OPC UA Aggregation Servers - Enabling Vertical Integration of Production Machines for Digital Manufacturing

Production devices often lack interoperability, making it difficult to integrate them into a centralized system. Open Platform Communications Unified Architecture tackles this issue, combining a communication stack with an information modeling approach suited for production devices. While OPC UA standardizes server protocols and information models, it does not have the ability to integrate the device models and servers into a larger overarching structure with centralized access, e.g., for whole production processes. This gap is filled by OPC UA Aggregation Servers. The OPC UA Aggregation Server acts as an intermediary between clients and the underlying OPC UA servers deployed on production devices, constructing a unified information model. On the communication side, OPC UA service requests and responses are routed to the appropriate lower-level server. On the model side, OPC UA namespaces, types and instances of the underlying source servers are brought together, forming a unified OPC UA address space. This integrated model enables centralized access, analyses and processes. The objective of this paper is to unify, complement and expand existing approaches by providing a detailed view of the theory and implementation of OPC UA Aggregation Servers. This includes methods employed for type and instance aggregation as well as service delegation. Additionally, novel approaches are proposed: a hash-based type aggregation method, an instance aggregation method using reference injection and a mapping database design. The Aggregation Server is then demonstrated on a robotic assembly station. • Comprehensive overview of the different aspects of an OPC UA aggregation server. • Analysis of the current state of research regarding OPC UA aggregation servers. • A novel hash-based type aggregation method for improved reliabality of type matching. • A new method to create object structures spanning nodes from multiple source servers.