AN OBJECT-ORIENTED APPROACH TO DEVELOPING A VIRTUAL MANUFACTURING SYSTEM MODEL

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1 205 Journal of Technology, Vol. 32, No. 3, pp (2017) AN OBJECT-ORIENTED APPROACH TO DEVELOPING A VIRTUAL MANUFACTURING SYSTEM MODEL Ming-Shan Lu Zong-Sian He Department of Industrial Engineering and Management National Yunlin University of Science & Technology Yunlin, Taiwan 640, R.O.C. Key Words: virtual manufacturing system, object-oriented, petri net, virtual device. ABSTRACT Recently, virtual manufacturing has emerged as an important concept in the development of manufacturing systems. To implement a virtual manufacturing system, it is necessary to construct digital models for all physical and logical elements of the real manufacturing system. In this research, an efficient and systematic methodology for creating an object-oriented virtual manufacturing system model is proposed. The proposed approach for developing a virtual manufacturing system includes four stages: (1) functional analysis, (2) behavior analysis, (3) virtual system model construction and (4) virtual system implementation. IDEF0 and object-oriented petri net (OPN) are integrated into the corresponding stages based on their characteristics. In addition, the concept of a virtual device is proposed as a reference model for constructing the virtual manufacturing system model. In order to develop the system rapidly and efficiently, transformation rules between the IDEF0 and OPN and between the OPN and the virtual device are constructed. Finally, an assembly and packaging system is produced to illustrate the proposed approach for developing a virtual manufacturing system model. I. INTRODUCTION In modern industry, a manufacturing system is required to produce products efficiently and to respond rapidly to changing market needs. In order to meet these requirements, virtual manufacturing (VM) has emerged as an important concept in the development of manufacturing systems. According to Banerjee and Zetu [1], VM can be defined as the modeling of manufacturing systems and components via effective use of computers, audiovisual and sensory displays to simulate or design alternatives for a manufacturing environment. The main objective is to support the design of new systems and to predict potential problems and inefficiencies in production functionality and manufacturability before real manufacturing is realized [2]. The VM environment, potential application areas, and resulting benefits were presented by Souza et al. [3]. Ghani et al. [4] presented the development of a package known as virtual-driven discrete event simulation (VDSim), used to establish an integration between VE (virtual engineering) and DES (discrete event simulation) domains. VDSim integration helps productivity planners and schedulers realize the best possible options for resource selection at different stages. Peng et al. [5] proposed a net-vm to share usage of the latest integrated design and manufacturing facilities. Distributive, collaborative, and interactive operations are the major features of the net-vm. A novel approach to constructing complex virtual manufacturing environments based on domain analysis and top-down construction was proposed by Xu et al. [6]. The software system developed provides easy-to-use * Corresponding author: Ming-Shan Lu, mslu@yuntech.edu.tw

2 206 Journal of Technology, Vol. 32, No. 3 and flexible tools that enable users to construct virtual environments rapidly and with minimum modeling effort. Lee and Park [7] discussed issues in application of virtual commissioning technology for automated manufacturing systems. Two issues were identified: the physical and logical model construction of a virtual device. It still requires significant time and effort to construct virtual models for automated manufacturing systems. Thus, to implement a virtual manufacturing system, it is necessary to construct digital models for all physical and logical elements of the real manufacturing system. When facing increasingly complicated manufacturing systems, an efficient and systematic methodology for developing reusable, configurable and scalable models of virtual manufacturing systems becomes an important issue. The object-oriented modeling (OOM) paradigm is a way of thinking based on modeling objects from the real world. It can provide a natural one-to-one correspondence between components in a manufacturing system and objects that represent them. Models developed based on object-oriented technology achieve reusability, maintainability and modularity. A tutorial and review of object-oriented design of manufacturing systems is given by Usher [8]. Applications of object-oriented technology include product modeling and design, process planning, manufacturing system modeling and simulation, planning and control, and manufacturing databases. Mize et al. [9] applied the object-oriented approach to model integrated manufacturing systems. Three categories of objects are classified in the manufacturing system: physical, information and control/decision objects. Park et al. [10] proposed an object-oriented framework, called a JR-net modeling framework, for an automated manufacturing system design. An automated manufacturing system is constructed based on a three-phase modeling approach: static layout modeling (object model), job flow modeling (functional JR-net model), and supervisory control modeling (dynamic model). Lai [11] applied activity flow diagrams (AFD) and object-oriented methods to analyze an automated manufacturing system. AFDs and object models are used to specify functional and behavioral specifications in the system abstraction stage. Ou-Yang et al. [12] addressed the analytical process for constructing a shop floor controller model by using the object model technique (OMT). There are three modules in the model: planning and scheduling, dispatch and coordination, and data monitoring and analysis. An objected-oriented modeling of the control system for agile manufacturing cells is discussed by Zhang et al. [13]. Manufacturing entity objects (MEOs) are defined as reusable building blocks of modeling and their structures are also addressed in the paper. An object-oriented methodology to creating a virtual flexible manufacturing system (FMS) model was presented by Park [14]. The proposed model consisted of four types of objects: the virtual device, the transfer handler, the state manager, and the flow controller. DEVSIM was employed for the implementation of the virtual flexible manufacturing system model. Petri net (PN) is a powerful tool for dynamic behavior analysis and verification of manufacturing systems and is adopted here due to the characteristics of graphical representation and mathematical analysis of control logic [15]. Traditional PN modeling is highly system dependent and lacks properties such as simplicity, modularity, reusability and maintainability that are commonly required in the modeling and control of a large and complex manufacturing system. Thus, a number of research reports have focused on developing extended petri nets, such as colored petri nets (CPNs), hierarchical petri nets (HPNs) and object-oriented petri nets (OPNs), to overcome the afore mentioned limitations of traditional PNs. A CPN has been proposed by adding color attributes into the traditional PN to represent manufacturing flows in a simplified and generalized way [16]. A HPN organizes a PN into hierarchies of layers to restrain complexity and enhance modularity of the model [17]. The concept of object-oriented paradigms such as encapsulation, modularity, inheritance, etc., has been widely used in system modeling because they allow us to describe systems easily, intuitively and naturally. Thus, in order to gain the benefits of object-oriented features, a number of research works in the field of unifying object-oriented concepts and PN theory are proposed. Lee and Park [18] developed an object-oriented high-level petri net (OPNet) model based on the concept of object and message passing in objectoriented programming (OOP) to increase maintainability and reusability in PN modeling. An OPN approach and analysis method applied to automated manufacturing systems has been presented by Wang [19]. The characteristics of a PN and OOP are included in the OPN paradigm and the decision knowledge is also incorporated in the control logic. In addition, a number of transformation rules have been developed to convert the specifications of the OPN model into rule-based control software coding. Liu et al. [20] have proposed an extended objected-oriented petri net (EOPN) to manage the complexity involved in the modeling of semiconductor wafer fabrication systems (SWFSs). The EOPN models are constructed in a hierarchical way to make the system modes more concise and authentic. An object-oriented approach for design and analysis of an agile

3 Lu M. S., & Z. S. He: An Object-Oriented Approach to Developing a Virtual Manufacturing System Model 207 manufacturing control system has been proposed by Lu and Tseng [21]. The proposed development process is divided into four stages: functional analysis, static structural analysis, behavior analysis and verification, and system implementation. OPN and unified modeling language (UML) were integrated into the process development stages. Leitão et al. [22] presented the integration of 2D/3D digital software tools with PN based service-oriented frameworks for design, configuration, analysis, validation, simulation, monitoring and control of manufacturing systems in a virtual environment. The IDEF is another standard graphic language supporting manufacturing/business process modeling. The IDEF family languages (IDEF0 to IDEF14) originate from integrated computer aided manufacturing (ICAM), which aimed to use computer technology to improve manufacturing productivity [23, 24]. IDEF0 is an activity-based modeling technique usually used for describing the functional aspects of a system. IDEF3 supports the description of process flows of a system and object state transitions in a specific process. Both of them can be specified with a top-down hierarchically decomposing process to present complex manufacturing functions and processes. Although IDEF0 and IDEF3 modeling requires considerable time and effort to construct a functional process model of a system, the model usually cannot be used directly in further system analysis such as dynamic behavior analysis and quantitative performance analysis [25]. Thus, some research has integrated PNs into the IDEF0 and IDEF3 to overcome these problems. Lee and Hsu [26] proposed a systematic approach to integrate IDEF0 and PNs for emulator design. IDEF0 and PNs are applied to perform the functional and behavior analysis of the equipment. Lee et al. [25] built a PN model from IDEF0 and IDEF3 models and steady state analysis of the PN model is employed to measure system performance. Kim et al. [27] have proposed the integrated use of IDEF0, IDEF3 and PNs to support the specification, capture and ongoing development of business process models that are created and used by system developers from their particular points of view. In addition, transformation rules that allow mapping between these three modeling approaches are also developed to reduce the effort involved in building models and to improve consistency of models. In this research, an efficient and systematic methodology for developing an object-oriented virtual manufacturing system model is proposed. The proposed development process is divided into four stages: (1) functional analysis, (2) behavior analysis, (3) system virtual model construction and (4) virtual system implementation. IDEF0 and OPN are integrated into the corresponding stages based on their characteristics. The OPN model is developed for behavior analysis according to the functional requirements specified in the IDEF0. This procedure eliminates the need to construct the OPN directly from manufacturing processes. In order to develop the system rapidly and efficiently, transformation rules between the IDEF0 and OPN and between the OPN and virtual model are constructed. At the virtual model construction stage, the concept of a virtual device is proposed as a reference model for construction of the virtual manufacturing system model. For the implementation of the proposed virtual manufacturing system model, Solid- Works CAD software, and Visual Components, a 3D objectoriented simulation software for manufacturing systems, are employed. Finally, an assembly and packaging system is created to illustrate how the integrated object-oriented approach is implemented in developing the virtual manufacturing system model. II. PROPOSED APPROACH FOR DEVELOPING A VIRTUAL MANUFACTURING SYSTEM The proposed object-oriented approach for developing a virtual manufacturing system includes four stages: (1) functional analysis, (2) behavior analysis, (3) system virtual model construction and (4) virtual system implementation. System functional analysis requires engineers to clearly define the expectations for the functional requirements provided by the system s objects. The functional requirements can be constructed from the scenario description of system activities. The IDEF0 is introduced to define the functional requirements of the system in this stage. In general, the dynamic behavior of the system includes interaction behavior between the objects and reaction behavior inside the objects. In the behavior analysis stage, the OPN is used to model these behaviors, and PN properties such as safeness, liveness, conservativeness, and reversibility are analyzed. After the desired and correct OPN model is achieved, the next stage is virtual model construction for the system. The virtual model consists of the static layout of manufacturing devices and their behaviors. In order to represent such as a manufacturing device, the concept of a virtual device is employed. A virtual device is a digital model imitating the physical and behavior of a real device. The behavior of the virtual device is developed based on the OPN model obtained from the behavior analysis stage. Once the virtual model is constructed, it can be implemented into the virtual

4 208 Journal of Technology, Vol. 32, No. 3 (signals or parameters) Control 1 Control 2 (signals or parameters) Input Function Output Manufacturing System Development Step 1: IDEF0 for Functional Analysis Step 2: Object-oriented Petri Nets for Behavior Analysis (materials or information) (materials or information) Manufacturing System Implementation Fig. 1 Step 4: Virtual Manufacturing System for Simulation and Verification Step 3: Virtual Device for Virtual Model Construction Proposed object-oriented approach for developing virtual manufacturing systems manufacturing system by object-oriented programming languages or commercial simulation software. Then, based on the virtual manufacturing system, the physical behavior of the system can be visualized and verified, and performance can be analyzed. The proposed object-oriented approach for developing virtual manufacturing systems is shown in Fig. 1. The detailed approaches are described in the following sections. 1. Functional Analysis At this stage, the objective of functional analysis is to define functional requirements of the control system. The functional requirements are specified with a top-down hierarchically decomposing process by using the IDEF0 technique. IDEF0 is a function modeling method for analyzing the functional perspectives of a system. The reason for using IDEF0 for functional modeling is that manufacturing systems usually contain functions/operations supported by devices/ machines, the controls or conditions needed for the functions/ operations, and the information/material that flows between the functions/operations. These elements and the relationships between them can easily and precisely be specified in the IDEF0. In the graphics of an IDEF0 diagram, the function is shown as a box and the interfaces to or from the function as arrows entering or leaving the box. These arrows are classified as inputs, outputs, controls and mechanisms. The box can be further decomposed into another IDEF0 diagram to describe more detailed activities in the box. For the implementation of the IDEF0 in developing an object-oriented control system, the functions of the system are analyzed and decomposed into the function of each equipment resource based on the IDEF0 decomposition technique. Each equip- Input (materials or information) (equipment resource 1) Mechanism 1 Mechanism 2 (equipment resource 1) Control 1 (signals or parameters) (equipment resource 2) Control 2 Function 1 Function 2 Mechanism 1 Mechanism 2 (signals or parameters) Output (materials or information) (equipment resource 2) Fig. 2 Basic scheme of the IDEF0 model ment resource, such as machines or devices, is defined as an object in the system. The mechanisms in the IDEF0 can be used to represent the equipment resources performing the functions. The inputs/outputs are materials or information transformed by the functions. The controls are the control messages such as signals and parameters sent from other equipment resources for controlling functions. The basic scheme of the IDEF0 model is shown in Fig Behavior Analysis Because the IDEF0 only represents an object s functions and their interrelationships, an OPN model developed from the IDEF0 is needed to analyze the dynamic behavior of the system. A traditional PN consists of four elements: place (circle), transition (bar), directed arc (arrow) and token (dot). The dynamic behavior of the PN is given by the flow of tokens through the net. This flow is realized by the firing of transitions, i.e., the change of its marking. This graphic interface makes the PN easy to represent and understand. However, a considerable number of states and transitions are needed when the system becomes complex. In addition, properties such as modularity, reusability and maintainability are required in an agile manufacturing system. Thus, an OPN is introduced to model the dynamic behavior of the manufacturing system and is extended for design of virtual system models. In an OPN approach, a physical object is defined as a PN object. The dynamic behavior of the PN object is characterized by external messages communicating with other PN objects and its internal behavior transitions reacting to

5 Lu M. S., & Z. S. He: An Object-Oriented Approach to Developing a Virtual Manufacturing System Model 209 State place Transition Directed arc Token Message place Gate Message flow PN object Fig. 3 Icon definition of an OPN the external message passing. The former is represented by a number of message places for sending and receiving messages (i.e., tokens) and message gates for transmitting messages with other PN objects. The latter is expressed by the state places and activity transitions inside its corresponding PN object. An object-oriented system, S, mathematically may be defined [19]: where S ( O, R) (1) O = a set of physical objects in the system = {O i, i = 1, 2,, I} (i = index of object and I = the total number of physical objects in the system) R = a set of message passing relations among physical objects = {R ij, i, j = 1, 2,, I; i j} Moreover, the OPN for the object i, O i, can be defined as follows: O ( P, T, IP, OP, F, M ) (2) i i i i i i i where P i = a finite set of state places for O i. T i = a finite set of activity transitions for O i. IP i = a finite set of input message places for O i. OP i = a finite set of output message places for O i. F i = an input and output relationship between state/ message places and transitions in O i M i = a marking whose jth component represents the number of tokens in the jth state/message place. The icon definition of an OPN is shown in Fig. 3. i. Transformation between the IDEF0 and OPN Since the IDEF0 is decomposed to define the function of each equipment resource and to represent the information/ material flow between the functions, the IDEF0 may be transferred into the OPN for dynamic behavior analysis. The transformation rules between the IDEF0 and OPN are defined as follows: (1) At the IDEF0 functional analysis stage, the functions of the system are analyzed and decomposed into the function of each equipment resource. Thus, the function box of each equipment resource in the IDEF0 is transferred into a PN object in the OPN. (2) The input and output arrows of a function box represent an object receiving and sending information/material messages. Thus, the input and output arrows of a function box are transferred into the receiving and sending message places of a PN object. The connection of the input and output arrow between two function boxes is transferred into the message gate and the message flow connecting the receiving message place and sending message place. (3) The control arrows of a function box represent an object receiving control messages sent from other objects. Thus, the control arrow of a function box is transferred into the receiving control message place of a PN object. The connection of the control input and output arrow between two function boxes is also transferred into the message gate and the message flow connecting the receiving message place and sending message place. The transformation between the IDEF0 and OPN is shown in Fig. 4. ii. Dynamic Behavior of a PN Object In an OPN approach, a physical object is defined as a PN object. The dynamic behavior of a PN object is characterized by external messages communicating with other PN objects and its internal behavior transitions reacting to the external message passing. The former is represented by a number of message places for sending and receiving messages (i.e., tokens) and message gates for transmitting messages with other PN objects. Thus, the OPN model transferred from the IDEF0 in the previous section represents the basic external dynamic behavior of PN objects. For the internal dynamic behavior of each PN object, a PN model needs to be constructed. After the OPN model is obtained, the correctness of the operation order and the properties of PNs such as safeness, liveness, conservativeness, and reversibility are analyzed based on invariant analysis and reachability analysis [19, 28].

6 210 Journal of Technology, Vol. 32, No. 3 Input i-1 Control i-2 Output i-3 Output i-4 Input j-1 Equipment resource i Control j-2 Output j-3 Interface Shell Core Geometry: Behavior: Nodes Actions States Features Kinematics logic Interface Material flow Connections Signals Equipment resource j CIi-1 Fig. 6 Concept of a virtual device MIi-1 Object i COi-1 MOi-1 Gij-1 Gij-2 MIj-1 Object j CIj-1 MIi-1: Receiving information/material message MOi-1: Sending information/material message CIi-1: Receiving control message COi-1: Sending control message MIj-1: Receiving information/material message CIj-1: Receiving control message MOj-1: Sending information/material message Gij-1: Message gate Gij-2: Message gate MOj-1 Fig. 4 Transformation between the IDEF0 and OPN MIi-1 Ti Pi CIi-1 Pi+1 Ti+1 COi-1 MOi-1 Fig. 5 Typical OPN model for an equipment object In general, typical internal dynamic behavior of an equipment object is shown in Fig. 5. When an equipment object is at idle status (P i 1) and receives a message of material being ready for the operation (MI i -1) and a control message (CI i -1) for triggering the operation from another equipment object, it starts the operation (T i ). After finishing the operation (P i ), it stops the operation (T i 1), returns to idle status and sends the material and control messages (MO i -1 and CO i -1) through the message gates to the following equipment object for the next operation. 3. Virtual Model Construction To develop a virtual model for a manufacturing system, a reference model needs to be established. In this research, the concept of a virtual device, as proposed by Park [14] and shown in Fig. 6, is introduced as a reference model. In designing a virtual device model, reusability, reconfigurability and scalability of the components are important features because a virtual device can be used for many different manufacturing configurations. To achieve these features, a virtual device is split into two parts, the shell and the core. The shell part is the interface of a virtual device and can adapt to different manufacturing configurations. The core part encompasses inherent properties of the device and usually consists of the geometry and behavior of a virtual device. The interface defines how devices communicate with each other. Material flow, connections and signals are three basic functions for describing the behavior of the interface. The material flow defines the input and output ports for the interfaces to receive and output material flow and how the material flow is routed between devices. The connections define the links between the output of the sending device and the input of the receiving device. The signals mapped in the input and output ports are used for communication between devices and transfer of values to connected behaviors when triggered. The geometry created by CAD software represents a visual device that can be seen in a 3D world. The geometry usually consists of two parts: nodes and features. The nodes are the skeleton of the device and are arranged into a tree-like structure called a node tree. Simple devices have only one node, but more complex devices, like robots, have several nodes. The features are basic geometry building blocks like blocks, cones, spheres and wedges and are also arranged into a tree structure called a feature tree. Every node in the node tree has its own feature tree. The feature tree is the visual representation of the node in a 3D world. The behavior determines how a device is simulated. Behaviors of a device are linked together to

7 Lu M. S., & Z. S. He: An Object-Oriented Approach to Developing a Virtual Manufacturing System Model 211 create the overall behavior for the device. The behaviors consist of actions, states, kinematics and rules. The actions define the movement and process of devices. A movement is a predefined path that can be a one-directional path, twodirectional path or bucket path. The states are the sensors for indicating the end of movements and processes and for triggering actions of devices. The kinematics provides kinematic solvers for forward and inverse kinematics. The logic defines the logical rules between signals and actions. Based on the proposed concept, Zeigler s discrete event systems speci cations (DEVS) formalism [29] is employed to specify the behavior formalism of the virtual model. The semantics of formalism are highly compatible with object-oriented specifications so it can be easily realized by object-oriented programming language or commercial simulation software. Behavior formalism consists of two types of sub-models: (1) the atomic model of the virtual device and (2) the coupled model which defines how atomic models are linked together to form a system model. An atomic model VD is defined as: VD ( X, Y, S, A,,,,, t) (3) where X: a set of input events (signals) Y: a set of output events (signals) S: a set of states A: a set of actions : X S A, external transition function : S A, internal transition function from states to actions : A S, internal transition function from actions to states : A Y, output transition function t: A Real, time advance function A couple model CP is defined as: CP ( X, Y, VD, EIC, EOC, IC, SELECT ) (4) where X: a set of input events Y: a set of output events VD: a set of all virtual device models EIC CP.IN VD.IN: external input coupling relation EOC CP.OUT VD.OUT: external output coupling relation IC VD.OUT VD.IN: coupling relation between virtual devices SELECT: 2 M M: tie-breaking selector i. Transformation between the OPN and the Virtual Model The virtual model of a system can be directly transferred from the corresponding OPN model. The transformation rules are defined as follows: (1) The PN objects are mapped into the device atomic models. (2) The message places of the PN object for receiving materials/information or control messages are mapped into the input events of the virtual device. (3) The message places of the PN object for sending materials/information or control messages are mapped into the output events of the virtual device. (4) The state places in the PN object are mapped into the states of the virtual device. (5) The transitions in the PN object are mapped into the actions of the virtual device. (6) The input and output relationships between state/ message places and transitions are mapped into the external, internal, and output transition functions of the virtual device. (7) The gates and message flows between the PN objects are mapped to the coupling relations between the virtual devices. For example, an OPN model of a system is shown in Fig. 7. The OPN model consists of three PN objects: object i, object j and object k. Thus, these three objects are mapped into three atomic models: VDi, VDj and VDk. The PN object i has one message place (MIi-1) for receiving a material message and another message place (CIi-1) for receiving a control message. By using Rule 2, one can obtain the set of input event {Xi-1, Xi-2}. The message place (MOi-1) for sending a material message and the message place (COi-1) for sending a control message are mapped into the set of output events {Yi-1, Yi-2} by using Rule 3. Two state places, end of operation (P i ) and idle (P i 1), are mapped into the set of states {Si-1, Si-2} by using Rule 4. The transitions T i and T i 1 for starting and stopping operation are mapped into the set of actions {Ai-1, Ai-2} by using Rule 5. By using Rule 6, the input and output relationships between state/message places and transitions are mapped into the external function ( Xi-1 and Xi-2, Si-1) = Ai-1, internal functions (Si-2) = Ai-2 and (Ai-1) = Si-2, and output transition function (Ai-2) = Yi-1and Yi-2. Finally, the gates Gji-1, Gji-2, Gik-1 and Gik-2 and their associated message flows are mapped into the coupling relations {(Yj-1 Xi-1), (Yj-2 Xi-2), (Yi-1 Xk-1), (Yi-2 Xk-2)} by using Rule 7. The atomic model of the virtual device VDi transferred from the OPN model in Fig. 7 is obtained as: VDi ( X, Y, S, A,,,,, t) (5)

8 212 Journal of Technology, Vol. 32, No. 3 COj-1 Gji-1 CIi-1 MIi-1 Pi+1 Gik-2 MOj-1 MIk-1 Gji-2 Ti Pi Ti+1 MOi-1 COi-1 Gik-1 CIk-1 Yj-1 Yj-2 - Xi-1 Xi-2 Si-1 Ai-1 Ai-2 Si-2 Yi-1 Xk-1 Yi-2 Xk-2 Fig. 7 The virtual model transferred from the OPN model where X = {Xi-1, Xi-2} Y = {Yi-1, Yi-2} S = {Si-1, Si-2} A = {Ai-1, Ai-2} (Xi-1 and Xi-2, Si-1) = Ai-1. (Si-2) = Ai-2 (Ai-1) = Si-2 (Ai-2) = Yi-1and Yi-2 t(ai-1) = dt and t(ai-2) = 0 The couple model CP for the OPN model in Fig. 7 is obtained as: CP ( X, Y, VD, EIC, EOC, IC, SELECT ) (6) where X = {}, Y = {} VD = {VDj, VDi, VDk} EIC = {}, EOC={} IC ={(Yj-1 Xi-1), (Yj-2 Xi-2),(Yi-1 Xk-1),(Yi-2 Xk-2)} SELECT: 2 M M: tie-breaking selector The virtual model transferred from the OPN model is shown in Fig Virtual Manufacturing System After the virtual model is constructed, it can be implemented in construction of the virtual manufacturing system by using object-oriented programming languages or commercial simulation software. Based on the virtual manufacturing system, the physical behavior of the system can be visualized and verified and the performance can be analyzed. In order to verify the proposed concept, Solid- Works CAD software and Virtual Components 3D manufacturing simulation software are employed for the implementation of the virtual manufacturing system model. The reason for selecting Visual Components is that it provides an object-oriented simulation environment for constructing manufacturing systems. The concept of visual components as defined in Visual Components is similar to the proposed visual devices. The proposed visual models are easily transferred into the visual components so the visual manufacturing system models can be efficiently constructed. The input and output events, states, internal and external transition functions of the atomic model correspond to the signals and sensors, signal maps and logic executors in Visual Components, respectively. The coupling relationship between virtual devices in the couple model corresponds to the interfaces in Visual Components. In addition, the 3D geometric models of components created by SolidWorks can be imported into Visual Components to present visual devices in a 3D world. After the visual manufacturing system is constructed, it can be simulated and verified in a 3D world. III. AN EXAMPLE: AN ASSEMBLY AND PACKAGING SYSTEM In this research, an assembly and packaging system created in the Virtual Manufacturing Laboratory at National

9 Lu M. S., & Z. S. He: An Object-Oriented Approach to Developing a Virtual Manufacturing System Model 213 Assembly Input Packaging Robot Mounted on Linear Slide Output Fig. 8 Layout of the assembly and packaging system MI1-1 Input MO1-1 G12-1 CI2-1 CI2-2 MO2-1 MI2-1 MO2-2 MI2-2 Robot MI2-3 MO2-3 G23-1 CO2-3 CO2-2 CO2-1 MI3-1 G25-2 Assembly G25-1 G24-1 CI3-1 G23-2 G24-2 G42-2 G32-1 CO3-1 MO3-1 CO4-1 G32-2 Packaging MI4-1 CI4-1 MO4-1 MI5-1 G42-1 Output CI5-1 MO5-1 Completing Assembly Completing Packaging Completing Movement to Assembly Completing Movement to Packaging Completing Movement to Output G51-1 Fig. 10 OPN for the assembly and packaging system Part Input Input Part Transport Robot and Slide Part Product Assembly Box Packaging Product Box Output Box Assembly Packaging Output Fig. 9 Functions of the assembly and packaging system Yunlin University of Science & Technology (NYUST) is used to illustrate the proposed concept. The assembly and packaging system consists of a robot mounted on a linear slide, an assembly machine, a packaging machine, an input conveyor and an output conveyor. The assembly and packaging system is shown in Fig. 8. The assembly and packaging process is described as follows: 1. The input conveyor feeds the part. 2. The robot loads the part from input conveyor into the assembly machine. 3. The assembly machine assemblies the parts into the product. 4. The robot unloads the product from the assembly machine and puts it into a box on the packaging machine. 5. The packaging machine packs the box. 6. The robot unloads the box from the packaging machine and moves it to the output conveyor. 7. The output conveyor outputs the box. 1. IDEF0 Functional Analysis for the Assembly and Packaging System According to the description of the assembly and packaging process, the functions of the system can be divided into five functions: input, transport, assembly, packaging and output, as shown in Fig. 9. The equipment providing the functions of input, assembly, packaging and output are an input conveyor, robot and slide, assembly machine, packaging machine and output conveyor. Two control signals, completing assembly and completing packaging, are input into the transport function. The completing movement for the assembly machine, completing movement for the packaging machine and completing movement for the output conveyor are the control signals input into the assembly, packaging and output functions, respectively. 2. OPN and Virtual Models for the Assembly and Packaging System The five equipment resources in IDEF0 are transferred into the associated five objects in the OPN. The completing assembly and completing packaging are two control signals sent from the assembly machine and packaging machine and input into the robot, so they are defined as the control messages between the robot and the assembly machine and between the robot and the packaging machine in the OPN, respectively. Because the completing movement to the assembly machine, completing movement to the packaging machine and completing movement to the output conveyor are three control signals sent from the robot for the operations of assembly, packaging and output, they are defined as the control messages between the robot and assembly machine, between the robot and packaging machine, and between the robot and output conveyor in the OPN, respectively. The OPN for the assembly and packaging system is shown in Fig. 10. In addition, the detailed OPN models for each equipment object in Fig. 10 are shown in Fig. 11. Figs. 11(a)-(e) are the OPN models of the input conveyor, output conveyor, assembly machine, packaging machine,

10 214 Journal of Technology, Vol. 32, No. 3 CI5-1 CI3-1 P1-1 P5-1 P3-1 MI1-1 T1-1 P1-2 T1-2 MO1-1 MI5-1 T5-1 P5-2 T5-2 MO5-1 MI3-1 T3-1 P3-2 T3-2 MO3-1 (a) (b) (c) CO3-1 CI2-2 CI2-1 P2-1 CI4-1 MI2-1 P2-2 MO2-1 MI4-1 P4-1 T4-1 P4-2 T4-2 MO4-1 MI2-2 MI2-3 T2-1 T2-3 P2-3 P2-4 T2-2 T2-4 MO2-2 MO2-3 CO4-1 (d) T2-5 T2-6 CO2-3 CO2-2 CO2-1 (e) Fig. 11 OPN models for the: (a) input conveyor; (b) output conveyor; (c) assembly machine; (d) packaging machine; and (e) robot X1-1 Input Y1-1 X2-1 X2-2 X2-3 X2-4 X2-5 Y2-1 Y2-2 Robot Y2-3 Y2-6 Y2-5 Y2-4 X3-1 Assembly Y3-1 Y3-2 Y4-1 X3-2 X4-1 Packaging Y4-2 X4-2 X5-1 Output Y5-1 X5-2 Fig. 12 Virtual model for the assembly and packaging system and robot, respectively. Each object has similar behavior as described in Section 2-ii. The complete OPN model for the assembly and packaging system can be obtained by combining the OPN models in Fig. 10 and Fig. 11. According to the transformation rules described in Section 3-i, the OPN model can be transformed into the virtual model for the as-

11 Lu M. S., & Z. S. He: An Object-Oriented Approach to Developing a Virtual Manufacturing System Model 215 X1-1 X5-1 S1-1 S5-1 A1-1 A1-2 S1-2 Y1-1 X5-2 A5-1 A5-2 S5-2 Y5-1 (a) (b) X3-1 Y3-1 X4-1 Y4-1 S3-1 S4-1 X3-2 A3-1 A3-2 S3-2 Y3-2 X4-2 A4-1 A4-2 S4-2 Y4-2 (c) (d) Y2-4 S2-1 X2-1 A2-1 S2-2 A2-2 Y2-1 X2-2 X2-4 A2-3 S2-3 A2-4 Y2-2 Y2-5 X2-3 X2-5 A2-5 S2-4 A2-6 Y2-6 Y2-3 (e) Fig. 13 Atomic models of the: (a) virtual devices for the input conveyor, (b) output conveyor, (c) assembly machine, (d) packaging machine, and (e) robot in Fig. 13. The interpretation of each element in Figs is listed in Table 1 and Table 2. Based on the virtual model, SolidWorks and Virtual Components are employed to construct the virtual assembly and packaging system. The 3D virtual model of the assembly and packaging system created in Virtual Components is shown in Fig. 14 and the behavior and performance of the system can be visualized and verified in a 3D world. IV. CONCLUSIONS Fig. 14 3D virtual model of the assembly and packaging system sembly and packaging system. The virtual model for the assembly and packaging system is shown in Fig. 12, and the atomic model of each virtual device in Fig. 12 is shown Critical to the success of construction of digital models is to develop reusable, configurable and scalable models of virtual manufacturing systems. Object-oriented modeling provides a natural one-to-one correspondence between components in a manufacturing system and the software objects that represent them. In this research, an efficient and systematic methodology for creating an object-oriented virtual

12 216 Journal of Technology, Vol. 32, No. 3 Table 1 Interpretations of message places and gates of Fig. 10, and input and output events of Fig. 12 Message places and gates Input and output events Interpretation MI1-1 X1-1 Part arriving at input conveyor MI2-1 X2-1 Part in input conveyor MI2-2 X2-2 Product in assembly machine MI2-3 X2-3 Box in package machine MI3-1 X3-1 Part in assembly machine MI4-1 X4-1 Product in packaging machine MI5-1 X5-1 Box in output conveyor MO1-1 Y1-1 Sending message of part in input conveyor MO2-1 Y2-1 Sending message of part in assembly machine MO2-2 Y2-2 Sending message of product in packaging machine MO2-3 Y2-3 Sending message of box in output conveyor MO3-1 Y3-1 Sending message of product in assembly machine MO4-1 Y4-1 Sending message box in packaging machine MO5-1 Y5-1 Sending message of completing outputting box CI2-1 X2-4 Completing assembly CI2-2 X2-5 Completing packaging CI3-1 X3-2 Completing movement to assembly machine CI4-1 X4-2 Completing movement to packaging machine CI5-1 X5-2 Completing movement to output conveyor CO2-1 Y2-4 Sending message of completing movement to assembly machine CO2-2 Y2-5 Sending message of completing movement to packaging machine CO2-3 Y2-6 Sending message of completing movement to output conveyor CO3-1 Y3-2 Sending message of completing assembly CO4-1 Y4-2 Sending message of completing packaging Table 2 Interpretations of places and transitions of Fig. 11, and actions and states of Fig. 13 Place and State Interpretation Transition and actions Interpretation P1-1 T1-1 Idle S1-1 A1-1 Start inputting part P1-2 T1-2 Inputting part S1-2 A1-1 Stop inputting part P2-1 T2-1 Idle S2-1 A2-1 Start moving part to assembly machine P2-2 T2-2 Moving part to assembly machine S2-2 A2-1 Stop moving part to assembly machine P2-3 T2-3 Moving product to packaging machine S2-3 A2-3 Start moving product to packaging machine P2-4 T2-4 Moving box to output conveyor S2-4 A2-4 Stop moving product to packaging machine P3-1 T2-5 Idle S3-1 A2-5 Start moving box to output conveyor P3-2 T2-6 Assembling product S3-2 A2-6 Stop moving box to output conveyor P4-1 Idle T3-1 Start assembling product

13 Lu M. S., & Z. S. He: An Object-Oriented Approach to Developing a Virtual Manufacturing System Model 217 Place and State Interpretation Transition and actions Interpretation S4-1 A3-1 P4-2 T3-2 Packaging box S4-2 A3-2 Stop assembling product P5-1 T4-1 Idle S5-1 A4-1 Start packaging box P5-2 T4-2 Outputting box S5-2 A4-2 Stop packaging box T5-1 A5-1 Start outputting box T5-2 A5-2 Stop outputting box and between the CTPN and virtual device are constructed to save time in developing the model and to improve the consistency of the models. In the proposed approach, as design specifications change, the needs of changing design specifications can be specified or modified in the IDEF0 at the functional analysis stage and then implemented by following the rest of the development process. The results of the integrated IDEF0/OPN/virtual device systematically lead to the design of object-oriented models for virtual m- anufacturing systems. A 3D virtual model of an assembly and packaging system was successfully implemented by the proposed integrated object-oriented approach. REFERENCES 1. Banerjee, P., and D. Zetu Virtual Manufacturing. New York, NY: John Wiley & Sons. 2. Xie, G. R. and W. A. Xie Advanced Manufacturing Technology-Virtual Manufacturing. Applied Mechanics and Materials ( ): doi: / 3. Souza, M. C. F., M. Sacco, and A. J. V. Porto Virtual Manufacturing as a Way for the Factory of the Future. Journal of Intelligent Manufacturing 17 (6): Ghani, U., R. Monfared, and R. Harrison Integration Approach to Virtual-driven Discrete Event Simulation for Manufacturing Systems. International Journal of Computer Integrated Manufacturing 28 (8): Peng, Q., C. Chung, C. Yu, and T. Luan A Networked Virtual Manufacturing System for SMEs. International Journal of Computer Integrated Manufacturing 20 (1): doi: / Xu, Z., Z. Zhao, and R. W. Baines Constructing Virtual Environments for Manufacturing Simulation. International Journal of Production Research 38 (17): doi: / Lee, C. G. and S. C. Park Survey on the Virtual Commissioning of Manufacturing Systems. Journal of Computational Design and Engineering 1 (3): doi: /JCDE Usher, J. M A Tutorial and Review of Objectoriented Design of Manufacturing Software Systems. Computers & Industrial Engineering 30 (4): doi: / (96) Mize, J. H., H. C. Bhuskute, D. B. Pratt, and M. Kamath Modeling of Integrated Manufacturing Systems Using an Object-oriented Approach. IIE Transaction 24 (3): doi: / Park, T. Y., K. H. Han, and B. K. Choi An Objectoriented Modeling Framework for Automated Manufacturing System. International Journal of Computer Integrated Manufacturing 10 (5): doi: / Lai, H. F A Study on Techniques of Computer Aided Engineering Applying AFD/OM Method to Analyze Automated Manufacturing System. Journal of Technology 16 (4): Ou-Yang, C., T. Guan, and J. S. Lin Developing a Computer Shop Floor Control Model for a CIM System-Using Object Modeling Technique. Computers in Industry 41 (3): doi: / S (99) Zhang, J., J. Gu, P. Li, and Z. Duan Objectoriented Modeling of Control System for Agile Manufacturing Cells. International Journal of Production Economics

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