Synthesis Of Controllers For Discrete Event Systems Research Articles

Model abstraction for discrete-event systems by binary linear programming with applications to manufacturing systems.

Model abstraction for finite state automata is helpful for decreasing computational complexity and improving comprehensibility for the verification and control synthesis of discrete-event systems (DES). Supremal quasi-congruence equivalence is an effective method for reducing the state space of DES and its effective algorithms based on graph theory have been developed. In this paper, a new method is proposed to convert the supremal quasi-congruence computation into a binary linear programming problem which can be solved by many powerful integer linear programming and satisfiability (SAT) solvers. Partitioning states to cosets is considered as allocating states to an unknown number of cosets and the requirement of finding the coarsest quasi-congruence is equivalent to using the least number of cosets. The novelty of this paper is to solve the optimal partitioning problem as an optimal state-to-coset allocation problem. The task of finding the coarsest quasi-congruence is equivalent to the objective of finding the least number of cosets. Then the problem can be solved by optimization methods, which are respectively implemented by mixed integer linear programming (MILP) in MATLAB and binary linear programming (BLP) in CPLEX. To reduce the computation time, the translation process is first optimized by introducing fewer decision variables and simplifying constraints in the programming problem. Second, the translation process formulates a few techniques of converting logic constraints on finite automata into binary linear constraints. These techniques will be helpful for other researchers exploiting integer linear programming and SAT solvers for solving partitioning or grouping problems. Third, the computational efficiency and correctness of the proposed method are verified by two different solvers. The proposed model abstraction approach is applied to simplify the large-scale supervisor model of a manufacturing system with five automated guided vehicles. The proposed method is not only a new solution for the coarsest quasi-congruence computation, but also provides us a more intuitive understanding of the quasi-congruence relation in the supervisory control theory. A future research direction is to apply more computationally efficient solvers to compute the optimal state-to-coset allocation problem.

Finite Automata as an Information Model for Manufacturing Execution System and Supervisory Control Integration

Abstract The use of MES (Manufacturing Execution System) is an alternative to link the ERP (Enterprise Resource Planning) to the activities at the shop floor. Due to the difference between the transactional feature of the ERP systems and the real-time features of the operations of the shop floor, MES becomes an important tool. SCADA (Supervisory Control And Data Acquisition) are responsible for monitoring the behaviour of shop floor and can interfere or alert in case of failures. The SCT (Supervisory Control Theory) is a formal methodology for modelling and control synthesis of DES (Discrete Event Systems). SCT appears to be an appropriate tool for this task, ensuring an optimal control logic that is minimally restrictive and non-blocking. The simultaneous use of MES, SCADA, ERP and controllers synthesized by SCT requires the creation of an interface between them. This creation requires development time, delays the project, being inflexible to change and is subject to failure. One of the formal models for the synthesis of DES controllers in the SCT is the DFA (Deterministic Finite Automaton). DFA is one of the computational models within the theory of automata. This paper shows the use of the DFA models to build the DES controller to interface SCADA and MES to save development time and to deal with flexibility and failure-tolerant issues.

Supervisory control synthesis of discrete-event systems using a coordination scheme

Supervisory control of distributed DES with a global specification and local supervisors is a difficult problem. For global specifications, the equivalent conditions for local control synthesis to equal global control synthesis may not be met. This paper formulates and solves a control synthesis problem for a generator with a global specification and with a combination of a coordinator and local controllers. Conditional controllability is proven to be an equivalent condition for the existence of such a coordinated controller. A procedure to compute the least restrictive solution within our coordination control architecture is provided and conditions under which the result coincides with the supremal controllable sublanguage are stated.