CPS-based hierarchical and self-similar automation ... - IEEE Xplore

CPS-based hierarchical and self-similar automation architecture for the control and verification of reconfigurable manufacturing systems Alessandro Brusaferri, Andrea Ballarino Institute of Industrial Technologies and Automation National Research Council Via Bassini, 15 – 20133 Milano, Italy {alessandro.brusaferri, andrea.ballarino}@itia.cnr.it

Abstract— Fast evolving requirements for the manufacturing sectors, which must now take much more into account problems such as product customization, markets volatility and shortening life cycles, imposes to focus the attention of innovation towards a new generation of automation systems based on the CPS paradigm. The work here presented therefore suggest a new extension of the Cyber-Physical Systems functionalities through the definition of their Avatar, virtual counterpart of the physical system that, based on semantic, simulation and control models, enables a much more reconfigurable and performance optimizing self-similar and hierarchical automation architecture. Having identified the major research challenges to be tackled to create this vision, an intermediate step of implementation is presented, where state-of-the-art technologies have been adopted as partial enablers of the proposed CPS architecture and corresponding engineering tools. An industrial-level application serves as testbed for the approach. Keywords—Cyber Physical Systems; Reconfigurable Manufacturing Systems; Control Verification and Optimization.

I.

INTRODUCTION

European leadership and excellence in manufacturing are being significantly threatened by the huge economic crisis that hit the Western countries over the last years. This situation actually translates in fast evolving requirements for the manufacturing sectors, which must now take much more into account: rising product variety; product individualization; volatile markets; increasing relevance of value networks; shortening product life cycles. Companies have therefore to invest on new technological solutions, and to focus the efforts on the conception of new automation platforms that could grant to the shop floor systems the flexibility and re-configurability required to optimize their manufacturing processes, whether they be continuous, discrete or a combination of both. An essential ingredient for a winning innovation path is a more aware and widespread use of ICT in manufacturing-related processes. The key enabling technology that the industrial research world (both in Europe and in the US) foresees as essential to tackle these challenges is that of the Cyber-Physical Systems

Franco Antonio Cavadini, Diego Manzocchi, Mauro Mazzolini Synesis Consortium ComoNExT science and technology park Via Cavour, 2, 22074 Lomazzo Como, Italy {franco.cavadini, diego.manzocchi, mauro.mazzolini}@synesis-consortium.eu [1], powerful and autonomous microcomputers networked (with the internet), resulting in the convergence of the physical and the virtual worlds [2][3]. In the realm of manufacturing (where CPS comprise smart machines, storage systems and production facilities capable of autonomously exchanging information, triggering actions and controlling each other independently), this technological evolution is described as the fourth industrial revolution [4]. From the point of view of production automation, adopting the CPS approach means that the classical paradigms of control technology grounding on a signal-based view and designing closed systems are not suited anymore; new open approaches must be developed. One potential approach is the concept of Service-oriented Architectures (SOA). The usage of the SOA paradigm in the context of industrial automation systems is intended to decrease significantly the effort for integration and programming of automation components. SOA relies on the paradigm of service-orientation, i.e. the different software components provide their functionalities as loosely coupled services over a network. SOA in the field of production automation is characterized by the technical realization based on web standards. Currently the two most important technologies for creating service-oriented industrial applications are OASIS’s Devices Profile for Web Services (DPWS) [5] and OPC Unified Architecture (OPC-UA) [6]. Even though both solutions are based on Web Services, they follow different approaches as reflected in the comparative analysis developed by [7]. Integrated approaches are being used to cover from the process planning and operation phase thorough the execution of the process control logic, as in [8] or even more, supporting also the automatic code generation for embedded real-time services [9]. As much as a problem of architectural approach to the implementation of CPS-based automation systems, another clear challenge to be addressed is the enhancement of CPS functionalities thanks to their connection to the cyber world. To avoid costly trial-and-error adaptation routines, manufacturing systems need a mean to predict and evaluate the effects of

2014 IEEE Emerging Technology and Factory Automation (ETFA)

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actions and response strategies. In this context, simulation has been identified as a key factor for future adaptive manufacturing systems and a number of simulation-based approaches to enhance the adaptive capabilities of manufacturing systems have recently been proposed [10]. During the last few years, many research efforts have been focusing on exploring and developing new methodologies and tools supporting the integration between simulation environments and real controller dedicated to the run-time execution of the control code for the purpose of Virtual Commissioning (VC) [11]. Despite the evidence of benefits that could be achieved with the adoption of VC procedures, their adoption is nowadays still not widely diffused within the industrial domain mainly due to the request for an high level of training and specialization for dealing with the integrated use of advanced design environments and communication technologies [12]. Overall, what is still missing is a comprehensive view on how CPS should be effectively integrated into the next generation of shop floors, knowing that, for them to be a really disruptive innovation, special leverage on their “internet of things” potentialities must be found. The following sections therefore proposes an innovative self-similar, modular and hierarchical architecture of CPS based on the newly-introduced concept of Virtual Avatar (section-2). Such architecture find a first instance of application through state-of-the-art technologies in section-3, and a concrete, industrial-level application in section-4. Conclusions finally foresees the next step of this research with a future, complete application of this CPS architecture. “VIRTUAL AVATAR”-BASED HIERARCHICAL AND SELFII. SIMILAR CPS ARCHITECTURE Though it is somewhat difficult to provide an exact definition due to its broadness, in general, it is possible to say that CPS stay in the border of cyber and physical worlds, requiring more collaboration between these two worlds than in the past. As an intellectual challenge, the concept of CPS is in fact about the intersection, not the union, of the physical and the cyber. It is not sufficient to understand separately the physical components and the computational components. It is their full interaction that must be understood [13]. The main problem is to find how the extensive distribution of computational power to the systems of the plant through CPS can be exploited to: enable effective re-configurability of the production systems; optimize the performance of such systems; enhance communication for a more effective collaboration among all the stakeholders of the factory value chain. This can be tackled only if the presently partial vision of a CPS is completed by investigating what its “cyber” part effectively is: which kind of “glocal” intelligence should leverage on the embedded computational power to monitor and control the physical system, while communicating (what?) with the other CPSs of the factory? For CPS to operate dependably, safely, securely, efficiently, and in real-time, some if not all components (whether cyber or physical) must be able to interact and communicate. On one hand, it is quite clear that the physical

system, through its I/O(s), must be governed by the on-board intelligence in order for its actions to be coordinated with those of the other systems it interacts with. Things become much less clear when dealing with what that same intelligence should do with the “cyber-“ component of the CPS. The work here presented proposes an innovative concept, envisioning a more comprehensive acceptation of a CPS, where its physical component, operating and interacting within the real shop floor, is completed through the introduction of its cyber counterpart: the Virtual Avatar. This means identifying which aspects of the CPS should acquire a higher level of concreteness within the cyber world, in order to exhibit the advanced functionalities that are needed to satisfy the aforementioned requirements of the next generation of shop floors. The first element to be considered concerns the virtual characterization of the CPS physical properties and of how they allow interactions (even passive, when dealing with purely sensing CPS) with the physical world, without having the need to specify its inner working mechanisms. The first layer of a CPS Avatar is thus set up through the definition of its semantic model, considering the following aspects: characterization of the CPS input and output (both the data and the material/resources flows); description of the CPS available functionalities, that is, the services it can execute to operate within the shop floor; definition of the extended characteristics that the CPS Avatar offers within the cyber world with respect to the physical one (through the other composing parts of the Avatar, described in the following paragraphs). Once established its (functional) appearance through the semantic model, the CPS Avatar must be “animated” by integrating the mathematical and numerical elements required to mimic, within the cyber world, the physical behavior of the CPS. This means that the second constituting part of the Avatar is its simulation model, the representation of the physical dynamics of the CPS, through hybrid (continuous and discrete) numerical techniques, which should allow studying the most relevant aspects of its operations within the shop floor without executing them concretely. The simulation model should be a perfect mirroring of its real counterpart, with its semantic model thus being representative for both the physical and the virtual components of the CPS. The accuracy of the simulation model obviously depends on the specificities of the CPS and of its usage. The just introduced simulation model of the CPS Avatar is conceived to be a virtual substitute of the physical part of the CPS, whenever anticipating its behavior could be useful for off-line decision making on the configuration of the shop floor. On the other hand, the last element of the Avatar to be considered is built to support the optimization of the runtime performance of CPS through the enhancement of the algorithms controlling their real-time behavior. For this reason, it is called the control model. The increasing scale and complexity of control applications impose qualitatively new demands on control systems technology, with embedded models becoming a prerequisite for advanced control.

CPSs therefore instantiated into the shop floor could belong to different levels of aggregation (or reasons at the different cycle-time). Layer-0 CPS will be those “un-openable”, directly supplied by automation systems providers, whose inner working mechanisms are hidden behind the corresponding CPS avatar, and, in particular, its interfaces described through the semantic model. This will grant the required level of interoperability without obliging producers to expose their proprietary technologies. Depending on the specific industrial domain, layer-0 CPS will be of different nature, ranging from local components (such as a single actuators) to more complex equipment for which it would not be reasonable to have a lower decomposition (a melting furnace of a foundry, for instance). Fig. 1. Hierarchical CPS architecture

However, modelling for systems of systems (SoS), such as those introduced through the CPS paradigm, brings complexities that are often not encountered at the subsystem level. Thus, the emphasis with systems of systems must shift toward more data-driven, empirical techniques such as identification, learning, and adaptation. The control model of a CPS Avatar is exactly that, a simplified representation of the most relevant dynamics of the CPS that could be used directly by advanced control techniques, in order to internally forecast its expected behavior (on a short prediction horizon) and consequently correct its real-time actions towards the achievement of optimal production performance. One important aspect to be considered is that the realization of this Avatar-based approach requires dealing with the problem of “synchronization” between the physical and virtual components of a CPS. A specific communication infrastructure must be realized to support the complete extension of the CPS into the cyber world through its Avatar. This means that all relevant data flows, generated within the physical world during the operations of the CPS, must be channeled efficiently (managing bandwidth constraints) and effectively (granting reliability and safety of the connection) towards requesting Avatar models. On the other hand, raw data coming from the physical world are not enough to enable an effective synchronization; they have to be interpreted, elaborated and then transformed in adaptation of the Avatar models. This means that specific learning and identification techniques must be integrated in the CPS intelligence in order to grant that the parameters characterizing the semantic, simulation and control models assume a value that minimize discrepancies from the physical behavior of the CPS. Based on this Avatar-extended acceptation of a CPS, this work proposes a specific hierarchical and self-similar functional architecture to manage the overall runtime execution of the operations from a generic set of CPS. Adopting the CPS paradigm for the automation systems of a factory effectively transforms it into a System of Systems, since it distributes intelligence into embedded devices, delocalizing part of the production real-time decision-making. On the other hand, the

From these layer-0 CPS, it is possible to grow a hierarchical architecture (Fig. 1) building “composite” CPS, which have exactly the same functional model but where the physical system under control is effectively composed by a set lower level CPS. This means that each CPS, independently from the layer it belongs to, must have the following characteristics: it has the responsibility of controlling the degrees of freedom of the physical system(s) it encompasses (which could be lower-level CPS), directing their behavior in order to satisfy its local production requirements; it has a complete virtual representation through an Avatar with explicit semantic, simulation and control models; if existing, it exposes to the higher level of aggregation, its own (still un-managed) degrees of freedom, through which its real-time execution can be supervised, effectively proposing again, in a self-similar way (Fig. 2), the same structure of the lower layers.

Fig. 2. Self-similarity within the different layers of the architecture

Through its progressive aggregation of CPS, this runtime hierarchical architecture effectively creates also an equivalent activity of abstraction, with the result that information are aggregated and filtered (on relevance criteria) when “climbing” the architecture, and control decisions (each at its own “cycletime”) propagates correctly their effects when descending it The concrete implementation of such approach must obviously be based on a specific engineering procedure directly related to the Avatar of a CPS, whose three models, independently from its layer of aggregation, needs to be designed for the CPS to work correctly. This means that software tools must be adopted to create the semantic,

simulation and control models of the avatar, linked together by the fact that they relates to the same CPS. These tools will be used by the engineers that are developing a factory automation to define the overall architecture of CPS and all of its composing elements. Only layer-0 will not see this activity, since the avatar of its CPS should be provided directly by the technology vendors. It is important to remark that through this procedure, the instance of a CPS architecture for a specific manufacturing case is progressively built up through a modular, decentralized approach. From layer-0 CPS, their aggregation into higherlevel ones is designed by engineers who are expert of that particular “section” of the factory, know which behavior it should have, and are capable of proposing to the higher levels (through the design of the Avatar) the most appropriate and relevant masking of the CPS of their competence. The other strength of this approach regards the verification and validation of the on-board intelligence of a generic CPS. Thanks to the availability of the Avatars of all its internal, lower-level CPS, it is in fact possible to leverage on their simulation models to execute a cyber-commissioning procedure. The effects of the newly conceived intelligence of the CPS will therefore be (at least partially) verified and its forecasted performance evaluated, reducing consistently the time required for the ramp-up of the manufacturing process, at the beginning of its life cycle, but, above all, after reconfiguration phases. III.

KEY ENABLING TECHNOLOGIES, STANDARDS AND TOOLS FOR CPS ARCHITECTURE

A first prototype implementation of the presented CPS architecture is presented in the following section as a proof of concept. To such an aim, available state-of-the-art technologies, standards and tools have been adopted. Starting from this proof of concept implementation, research and development activities are needed in order to set up the most suitable methodologies, standards and tools that allow to fully harness the benefits of the introduced new generation of CPSbased automation solution. The backbone of this first implementation of the architecture is the integration between the IEC 61499 standard [14] [15], for modular and distributed automation solution development, and the OPC Unified Architecture (OPC UA) [16] specification for setting up the communication infrastructure within the different layers of the architecture. In particular, the adoption of IEC 61499 standard addresses various concepts of the presented self-similar CPS approach basically thanks to the distributed application development support. In addition, IEC 61499 standard enhances the definition of reusable models, since the principles of modularity, encapsulation and standardization of interfaces are strongly exploited. The first basic element introduced by such a standard, the modularity, introduces the use smaller and manageable autonomous functional modules (Function Blocks) that interact with each other materializing an effective distributed approach where the intelligence is executed across multiple resources (processing units embedded within the CPS). Moreover, every modules hide their internal dynamics

exposing the available functionalities through a defined set of events and exchanging information through appropriate set of data. The advanced algorithms required for the optimization of the system behavior can be therefore encapsulated within the modules enhancing the readability and usefulness of the whole architecture. FBs of the IEC 61499 standard are structured on different hierarchical levels in a top-down functional decomposition approach enabling the hierarchical nature of the presented CPS architecture in which the elements that exist at different levels of aggregation can interact thanks to the use of the same semantic model. Major benefits in the adoption of the IEC 61499 standard for the automation software development phase includes: distributed download of the control application onto the overall multi-target architecture, automatic binding and the possibility to debug on-line the overall distributed system by a single environment reduce deployment, commissioning and reconfiguration efforts. Finally the possibility to obtain main modules of the architecture by the aggregation of simpler sub-modules completes the set of IEC 61499’s features that accomplish with the presented CPS-based approach. Furthermore, the usage of OPC UA allows an easier communication and interaction within the whole CPS architecture. Thanks to its Service Oriented approach it is used to implement a seamless interaction between the different elements both in terms of virtual and real components. OPC UA enables an effective distributed hierarchical intelligence within the CPS architecture allowing the actual communication between the different devices. OPC is an open connectivity standard that finds an effective application in the industrial automation and IT systems of industrial enterprises. It represents an important step ahead in combining specific needs of the automation industry with a standard interface for secure and more efficient information exchange of complex data. Major features that OPC UA introduces are the following: • Object-oriented techniques supporting type hierarchies and inheritance enable agile handling of instances of the same type on client side. • Type information is treated, exposed and can be accessed with the same mechanisms used to access instances, similarly to the case of information schema in relational database systems. • Interlaced network of nodes allowing information to be exposed and connected in various ways. • Extensibility regarding the type hierarchies as well as the types of references between nodes. OPC UA servers targeting a system that already contains a rich information model can expose that model “natively” in OPC UA, avoiding its mapping in to a different model. Furthermore OPC UA information models always exist on OPC UA servers, not on client-side. Undoubtedly, OPC-UA has been adopted as a proof of concept solution. Real-time communication requirements have to be further exploited before the effective adoption of such service oriented technology within industrial real-time control field.

In the context of this first implementation of the architecture the integration between IEC 61499 standard and OPC UA is directly implemented using NxtControl development tools: NxtOne [17]. It fully integrates IEC 61499 standard through the establishment of software objects called Composite Automation Type (CAT). A CAT incorporates the control logic according to IEC 61499, a visual representation of the system, in term of advanced HMI and SCADA facilities, and the management of the connection with the hardwarespecific input/output. Then all these aspects are integrated within a unique software object type that can be instantiated and aggregated with the aim to obtain the whole application for the specific CPS considered. Moreover, it is fully compliant with the distribution concept because it allows to map the execution of the CAT to the specific resource among those are available within the CPS architecture and to automatically handle with the events and variables binding between the different hardware resources. NxtControl solution natively supports the OPC UA-based communication offering the basic elements to implement the interaction infrastructure between the different component of the presented architecture. Using NxtOne facilities is then possible to design the control solution and to set up the communication infrastructure, according to the IEC 61499 standard and OPC UA specification, within a unique integrated development environment.

system states is implemented. This has been realized by using specific service calls over the OPC UA infrastructure. In such a way the Virtual Avatar can be intended as an effective element with the right interfaces to provide an easy access to all high level factory applications. The mapping between resources described within communication architecture and the Virtual Avatar is addressed at design phase as well as the definition of the database model. It is important to remark that at control system design phase, the control and sensor variables will be linked to the simulation model for validating the control code in virtual commissioning. Thus, at the end of this phase, is simple to have a direct correspondence between database records and simulated variables through an XML configuration file. The interaction layer loads the XML configuration file setting up the client application, accessing to the desired value corresponding to the desired variables. Finally, this layer has also to meet different high level application requests. In particular it has to provide a set of possible usages of the Virtual Avatar based on the needs of the requesters. This is possible extending the communication architecture and providing a mechanism to enable cooperation, information exchange and interaction between the Virtual Avatars and other high level applications used within the factory. A specific area of the database, contains the tables needed to describe new resources that high level applications want to share.

A further key enabling element for this first implementation of the CPS architecture is represented by the adoption of virtual modeling and simulation tools for the extension of the cyber part of CPS with the Virtual Avatar. Many different virtual modeling and simulation tools are normally exploited for different purposes within all the phases of the manufacturing system lifecycle, mainly in the engineering and prototyping but also in management and production planning. Computer Simulation, especially Discrete Event Simulation (DES) tools, are well used for analysis and forecasting purposes. Advanced previsions of the system performance, plant configuration and material flow simulation for facility design are nowadays widely considered.

Finally, the Virtual Avatar has to be able to communicate, through proper interfaces, with the IEC 61499 compliant control run-time allowing validation, commissioning and optimization. A plant model composed with the correct details level can be mapped over the automation execution environment, in order to support the development and the validation of the control solution. At this level the virtual model cooperating with the real control system allows to effectively implement optimization algorithms. Such algorithms (e.g. for minimizing the resources utilization or maximizing throughput) can be included within the control system and verified through simulations providing on-line decision capabilities according to the actual boundary conditions of the system. A loop of continuous adaptation of the control parameters to the plant, testing the control and optimizing the behavior of the model is made, satisfying the optimization criterions chosen.

With regard to the goals of this implementation such tools are integrated with a 3D visualization and dynamic data storage capabilities in order to obtain an effective mirror of the physical part that forms the Avatar. In addition, the possibility to exploit the Virtual Avatar synchronized with the physical part of CPS has to be enabled. Again by taking advantage of the capabilities integrated within the OPC UA-based communication infrastructure it is possible to keep updated the CPS Avatar with the current state of the physical plant. For instance one of the enabled feature is that the automation system is able to expose in a structured and hierarchical way both control and sensor data through OPC UA client/server architecture. Within the prototype implementation performed, the gathered data, through OPC UA server, are stored in a central MySQL database designed for the specific purpose as the data storage component of the Virtual Avatar. As OPC UA provides just-in-time data, any information between samples could be provided, thus it is important that the OPC UA server samples each data coherently with the physical phenomenon. Furthermore an interaction layer responsible to maintain the Virtual Avatar updated in terms of control behaviors and real

IV.

CONCEPT APPLICATION TO A REAL SYSTEM

In present section, the developed solution based on the adoption CPS approach is discussed with reference to the control and monitoring of a pilot plant for mechatronic products remanufacturing (Fig. 3). Specifically, the Remanufacturing pilot plant, described in [18], represents a fully automated, modular, and re-configurable solution involving integrated Printed Circuit Boards analysis, re-work, and testing, which allows treating efficiently a large spectrum of populated and unpopulated PCBs. Moreover, the process integrates an automatic conveyor for moving PCBs - mounted on proper pallets – between required operating stations. In order to properly cope with planned as well as unforeseen product and process variations, such transport system has to guarantee scalability, integrability and agile re-configurability. Therefore, a modular architecture has been considered, exploiting the

Plug-and-Play (PnP) concept. To such an aim, the transport system has been designed to be composed of mechatronic components, integrating dedicated sensors and actuators as well as the related control system. Furthermore, advanced hardware and software solutions have been adopted to support the agile mechatronic component integration within the overall system, as described within the following paragraphs.

Fig. 3. Remanufacturing pilot plant

A. Reconfigurable Conveyor architecture The transport system modules can be connected back to back to form the desired conveyor layout. Cross transfer modules can be incorporated as required (i.e. to provide the pallet transfer to a connected machine or between parallel transfer modules). Thus, complex routing strategies can be agilely developed, including overtaking policies, as shown in Fig. 4.

Fig. 4. Pallet routings within the conveyor

In order to properly guarantee agile transport system reconfigurations the transport system modules have been structured in: • a main backbone mechatronic component (namely Straight Conveyor), providing the straight pallet transfer capability in both forward (F) and backward (B) direction.

• a dedicated mechatronic component for each cross transfer position (namely Cross Translator), capable of supporting both right and left pallet movements. To support agile reconfigurations each mechatronic module composing the conveyor integrates a dedicated embedded controller. The communication framework between the controllers has been implemented as an Industrial Ethernet network. Indeed, the adoption of such technological solution guarantee the resiliency and network security of traditional fieldbus solutions, as well as the improved bandwidth, open connectivity, and standardization that Ethernet provides. In order to properly optimize synchronous data access, Moxa EDS 405A Industrial Ethernet switches have been considered, providing the utilities for multicast control (IGMP Snooping), Quality of Service (QoS), and virtual LANs (VLANs) configurations. Furthermore, to properly balance communication network robustness and agile cables connection, a ring topology has been implemented, installing a Moxa EDS switch in each cabinet to support the agile integration of Cross Translators due to transport system reconfigurations. Such solution provides also the capability of defining Quality of Service policies so that the real-time information exchange at the conveyor module layer (between mechatronic components) has the highest priority, followed by the communication at a system level, between the conveyor modules and between conveyor modules and machines. Within each automation system component (i.e. conveyor module) an NxtOne firmware have been integrated and linked to an embedded OPC-UA server supporting run-time architecture interoperability. Thus, the control logic of each module have been implemented, as detailed in the next section, by a composite IEC61499 function block. The interface of each function block towards the overall architecture (in terms of service oriented distributed controllers communication mean) have been developed by means of OPC-UA technology. Thus, IEC61499 formalisms have been adopted as a tool for both low level control logic developement and services orchestration modeling and implementation at higher architectural levels, e.g. for pallets routing control, conveyor modules pallet exchange, operating machines pallets exchange, etc. Moreover, the integrated servers support a general data abstraction layer, independent from underlying control technology. Therefore, the automation technology adopted for controlling the plant can be dynamically reconfigured during the plant lifecycle, guaranteeing the data collection and aggregation interoperability and minimizing the time consuming remapping activity, critically impacting the commissioning and reconfiguration phases. Thus, the designed data classes, structuring the automation system raw data, can be maintained and agilely readapt on different installations, independently from the underlying control technology adopted. The overall control architecture of the conveyor is represented in Fig. 5. As represented, each mechatronic module controller (i.e. straight conveyor, cross translator) include related instances of semantic, simulation and control models.

SCADA

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SECURE ACCESS TO THE FIELD

HMI Line Virtual Model

SWITCH

SWITCH

STRAIGHT CONTROL

CROSS 1 CONTROL

TRANSPORT SYSTEM MODULE 12 CONTROL CABINET

SWITCH

STRAIGHT CONTROL

CROSS 1 CONTROL

CROSS 2 CONTROL

TRANSPORT SYSTEM MODULE 13 CONTROL CABINET

Fig. 5. Conveyor control architecture

B. Conveyor control solution A control logic block for each functional component of the conveyor has been defined according to its modular composition and to control functionalities. In such a context, each mechatronic device is meant as a self-contained module. The execution is regulated by a dedicated management utility implemented within the Execution Control Chart. Hence, an event-driven design approach has been exploited. For each submodule, the data structure, the software interface and the control functionalities have been designed to guarantee reusability and to enable agile integration within the control application. To such an aim, each function block exposes: • an event type input request for each task (i.e. automation function) the module is capable of performing; • a data type input for each configuration parameter related to module’s tasks execution; • an event type output for each module’s task execution acknowledgment (including un-nominal and failure conditions) and for each request for task execution towards subsequent modules; • a data type output for each variable representing the module internal state and tasks execution. As an example, the automation tasks exposed by a conveyor module including one Cross Translator consist of: MOVE_F (move pallet forward), MOVE_B (move pallet backward), MOVE_RC1 (cross the pallet to the right at position 1), MOVE_LC1 (cross the pallet to the left at position 1). Moreover, the control algorithms to be executed during specific operating conditions has been encapsulates within the related state of the Execution Control Chart.. Thanks to the adoption of such modeling criteria, a clear and readable model structure has been obtained, organized by functionalities and competencies. Indeed, a function block class has been developed for the Cross Translator and for the Straight conveyor logic. Then, the conveyor module composite function block is deployed by integrating an instance of a Straight

conveyor function block and one instance of Cross Translator function block for each installed device. Despite the reactive logics reported above, the conveyor control solution required the integration of advanced dynamic optimization capabilities. In fact, the destination within the Remanufacturing plant of each PCB depends on the results of the testing stage and on the required repair operations identified. Thus, multiple run-time changing flows have to be properly real-time managed by the conveyor control system. Moreover, the weighted sum of the total PCBs tardiness and the idle time of the PCB Rework machine have to be minimized. The first is required for respecting production constraints defined by the production schedule provided by the Manufacturing Execution System. The latter is required to reduce energy consumption for heat dissipation within the printed circuit board Rework machine. To such an aim, a Particle Swarm Optimization based manager have been implemented for the dynamic solution of the PCBs transport problem by means of the PSO toolbox (see [19] for details). Afterwards, the solution have been deployed as C++ code and integrated within the embedded run-time by means of the NxtOne firmware interface. C. Conveyor Avatar and Virtual commissioning The development of the Virtual Avatar for the Remanufacturing conveyor has been based on the same granularity of the control system, exploiting the CPS selfsimilar approach. To such an aim, virtual modules have been implemented for Cross Translator and Straight conveyor logic. Besides, the conveyor virtual module have been developed by integrating a Straight conveyor model and an instance of Cross Translator model for each installed device. The virtual models development have been performed within the SIMIO platform [20]. In particular, every above mentioned modules have been implemented in order to obtain a custom SIMIO object library through which it is possible to create the whole virtual model of the plant by instantiating the various modules according to the real equipment. Exploiting the features of such a simulation platform the elements that have to be considered for the synchronization with the real plant have been modeled as “State Properties”. They are specific type of SIMIO property that can be used as connectors to the simulation data. Then a custom add-in program has been implemented for analyzing the instantiated modules and their state properties in order to produce an XML file that describes the interfaces of the virtual model that have to be exploited to maintain the synchronization with the real plant. Such XML file has been therefore used for the configuration of the MySQL database which is the dynamic storage facility that allows to complete the features of the Virtual Avatar. Furthermore, exploiting such synchronized interaction between the SIMIO models and the database further applications, or if needed other Avatars, can be connected for exchanging specific information or data. Fig. 6reports the overall plant representation. Moreover, the virtual models have been connected to the automation system by means of OPCUA interface. Therefore, model parameters (i.e. conveyor speed, run-time cross transfer dynamics, etc) are dynamically updated by collecting shop floor data. Indeed, the overall control application have been validated by closed loop

simulation before final deployment onto the real plant including nominal process execution, failure conditions and plant/conveyor reconfiguration scenarios.

Contract number: 314805) European Seventh Framework Programme. REFERENCES [1]

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[4] Fig. 6. Remanufacturing virtual model

V.

CONCLUSIONS AND NEXT STEPS

Current manufacturing plants are based on automation engineering concepts that were defined and developed decades ago, with a very sharp horizontal division between concurrent production assets, and vertical separation from the higher levels of a factory decision-making hierarchy. These limitations have always been acceptable because functionalities of the systems were more important than strictly optimized performance, and the automation architecture of a factory was designed and implemented only once, at the beginning of the life cycle of a plant. Today, this is no longer acceptable, and for factories to sustain the challenges of the market, a completely new approach to the realization of automation architectures must be developed. This work has therefore presented an innovative hierarchical and self-similar CPS architecture, based on the concept of Virtual Avatar, the extension into the cyber-world of the functionalities of a modern manufacturing system. This approach has seen a “first-tier” implementation through state of the art technologies, validating its effectiveness on an industrial test-bed that encompasses all the requirements of a FoF shop floor. IEC-61499 standard has in fact been adopted has a tool to model and implement both the low-level control modules and the system orchestration, thanks to the integration of OPC UA to support communication between distributed elements of the CPS architecture. In order to enable this real world testing, a few compromises have been obviously accepted, opening already a clear path towards the next steps of this research. Alternative, beyond the state of the art technologies will be in fact investigated to implement the hierarchical and self-similar architecture in a much more flexible and interoperable way, paying particular attention to satisfy the always present realtime constraints that an automation system must consider. ACKNOWLEDGMENT The work presented in this paper has been partially supported by the Factory-ECOMATION project (Factory ECO-friendly and energy efficient technologies and adaptive autoMATION solutions – Call identifier: FoF.NMP.2012-1,

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