech- with the special attention paid to change propagation and online anisms trigger the addition, update, or deletion of the traces. changes. The simulation capabilities offered in the PLC IDEs Table I shows that many PLC tools persist the traces based on help check the accuracy of the existing trace links and provide the unique identifiers, such as input/output variables and hard- feedback for potential link maintenance. However, contempo- ware component addresses. In our opinion, a trace link (i.e., rary PLC simulations offer more support in verification (i.e., a relationship) that is not persistently stored needs little main- whether the code is written right) and less in validation (i.e., tenance. In the PLC environments, we observe that only very whether the right code is written). To properly validate the engi- primitive links are automatically stored, especially those infer- neering solution, systems-level models such as those specified able directly from the identifier terms. As a result, a change in SysML would need to be considered. in one PLC model can be automatically propagated to other models in order to ensure consistency. Other than “propagate IV. TRACEABILITY WITHIN SYSML ENVIRONMENTS change”, “online/offline change” also drives link maintenance. Our selection of the SysML IDEs had the same empha- Online refers to making the change without any downtime to the sis of industrial adoption as the PLC counterparts. Similarly, machine, whereas offline requires the PLC program to restart. A we used a variety of online resources to guide our tool se- majority of PLC tools support offline change of a trace link, e.g., lection and in-place traceability survey. Six SysML tools are modifying the PLC logic as it corresponds to the input/output reviewed. variables. TwinCAT 3, in particular, allows online changes to 1) Enterprise Architect is a popular tool for industrial au- the PLC programs while running so as to observe the change tomation. It was rated to have best values among the effect immediately. SysML tools [25] and has been implemented by over The link usage surveyed in Table I focuses on in-place uses 650 000 users. inside the PLC IDEs. The visualization of the links correlates 2) Cameo Systems Modeler is a model-based systems engi- strongly with this usage. In many PLC IDEs, simulator provides neering tool used widely in the transportation, healthcare, effective ways to test the logical correctness of PLC code with- aerospace industries, as well as navy defense [26]. As an out in-field hardware configurations and deployment. To build example, Cameo Systems Modeler assisted Bombardier, a simulator, the right set of PLC models need to be integrated, the world’s largest manufacturer of both planes and trains, which requires accurate trace links to specify the subtle rela- in engineering SysML and other systems models [27]. WANG et al.: IN-PLACE TRACEABILITY FOR AUTOMATED PRODUCTION SYSTEMS: A SURVEY OF PLC AND SYSML TOOLS 3159

TABLE II TRACEABILITY SUPPORT WITHIN SYSML DEVELOPMENT ENVIRONMENTS

†Empty cell indicates the information that our survey fails to identify. Trace link types (source → target). (1) Requirements → use cases. (2) Use cases → implementation models. (3) Functional requirements → SysML models. (4) Nonfunctional requirements → SysML models. (5) Requirements → SysML models. (6) Requirements → test cases. (7) High-level SysML designs → low-level SysML models. Tasks: (a) simulating, (b) navigation, (c) reporting, and (d) traceability editing.

3) Rational Rhapsody Architect is IBM’s integrated systems between the central model and all of the related models and engineering environment that uses UML and SysML for diagrams [32]. Note that the majority of our surveyed SysML requirements analysis, as well as visual, model-based de- tools lack the capability of deﬁning trace links involving non- sign. Its industrial users span ﬁnancial services, commu- SysML elements in an in-place manner. This motivates our nications, distribution, and other domains [28]. proposed tool-integration solution presented in Section V. 4) Astah SysML, formerly known as Java and UML devel- Similar to the PLC tools, simulation plays an important role opers’ environment, was created by Change Vision [29]. in SysML development environments. More than half of our It is a lightweight tool for modeling SysML diagrams, surveyed SysML IDEs offer such support, e.g., simulation of e.g., package diagram is not supported and it also lacks activity diagram. Ports in SysML facilitate the simulation fea- model-based simulation capabilities. ture. SysML supports two ports. Full port represents a physical 5) Modelio is an open-source modeling tool that supports access point (e.g., a mechanical interface,) while proxy port UML, SysML, and Java code generation [30]. The IN- exposes the parts visible to external connectors. Simulation, to- COSE 2012 Tool Vendor Challenge was solved using gether with the different matrices and maps, helps engineers Modelio environment and the proposed solution was uncover inconsistency and incompleteness of SysML models, based on SysML models. which in turn helps improve the quality of the trace links. 6) Papyrus is another open-source tool. In 2015, Papyrus In short, the in-place traceability of SysML tools is explicit became an Eclipse project aiming to achieve industrial and comprehensive. The traceability strategy of our surveyed grade. For example, Papyrus was selected by Sherpa as SysML IDEs is to store links in some relational form (e.g., ma- the underlying model-driven engineering platform be- trix) to ensure model completeness and consistency. However, cause of its full coverage of SysML [31]. model-level consistency may overlook operation-level details. We present the in-place traceability support of the SysML In the PPU example, for instance, if PLC code overrides stamp’s tools in Table II. Compared to the PLC tools, the SysML coun- default pressure, then the model-level consistency checked with terparts offer much more and stronger traceability features, default pressure settings must be rechecked. demonstrated by the built-in representation(s) inside each of the six tools that we surveyed. For this reason, we use such in-place trace forms to unify link creation and link maintenance V. D ISCUSSION in Table II. This echoes our view that a trace link that is not Our objective of surveying the in-place traceability support persistently stored requires little maintenance effort. of PLC and SysML development environments is to understand It turns out that the SysML tools explicitly store the links as the nuances of the state-of-the-practice tools and to enable prin- native elements in trace matrix, relation maps, or other relational cipled ways to enhance collaborative traceability management forms. An example is the lightweight Astah SysML tool that for aPS. In this section, we discuss the limitations of our study, uses a “traceability map” to place a model at the center of draw similarities and differences from our survey results, and the visualization and further presents coarse-grained trace links present an improved design. 3160 IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 15, NO. 6, JUNE 2019

Fig. 3. Design of a distributed trace coordination prototype where a persistently stored SysML trace matrix is integrated into a PLC IDE: ❶ shows the hardware conﬁguration, ❷ shows the symbol list, ❸ shows the function blocks, ❹ shows the simulator, and ❺ shows the integrated SysML trace matrix.

The sources of our survey limit how the results could be gen- and other tasks by attending to their immediate concerns [13]. eralized. While we intend to select the representative PLC and Our investigation also indicates that all the surveyed PLC and SysML tools in industrial practice, our choices might be influ- SysML tools provide interfaces allowing external traceability to enced by admittedly subjective materials like the list of PLC be exchanged. Three main means exist: tools compiled by Pedersen [17] and the case studies document- 1) APIs for extending the IDE’s features, e.g., Astah ing successful stories by the tool vendors (e.g., [18], [31]). A SysML [34]; more comprehensive set of tools could be identified by perform- 2) service customizations for constructing customer- ing systematic literature reviews or mapping studies. Therefore, specific plug-ins, e.g., CODESYS [35]; and our survey results, especially the commonalities, should be in- 3) common infrastructures such as Eclipse Safety Frame- terpreted in the context of our selected tools. Another limita- work [36], which would host a family of plug-ins. tion lies in our inability to fully access some of the proprietary We identify three discrepancies of in-place traceability be- tools, though in a few cases the trial versions helped us to test tween PLC and SysML IDEs. First, the trace links within PLC out the tools’ in-place traceability features. In Tables I and II, development are finer grained and point-to-point (e.g., address- consequently, several empty cells could be filled with inside to-input), whereas the SysML traces tend to be encapsulated at knowledge. the model level. Second, the PLC changes aim to be done online Similarities emerge between the traceability support within without machine downtime, which require precise trace links to the PLC and the SysML tools. Using the trace links to support localize a small set of artifacts while leaving many other artifacts querying, navigation, and reporting is common in Tables I and II. intact. In contract, SysML changes are not only offline but often In addition, simulation is frequently supported. The most impor- broadly scoped, such as enhancing the nonfunctional require- tant similarity, in our opinion, is the linking of multiple models ments. Third, PLC tools store the trace links primitively (e.g., as to one another inside either development environments. All the variable names or interface addresses), while SysML tools store seven PLC tools that we surveyed support the five programming them separately in relational forms. It is important to point out language standards defined in IEC 61131-3. Similarly, the six that the above-mentioned discrepancies are not isolated but re- tools of Table II all support SysML requirements, behavioral, lated. For example, the primitive, finer-grained traces facilitate and structure models. Although the set of PLC models and the the precise linking of PLC parts, which in turn helps to achieve set of SysML models are traced in place, connecting the two sets localized online changes. In this way, the in-place traceability presents a significant challenge. As shown in a systematic liter- of the PLC and SysML environments is indeed purposed [13]. ature review recently conducted by Mustafa and Labiche [33], The differences resulted from our survey lead to a design there is a gap of tracing the artifacts that come from different sketched in Figs. 3 and 4, which attempts to support tasks like domains of expertise, and such a gap is reflected in inadequate validating PLC source code and consistency checking in SysML. traceability tools. We use the PPU running example from Section II to illustrate Our survey results help discover the causes of the gap between our design. The essence here is to embed the persistently stored PLC and SysML IDEs. Before presenting our design to bridge trace links from one tool into another. Fig. 3, for instance, inte- the gap, we argue that the distributed coordination architecture grates a SysML trace matrix into a PLC IDE. The trace links of is better than the centralized ones (pull or push) [7] because the artifacts ❶, ❷, ❸, and ❹ of Fig. 3 are handled natively within individual trace engines already exist in the PLC and SysML the PLC IDE. The address “2” in ❷ links the input in both the IDEs and we can interconnect these engines rather than reinvent- symbol list ❸ and the simulator ❹. A novel feature in Fig. 3 is ing new ones. Meanwhile, compared to the third-party plug-ins, to provide some contextual information in the function blocks distributed trace coordination can support engineers’ in place ❸ to enrich the understanding of the point-to-point traceability WANG et al.: IN-PLACE TRACEABILITY FOR AUTOMATED PRODUCTION SYSTEMS: A SURVEY OF PLC AND SYSML TOOLS 3161

coordination helps tackle the task speciﬁcity problem by treat- ing trace links from one tool to another as task’s contextual information in our design. Therefore, the design demonstrates the value of our survey, both of which serve as stepping stones toward new and practical progresses. In addition to the above-mentioned three challenges, researchers and tool vendors could address the different types of traceability artifacts, various tasks the engineers face, and automated ways to create the trace links and maintain their consistency. Tables I and II provide information related to these challenges.

VI. CONCLUSION The development and evolution of aPS involve multiple engineers with diverse expertise. One challenge of collaborative traceability management is tool integration. We have surveyed in this paper the state-of-the-practice tools in aPS development (seven tools for PLC and six for SysML). By comparing and Fig. 4. Continued design of Fig. 3 that embeds a PLC simulator ❻ into contrasting the tools’ in-place traceability support along the link the simulation of an activity diagram ❼ inside a SysML IDE. creation, maintenance, usage, and strategizing dimensions, we derive a distributed trace coordination design aiming to bridge the gap of linking sets of heterogeneous artifacts [33]. (i.e., address “2” is allocated to variable “WPReady”). The sta- Our future work includes expanding and extending our sur- tus indicating whether a work piece is ready, combined with the veys in the context of aPS development, possibly by explor- context of crane’s position, improves the throughput of the PPU ing the role of traceability in refactoring [37], viewpoint merg- in that the crane could reduce its idle time as well as fruitless ing [38], [39], and model testing [40]. We also plan to investigate moves [14]. The effect of satisfying the nonfunctional require- ways to persistent simulations as behavioral trace links, and fur- ment of increasing throughput is contextualized in the SysML ther use open-source tools like OpenPLC and Papyrus to test trace matrix ❺ of Fig. 3. the feasibility of our design. Fig. 4 illustrates the integration of PLC’s traceability information into a SysML IDE. This time, rather than incorporating the point-to-point traces, we link the PLC simulation model ❻ REFERENCES ❼ with SysML’s own simulation to offer operational insights [1] B. Vogel-Heuserand D. Hess, “Guest editorial Industry 4.0—prerequisites into a SysML activity’s execution. Note that storing simulation and visions,” IEEE Trans. Automat. Sci. Eng., vol. 13, no. 2, pp. 411Ð413, as a behavioral trace link has yet to be supported in the PLC Apr. 2016. [2] W. Wang, A. Gupta, N. Niu, L. D. Xu, J.-R. C. Cheng, and Z. Niu, “Au- tools that we surveyed. We believe persisting such links and tomatically tracing dependability requirements via term-based relevance making them portable to other (SysML) IDEs will be valuable. feedback,” IEEE Trans. Ind. Inf., vol. 14, no. 1, pp. 342Ð349, Jan. 2018. For example, the persistent links can help engineers to identify [3] N. Niu, S. Brinkkemper, X. Franch, J. Partanen, and J. Savolainen, “Re- quirements engineering and continuous deployment,” IEEE Softw., vol. 35, missing or incorrect information. In Fig. 4, if the PLC simu- no. 2, pp. 86Ð90, Mar./Apr. 2018. lation model ❻ is about metallic work pieces and the activity [4] N. Niu, J. Savolainen, Z. Niu, M. Jin, and J.-R. C. Cheng, “A systems diagram ❼ is activated in the state of white work pieces, then approach to product line requirements reuse,” IEEE Syst. J., vol. 8, no. 3, pp. 827Ð836, Sep. 2014. the trace links between the tools are likely to be incorrectly con- [5] B. Vogel-Heuser, J. Fischer, S. Rosch,¬ S. Feldmann, and S. Ulewicz, nected. If the simulation ❻ does not include stamp’s pressure “Challenges for maintenance of PLC-software and its related hardware that is explicitly specified in the activity diagram ❼, then some for automated production systems: Selected industrial case studies,” in Proc. Int. Conf. Softw. Maintenance Evol., Bremen, Germany, Sep./Oct. missing traces should be noted. 2015, pp. 362Ð371. Our conceptual sketch presented in Figs. 3 and 4 is only one [6] N. Niu, W. Wang, and A. Gupta, “Gray links in the use of requirements design that concretizes the distributed tracing architecture [7]. traceability,” in Proc. Int. Sympo. Found. Softw. Eng., Seattle, WA, USA, Nov. 2016, pp. 384Ð395. However, this design is derived from our survey results in a [7] A. Czauderna, J. Cleland-Huang, M. Ç inar, and B. Berenbach, “Just- principled way by recognizing the asymmetric aspects of trace- in-time traceability for mechatronics systems,” in Proc. Int. Workshop ability within PLC and SysML: granularity (finer grained ver- Requirements Eng. Syst., Services, Syst.-of-Syst., Chicago, IL, USA, Sep. 2012, pp. 1Ð9. sus model encapsulated), change (locally online versus broadly [8] S. Feldmann, M. Wimmer, K. Kernschmidt, and B. Vogel-Heuser, “A scoped), and persistency (term based versus relational). The first comprehensive approach for managing inter-model inconsistencies in au- difference may cause extra work on adjusting link granularity tomated production systems engineering,” in Proc. Int. Conf. Automat. Sci. Eng., Fort Worth, TX, USA, Aug. 2016, pp. 1120Ð1127. and the latter two may lead to additional effort to support spe- [9] J. Cleland-Huang, B. Berenbach, S. Clark, R. Settimi, and E. Romanova, cific tasks. Instead of adjusting granularity, our design embeds “Best practices for automated traceability,” IEEE Comput., vol. 40, no. 6, PLC simulations into SysML models. Meanwhile, distributed pp. 27Ð35, Jun. 2007. 3162 IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 15, NO. 6, JUNE 2019

[10] M. Alenazi, N. Niu, W. Wang, and A. Gupta, “Traceability for auto- [38] N. Niu, J. Savolainen, and Y. Yu, “Variability modeling for product line mated production systems: a position paper,” in Proc. Int. Model-Driven viewpoints integration,” in Proc. Annu. Comput. Softw. Appl. Conf., Seoul, Requirements Eng. Workshop, Lisbon, Portugal, Sep. 2017, pp. 51Ð55. Korea, Jul. 2010, pp. 337Ð346. [11] No Magic, “Clients of No Magic,” 2018. [Online]. Available: https://www. [39] N. Niu, A. Koshoffer, L. Newman, C. Khatwani, C. Samarasinghe, and J. nomagic.com/company/our-clients. Accessed on: Oct. 2018. Savolainen, “Advancing repeated research in requirements engineering: A [12] International Organization for Standardization, “ISO/IEC 19514,” 2017. theoretical replication of viewpoint merging,” in Proc. Int. Requirements [Online]. Available: https://www.iso.org/standard/65231.html. Accessed Eng. Conf., Beijing, China, Sep. 2016, pp. 186Ð195. on: Oct. 2018. [40] M. Alenazi, N. Niu, W. Wang, and J. Savolainen, “Using obstacle analysis [13] J. Cleland-Huang, O. Gotel, J. H. Hayes, P. Mader,¬ and A. Zisman, “Soft- to support SysML-based model testing for cyber physical systems,” in ware traceability: Trends and future directions,” in Proc. Future Softw. Proc. Int. Model-Driven Requirements Eng. Workshop, Banff, Canada, Eng., Hyderabad, India, May/Jun. 2014, pp. 55Ð69. Aug. 2018, pp. 46Ð55. [14] B. Vogel-Heuser, C. Legat, J. Folmer, and S. Feldmann, “Researching evolution in industrial plant automation: Scenarios and documentation of the pick and place unit,” Tech. Univ. Munich, Munich, Germany, Tech. Rep. TUM-AIS-TR-01-14-02, 2014. Wentao Wang (S’15) received the B.Sc. de- [15] M. Alenazi, D. Reddy, and N. Niu, “Assuring virtual PLC in the context gree in computer science from Shanghai Mar- of SysML models,” in Proc. Int. Conf. Softw. Reuse, Madrid, Spain, May itime University, Shanghai, China, in 2007, and 2018, pp. 121Ð136. the M.Eng. degree in software engineering from [16] S. Nejati, M. Sabetzadeh, C. Arora, L. C. Briand, and F. Mandoux, “Au- Beijing Institute of Technology, Beijing, China, in tomated change impact analysis between SysML models of requirements 2010. He is currently working toward the Ph.D. and design,” in Proc. Int. Sympo. Found. Softw. Eng., Seattle, WA, USA, degree in the Department of Electrical Engineer- Nov. 2016, pp. 242Ð253. ing and Computer Science, University of Cincin- [17] J. M. Pedersen, “PLC Comparison Chart,” 2017. [Online]. Available: nati, Cincinnati, OH, USA. https://www.plcs.net/downloads. Accessed on: Oct. 2018. His research interests include software [18] Rockwell Automation, “Case studies,” 2018. [Online]. Available: https:// engineering, requirements engineering, and www.rockwellautomation.com/global/news/case-studies/. Accessed on: cybersecurity. Oct. 2018. [19] GE Automation, “Automotive manufacturing,” 2015. [Online]. Available: http://www.geautomation.com/industries/automotive-manufacturing. Ac- cessed on: Oct. 2018. Nan Niu (M’08–SM’13) received the B.Eng. de- [20] E. Zeidler, “GE launches its automation & controls solution platform to gree in computer science and engineering from bring the benefits of the industrial internet to power industries,” 2015. Beijing Institute of Technology, Beijing, China, [Online]. Available: http://www.geindustrial.com/print/node/220856. Ac- in 1999, the M.Sc. degree in computing sci- cessed on: Oct. 2018. ence from the University of Alberta, Edmonton, [21] Beckhoff Automation, “TwinCAT 3 eXtended Automation,” 2012. AB, Canada, in 2004, and the Ph.D. degree in [Online]. Available: http://download.beckhoff.com/download/Document/ computer science from the University of Toronto, catalog/. Accessed on: Oct. 2018. Toronto, ON, Canada, in 2009. [22] Beckhoff Automation, “TwinCAT PLC and motion control on the He is currently an Associate Professor with PC,” 2017. [Online]. Available: https://www.beckhoff.com/english.asp? the Department of Electrical Engineering and twincat/. Accessed on: Oct. 2018. Computer Science, University of Cincinnati, [23] CODESYS, “Device directory,” 2016. [Online]. Available: http://devices. Cincinnati, OH, USA. His research interests include software require- codesys.com/device-directory.html. Accessed on: Oct. 2018. ments engineering, information seeking in software engineering, and [24] T. R. Alves, “OpenPLC,” 2016. [Online]. Available: http://www. human-centric computing. openplcproject.com. Accessed on: Oct. 2018. Dr. Niu is the recipient of the U.S. National Science Foundation Faculty [25] PivotPoint Technology, “SysML modeling tool reviews,” 2018. [Online]. Early Career Development Award, and the IEEE International Require- Available: http://sysml.tools/review-sparx-ea. Accessed on: Oct. 2018. ments Engineering Conference’s Best Research Paper Award in 2016, [26] No Magic, “Case studies,” 2018. [Online]. Available: https://www. and the Most Influential Paper Award in 2018. nomagic.com/mbse/resources/case-studies.html. Accessed on: Oct. 2018. [27] M. Chami, P. Oggier, O. Naas, and M. Heinz, “MBSE at Bombardier transportation,” 2015. [Online]. Available: https://www.nomagic.com/ mbse/images/images/MBSE_at_BT.pdf. Accessed on: Oct. 2018. [28] IBM, “Rational Rhapsody Architect for systems engineers,” 2018. Mounifah Alenazi (S’18) received the B.Sc. degree in computer science [Online]. Available: http://www-03.ibm.com/software/products/en/ from the University of Dammam, Dammam, Saudi Arabia, in 2009, and ratirhaparchforsystengi. Accessed on: Oct. 2018. the M.Sc. degree in computer science from the Kennesaw State Univer- [29] Change Vision, “Astah SysML,” 2018. [Online]. Available: http://astah. sity, Kennesaw, GA, USA, in 2016. She is currently working toward the net/editions/sysml. Accessed on: Oct. 2018. Ph.D. degree in the Department of Electrical Engineering and Computer [30] Modeliosoft, “Modelio open source modeling environment,” 2018. Science, University of Cincinnati, Cincinnati, OH, USA. [Online]. Available: https://www.modelio.org/categories/about-modelio- Her research interests include software engineering, requirements 2.html. Accessed on: Oct. 2018. engineering, and model-driven engineering. [31] Eclipse, “Case study,” 2016. [Online]. Available: https://eclipse.org/ papyrus/resources/sherpa-usecasestory.pdf. Accessed on: Oct. 2018. [32] Change Vision, “Traceability map,” 2018. [Online]. Available: http://astah. net/features/traceability-map. Accessed on: Oct. 2018. Li Da Xu (M’86–SM’11–F’16) received the B.S. [33] N. Mustafa and Y. Labiche, “The need for traceability in heterogeneous and M.S. degrees in information science and en- systems: a systematic literature review,” in Proc. Annu. Comput. Softw. gineering from the University of Science and Appl. Conf. (COMPSAC), Turin, Italy, Jul. 2017, pp. 305Ð310. Technology of China, Hefei, China, in 1978 [34] Change Vision, “JIRA mind map planner,” 2014. [Online]. Available: and 1981, respectively, and the Ph.D. degree in http://astah.net/features/jira-plugin. Accessed on: Oct. 2018. systems science and engineering from Portland [35] CODESYS, “Development services,” 2018. [Online]. Available: https:// State University, Portland, OR, USA, in 1986. www.codesys.com/products/codesys-services/development-services. He is academician of the European Academy html. Accessed on: Oct. 2018. of Sciences and the Russian Academy of Engi- [36] PolarSys, “Eclipse Safety Framework,” 2014. [Online]. Available: neering (formerly USSR Academy of Engineer- https://www.polarsys.org/esf/. Accessed on: Oct. 2018. ing). [37] N. Niu, T. Bhowmik, H. Liu, and Z. Niu, “Traceability-enabled refactoring Dr. Xu is a 2016, 2017, and 2018 Highly Cited Researcher in the field for managing just-in-time requirements,” in Proc. Int. Requirements Eng. of engineering named by Clarivate Analytics (formerly Thomson Reuters Conf., Karlskrona, Sweden, Aug. 2014, pp. 133Ð142. Intellectual Property & Science).