Sheng-Luen Chung

Deadlock prevention policy based on Petri nets and siphons

This paper presents a deadlock prevention method for a class of flexible manufacturing systems where deadlocks are caused by unmarked siphons in their Petri net models. This method is an iterative approach consisting of two main stages. At each iteration, a fast deadlock detection technique developed by mixed integer programming is used to find an unmarked maximal siphon. An algorithm is formalized that can efficiently obtain an unmarked minimal siphon from the maximal siphon. The first stage, called siphons control, of the proposed method is to add, for each unmarked minimal siphons, a control place to the original net with its output arcs to the sink transitions of the minimal siphon. The objective is to prevent a minimal siphon from being unmarked. The second stage, called augmented siphons control, is to add a control place to the modified net with its output arcs to the source transitions of the resultant net if the resource places are removed. The second stage is required since adding control places in the first stage may create new unmarked siphons. In addition, the second stage assures that there are no new unmarked siphons generated. The relation of the proposed method and the liveness and reversibility of the controlled net have been obtained. Finally, manufacturing examples are presented to illustrate the method and to allow comparison with earlier methods.

Limited lookahead policies in supervisory control of discrete event systems

A supervisory control scheme which is based on limited lookahead control policies is proposed, using an online scheme where after the occurrence of an event, the next control action is determined on the basis of an N-step-ahead projection of the behavior of the process (represented as an N-level tree). This procedure then repeats after the execution of the next event. Different attitudes can be adopted to calculate a control action for the given N-level tree in order to resolve uncertainties about the process behavior beyond N steps. Two such attitudes, termed conservative and optimistic. are studied. Results are presented pertaining to monotonicity and convergence properties of the optimistic and conservative N-step policies in terms of N, and comparison of these policies with the optimal offline solution; closed-form expression for the language generated by the N-step conservative policy for prefix-closed legal languages; and lower bounds for N that guarantee that the online scheme performs as well as an offline scheme with complete information. >

Robust static output-feedback stabilization for nonlinear discrete-time systems with time delay via fuzzy control approach

This paper addresses the problem of designing robust static output-feedback controllers for nonlinear discrete-time interval systems with time delays both in states and in control input. In the approach, we do not directly employ the Lyapunov approach, as do in most of traditional fuzzy control design approaches. Instead, sufficient conditions for guaranteeing the robust stability for the considered systems are derived in terms of the matrix spectral norm of the closed-loop fuzzy system. The stability conditions are further formulated into linear matrix inequalities so that the desired controller can be easily obtained by using the Matlab linear matrix inequality (LMI) toolbox. Finally, an example is provided to illustrate the effectiveness of the proposed approach.

A deadlock prevention policy for flexible manufacturing systems using siphons

We present a new deadlock prevention algorithm for a class Petri nets. A new class of net that is extended from S/sup 3/PR, called ES/sup 3/PR, where deadlocks are related to unmarked siphons. This method is an iterative approach by adding two kinds of control places, called the ordinary control place and weighted control place, to the original model to prevent siphons from being unmarked. We obtained the relation of the algorithm, liveness and reversibility of the controlled net. Finally, a flexible manufacturing example is presented for illustrating the method.

Diagnosing PN-based models with partial observable transitions

Failure diagnosis back-traces failures based on an observed system behaviour when a failure occurs. Ushio et al. (1998) first studied the diagnosability of Petri net (PN) models, a common mathematical modelling structure for CIM systems. With transitions all assumed to be unobservable, the diagnosis process of Ushio et al. relies solely on marking changes at observable places, making diagnosability analysis of most non-trivial PN non-diagnosable. However, in the context of manufacturing process, the initiation of a command and the response of a process are readily available to supervisory controller. This paper, in contrast, assumes that part of the transitions of the PN modelling is observable. Under this assumption, the first contribution of this paper is to investigate how both the label propagation function and the range function, used to construct a diagnoser, are to be revised in order to take advantage of the newly available information provided by observable transitions. The second contribution is to present a procedure to construct, for PN modelling, the associated verifier, first proposed by Yoo and Lafortune (2001) as a polynomial check mechanism on diagnosability but for finite state automata models. As shown by examples, the additional information from observed transitions in general adds diagnosability to the analysed system.

Supervisory Control Using Variable Lookahead Policies

This paper deals with the efficient on-line calculation of supervisory controls for discrete event systems (DESs) in the framework of limited lookahead control policies (or LLPs) that we introduced in previous papers. In the LLP scheme, the control action after a given trace of events has been executed is calculated on-line on the basis of anN-step ahead projection of the behavior of the DES. To compute these controls, one must calculate after the execution of each event the supremal controllable sublanguage of a finite language with respect to another finite larger language. In our previous work, we showed how the required supremal controllable sublanguage calculation can be performed by using a backward dynamic programming algorithm over the nodes of the tree representation of these two languages. In this paper, we pursue the same approach for the calculation of LLP controls, but instead we adopt a forward calculation procedure over theN-level tree of interest. This forward procedure improves upon previous work by avoiding the explicit consideration of all the nodes of theN-level tree, while still permitting tree-to-tree recursiveness as enabled events are executed by the system. The forward search ends whenever a control decision can be made unambiguously or whenever the boundary of theN-level tree is reached, whichever comes first. This motivates the name “Variable Lookahead Policy” (or VLP) for this implementation of the LLP supervisory control scheme. This paper presents a general VLP algorithm and studies the properties of several special cases of it. The paper also discusses the implementation of the VLP algorithms and presents computational results regarding the application of these algorithms to a “time-varying” DES.

Failure diagnosis: a case study on modeling and analysis by Petri nets

Failure diagnosis in the context of DES was first formulated by Sampath et al. Where the notion of diagnosability and the associated diagnoser are proposed. Ushio et al. extended Sampaths study to systems modeled by Petri nets. This paper further assumes some of the transitions in a Petri net model are observable in the sense that its occurrence can be observed. The main contribution of this study shows how diagnosers and verifiers for systems modeled by Petri net are constructed accordingly. As shown by examples, the additional information from observed transitions adds diagnosability to the system.

An overview of semiconductor fab automation systems

A typical semiconductor wafer fabrication involves hundreds of complex manufacturing steps on capital-intensive equipment. In the persistent pursuit of near perfect hundred percents of production yield and equipment utilization, automation systems have been designed implemented to provide the functions of: direct equipment control, automated material transportation, and real-time lot dispatching. This paper presents an overview of typical semiconductor fab automation systems, of which two key systems are presented: (1) manufacturing executions systems (MES) that formulates manufacturing methods and procedures, and (2) cell controller that serves as the automation link between the upper MES and the lower equipment. At the end of the paper, lot operation of the lower equipment. At the end of the paper, lot operation of one manufacturing step is detailed to highlight the interaction between MES and cell controller within a fully automated semiconductor fab.

Recursive computation of limited lookahead supervisory controls for discrete event systems

We continue the study of limited lookahead policies in supervisory control of discrete event systems undertaken in a previous paper. On-line control of discrete event systems using limited lookahead policies requires, after the execution of each event, the calculation of the supremal controllable sublanguage of a given language with respect to another larger language. These two languages are finite and represented by their tree generators, where one tree is a subtree of the other. These trees change dynamically from step to step, where one step is the execution of one event by the system. We show in this paper how to perform this calculation in a recursive manner, in the sense that the calculation for a new pair of trees can make use of the calculation for the preceding pair, thus substantially reducing the amount of computation that has to be done on-line. In order to make such a recursive procedure possible from step to step, we show how the calculation for a single step (i.e., for a given pair of trees) can itself be performed recursively by means of a backward dynamic programming algorithm on the vertices of the larger tree. These two nested recursive procedures are also extended to the limited lookahead version of the “supervisory control problem with tolerance.”

Design, analysis and implementation of a real-world manufacturing cell controller based on Petri nets

The primary function of a flexible manufacturing cell controller is to coordinate equipment within a cell to perform a sequence of manufacturing operations required by flexible production. In this paper, we present the modelling, analysis and implementation of a real-world cell controller. A Petri net-based development environment for cell controller design and implementation is described. The intended advantage of the proposed environment is that, users can define and/or change operation flow directly on the modelled Petri net, without programming efforts, otherwise required for general cell controller implementation. The proposed environment is used to design and implement a real-world flexible manufacturing cell controller at National Taiwan University of Science and Technology (NTUST). A scenario-based approach is adopted to model the operation flows in the cell, and from the scenario, we construct its Petri net model. To our knowledge, this is the first work that formalizes the conversion of scenarios to Petri nets. The resultant net is an augmented marked graph (Chu and Xie, 1997, IEEE Trans. Robotics Automat., 13(6), 793 - 804), whose liveness and reversibility are related to empty siphons. For qualitative analysis, after we reduce the net, we are able to generate all minimal siphons and traps, and show that the cell model is live and reversible.