Interlocking Design For A Railway Yard-A Symbolic Supervisory Synthesis Approach For Discrete Event Systems

Discrete Event Systems (DESs) refer to those systems that changing of their states is driven by asynchronous events at discrete time slots with unpredictable characteristic. Therefore, proper operation of DESs relies on effective supervisors[1]. The Supervisory Control Theory, proposed by Ramadge and Wonham, focuses on managing DESs with formal languages and automata theory[2],[3].Events and States, as the most essential elements of DESs, should always be on the top of list in our mind when dealing with DESs. Formal languages are powerful tools to describe the concurrence and dynamic behavior of DESs; on the other hand, logical computing implements the transitions by states feedback. Apparently, both of them focus on discrete variables, which lead us to have to keep an eye on the complexity of computing from the beginning of the supervisor design[4].As we all known, the complexity of a model is often measured by its number of states. The more states mean the larger state space. In order to moderate the state-explosion problem, we utilize an efficient data structure-Binary Decision Diagrams, BDD[5]-to compress the demand for storage when computing the reachability of a DES. The supervisor design follows after this step.Then two cases with distinct types and complexities are studied in this paper. The first case concentrates on algorithm. The second, railway yards have many tracks in parallel for keeping rolling stock stored off the mainline, so that they do not obstruct the flow of traffic where signal lights are used to prevent conflicting movements through an arrangement of tracks. In order to work smoothly, assigned route, switches and signal apparatus should coordinate and restrict each other. Presenting this kind of restriction into a table formats an interlocking table, which is the basis of control policies. This paper presents a guards-based supervisory method to coordinate each component, keeping the safety and reliability of a large system, especially the interlocking system of a railway yard within the context; meanwhile permitting the maximally liveness for the system. This paper starts with explaining some basic important mathematic concepts that must be known in order to understand the essence of this work. In following chapters, supervisory control theory is introduced, implementation is verified both in an academic model and an industry model, and finally make conclusions for this work.