Yongmei Gan

Distributed supervisory control of discrete-event systems with communication delay

This paper identifies a property of delay-robustness in distributed supervisory control of discrete-event systems (DES) with communication delays. In previous work a distributed supervisory control problem has been investigated on the assumption that inter-agent communications take place with negligible delay. From an applications viewpoint it is desirable to relax this constraint and identify communicating distributed controllers which are delay-robust, namely logically equivalent to their delay-free counterparts. For this we introduce inter-agent channels modeled as 2-state automata, compute the overall system behavior, and present an effective computational test for delay-robustness. From the test it typically results that the given delay-free distributed control is delay-robust with respect to certain communicated events, but not for all, thus distinguishing events which are not delay-critical from those that are. The approach is illustrated by a workcell model with three communicating agents.

Exploiting symmetry of state tree structures for discrete-event systems with parallel components

We consider discrete-event systems (DES) consisting of parallel arrays of machines and buffers. The machines are divided into groups in each of which the members have identical structure, i.e. same state set and isomorphic transitions. In these systems, to avoid the underflow or overflow of the buffers, the controller only needs the information of the total numbers of components at each state and the numbers of workpieces in the buffers. By exploiting the identical structure of each group, we extract such control information from the control functions computed by the state tree structures (STS) to generate abstract control functions. Thanks to the symmetry of the system, we show that all controllable events relabeled to the same symbol share an invariant abstract control function, which is independent of the total number of machines, as long as the buffer sizes are fixed. The approach is illustrated by two examples.

Distributed supervisory control solution for under-load tap-changing transformers

Supervisory control theories of Discrete-Event Systems (DES) and Timed Discrete-Event Systems (TDES) are introduced to solve different control problems in power systems, e.g. the voltage control problem of under-load tap-changing transformers(ULTC), which are widely used in transmission systems to take care of instantaneous variations in the load conditions in substations. This paper employs the recently developed supervisor localization technique of TDES to solve the voltage control problem in a distributed fashion. Compared with the previous (monolithic) supervisory control approach, the newly designed distributed supervisory controllers have simpler and more transparent structures, but importantly achieve the same optimal and nonblocking behavior.

Exploiting symmetry of discrete-event systems with parallel components by relabeling

We consider discrete-event systems (DES) consisting of parallel arrays of machines and buffers. The machines are divided into groups in each of which the members have identical (isomorphic) structure. This feature allows event relabeling of the machines in a given group to a standard prototype machine. With respect to buffer specifications (prohibiting overflow and underflow) it is shown that optimal supervisory control of the original DES (with many machines) can be reduced to control of the much smaller collection of prototype machines. The result is a small “template” supervisor which is proved to be independent of the total number of original component machines, as long as the buffer sizes are held fixed. This result is applied to efficient reconfiguration triggered by the addition or removal of machines.

Delay-robustness in distributed control of timed discrete-event systems based on supervisor localisation

ABSTRACT Recently, we studied communication delay in distributed control of untimed discrete-event systems based on supervisor localisation. We proposed a property called delay-robustness: the overall system behaviour controlled by distributed controllers with communication delay is logically equivalent to its delay-free counterpart. In this paper, we extend our previous work to timed discrete-event systems, in which communication delays are counted by a special clock event tick. First, we propose a timed channel model and define timed delay-robustness; for the latter, a verification procedure is presented. Next, if the delay-robust property does not hold, we introduce bounded delay-robustness, and present an algorithm to compute the maximal delay bound (measured by number of ticks) for transmitting a channelled event. Finally, we demonstrate delay-robustness on the example of an under-load tap-changing transformer.

On computation of distributed supervisory controllers in discrete-event systems

This paper refines an earlier localization procedure used to design optimal nonblocking distributed supervisory controllers in discrete-event systems (DES). The earlier algorithm needed a control consistency relation involving two conditions, and introduced superfluous selfloops. We propose a refined procedure, including a newly defined weak control consistency relation, and a cleanup algorithm, which results in simpler and cleaner controllers. It is proved to be correct, and illustrated by a Transfer Line plant.

On the reconfiguration of flexible manufacturing workcells

Most supervisors of discrete-event systems we designed before are inadaptable to reconfiguration due to the change of plant structures or the modification of specifications. However, in real manufacturing process, such reconfiguration is needed to make the discrete-event systems more intelligent. To improve the resilience and adaptability of discrete-event systems, this paper proposes a methodology for reconfiguration. The reconfiguration details are shown in the supervisory control of a flexible manufacturing workcell, including the reuse of the original supervisor and the seamless switch between the original supervisor and the reconfigured one. Being built in this way, the original system can work as usual before the new supervisor is computed and then makes a smooth switch to the corresponding state of the new supervisor without waiting for the old supervisor returning to its initial state.

Representation of supervisory controls using state tree structures, binary decision diagrams, automata, and supervisor reduction

In the synthesis of an optimal nonblocking supervisor for a discrete-event system (DES), the problem of state explosion is a well-known computational obstacle. This problem can often be managed successfully by the use of state-tree structures (STS) and binary decision diagrams (BDD). Unfortunately BDD control functions may become quite large, and as such difficult to represent and interpret. In some cases it may, therefore, be convenient to convert an STS/BDD based controller to automaton form, and then apply a well known algorithm for supervisor reduction. In this paper we illustrate the advantage of this approach with a concrete example.