Guarded state machines in OPC UA

Abstract

Currently, an increasing number of initiatives are observed aiming at enriching automation environment with modern Information and Communication Technologies (ICT). The desired solutions should lead to increasing and facilitating interoperability of production systems and their components. To realize this vision, a key component is an open and standardized communication platform. Meanwhile, OPC Unified Architecture (OPC UA) is accepted as communication solution, especially because of its capabilities to create extensible information models. As part of the ongoing research project OPC4Factory, semantically rich OPC UA interfaces based on specific information models for a robotized manufacturing cell are developed. In addition to the static hierarchical structure, the information model should also describe the dynamic behavior of the modeled system. State machines provide proven means to model the dynamic behavior of manufacturing machines. However, whether a transition of a state machine can be executed often depends on certain conditions. In UML, these conditions are reflected by guards. Unfortunately, a similar functionality is neither provided by the current OPC UA specification nor by companion standards. For this reason, an amendment to the OPC UA information model is introduced, enabling guards in OPC UA. Thereby, new object and reference types allow to model operations in an OPC UA server's address space. Exposing this information has several advantages over hiding it in the server's source code regarding the required implementation effort as well as the client's possibilities to react to rejected transition requests.