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Generic automation task description for flexible assembly systems
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52nd 52nd CIRP CIRP Conference Conference on on Manufacturing Manufacturing Systems Systems
Generic for flexible systems 28th CIRP task Designdescription Conference, May Nantes,assembly France Generic automation automation task description for2018, flexible assembly systems Rainer Matthias Scholer, Karkowski* Rainer Müller, Matthias Scholer, Martin Martin Karkowski* A new methodology toMüller, analyze the functional and physical architecture of ZeMA gGmbH – Zentrum für Mechatronik und Automatisierungstechnik, Eschberger Weg 46, D-66121 Saarbrücken, Germany ZeMA gGmbH – Zentrum für Mechatronik und Automatisierungstechnik, Eschberger Weg 46, D-66121 Saarbrücken, Germany existing products for an assembly oriented product family identification
* Corresponding author. Tel.: +49 (0) 681 85787 529; fax: +49 (0) 681 85787 11. E-mail address: [email protected] * Corresponding author. Tel.: +49 (0) 681 85787 529; fax: +49 (0) 681 85787 11. E-mail address: [email protected]
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
Abstract Abstract
École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France
*To Corresponding author. Tel.: +33 3 87 37for 54mass 30; E-mail address: [email protected] face the increased requirements customization and frequent technological changes, currently used assembly systems require a high
To face the increased requirements for mass customization and frequent technological changes, currently used assembly systems require a high level of flexibility and re-configurability. These requirements lead to increased time-to-manufacture and manufacturing costs, which goes against level of flexibility and re-configurability. These requirements lead to increased time-to-manufacture and manufacturing costs, which goes against manufacturer’s goals. To mitigate the impact of these challenges a holistic approach for planning and programming an assembly system is manufacturer’s goals. To mitigate the impact of these challenges a holistic approach for planning and programming an assembly system is presented in this paper. The approach will minimize the implementation effort of flexible automation tasks suitable for industrial applications by presented in this paper. The approach will minimize the implementation effort of flexible automation tasks suitable for industrial applications by Abstract utilizing a combination of an abstract process description, a virtual model of the assembly system, and a standardized control system utilizing a combination of an abstract process description, a virtual model of the assembly system, and a standardized control system © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2019 Authors. Published by by Ltd. This is an open access under the CC BY-NC-ND In© business environment, theElsevier trend towards more product varietyarticle and customization is unbroken. license Due to this development, the need of ©today’s 2019 The The Authors. Published Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/) (http://creativecommons.org/licenses/by-nc-nd/3.0/) agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production This is an open access article under the scientific CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the committee of the 52nd CIRP Conference on Manufacturing Systems. Peer-review under responsibility the committee of CIRPmethods Conference on Manufacturing Manufacturing Systems. Peer-review under responsibility of the scientific scientific of the the 52nd 52nd CIRP Conference Systems. systems as well as to choose the of optimal productcommittee matches, product analysis are on needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and Keywords: assembly systems, control system, modularity, workflow, design methodology Keywords: assembly systems, system, modularity, designand methodology nature of components. This control fact impedes an efficientworkflow, comparison choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies areinstruction identified, and 1. Introduction process. The specification is later later used as as work work for Introduction process. The specification is used instruction for a1.functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which the aa standardized standardized control control system. system. As As aa consequence consequence the thedepicts assembly assembly similarity between providing designflexible supportand to both,process production system planners and product designers. illustrative To handle handle theproduct needs families for mass massbycustomization, customization, is expressed expressed as aa sequence sequence of skills skills and is isAndivided into To the needs for and Anprocess of example of a nail-clipper is used to explain the proposedflexible methodology. industrialiscase study onastwo product families of and steeringdivided columnsinto of adaptable assembly systems are required [1]. Customized a program-flow – the logic of the process – and a data-flow adaptable assembly systems are required [1]. Customized a program-flow – theapproach. logic of the process – and a data-flow –– thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed automation, human human robot robot cooperation cooperation (HRC) (HRC) and and modular modular the flow flow of of information information during during the the process. process. Based Based on on this this the ©automation, 2017 The Authors. Published by Elsevier B.V. assembly systems are utilized to face these challenges. This generic task description, the suitable production equipment Peer-review under responsibility of the scientific committee of theThis 28th CIRP Designtask Conference 2018. the suitable production equipment assembly systems are utilized to face these challenges. generic description,
results in in an an increased increased complexity complexity of of the the already already demanding demanding results while while widely used, Programmable Logic Controllers (PLCs) only widely used, Programmable Logic Controllers (PLCs) only offer a low level of abstraction, which complicates the offer a low level of abstraction, which complicates the development of reusable, maintainable and well-structured of reusable, maintainable and well-structured 1.development Introduction software components even even more. more. [3] [3] software components Because of changing market situations, the assembly assembly system system Because changing market situations, Due to ofthe fastsoftware development in thethe domain of – and thereby its – remains subject of many – and thereby and its an software – remains subject of many communication ongoing trend of digitization and technological changes. changes. The The thereby thereby extended extended implementation implementation technological digitalization, manufacturing enterprises are commissioning facing important process goes against the demand for faster of process goes themarket demandenvironments: for faster commissioning of challenges in against today’s a continuing assembly systems, reduced costs and a shortened time-toassemblytowards systems, reducedofcosts and a shortenedtimes time-totendency manufacture [1], [1], reduction and leads leads to toproduct the need needdevelopment for holistic holistic design designand of manufacture and the for of shortened product lifecycles. In addition, there is an increasing assembly systems. assembly systems. demand customization, at the same time a global This ofpaper paper presents aabeing holistic approach to in reduce the This presents holistic approach to reduce the competition with competitors all over the world. This trend, implementation effort of flexible assembly systems suitable for implementation effort flexible assembly systems for which is inducing theof development from macrosuitable to micro industrial applications. Therefore the specification of an industrialresults applications. Therefore the specification of an markets, inand diminished lot sizes duefrom to augmenting assembly system processes is obtained an iterative assembly system(high-volume and processes obtained from an iterative product varieties to is low-volume production) [1]. implementation ofDesign an automation automation taskidentification [1,2]. In In addition, addition, Keywords: Assembly; method; Family implementation of an task [1,2].
implementing those those skills skills via via services services can can be be determined. determined. This This implementing generic task description is then detailed and adapted. To generic task description is then detailed and adapted. To execute the assembly task a virtual model of the assembly execute the assembly task a virtual model of the assembly system –– describing describing its its independencies independencies and and properties properties –– is is system designed. Afterwards the virtual model and the task description designed. Afterwards the virtual model and the task description of product range characteristics and/or arethe exported into the and standardized controlmanufactured system, to to perform perform are exported into the standardized control system, assembled in this system. In this context, the main challenge in the assembly assembly process. process. the modelling and analysis is now not only to cope with single Section 22 presents presents the the theoretical theoretical basics basics of of this this research research as as Section products, a limited to product range orThe existing product families,3 an introduction the topic. following section an also introduction to analyze the topic. The following section 3 but to be currently able to and toapproaches compare products to define summarizes published to simplify simplify the summarizes currently published approaches to the new product families. It of cananbeassembly observedprocess. that classical existing implementation process Based on on those implementation process of an assembly process. Based those product families are regrouped in function of clients or features. approaches, the research needs are identified (section 4) and the approaches, the research needs are identified (section 4) to and the However, assembly oriented product families are hardly find. developed approach is presented in detail (section 5). The developed approach is presented in detail (section The On the modular product family level, differ mainly5). in two designed control systemproducts and its its main main components are designed modular control system and components are main characteristics: (i) the number of components and (ii) the briefly introduced in section 6. briefly introduced in(e.g. section 6. type of components mechanical, electrical, electronical).
Classical methodologies considering mainly single products or solitary, already existing product families analyze the To cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which 2212-8271 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2212-8271 possible © 2019 The optimization Authors. Publishedpotentials by Elsevier Ltd. This is existing an open access causes article under the CC BY-NC-ND license an efficient definition and identify in the difficulties regarding (http://creativecommons.org/licenses/by-nc-nd/3.0/) (http://creativecommons.org/licenses/by-nc-nd/3.0/) production system, it is important to have a precise knowledge comparison of different product families. Addressing this Peer-review under responsibility of the scientific committee of the 52nd CIRP Conference on Manufacturing Systems. Peer-review under responsibility of the scientific committee of the 52nd CIRP Conference on Manufacturing Systems.
2212-8271 © 2019 The Authors. Published by Elsevier Ltd. This is an©open article Published under theby CC BY-NC-ND 2212-8271 2017access The Authors. Elsevier B.V. license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of scientific the scientific committee theCIRP 52ndDesign CIRPConference Conference2018. on Manufacturing Systems. Peer-review under responsibility of the committee of the of 28th 10.1016/j.procir.2019.03.185
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2. Theoretical Basis 2.1. Software Architectures Architectural patterns are reusable solutions for commonly occurring problems which divide systems into smaller sub systems. Architectural patterns may also contain design patterns, which define the relations between different elements of the system to achieve the desired goals. [4] Currently used development models are often service- or agent-based. A Service Orientated Architecture (SOA) describes how systems provide and consume loosely coupled services (methods and applications) – the orchestration of the system – to achieve a given goal instead [3,4]. 2.2. Programmable Logic Controller (PLC) In most production systems, process state changes are triggered by events, for instance changed pressure values. By now PLCs are utilized as economical solutions for controlling those production systems. Next to the economic benefits, machine manufactures are facing constraints related to the characteristics of PLCs. PLCs are characterized by their cyclic data processing and real-time execution, which results in a state based design for sequencing control. [5,6] Due to the low level of abstraction, the fail supporting high-level programming. [7] 2.3. Petri nets Petri nets represent discrete event dynamic systems and consist of four basic elements – places, transitions, arcs and tokens. Whereas places model states of a system or subsystem, transitions model the activities and events of a system. To indicate that a modelled state is met, places can contain tokens. These tokens are consumed or produced by the transitions of a net. Arcs connect places with transitions or vice versa and define the flow of the tokens. [8] The explicit state-based modelling of a system enables simplified analysis of the system. In addition, petri nets have a formal mathematical description and support simple methods for model-checking. For modelling a petri net, their graphical representation is utilized. [9] 3. Approaches to simplify the Implementation Process of Automation Systems Approaches to simplify the implementation of automation system are often based on standardization, the utilization of (customizes) middleware, and (multi-) agent-systems for process planning and execution. These tools are summarized in the following sections. 3.1. Standardization of Programming Languages, Communication Protocols and Exchange Formats Despite the standardization of textual and graphical programming languages of PLCs (IEC 61131-3), it remains a manufacturer’s task to apply this standard. (compare [10,11]). The PLCopen XML standard was developed on top of
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IEC 61131-3 standard, to be a distribution format for function block libraries, which could be used on different platforms [12]. An event based programming paradigm for PLCs is provided by the IEC 61499. It utilizes concepts and methodologies of modern software, but it is widely claimed that it doesn’t satisfy the requirements of automation systems. [13,14] Next to a standardization of the utilized Programming Languages, standards for communication layers and protocol – like the Open Platform Communications - Unified Architecture (OPC-UA) – have been developed. OPC-UA combines a language and platform independent communication model with a generic information model. This enables a platform independent implementation of an automatic task communication with other control systems or software like ERP- or MES-Systems. [15] Thereby OPC-UA simplifies and unifies the distribution and communication of controllers. AutomationML™ – a XML-based neutral and object orientated data exchange format to save interdisciplinary planning data – is capable of storing geometric, kinematic, logical and structural information of a technical facility. It utilizes already existing data formats like Collada (geometry and kinematic information) and CAEX (structural information). [16] 3.2. Utilizing Middleware Middleware is defined as a generic software layer for distributed software, which simplifies its distribution and unifies communication between the nodes. [4] Due to its generic approach, middleware enables high-level programming. In addition, it often offers various packages for common domain specific tasks like path planning, navigation and machine vision. [17] Nevertheless, the development of complex systems often becomes challenging due to the overhead caused by required prior knowledge of the setup, configuration and integration [18]. 3.3. Utilizing (Multi-)Agent-Systems Distributed assembly system controllers relying on middleware and standard communication models only shift the effort of implementing the required system behavior. [19] To support the implementation process, projects like MyJoguhrt [20] or SkiROS [21] utilize self-organizing agent systems on top of middleware. Unfortunately Wooldridge [22] describes the behavior of distributed agent systems as hardly predictable, which contradicts the requirements for maintainability and safety. The high safety requirements and standards of semiautomated systems prevents using those approaches in an industrial applications without further adaptation. 3.4. Published Research Projects On top of the presented tools, concepts such as “Plug and Produce” [23,24] – a vision, enabling structural changes in automatic systems with minimal manual adaption and reconfiguration of the software – have been developed. Based on the conceptual implementation of this vision different aspects such as automatic integration of real-time
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communication systems [25,26], “high-level” programming [23,27] or automatic programming [3,25,28] are considered in different research projects. To create more versatile systems with automatic configuration, the concepts of skills and selfdescribing assets are introduced (e.g. [24], [28] or [29]). This enables automatic orchestration of processes based on the current configuration of the system and a corresponding task description. The mentioned approaches following the Product Process Resource (PPR)-based modelling, utilize a stateless description of the assembly process, which complicates the consideration of relevant aspects for an industrial application, such as processes having encountered errors or disturbances caused by human workers. In addition, these concepts are hardly transferable to the standards regarding hardware and software used by the industry. 4. Research Needs One significant challenge is to provide a simple, predictable, flexible and maintainable task description, which satisfies standards used by the industry while reducing the implementation effort of the assembly process and system. In particular, mid-life cycle adaptations of the system and interaction with a human worker must be considered. The main research questions are: • How can a holistic planning method for an assembly system and its corresponding assembly process be designed to support a developer during the specification, configuration, (re-) implementation and adaptation stages? • How can an assembly process and system be modelled in such a way that it helps to identify critical states and events which can occur as well as to model the disturbances caused by human workers? • How can the assembly process be expressed to become a deterministic work instruction for a standardized control system? 5. Methodology In order to address these research questions, a holistic and iterative methodology is introduced (Fig. 1). The service based approach seems to be appropriate to describe a distributed, flexible and loosely coupled assembly system, whereas utilizing the concept of skills provides a more generic highlevel description of the process and system. The solution oriented approach separates the technical implementation and the design of the assembly process. Utilizing petri nets as a formal description of an assembly process – and thereby the orchestration of the services – allows for predictable behavior during runtime. To interpret the generated process specification and thereby speed up the virtual commission and simplify the implementation, a standardized control concept is utilized.
Fig. 1. Graphical representation of the Methodology.
5.1. Determining the assembly process and –system Beginning with a detailed analysis of the product being considered, the product-related requirements for the assembly process are derived. Based on those requirements, the assembly process is planned. This results in a process sequence, structure as well as a list of assembly operations for each assembly station. In a next step this information is analyzed to determine the necessary operations of the assembly stations. In doing so, the assembly operations are constructed from smaller and generic skills. The skills are combined into program-flows and data-flows. Whereas the program-flow describes the required logic to perform the assembly process, the data-flow is used to model the flow of information during the process. Combined, the data-flow and program-flow will be called workflow. The program-flow is expressed as a petri net, wherein each place represents a state while performing the described process. These transitions between states can occur by performing a skill (for instance open a valve during a filling process) or a status change (for instance a changed pressure value during a filling process). These changes are modelled with transitions and their conditions. The defined conditions have to be fulfilled to change the state of the process. Utilizing petri nets as formal and simple descriptions of the logic, simplifies parallelization and synchronization of assembly operations and enables later execution, analysis and simulation of the process. Due to the state based modelling, petri nets support situational definition of the assembly system’s behavior. Furthermore, petri nets provide an intuitive and simple visual model of the process. Next to the program-flow, the data-flow – the flow of information between the used skills of the workflow – has to be modelled. This enables connecting information from different software functionalities or the utilized production equipment to other elements. An additional semantic model for describing the assembly process allows a more abstract, reusable and generic
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description. The semantic model is used to determine the required production equipment based on the required skills. For this purpose, a predefined modular system is utilized, whose elements implement skills via services. An assembly station is obtained by a skill-based matching process. During this process, a set of appropriated elements, capable to performing the required skills of the workflow, is generated, whereas skills, on the other hand, can be established by combining multiple skills or services. The elements must respect the requirements of the product and process and must be compatible with one another. 5.2. Detailing the workflow Process Planning Pick & Place Object A from “Pos. 1“ to “Pos. 2“
Object A at “Pos. 1“
Object A at “Pos. 2“
Abstract Process
Assigning the Pick and Place Task to a Robot Object A at „Pos. 1“
Close Gripper
Robot at „Pos. 2“
Object A grasped
Open Gripper
Detect Object A
“Pos.1“ known
Robot at “Pos. 1“
Command Robot to “Pos. 1“
Command Robot to “Pos. 2“
Object A at “Pos. 2“
More detailed Program-Flow to perform the Pick & Place Task Specification of the sub process Object A detected Position ID
Command Robot to “Pos. 1“ Target Acceleration Speed …
Human enters Robot is moving
Move-Task completed
“Pos. 1“ known
1.0 m/s² 1.0 m/s
Collision
Robot is idling
Robot at “Pos. 1“
Fully specified Workflow with highlighted Data-Flow Fig. 2. Example of detailing the workflow. The detailed operation contains the required process-logic and data-flow.
After obtaining the assembly process and station, the generic skills of the workflow are replaced with the actual services of
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the determined production equipment. This results in detailing the workflow by adding additional states and required skills into the workflow. As a result, the designing process of the assembly system and process becomes iterative and must be carried out until appropriate production equipment have been determined for all required skills. An example is given in Fig. 2, in which “Object A” must be picked at its current position and placed at a desired position. In the given example, the “Pick-and-Place” operation should be performed by a robotic system. Therefore the position of the object must be detected. Then the robot must move to that position and grab the object by closing its gripper. After grabbing the object, the robot must move to the desired position and can open the gripper. To obtain all states and events of the assembly process, the description is expanded and specified until every transition of the process is able to fire without consuming time. From the given example, the highlighted transitions and states are considered and detailed. After a successful detection of the object’s position, an idling robot can be commanded to the determined position. Therefore the robot changes its state to “moving robot”. To fully specify the process, the newly added states have to be analyzed. In doing, critical events (highlighted in yellow) of the process regarding the application and the current state are determined and actions to prevent those unwanted states system must be derived. For instance, a human could enter the robot’s workspace while it is moving. In this example, the robot must stop, which could require fencing off the robot’s workspace or integrating additional sensors to detect the human being. In addition to the logic of the assembly process the information flow has to be modelled. Therefore the inputs and outputs of services and skills are connected among each other to specify the required input parameters of a service. In the given example, the skill to detect the object extracts the object’s id and position. This position is used as target position to command the following service. Next to dynamic values, constants can be used as inputs. For instance, speed and acceleration are limited to the specified values. To allow data aggregation of multiple data sources, partial connections are allowed. This allows for e.g. using only the translation of a pose as an input parameter. Thereby integrating technical upgrades into the assembly system results inter alia in adapting the dataflow. 5.3. Defining a virtual model Next to the assembly process and a data model, the physical assembly system’s electrical and mechanical design has to be modelled. Based on this model, certain connections of the production equipment like their kinematic coherences and interdependencies can be obtained. To match the physical assembly system, the corresponding parameters of the model can be adapted. This allows adapting the system in its mechanical design without adapting the workflow. In the given example mounting a previously static installed camera – for object detection – to the robot’s end-effector requires only adapting the virtual model without adapting the workflow.
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An additionally semantic description of sensor-values, datapoints and events complete the virtual model. 5.4. Performing the assembly process After the mechanical and electrical setup of the assembly system, the virtual model has to be transferred into the standardized control system. An assembly operation is performed by loading and starting the corresponding worklist. Thereby its stored assembly operations are executed predictable by the control system. Alternating product variants or production equipment of the system is performed by adapting the service or workflow and if necessary the virtual model of the assembly system. 6. Architecture of the Control System Virtual Model
Workflow
A.1
A
Object A detected Position ID
E.1
B.1
C.1
B.2
D
Command Robot to “Pos. 1“ Target Acceleration Speed …
Human enters Robot is moving
B
E.2
Collision Move-Task completed
„Pos. 1“ known
1.0 m/s² 1.0 m/s
Robot is idling
Object A at „Pos. 2“
Modular Workflow-Engine Dynamic Logic Interpreter
Volatile Memory System
Modular Broker System
➔ PROGRAM-FLOW
➔ DATA-FLOW
➔ EXECUTION
CONNECTOR
Internal Module
MQTT
OPC-UA
ROS
…
Communication Layers Service Providers
Fig. 3. Elements of the modular control system.
To utilize the virtual model and workflow of the assembly system and process, the modular control system consists of the following main elements (Fig. 3): • A Dynamic Logic Interpreter (DLI) for interpreting, analyzing and executing the given logic of a workflow. • A Volatile Memory System (VMS) for verifying the configured data-flow and thereby connecting the inputs and outputs of the services. • A Modular Broker System (MBS) for processing the requested service defined in the workflow. • Communication Layers to connect to the services • Modules providing at least one required service
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6.1. Dynamic logic interpreter –DLI To analyze and execute the given program-flow of the assembly process, the DLI generates the corresponding petri net. The transitions are designed as agents sensing their surrounding for changes. Thereby they are able to check their firing conditions autonomously. Based on the distributed design of the DLI, the transitions have to request firing at a supervised manager if they are able to fire to prevent runtime related synchronization errors. On firing a transition, the supervised manager asynchronously request the execution of linked services at the modular broker system. 6.2. Volatile Memory System – VMS The VMS is used to represent the information model of the assembly system and processes. Therefore all data of the utilized modules and results of performed services are merged into a shared data space. Its data is accessed and manipulated with text-based data-pointers. The shared data space of the VMS includes a topic-based publish-and-subscribe pattern for the contained data-points. By utilizing a publish-and-subscribe pattern, changing the content of data-points results in adapting the information model as well as informing its subscribers. 6.3. Modular Broker System – MBS The extendable MBS delegates and observes the execution of a service at its provider. Therefore the MBS determines the required service providers at its service registry and forwards the requests to the service providers utilizing the specified communication layers. During the execution of a service, its status, errors and results are forwarded to the VMS, which leads to updating the information model. To cancel the execution of a service, the MBS keeps track of currently active services. 6.4. Communication Layers Communication layers are used to enable extension of the control system by providing generic interfaces for different communication protocols. Therefore communication protocols are connected with the VMS. This allows bridging between different protocols. The integration of different communication protocols decouples the control system from specific communication models. 6.5. Modules The required skills and thereby services of a workflow are provided and performed by modules. These modules are used to implement a required behavior of the production equipment or logical functionalities in services. The modules extend the functionality of the control system. Modules are developed independently and can be distributed to other computational units. Therefore they are integrated over the communication layers.
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7. Conclusion and Outlook To decrease the implementation effort of flexible assembly systems and their processes, a new development method and the architecture of a generic modular control system are presented. The service based approach and concept of skills are utilized to obtain a more generic task description and separate the technical implementation of the assembly system from the assembly operations. The assembly process is expressed as an orchestration of skills, called “generic workflow”. The orchestration is defined as a petri net and consists of a programflow – describing the logic of the process – and an information flow – called data-flow. The state based modelling of the assembly process supports identifying critical and unwanted states. Preventing those states supports developing safe processes suitable for industrial applications. The generic task description is used to derive the required production equipment, which will implement the skills via services. These assets and their interdependencies are modelled in the virtual model. Based on the application, selecting production equipment results in iterative adaptation and detailing of the workflow and virtual model by adding additional skills, states, transitions, and, if necessary, production equipment. To perform the assembly process the virtual model and the workflow – thereby the specification of the assembly system and its process – are transferred into a standardized control system. This reduces the time-to-manufacture by providing a basically programming-free implementation process. Small case studies proved the flexibility of the control system and the development method. Currently the validation and optimization of the methodology is done with industrial application partners in a complex use-case with requirements for real-time. Therefore a web-based graphical editor will be used to model the workflow and virtual model, which is currently in development. In the future a more precise semantic model of the assembly system and process will be examined. References [1] G. Michalos, S. Markis, N. Papakotas et al., “Automotive assembly technologies review: challenges and outlook for a flexibleand adaptive approach,” CIRP: Journal of Manufactoring Sience and Technology, vol. 2010, no. 2, pp. 81–91. [2] F. Jammes and H. Smit, “SERVICE-ORIENTED PARADIGMS IN INDUSTRIAL AUTOMATION,” vol. 2015, pp. 716–723, 2015. [3] M. Loskyll, I. Heck, J. Schlick et al., “Context-Based Orchestration for Control of Resource-Efficient Manufacturing Processes,” Future Internet, vol. 4, no. 3, pp. 737–761, 2012. [4] J. Goll, Methoden und Architekturen der Softwaretechnik, Vieweg+Teubner Verlag, Wiesbaden, 2011. [5] B. Vogel-Heuser, J. Fischer, S. Feldmann et al., “Modularity and architecture of PLC-based software for automated production Systems: An analysis in industrial companies,” Journal of Systems and Software, vol. 131, pp. 35–62, 2017. [6] G. Olsson, “Programmable Logic Controllers,” in Handbook of Networked and Embedded Control Systems, D. Hristu-Varsakelis, R. Alur, K.-E. Årzén et al., Eds., pp. 259–278, Birkhäuser Boston, Boston, MA, 2005.
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[7] Z. Mammeri, ed., IEEE 16th Conference on Emerging Technologies & Factory Automation (ETFA), 2011: Toulouse, France, 5 - 9 Sept. 2011, IEEE, Piscataway, NJ, 2011. [8] W. Reisig, Understanding Petri Nets, Springer Berlin Heidelberg, Berlin, Heidelberg, 2013. [9] W.M.P. van der Aalst, “Geschäftsprozessmodellierung: Die ,,KillerApplikation“ für Petrinetze,” Informatik-Spektrum, vol. 37, no. 3, pp. 191–198, 2014. [10] Siemens, “Standards compliance according to IEC 61131-3 (3rd Edition),” vol. 2015. [11] International Electrotechnical Commission, “IEC International Standard IEC 61131–3,” 2013-02, 61131–3. [12] PLCopen Technical Committee 6, “XML Formats for IEC 61131-3,” http://www.plcopen.org/pages/tc6_xml/downloads/tc6_xml_v201_technic al_doc.pdf. [13] K. Thramboulidis, “IEC 61499 vs. 61131: A Comparison Based on Misperceptions,”. [14] L. H. Yoong, P. S. Roop, Z. E. Bhatti et al., Model-Driven Design Using IEC 61499: A Synchronous Approach for Embedded and Automation Systems, Springer International Publishing, Cham, s.l., 2015. [15] OPC Foundation, “PLCopen for efficiency in automation: Introduction in the PLCopen and OPC UA Communications Model,” OPC Foundation, http://www.plcopen.org/pages/tc4_communication/downloads/intro_plco pen_opc_v3.pdf. [16] “: standardized data exchange in the engineering process of production systems,” vol. 2018. [17] M. Quigley, K. Conley, B. P. Gerkey et al., “ROS: an open-source Robot Operating System,” ICRA Workshop on Open Source Software, 2009. [18] R. Hoogervorst, “VIKI—More Than a GUI for ROS,” in ROBOT OPERATING SYSTEM: The complete reference, pp. 633–655, Springer, [S.l.], 2017. [19] J. Jasperneite, S. Hinrichsen, and O. Niggemann, “,,Plug-and-Produce“ für Fertigungssysteme,” Informatik-Spektrum, vol. 38, no. 3, pp. 183– 190, 2015. [20] D. Pantförder, F. Mayer, C. Diedrich et al., “Agentenbasierte dynamische Rekonfiguration von vernetzten intelligenten Produktionsanlagen: Evolution statt Revolution,” in Handbuch Industrie 4.0, M. ten Hompel, B. Vogel-Heuser, and T. Bauernhansl, Eds., 2016. [21] F. Rovida, M. Crosby, D. Holz et al., “SkiROS—A Skill-Based Robot Control Platform on Top of ROS,” in ROBOT OPERATING SYSTEM: The complete reference, pp. 121–160, Springer, [S.l.], 2017. [22] M. J. Wooldridge, An introduction to multiagent systems, Wiley, Chichester, 2009. [23] A. Zoitl, Dr., “AutoPnP - Plug&Play für Automatisierungssysteme: Schlussbericht - Konsortialbericht,”. [24] P. Ferreira and N. Lohse, “Configuration Model for Evolvable Assembly Systems,” 2012. [25] C. Buckl, G. Kainz, N. Keddis et al., “Technische Umsetzung des Plug&Play-Konzepts auf Basis von CHROMOSOME im Projekt AutoPnP: Plug&Play-Umsetzung in Chromosome,” 2014. [26] J. Jatzkowski, P. Adelt, and A. Rettberg, “Hierarchical Scheduling for Plug-and-Produce,” Procedia Technology, vol. 26, pp. 227–234, 2016. [27] N. Antzoulatos, E. Castro, D. Scrimieri et al., “A multi-agent architecture for plug and produce on an industrial assembly platform,” Production Engineering, vol. 8, no. 6, pp. 773–781, 2014. [28] P. Danny, P. Ferreira, N. Lohse et al., “An AutomationML model for plug-and-produce assembly systems,” in 2017 IEEE 15th International Conference on Industrial Informatics (INDIN): University of Applied Science Emden/Leer, Emden, Germany, 24-26 July 2017 : proceedings, pp. 849–854, IEEE, Piscataway, NJ, 2017. [29] J. Pfrommer, D. Stogl, K. Aleksandrov et al., “Modelling and orchestration of service-based manufacturing systems via skills,” in IEEE [International Conference on] Emerging Technologies and Factory Automation (ETFA), 2014: 16 - 19 Sept. 2014, Barcelona, Spain, pp. 1–4, IEEE, Piscataway, NJ, 2014
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52nd 52nd CIRP CIRP Conference Conference on on Manufacturing Manufacturing Systems Systems
Generic for flexible systems 28th CIRP task Designdescription Conference, May Nantes,assembly France Generic automation automation task description for2018, flexible assembly systems Rainer Matthias Scholer, Karkowski* Rainer Müller, Matthias Scholer, Martin Martin Karkowski* A new methodology toMüller, analyze the functional and physical architecture of ZeMA gGmbH – Zentrum für Mechatronik und Automatisierungstechnik, Eschberger Weg 46, D-66121 Saarbrücken, Germany ZeMA gGmbH – Zentrum für Mechatronik und Automatisierungstechnik, Eschberger Weg 46, D-66121 Saarbrücken, Germany existing products for an assembly oriented product family identification
* Corresponding author. Tel.: +49 (0) 681 85787 529; fax: +49 (0) 681 85787 11. E-mail address: [email protected] * Corresponding author. Tel.: +49 (0) 681 85787 529; fax: +49 (0) 681 85787 11. E-mail address: [email protected]
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
Abstract Abstract
École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France
*To Corresponding author. Tel.: +33 3 87 37for 54mass 30; E-mail address: [email protected] face the increased requirements customization and frequent technological changes, currently used assembly systems require a high
To face the increased requirements for mass customization and frequent technological changes, currently used assembly systems require a high level of flexibility and re-configurability. These requirements lead to increased time-to-manufacture and manufacturing costs, which goes against level of flexibility and re-configurability. These requirements lead to increased time-to-manufacture and manufacturing costs, which goes against manufacturer’s goals. To mitigate the impact of these challenges a holistic approach for planning and programming an assembly system is manufacturer’s goals. To mitigate the impact of these challenges a holistic approach for planning and programming an assembly system is presented in this paper. The approach will minimize the implementation effort of flexible automation tasks suitable for industrial applications by presented in this paper. The approach will minimize the implementation effort of flexible automation tasks suitable for industrial applications by Abstract utilizing a combination of an abstract process description, a virtual model of the assembly system, and a standardized control system utilizing a combination of an abstract process description, a virtual model of the assembly system, and a standardized control system © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2019 Authors. Published by by Ltd. This is an open access under the CC BY-NC-ND In© business environment, theElsevier trend towards more product varietyarticle and customization is unbroken. license Due to this development, the need of ©today’s 2019 The The Authors. Published Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/) (http://creativecommons.org/licenses/by-nc-nd/3.0/) agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production This is an open access article under the scientific CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the committee of the 52nd CIRP Conference on Manufacturing Systems. Peer-review under responsibility the committee of CIRPmethods Conference on Manufacturing Manufacturing Systems. Peer-review under responsibility of the scientific scientific of the the 52nd 52nd CIRP Conference Systems. systems as well as to choose the of optimal productcommittee matches, product analysis are on needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and Keywords: assembly systems, control system, modularity, workflow, design methodology Keywords: assembly systems, system, modularity, designand methodology nature of components. This control fact impedes an efficientworkflow, comparison choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies areinstruction identified, and 1. Introduction process. The specification is later later used as as work work for Introduction process. The specification is used instruction for a1.functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which the aa standardized standardized control control system. system. As As aa consequence consequence the thedepicts assembly assembly similarity between providing designflexible supportand to both,process production system planners and product designers. illustrative To handle handle theproduct needs families for mass massbycustomization, customization, is expressed expressed as aa sequence sequence of skills skills and is isAndivided into To the needs for and Anprocess of example of a nail-clipper is used to explain the proposedflexible methodology. industrialiscase study onastwo product families of and steeringdivided columnsinto of adaptable assembly systems are required [1]. Customized a program-flow – the logic of the process – and a data-flow adaptable assembly systems are required [1]. Customized a program-flow – theapproach. logic of the process – and a data-flow –– thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed automation, human human robot robot cooperation cooperation (HRC) (HRC) and and modular modular the flow flow of of information information during during the the process. process. Based Based on on this this the ©automation, 2017 The Authors. Published by Elsevier B.V. assembly systems are utilized to face these challenges. This generic task description, the suitable production equipment Peer-review under responsibility of the scientific committee of theThis 28th CIRP Designtask Conference 2018. the suitable production equipment assembly systems are utilized to face these challenges. generic description,
results in in an an increased increased complexity complexity of of the the already already demanding demanding results while while widely used, Programmable Logic Controllers (PLCs) only widely used, Programmable Logic Controllers (PLCs) only offer a low level of abstraction, which complicates the offer a low level of abstraction, which complicates the development of reusable, maintainable and well-structured of reusable, maintainable and well-structured 1.development Introduction software components even even more. more. [3] [3] software components Because of changing market situations, the assembly assembly system system Because changing market situations, Due to ofthe fastsoftware development in thethe domain of – and thereby its – remains subject of many – and thereby and its an software – remains subject of many communication ongoing trend of digitization and technological changes. changes. The The thereby thereby extended extended implementation implementation technological digitalization, manufacturing enterprises are commissioning facing important process goes against the demand for faster of process goes themarket demandenvironments: for faster commissioning of challenges in against today’s a continuing assembly systems, reduced costs and a shortened time-toassemblytowards systems, reducedofcosts and a shortenedtimes time-totendency manufacture [1], [1], reduction and leads leads to toproduct the need needdevelopment for holistic holistic design designand of manufacture and the for of shortened product lifecycles. In addition, there is an increasing assembly systems. assembly systems. demand customization, at the same time a global This ofpaper paper presents aabeing holistic approach to in reduce the This presents holistic approach to reduce the competition with competitors all over the world. This trend, implementation effort of flexible assembly systems suitable for implementation effort flexible assembly systems for which is inducing theof development from macrosuitable to micro industrial applications. Therefore the specification of an industrialresults applications. Therefore the specification of an markets, inand diminished lot sizes duefrom to augmenting assembly system processes is obtained an iterative assembly system(high-volume and processes obtained from an iterative product varieties to is low-volume production) [1]. implementation ofDesign an automation automation taskidentification [1,2]. In In addition, addition, Keywords: Assembly; method; Family implementation of an task [1,2].
implementing those those skills skills via via services services can can be be determined. determined. This This implementing generic task description is then detailed and adapted. To generic task description is then detailed and adapted. To execute the assembly task a virtual model of the assembly execute the assembly task a virtual model of the assembly system –– describing describing its its independencies independencies and and properties properties –– is is system designed. Afterwards the virtual model and the task description designed. Afterwards the virtual model and the task description of product range characteristics and/or arethe exported into the and standardized controlmanufactured system, to to perform perform are exported into the standardized control system, assembled in this system. In this context, the main challenge in the assembly assembly process. process. the modelling and analysis is now not only to cope with single Section 22 presents presents the the theoretical theoretical basics basics of of this this research research as as Section products, a limited to product range orThe existing product families,3 an introduction the topic. following section an also introduction to analyze the topic. The following section 3 but to be currently able to and toapproaches compare products to define summarizes published to simplify simplify the summarizes currently published approaches to the new product families. It of cananbeassembly observedprocess. that classical existing implementation process Based on on those implementation process of an assembly process. Based those product families are regrouped in function of clients or features. approaches, the research needs are identified (section 4) and the approaches, the research needs are identified (section 4) to and the However, assembly oriented product families are hardly find. developed approach is presented in detail (section 5). The developed approach is presented in detail (section The On the modular product family level, differ mainly5). in two designed control systemproducts and its its main main components are designed modular control system and components are main characteristics: (i) the number of components and (ii) the briefly introduced in section 6. briefly introduced in(e.g. section 6. type of components mechanical, electrical, electronical).
Classical methodologies considering mainly single products or solitary, already existing product families analyze the To cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which 2212-8271 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2212-8271 possible © 2019 The optimization Authors. Publishedpotentials by Elsevier Ltd. This is existing an open access causes article under the CC BY-NC-ND license an efficient definition and identify in the difficulties regarding (http://creativecommons.org/licenses/by-nc-nd/3.0/) (http://creativecommons.org/licenses/by-nc-nd/3.0/) production system, it is important to have a precise knowledge comparison of different product families. Addressing this Peer-review under responsibility of the scientific committee of the 52nd CIRP Conference on Manufacturing Systems. Peer-review under responsibility of the scientific committee of the 52nd CIRP Conference on Manufacturing Systems.
2212-8271 © 2019 The Authors. Published by Elsevier Ltd. This is an©open article Published under theby CC BY-NC-ND 2212-8271 2017access The Authors. Elsevier B.V. license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of scientific the scientific committee theCIRP 52ndDesign CIRPConference Conference2018. on Manufacturing Systems. Peer-review under responsibility of the committee of the of 28th 10.1016/j.procir.2019.03.185
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2. Theoretical Basis 2.1. Software Architectures Architectural patterns are reusable solutions for commonly occurring problems which divide systems into smaller sub systems. Architectural patterns may also contain design patterns, which define the relations between different elements of the system to achieve the desired goals. [4] Currently used development models are often service- or agent-based. A Service Orientated Architecture (SOA) describes how systems provide and consume loosely coupled services (methods and applications) – the orchestration of the system – to achieve a given goal instead [3,4]. 2.2. Programmable Logic Controller (PLC) In most production systems, process state changes are triggered by events, for instance changed pressure values. By now PLCs are utilized as economical solutions for controlling those production systems. Next to the economic benefits, machine manufactures are facing constraints related to the characteristics of PLCs. PLCs are characterized by their cyclic data processing and real-time execution, which results in a state based design for sequencing control. [5,6] Due to the low level of abstraction, the fail supporting high-level programming. [7] 2.3. Petri nets Petri nets represent discrete event dynamic systems and consist of four basic elements – places, transitions, arcs and tokens. Whereas places model states of a system or subsystem, transitions model the activities and events of a system. To indicate that a modelled state is met, places can contain tokens. These tokens are consumed or produced by the transitions of a net. Arcs connect places with transitions or vice versa and define the flow of the tokens. [8] The explicit state-based modelling of a system enables simplified analysis of the system. In addition, petri nets have a formal mathematical description and support simple methods for model-checking. For modelling a petri net, their graphical representation is utilized. [9] 3. Approaches to simplify the Implementation Process of Automation Systems Approaches to simplify the implementation of automation system are often based on standardization, the utilization of (customizes) middleware, and (multi-) agent-systems for process planning and execution. These tools are summarized in the following sections. 3.1. Standardization of Programming Languages, Communication Protocols and Exchange Formats Despite the standardization of textual and graphical programming languages of PLCs (IEC 61131-3), it remains a manufacturer’s task to apply this standard. (compare [10,11]). The PLCopen XML standard was developed on top of
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IEC 61131-3 standard, to be a distribution format for function block libraries, which could be used on different platforms [12]. An event based programming paradigm for PLCs is provided by the IEC 61499. It utilizes concepts and methodologies of modern software, but it is widely claimed that it doesn’t satisfy the requirements of automation systems. [13,14] Next to a standardization of the utilized Programming Languages, standards for communication layers and protocol – like the Open Platform Communications - Unified Architecture (OPC-UA) – have been developed. OPC-UA combines a language and platform independent communication model with a generic information model. This enables a platform independent implementation of an automatic task communication with other control systems or software like ERP- or MES-Systems. [15] Thereby OPC-UA simplifies and unifies the distribution and communication of controllers. AutomationML™ – a XML-based neutral and object orientated data exchange format to save interdisciplinary planning data – is capable of storing geometric, kinematic, logical and structural information of a technical facility. It utilizes already existing data formats like Collada (geometry and kinematic information) and CAEX (structural information). [16] 3.2. Utilizing Middleware Middleware is defined as a generic software layer for distributed software, which simplifies its distribution and unifies communication between the nodes. [4] Due to its generic approach, middleware enables high-level programming. In addition, it often offers various packages for common domain specific tasks like path planning, navigation and machine vision. [17] Nevertheless, the development of complex systems often becomes challenging due to the overhead caused by required prior knowledge of the setup, configuration and integration [18]. 3.3. Utilizing (Multi-)Agent-Systems Distributed assembly system controllers relying on middleware and standard communication models only shift the effort of implementing the required system behavior. [19] To support the implementation process, projects like MyJoguhrt [20] or SkiROS [21] utilize self-organizing agent systems on top of middleware. Unfortunately Wooldridge [22] describes the behavior of distributed agent systems as hardly predictable, which contradicts the requirements for maintainability and safety. The high safety requirements and standards of semiautomated systems prevents using those approaches in an industrial applications without further adaptation. 3.4. Published Research Projects On top of the presented tools, concepts such as “Plug and Produce” [23,24] – a vision, enabling structural changes in automatic systems with minimal manual adaption and reconfiguration of the software – have been developed. Based on the conceptual implementation of this vision different aspects such as automatic integration of real-time
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communication systems [25,26], “high-level” programming [23,27] or automatic programming [3,25,28] are considered in different research projects. To create more versatile systems with automatic configuration, the concepts of skills and selfdescribing assets are introduced (e.g. [24], [28] or [29]). This enables automatic orchestration of processes based on the current configuration of the system and a corresponding task description. The mentioned approaches following the Product Process Resource (PPR)-based modelling, utilize a stateless description of the assembly process, which complicates the consideration of relevant aspects for an industrial application, such as processes having encountered errors or disturbances caused by human workers. In addition, these concepts are hardly transferable to the standards regarding hardware and software used by the industry. 4. Research Needs One significant challenge is to provide a simple, predictable, flexible and maintainable task description, which satisfies standards used by the industry while reducing the implementation effort of the assembly process and system. In particular, mid-life cycle adaptations of the system and interaction with a human worker must be considered. The main research questions are: • How can a holistic planning method for an assembly system and its corresponding assembly process be designed to support a developer during the specification, configuration, (re-) implementation and adaptation stages? • How can an assembly process and system be modelled in such a way that it helps to identify critical states and events which can occur as well as to model the disturbances caused by human workers? • How can the assembly process be expressed to become a deterministic work instruction for a standardized control system? 5. Methodology In order to address these research questions, a holistic and iterative methodology is introduced (Fig. 1). The service based approach seems to be appropriate to describe a distributed, flexible and loosely coupled assembly system, whereas utilizing the concept of skills provides a more generic highlevel description of the process and system. The solution oriented approach separates the technical implementation and the design of the assembly process. Utilizing petri nets as a formal description of an assembly process – and thereby the orchestration of the services – allows for predictable behavior during runtime. To interpret the generated process specification and thereby speed up the virtual commission and simplify the implementation, a standardized control concept is utilized.
Fig. 1. Graphical representation of the Methodology.
5.1. Determining the assembly process and –system Beginning with a detailed analysis of the product being considered, the product-related requirements for the assembly process are derived. Based on those requirements, the assembly process is planned. This results in a process sequence, structure as well as a list of assembly operations for each assembly station. In a next step this information is analyzed to determine the necessary operations of the assembly stations. In doing so, the assembly operations are constructed from smaller and generic skills. The skills are combined into program-flows and data-flows. Whereas the program-flow describes the required logic to perform the assembly process, the data-flow is used to model the flow of information during the process. Combined, the data-flow and program-flow will be called workflow. The program-flow is expressed as a petri net, wherein each place represents a state while performing the described process. These transitions between states can occur by performing a skill (for instance open a valve during a filling process) or a status change (for instance a changed pressure value during a filling process). These changes are modelled with transitions and their conditions. The defined conditions have to be fulfilled to change the state of the process. Utilizing petri nets as formal and simple descriptions of the logic, simplifies parallelization and synchronization of assembly operations and enables later execution, analysis and simulation of the process. Due to the state based modelling, petri nets support situational definition of the assembly system’s behavior. Furthermore, petri nets provide an intuitive and simple visual model of the process. Next to the program-flow, the data-flow – the flow of information between the used skills of the workflow – has to be modelled. This enables connecting information from different software functionalities or the utilized production equipment to other elements. An additional semantic model for describing the assembly process allows a more abstract, reusable and generic
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description. The semantic model is used to determine the required production equipment based on the required skills. For this purpose, a predefined modular system is utilized, whose elements implement skills via services. An assembly station is obtained by a skill-based matching process. During this process, a set of appropriated elements, capable to performing the required skills of the workflow, is generated, whereas skills, on the other hand, can be established by combining multiple skills or services. The elements must respect the requirements of the product and process and must be compatible with one another. 5.2. Detailing the workflow Process Planning Pick & Place Object A from “Pos. 1“ to “Pos. 2“
Object A at “Pos. 1“
Object A at “Pos. 2“
Abstract Process
Assigning the Pick and Place Task to a Robot Object A at „Pos. 1“
Close Gripper
Robot at „Pos. 2“
Object A grasped
Open Gripper
Detect Object A
“Pos.1“ known
Robot at “Pos. 1“
Command Robot to “Pos. 1“
Command Robot to “Pos. 2“
Object A at “Pos. 2“
More detailed Program-Flow to perform the Pick & Place Task Specification of the sub process Object A detected Position ID
Command Robot to “Pos. 1“ Target Acceleration Speed …
Human enters Robot is moving
Move-Task completed
“Pos. 1“ known
1.0 m/s² 1.0 m/s
Collision
Robot is idling
Robot at “Pos. 1“
Fully specified Workflow with highlighted Data-Flow Fig. 2. Example of detailing the workflow. The detailed operation contains the required process-logic and data-flow.
After obtaining the assembly process and station, the generic skills of the workflow are replaced with the actual services of
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the determined production equipment. This results in detailing the workflow by adding additional states and required skills into the workflow. As a result, the designing process of the assembly system and process becomes iterative and must be carried out until appropriate production equipment have been determined for all required skills. An example is given in Fig. 2, in which “Object A” must be picked at its current position and placed at a desired position. In the given example, the “Pick-and-Place” operation should be performed by a robotic system. Therefore the position of the object must be detected. Then the robot must move to that position and grab the object by closing its gripper. After grabbing the object, the robot must move to the desired position and can open the gripper. To obtain all states and events of the assembly process, the description is expanded and specified until every transition of the process is able to fire without consuming time. From the given example, the highlighted transitions and states are considered and detailed. After a successful detection of the object’s position, an idling robot can be commanded to the determined position. Therefore the robot changes its state to “moving robot”. To fully specify the process, the newly added states have to be analyzed. In doing, critical events (highlighted in yellow) of the process regarding the application and the current state are determined and actions to prevent those unwanted states system must be derived. For instance, a human could enter the robot’s workspace while it is moving. In this example, the robot must stop, which could require fencing off the robot’s workspace or integrating additional sensors to detect the human being. In addition to the logic of the assembly process the information flow has to be modelled. Therefore the inputs and outputs of services and skills are connected among each other to specify the required input parameters of a service. In the given example, the skill to detect the object extracts the object’s id and position. This position is used as target position to command the following service. Next to dynamic values, constants can be used as inputs. For instance, speed and acceleration are limited to the specified values. To allow data aggregation of multiple data sources, partial connections are allowed. This allows for e.g. using only the translation of a pose as an input parameter. Thereby integrating technical upgrades into the assembly system results inter alia in adapting the dataflow. 5.3. Defining a virtual model Next to the assembly process and a data model, the physical assembly system’s electrical and mechanical design has to be modelled. Based on this model, certain connections of the production equipment like their kinematic coherences and interdependencies can be obtained. To match the physical assembly system, the corresponding parameters of the model can be adapted. This allows adapting the system in its mechanical design without adapting the workflow. In the given example mounting a previously static installed camera – for object detection – to the robot’s end-effector requires only adapting the virtual model without adapting the workflow.
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An additionally semantic description of sensor-values, datapoints and events complete the virtual model. 5.4. Performing the assembly process After the mechanical and electrical setup of the assembly system, the virtual model has to be transferred into the standardized control system. An assembly operation is performed by loading and starting the corresponding worklist. Thereby its stored assembly operations are executed predictable by the control system. Alternating product variants or production equipment of the system is performed by adapting the service or workflow and if necessary the virtual model of the assembly system. 6. Architecture of the Control System Virtual Model
Workflow
A.1
A
Object A detected Position ID
E.1
B.1
C.1
B.2
D
Command Robot to “Pos. 1“ Target Acceleration Speed …
Human enters Robot is moving
B
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Collision Move-Task completed
„Pos. 1“ known
1.0 m/s² 1.0 m/s
Robot is idling
Object A at „Pos. 2“
Modular Workflow-Engine Dynamic Logic Interpreter
Volatile Memory System
Modular Broker System
➔ PROGRAM-FLOW
➔ DATA-FLOW
➔ EXECUTION
CONNECTOR
Internal Module
MQTT
OPC-UA
ROS
…
Communication Layers Service Providers
Fig. 3. Elements of the modular control system.
To utilize the virtual model and workflow of the assembly system and process, the modular control system consists of the following main elements (Fig. 3): • A Dynamic Logic Interpreter (DLI) for interpreting, analyzing and executing the given logic of a workflow. • A Volatile Memory System (VMS) for verifying the configured data-flow and thereby connecting the inputs and outputs of the services. • A Modular Broker System (MBS) for processing the requested service defined in the workflow. • Communication Layers to connect to the services • Modules providing at least one required service
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6.1. Dynamic logic interpreter –DLI To analyze and execute the given program-flow of the assembly process, the DLI generates the corresponding petri net. The transitions are designed as agents sensing their surrounding for changes. Thereby they are able to check their firing conditions autonomously. Based on the distributed design of the DLI, the transitions have to request firing at a supervised manager if they are able to fire to prevent runtime related synchronization errors. On firing a transition, the supervised manager asynchronously request the execution of linked services at the modular broker system. 6.2. Volatile Memory System – VMS The VMS is used to represent the information model of the assembly system and processes. Therefore all data of the utilized modules and results of performed services are merged into a shared data space. Its data is accessed and manipulated with text-based data-pointers. The shared data space of the VMS includes a topic-based publish-and-subscribe pattern for the contained data-points. By utilizing a publish-and-subscribe pattern, changing the content of data-points results in adapting the information model as well as informing its subscribers. 6.3. Modular Broker System – MBS The extendable MBS delegates and observes the execution of a service at its provider. Therefore the MBS determines the required service providers at its service registry and forwards the requests to the service providers utilizing the specified communication layers. During the execution of a service, its status, errors and results are forwarded to the VMS, which leads to updating the information model. To cancel the execution of a service, the MBS keeps track of currently active services. 6.4. Communication Layers Communication layers are used to enable extension of the control system by providing generic interfaces for different communication protocols. Therefore communication protocols are connected with the VMS. This allows bridging between different protocols. The integration of different communication protocols decouples the control system from specific communication models. 6.5. Modules The required skills and thereby services of a workflow are provided and performed by modules. These modules are used to implement a required behavior of the production equipment or logical functionalities in services. The modules extend the functionality of the control system. Modules are developed independently and can be distributed to other computational units. Therefore they are integrated over the communication layers.
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7. Conclusion and Outlook To decrease the implementation effort of flexible assembly systems and their processes, a new development method and the architecture of a generic modular control system are presented. The service based approach and concept of skills are utilized to obtain a more generic task description and separate the technical implementation of the assembly system from the assembly operations. The assembly process is expressed as an orchestration of skills, called “generic workflow”. The orchestration is defined as a petri net and consists of a programflow – describing the logic of the process – and an information flow – called data-flow. The state based modelling of the assembly process supports identifying critical and unwanted states. Preventing those states supports developing safe processes suitable for industrial applications. The generic task description is used to derive the required production equipment, which will implement the skills via services. These assets and their interdependencies are modelled in the virtual model. Based on the application, selecting production equipment results in iterative adaptation and detailing of the workflow and virtual model by adding additional skills, states, transitions, and, if necessary, production equipment. To perform the assembly process the virtual model and the workflow – thereby the specification of the assembly system and its process – are transferred into a standardized control system. This reduces the time-to-manufacture by providing a basically programming-free implementation process. Small case studies proved the flexibility of the control system and the development method. Currently the validation and optimization of the methodology is done with industrial application partners in a complex use-case with requirements for real-time. Therefore a web-based graphical editor will be used to model the workflow and virtual model, which is currently in development. In the future a more precise semantic model of the assembly system and process will be examined. References [1] G. Michalos, S. Markis, N. Papakotas et al., “Automotive assembly technologies review: challenges and outlook for a flexibleand adaptive approach,” CIRP: Journal of Manufactoring Sience and Technology, vol. 2010, no. 2, pp. 81–91. [2] F. Jammes and H. Smit, “SERVICE-ORIENTED PARADIGMS IN INDUSTRIAL AUTOMATION,” vol. 2015, pp. 716–723, 2015. [3] M. Loskyll, I. Heck, J. Schlick et al., “Context-Based Orchestration for Control of Resource-Efficient Manufacturing Processes,” Future Internet, vol. 4, no. 3, pp. 737–761, 2012. [4] J. Goll, Methoden und Architekturen der Softwaretechnik, Vieweg+Teubner Verlag, Wiesbaden, 2011. [5] B. Vogel-Heuser, J. Fischer, S. Feldmann et al., “Modularity and architecture of PLC-based software for automated production Systems: An analysis in industrial companies,” Journal of Systems and Software, vol. 131, pp. 35–62, 2017. [6] G. Olsson, “Programmable Logic Controllers,” in Handbook of Networked and Embedded Control Systems, D. Hristu-Varsakelis, R. Alur, K.-E. Årzén et al., Eds., pp. 259–278, Birkhäuser Boston, Boston, MA, 2005.
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