Analysis And Application Of Deadlock Mechanism In Automated Manufacturing Systems

As an important part of industrial manufacturing,it is of great significance to ensure the ef-ficient and stable operation of automated manufacturing systems(AMSs).AMSs automate processing by machining raw materials into products with less direct or indirect human in-tervention.AMSs are widely used in industrial field since they not only facilitate industry automation but also promote diversified production.In recent years,there has been an in-creasing demand for customization,small batches,and high quality.In order to meet diverse product requirements,AMSs can dynamically configure and allocate resources in accordance with product specifications.In general,AMSs have gained increasing popularity in both scientific research and engineering practice due to the advantages in equipment utilization,capacity,quality and economic benefits.Deadlocks lead certain processes to wait indefinitely,preventing them from moving forward and directly causing a sharp decline in productivity.Moreover,a process unnecessarily using the scrambled resources can be blocked by these processes in stagnation.In the worst case,a whole system may fall into a deadlock.Therefore,deadlocks are particularly important problems in large-scale AMSs.Many approaches have been proposed to deal with this issue.However,most approaches still hold some defects such as narrow application extent and high computational complexity since they necessarily rely on the existing limited deadlock mechanism.This dissertation aims to analyze the deadlock mechanism in AMSs with the paradigm of Petri nets(PNs),which means to explore the fundamental cause of deadlocks.On this basis,a series of deadlock control policies are proposed to ensure the liveness of systems.The main results of this dissertation are as follows:1.For a variety of complex AMSs,a new kind of circular waits is proposed as a necessary condition for deadlocks to occur,i.e.,event circular waits(EWs).EWs can be used to revise the limited resource circular waits(RWs)and explore the deadlock mechanism in AMSs.The research is developed in a type of general PNs with resources(GPRs),which benefits from fewer constraints and wider applicability.First,the fundamental cause of deadlocks can be explored by deeply comparing the differences between the two kinds of circular waits.Second,a necessary and sufficient condition between the existence of deadlocks and EWs is clarified.This further illustrates the applicability and efficiency of EWs in studying the liveness of systems.In general,EWs not only serve as the theoretical basis for deadlock control,but also provide technical support for subsequent deadlock prevention policy and deadlock avoidance policy.2.A large number of deadlock control policies are presented in the existing literature.How-ever,how to solve deadlocks in an efficient way is still one of the major obstacles.In this dissertation,a new kind of structural objects of PNs is proposed,i.e.,event circuit struc-tures(ESs),based on EWs to overcome this difficulty.First,the concepts,definitions and properties related to ESs are introduced in the systems of sequential systems with shared resources(S~4Rs).And another kind of structural objects based on RWs,i.e.,siphons,can be shown a novel classification in S~4Rs.Second,this dissertation explores the difference in the accuracy of using ESs and siphons when analyzing the liveness of systems.It can be ob-tained that ESs are more accurate than siphons in explaining the cause of deadlocks.Third,a deadlock control policy based on ESs is proposed to ensure the liveness of systems by preventing S~4Rs from being deadlocked.This approach provides a guarantee for the stable operation of systems.3.This dissertation focuses on deadlock avoidance,which aims at predicting and avoiding deadlocks before they occur.This not only improves the operating efficiency of AMSs but also avoids unnecessary waste of resources.However,most existing approaches for dead-lock avoidance suffer from formidable computational difficulty,especially for large systems.This dissertation proposes an approach for avoiding evolving dead markings,where an evolv-ing dead marking can be determined as a dead one once it may lead inevitably to deadlocks.First,a necessary and sufficient condition between ESs and evolving dead markings in S~4Rs is established.Second,this dissertation describes how ESs can be applied to avoid evolving dead markings in S~4Rs.Only structure information is needed during this procedure,thereby improving the efficiency and convenience of avoiding evolving dead markings.This pro-vides a practical basis for subsequent deadlock avoidance policy.4.Based on the approach for avoiding evolving dead markings mentioned above,an im-proved deadlock avoidance policy based on ESs is proposed in a special type of PNs,i.e.,weighted augmented marked graphs(WAMGs).Due to the structural particularity of such PNs,assembly operations in AMSs can be better simulated.First,explore the differences in liveness analysis between two different kinds of structural objects.Specifically,com-pared with ESs,there are certain limitations in studying the liveness of systems by siphons in WAMGs.Next,after optimizing and improving the approach mentioned above,a new deadlock avoidance policy based on ESs is proposed.This policy can avoid deadlocks in a timely and comprehensive manner to ensure the liveness of systems.