Development of Flexible Manufacturing System using
Virtual Manufacturing Paradigm
Sung-Chung Kim* and Kyung-Hyun Choi
School of mechanical engineering, Chungbuk National University, Cheongju, South Korea,
School of mechanical engineering, Cheju National University, Cheju, South Korea
ABSTRACT
The importance of Virtual Manufacturing System is increasing in the area of developing new manufacturing processes, implementing automated workcells, designing plant facility layouts and workplace ergonomics. Virtual manufacturing system is a computer system that can generate the same information about manufacturing system structure, states, and behaviors as is observed in a real manufacturing. In this research, a virtual manufacturing system for flexible manufacturing cells (VFMC), (which is a useful tool for building Computer Integrated Manufacturing (CIM),) has been developed using object-oriented paradigm, and implemented with software QUEST/IGRIP. Three object models used in the system are the product model, the facility model, and the process model. The concrete behaviors of a flexible manufacturing cell are represented by the task-oriented description diagram, TID. An example simulation is executed to evaluate applicability of the developed models, and to prove the potential value of virtual manufacturing paradigm.
Key Words : FMS, virtual manufacturing system, CIM, object-oriented paradigm, TID
Recent trends in manufacturing systems, such as the need for customized products by small batches and for fast product renewal rates, have been demanding new paradigms in manufacturing. Therefore, the modern manufacturing systems are needed to be adaptable, and have the capability to reconfigure or self configure their own structure. Flexible Manufacturing Cells (FMCs) are generally recognized as the best productivity tool for small to medium batch manufacturing, and are also basic unit to construct a shop floor which is an important leve for developing computer integrated manufacturing (CIM). However, due to its complexity, the modeling and operation methodology related to FMC should be verified before implementation.
As one of approaches to these requirements, Virtual Manufacturing
(VM) approach has been introduced, and known as a effective paradigm for generating a model of manufacturing systems and simulating manufacturing processes instead of their operations in the real world. VM pursues the informational equivalence with real manufacturing systems. Therefore, the concept of Virtual Manufacturing System is expected to provide dramatic benefits in reducing cycle times, manufacturing and production costs, and improving communications across global facilities to launch new products faster, improve productivity and reduce operations costs for existing product shop [1,2].
With an object-oriented paradigm, computer-based technologies such
as virtual prototyping and virtual factory are employed as a basic concept for developing the manufacturing processes, including the layout of the optimal facility, to produce products. Virtual prototyping is a process by which advanced computer simulation enables early evaluation of new products or machines concept without actually fabricating physical machines or products. Bodner, et al.,[3] concentrated on the decision problems associated with individual machines that assemble electronic components onto printed circuit boards (PCBs). Virtual factory is a realistic, highly visual, 3D graphical representation of an actual factory floor with the real world complexity linked to the production controlling system and the real factory. Virtual factories are increasingly used within manufacturing industries as representations of physical plants, for example, VirtualWork system for representation of shop floor factory[4].
Despite its benefits and applicability, VM systems should deal with a
number of models of various types and require a large amount of computation for simulating behavior of equipment on a shop floor. To cope with this complexity in manufacturing, it is necessary to introduce open system architecture of modeling and simulation for VM systems.
In this paper, three models, which are product, device, and process
models will be addressed. Especially process model for FMC will be emphasizedusing QUEST/IGRIP as an implementation issue. The open system architecture consists of well-formalized modules for modeling and simulation that have carefully decomposed functions and well-defined interface with
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other modules.
2. Concept of virtual manufacturing
Virtual Manufacturing System is a computer model that represents the precise and whole structure of manufacturing systems and simulates their physical and logical behavior in operation, as well as interacting with the real manufacturing system. Its concept is specified as the model of present or future manufacturing systems with all products, processes, and control data. Before information and control data are used in the real system, their verification is performed within virtual manufacturing environment. In addition, its status and information is fed back to the virtual system from the real system.
Virtual environments will provide visualization technology for virtual manufacturing. The virtual prototype is an essential component in the virtual product life cycle, while the virtual factory caters for operations needed for fabricating products. Therefore, the developments in the area of virtual prototyping and virtual factory will enhance the capabilities of virtual manufacturing.
The major benefit of a virtual manufacturing is that physical system components (such as equipment and materials) as well as conceptual system compvonents (e.g., process plans and equipment schedules) can be easily represented through the creation of virtual manufacturing entities that emulate their structure and function. These entities can be added to or removed from the virtual plant as necessary with minimal impact on other system data. The software entities of the virtual factory have a high correspondence with real system components, thereby lending validity to simulations carried out in the virtual system meant to aid decision-makers in the real system.
For virtual manufacturing, three major paradigms have been proposed, such as Design- centered VM, Production-centered VM, and Control- centered VM. The design-centered VM provides an environment for designers to design products and to evaluate the manufacturability and affordability of products. The results of design-centered VM include the product model, cost estimate, and so forth. Thus, potential problems with the design can be identified and its merit can be estimated. In order to maintain the manufacturing proficiency without actual building products, production-centered VM provides an environment for generating process plans and production plans, for planning resource requirements (new equipment purchase, etc.), and for evaluating these plans. This can provide more accurate cost information and schedules for product delivery. By providing the capability to simulate
actual production, control-centered VM offers the environment for engineers to evaluate new or revised product designs with respect to shop floor related activities. Control-centered VM provides information for optimizing
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manufacturing processes and improving manufacturing systems.
The virtual manufacturing approach in this paper is close to Control-centered VM. Fig.1 illustrates the viewpoint of the functional model of the virtual flexible manufacturing cell. Since the activity Execute real manufacturing systems depicts a model of real factory, it possibly replaces real factory. All manufacturing processes except physical elements of virtual manufacturing, such as design, process planning, scheduling, are included in the activity Operation of Virtual factory. The activity Execute simulation for virtual factory is a separate simulation model of VM system. With this virtual factory, parameters (e.g, utilization, operation time, etc.,) associated with operating a flexible manufacturing cell are simulated. And these results can provide the possibility of controlling manufacturing processes and predicting potential problems in the real manufacturing.
3. Object modeling for virtual flexible
manufacturing cells
Object-oriented technology may provide a powerful representation and classification tools for a virtual flexible manufacturing cell. It may also provide a common platform for the information sharing between sub-modules, and provide a richer way to store/retrieve/modify information, knowledge and models and reuse them. In the context of an object oriented approach, a model is simply an abstraction, or a representation of an objects or process.
VFMC requires a robust information infrastructure that comprises rich information models for products, processes and production systems. As shown in Fig. 2, three models, that is product model, facility model, and process model, are developed for virtual flexible manufacturing cells. A product model is a generic model used for representing all types of artifacts, which appear in the process of manufacturing. It represents target products, which include conceptual shape information as well as analysis module for a specification, productivity, and strength. A facility model contains information about machines consisted of a virtual flexible manufacturing cell. By using the model, innovative tooling and methods can be evaluated without the cost of physical machine prototypes and fixture mock-ups. A process model is used for representing all the physical processes that are required for representing product behavior and manufacturing processes. 3.1 Product model
A product model holds the process and product knowledge to ensure the correct fabrication of the product with sufficient quality. It acts as an information server to the other models in the VFMC. It also provides consistent and up-to-date information on the product lifecycle, user requirements, design, and process plan and bill of material. An instance of
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Class Part provides detailed information about a part to be fabricated in VFMC. Sub-classes like ProcessPlan, BOM, and NcCode, are aggregated into the class Part. Classes Process Plan and BOM manipulate information and data associated with process plans and bill of materials, respectively. Class NcCode deals with NC programs, which interacts with CAD/CAM systems. With incorporation with the facility model, this developed NC programs can be verified and checked for collisions and interference with any workpiece or tooling in the fixture. This can avoid costly machine crashes and reduce risk during initial equipment installation and produce launch. Furthermore, productivity can be improved by avoiding nonproductive time for program prove out on the machine tool and by using the simulation environment to train operators of new machines. 3.2 Facility model
Real manufacturing cell may consist of NC machines, robots, conveyors, and sensory devices. The architecture of class corresponding to the real manufacturing cell is shown in Fig.3, and represents the factory model. In VFMC, characteristics of the factory model include a detailed representation of machine behavior over time, a structure to the model that can configure and reconfigure easily, and a realistic and three-dimensional animation of machine behavior over time. Virtual machines defined within this model may be used to estimate accurately the merit of a process plan, and, based on this evaluation, determine appropriate process conditions to improve (and even optimize) the plan. Virtual robot contributes to unload and load parts into/from machines, and is used to find optimal paths without any collisions. With virtual operation, the fidelity of the machining and robot utilizing time and cost estimates is expected to improve. In addition, accurate modeling will predict the quality of the machined part, which cannot be determined easily and reliably without producing several physical prototypes. This information is invaluable to both the designer and the process planner. Physical entities such as machines and workpieces have the explicit representation as 3-D models for their shapes, positions, and orientations. 3-D models are conveniently used for calculating, geometrical attributes, checking spatial relations, and displaying computer graphics. 3.3 Process model
By assigning a finite set of states to each device in a cell (idle, busy, failed, etc.), the process of cell control can be modeled as a process of matching specific state change events to specific cell control actions, decision algorithms, or scripts. With this model, cell processes are represented a Task Initiation Diagram (TID) using an object-oriented approach. The methodology behind developing TID regards the tasks to be performed by the cell or any of its constituent machines for being primal, and employs the multi-layered approach. Sensory signals indicating the change of state of machines are used to trigger or initiate tasks. A task may be simple and require a relatively short
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机械设计制造及其自动化外文翻译-----柔性制造系统的发展运用在实际制造中的范例
