Research On Dynamic Scheduling Of Part Flow And Tool Flow In FMS

Dynamic scheduling is the main problem in flexible manufacturing systems' integrated production supervision, and is also the core of FMS controlling system. It shows great effect on the running efficiency and beneficial result for FMS application.Dynamic scheduling of FMS mainly includes two aspects: part-flow scheduling and tool-flow scheduling. In recently two decades, there is already a significant development in the study of part-flow scheduling, especially the recent proposal for the heuristic scheduling methods which fully utilizes the systemic flexibility and improves productivity moderately. While the traditional heuristic scheduling research mostly fixes eye on the ordinary process of scheduling, it lacks the study of exceptional situation such as machine block and breakdown. Actually the existence of these abnormal cases will critically affect system operation. Sometimes they can even cause the whole system paralyzed. It is therefore rather important to do necessary research in block and breakdown scheduling. Additionally, traditional research of part-flow scheduling takes no consideration about the influence of the systemic tool-flow, which is always supposed to be in order at any time. While this assumption is acceptable at an early stage of simple FMS, it is untenable for modern FMS owing to the boost of the diverse requirements towards products and the limitation in the volume capacity of machine tool magazine in system. Therefore, with careful consideration of tool-flow, it is an emergent task to realize the heuristic scheduling of part-flow. The scheduling of tool-flow refers to automatic control of kinds of tools, which is an important part in the scheduling of FMS operation. Because of the long-lasting neglect of the study oftool-flow and the conservative strategies of techniques in most industrial countries, theory about tool-flow scheduling is very limited nowadays. The inferior level of recent automatic tool-flow in use has greatly influenced the further development of FMS productive capacity. Although some scholars have already set to work on it, they mainly focus on the structure and the functional system of tool-scheduling system. As regards towards tool-flow scheduling, they mostly cope with the primitive analysis of the deadlock in single process operation. These analyses are only within tool-flow and regardless of influence of part-flow. Besides, multi-process and successive process operation is the common case in actual FMS production. It is still an unsolved problem for how to achieve tool-flow scheduling under that circumstance, especially how to achieve the detection and avoidance of tool-flow deadlock. Based on the study of tool-flow this dissertation realizes tool-flow dynamic scheduling with deadlock avoidance and sets up the synthetic system of scheduling both for part-flow and tool-flow, satisfying the urgent need of flexibility and requirement of machining in FMS. It presents reasonable theoretical importance and engineering value.This dissertation starts with FMS modeling, in light of recent research of part-flow and tool-flow concerned with the dialectical relationship of their interaction and their inter-restriction, and discusses in the following three aspects:1. ModelingThe research about FMS scheduling are under the guidance of simulating models. Actually the analysis of FMS operation is influenced directly by the performance of system models. Based on object-oriented technology and Petri-net theory OOP-Petri net model isestablished for a typical FMS which is superior for its stability in the ever-changing scale of the system. Not only the handling of machine breakdowns is considered but also tool-flow is organically integrated in the model strongly directing the establishment of the complex scheduling system.2. Research of part-flow dynamic scheduling1) On the basis of virtual-list-process technology the principles of scheduling is improved in this paper, a module of scheduling for system block is established, a method dealing with machine breakdowns by degrees put forward, a module of scheduling for such breakdowns build up, and then a simulating experiment followed. The results of the experiment indicate: the module of scheduling for block can eliminate the blocks quite well; the module of scheduling for breakdowns helps part-flow by-pass such machines and maintains the continual operation ofFMS.2) Under the traditional heuristic scheduling method, the influence of tool-flow is examined sufficiently and an entirely new method to decide system decision point is provided which suggests some formula for system decision points calculating with regard to various systemic situations. Therefore, it is more suitable to meet part-flow heuristic scheduling actually. The result of scheduling shows: the new method is extremely valid for the scheduling control of FMS, and the systemic productiveness is considerably high.3) With the consideration of the part-flow scheduling influenced by tool-flow, the rules to select parts through machining are analyzed and the minimal-tools-borrowed rule is put forward. The results of experiment present: the new rule may dramatically reduce the total replacement of tools of the system, which effectively decreases thetoo-waiting time and improves the systemic productiveness. 3. The research of tool-flow scheduling1) Method of pre-preparation for tools is proposed at first and tool-flow process is defined always preparing tools for the later process. Based on, this method the two processes: the process of pre-preparation of tools by tool-flow and the process of machining, are overlapped fully. Tool-waiting time is lowered and the efficiency of machines improved. Assuming pre-preparation of tools, the principle of tool-flow scheduling is recommended which is superior to others. It is the first time to consider the dialectical relationship between part-flow and tool-flow, and to study the limitation of part-flow influenced by tool-flow scheduling. The modules of part-selection and tool-distribution are combined so as to achieve a more advancing scheduling control of tool-flow.2) Some important new concepts are proposed such as tool magazine partition, applying line and tool applying allocation graph. Deadlock of tool-flow and deadlock graph are defined seriously. Through all of these notions essential nature of the deadlock of tool-flow is revealed by linking deadlock to deadlock graph.3) The property of the deadlock of tool-flow is analyzed further and three critical principles are proposed and testified against the actual situation of multi-process successive processing which develop the relative theories about tool-flow.4) Two practical algorithms for deadlock detection and one for tool distribution are given. In the complex scheduling of part-flow and tool-flow, the algorithms of deadlock detection given in this paper are employed to achieve the deadlock control of the module of part selection on one hand. Tool distribution algorithm is also taken tocalculate the feasible tool-distributed plan in order to fulfill the function of deadlock avoidance. Finally a dynamic synthetic scheduling system of FMS based on tool-flow deadlock avoidance is set up and computer simulation of scheduling is taken to testify both for the FMS with center crib supported and not supported. The results of the simulation shows: this synthetic scheduling system may realize the whole scheduling of part-flow and tool-flow fairly well and the properties of the FMS is fine.