روشی برای ایجاد یک مدل مجازی برای سیستم های تولید انعطاف پذیر

Proposed in the paper is an object-oriented methodology to create a virtual flexible manufacturing system (FMS) model. The proposed virtual FMS model consists of four types of objects: the virtual device model (object model), the transfer handler model (functional model), the state manager model, and the flow controller model (dynamic model). A virtual device model consists of two parts: shell and core. To improve the reusability of a virtual device model, the shell part is designed to adapt to different FMS configurations. For the fidelity of the virtual FMS model, a transfer handler model has a set of device-level commands imitating the physical mechanism of a transfer. As a result, we can expect more accurate simulation results. The flow controller model makes decisions on firable transfers based on decision variables, which are maintained by the state manager model. For the implementation of the proposed virtual FMS model, this paper employs Discrete Event Systems Specifications (DEVS) formalism, which supports the specification of discrete event models in a hierarchical, modular manner. The proposed virtual FMS model has been implemented and tested with many examples.

As product life cycles are reduced in the continuously changing marketplace, modern manufacturing systems must have sufficient responsiveness to adapt their behaviors efficiently to a wide range of circumstances [7]. To respond to these demands, including high productivity and production flexibility, the use of a flexible manufacturing system (FMS) has been widely accepted. A flexible manufacturing system is an integrated production system composed of automated workstations such as computer numerically controlled (CNC) machines with tool change capability, a hardware handling system and storage system, and a computer control system which controls the operations of the whole system. The design of an FMS requires high investment, and decisions at this stage have to be made very carefully to ensure that the highly automated manufacturing system will successfully satisfy the demands of an ever-changing market. Simulation is an essential tool for the design and analysis of complex systems that cannot be easily described by analytical or mathematical models [3] and [4]. It is useful for calculating utilization statistics, finding bottlenecks, pointing out scheduling errors and even for creating manufacturing schedules. Traditionally, various simulation languages, including ARENA® and AutoMod®, have been used for the simulation of manufacturing systems [2]. Since those simulation languages focus on the representation of independent entity flows (job flows) between processes, their approach is commonly referenced to as a transaction-oriented approach. On the other hand, the object-oriented approach is based on a set of object classes that model the behavior of real system components.

This paper presents an object-oriented methodology for the modeling and simulation of a virtual FMS. For the implementation of the proposed virtual FMS model, the paper employs DEVSIM++ which is an object-oriented simulation language based on Zeigler's DEVS formalism. The OOM paradigm of Rumbaugh uses three kinds of models to describe a system: the object model, describing the objects in the system and their relationships; the dynamic model, describing the interactions among objects in the system; and the functional model, describing the data transformations of the system. The proposed virtual FMS model follows the OOM paradigm and uses four types of components: the virtual device model corresponding to the object model, the transfer handler model corresponding to the functional model, and the state manager and flow controller model corresponding to the dynamic model. A virtual device is designed considering reusability, and the device consists of two parts: shell and core. The shell part allows a virtual device model to adapt to different FMS configurations, and it encloses the core part which contains the inherent properties of a device, such as kinematics, geometric shape, and the execution of device-level commands. For the fidelity of the virtual FMS model, a transfer handler model has a set of device-level commands imitating the physical mechanism of a transfer. As a result, we can check the mechanical validity of each transfer and also expect more accurate simulation results, because the total time of each transfer will be computed from its physical mechanism. The flow controller model makes decisions on firable transfers based on decision variables, which are maintained by the state manager model. To maintain the decision variables, the state manager stores mapping relations between decision variables and the states of virtual devices in the system. The mapping relations can serves as the guidelines for the planning of a real FMS implementation.