November/December 2020

36 n CONTROLS, HMIs AND SOFTWARE November/December 2020 www.drivesncontrols.com OPC UA: the protocol of choice for IIoT? A s the number of Industrial Internet of Things (IIoT) applications continues to multiply, OPC Unified Architecture (OPC UA) has rapidly become the communication protocol of choice for PLCs and PACs. It provides users with a secure, platform-independent, scalable, and object-oriented client- server architecture for representing and communicating information. Whenever an organisation collects, analyses, stores or moves data, security is essential, which is why it is an integral component of the OPC UA technology. OPC UA provides a mechanism for protecting the confidentiality and integrity of information, and for determining whether applications are trustworthy – a fundamental requirement of Industry 4.0. Specifically, the OPC UA server offers a set of services dedicated to creating secure connections. Once created, it applies the security protocol to messages between a client and server to ensure the integrity and confidentiality of the messages. UA security consists of authentication and authorisation, encryption and data integrity via signatures. Platform independence Inherent platform independence and scalability make OPC UA ideal for the industrial Internet. This enables intelligent devices with embedded OPC UA servers and clients to achieve bidirectional communications with other intelligent devices. An aggregation server can concentrate, normalise and enrich information from underlying servers, before making the aggregated information available to higher- level clients. This helps to reduce the number of connections that resource- limited devices need to manage. OPC UA’s platform independence is crucial for IIoT, which includes multiple components, often running on different operating systems. For example, an OPC UA aggregation server might run on Windows, while an OPC UA embedded server might run on a real-time operating system, such as VxWorks. The ability of OPC UA to provide secure data exchange, independent of the platform and operating system, is essential to converge disparate systems into one secure system. The resulting chain of systems – from low- level devices, to PLC and PAC systems, to enterprise applications – ultimately integrates with OPC UA to form a system of systems. Object oriented OPC UA servers are designed to expose information for clients to find and consume. The collection of information that servers make available to clients is called the AddressSpace, which standardises object representation. It defines objects in terms of variables, methods and their relationships to other objects. Furthermore, OPC UA AddressSpace unifies the three classic data models (data access, alarms and events, and historical data access) into one information model. This makes it easy to connect the dots between data values that are read, and events that are raised based on those data values. Being object-oriented means servers can provide type definitions for objects and their components. Because OPC UA uses object- oriented techniques, it can formulate an information model that serves a specific problem domain. By using ObjectType, VariableType, DataType, and ReferenceType OPC UA offers a secure, platform-independent, scalable, and object-oriented client-server architecture for communicating information. Steve Ward, Emerson’s director of application engineering for the EMEA region, explains how it is becoming the protocol of choice for creating IoT environments. The inner loop provides deterministic control and outer loop provides non-deterministic advice to the inner loop to achieve a goal – such as minimising a cost function. OPC UA as a secure, platform-independent and objected-oriented protocol, is an obvious choice for communicating between the loops. Coupling the knowledge of OPC UA type definitions with the OPC UA services for discovering information, the outer loop knows what information to seek and has the means to find that information. This allows outer-loop applications to be configured and deployed in a semi-automated manner.