On Object Oriented Nondeterministic Supervisory Control

Doctoral thesis, 1995

Implementation of complex discrete event fabrication processes can be considerably simplified by use of general reusable software modules representing the physical components. At the same time, construction of the control system can be facilitated by applying the supervisory control theory for the automatic generation of control laws. These two aspects can be joined into a general concept with object-oriented modeling and control law synthesis as foundations. The goal is to allow an operator to specify operation lists describing the required sequences of operations for the manufacturing of the product, independently of constraints given by a specific plant. With a suitable model of the capabilities and constraints of the resources of that plant, a product route can be automatically generated from the operation list. Such a product route describes all available paths through the system, for each type of product, irrespective of any other type of product that may be simultaneously present within the production system. Given a set of product routes and a model of the plant, control laws guaranteeing production according to those product specifications can be synthesized. Based on the supervisory control theory, using interleaved product routes as specification, we show how such control laws can be synthesized. An added complexity is that the specification becomes non-deterministic, in the sense that the same string of events can lead to different system states. We show that the supervisory control theory can be used with non-deterministic specifications assuming certain properties. An algorithm for synthesis of a non-deterministic supervisor is presented. We also describe an object-oriented modeling approach to discrete event fabrication processes. It is shown that the properties that have been defined as necessary for the non-deterministic supervisory approach are immediate by the modeling approach. Thus, we show that the approach to non-deterministic supervisory control can be combined with object-oriented modeling techniques, and so we have a powerful framework for implementing control of large and complex discrete event fabrication processes.